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Effect of nanostructuring and Al alloying on friction and wear behaviour of thermal sprayed WC–Co coatings
Abstract
Nanostructured WC–Co and WC–Co–Al coatings, with about 300-μm as-deposited coating thickness, were deposited by high velocity oxy-fuel (HVOF) spraying. Agglomerated nanostructured cermet powders produced by the Mechanomade® process was used for HVOF spraying. Dense and well-adherent coatings with crystal sizes below 30 nm were deposited on stainless steel 304 substrate. Porosity was less than 5% and the bond strength with the substrate was around 60 MPa. Experimental data on friction, wear, and abrasion resistance revealed that nanostructured WC–Co based coatings containing some Al as alloying element, exhibit improved tribological characteristics in comparison to nanostructured and micron-sized WC–Co coatings. This was attributed to a carbide particle distribution within the coating revealed by SEM, the absence of brittle W2C-like phases revealed by XRD, and the presence of Al at particle/matrix boundaries revealed by TEM.
... Low-and high-water pressure levels combined with high traverse speed seem to decrease the surface quality by increasing the damages, while increasing the abrasive feed rate reduces the delamination and fibre pull out, which could be attributed to the increased cutting ability of the waterjet. This cavitation damage is somewhat similar to what also occurs in the metallic/ceramic system in the presence of abrasives [34,35]. Other surface damage occurred in the form of cavitation and pitting ( Figure 6), as reported earlier. ...
... Low-and high-water pressure levels combined with high traverse speed seem to decrease the surface quality by increasing the damages, while increasing the abrasive feed rate reduces the delamination and fibre pull out, which could be attributed to the increased cutting ability of the waterjet. This cavitation damage is somewhat similar to what also occurs in the metallic/ceramic system in the presence of abrasives [34,35]. behaviour from the waterjet would also be responsible for the pitting within the internall cut surface. ...
... Low-and high-water pressure levels combined with high traverse speed seem to decrease the surface quality by increasing the damages, while increasing the abrasiv feed rate reduces the delamination and fibre pull out, which could be attributed to th increased cutting ability of the waterjet. This cavitation damage is somewhat similar t what also occurs in the metallic/ceramic system in the presence of abrasives [34,35]. Based on the above experimental results and analysis, it could be summarized that the best overall results to reduce the circularity and cylindricity deviation and the kerf taper ratio were seen at a low level of the abrasive feed rate (200 g/min), a low level of the traverse speed (60 mm/min), and at all three levels of the water pressure (275, 345, and 413 MPa). ...
The effect of the water pressure, traverse speed, and abrasive feed rate on the circularity, cylindricity, kerf taper, and surface roughness of holes produced by abrasive waterjet machining (AWJM) of a carbon-fibre-reinforced polymer (CFRP) composite was investigated in the current study. It was found that the circularity deviation decreased as the water pressure was increased. Cylindricity was affected by all three parameters, although the abrasive feed rate caused the largest deviations as it increased. The surface roughness was affected by all three, but a clear connection was not able to be concluded. The kerf taper ratio reduced with an increase in water pressure, while it increased with an increase in the abrasive feed rate and an increase in the traverse speed. To obtain optimum results, the water pressure should be increased, the traverse speed should be decreased, and the abrasive feed rate can remain constant but is recommended to be slightly reduced.
... Among different coating deposition processes, high-velocity oxy-fuel (HVOF) spraying and air plasma spraying (APS) are widely used. In recent years, thermal spraying using nanostructured feedstock powders has yielded coatings with higher hardness, strength, and corrosion resistance than the corresponding conventional coatings (Basak et al., 2012). Moreover, materials in their wrought or cast form (e.g., stainless steel) are generally less wear resistant than their thermally sprayed counterparts (Basak et al., 2007). ...
... Thermal spraying is very flexible and allows buildup of thick coatings from many materials, as shown in Figure 14.3. During such deposition, a very limited crystal growth can be achieved that allows for a buildup of coatings with a refined structure, down to a size of 30 nm in the case of nanostructured WC-Co-based coatings (Basak et al., 2012). ...
... Figure 14.5 shows the polished sections of APS nanostructured Fe-Al-Al 2 O 3 coatings (Basak et al., 2008). Basak et al. (2008) has done extensive studies on this and proven the superiority of nanostructured WC-12Co coatings compared with microsized counterparts (Basak et al., 2008(Basak et al., , 2010(Basak et al., , 2012. ...
The quest for advanced functional materials with superior functionality is being driven in various frontiers of engineering materials. One such frontier is the research of metal matrix composites (MMCs), which has already been proven as one of the most productive fields. With advance technology, it is possible to reinforce the MMCs with nanosized particles compared with conventional micron-sized counterparts. However, the addition of nanoparticles to MMCs to improve their mechanical properties is not unconditional, and there are several factors that should be taken into consideration. This present chapter gives an overview of such considerations along with recent trends in this field.
... Among them, the Al-12Si (wt.%) alloy is of particular interest due to its outstanding castability in the development of parts of intricate geometries. The high percentage of silicon in alloy composition improves castability, whereas the eutectic composition of Al-Si alloys have the low melting point and good melt fluidity [9]. It also reduces the thermal elongation coefficient and enhances resistance to wear [9] and corrosion-wear [10], meanwhile also contributing to a reasonable improvement in mechanical strength [11]. ...
... The high percentage of silicon in alloy composition improves castability, whereas the eutectic composition of Al-Si alloys have the low melting point and good melt fluidity [9]. It also reduces the thermal elongation coefficient and enhances resistance to wear [9] and corrosion-wear [10], meanwhile also contributing to a reasonable improvement in mechanical strength [11]. ...
Additive manufacturing (AM) has evolved as an innovative approach to building 3D objects by addition of layer-upon-layer of material, which has brought some added industrial benefits from the microstructural processes and mechanical behavior aspects. In the current study, Al-12Si (wt.%) alloy has been fabricated based on AM utilizing the laser powder bed fusion (L-PBF) technique. Its mechanical behavior and microstructural processes formation were investigated and compared to those of a cast alloy with a similar elemental composition. It was observed that the former alloy exhibits a hierarchical microstructure consisted of cellular sub-structures (~ 600 nm), melt pool, and grain boundaries which is outstandingly different from that of cast alloy. The cast form of this alloy exhibit a needle-like eutectic Si structure. However, L-PBF processed of the same alloy shows a finely dispersed phase of Si. This difference in microstructure substantially affect the mechanical properties (elastic-plastic behaviour) of the material. Based upon indentation examination (hardness testing), the nanohardness of the L-PBF fabricated alloy (1.3 – 1.09 GPa) was found to be about 1.85 times higher than that of the cast alloy (0.62 GPa). Furthermore, a remarkable improvement in shear strength of the L-PBF processed alloy, between 0.40 -0.41 GPa, was achieved in comparison to that 0.32 GPa of the cast alloy.
... However, with the decreases of carbide size, nanostructured WC-Co powder decarburized in the spraying process, which should be avoided as much as possible. Bimodal powders have been produced to minimize this effect, keeping higher resistance to corrosion when compared with conventional ones [22][23][24]. ...
... Moreover, the nano-sized WC particles are evenly distributed around the WC, which acts as a dispersion strengthening and fine grain strengthening. In addition, the bond strength test of the WC-10Co4Cr coatings shows that the average bond strength is approximately 68 MPa, indicating the WC-10Co4Cr coatings prepared by HVOF have a good bond strength [24]. ...
Two kinds of WC-10Co4Cr composite coatings with conventional (micron-sized WC particles) and bimodal (a mixture of nano-sized and micron-sized WC particles) structures were successfully prepared on 35CrMo steel (ANSI/ASTM 4135) substrate by high velocity oxygen fuel (HVOF) technology. Scanning electron microscope (SEM) equipped with the energy dispersive spectroscopy (EDS), mechanical testing machine, microhardness tester were used to analyze the characteristics of the coatings. The coatings were immersed in the simulated seawater drilling fluid under the high pressure to simulate the deep-sea environment, and then electrochemical impedance spectroscopy (EIS), potentiodynamic polarization were carried out to investigate the electrochemical properties of the coatings and substrate. The results show the WC-10Co4Cr coating has excellent corrosion resistance and can effectively protect the substrate under the high pressure. Besides, compared with the conventional coating (CC), the bimodal coating (BC) has a denser microstructure, superior mechanical properties, lower porosity, and better corrosion resistance. The corrosion mechanism of the coatings in the simulated seawater drilling fluid under high the pressure is as follows: The micro-galvanic corrosion between the WC phase and the CoCr binder phase is the primary corrosion mechanism. Besides, the electrolyte under the high pressure is more penetrable, so the crevice corrosion also exists.
... Great efforts have been made to improve the wear resistance of WC-Co coatings in order to satisfy the growing demands for high-performance surface protection materials. The methods that are used to enhance the properties of WC-Co coatings can be summarized as (i) introduction of metals or ceramics to the starting powder [5][6][7][8][9][10], (ii) decreasing the WC grain size [11][12][13] or optimizing the combination of WC grains with different sizes [14][15][16], (iii) structural modification and improvement of the physical properties of the spray feedstock [17][18][19][20], (iv) innovation of thermal spraying technologies and optimization of spray parameters [21][22][23][24][25], and (v) post-treatments for coatings [26][27][28]. In most cases, these methods were used in combination. ...
... Basak et al. demonstrated that the wear resistance of the nanostructured WC-Co coating with 1 wt.% Al was more than twice as high as that without Al, and three times higher than that of the micron-sized WC-Co coating [8]. The presence of Al mainly increases the cohesion within the coating, forms Al 2 O 3 as well as reduces the porosity of the nanostructured coating. ...
A new type of WC-based coating with high oxidation- and wear-resistance at elevated temperature was fabricated by thermal spraying the pre-treated WC-Co powder doped with WB. Addition of WB led to in situ formation of WCoB, which acted as a substitute for Co in the powders and the resultant coatings. It was shown by thermal analysis that WCoB has obviously higher oxidation resistance at high temperatures than that of WC and Co. Thus, the oxidation of the WC-WCoB coating was mainly initiated from WC, rather than from Co in the conventional WC-Co coatings. Most of WCoB was preserved in the coating after high-temperature wear tests. Particularly, with an addition of 40 wt.% WB, the wear rates of the WC-Co coating were dramatically decreased by 90% and 77% at the room and elevated temperatures, respectively.
... The present paper aims to characterize the properties of coatings obtained by High-Velocity Oxygen Fuel (HVOF) spraying of a TiC-based hardmetal powder with Fe-based alloy matrix, produced through an industrial high-energy ball milling process already in use for large-scale powder production, as previously illustrated in [42,43]. The production of the feedstock presented in this study is therefore amenable for direct, cost-effective scale-up. ...
... As a term of comparison, commercially available WC-10Co-4Cr [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] (WOKA 3652) and Cr 3 C 2 -25(Ni-20Cr) (WOKA 7102) powders (both from Oerlikon Metco WOKA GmbH, Barchfeld, Germany) were also sprayed by the same HVOF torch, using previously optimized process parameters listed in Table 1. ...
As an alternative to WC-CoCr and Cr3C2-NiCr coatings for wear and corrosion protection, a TiC – 25 vol% (Fe-20 wt%Cr-5 wt%Al) powder, free from hazardous and/or supply-critical elements (Ni, Co, W), was produced by high-energy ball-milling and processed by High Velocity Oxygen-Fuel (HVOF) spraying, obtaining dense (<1 vol% porosity), hard (HIT > 12 GPa) layers with reasonably good deposition efficiency of ≈ 54%.
Tribological testing revealed that the TiC-FeCrAl coatings are particularly promising for sliding contacts, as their ball-on-disc wear rates against an Al2O3 counterpart were lower than those of an HVOF-sprayed Cr3C2-NiCr reference, both at room temperature and at 400 °C, although they could not match the performance of WC-CoCr. At room temperature, brittle fracture along oxidized lamellar boundaries caused localized spallation, releasing debris in the contact region, but, in the incubation period before spallation cracks could propagate, remarkably low friction (≈0.27) was recorded. At 400 °C, spallation was largely suppressed by thermal softening, whilst coarser abrasive grooving became the dominant wear mechanism.
TiC-FeCrAl coatings appeared less suited to high-stress abrasion, since extensive brittle fracture resulted in higher wear rates than HVOF-sprayed Cr3C2-NiCr, and to (acidic) corrosive environments. Electrochemical polarisation tests in 0.1 M HCl indeed revealed limited corrosion resistance of the FeCrAl matrix.
... Z uwagi na szczególne właściwości fizyczne i mechaniczne znacznie przekraczające właściwości materiałów krystalicznych i mikrokrystalicznych, materiały nanostrukturalne i nanopowłoki stanowią od wielu lat przedmiot wszechstronnych badań [13,14]. Na przestrzeni minionych lat wielokrotnie wykazano, że zastosowanie materiałów nanostrukturalnych do wytwarzania wysokojakościowych powłok zapewnia im dużą odporność na ścieranie, niski współczynnik tarcia (również w wysokich temperaturach) i odporność na chemiczne oddziaływanie środowiska [9,13]. ...
... 7a) wskazują, że powłoki te charakteryzują się również znacząco mniejszym zużyciem ściernym (wyrażonym przez głębokość wytarcia) niż inne badane materiały odniesienia. Szczegółowe wyniki badań przedstawiono we wcześniejszych publikacjach autorów [9,14]. Testom odporności na zużycie ścierne poddano również powłoki cermetalowe WC-CoAl natryskiwane zimnym gazem (rys. ...
... However, decomposition and decarburisation of the WC still occur during the spraying process. The undesirable W 2 C phases, tungsten oxide, and amorphous or nanostructured Co-W-C compounds are formed during decarburisation of the WC leading to brittleness coating [15][16][17]. Several investigations have been conducted to evaluate the harmful effects of these phases on the friction properties and thus suggested many solutions for reducing these harmful effects [15][16][17][18][19]. ...
... The undesirable W 2 C phases, tungsten oxide, and amorphous or nanostructured Co-W-C compounds are formed during decarburisation of the WC leading to brittleness coating [15][16][17]. Several investigations have been conducted to evaluate the harmful effects of these phases on the friction properties and thus suggested many solutions for reducing these harmful effects [15][16][17][18][19]. Some of these solutions include the usage of the inert gasses [20,21] and optimisation of HVOF parameters such as in-flight particle velocity and fuel/oxygen ratio [22,23]. ...
In this study, electroless nickel–phosphorus plating was optimised for WC–Co powder using different bath loads and then subjected to a high-velocity oxygen fuel process. The produced Ni–P-coated WC–12Co powders were subsequently used as HVOF feedstock materials. Based on the SEM results, an increase in the bath load resulted in a decrease in the thickness of the Ni–P coating formed on the WC–12Co powders. Moreover, applying the HVOF process led to a lower porosity and roughness and a higher thickness of nickel coating, whereas applying a suitable thickness of this coating can prevent the destructive W2C phase. Finally, a bath load of 16 g L–1 was determined as the optimum value to achieve a proper thickness of the Ni–P, extremely low porosity (0.3%), good surface roughness (5 μm), and high microhardness (1112 Hv) in the produced coating.
... Great efforts have been made to improve the wear resistance of WC-Co coatings in order to satisfy the growing demands for high-performance surface protection materials. The methods that are used to enhance the properties of WC-Co coatings can be summarized as (i) introduction of metals or ceramics to the starting powder [5][6][7][8][9][10], (ii) decreasing the WC grain size [11][12][13] or optimizing the combination of WC grains with different sizes [14][15][16], (iii) structural modification and improvement of the physical properties of the spray feedstock [17][18][19][20], (iv) innovation of thermal spraying technologies and optimization of spray parameters [21][22][23][24][25], and (v) post-treatments for coatings [26][27][28]. In most cases, these methods were used in combination. ...
... Basak et al. demonstrated that the wear resistance of the nanostructured WC-Co coating with 1 wt.% Al was more than twice as high as that without Al, and three times higher than that of the micron-sized WC-Co coating [8]. The presence of Al mainly increases the cohesion within the coating, forms Al 2 O 3 as well as reduces the porosity of the nanostructured coating. ...
A new kind of WC-based coating with superhard WCoB compound as the binder was fabricated by the high velocity oxy-fuel spraying of WC-WB-Co powder. The microstructure, mechanical and tribological properties of the WC-WCoB coating were investigated, together with those of the conventional WC-Co coating for comparison. The results demonstrated that the WC-WCoB coating has simultaneously improved hardness and fracture toughness, and thus remarkably decreased wear rate as compared to the conventional coating. The enhanced tribological properties of the WC-WCoB coating are attributed to the low plastic deformation and the resultant inhibition of the micro-ploughing wear and the increased fracture toughness and interfacial bonding, which can reduce the amount of large cracks. Moreover, the high intrinsic hardness of WC and WCoB, as well as their good interfacial bonding, are more effective in resisting against wear as compared with the conventional coating.
... The mechanism of W 2 C formation in mc-WC coating is delineated by schematic in Fig. 4d. The extensive grain growth of WC in nc-WC coating is caused by the high temperature of HVOF flame resulting in the coalescence of adjacent nano-sized WC to form larger particles (more details on grain growth mechanisms of nano-sized WC are provided in [36,37]). Despite the decarburized region that indicated a core-shell structure of mc-WC (Fig. 4), W-2 region of nc-WC coating is characterized by a large number of discrete W 2 C particles with a rounded morphology (Fig. 5b). ...
... Apart from the protective role of the Ni layer against decarburization, this layer was found to be effective in suppressing WC grain growth during HVOF spraying of Ni/mc-WC and Ni/nc-WC as compared to mc-WC and nc-WC ( Supplementary Information, Fig. S3). The reason is that, due to the presence of Ni outer layer, WC-Co particles located in the core position of Ni/mc-WC and Ni/nc-WC powders experience lower temperatures during HVOF process causing the WC grain growth, which mainly occurs by coalescence mechanism [37], to be highly retarded. ...
In this study, we addressed the effects of Ni-plating and nanostructuring of WC-Co powders on the microstructure-property relations and WC decarburization of WC-Co composite coatings. By high velocity oxygen fuel (HVOF) spraying, the coating materials were produced from four different powders, referred to as microstructured WC-Co (mc-WC), nanostructured WC-Co (nc-WC), Ni-plated microstructured WC-Co (Ni/mc-WC) and Ni-plated nanostructured WC-Co (Ni/nc-WC). We found that the coatings, deposited from Ni-free powders, undergo a high level of decarburization, thereby yielding W2C phase formation in brittle amorphous W-rich Co matrix that could be preferential site for crack propagation. On the contrary, the Ni-plating treatment could reduce the amount of decarburization of the coatings, resulting in the uniform distribution of WC phase in the W-depleted Co matrix. Consequently, we revealed the underlying mechanisms responsible for WC decarburization in the coatings, and further suggested that the coating deposited from Ni-plated and nanostructured powders exhibits a superior combination of hardness and fracture toughness as compared to other coatings.
... Aluminum metal matrix composites (MMCs) play a big part in composite materials and are widely used in various structural applications as MMCs possess strong advantages in their mechanical properties [1,2], such as being lightweight, high stiffness, specific strength, and wear [3] and corrosion-wear [4] resistance. Despite all these advantages, the machining of MMCs is challenging due to the reinforced particles, which are hard and highly abrasive by nature [5][6][7][8]. ...
The extensive industrial applications of aluminum (Al) metal matrix composites (MMCs) require optimal drilling conditions in the manufacturing industries. The effects of drilling variables, for instance, the point angle, spindle speed and feed rate on torque, thrust force, burr, surface quality, and dimensional errors were analysed while producing holes in MMC. The results showed that the torque and thrust force decreased with an increase in point angle and speed but increased with the rise in feed rate. Similar trends were also noticed for the chip thickness as these parameters vary. The burr height, circularity, and diameter error at the exit and entrance of the workpiece material were highly impacted by the point angle. In addition, the feed rate showed a strong influence on torque, burr height at the exit of the workpiece, and chip thickness. Thrust force and circularity as well as the diameter error at the exit of the workpieces were highly affected by the feed rate. The variation in surface roughness was not significant with the variations in the parameters considered in this investigation.
... The strong tendency of these coatings to decarburize and dissolve during the thermal spraying process, which can impair their mechanical qualities, has long been a challenge for nanostructured WC-Co-based coatings [42]. In addition, the results of studies showed that HVOF sprayed nanostructured WC-Co based coatings had higher wear, corrosion, and cavitation erosion resistance than conventional ones and were better bonded to the substrate with a high fraction of retained near-nano WC grain, low porosity, and a low quantity of harmful reaction products [43]. ...
Application of Kaplan turbines is widespread in low-water-head and large-capacity hydropower plants. An understanding of the failure mechanism of Kaplan Turbines is a key factor to provide useful solutions for their prevention or early treatment and to guarantee their workability. The long-term performance of Kaplan turbines depends on many factors such as cavitation, erosion, fatigue, and material defects. Cavitation in Kaplan turbines leads to flow instability, vibrations, surface damage, and reduce the machine performance. Therefore, this paper investigates the factors leading to cavitation in Kaplan turbine and presents practical solutions for it. Thermal-sprayed coatings are frequently applied due to their high wear resistance, cost effectiveness, weight reduction, and less negative impacts on base metal. Moreover, HVOF is used to create coatings with a high density and bonding strength. At high temperatures, cermet coatings, including nanoparticles, exhibit exceptional wear resistance. WC-based nanostructured and multifaceted coatings are utilized due to their high wear resistance. In addition, chromium carbide in WC-based coatings increases their oxidation and wear resistance.
... Fig. 7 confirmed the occurrence of voids, pores and unmelted powders with the evidence of a 'cup and cone' like fracturing. Such unconsolidated regions may be detrimental to subsequent applications of materials that involve corrosion-wear [30] and wear [31], as reported in the literature. All other fractured surfaces, regardless of printing variables, displayed identical morphology, that is, predominantly ductile fracture [32], except along fusion lines, where a cleavage-type fracture was dominant. ...
... However, with the advancement of the research in that area, other fabrication routes, such as 'magnetron sputtering' [6], 'laser cladding' [34], and 'thermal spraying' [35] are gaining more and more attention to deposit this HEA as a coating form on a given substrate. Among different 'thermal spraying', atmospheric plasma spray (APS) [36] and high-velocity oxy-fuel (HVOF) [37] were suggested for HEAs coating deposition [38]. In our recent communications [39], successful deposition of AlCoCrFeNi coating on stainless steel substrate has been reported, together with their physical, tribological [35,40] and electro-chemical [41] behaviour explorations. ...
Atmospheric plasma spraying (APS) was used to deposit an AlCoCrFeNi high entropy alloy (HEA) coating on stainless steel substrate. The as-deposited coating was about 300 μm thick and the microstructure consisted of a nickel solid-solution matrix, together with a number of secondary phases/intermetallics. In addition, phase and splat boundaries were also prevailing. However, the extent of intermetallics and secondary phases were supressed, compared to other processing techniques (e.g., melting) of HEA, due to fast solidification in the APS process. In-situ micro-pillar compression was employed to evaluate the mechanical properties of the coating. The experimental results show that, mechanical properties of the coating in the cross-sectional direction (963.14 ± 28.58 MPa of yield strength and 1005.58 ± 22.08 MPa of compressive strength) is marginally higher than that of planar direction (802.33 ± 43.76 MPa of yield strength and 817.73 ± 43.84 MPa of compressive strength). The presence of secondary phases and/or intermetallics in the coating microstructure acts as a reinforcement medium to allow effective load bearing capacities of the coating. On the hindsight, phase and splat boundary areas are the ‘weakest link’, that acts as a slip/shear plan initiation site. The orientation of these phase/splat boundaries to that of direction of loading plays a significant role on the coating failure mode.
... Traditionally, the HEAs are formed via casting into bulk form, which forms different detrimental oxide phases in the alloy. This can be avoided when the HEAs are deposited in the form of coating by 'vacuum plasma spraying' (VPS) [18,19], 'atmospheric plasma spraying' (APS) [20], or 'high-velocity oxygen fuel' (HVOF) spraying [21][22][23]. Among them, APS process is simple in operation and less expensive. ...
NiCoCrAlY high entropy alloy (HEA) coating (47.1 wt.% Ni, 23 wt.% Co, 17 wt.% Cr, 12.5 wt.% Al, and 0.4 wt.% Y) was deposited on a stainless steel subtract by atmospheric plasma spraying (APS). The as-deposited coating was about 300 μm thickness with <1% porosity. The microstructure of the coating consisted of dispersed secondary phases/intermetallics in the solid solution. The stress–strain behaviour of this coating was investigated in micro-scale with the help of in situ micro-pillar compression. The experimental results show that yield and compressive stress in the cross-section of the coating was higher (1.27 ± 0.10 MPa and 2.19 ± 0.10 GPa, respectively) than that of the planar direction (0.85 ± 0.09 MPa and 1.20 ± 0.08 GPa, respectively). The various secondary/intermetallic phases (γ′–Ni3Al, β–NiAl) that were present in the coating microstructure hinder the lattice movement during compression, according to Orowan mechanism. In addition to that, the direction of the loading to that of the orientation of the phase/splat boundaries dictate the crack propagation architecture, which results in difference in the micro-mechanical properties.
... As a member of the Nickel-based superalloys family, Inconel 718 is well known to maintain its high strength in severe working conditions, including high temperatures, and possess excellent resistance against oxidation [1] , corrosion [2], wear [3] and corrosion-wear [4]. Thus, it is a sought-after material for a number of high-end applications, such as engines for aircrafts and rockets, the structural frame of rockets, cryogenic and petrochemical structures, turbines for power stations and chemical processing industries [5]. ...
Inconel 718, a Ni-based superalloy, was prepared by powder laser bed fusion (P-LBF) additive manufacturing (AM) method. AM of the Inconel 718 is challenging, as this particular alloy possesses a higher melting point, as well as resistance to temperature, which induces ‘hot-cracking’. Thus, the selection of proper input parameters are essential, which was successfully achieved in this present study. The microstructure exhibits a hierarchical structure, that starts with equiaxed and sub-cellular (∼100–600 nm) grains, and ends up on the formation of melt-pool and grain boundaries. This structure is exceptional and completely dissimilar to that of wrought alloy, which is infested with twins and precipitates of different secondary phases. However, the effectiveness of such microstructure of L-PBF alloy towards material's strength was undermined, as it lacks the presence of abundant precipitates, which is the major contributor to the strength of such alloys. The strength of the L-PBF processed alloy was in the range of 413–491 MPa and 562–665 MPa respectively, for yield and ultimate compressive strength. This was marginally higher than that of wrought alloy (408 MPa of Yield and 656 MPa of compressive strength). This was attributed to the absence of abundant gamma/gamma double prime (γ'/γ'') phases, and the presence of laves and delta (δ) phases, that are inferior towards the overall strengthen of the Inconel 718. Cross-sectional SEM and TEM investigation of the deformed micro-pillars confirmed the prevailing deformation mechanism that can be attributed to the slip/share plane formation in the presence of dislocation loops.
... It enables us to reduce the number of layers required and print time. Topology optimisation (TO) and hybrid analytical thermal topology optimisation (HATTO) of cermet composites and multi-material printed components have been implemented to optimise specific properties such as maximising stiffness while minimising stress concentration, as well as material usage and weight [66][67][68][69]. Material properties and design constraints are considered, and optimal material distribution is also identified [69]. ...
The advent of additive manufacturing (AM) in Australian small and medium-sized enterprises offers the direct benefits of time-saving and labour cost-effectiveness for Australian manufacturing to be highly competitive in global markets. Australian local businesses can tailor their products to a diverse range of customers with a quicker lead time on the sophisticated design and development of products under good quality control in the whole advanced manufacturing process. This review outlines typical AM techniques used in Australian manufacturing, which consist of vat polymerisation (VP), environmentally friendly AM, and multi-material AM. In particular, a practical case study was also highlighted in the Australian jewellery industry to demonstrate how manufacturing style is integrated into their manufacturing processes for the purpose of reducing lead time and cost. Finally, major obstacles encountered in AM and future prospects were also addressed to be well positioned as a key player in the revolutionised Industry 4.0.
... Li et al. [54] used LBM and EDM in sequence to produce micro-hole in fuel injecting nozzle. This hybrid methodology eradicated the heat-affected zones and recast layer usually generated in laser drilling which is a kind of nanostructured composite [55,56] as it contains the elements from the base material along with surrounding medium. This hybrid procedure reduced drilling time by 70% compared to that by EDM drilling alone. ...
This study investigates the laser beam machining mechanism, surface formation mechanisms, heat-affected zone, taper formation, and the dimensional deviation of the titanium alloy, based on the information available in literature. The heat induced by the laser beam melts and vaporises titanium alloy, which is removed by a high pressure-assisted gas. The machined titanium alloy surface is expected to have craters and resolidified materials which were contributed by the low thermal conductivity of the titanium alloy. Taper and circularity error can be minimised by optimising the laser parameter, but it cannot be avoided in the laser beam machining of titanium alloy. Laser beam machining induces a non-diffusion phase transformation, which slightly changes the surface mechanical properties of the titanium alloys. Laser beam machining is gaining popularity as a way to improve the surface finish quality and properties of titanium components manufactured by additive manufacturing processes. To enhance the machining efficacy of titanium alloys, several hybrid machining processes were proposed.
... These discrete oxide particles formed due to the oxidation of the magnesium in course of the stirring process as well as break down of any residual oxide particles incorporated during the casting process. This gives rise to a structure somewhat resemble to particle matrix composites [29,30]. The melting point of Mg is 923 K [31] and the temperature can rise up to 700 K [12] during FSP. ...
This investigation deals with the microstructural and mechanical behaviours of friction stir processed (FSPed) commercially available pure magnesium with different tool profile geometry. The micro-level properties were assessed through in-situ micro-pillar compression which showed that FSPed material experienced significant plastic deformation and even re-distribution of residual oxide particles which was present in the cast magnesium together with grain refinement (from 3.4 μm to 2.6–2.7 μm). These changes in microstructure contributed to raise the compressive and yield strengths of the FSPed magnesium, up to 99 MPa and 94 MPa respectively, compared to 33 MPa of cast magnesium, when triangular tool profile was employed. Furthermore, the deformed material acquires ductile breakage with improved plasticity (micro-necking) irrespective of processing conditions, that is, either cast or friction stir processed.
... The strong tendency of these coatings to decarburize and dissolve during the thermal spraying process, which can impair their mechanical qualities, has long been a challenge for nanostructured WC-Co-based coatings [42]. In addition, the results of studies showed that HVOF sprayed nanostructured WC-Co based coatings had higher wear, corrosion, and cavitation erosion resistance than conventional ones and were better bonded to the substrate with a high fraction of retained near-nano WC grain, low porosity, and a low quantity of harmful reaction products [43]. ...
Cavitation in Kaplan turbines leads to flow instability, vibrations, surface damage, and reduce the machine performance. Therefore, this paper investigates the factors leading to cavitation in Kaplan turbine and presents practical solutions for it. Thermal-sprayed coatings are frequently applied due to their high wear resistance, cost effectiveness, weight reduction, and less negative impacts on base metal. Moreover, HVOF is used to create coatings with a high density and bonding strength. At high temperatures, cermet coatings, including nanoparticles, exhibit exceptional wear resistance. WC-based nanostructured and multifaceted coatings are utilized due to their high wear resistance. In addition, chromium carbide in WC-based coatings increases their oxidation and wear resistance.
... The friction coefficient profile of the ST-HVOF coating indicates a short running-in period in the sliding distance of 13 m, in which the friction coefficient reached 0.59 due to applying static frictional forces [56]. After this period, the friction coefficient declined sharply to 0.5 in 200 m and then increased to 0.6 with sharp fluctuations. ...
In this study, the Stellite 6 coating was applied on 316 stainless steel substrates through the high-velocity oxygen fuel method (ST-HVOF). Then, the laser-glazed sample (ST-Glazing) was deposited on the as-sprayed Stellite 6 coating using optimal conditions. Afterward, the Stellite 6 powder was optimized on the stainless-steel substrate by laser cladding process (ST-Clad) using 300 W and 5 mm/s of laser power and beam scanning speed, respectively. The microstructural characterization and phase analysis of ST-HVOF, ST-Clad, and ST-Glazing coatings were performed by field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) techniques. The microscopic observation results revealed that laser glazing significantly decreased the porosity of ST-HVOF (from about 2.3% to about 0.3%) and formed a dense coating with a uniform microstructure and strong adhesion to the substrate. Also, ST-Clad coating showed a porosity of 0.2% and a compact and uniform microstructure with a high-strength bonding to the substrate. Tribological evaluation of the coatings indicated a 67% and 58% increase in wear resistance of ST-Clad and ST-Glazing coatings compared to ST-HVOF coating, respectively. The high-temperature wear results suggested the following mechanisms for the studied coatings: plastic deformation mechanisms, wear adhesion, brittle fracture, and delamination.
... The addition of the Al in a small amount into the nanostructures WC-Co coatings improved the wear resistance of the coatings. Al helps to improve the cohesion between the coatings resulting in the reduction of porosity, and also helps by forming Al 2 O 3 [123]. ...
Components working under harsh environments in power generation, marine, and aerospace sectors are subjected to severe surface degradation because of wear, corrosion, and erosion by solid particles, slurry, silt, and cavitation. Carbide-based materials exhibit high resistance to degradation under such conditions because of their high hardness and chemical stability. These carbides can be effectively deposited as coatings on the components using advanced thermal spray techniques such as plasma spraying, HVOF, and HVAF. The carbide-based thermal spray coatings are majorly based on either WC or Cr3C2 or a combination of these materials. However, the composition of the carbides, the type and percentage of binders, and process parameters significantly affect the performance of these coated components. In this article, the degradation behavior and performance of the different carbide-based coatings as a function of carbide grain size and type of metallic binders, spray process parameters, and working conditions have been critically reviewed. On the other hand, the post-processing of carbide coatings is also emerging as a promising strategy to enhance the performance by modifying and refining the structure of coatings. Hence, a comprehensive summary of the post-processing techniques such as heat treatment, laser treatment, and cryogenic treatment of the carbide coatings is also provided.
... Application of suitable tribological coatings (including engineered soft coatings and solid lubricants) on the mating components [9]; and iii. ...
To simulate today’s complex tribo-contact scenarios, a methodological breakdown of a complex design problem into simpler sub-problems is essential to achieve acceptable simulation outcomes. This also helps to manage iterative, hierarchical systems within given computational power. In this paper, the authors reviewed recent trends of simulation practices in tribology to model tribo-contact scenario and life cycle assessment (LCA) with the help of simulation. With the advancement of modern computers and computing power, increasing effort has been given towards simulation, which not only saves time and resources but also provides meaningful results. Having said that, like every other technique, simulation has some inherent limitations which need to be considered during practice. Keeping this in mind, the pros and cons of both physical experiments and simulation approaches are reviewed together with their interdependency and how one approach can benefit the other. Various simulation techniques are outlined with a focus on machine learning which will dominate simulation approaches in the future. In addition, simulation of tribo-contacts across different length scales and lubrication conditions is discussed in detail. An extension of the simulation approach, together with experimental data, can lead towards LCA of components which will provide us with a better understanding of the efficient usage of limited resources and conservation of both energy and resources.
... Qiao et al. [20] reported that the hardest and toughest WC-Co coatings with superior tribological performance are obtained with a hot, neutral flame during the HVOF spraying process. Furthermore, the addition of Al into an HVOF-sprayed nanostructured WC-12Co coating is an effective method to enhance wear resistance [21]. However, till now, there is only limited works focus on sliding wear of HVOF-sprayed WC-based coatings at different temperatures [22,23]. ...
The main objective of this study was to develop an efficient coating to increase the wear resistance of cold work die steel at different temperatures. The microstructures of high-velocity oxygen-fuel (HVOF)-sprayed WC-CoCr coatings were evaluated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effect of temperature on the tribological properties of the coatings and the reference Cr12MoV cold work die steel were both investigated by SEM, environmental scanning electron microscopy (ESEM), X-ray diffraction (XRD), and a pin-on-disk high-temperature tribometer. The coating exhibited a significantly lower wear rate and superior resistance against sliding wear as compared to the die steel at each test temperature, whereas no major differences in terms of the variation tendency of the friction coefficient as a function of temperature were observed in both the coatings and the die steels. These can be attributed to the presence of nanocrystalline grains and the fcc-Co phase in the coating. Moreover, the wear mechanisms of the coatings and the die steels were compared and discussed. The coating presented herein provided a competitive approach to improve the sliding wear performance of cold work die steel.
... In terms of substance, these various thermal spraying processes are all based on the same spraying principle: the coating material is rapidly heated in a hot gaseous medium while being projected onto the surface at a high speed to create a coating. The reason why thermal spray coatings are used to prevent fretting damage is generally that some oxides in the coating can prevent or delay adhesion between the two contact surfaces; the laminated structure of the coating also delays the expansion of the fatigue crack from the fretting site [25]. ...
This paper summarises the regulation mechanism of the traditional and emerging surface strengthening treatments – mechanical shot peening (SP) and laser shock processing (LSP) treatment, respectively – on the fretting fatigue behaviour of the tenon joint materials of aero-engine cold-end parts, as surface strengthening treatment contributes to the improvement of service performance. From the point of view of the actual service environment of turbine components, this paper also expounds on the key problems faced by surface strengthening treatments in improving the mechanical properties of joint materials for aero-engine hot-end components. The application of LSP in the fretting fatigue life extension of turbine joint materials has strong feasibility and development prospects.
... In the case of carbide coatings, feedstock powders of tungsten carbide (WC) with a 10-15 wt % Co-based metal binder fraction [21][22][23] and chromium carbide (Cr 3 C 2 ) with a 25-35 wt % NiCr-based metal binder fraction [24][25][26][27] are used to elaborate extremely wear-resistant and ultra-hard coatings, via High Velocity Oxygen Fuel spraying, a process that controls the in-flight carbide decomposition phenomena. Although these coating types can provide excellent anti-wear protection of metallic components, their deposition processes require large, stationary installation facilities, a fact that limits their industrial applications to parts of relevant small dimensions and hinders their in-situ deposition on non-removable metallic systems. ...
The technique of thermal spraying has been proposed since several years ago, as an alternative to Cr electrodeposition, a process characterized by the need of post-deposition handling of a large amount of toxic slurry wastes. Chromium oxide and chromium carbide coatings, as well as Cr electrodeposits find applications, mainly, on the wear protection of metallic components participating in several tribosystems. In the present study, three different thermal spraying techniques were applied for the deposition of such ceramic coatings onto stainless steel substrates; namely, Flame Spraying (FS) and Atmospheric Plasma Spraying (APS) were employed for the deposition of chromium oxide coatings, whilst High Velocity Oxygen Fuel (HVOF) technique for the elaboration of chromium carbide ones. Post-deposition evaluation of the coatings with respect to dry sliding against an Al2O3 ball and a cBN-coated conical insert, as well as three-body abrasion performance according to the ASTM G65 technical specification, demonstrated that the APS oxide coatings exhibited superior tribological behavior during both tests, despite the fact that their microstructure was not free of flaws. Compared to APS ones, FS oxide coatings exhibited lower three-body abrasion resistance; however, their dry sliding wear resistance was of the same order of magnitude, being only marginally lower. This last characteristic advocates for the use of the much more flexible FS technique for the elaboration of chromium oxide coatings for applications where relatively low shear stresses are expected to be encountered. Finally, the HVOF carbide coatings showed intermediate three-body abrasion resistance but high sliding wear, the latter attributed to both microstructure flaws and in-flight decomposition of the feedstock carbide powder during deposition.
... 1. Running-in: During this stage, the sliding occurs between the ceramic coating and the abrasive ball surface. As seen in Figure 5, the surface of the coatings has primarily some microscopic asperities due to the differences in particles melting points and splats flattening degrees [44,45]. At the initial stage of the wear test the COF value is high, due to the high coating roughness and the smaller contact areas between the ball and coatings and therefore higher applied stress on the contact surface. ...
In this study, the microstructure and mechanical properties of the atmospheric plasma-sprayed Cr2O3 (C), Cr2O3-20YSZ (CZ), and Cr2O3-20YSZ-10SiC (CZS) coatings were evaluated and also compared with each other, so as to explain the coatings wear behavior. Microstructural evaluations included X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive X-ray spectroscopy (EDX) and porosity measurements. Mechanical tests including bonding strength, fracture toughness, and micro-hardness tests were used to advance our understanding of the correlation between the coatings properties and their wear behavior. The sliding wear test was conducted using a ball-on-disk configuration against an alumina counterpart at room temperature. Addition of multimodal YSZ and subsequent SiC reinforcements to the Cr2O3 matrix resulted in an increase in the fracture toughness and Vickers micro-hardness, respectively. It was found that the composite coatings had comparable coefficients of friction with pure Cr2O3 coatings. When compared with the C coating, the CZ and CZS composite coatings with higher fracture toughness exhibited superior wear resistance. Observation of the wear tracks of the coatings indicated that the lower wear rates of the CZ and CZS coatings were due to the higher plastic deformation of the detached materials. In fact, improvement in the wear resistance of the composite coatings was attributed to a phase transformation toughening mechanism associated with tetragonal zirconia which created more ductile tribofilms during the wear test participated in filling the pores of coatings.
... Hong et al. [10] prepared nanostructured WC-10Co-4Cr coating by adjusting HVOF process, and the coating exhibited better corrosion resistance in 3.5% NaCl solution. Investigation by Basak et al. [11] showed nanostructured WC-Co coatings (with and without Al) exhibited better corrosion in comparison with micron-sized coating. Reference [12] reported that the addition of TiC improves the corrosion resistance of WC-Co in 1 mol L −1 NaOH compared with the base alloy. ...
There is an urgent need to improve the corrosion resistance of WC-based cermet coatings in different corrosive environments. The main objective of this work was to investigate the microstructure and evaluate the corrosion resistance in neutral, acidic, and alkaline electrolytes of the WC-based cermet/Fe-based amorphous alloy composite coating. Thus, a composite coating of WC–CoCr/Fe-based amorphous alloy and a single WC–CoCr coating were fabricated using the high-velocity oxygen fuel (HVOF) process. The phase composition, microstructure of the original powders, and as-sprayed coatings were studied. The detailed interface information between different compositions of the composite coating was observed by high-resolution transmission electron microscopy (HRTEM). The corrosion resistance of the coatings was studied comparatively by electrochemical tests in 3.5 wt % NaCl, 1 M HCl and 1 M NaOH solutions, respectively. Results showed that the composited coating had a dense layered structure with a composition of WC, Fe-based amorphous alloy, and small amount of W2C. It was revealed that obvious inter-diffusion exists between the interfaces of tungsten carbide/Co, Cr binder and WC–CoCr/Fe-based amorphous alloy. The electrochemical test results showed that the composite coating displayed better corrosion resistance than single WC–CoCr coating both in 3.5 wt % NaCl solution and in 1 M NaOH solution.
... The recent studies have reported that the hardness and toughness of the nanostructured WC-based coatings can be simultaneously improved [10][11][12][13], indicating the very promising applications of the nanostructured WC-based coatings. However, there is still much uncertainty regarding the wear resistance property and its influencing factors of the nanostructured cermet coatings [14][15][16][17][18]. The challenge is to disclose which kind of interfacial bonding state is beneficial to enhance the wear resistance of the nanostructured cermet coatings. ...
Dense near-nanostructured WC-Co coatings with hardness above 1400 HV0.3 were fabricated by high velocity oxy-fuel spraying using the in situ synthesized WC-Co composite powders containing VC and Cr3C2. The carbide additives inhibited WC coarsening effectively and facilitated formation of the clustered structures consisting of WC grains and amorphous Co. Owing to the dragging effect of clusters, the connected WC particles are much more difficult to be peeled off than the isolated WC particles. Therefore, the wear resistance of the near-nanostructured coating was significantly enhanced. The mechanisms proposed the importance of interfacial characteristics for the wear resistance behavior of the cermet coatings.
... Furthermore, pores and voids are unavoidable in plasma sprayed coatings, decreasing the coating performance [12]. In such a case, metals such as Cr, Co, Ni and Al are widely used as a binder metal in ceramic coatings because of its good wetting behavior [13][14][15], with the aim to enhance bonding strength between ceramic phases as well as the coating/substrate interface, and low porosity of coating is also achieved. ...
Nanostructured TiCN based composite coatings with various Cr content were prepared by reactive plasma spray (RPS) from mixed powder (Ti-graphite + Cr) under nitrogen atmosphere. Results showed that composite coatings consisted mainly of TiC0.7N0.3 phase and residual metal Cr. Metal Cr plates were homogeneously embedded in TiCN matrix with good interface bond. The TiCN-Cr composite coatings exhibited lower porosity than TiCN coatings, but increasing porosity with excess Cr addition (30 wt.%). The TiCN–20 wt.% Cr coating showed the highest hardness (1309 HV0.2) among composite coatings, slight lower than the TiCN matrix coating (1526 HV0.2). Compared with the TiCN matrix coating, the TiCN-Cr composite coatings showed higher variability in surface microhardness distribution. The TiCN-Cr composite coatings showed slight higher friction coefficients (0.4–0.6) than TiCN matrix coating (0.35). The wear resistance of TiCN-Cr composite coatings was improved with less mass loss compared with TiCN coating under the test load of 400 N. The TiCN-Cr composite coatings with high Cr content showed the mixture of abrasive and adhesive wear.
... Various thermal spray techniques including atmospheric plasma spray (APS) and high velocity oxygen fuel (HVOF) have been employed to deposit WC-Co coatings. The HVOF spraying provides several advantages over APS, such as higher velocity and lower temperature, resulting in the deposition of a highly compacted coating with negligible porosity, a very high adhesive strength to the substrate and limited WC decomposition during spraying [6][7][8][9][10]. ...
WC-Co coatings with low degree of WC decomposition were deposited from electroless Ni-plated micro- and nano-structured feedstock powders using high velocity oxygen fuel (HVOF) spraying. Dry sliding friction and wear behavior of the resultant coatings, referred to as Ni/mc-WC and Ni/nc-WC, were investigated by using sintered alumina (Al2O3) as the mating material at 700 °C. For the purpose of comparison, similar experiments were carried out on conventional micro- and nano-structured coatings, denoted as mc-WC and nc-WC. The worn scars were examined by field emission scanning electron microscopy (FESEM) equipped with energy dispersive spectroscopy (EDS). The wear resistance of Ni/mc-WC and Ni/nc-WC coatings at 700 °C was found to be 45% and 72% greater than that of mc-WC and nc-WC coatings, respectively. Moreover, Ni/mc-WC and Ni/nc-WC coatings exhibited a significantly lower friction coefficient with negligible fluctuations as compared to mc-WC and nc-WC. The microscopic analyses of mc-WC and nc-WC worn surface revealed the presence of plastically deformed and discontinuous tribofilms, made up of a mixture of MWO4/WO3 (M = Co) and Al2O3, severe cracking and delamination especially at the interface of tribofilm and the underlying surface. In contrast, examination of Ni/mc-WC and Ni/nc-WC substantiated that a large area of the coatings surface is covered by a dense and adhered MWO4-type oxide layer (M = Co, Ni), which provides a more protection against wear at elevated temperature.
... Recently, Wu et al. [18] reported an Al/WC composite coating prepared by high energy milling and found that Al can react with WC, which was beneficial to bond Al and WC. Basak et al. [19] also reported that Al alloying can improve the wear resistance of the WC-Co coatings. However, there is still no evidence in literature regarding the deposition behavior of nanostructured WC-Co/Al powder and the details of the microstructural evolution during preparation of these powders. ...
Nanostructured WC-Co/Al powder was synthesized from WC-12Co powder and pure Al powder by mechanical alloying (MA). The morphology and microstructural evolution of WC-Co/Al powder were investigated by a series of characterization methods. The results showed that the β-Co phase in the initial WC-12Co powder was replaced by the Al
xCo phases (such as Al9Co2 and Al13Co4). As the ball milling time increased, the average grain size of WC in the WC-Co/Al powder decreased firstly and then remained at a constant value of around 40 nm. The deposition behavior of powders sprayed by high velocity oxygen fuel (HVOF) spraying was investigated. During spraying, the WC-Co/Al powder had a better flattening than the WC-12Co powder without ball milling, which is beneficial to fabricate compact coatings with lower porosity.
... In the case of oxide coatings, alumina (Al 2 O 3 ) ones are the most widely investigated with respect to both the quality of the feedstock powders used, as well as their wear resistance under various tribo-pairs configurations [11][12][13][14][15] whilst the most suitable thermal spraying technique to obtain oxide coatings is Atmospheric Plasma Spraying (APS). In the case of carbide coatings, feedstock powders of tungsten carbide (WC) with a metal binder fraction of ~10% are used to elaborate extremely wear-resistant and ultra-hard coatings, via High Velocity Oxy-Fuel (HVOF) spraying [16][17][18][19][20][21], a process that controls the in-flight carbide decomposition phenomena. Although these coating types can provide excellent anti-wear protection of metallic components, their deposition processes require large, stationary installation facilities, a fact that limits their industrial applications to parts of relevant small dimensions and does not allow in-situ deposition on non-removable metallic systems. ...
Abstract: The paper presents the applications of thermal spray (TS) coatings, when a high tribological performance (wear resistance, friction, lubrication) is necessary. For many decades Cr plating was applied when high wear resistance was requested. However this technique is characterized by the hazardous emission of Cr6+ ions, and the need of management of a large amount of toxic wastes. Several coating techniques are proposed as alternative to hard Cr. Among them, thermal spray seems to be the most appropriate, since it combines cost effectiveness, short production time and a large flexibility in the choice of coating material. In addition, the use of nanostructured materials in TS adds a new dimension.
Keywords: Coatings, Thermal spray, Tribological performance, Cr replacement, Industrial applications
... In the case of oxide coatings, alumina (Al 2 O 3 ) ones are the most widely investigated with respect to both the quality of the feedstock powders used, as well as their wear resistance under various tribo-pairs configurations [4][5][6][7][8] whilst the most suitable thermal spraying technique to obtain oxide coatings is Atmospheric Plasma Spraying (APS). In the case of carbide coatings, feedstock powders of tungsten carbide (WC) with a metal binder fraction of ~10% are used to elaborate extremely wear-resistant and ultra-hard coatings, via High Velocity Oxy-Fuel (HVOF) spraying [9][10][11][12][13][14], a process that controls the in-flight carbide decomposition phenomena. Although these coating types can provide excellent anti-wear protection of metallic components, their deposition processes require large, stationary installation facilities, a fact that limits their industrial applications to parts of relevant small dimensions and does not allow in-situ deposition on non-removable metallic systems. ...
Abstract: The present study is focused on the evaluation of the in-service tribological performance of Cr2O3 coatings deposited by Flame Spraying, a method that allows in-situ deposition and surface enhancement of metallic components of large dimensions. The relevant experimental results concern ball-on-disc and three-body abrasion testing and are compared to those obtained for APS Cr2O3 and HVOF Cr3C2 coatings. The strategic target of the work is to investigate the feasibility of replacing Cr electrodeposition, a technique characterized by the need of post-deposition management of a large amount of toxic slurry wastes, by thermal-sprayed Cr compounds using flexible and mobile spraying equipment.
Keywords: Thermal spraying, Cr-oxide coatings, Cr-carbide coatings, Tribological performance.
The hybrid S-phase layer was formed on the surface of an austenitic stainless steel by the thermomechanical heat treatment, which was carried out at a low temperature (475 °C). The thickness of the layer depends on the methane (CH4) gas content in the heat treatment chamber. The microstructure of the layer contains plenty of microstructural defects, such as slip bands, twins, stacking faults, and entangle dislocations, as confirmed by detail electron microscopy investigations. The S-phase layers were subjected to nanoindentation and nanoscratching to investigate their micro-mechanical properties and deformation aspects; and compared against the substrate (bulk austenitic) material. This layer exhibit about 6 folds increase in hardness (11.65–14.65 GPa) to that of bulk austenitic (2.33 GPa), whereas the Young's modulus increased by 1.70 times (124.84–142.34 GPa for the S phase layers and 83.25 GPa for bulk material). This increase in hardness caused relatively less plastic deformation in the layer, compared to bulk austenitic under identical loading conditions. This induced different sub-surface deformation aspects, not only in terms of deformation depth, but also in microstructural aspects, as reviled through a transmission electron microscopy investigation on the residual nanoindentation imprint and scratch tracks.
The present work compares WC-10Co-4Cr coatings fabricated by HVOF spraying of a conventional powder (1.33 ± 0.67 μm) and a nanostructured powder (0.17 ± 0.06 μm) on Al7075-T6, in terms of microstructure and phase composition, microhardness and fracture toughness, corrosion performance in 3.5 wt% NaCl (potentiodynamic polarization, chronoamperometry, galvanic effect measurements and salt spraying) and sliding wear performance under lubrication. Τhe nanocoating, despite its greater microporosity and decarburization, displayed higher microhardness, fracture toughness and surface roughness compared to its conventional counterpart. A superiority of the corrosion behaviour and sliding wear behaviour of the nanocoating as compared to its conventional analogue was observed in terms of a higher resistance to general/localized corrosion, lower passive film conductivity, lower wear rate, subtler wear scars and greater occurrence of islands of tribofilms. The reasons for such superiority are being clarified. A good galvanic compatibility between the nanocoating and its substrate was manifested. Both coatings exhibited high corrosion resistance during salt spraying for 45 days. The mechanisms of corrosion and wear degradation are being proposed.
Ball end magnetorheological finishing (BEMRF) is a nanofinishing process for finishing 3D surfaces of a large variety of materials such as glass, steel, copper, polycarbonates, silicon, etc. Under the influence of magnetic field, abrasive-laden ball of magnetorheological polishing fluid present at the tip of the tool removes material from the workpiece surface. The knowledge of forces associated with the process aids in understanding the material removal mechanism and the process physics. Also, the prediction of finishing spot plays a vital role in increasing the process capabilities of BEMRF process in the area of localized/selective finishing. In this work, a theoretical model of finishing forces is presented that helps in improving the in-depth understanding of the nanofinishing process. In addition to it, a theoretical model of finishing spot size is also proposed. Depending upon the area of workpiece to be finished locally/selectively, the finishing spot model provides a deterministic way to alter the size of finishing spot of BEMRF process by changing the finishing parameters.
During the machining of hard coatings, debris generated due to machining causes abrasive wear of machine components as well as workpiece. The present chapter investigates such abrasive wear effect by simulating such conditions that took place during machining in sand abrasion test instruments. Hard nanostructured cermet coatings, namely nanostructured WC-Co and FeCu/Al2O3/Al, with about 250 μm thickness, were deposited by high velocity oxy-fuel (HVOF) and atmospheric plasma spraying techniques on stainless steel 304 substrate. Experimental data on abrasive wear of such coatings were reported under both dry and lubricated abrasion conditions. Relatively higher abrasion resistance of such coatings, compared to reference metallic materials, is due to the formation of small debris that rolls between the moving surfaces and thus limits the wear of the workpiece materials. Another aspect of higher abrasive wear resistance is homogeneous distribution of hard (WC and Al2O3) particles within relatively soft matrix (Co and FeCu) as revealed by scanning electron microscopy investigation on wear scars of the samples after abrasion tests.
The tribological properties and behaviors in the nanometric machining play a critical role in the high surface quality and low subsurface damage for the machined materials. However, in situ TEM experiments have the limitation of length and time scales to investigate the dynamic nanomachining process. Molecular dynamics (MD) simulation is widely employed to describe the nanomachining at atomic scale, and provides some dynamic deformation details which can be hardly revealed by the experiment. In this chapter, we review recent works on the nanometric machining to understand the tribological behaviour. The fundamentals of nanometric machining in term of the friction, material removal, tool wear, and lubrication, are discussed for deeply understanding of the physical mechanisms of nanomachining induced tribological behaviour.
Abrasive wear behaviour of WC-Co and Fe/Cu-based nanostructured coatings has been investigated under dry and slurry conditions in this study. The retention of nanostructured coating constituents was confirmed together with the formation of various metallic and ceramic phases. Abrasive wear rate of nanostructured WC-Co coating was one order lower than that of micro-structured coatings in both conditions. Relatively higher abrasion wear resistance of such coatings compare to reference metallic material was because of comparatively smaller debris formation. These debris roll in between the moving surfaces and thus limit the wear of coating materials. Another aspect of higher abrasive wear resistance is homogeneous distribution of hard (WC and Al2O3) particles within relatively soft matrix (Co and FeCu) as revealed by SEM investigation.
Recent work indicates that the high-velocity oxy-fuel (HVOF) thermal spraying WC–Co coatings have been used to enhance the wear resistance of various engineering components in a variety of industrial environments. In the present work, WC–Co powder, containing Cr particles in an amount of 10%, was deposited on ductile cast iron with the HVOF thermal spray coating technique. An investigation was conducted to determine the role of Cr particles in the WC–Co coating produced with the HVOF technique on microstructure, mechanical, and wear properties in a system of type: WC-Co coating/ductile cast iron. The microstructure of the HVOF-sprayed WC–Co + Cr coating was characterised by light microscopy, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and energy-dispersive X-ray spectroscopy (EDS). The analysis of the microstructure showed the formation of a coating with low porosity, compact structure, and good adhesion to the substrate with a typical lamellar structure composed of fine molten Cr particles and finely fragmented WC grains embedded in a Co matrix, reaching the size of nanocrystalline. The scratch test was applied for the analysis of the adhesion of coatings to the substrate. The erosion behaviour and mechanism of material removal was studied and discussed based on microstructural examinations. Moreover, the results were discussed in relation to the bending strength test, including cracks and delamination in the system of the WC–Co + Cr/ductile cast iron, as microhardness and erosion resistance of the coating. It was found that the addition of Cr particles to the WC–Co powder, which causes hardening of the binder phase is a key influence on increased mechanical and wear properties in the studied system. Additionally, due to the construction of nanostructured coatings, suitable proportion of hard and soft phases, the technique sprayed HVOF coatings have advantageous properties such as high density and good slurry erosion resistance.
In this study, nanostructured WC-12Co powder was produced by ball milling and then agglomerating by spray drying. The produced nanostructured WC-12Co powder was used for laser cladding of AISI 321 stainless steel substrate. In order to evaluate the effect of argon shroud protection, the cladding process was performed with and without argon shroud protection. Microstructural and phase characterization showed that argon shroud protection can reduce the WC decarburization and the relative fraction of WC phase in clads reached up to 74%. In addition, microhardness and porosity percentage of the clads with argon shroud protection showed a significant improvement in comparison with the clads without argon shroud protection. The microhardness (to 2200 HV) and wear resistance increased and the porosity percentage decreased to less than 1%.
In this study, the effect of TiCN and WC-Co coatings on the tribological performance of spheroidal graphite cast iron materials was investigated by wear tests under dry and lubricated conditions. The mechanical and physical effects of the coating processes were analysed by microhardness measurements and visual methods (SEM, EDX, XRD). The tribological effect of WC-Co and TiCN coated specimens were investigated by ball-on-plate wear tests carried out under dry and boron oil emulsion (5% concentration) conditions at 50 N load and 60 rpm sliding speed parameters. The friction coefficient and wear volume results were obtained to investigate the effect of the coating process and boron oil emulsion on friction and wear behaviour. The wear mechanisms were determined with SEM images obtained from the worn surfaces and surfaces analysed elementally with EDX images. Additionally, surface morphology was examined with surface roughness measurements and 2D-3D topography images. According to the microhardness measurement results, it was observed that the spheroidal graphite cast iron with 290 HV hardness reached 1559 HV with TiCN coating and 1440 HV with WC-Co coating. According to the wear test results, it was determined that TiCN and WC-Co coatings decreased the friction coefficient by 32.51% and 12.80%, respectively. When the effect of boron oil usage compared to dry environmental conditions, it was concluded that the friction coefficient and wear volume decreased by a maximum of 54.80% and 53.70%, respectively. Additionally, the roughness values decreased in the experiments where the boron oil emulsion was used. However, although the TiCN coating improved the surface quality, the WC coating negatively affected the surface quality. Surface form images obtained with 2D and 3D topography also support the alteration in roughness values.
A Ni-20 wt% Sn coating was deposited by the cold spraying method on steel substrate. The thickness of the deposited coating was approximately 50 μm and porosity was <1%. The coating microstructure consisted of a Ni solid solution with the presence of various intermetallics, which was confirmed by x-ray diffraction and electron microscopy. Stress-strain behaviour of this coating, both in planar and cross-sectional direction, was investigated under in-situ micro-pillar compression. It was found that strength and elastic modulus was slightly higher in the cross-sectional direction of the coating compared to that of the planar direction. The presence of intermetallics in the coating microstructure acts as a reinforcement medium to allow effective load bearing capacity of the coating. Loading direction during compression with respect to splat boundary orientation plays an important role in the mechanism of coating deformation. There was no substantial work hardening, due to the lubricating effect of SnO2 in the coating system.
A WC-17 wt % Co coating was deposited onto a ST37 mild steel substrate by HVOF-spray technique and then vacuum heat treated at 1100°C. The cross sectional microstructure of the produced coating was characterized before and after heat treatment by optical microscopy. X-ray diffraction (XRD) was also utilized to evaluate the effects of heat treatment on the phase composition of the WC-17Co coating. To study the electrochemical corrosion behavior of both as-produced and heat treated samples, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods were performed in 3.5 wt % NaCl solution. Our results reveal that the WC-17Co coating had a dense structure with an average thickness of about ~500 μm. However, heat treating the coating resulted in an adhesion failure and therefore partial separation of the coating from the substrate. The as-produced coating was also composed of both crystalline WC and amorphous phases whilst the heat treated layer was fully crystalline. According to the corrosion tests, the WC-17Co coating improved the corrosion resistance of the substrate. However, heat treating the coating at 1100°C decreased anticorrosion performance which was due to the precipitation of η-phases with different electrochemical potential than the WC phase together with the formation of microgalvanic cells between the crystalline phases with different compositions.
Mechanical equipment operating in the ocean is vulnerable to cavitation erosion damage. The coatings fabricated by high velocity oxygen fuel (HVOF) spraying technology can remarkably improve the resistance of cavitation erosion. In this study, WC-10Co4Cr coatings with two structures (conventional and bimodal) were prepared on 35CrMo steel substrate, respectively. The morphologies and structures of the powders and as-sprayed coatings were analyzed by SEM and XRD, and microhardness, porosity and surface roughness were also studied. The corrosion resistance of the coatings was evaluated by using potentiodynamic polarization. More importantly, the cavitation erosion resistance in simulated sea water was studied by the ultrasonic cavitation vibratory apparatus and the cavitation erosion failure mechanisms were also explored. The results show that the bimodal coating has better quality and cavitation erosion resistance. Moreover, the effect of corrosion on cavitation erosion is not obvious.
In this research, plasma nitridation is carried out on the as-sintered and hardened Astaloy Mo in order to improve the wear behavior of PM parts. The starting materials including pre-alloyed powders with 0.8% graphite were pressed at different pressures. Next, the specimens were sintered at 1120 °C for 30 min, in the N2/H2 atmosphere. The effects of density and cooling rate were studied on the microhardness, microstructure, and wear performance of as-sintered and hardened specimens. X-ray technique, scanning electron microscopy, and energy dispersive spectroscopy were used for phase transformation, morphology characterization, and elemental analysis, respectively. The results showed that plasma nitridation and an increase in cooling rate and density up to 6.8 g/cm³ resulted in a change in wear resistance. Moreover, the wear performance of dense samples was decreased due to the enhanced three-body abrasive mechanism.
The flexural strength and residual stresses significantly influence the mechanical, structural and tribological performance of the coatings/claddings on metallic substrates. In this study, micrometric and nanometric WC-12Co clad were developed on stainless steel substrates using microwave hybrid heating technique. The flexural strength of the WC-12Co clads was evaluated using a three-point bend test. The nanometric clads exhibited approximately 14% higher flexural strength compared to the micrometric clads. The presence of eutectics of nanocarbides in the nanometric clads restricted crack propagation in the decohered ductile metallic matrix. The residual stresses in the micrometric and nanometric clads were evaluated through the sin2ψ technique using X-ray diffraction. The nature of residual stresses was observed to be compressive in the micrometric and nanometric clads. However, the magnitude of the stresses in the nanometric clads was observed to be ~ 68% higher than the micrometric clads.
The nanostructured WC–10Co–4Cr coatings were fabricated by high velocity oxy-fuel spraying using the in situ synthesized WC–Co nanocomposite powder with size of 70–200 nm and Cr addition. Through optimization of the processing conditions, the nanostructured WC–Co–Cr coating has only a small amount of decarburized phase, a dense microstructure and an excellent combination of hardness, fracture toughness and wear resistance. A series of sliding wear tests were performed to investigate the wear behavior of the nanostructured cermet coating. The evolution of the friction coefficient, wear characteristics and their mechanisms were studied for the nanostructured WC–Co–Cr coating with the change of the load. The present study proposes a new understanding of the occurrence and the related mechanisms of the wear of the cermet coatings.
WC–Co coatings obtained by high velocity oxy fuel spraying (HVOF) are widely used in applications where good abrasion, erosion or sliding wear resistance is required, due to the high toughness that the cobalt matrix provides combined with the wear resistance that the hard carbides give to the coating. The sliding properties of this coating are excellent due to the formation of oxides during the sliding process which act as a solid lubricant and give the coatings low friction coefficients. In the present paper the effect of the counterfaces (Al2O3, hard metal, Si3N4 and martensitic steel) on the friction coefficient and wear damage of the coating in the sliding tests carried out have been studied. For this purpose SEM and atomic force microscopy (AFM) were used to obtain information on the wear mechanisms. Also the results of friction coefficients and wear damage for both the balls and the coating are reported. A relationship between the quantity of debris produced by the counterfaces during sliding and the friction coefficient and wear resistance of the coating was found.
Comparative wear and corrosion properties of Cr3C2-NiCr (CC-TS) (a high-velocity oxyfuel [HVOF]) and hard chromium (HC) coatings obtained on a steel substrate have been studied.
The structural characterization was done before and after measurements by optical microscopy, scanning electron microscopy,
and scanning white light interferometry. Wear and corrosion properties were evaluated by ball on disk (ASTM G99-90), rubber
wheel (ASTM G65-91), and electrochemical measurements of open circuit and polarization curves. The best corrosion and wear
resistance was for the CC-TS obtained by HVOF. The open-circuit potential values measured for both samples after 18 h of immersion
were: −0.240 and −0.550 V, respectively, for CC-TS and HC, versus Ag/AgCl,KClsat. Three orders of magnitude lower volume loss were found for CC-TS (HVOF) after friction tests compared with HC.
The effect of carbon addition on tribological properties of a series of Fe-Al alloys was discussed. The addition of carbon in Fe-Al alloys resulted into the formation of Fe3AlC0.5 or graphite in the matrix, depending on the Al content. The tribological properties of high carbon Fe-Al alloys were compared with those of cast iron. Optical micrographs of the carbon-added Fe-Al alloys after homogenizing treatment were also presented. The addition of carbon was found to be effective in improving the room-temperature tribological properties of Fe-Al alloys, but had negative effect on the tribological properties at elevated temperatures.
It is proposed that a correlation exists between the d state electronic configuration of transition metal based hardmetals and the thermokinetic reactions that take place between binder and hardmetal during sintering.MST/1749
The tribological behavior of Al2O3/TiO2 and WC/Co plasma-sprayed coatings was studied under dry sliding conditions. X-ray photoelectron spectroscopic analyses have shown the presence of graphite in WC/17 wt% Co coatings and a thin WO3 layer in the contact. These phases could explain the improvement in the friction behavior of the cermet/ceramic couples versus the ceramic/ceramic couples.
Materials can be processed by grinding not only for comminution, but also to obtain a variety of structures (amorphous, nanophased), fine mixing of phases, alloys and to directly synthesize compounds such as carbides. Nanocomposites can also be synthesized by reduction, exchange and mixing reactions driven by grinding, as well as by combining the above processes. Large scale economical production of such materials is feasible by grinding in mills designed for that purpose. The mechanical properties of materials are enhanced by the nanophase structure.
Conventional and nanostructured WC–12%Co coatings were deposited on 1Cr18Ni9Ti stainless steel substrate using air plasma spraying. The hardness of the coatings was measured, while their friction and wear behavior sliding against Si3N4 at room temperature and elevated temperatures up to 400 °C was comparatively studied. The microstructures and worn surface morphologies of the coatings were comparatively analyzed as well by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDXA). It was found that the as-sprayed WC–12%Co coatings were composed of WC as the major phase and W2C, WC1−x, and W3Co3C as the minor phases. The plasma sprayed nanostructured WC–12%Co coating had much higher hardness and refined microstructures than the conventional WC–12%Co coating. This largely accounted for the better wear resistance of the nanostructured WC–12%Co coating than the conventional coating. Besides, the two types of WC–12%Co coatings showed minor differences in friction coefficients, though the nanostructured WC–12%Co coating roughly had slightly smaller friction coefficient than the conventional coating under the same sliding condition. Moreover, both the conventional and nanostructured WC–12%Co coatings recorded gradually increased wear rate with increasing temperature, and the nanostructured coating was less sensitive to the temperature rise in terms of the wear resistance. The worn surfaces of the conventional WC–12%Co coating at different sliding conditions showed more severe adhesion, microfracture, and peeling as compared to the nanostructured WC–12%Co coating, which well conformed to the corresponding wear resistance of the two types of coatings. The nanostructured WC–12%Co coating with a wear rate as small as 1.01 × 10−7 mm3/Nm at 400 °C could be promising candidate coating for the surface-modification of some sliding components subject to harsh working conditions involving elevated temperature and corrosive medium.
Mechanical alloying is a method of synthesis of advanced materials, often with non-equilibrium structures, and, remarkably, of crystalline materials with nanometersized grains. Grinding is also a way of inducing or activating chemical reactions. After having described some general characteristics and some applications of high-energy ball-milling, we sketch out some contributions which Mssbauer spectroscopy has made to gaining a deeper understanding of synthesis mechanisms and of mechanosynthesized materials.
Ten WC–Co (12, 17, and 20 wt.% Co) powders manufactured by different techniques (sintering, atomization, coating) were studied. Both open atmosphere Ar–H 2 plasma spraying and controlled atmosphere Ar–He chamber plasma spraying were performed. Pressures of 60, 150, 400, and 700 Torr were employed. To optimize controlled atmosphere spray conditions, particles splatted on glass slides rapidly traversing the jet at different distances from the nozzle exit were studied. The melting conditions were determined as a function of the plasma power level, the particle composition, and structure for particles with the same size distribution. Crystal structure analysis showed a strong decomposition of the carbides when sprayed in open atmosphere. Less decomposition occurred in the coated or atomized particles. The decomposition was reduced and coating density increased by reduced pressure spraying. The best results were obtained at 60 Torr. The hardness depended on the cobalt content, the degree of decomposition and on the density of the coatings.
WC–Co cermets have been used traditionally as wear-resistant materials. Recent work has shown that nanostructured cermets offer improved properties over their conventional counterparts. This work examines the performance of such conventional and nanostructured materials in the form of coatings deposited by high velocity oxy-fuel (HVOF) thermal spraying. WC–Co coatings were deposited under identical conditions using both conventional sintered and crushed and nanocomposite powder feedstocks. Both powders consisted of tungsten carbide (WC) grains in a cobalt binder. Characterisation of the coatings by a range of techniques showed that both coatings not only contained WC but also reaction products such as tungsten hemicarbide (W2C) and W and an amorphous Co-rich binder phase containing W and C. Due to differences in the morphology of the powder feedstock and the WC grain size, the nanocomposite coating contained a smaller fraction of unreacted WC than the conventional coating. Three body abrasive wear tests were performed using a modified dry sand rubber wheel apparatus with alumina and silica abrasives. A range of abrasive particle sizes and loads were used to assess the wear resistance of both coatings. It was found that the nanocomposite had a poorer wear resistance than the conventional coating under all the conditions examined. Wear was dominated by the loss of ductility in the Co-rich binder phase due to its amorphisation. The differences in the wear behaviour of the coatings could, thus, be explained in terms of differences in powder characteristics, the extent of reaction and decarburisation during spraying, and the subsequent development of the microstructure in the coating during splat solidification at high cooling rates.
The plasma spraying of TiC-reinforced wear resistant coatings, and a study of their structure and tribological behaviour are the subject of this paper. HVOF WC-Co coatings, which are well established hard facing coatings, were investigated for comparison with the coatings tested in the present study. Based on scientific criteria, modifications of the carbide and the binder phase of the NiCrTiC system were realised. Strengthening of the NiCr matrix, achieved by alloying the matrix with Fe, Al and Ti, proved to be beneficial for the coating tribological behaviour, as evaluated by pin-on-disk tests. The NiCr-based coatings reinforced with a solid solution (Ti,Ta)C showed better tribological behaviour than the coatings reinforced with just TiC particles. In unlubricated sliding under ambient conditions, the WC-Co coatings showed better tribological behaviour than the TiC-based coatings. The coating structure, wear track, counterbody and debris investigation contributes in better understanding the coating wear performance.
WC–Co hardmetals have been used traditionally as wear resistant materials and a significant body of research has demonstrated that sintered hardmetals with WC particles in the nanoscale range offer improved properties over their conventional counterparts. This work examines the performance of such conventional and nanostructured materials in the form of coatings deposited by high velocity oxy-fuel (HVOF) thermal spraying. WC–Co coatings were deposited using both conventional (sintered and crushed) and nanostructured powder feedstocks. Characterisation of the coatings by a range of techniques showed that both coatings not only contained WC but also reaction products such as W2C and W and an amorphous Co-rich binder phase containing W and C. The nanostructured coating exhibited higher levels of these decomposition products due to the enhanced kinetics of dissolution of the small WC particles into the molten binder during HVOF spraying. The performance of the coatings were examined under sliding wear conditions against an alumina ball and compared with the behaviour of a conventional sintered WC–Co hardmetal. It was found that the nanostructured coating exhibited the highest wear rate of the three materials examined. Microstructural examination showed that wear was dominated by the loss of ductility in the Co-rich binder phase (due to decomposition during spraying) leading to subsurface fracture and the generation of wear debris.
In order to examine the effect of carbide grain size on the wear behavior of WC–Co coatings, coatings with low degree of decomposition of WC were thermally sprayed by a high-velocity oxy-fuel (HVOF) system from three agglomerated WC–12% Co powders with various carbide size distributions. Characterization of the coating showed that the average carbide grain sizes of the coatings were 0.8, 1.4 and 2.8 μm and that a decrease in carbide grain size led to slightly higher degree of decomposition of WC. Dry sliding friction and wear tests using sintered alumina (Al2O3) as the mating material were performed. The coefficient of friction of the coatings was nearly constant regardless of the test conditions and carbide grain sizes. The specific wear rate of the coatings was very low ∼10−6 mm3/(Nm) and increased with increasing carbide grain size. The microscopic analyses of the worn tracks have shown that binder extrusion followed by carbide removal or carbide fracture are the dominant material removal mechanisms. The extruded cobalt acts as binder to form a ductile, dense and well cohered tribofilm on the worn surface to protect the surface from further wear, decreasing the wear rate of the coatings. Because a pull-out of single carbide particle provides less damage to the finer coating and also because the debris consisting of finer carbides are less effective as the third-body abrasions, the wear rate decreases with decreasing carbide size in the coatings.
The sliding wear mechanisms of cemented carbide and the effects of the microstructure scale on the wear resistance were investigated by performing a series of unlubricated sliding wear tests in air with pins of WCCo composites sliding against silicon nitride disks. In the first approximation, the wear rate is proportional to the hardness with a wear coefficient k = 6.9 × 10−6 for all materials. In the conventional cermets, the wear coefficient k also depends on the grain size; materials with smaller WC grains exhibit a smaller wear resistance. This reduction, however, does not extend to the nanostructured materials which exhibit the above value for k: their wear resistance is higher than that of conventional cermets in proportion to their hardness. The data can also be expressed in terms of cobalt content; the lower the cobalt content, the lower the wear; but two different such dependencies exist, one for the conventional and one for the nanostructured materials with lower wear. The sliding wear of WCCo composites occurs on a very small scale: the worn surfaces show no evidence of fracture or plastic deformation. This wear behavior is explained by the hexagonal structure and the anisotropic mechanical behavior of the WC grains that are capable of shear in a limited number of planes but are not capable of triaxial deformation. The higher wear resistance of the nanostructured composites is related to their hardness which decreases the real area of contact.
Nanophase materials can be synthesized by high energy milling. However, processing of large quantities finds a limitation in the devices available commercially, which are usually designed mainly for comminution and for laboratory-scale operations. We propose the design of a high energy, high capacity ball-mill which can be easily scaled up. An experimental device capable of processing up to about 250 g of powders with about 5 kg of balls has been realized. Impact velocities of about 3.5 m s−1 has been measured with operating frequencies of 17 Hz. The synthesis of nanophase iron carbides (assumed as a test system) is shown to be feasible with the new mill, with kinetics comparable with the best obtained in a Spex mill.
Nanostructured and conventional WC–Co coatings were deposited by vacuum plasma spraying. The wear and friction properties of the two coatings against alumina under dry friction conditions were comparatively studied. It was found that the wear resistance of the nanostructured WC–Co coating is superior to that of conventional WC–Co coatings, especially under high load conditions. The improved wear resistance of the nanostructured coating is attributed to its higher hardness and toughness. The wear mechanism of the nanostructured WC–Co coating is plastic deformation with slight surface fracture, whilst that of a conventional WC–Co coating is the initial removal of a binder phase followed by fragmentation or uprooting of carbide grains. Their tribological properties are discussed in relation to the microstructure of the two coatings. It is concluded the decomposition is a fatal factor influencing the wear resistance of thermal sprayed nanostructured WC–Co coatings.
Processing of cermet such as WC–Co is not easy by cold spray deposition, although cold spray process can eliminate the degradation of the WC phase as compared to conventional high velocity oxygen fuel (HVOF) or plasma spraying process. In this study, WC–12%Co powders with nano-sized WC were deposited by cold spray process using helium gas. Microstructural characterization and phase analysis of feedstock powders and as-deposited coatings were carried out by SEM and XRD. The results show, as expected, that there is no detrimental phase transformation and/or decarburization of WC by cold spray deposition. It is also observed that nano-sized WC in the feedstock powder is maintained in the cold sprayed coatings. It is demonstrated that it is possible to fabricate the nano-structured WC–Co coatings with low porosity and very high hardness (∼2050 HV) by cold spray deposition with reasonable powder preheating.
A conceptually new simulation system for the laboratory investigation of fretting wear is described. Fretting vibrations are generated either directly by oscillating a linear relative displacement device of constant stroke between the contacting bodies or indirectly by oscillating the applied contact load resulting in a cyclic radial expansion of the contact zone. The principles of both actuation mechanisms are outlined in detail, indicating the precision and performance range of the simulation system. A data acquisition and evaluation strategy has been developed for the on-line characterization of the mechanical contact response. It is based on the measurement of the contact displacement, tangential force and normal load. Typical experimental results obtained under different testing conditions are presented for hard coatings such as TiN and diamond deposited on flat samples and vibrating against corundum counterbody balls.
In this paper we propose a prefetching algorithm called STAP (Spatial and Temporal Association based Prefetching algorithm).
Our methods are based on the analysis of the spatial and temporal associations of the user’s request using data mining techniques.
First, we exploit an ”associative class set” consisting of an itemset of service classes that is close both spatially and
temporally and frequently requested together. With the first method, our prefetching algorithm can select a candidate set
that is spatially and temporally associated with the previous request of a user. It is shown that through performance experiments
STAP is effective in improving system performance.
Nanostructured WC-Co based coatings were investigated. The main focus was given to the effect of Al and Cr addition on their tribological behaviour. These coatings were successfully deposited from engineered nanosized powders using high velocity oxy fuel (HVOF) and atmospheric plasma spraying (APS). Porosity level was < 3%. Crystal sizes of around 20-30 nm determined by TEM in such coatings, confirm the retention of a nanosize after thermal spraying. The nanostructured coatings were tested for their tribological characteristics and compared to industrial micrometre sized WC-Co coatings and common wear resistant engineering materials. It was found that decarburisation of the coating constituents is a critical issue and has a large impact on the tribological behaviour of the coatings. Proper selection of spraying technique, spraying parameters and distribution of phases are shown to be key factors for achieving nanostructured coatings with high wear resistance.
High-velocity oxyfuel (HVOF) spraying of WC-12Co was performed using a feedstock in which the WC phase was either principally in the micron size range (conventional) or was engineered to contain a significant fraction of nanosized grains (multimodal). Three different HVOF systems and a wide range of spray parameter settings were used to study the effect of in-flight particle characteristics on coating properties. A process window with respect to particle temperature was identified for producing coatings with the highest resistance to dry abrasion. Although the use of a feedstock containing a nanosized WC phase produced harder coatings, there was little difference in the abrasion resistance of the best-performing conventional and multimodal coatings. However, there is a potential benefit in using the multimodal feedstock due to higher deposition efficiencies and a larger processing window.
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