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“Effect of nanostructuring and Al alloying on corrosion behaviour of thermal sprayed WC-Co coatings”
Abstract
Nanostructured WC–12Co coating with crystal sizes below 30 nm were deposited on low carbon steel substrate by high velocity oxy-fuel spraying. In this study, corrosion behavior of such coatings was investigated in artificial sea water electrolyte and compared with micron-sized (internal structure) coating of similar composition. Besides that, role of small amount Al alloying on corrosion behavior of such nanostructured coating was also investigated. Nanostructured WC–12Co coatings with or without Al exhibit lower corrosion potential and lower polarization resistance compared to micron-sized (internal structure) coating as well as higher Co dissolution as revealed by electrochemical impedance spectroscopy (EIS) together with other complementary electrochemical techniques.
... The outcome of their comprehensive investigation underscored a conspicuous augmentation in corrosion resistance, distinctly attributable to the introduction of TiC hard-phase constituents vis-à -vis the conventional WC-Co coating. Further, BASAK et al [23] forged an analogous trajectory by interrogating the multifarious influence of aluminum-based alloying and particle granularity on the corrosion-prone efficacy of the deposited WC-Co coatings. Their meticulous inquiry unveiled a salient revelation, affirming that nanostructured WC-12Co-based coatings, irrespective of the incorporation of aluminum alloy moieties, yielded a preeminent synergy of wear and corrosion resistance attributes. ...
... Upon closer inspection, it was evident that sample VC+50C had numerous micro-cavities, micropores, and corrosion products. These features are believed to be a consequence of localized pitting corrosion within the coating, leading to the release of hard phases, such as VC particles [23]. Recent studies have also reported that cracks in the binders can cause the pull-out of carbide particles, contributing to the corrosion process [24,25]. ...
In this investigation, the high-velocity oxygen fuel (HVOF) deposition technique was implemented to administer vanadium carbide (VC) and cupronickel-chromium (CuNiCr) composite coatings onto SS316 stainless steel. The significance of this research lies in its direct relevance to addressing corrosion-related challenges in marine environments. Preceding and subsequent to the execution of electrochemical corrosion examinations within a 3.5% sodium chloride (NaCl) medium at ambient temperature, a comprehensive scrutiny of the surface topographies of both the coated and uncoated specimens was conducted through scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The outcomes manifest that the intermetallic binder composed of copper (Cu), nickel (Ni), and chromium (Cr) within the coatings undergoes deterioration under the influence of the NaCl medium, thereby inducing localized pitting corrosion phenomena across the substrate. Intriguingly, the incorporation of VC within the coating formulation conspicuously amplifies the corrosion resistance attributes of the treated surface, thereby ameliorating the occurrence of confined corrosive pits. Amidst the assortment of coatings subjected to scrutiny, the VC imbued surface attains the most favorable outcome, showcasing minimal corrosion rate of 72.38×10−3 mm/a. In contrast, the SS316 base substrate exhibits the most escalated corrosion rate calculated at 783.82×10−3 mm/a.
... A relatively low-magnification TEM bright field image (Figure 7a) exposes the lamellar-type microstructure of the thermal-sprayed coatings. As stated in the literature, exposure of the agglomerated powders at high temperatures (even for a fraction of seconds) and kinetics during thermal spraying [44][45][46] flattens individual molten/semi-molten droplets of powders against the substrate, producing thin layers or lamellae, often called "splats" [47]. These splats consequently stick on substrate by means of mechanical interlocking as substrate surface is at relatively low temperature than that of the splats. ...
... A relatively low-magnification TEM bright field image (Figure 7a) exposes the lamellar-type microstructure of the thermal-sprayed coatings. As stated in the literature, exposure of the agglomerated powders at high temperatures (even for a fraction of seconds) and kinetics during thermal spraying [44][45][46] flattens individual molten/semi-molten droplets of powders against the substrate, producing thin layers or lamellae, often called "splats" [47]. ...
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.
... 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.
... Carbide-based thermal spray coatings are mostly used in wear and corrosion resistance applications in the area of automotive, aerospace, power plant, oil, and gas, petrochemical plants, drilling, mining operation, etc. [1,2]. Carbide coatings can be deposited through the plasma spray process but as it possesses higher flame temperature and low particle velocity, the chances of loss of carbon are more, which results in the formation of undesirable new phases and the presence of porosity is also on the higher side [3]. ...
The WC–12Co coatings were deposited on SS 410 substrates using a high-velocity oxygen fuel (HVOF) process and the coatings were heat-treated at 750°C for 1 h in argon environment. Further, the coatings were subjected to cryogenic treatment for 1, 2, 8 and 24 h, and its influence on the reciprocating sliding wear and corrosion characteristics was studied. The structural changes in the coatings after post-treatment were assessed by X-ray diffraction analysis and Raman spectroscopy. Microhardness was improved for cryogenically treated coatings due to the α-Co transformation into ϵ-Co. Cryogenic treatment duration was not having a significant effect on the microhardness values. However, the specific wear rate was influenced by the cryogenic treatment duration. Also, corrosion resistance was increased with the increased cryogenic treatment duration. The protective layers consisting of WO3 and Co3O4 phases formed during the cryogenic treatment are attributed to the improved corrosion resistance of the coatings.
... The findings demonstrated that the inclusion of TiC hard metals significantly improved the corrosion resistance compared to WC-Co coating. Basak et al. [9] studied the influence of aluminum-based alloying and their particle size on the corrosive performance of WC-Co deposited coatings. The findings indicated that nano-sized WC-12Co-based coatings with/without aluminum alloy exhibited better wear and corrosion resistance. ...
... The findings demonstrated that the inclusion of TiC hard metals significantly improved the corrosion resistance compared to WC-Co coating. Basak et al. [9] studied the influence of aluminum-based alloying and their particle size on the corrosive performance of WC-Co deposited coatings. The findings indicated that nano-sized WC-12Co-based coatings with/without aluminum alloy exhibited better wear and corrosion resistance. ...
... For the case where α =0, the equation describes the impedance response of an ideal resistor with Q=R [15], [30]. If α =0.5, it is indicative of a diffusion controlled process that is feasible to find in coatings with high porosity [31]. Many authors have used CPE for the deviations of an ideal resistor and capacitor in response to different physical phenomena, such as inhomogeneity and surface roughness, which are commonly present in the microstructure of ceramic coatings [7], [8], [15], [27], [28], [32], [33]. ...
Cr2O3 coatings were deposited on carbon steel through the flame spraying technique using two types of flames (neutral and oxidizing). The protective and morphological characteristics of the coatings were determined. The coatings had layer thickness values of 114 and 214µm for oxidizing and neutral samples, respectively. Porosity percentages of 4.5 % and 2.5 % were determined, where the neutral sample presented the greatest porosity due to the insufficient fusion of the oxide particles during the process, resulting in the formation of a heterogeneous and less compact layer. Microcracks and pores were found on the surface and cross-section of the coatings, due to the thermal expansion generated during the solidification process. The coating protective capacity was evaluated by electrochemical techniques over 672 hours in a 3.5 %wt NaCl saline solution. The results evidenced that the coatings manufactured with the oxidizing flame presented more corrosion resistance compared to those prepared with the neutral flame. The corrosion products were more evident in the neutral flame coatings, because of the diffusion mechanisms from the substrate to the surface coating through the interconnected pores. Finally, the wettability of the sodium chloride solution in the Cr2O3 coatings was measured by the contact angle technique, finding that the oxidizing flame coatings exhibited a higher angle contact value (64.8°) in contrast to the neutral flame coatings (35°).
... Therefore, higher phase angle (720) for L12 coating indicates better corrosion resistance of the coating as compared to L17 coating. EIS spectrum recorded on the cermet based coatings is strongly influenced by coating morphology, that is WC/Co ratio and porosity and dissolution of the substrate strictly depends on the presence of ‗through coating pores' [23]. Better corrosion performance of L12 coating as compared to L17 coating can be explained due to higher oxidation dissolution regions of carbides leading to amorphization, and hence higher resistance to corrosion. ...
Substrate preparation plays a vital role in the performance of thermal spray coatings. Adherence of the coating to the substrate depends on surface characteristics like; mechanical interlocking, physio-chemical properties of surface and metallurgical bonding. The surface characteristics depend to great extent on the method used for substrate preparation. In the present study, WC-12Co and WC-17Co coatings were deposited on AZ91D samples prepared using laser texturing as substrate preparation technique. Laser textured substrate was characterized using scanning electron microscopy and surface roughness tester. Bond strength of the coating was determined using ASTM 633C pull off test. SEM was used to study the morphology of the coating; XRD/EDAX techniques were used to study the structure of the coatings. Porosity and micro-hardness of the coatings were also measured. Corrosion performance of the coatings was determined using Potentiodynamic and EIS electrochemical corrosion testing techniques. The coatings are found to have good surface and mechanical properties. Both WC-12Co and WC-17Co demonstrated good corrosion performance during electrochemical testing. However, WC-12Co was found to better protect the AZ91D compared to WC-17Co.
... Electrochemical impedance spectroscopy (EIS) is widely used [46,47] to evaluate the corrosion resistance property of anodic films. Figure 7 shows the impedance spectra of the SS weld and the heat-treated anodized welded SS. ...
In this study, we fabricated a nanoporous oxide layer by anodization to improve corrosion resistance of type 304 stainless steel (SS) gas tungsten arc weld (GTAW). Subsequent heat treatment was performed to eliminate any existing fluorine in the nanoporous oxide layer. Uniform structures and compositions were analyzed with field emission scanning electron microscope (FESEM) and X-ray diffractometer (XRD) measurements. The corrosion resistance of the treated SS was evaluated by applying a potentiodynamic polarization (PDP) technique and electrochemical impedance spectroscopy (EIS). Surface morphologies of welded SS with and without treatment were examined to compare their corrosion behaviors. All results indicate that corrosion resistance was enhanced, making the treatment process highly promising.
... These improvements have been attributed to various reasons, such as more uniform composition of the binder, denser distribution of the carbide phase and higher dissolution of W and C into the binder [21]. However, with the decreases of carbide size, nanostructured WC-Co powder decarburized in the spraying process, which should be avoided as much as possible. ...
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.
... 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.
... 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. ...
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.
... 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. ...
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.
... 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.
... However, the main drawbacks of thermal-sprayed coatings were their high porosity [3], common characteristic of all ceramic coatings elaborated by thermal spraying, and especially in the case of carbide cermets, the decarburization/ dissociation of the carbide particles due to the high temperatures developed during deposition. The issue of high porosity was tackled in a satisfactory way in the one hand by applying High Velocity Oxy-Fuel (HVOF) spraying and in the other hand by employing powder feedstocks of sub-micron and nano-size [4][5][6][7][8]. In HVOF spraying, oxygen and fuel gas are mixed and burnt in a combustion chamber at high flow-rates and pressures up to 12 bar, producing a high-speed jet. ...
The influence of the cermet fraction in cermet/ metal composite coatings developed by High-Velocity Oxyfuel Flame (HVOF) spraying on their tribological behaviour was studied. Five series of coatings, each one containing different proportion of cermet-metal components, prepared by premixing commercially available feedstocks of NiCrFeBSiC metallic and WC-Co/Cr cermet powders were deposited on AISI 304 stainless steel substrate. The microstructure of as-sprayed coatings was characterized by partial decomposition of the WC particles, lamellar morphology and micro-porosity among the solidified splats. Tribological behavior was studied under sliding friction conditions using a Si3N4 ball as counterbody and the friction coefficient and volume loss were determined as a function of the cermet fraction. Microscopic examinations of the wear tracks and relevant cross sections identified the wear mechanisms involved. Coatings containing only the metallic phase were worn out through a combination of ploughing, micro-cracking and splat exfoliation, whilst those containing only the cermet phase primarily by micro-cracking at the individual splat scale. The wear mechanisms of the composite coatings were strongly affected by their randomly stratified structure. In-depth cracks almost perpendicular to the coating/ substrate interface occurring at the wear track boundaries resulted in cermet trans-splat fracture.
... WC-Co and Ytria-stabilized Zirconia coatings have paid much attention in the field of power plant sectors and as thermal barrier coatings in gas turbine applications as theyexhibitexcellent wearresistance and Fracture toughness (Basak et al;2012, Xiao-Qin Zhao et al;. Besides this, these coatings must also exhibit excellent corrosion resistance when it is being employed on the ship structures in marine atmosphere (Li-Jun Wang et al;2013).Yang et al have reported that there has been a drastic reduction in the specific wear rate of WC-12%Co coating at elevated temperature when they are allowed to slide against the alumina ball (Yang et al;. ...
licenses/by-nc-nd/3.0/). Selection and peer review under responsibility of the Gokaraju Rangaraju Abstract This paper deals with the fabrication of WC-12wt%Co and Yttria-stabilized Zirconia coatings fabricated using Atmospheric Plasma Spraying technique.The Microstructural Characterization of the coatings was carried out using Scanning electron microscope(SEM).Potentiostat was employed to perform corrosion testing for the bare substrate and the coatings in 5 wt %Nacl solution. The adhesion between the coating and the substrate was assessed using Scratch testing machine. The results suggested that WC-12wt%Co coating offered enhanced Corrosion and Scratch resistance when compared to that of the Yttria stabilized Zirconia coatings. This enhancement could be mainly ascribed to the presence of few amounts of interconnected pores and hard tungsten carbide particles embedded in the cobalt matrix. © 2014 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the Gokaraju Rangaraju Institute of Engineering and Technology (GRIET).
WC-based wear-resistant coatings are usually corroded in salt spray environment, which seriously affects their performances. In this work, the corrosion behavior and corrosion mechanism of WC-12Co coating exposed to salt spray conditions were studied. The results showed that W2C and W phases were produced on the surface of WC-12Co coating via decarburization during spraying and participated in the corrosion process of the coating. The existence of pores was an important factor in the occurrence and development of coating corrosion. The Co in the WC-12Co coating was corroded to the most extent, followed by other components in the coating.
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.
Volume 18 addresses friction and wear from a systems perspective, while providing a detailed understanding of why it occurs and how to control it. It explains the basic theory of friction and wear, and offers valuable insight on the forces, mechanisms, and interactions that are involved. It examines common wear scenarios, including wear by particles or fluids, rolling-contact wear, sliding wear, impact wear, and both chemical and environmentally assisted wear. It also covers operational wear, addressing several cases, including tool and die wear, bearing wear, engine wear, turbine wear, pump wear, and seal wear. In addition, the volume provides information on lubricants and lubrication, coatings, surface treatments and modifications, and the tribology of irons and steels, cobalt-base alloys, titanium, aluminum alloys and composites, cemented carbides, ceramics, polymers, and polymer composites. It also introduces the topic of condition monitoring, addressing wear particle analysis, vibroacoustic monitoring, and motor current signature analysis. For information on the print version of Volume 18, ISBN 978-1-62708-141-2, follow this link.
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.
The WC-10Co4Cr coatings with conventional structure and bimodal structure were sprayed by high velocity oxygen fuel (HVOF) technology. The phase compositions and morphologies of the WC-10Co4Cr powders and coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The microhardness, porosity, bonding strength, elastic modulus and indentation fracture toughness of the conventional coating (Conventional) and the bimodal coating (Bimodal) were also studied. The sliding wear properties of the Conventional and the Bimodal against Si3N4 counterballs under different loads at room temperature (~25 °C) were investigated using a friction and wear tester. Compared with the Conventional, the Bimodal has denser microstructure, lower porosity, more excellent mechanical properties, and the Bimodal has better wear resistance than the Conventional under different loads. The two coatings under 15 N and 30 N only exhibit abrasive and slightly adhesive wear mechanism, while in the load application of 45 N, additional mechanism which is fatigue is detected and causes flaking of the coating.
To evaluate the potential of high entropy alloys for marine applications, a new high entropy alloy coating of AlCrFeNiW0.2Ti0.5 was designed and produced on Q235 steel via laser cladding. The microstructure, microhardness and tribological performances sliding against YG6 cemented carbide, GCr15 steel and Si3N4 ceramic in seawater were studied in detail. The AlCrFeNiW0.2Ti0.5 coating showed an anomalous ‘sunflower-like’ morphology and consisted of BCC and ordered B2 phases. The microhardness was approximately 692.5 HV, which was 5 times higher than substrate. The coating showed more excellent tribological performances than Q235 steel and SUS304, a typical material used in seawater environment, sliding against all three coupled balls in seawater. Besides, the wear and friction of AlCrFeNiW0.2Ti0.5 coating sliding against YG6 in seawater were most mild. The main reason was the generation of Mg(OH)2, CaCO3, metal oxides and hydroxides and the formation of protective tribo-film on the worn surface of AlCrFeNiW0.2Ti0.5 coating in the process of reciprocated sliding. This would effectively hinder the direct contact between the worn surfaces of AlCrFeNiW0.2Ti0.5 coating and YG6 ball, resulting in a decrease of friction coefficient and wear rate. Thus the YG6 was an ideal coupled material for AlCrFeNiW0.2Ti0.5 coating in seawater, and the coating would become a promising wear-resisting material in ocean environment.
Cavitation is a major problem in the hydraulic reaction turbine because it damages turbine components, affects turbine performance and increases maintenance. Hence there is a need for further research and development on the minimization of cavitation in turbines based on previous research work. Cavitation occurs due to the fall in pressure below vapour pressure of water, which may be due to inappropriate design, frequent change in operating conditions or improper setting of the turbine runner to the tailrace level. The cavitation prediction in the turbine is a challenging task and its prediction is useful in adopting appropriate techniques for its mitigation. The theoretical and computational techniques are commonly used to predict cavitation at the design stage, but the experimental techniques can be used during design as well as operating conditions. This paper presents a review of cavitation prediction techniques and performance reduction of hydraulic turbines. Cavitation is inevitable in turbines, but it can be mitigated by some techniques which avoid drastic pressure reduction in turbine components having presumable cavitation. In addition, the material damage due to cavitation can be minimized by using cavitation resistant materials and polishing surfaces. The cavitation prevention or mitigation techniques have also been discussed.
In this work, a 10Ni-WC/NiCrBSi composite coating was fabricated on a Q235 substrate by using vacuum brazing. The bonding strength, surface hardness and corrosion behavior of the coating were characterized by using shearing, a Vickers hardness test and an electrochemical measurement. The results showed that the bonding strength was 365.1 MPa and that the surface hardness of the coating reached nearly 2500 HV, which was ten times that of the Q235 substrate. The corrosion trend of the coating was analyzed by electrochemical impedance spectroscopy and potentiodynamic polarization curves. The results showed that the electrochemical system of the coating was a chargetransfer control system with a passive behavior. Combined corrosion micro-morphology and the energy dispersive X-ray spectroscopy (EDS) showed that the 10Ni-WC/NiCrBSi composite coating had superior corrosion resistance, but the pitting corrosion resistance of the coating was poor. In addition, after corrosion, the hard phase particles inside the coating were shown not to fail. The corrosion resistance of the vacuum-cladded 10Ni-WC/NiCrBSi coating was obtained.
Purpose
This study aims to investigate the electrochemical corrosion performance of high velocity oxygen fuel (HVOF) sprayed WC–12Co coating in 3.5 Wt.% NaCl solution, which provided a guiding significance on the corrosion resistance of H13 hot work mould steel.
Design/methodology/approach
A WC–12Co coating was fabricated on H13 hot work mould steel using a HVOF, and the electrochemical corrosion behaviors of WC–12Co coating and substrate in 3.5 Wt.% NaCl solution was measured using open circuit potential (OCP), potentiodynamic polarization curve (PPC) and electrochemical impedance spectroscopy (EIS) tests.
Findings
The OCP and PPC of WC–12Co coating positively shift than those of substrate, its corrosion tendency and corrosion rate decrease to enhance its corrosion resistance. The curvature radius of capacitance curve on the WC–12Co coating is larger than that on the substrate, and the impedance and polarization resistance of WC–12Co coating increase faster than those of substrate, which reduces the corrosion process.
Originality/value
The electrochemical corrosion behaviors of WC–12Co coating and substrate in 3.5 Wt.% NaCl solution is first measured using OCP, PPC and EIS tests, which improve the electrochemical corrosion resistance of H13 hot work mould steel.
Corrosion properties of nanostructured coatings deposited by suspension high-velocity oxy-fuel (S-HVOF) via an aqueous suspension of milled WC-Co powder were compared with conventional HVOF-sprayed coatings. Microstructural evaluations of these coatings included x-ray diffraction and scanning electron microscopy equipped with an energy-dispersive x-ray spectroscopy. The corrosion performance of AISI440C stainless steel substrate and the coatings was evaluated in a 3.5 wt.% NaCl aqueous solution at ~ 25 °C. The electrochemical properties of the samples were assessed experimentally, employing potentiodynamic polarization and electrochemical impedance spectroscopy. The potentiodynamic polarization results indicated that coatings produced by S-HVOF technique show lower corrosion resistance compared with the coatings produced by HVOF-JK (HVOF Jet Kote) and HVOF-JP (HVOF JP5000) techniques. Results are discussed in terms of corrosion mechanism, Bode and Nyquist plots, as well as equivalent circuit models of the coating–substrate system.
In this study, a composite coating was formed by adding Fe-based alloy powder to WC powder using a high velocity oxygen fuel thermal spraying process. The microstructure, wear, and corrosion properties of the single WC coating and composite coating were studied in detail. It was observed that the composite coating displays a partial amorphous character and closely adheres to the WC matrix. The composite coating exhibited an extremely dense structure with a porosity of 1.5%. Polarization tests demonstrated that the composite coating exhibited better corrosion resistance than a single WC coating in simulated seawater. The reasons for the observed enhanced corrosion resistance have been discussed from the perspective of structure and the elemental composition of the alloys.
Abstract：Tribological performances of the graphite-like carbon (GLC) film sliding against WC balls in distilled water (DW), artificial seawater (SW) and four kinds of saline solutions related to seawater were investigated, comparatively. The GLC film was deposited by magnetron sputtering technique. The microstructure, mechanical properties and tribological performances of the GLC film were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, nano-indention and reciprocating ball-on-disk tribo-meter. Results showed that the smooth, dense and hard GLC film with significant sp2-hybridized carbon exhibited good tribological performance with low friction and wear not only in distilled water but also in seawater. The tribological performance of GLC film in seawater was closely related to the nature and constitute of seawater as well as the nature and performance of counterparts. Bulky wear debris with hard particles would generate three-body wear regime, which increased friction coefficient and wear rate of the GLC film slightly in seawater environment. While, the divalent metal salts of seawater were apt to decrease friction and wear between the two contact surfaces. Synergistic effect led to the relatively higher friction coefficient and wear rate of GLC film against WC counterpart in seawater than that in distilled water.
Conventional and nanostructured WC-12 %wt Co powders were HVOF sprayed on Al 7075-T6. The nanocoatings presented slightly less microporosity, higher microhardness, higher fracture toughness but higher decarburization, than the conventional ones. The nanocoatings exhibited greater resistance to general corrosion and localized corrosion in 3.5% NaCl than their conventional counterparts. Other than an occasional removal of WC nanoparticles, corrosion modes appeared similar for both types of coatings, characterized by selective dissolution of Co, formation of protective surface films and crevice corrosion along interlayer boundaries. Both types of coatings exhibited high dry sliding wear resistance. Nevertheless, the wear performance of the nanocoatings was even superior than that of their conventional counterparts. Main wear modes included plastic deformation of binder, oxidative wear leading to the formation of lubricating oxides and adhesive wear leading to the formation of alumina-rich surface depositions. Coating of Al 7075 with nano WC-12Co was proved effective in terms of microstructural features, corrosion and wear behaviour.
Cermets coatings are extensively used in energy applications both because of their high wear resistance as required, for example, in components like gas turbine sealants, and because of their specific functionality as required in solar absorbers. So far, high-temperature thermal spraying and physical vapor deposition have traditionally been used to deposit this kind of coatings. In this study, Ni-Al2O3 coatings have been deposited using a Kinetic®3000 cold-spray system starting from Ni and Al2O3 powders blend; five blends have been prepared setting the alumina content in the feedstock to 10, 25, 50, 75, and 90 wt.%. The embedded alumina ranges between a few percent weight up to 16 and 31 wt.%, while the microhardness shows a deep increase from 175 Vickers in the case of pure Ni coatings up to 338 Vickers. The spray and coating growth mechanism have been discussed, with special attention to the fragmentation of the ceramic particles during the impact. Finally, the coating behavior at high temperature was analyzed by oxidation tests performed in air at 520 °C emphasizing a good oxidation resistance that could represent a very promising basis for application in power generation systems.
This review is based essentially on the results in the field of synthesis and characterization of nanostructured coatings obtained by the authors themselves or their colleagues. Characteristics of feedstock powders for synthesizing nanostructured coatings such as particle size and morphology, changes in chemical composition and grain size are summarized. The evolution of microstructure caused by mechanical milling in two typical powder systems, Cr3C2–NiCr and Inconel 625, and mechanisms governing the development of the nanostructure are discussed. Mechanical properties and microstructure of several nanostructured coatings are evaluated by using microhardness testing, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. As background information, a review of the agglomeration process for milled powders and of thermal spraying technologies to synthesize nanostructured coatings are also included. In addition, the methodologies used to characterize the performance of the milled powders and nanostructured coatings, as well as the practice techniques for sample preparation, are described in detail.
The electrochemical behavior of 55% Al-Zn coated steel sheet, on which a very thin polymeric film (â 1 μm) was applied, was studied using electrochemical impedance spectroscopy (EIS). Use of a straightforward electric model to analyze the spectra obtained with different polymer films allowed correlation of the coating parameters and the physicochemical processes occurring on a microscale. Exposure to a 0.5% sodium chloride solution for variable time periods gave new insight into the mechanisms of protection and short-term degradation of the coating.
WC–Co hardmetals are used for their combined high hardness and toughness. However, their poor corrosion resistance in aqueous solutions reduces the spectrum of their application. The goal of this work was a systematic investigation of the corrosion mechanisms of the WC–Co composite with electrochemical methods and analytical chemistry solution analysis characterization. WC–Co, Co and WC samples were investigated by electrochemical impedance spectroscopy, potentiodynamic polarization and inductively coupled plasma mass spectroscopy (ICP-MS). Concerning the corrosion susceptibility, the solution pH dominates the effect of specific ions. In neutral and acidic solution, the corrosion process of WC–Co consists mainly of Co dissolution. WC dissolution becomes more significant at alkaline pH. Degradation is mainly the result of selective uniform dissolution of the phases (Co or WC) not of localized corrosion because of the poor passivating ability of Co (except in alkaline pH). Synergistic effects due to galvanic coupling between the Co binder and WC are accelerating Co dissolution and hindering WC dissolution in the hardmetals compared to the pure compounds. This although the Co binder phase contains W and C, making it more corrosion resistant than pure Co. A further influence of the locally separated anodic and cathodic reactions is that the cathodic reduction on WC induces local pH increase which causes chemical dissolution of WC detected only by ICP-MS.
Nanomaterials are important due to their unique properties that may lead to new and exciting applications. Current scenario of application of nanotechnology in the field of corrosion prevention of metals is reviewed here. Recent research and developments in this area are discussed in designing efficient coating materials and alloys, which provide superior resistance to corrosion.
Nanocomposite powders (Fe or Fe-Cr alloy)/α-Al2O3 (75 and 85 vol.%) were obtained by room-temperature high-energy milling powder mixtures of hematite (and chromium oxide) with aluminum and alumina in a high-capacity mill for 8-10 h. The composition of iron and iron alloys was followed by Mössbauer spectroscopy, while the appearance of other phases was revealed by X-ray diffraction. The powder particles produced are assemblies of grains (10–20 nm in size) with a wide size distribution (from well below 1 μm up to several hundreds) and low porosity (fully dense particles). Both the metallic and ceramic phases have crystallite sizes below 15 nm for all the compositions investigated. Nano-nano type ceramic nanocomposites were, therefore, obtained.
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.
Cermet based coatings are being increasingly used to combat erosion–corrosion in oil sands pipelines and pumps where the degradation is caused by a slurry mix of sand particles and aqueous solution. This research assesses the erosion–corrosion resistance of cermet composite coatings obtained by HVOF thermal spraying of microcrystalline and ‘duplex cobalt coated’ near-nanocrystalline WC–17Co feedstock powders. Electrochemical measurements, surface characterization, and the extent of weight loss were studied through an impingement jet system. Results suggest that the erosion–corrosion mechanism in the coatings was dominated by pure erosion in the microcrystalline coating and corrosion-enhanced erosion in the near-nanocrystalline coating.
Oxygen reduction reaction (ORR) kinetics were investigated on synthesized Al-Cu, Al-Cu-Mg, and Al-Cu-Fe-Mn intermetallic phases and compared to AA2024-T3 (UNS A92024), as well as high-purity Al, Cu, and Au. These tests were conducted in 0.1 M sodium sulfate (Na2SO4) with and without either 0.005 M sodium chloride (Nacl, pH 6), 0.01 M sodium chromate (Na2CrO4. pH 8), or 0.0062 M Na2CrO4 + 0.0038 M chromic acid (H2CrO4. pH 6) additions using stationary and rotating disk electrodes. Mass-transport-rate-controlled ORR were near theoretical values on all Cu-bearing materials and Au in 0.1 M Na2SO4 with and without 0.005 M Nacl. Theoretical limiting current densities (CD) were not achieved in the mass-transport-controlled regime onAl because of the rate limitation of electron transfer through its oxide film. Four effects of chromate additions on such cathodic reaction rates were identified as follows: - Little intrinsic effect of chromate pretreatment on mass-transport-controlled ORR on high-purity Cu, Cu-containing Al-based intermetallics, and Au electrodes in the absence of open-circuit corrosion. Charge transfer-controlled and mixed-controlled ORR are lowered possibly by the blocking of O2 adsorption sites by chromate onions. - Decreased net cathodic kinetics on AA2024-T3 as a function of the degree to which chromate suppressed open-circuit pitting corrosion prior to cathodic polarization. - Increased net cathodic kinetics on high-purity Al. - Decreased ORR kinetics on S-Al2CuMg as a result of minimization of Al(Mg) dissolution and, subsequently, minimization of the formation of a high, Cu-rich surface. Comments are made regarding the influence of each of these phenomena on open-circuit pitting of AA2024-T3.
Sudden failures of cemented tungsten carbide (WC)-cobalt punching dies are rare but expensive. Corrosion of the cobalt matrix, or cobalt leaching, by water-based lubricants usually is considered the cause of such failures. The validity of this mechanism was evaluated by measuring the corrosion behavior of cobalt, WC, and WC-15% Co die material in water and seven made-up lubricants. In the worst lubricant, the cobalt matrix corroded at an estimated rate of < 3 mpy. In chlorinated, chloride (Cl)-bearing (> 0.01% sodium chloride [NaCl]), aerated water of pH 7, the rate could be 80 mpy. Such a rate still was not high enough to explain die failures because of the short contact time between lubricant and sliding surfaces during punching operations. Excessive adhesive wear caused by low lubricity and resulting in removal of large wear particles may have been responsible. Such particles wedged between sliding surfaces may have exerted forces sufficient to cause breakage of the dies.
The wear mechanism of plasma-sprayed alumina was studied by scratching a model alumina coating using a diamond indenter. Single scratching on polished virgin surface, repeated scratching over the same track and parallel interacting scratching were investigated. The predominant mechanism of material removal was found to be microfracture within the quasi-plastic zone, which takes place preferentially at splat boundaries and pre-existing cracks and is driven by inelastic strain. When scratching was made on a virgin surface the wear rate was relatively low, but when parallel scratches interacted, subsequent scratching was made on already heavily strained material, thus, the wear rate was much higher. The wear debris produced during single and parallel interacting scratching was in the form of small angular particles whose dimensions were related to the splat structure and pre-existing cracks. In repeated scratching over the same track, surface material experienced repeated inelastic deformation and, thus, crack development at the splat boundaries, and this resulted in the production of thin platelet wear debris.
The porosity of plasma sprayed coatings (which were either conductive when Co was incorporated in a WC matrix or insulating with Al2O3) was tested by impedance techniques in a NaCl aqueous medium. Models of active pores (conductive coating) based on transmission lines or inactive pores (insulating coating) based on lumped parameter circuits were proposed. A good agreement was found between the impedance predicted by the models and the impedances measured in a wide frequency range on both conductive and insulating coatings. In particular, the coatings can be compared on the basis of their porosity. For WC–Co coatings, it was found that the porosity decreases when the Co content increases.
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
This paper proposes an alternative model for fitting electrochemical impedance spectra of protective coatings. It describes broadening of the semicircle in the complex plane, in the absence of corrosion reactions. In addition the infinite Warburg impedance circuit element is represented, which bridges existing parallel elements of the plate capacitor and resistor of a classical equivalent electrical circuit. The Warburg impedance element is the result of the Fick’s second law on partial differential equation solution. The proposed model and, for comparison the model with the CPE element, are used on our epoxy protective system to describe EIS measurements. The proposed model shows a better quality of fitting for our EIS data in comparison to the model with CPE.
The corrosion protection afforded to a carbon steel substrate by two cermet coatings (WC/12 wt.% Co and WC/17 wt.% Co; 0.05, 0.01 and 0.2 mm coating thickness), applied by high velocity oxygen fuel (HVOF) technique, has been studied in 3.5% NaCl solutions. Potentiodynamic polarization curves of cermet constituents, substrate and coated samples, iron and cobalt dissolution kinetics under potentiostatic conditions and galvanic coupling tests have been carried out.Cermet layer hinders the anodic process and WC/17%Co is more protective than WC/12%Co, particularly at high coating thickness. It is likely that the increase in the matrix cobalt content changes the pore morphology, from interconnected to isolated pores, with enhanced protective efficiency.
The electrochemical behaviour of three cermet coatings applied on low alloyed steel has been studied in artificial seawater using Zero Resistance Ammetry techniques.The evolution of corrosion potential and galvanic current with immersion time reveals the existence of two processes involved in coating corrosion. The initial stage involves pore filling by electrolyte and steel corrosion due to oxygen reduction at the cermet coating. The galvanic effect between steel and cermet coating leads to local acidification of pore bottom that decreases galvanic effect and increases steel acid corrosion.EIS measurements on individual components of the steel/coating galvanic couple reveal that the overall impedance spectrum results from the parallel combination of both anodic and cathodic reactions. Steel corrosion defines the shape of the spectrum at the low frequency range.
HVOF-sprayed coatings (WC–17Co, WC–10Co–4Cr, Co–28Mo–17Cr–3Si) and electrolytic hard chrome (EHC) coatings corrosion resistances have been compared through electrochemical polarization tests (0.1 N HCl, 0.1 N HNO3) and Corrodkote test. EHC coatings passivate in HNO3, but undergo pitting corrosion in HCl and in Corrodkote test too. HVOF coatings do not passivate, but possess more noble corrosion potentials. Both in HNO3 and HCl, they undergo more generalized corrosion, with similar icorr; crevice corrosion along splat boundaries is sometimes detected after the HCl test. Their icorr in 0.1 N HCl solution is lower than in several of EHC coatings. No visible damage in the HVOF coatings has occurred after the Corrodkote test.
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.
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.
A series of the electrochemical and long-term immersion tests was carried out in a strong sulfuric acid (5 wt.% H2SO4) solution on thermal sprayed WC cermet coatings having various kinds of metallic binder in order to examine the effect of composition of binder materials on the corrosion behavior. In the present study, the coatings were processed via a high velocity oxygen fuel (HVOF) spraying technique with WC–Co, WC–Co–Cr, WC–CrC–Ni and WC–Ni composite powders. The experimental results revealed that a considerable galvanic corrosion occurred between WC particles and metallic binders in the aerated 5 wt.% H2SO4 solution. The corrosion resistance of the coatings containing Cr was better than that of the coatings without Cr. For the coatings without Cr, general corrosion occurred in binder materials in addition to galvanic corrosion between WC particles and metallic binders. By contrast, the formation of passive film in the form of surface oxide in the coatings containing Cr suppressed the binder material dissolution into the solution. However, the overall corrosion resistance of the WC–CrC–Ni coating was inferior to that of the WC–Co–Cr coating due to the presence of microcracks which act as the infiltration paths of the solution. Conclusively, it is found that chemical composition of metallic binder materials and control of microcracks are the most important factors influencing the corrosion resistance of the HVOF sprayed WC cermet coatings in the strong acidic environment. Also, the wear–corrosion results such as surface morphologies and friction coefficients have been presented.
The study of the tribological behavior of plasma sprayed coatings (Al2O3, Al2O3/TiO2, Al2O3/TiO2/Copper) in dry conditions was carried out in a cylinder/plane configuration. The effect of TiO2 and copper additions on the properties of coatings were investigated in terms of microhardness and fracture toughness, and were related to the friction behavior and wear resistance. It was found that TiO2 improves the fracture toughness of coatings and copper reduces friction and wear.
The electrochemical and corrosion behavior of PVD-TiN coatings was investigated through electrochemical impedance spectroscopy (EIS) in a 3% NaCl solution. Mild steel (active) and AISI316 stainless steel (passive) substrates were employed. The results indicated that it was possible to follow the corrosion of the active substrate using EIS over a period of 500 h immersion. The AISI316 substrate, however, remained effectively passive for the duration of the experiment. Based on this study, the EIS technique certainly shows promise in assessing the corrosion performance (and the coating quality) of electron-beam plasma-assisted PVD-TiN coatings on active substrates such as mild steel. Its usefulness on passive materials appears more limited, owing to the similarity of the electrochemical impedance response of the coating and substrate.
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.
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