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Effect of Al and Cr addition on tribological behaviour of HVOF and APS nanostructured WC-Co coatings
Abstract
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.
... 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 addition of chromium helped in enhancement of the corrosion resistance along with high-temperature wear properties of WC-Co coating. WC-Co-Cr coating had better high temperature resistance by minimizing the decarburization of WC [8,20,27,28]. ...
... This eta phase formed when W 2 C and W dissolve in binder phase to form ternary or eta carbides as Co x W y C z [25,44]. Earlier studies reported the formation of Cr 7 C 3 and Cr 3 C 2 phases after deposition of WC-Co-Cr coatings [13,28], which were not detected in the current work. The properties of WC-10Co-4Cr coating deposited by HVOF process are given in Table 3. ...
The effect of tribo-chemical reactions under varying sliding speed and load conditions on the friction and abrasive wear response of high-velocity oxy-fuel-sprayed WC–10Co–4Cr coating was studied. The abrasive wear rate and friction coefficient decreased with the increase in sliding speed while friction coefficient displayed increasing trend with increase in load. The decrease in friction coefficient and wear rate was attributed to formation of tribo-oxides and surface films with good lubricating properties. Severity of abrasive wear increased with increasing load which was associated with transition in wear mechanisms from plastic deformation and fatigue to delamination cracking, intergranular fracture and splat fracture. Increase in friction coefficient with load irrespective of sliding speed was due to increasing contribution of fracture-assisted mechanical wear as compared to oxidative wear. The nature, composition and properties of tribo-films imparted crucial role to influencing friction and abrasive wear of WC–10Co–4Cr coating.
... The hard particles are the loadbearing component in the structure, in which the roles of the metallic matrix are to hold the structure together and to provide the material toughness. Depending on the application and need, these composites can be fabricated by an in situ casting technique, thermal/plasma spraying of nanostructured powders, the sol-gel process, electrodeposition, physical/chemical vapor deposition, and other techniques (Basak et al., 2006). However, for the structural application, ex situ/in situ casting and thermal/ plasma spraying are most widely used and are discussed in the following sections. ...
... HVOF nanostructured WC-Co coatings on low-carbon steel substrate: (a) crosssection and (b) top views as well as (c) different magnifications(Basak et al., 2006). ...
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.
... The reason behind that was, during consolidation of the coating, individual splats got solidified almost instantaneously, which restricted the diffusion among them. The individual splats were mostly locked mechanically among them, rather than metallurgical bonding [58]. Therefore, during loading, these splat boundaries were prone to crack formation, as they were not able to transfer the load effectively in the surrounding areas. ...
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.
... Other than the mechanical properties, the presence of Co reduces the chances of decarburisation and formation of W 2 C [16]. Further, Cr is added to the WC-based coating to enhance the cohesive strength of the coating which in turn increases the wear resistance and provides enhanced corrosion-resistant properties to the coating [17]. Further many researchers have added graphene nano-platelets (GNPs) to enhance the resistance of coating towards wear and corrosion. ...
The aim of this work was to investigate the flexural strength and corrosion behaviour of graphene-reinforced WC-10Co-4Cr HVOF coating on sandblasted and laser-textured DH-36 naval-grade steel. In this regard, WC-10Co-4Cr and WC-10Co-4Cr + 3% graphene nanoparticles (GNPs) are applied on DH-36 substrates using the HVOF coating technique and their microhardness, flexural strength and corrosion behaviour are investigated. Amongst all, the flexural strength of WC-10Co-4Cr + 3%GNPs coating on laser-textured sample with pitch-to-diameter (p/d) ratio 2 was minimum (327.006 MPa) due to the effect of GNPs and better mechanical interlocking at the coating-substrate interface. Also, the corrosion resistance of coated DH-36 steel was evaluated by salt spray test; it revealed that corrosion resistance was improved with the application of WC-10Co-4Cr coating, which was further enhanced by the addition of GNPs as GNP has the property of being impermeable to gasses and salts. Further, microstructures and morphologies of the corroded surface were investigated using FE-SEM images.
... Based on field survey data, the most appropriate input parameters are selected to conduct lab-tests. Based on the results of lab-scale tests, components may be improved further by playing with different materials and coatings in hand [24]. Though lab-scale tests are most widely used and offer reliable results, it is not free from some flip sides [25]. ...
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.
... XRD pattern of the WC-Co coating Fig. 6 XRD pattern of the WC-CoMo coatingRegarding the porosity level, in the APS coatings a good value under 3% should be reached[8]. Chivavibul et al. studied the characteristics of the WC-Co powder deposited through HVOF and warm spraying. ...
In order to be competitive, it is demanded to have thin, tough and long lasting coatings. An important aspect is to use stable deposition technologies. As Cr assures wear, corrosion and high temperature resistance, the most employed coatings in industry generally contain Cr. Nevertheless, Cr is a hazardous element for the humans’ health, therefore, sustainable alternatives are needed to be implemented. The aim of this work is to investigate the microstructure, hardness, corrosion resistance and wear behavior of the novel WC-CoMo compared to conventional WC-Co coatings. So far, WC-CoMo coatings are not part of state of the art regarding the Atmospheric Plasma Sprayed (APS) coatings. WC-Co powder in plain form and mechanically mixed with Mo was deposited using the APS method on standardized Type A Almen Strips (C67 steel). The size of the powder grains varies between 5 µm and 30 µm. The obtained samples were investigated by means of Scanning Electron Microscopy, Energy Dispersive X-Ray Spectroscopy, X-Ray Diffraction, and hardness, wear and corrosion behavior were also evaluated. Results revealed formation of different intermetallic phases around the WC particles, which have a benefic influence on the coating characteristics and microstructure.
In this study, microstructural characterization was conducted on WC-17Co coatings produced via high-velocity oxygen fuel (HVOF), high-velocity air fuel (HVAF), and cold spraying (CS). All coatings exhibited good initial qualities such as relatively low porosity content, no surface cracking, and no delamination. The SEM study revealed important microstructural features and grain morphologies of each coating. While the composition of the feedstock material was similar, the CS-deposited coating showed higher Co content and lower WC content as determined by elemental composition using EDS. XRD analysis identified the formation of complex oxides and tungsten phases in coatings deposited using spraying technologies that result in melting of the feedstock powder such as HVOF and HVAF. These coatings include brittle phases such as W3Co3C or W2C due to decarburization of tungsten carbide particles. Microindentation test indicated that the CS-deposited coating had considerably lower hardness compared to the other two coatings, despite not having significantly lower porosity content. This discrepancy could be attributed to the dissociation and physical loss of the hard carbide phase during the high-velocity impact of particles in the CS process, which aligns with the detection of a higher amount of cobalt in the CS-deposited coating material. The obtained results contribute to understanding the microstructural characteristics and pave the way for future investigations on the thermo-mechanical properties of WC-Co coatings.
WC10Co4Cr and WC10Co4Cr+Graphene nano-platelets (GNPs) coatings were deposited on textured IS-2062 steel using the High Velocity Oxy-Fuel (HVOF) technique. Microhardness, wear-resistance, and bond-strength of coated specimens were investigated to assess coating soundness. In compared to WC10Co4Cr coating and pristine substrate, the microhardness of WC10Co4Cr+2%GNPs coating increases by 1.35 and 7.33 times, respectively. The wear-rate of WC10Co4Cr+GNPs coating decreases significantly as the percentage of GNPs increase. As WC10Co4Cr+4GNPs were compared to WC10Co4Cr coating and pristine substrate, the co-efficient of friction (COF) was found to be reduced by approximately 25% and 50%, respectively. Laser texturing considerably enhances the coating interface bond strength. The substantially improved performance of the WC10Co4Cr+2%GNPs coating could be attributed to a superior combination of microhardness and bond strength.
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.
The present work focuses on high-temperature solid particle erosion and sliding wear behaviours of WC–Cr3C2–Ni coatings deposited by atmospheric plasma spray (APS) and high-velocity oxy-fuel (HVOF) processes. The high-temperature erosion behaviour and the effects of temperature on the sliding wear performance of coated specimens were comparatively studied using an air-jet erosion tester and a ball-on-disc tribometer, respectively. Unidirectional dry sliding wear has been tested up to 800 °C, whereas the erosion tests were up to 650 °C. Interestingly, in a high-temperature sliding, the wear rates of both the coatings decreased after 500 °C due to the development of tribofilm. Furthermore, erosion resistance of both coatings was observed to be more prominent at 650 °C. HVOF coating offers higher erosion resistance than APS coating due to higher hardness, lower splat cohesion, lesser degree of decarburisation, and retained oxide layer. The HVOF-deposited WC–Cr3C2–Ni coating improves the high-temperature sliding wear and solid particle erosion resistance of the base material appreciably.
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.
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.
In the current study, WC-Co and Cr3C2-NiCr coatings deposited on 90MnCrV8 steel surface via an atmospheric plasma spray (APS) system were modified by the plasma transferred arc (PTA) welding method. Microstructural defects including micro-cracks, voids, pores, and non-uniform zones were determined in the APS deposited layers. The microstructural defects were terminated by the PTA melting process due to the dissolving pool at high temperature. Strong metallurgical bonding between the coating layer and substrate and columnar dendrites and inter-dendritic precipitates were observed during the PTA melting process. Following the PTA melting process, MC, M3C, and M7C3 hard phases were formed in the coating layers. The hardness and wear performance of the coating layers significantly increased due to the PTA surface modification. The main reason for the significant increases in wear performance corresponded to the newly formed hard carbide phases and elimination of microstructural defects via the PTA surface modification.
The objective of the present work is to investigate the effect of abrasive particle size on friction and abrasive wear performance of HVOF sprayed WC-10Co-4Cr coating. WC-10Co-4Cr coating was deposited by HVOF technique on 316 SS substrate. Friction coefficient and abrasive wear rates were measured with varying abrasive particle size, load and sliding speed. The microscopic observations, XRD and XPS analyses of worn surface were used to analyze friction and wear data. The increase in abrasive wear rate with an increase in load and abrasive particle size was attributed to the transition in wear mechanisms. The variation in friction coefficient with abrasive particle size as a function of load and sliding speed was due to change in the contribution of adhesion and abrasion, and varying degree of tribo oxidation. With increasing sliding speed abrasive wear rate and friction coefficient decreased, which was due to the dominance of oxidative wear. The prevalence of fracture dominated mechanical wear processes led to an increased coefficient of friction at higher load.
Coating deposition on many industrial components with good microstructural, mechanical properties and better wear resistance is always a challenge for the thermal spray community. A number of thermal spray methods are used to develop such promising coatings for many industrial applications; viz. arc spray, flame spray, plasma, and HVOF. All these processes have their own limitations to achieve porous free, very dense, high
performance wear resistant coatings. In this work, an attempt has been made to overcome this limitation. Molybdenum coatings were deposited on low carbon steel substrates using Wire-High Velocity Oxy-Fuel (W-HVOF; WH) thermal spray system (trade name-HIJET 9610®). For a comparison, Mo-coatings were also fabricated by arc spray (AS), flame spray (FS), plasma spray (PS) and powder-HVOF (PH) processes. As-sprayed coatings were analyzed using x-ray diffraction (XRD), scanning electron microscopy (SEM) for phase and
microstructural analysis respectively. Coating microhardness, surface roughness, and porosity were also measured. Adhesion strength and wear tests were conducted to determine the mechanical and wear properties of the as-sprayed coatings. Results show that the coatings deposited by W-HVOF have better performance in terms of microstructural, mechanical and wear resistance properties, in comparison to available thermal spray process (flame spray and plasma spray).
A comprehensive review allows the author to discern not only the various trends as they emerge within Metal Finishing, but also to track published articles in the most important journals serving the industry in terms of surface finishing and materials technology. That German and more generally, European surface finishing has continued to flourish, is ascribed, among other factors, to the ability within the industry to adopt new developments and a generally high level of competence. The superiority of the German metal finishing industry over, for example, its American counterpart, is evident. Noteworthy also, is a growth related to hitherto unimportant disciplines, such as medical technology. The review is based on a scan of 34 technical publications, of which 19 are German-language.
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.
Decarburization behavior of WC-Co particles in terms of transformation of WC to W2C and W, and formation of eta and gamma phases and microstructure evolution during plasma spraying have been systematically investigated in this study. The extent of the carbon loss of WC was tailored by either altering cooling conditions of substrate/pre-coating or spraying the particles into the media with different temperatures. It is revealed that loss of carbon of WC was alleviated by protection of Co during the coating formation stage. W2C exhibits epitaxial growth on the WC substrates in perpendicular direction and forms a nearly complete shell around the WC particles. eta phase was formed as a result of decar-burization and diffusion of associated phases and is located around WC-Co splats with its crystals being in cross shape. The gamma phase in rod-like shape with a size of 10-20 nm embeds within the binder Co and is clearly well separated from WC grains. Further decarburization-induced W was detected mainly in Co binder, being apart entirely from WC grains. The main advantage of Co for preventing decarburization in WC-Co particles is not associated with oxidation, but instead the diffusion-controlled carbon loss. These findings would facilitate fabrication of the WC-based cermet coatings with excellent mechanical properties in particular wear resistance for extreme wear applications.
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.
Protective WC–Co-based coatings containing solid lubricant Cu and MoS2 used in wear applications were investigated in this study. These coatings were deposited on mild steel substrates by atmospheric plasma spraying (APS). The feedstock powders were prepared by mechanically mixing the solid lubricant powders and WC–Co powder, followed by sintering and crushing the mixtures to avoid different particle flighting trajectories at plasma. The tribological properties of the coatings against stainless steel balls were examined by ball-on-disk (BOD) tribometer under normal atmospheric condition. The microstructure of the coatings was studied by optical microscope, scanning electron microscope and X-ray diffraction. It was found that the MoS2 composition in the feed powder was kept in WC–Co–Cu–MoS2 coatings, and the decomposition and decarburization of WC in APS process were improved, which were attributed to the protection of Cu around them. The friction and wear behaviors of all the WC–Co–Cu–MoS2 coatings were superior to that of WC–Co coating. Such behavior was associated to different wear mechanisms operating for WC–Co coating and the WC–Co–Cu–MoS2 coatings.
The effects of mixing powders with various particle sizes on the fracture toughness and wear resistance of thermally sprayed WC–10Co–4Cr coating layers fabricated by the HVOF (High-Velocity Oxygen Fuel) process on a S45C steel substrate were investigated. In order to obtain a high fracture toughness and wear resistance, the powder size and powder mixing ratio were varied. The microstructure and chemical composition of the phases in the coatings were characterized by means of the SEM and XRD techniques. Image analysis was used for the evaluation of the porosity of the coatings. Indentations tests were carried out on the cross sections of the coatings to evaluate the hardness and fracture toughness. The wear properties of the coatings were assessed using a pin-on-disk wear tester at ambient temperature without lubrication. The mixing of a small amount of coarse powders with fine powders resulted in the highest fracture toughness and wear resistance, due to the formation of coating layers having the lowest porosity.
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.
Near-nanostructured WC-18 pct Co coatings, with low amounts of non-WC carbide phases, have been synthesized using high velocity
oxygen fuel (HVOF) thermal spraying under spraying conditions of varying fuel chemistry, fuel-oxygen ratio, and powder particle
size. The results show that the temperature the particles experience during spraying depends on the preceding parameters.
Compared to available published results on WC-Co system coatings, nanostructured WC-18 pct Co coatings, synthesized in these
experiments, contain very low amounts of non-WC carbide phase (less than 10 pct vol). This is comparable to that of the conventional
WC-12 pct Co coating, prepared in the present study for comparison purposes. Regardless of whether the binder phase in the
agglomerated feedstock powder particles melt or not, the WC particles do not appear to experience significant growth as a
result of the spraying. The size of WC particles remains in the 200 to 500 nm range, consistent with that present in the feedstock
powder. The as-received near-nanostructured WC-18 pct Co feedstock powder exhibits morphological characteristics that lead
to low amounts of non-WC carbide phases in the coatings. The microstructure and phase constitution of the coatings depend
on particle size of the feedstock powder and flame characteristics of the fuels during spraying. A higher particle temperature
causes more decomposition of the WC phase but reduces porosity in the coatings, this occurs with higher flame temperature
and smaller particle sizes. Propylene fuel produces less decomposition of the WC phase despite the higher flame temperature
and, thus, provides the best combination of dense coating with low amount of non-WC phase.
FRED HASSAN COULD HAVE TAKEN the easy route and become vice chairman of Pfizer, the world's largest pharmaceutical company, after it spent $58 billion to buy his old business, Pharmacia. Instead, the 57-year-old executive signed on in April as chairman and chief executive officer of Schering-Plough, a midsized drug company plagued by an extraordinary convergence of manufacturing, legal, regulatory, and competitive problems.
The effects of nitrogen ion implantation on the properties and the characters of WC-Co cemented carbides were studied. The implanted layers were characterized by means of AES and XRD. The friction wear tests were undertaken with a pin-on-disc friction-wear tester. The worn surface morphologies of the samples were observed with SEM and EMPA. The mechanisms of surface modification of N ion implantation into WC-Co cemented carbides were considered. The results indicate that a carbon-rich layer and some nitrides form in the nitrogen ion implanted surface layer of WC-Co. A suitable dose of nitrogen ion implantation can increase the surface hardness of WC-Co and an excess dose can cause decrease in surface hardness of WC-Co. Nitrogen ion implantation reduces the friction adhesion transfer from stainless steel to implanted WC-Co.
This paper reports the evaluation of extreme-pressure activity of lubricants containing alkyl para substituted benzyl sulfides. These additives exhibited good activity by increasing seizure and welding load compared to the base fluid. A polyethoxylated chain in para position on additives has been found to enhance the load-carrying capacities of the emulsion. Electron probe microanalysis was conducted in order to detect the sulfur distribution on the wear scar surface of compounds compared to the behavior of a commercial disulfide.
One of the foremost coating methods for combating wear is thermal spraying, however, despite its widespread industrial use, little is known about the basic friction behavior and the mechanisms by which such coatings wear. The manner of processing in thermal spraying inevitably leads to inhomogeneities, such as unmelted particles, oxide inclusions and porosity, in the sprayed deposits resulting in a structure markedly different from that of cast, wrought or even powder metallurgy materials. It is expected that thermal spray coating behavior would be even more complicated than that of homogeneous, uncoated bulk materials. Results of an investigation to determine the effects of some wear test variables on high velocity oxy-fuel (HVOF) sprayed Cr3C2NiCr coatings using a pin-on-disk tribometer are presented. Room temperature sliding friction and wear behavior of coatings are discussed with respect to load, relative velocity and counterbody material. The present investigation shows that the tribological behavior of HVOF sprayed Cr3C2NiCr coatings is significantly affected by its microstructural constituents such as splats, porosity and form and dispersion of second phases. It is also shown that changes in imposed sliding wear test conditions varied the friction and wear behavior of thermally sprayed coatings considerably. The break-in sliding coefficient of friction is found to be more significantly affected by load than other test parameters. Results also indicate that friction decreased with increasing velocity but wear decreased then increased with increasing velocity. By proper control of test conditions and by selected changes of those conditions, the physical wear mechanisms involved in thermally sprayed coatings could be understood. This study showed that the pin-on-disk is a well controlled test and can be used to understand certain basic relationships between the sliding friction and wear behavior of thermally sprayed coatings.
The microstructure of WC-Co composites sintered with VC or with a mixture of VC and Cr3C2 is investigated by several techniques in order to understand the grain growth inhibition process. In this work, using the high-resolution transmission electron microscopy and high-resolution energy-filtering electron microscopy, we are able to study on the atomic scale the microstructure and composition of the phases present in the alloys. A thin (VW)Cx layer less than 1 nm thick covering all surfaces of WC grains and thin (VW)Cx platelets embedded in the WC grains are evidenced. Microsteps are observed at the interface between Co and WC along the directions. Small (VW)Cx precipitates lie on the (0001)WC and facets of these steps. On the (0001) surface of WC grains, other stacking sequences of the metal planes are sometimes observed and, in particular, the occurrence of the compound (VW)2C is shown. Owing to these observations a grain growth inhibition mechanism is proposed.
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.
Hard chrome plating is used to restore the original dimensions to worn surfaces of gas turbine shafts. However, its use is about to decrease due to some intrinsic limitations of its deposits and the toxic and carcinogenic characteristics of the hexavalent chromium. During the last decade high velocity oxy-fuel (HVOF) thermal sprayed cermet coatings play an important role in industrial applications where exceptional friction and wear resistance are required. The purpose of this study is to investigate and to compare the microstructure, wear resistance and potentials of HVOF sprayed Cr C –NiCr and WC–Co coatings for a possible 3 2 replacement of hard chromium plating in gas turbine components repair. It has been shown that coatings exhibit high hardness with a high volume fraction of carbides being preserved during the spraying, and have different wear behaviour.
Tungsten carbide coatings applied by the plasma spray process have been widely used in wear applications. In the W–C–Co ternary system, tungsten carbide can either be present as WC or W 2 C. Frequently WC transforms into W 2 C during the plasma spray process. In this study, tungsten carbide/17% cobalt coatings were applied by both the air plasma and the vacuum plasma spray processes using different power levels and plasma gases. The W 2 C phase was found by X‐ray diffraction techniques in the air plasma sprayed coatings. Decarburizing of the WC in the presence of the oxygen took place in the plasma. In this study air plasma sprayed coating hardness and microstructure are superior to those of the vacuum plasma sprayed ones. However the vacuum plasma coatings were found to be more wear and impact resistance than the air plasma coatings. This performance difference may be attributed to the presence of hard and brittle W 2 C phase in the air plasma sprayed tungsten carbide 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 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.
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.
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.
The corrosion and corrosion-wear behaviour of a thermal sprayed nanostructured FeCu/WC-Co coating were investigated in a Hank's solution and compared to stainless steel AISI 304 and nanostructured WC-Co coatings. Electrochemical noise and potentiodynamic polarization measurements were conducted along with an ex situ scanning electron microscopy to unfold the response of these materials under these corrosive and corrosion-wear test conditions. The multiphase structure of the FeCu/WC-Co coating induces a complex corrosion behaviour. Under corrosion-wear conditions, the nanostructured FeCu/WC-Co coating exhibits a depassivation/repassivation behaviour comparable to the behaviour of stainless steel AISI 304 and nanostructured WC-Co coatings.
- D A Stewart
- P H Shipway
- D G Mccartney
D. A. Stewart, P. H. Shipway and D. G. McCartney: Wear, 1999,
225-229, 789-798.
- T Sahraoui
- N E Fenineche
- G Montavon
- C Coddet
T. Sahraoui, N. E. Fenineche, G. Montavon and C. Coddet:
Mater. Des., 2003, 24, 309-311.
- J M Guillemany
- J M Miguel
- S Vizcaino
- F Climent
J. M. Guillemany, J. M. Miguel, S. Vizcaino and F. Climent: Surf.
Coat. Technol., 2001, 140, 141-146.
- M Mohanty
- R W Smith
- M De
- J P Bonte
- E Celis
- Lugscheider
M. Mohanty, R. W. Smith, M. De Bonte, J. P. Celis and
E. Lugscheider: Wear, 1996, 198, 251-266.
- Y Zhu
- K Yukimura
- C Ding
- P Zhang
Y. Zhu, K. Yukimura, C. Ding and P. Zhang: Thin Solid Films,
2001, 388, 277-282.