CoCr Coatings Research Articles

ChemInform Abstract: Behavior of a Sputtered Chromia-Dispersed Co-Cr Coating.

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Influence of feedstock powder characteristics and spray processes on microstructure and properties of WC–(W,Cr) 2C–Ni hardmetal coatings

The composition WC–(W,Cr) 2C–Ni (commercial designations WC–‘CrC’–Ni, WC–Cr 3C 2–Ni and WC–NiCr) is unique among the WC-based materials used for the preparation of thermally sprayed hardmetal coatings. These coatings show a significantly higher oxidation resistance and high-temperature sliding wear resistance than WC–Co and WC–CoCr coatings do. Unlike WC–Co and Cr 3C 2–NiCr, WC–(W,Cr) 2C–Ni is not a simple binary hard phase–binder metal composite as it is composed of two hard phases: WC and (W,Cr) 2C. Surprisingly this composition has been poorly investigated in the past. In this paper coating microstructures and properties obtained from five commercial feedstock powders of different origins using two different liquid-fuelled high velocity oxy-fuel (HVOF) systems (K2 and JP-5000) were investigated. Additional experiments were performed with one powder using atmospheric and vacuum plasma spraying (APS and VPS, respectively). The microstructures and phase compositions of the powders and the coatings were studied. Focus was on the appearance, composition and distribution of the (W,Cr) 2C phase which might form or might change its Cr/W ratio during the spray process. The composition of the (W,Cr) 2C phase was estimated from the lattice parameters. Hardness HV0.3 was measured for all coatings. The density, Young's modulus and abrasion wear resistance of HVOF-sprayed coatings were studied.

Influence of nano-WC–12Co powder addition in WC–10Co–4Cr AC-HVAF sprayed coatings on wear and erosion behaviour

WC–10Co–4Cr coatings deposited by activated combustion-high velocity air fuel (AC-HVAF) spray process are the most commonly used system for different components prone to wear/erosion. The aim of this work is to investigate the influence of nano-WC–12Co powder addition in WC–10Co–4Cr AC-HVAF sprayed coatings on wear and erosion behaviour. Therefore, in this work, nano-WC–12Co powder was added to the sub-micron WC–10Co–4Cr powder in order to improve the hardness of WC based AC-HVAF coatings. The result shows that 15% addition of nano-WC–12Co will improve the hardness of AC-HVAF coating from 1677 to 1873 HV0.3 due to the embedding of nano-WC–12Co powder. Sliding wear test and slurry erosion test were chosen to evaluate the effect of adding nano-WC–12Co powder. The wear and erosion resistance of AC-HVAF coated samples go up with the addition of nano-WC–12Co powder, in addition, the coating hardness has also increased. The coatings have been characterised by scanning electron microscope (SEM), microhardness tester and X-ray diffractometer (XRD). The results indicated that the sample with 15% addition of nano-WC–12Co to WC–10Co–4Cr based AC-HVAF coating has the best wear and erosion resistance. In addition, this also demonstrated that it is possible to fabricate the nanostructured WC–10Co–4Cr coating with low porosity and high hardness by AC-HVAF spray deposition with reasonable thermal spraying parameters.

Surface modification of HVOF thermal sprayed WC–CoCr coatings by laser treatment

In this work the affects of laser characteristics on microstructure and microhardness of high velocity oxygen fuel sprayed (HVOF) WC–CoCr coatings were investigated. The coating was deposited with a Sulzer Metco WokaJet™-400 kerosene fuel and the laser surface treatments were applied using CO2 laser with 10.6 μm wavelength. Large variations in surface properties were produced from variation in the laser processing parameters. In total, four levels of peak power (100, 200, 300 and 350 W), four levels of spot diameter (0.2, 0.4, 0.6 and 1 mm) and three levels of pulse repetition frequency (PRF) were investigated. An initial set of tests were followed by a more detailed 33 factorial design of experiments. Pulse repetition frequency and duty cycle were set in order to maintain the same overlap in the x and y directions for the raster scanned sample spot impact dimensions. Overlaps of 30% were used in the initial tests and 10% in the more detailed trials. The results have shown that care must be taken to keep the irradiance at a relatively low level compared to uncoated surfaces. High irradiance can in this case result in rough and porous surfaces. Lower levels of irradiance are shown to provide more uniform microstructures, reduced porosity and increased microhardness.

Effect of microstructure on abrasive wear behavior of thermally sprayed WC–10Co–4Cr coatings

Thermally sprayed WC–Co coatings have been widely used in the coatings industry for its superior sliding, abrasive and erosive wear properties. In applications where corrosion resistance is also required in addition to wear resistance, WC–10Co–4Cr is the preferred coating composition. The coatings produced by different thermal spray processes exhibit a broad range of coating hardness, porosity and microstructural features like grain size and volume fraction of individual phases. In this study, we have evaluated the coating microstructures of various WC–10Co–4Cr coatings produced from different spraying processes, such as high velocity oxy-fuel (HVOF) and pulsed combustion. The objective of our study is to explore the abrasive wear mechanism of WC–10Co–4Cr coatings in great detail, and determine how these mechanisms are influenced by the coating microstructure. Dry sand rubber wheel abrasion test rig (based on ASTM G65) is used for evaluating the three-body abrasive wear properties of the coatings, using alumina as the abrasive material. The coating microstructural parameters including WC grain size, volume fraction, binder mean free path have been quantitatively measured, and correlation between abrasive wear behavior of coatings and its microstructural parameters is sought. This study shows that binder mean free path of carbides (which is a function of WC grain size and volume fraction) is a very important parameter affecting the abrasion resistance of good quality coatings (containing low porosity). The lower the binder mean free path, the higher is the abrasive wear resistance offered by the coating. This is because the abrasive wear mechanism of these coatings is dominated by preferential removal of the binder phase, followed by pullout of WC grains.

Tribology of thermal sprayed WC–Co coatings

This paper looks at the tribology of thermal sprayed WC–Co based coatings and covers the high energy air–sand erosion resistance and slurry jet impingement erosion performance, dry and wet sliding tribology of thermal spray WC–Co based coatings as well as the abrasion and abrasion–corrosion of these coatings. The tribological and tribo-corrosion performance of the coatings will be related to their mechanical and corrosion properties as well as deposition parameters, microstructure and actual composition. For example, the anisotropic microstructure of thermally sprayed WC–Co–Cr coatings, in particular the low fracture toughness in a direction parallel to the substrate, has been observed to affect the nature of crack formation under 200 μJ air–solid particle erosion conditions. Voids and occasionally other microstructural features (i.e., cobalt lakes, splat boundaries, interfacial inclusions) in the coating act as crack initiation sites. The erosion rate was dominated by cracks within 5 μm of the surface and was relatively insensitive to total length of cracks, showing a near-surface damage front controls the erosion rate and this region is coincident with the region of maximum shear stress induced by erodent impacts. Optimisation of the deposition parameters of HVOF 86WC–10Co–4Cr coatings show an improvement in erosion resistance of more than 50% over the conventional D-gun applied coating of identical nominal composition. The variation in the slurry erosion performance of the thermally sprayed coatings is also linked to directional fracture toughness and crack propagation paths which are influenced by the presence of pores, inhomogeneous carbide distributions and substrate grit blast remnants. The influence of slurry jet angle is more pronounced under 0.4 μJ energy conditions where maximum erosion occurred at 90° and the minimum at 30° in contrast to 7 μJ slurry erosion rates which were independent of jet angle. This reflects the lower levels of fluctuating stresses imparted to the coating during low energy slurry impacts leading to the impact angle having a greater effect on sub critical crack growth rate than for higher energy conditions. The abrasion resistance of these coatings was found comparable to sintered cermets of the same composition. The synergistic effects between micro and macro abrasion and corrosion for detonation gun (D-gun) sprayed WC–10Co–4Cr coatings are shown to be significant and depend on the environment. The size effect of the abradant relative to the microstructure and splat size is important as well as the propensity for the various phases to passivate to control corrosion levels. Comparisons between exposed and freshly polished coating surfaces in strong NaOH solutions (pH 11) show that significantly lower wear rates were seen for the exposed surface due to a negative wear–corrosion synergy due to selective phase removal and the effects of localised passivation. Dry and wet sliding wear resistance of these coatings is shown to be high (wear rates of 10−16–10−18 m3/Nm) with modest coefficient of friction levels between 0.2 and 0.5. The presence of oxides on the binder phases appears to influence the friction and wear levels. Wear appears to be by carbide ejection and/or by tribo-chemical processes.

HVOF-sprayed WC–CoCr coatings on Al alloy: Effect of the coating thickness on the tribological properties

The microstructure, the micromechanical properties, the wear behaviour and the impact resistance of WC–CoCr cermet coatings, deposited onto an aluminium alloy substrate by High Velocity Oxygen-Fuel (HVOF) flame-spraying, were examined as a function of the coating thickness, which was varied between 50 μm and 150 μm by performing different numbers of scans of the HVOF torch in front of the substrate. The coatings became denser and significantly harder as the number of torch scans increased: the analysis of single WC–CoCr splats by combined SEM and Focused Ion Beam (FIB) techniques enabled the interpretation of the mechanisms underlying this phenomenon. In accordance to such densification, the sliding wear resistance increased with the number of torch scans, as abrasive grooving and brittle failure mechanisms were progressively suppressed. The resistance to cyclic impact was also enhanced. In comparison to anodised films, the WC–CoCr coatings appeared much more resistant against wear and cyclic impact; specifically, three torch scans seem enough to produce a coating having suitable characteristics.

Characterization of microstructure and surface properties of hybrid coatings of WC–CoCr prepared by laser heat treatment and high velocity oxygen fuel spraying

The microstructure and microhardness of high velocity oxygen fuel-sprayed WC–CoCr coatings were comparatively studied both before and after laser heat treatment of the coatings. Optical microscopy, scanning electron microscopy, X-ray diffraction and microhardness testing were applied to investigate the microstructure, phase composition, porosity and microhardness. The results indicate that WC is still present, and W 2C has appeared, while neither cobalt nor σ-CrCo is detectable. Co 4W 2C has appeared in the high velocity oxygen fuel-sprayed coating after laser heat treatment as compared to the coating before laser treatment. The relative content of the W 2C has not increased with laser treatment, but the laser treatment has essentially eliminated the porosity almost entirely, providing a more homogeneous and densified microstructure. The laser heat treatment has effected the formation of a denser compact coating on the substrate. After laser heat treatment, the thickness of the coating has decreased from 300 μm to 225 μm. This corresponds to an average porosity in the high velocity oxygen fuel-sprayed coating that is approximately five times greater than that in the subsequently laser heat-treated coating. The laser treatment has also resulted in an increased hardness of the coating near the surface, where the average value increased from Hv 0.2 = 1262.4 in the coating before laser heat treatment to Hv 0.2 = 1818.7 after laser heat treatment.

Exposure effects of strong alkaline conditions on the microscale abrasion–corrosion of D-gun sprayed WC–10Co–4Cr coating

The synergistic effects between abrasion and corrosion for detonation gun (D-gun) sprayed WC–10Co–4Cr coatings have been studied for alkaline conditions. Comparisons between exposed and freshly polished coating surfaces in strong NaOH solutions (pH 11) show that significantly lower wear rates were seen for the exposed surface due to a negative wear–corrosion synergy. The negative synergy was seen to increase with increasing sliding distances for both fresh and exposed surfaces. The physical mechanisms of this negative synergy are discussed. The effect of anodic surface treatments on wear–corrosion to elucidate the influence of corrosion on the negative synergy is also reported.

Exposure effects of alkaline drilling fluid on the microscale abrasion–corrosion of WC-based hardmetals

The microscale abrasion–corrosion performance has been examined for two sintered hardmetals, WC–6Co and WC–11Ni, and two WC–10Co–4Cr sprayed coatings after exposure to a water-based alkaline drilling fluid for 168 h. A series of tests were also performed on freshly polished sintered hardmetals and coatings using both NaOH (pH 11) and neutral SiC aqueous slurries. Contrary to expectations, micro-abrasion under alkaline conditions generally resulted in lower wear rates producing a negative 8–18% abrasion–corrosion synergy, with the exception of the sintered WC–11Ni. Typically, wear scars for the sintered hardmetals were 7–8 carbides deep and the wear mechanism was binder depletion and undermining of the carbides. While the corrosion performance of the sintered hardmetals was only slightly affected when exposed to the alkaline drilling fluid, there was a marked decrease in the anodic behaviour for the WC–10Co–4Cr coatings. SEM analysis revealed that exposure to the drilling fluid resulted in a localised corrosion zone at the binder–carbide interface approximately 100 nm wide. This selective corrosion will alter the surface composition in addition to influencing mechanical properties such as the surface roughness, hardness, surface compliance and synergistic effects. The synergy results for both sintered and sprayed coatings are discussed in terms of the wear mechanisms at the microscale, selective phase removal and the effects of localised passivation.

Evaluation of WC–17Co and WC–10Co–4Cr thermal spray coatings by HVOF on the fatigue and corrosion strength of AISI 4340 steel

It is known that chromium electroplating is related to the reduction in the fatigue strength of base metal. However, chromium results in protection against wear and corrosion combined with chemical resistance and good lubricity. Environmental requirements are an important point to be considered in the search for possible alternatives to hard chrome plating. Aircraft landing gear manufactures are considering WC thermal spray coating applied by the high-velocity oxygen-fuel (HVOF) process an alternative candidate, which shows performance at least comparable to results, obtained for hard chrome plating. The aim of this study is to compare the influence of WC–17Co and WC–10Co–4Cr coatings applied by HVOF process and hard chromium electroplating on the fatigue strength of AISI 4340 steel, with and without shot peening. S– N curves were obtained in axial fatigue test for base material, chromium plated and tungsten carbide coated specimens. Tungsten carbide thermal spray coating results in higher fatigue strength when compared to hard chromium electroplated. Shot peening prior to thermal spraying showed to be an excellent alternative to increase fatigue strength of AISI 4340 steel. Experimental data showed higher axial fatigue and corrosion resistance in salt fog exposure for samples WC–10Co–4Cr HVOF coated when compared with WC–17Co. Fracture surface analysis by scanning electron microscopy (SEM) indicated the existence of a uniform coverage of nearly all substrates.

Corrosion properties of nanocrystalline Co-Cr coatings.

Nanocrystalline and conventional Co-Cr (ASTM F75) coatings were prepared by plasma spraying for possible orthopedic implant applications. Scanning electron microscopy and transmission electron microscopy were used to study the macrostructure and microstructure of the resultant sprayed coatings. The corrosion resistance was characterized by an in vitro potentiodynamic anodic polarization technique in a pseudophysiological solution. The nanocrystalline coating has higher porosity, lower corrosion current density, and less localized damage than that of the conventional one, demonstrating better application potential for orthopedic implants. A change in the atomic compositional difference between the grain interior and the grain boundary, the presence of residual strain in the grain interiors, and a change in the repassivation kinetics are discussed as possible explanations for the enhanced corrosion behavior observed.

Mechanical and histological evaluations of cobalt-chromium alloy and hydroxyapatite plasma-sprayed coatings in bone

This study evaluated the mechanical and histological behavior of cobalt-chromium (CoCr) alloy and hydroxyapatite (HA) plasma-sprayed coatings in canine cortical bone after 6 and 12 weeks of implantation, using CoCr alloy as the substrate. the substrate was bond-coated with microtextured CoCr alloy coating to ensure adherence between the substrate and top coats. A macrotextured CoCr alloy top coat with surface roughness R a=34.25±5.50 μm was produced to create suitable pores ranging from 25 μm to 200 μm for bone ingrowth. For HA top coat, a relatively smooth surface (R a=15.14±3.21 μm) was prepared for bone apposition. Shear testing of bone/implant interfaces showed that the CoCr alloy top coat exhibited significantly lower (p<0.01) mean shear strength than the HA top coat at each time interval. The maximum shear strength was 10.88±0.38 MPa for HA-coated implants 12 weeks post-implantation. After histological evaluations, substantial differences in the extent of new bone formation and the types of implant/bone contact were found between two kinds of implants. Direct bone-to-HA coating contact was consistently observed, while a layer of fibrous tissue intervening at the bone-CoCr alloy coating interface was found. Occasionally, partial dissolution of HA coating was seen after 12 weeks of implantation. The results of this study suggested that plasma-sprayed macrotextured CoCr coatings may not be an effective alternative for biological fixation.

High chromium cobalt-base coatings for low temperature hot corrosion

Accelerated oxidation tests have been conducted on a number of cast Co-Cr-Al alloys coated with a thin film of Na 2SO 4 and exposed in an O 2-0.15%(SO 2-SO 3) gas mixture at 750°C. These tests show that the corrosion resistance of binary Co-Cr alloys increases with the chromium content of the alloys and that even small concentrations of aluminum have detrimental effects on their corrosion resistance. Tests on Co-Cr coatings on a René 80 substrate under the same conditions indicate that there is a transition from a high to a low corrosion rate at a chromium level of about 37.5 wt.%. These results can be explained on the basis of a mechanism that we have already proposed in the literature. Based on our laboratory studies, we have developed new Co-Cr coatings that provide excellent resistance to low temperature hot corrosion and are also acceptable for conventional, high temperature, hot corrosion.

A neutron depolarization study on dysprosium and terbium

The composition WC–(W,Cr)2C–Ni (commercial designations WC–‘CrC’–Ni, WC–Cr3C2–Ni and WC–NiCr) is unique among the WC-based materials used for the preparation of thermally sprayed hardmetal coatings. These coatings show a significantly higher oxidation resistance and high-temperature sliding wear resistance than WC–Co and WC–CoCr coatings do. Unlike WC–Co and Cr3C2–NiCr, WC–(W,Cr)2C–Ni is not a simple binary hard phase–binder metal composite as it is composed of two hard phases: WC and (W,Cr)2C. Surprisingly this composition has been poorly investigated in the past.In this paper coating microstructures and properties obtained from five commercial feedstock powders of different origins using two different liquid-fuelled high velocity oxy-fuel (HVOF) systems (K2 and JP-5000) were investigated. Additional experiments were performed with one powder using atmospheric and vacuum plasma spraying (APS and VPS, respectively). The microstructures and phase compositions of the powders and the coatings were studied. Focus was on the appearance, composition and distribution of the (W,Cr)2C phase which might form or might change its Cr/W ratio during the spray process. The composition of the (W,Cr)2C phase was estimated from the lattice parameters. Hardness HV0.3 was measured for all coatings. The density, Young's modulus and abrasion wear resistance of HVOF-sprayed coatings were studied.