WC-CoCr Coatings Research Articles

Comparison between oxidation kinetics of HVOF sprayed WC–12Co and WC–10Co–4Cr coatings

In this study, the high temperature oxidation behavior of HVOF-sprayed WC–12Co and WC–10Co–4Cr coatings were investigated. To explore the oxidation mechanism, thermo-gravimetric analysis (TGA) was applied for isothermal treatments in the range of 500–800°C for 3h. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the structural changes and microstructural evolutions during oxidation tests. The TGA experiments showed negligible oxidation mass gains at 500°C for both coatings. At higher temperatures, i.e. 700 and 800°C, the oxidation mass gains of WC–12Co were found to be much higher than those for WC–10Co–4Cr coating, respectively. The higher oxidation resistance of WC–10Co–4Cr coating probably results from the formation of compact chromium oxide layers and higher MWO4 type tungstate (M: Co and/or Cr) to tungsten trioxide (WO3) ratios which provide lower porosity and consequently more efficient passivation effect against oxidation. The time dependent mass gain of WC–12Co coating obeys the linear law within temperature range of 600–800°C with apparent oxidation activation energy of ~104kJ/mol. As for the oxidation of WC–10Co–4Cr coating, a negligible deviation from linear law was observed possibly due to the presence of chromium oxide and higher tungstate to tungsten trioxide ratio which hinders the diffusion process through the scales compared with WC–12Co coating. The apparent activation energy for oxidation of the WC–10Co–4Cr coating was found to be ~121kJ/mol.

Wear and corrosion performance of WC-10Co4Cr coatings deposited by different HVOF and HVAF spraying processes

This study compares three types of WC-10Co4Cr coatings deposited with high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) spraying processes. The experimental results indicated that the decarburisation of the WC in the WC-10Co4Cr coating was dramatically influenced by the spraying equipment, and the non-WC phase content in the as-sprayed coatings greatly influenced their performances. The HVAF-sprayed WC-10Co-4Cr coating revealed the lowest degree of decarburisation, achieving the best properties in terms of hardness, fracture toughness, abrasive and sliding wear as well as electrochemical corrosion resistance when compared to the two HVOF-sprayed WC-10Co-4Cr coatings.

WC-10Co4Cr으로 초고속 화염용사 코팅된 Cu 합금의 해수내 캐비테이션 손상 거동

Due to the good corrosion resistance and machinability, copper alloy is commonly employed for shipbuilding, hydroelectric power and tidal power industries. The Cu alloy, however, has poor durability, and the seawater application at fast flow condition becomes vulnerable to cavitation damage leading to economic loss and risking safety. The HVOF(High Velocity Oxygen Fuel) thermal spray coating with WC-10Co4Cr were therefore introduced as a replacement for chromium or ceramic to minimize the cavitation damage and secure durablility under high-velocity and high-pressure fluid flow. Cavitation test was conducted in seawater at <TEX>$15^{\circ}C$</TEX> and <TEX>$25^{\circ}C$</TEX> with an amplitude of <TEX>$30{\mu}m$</TEX> on HVOF WC-10Co4Cr coatings produced by thermal spray. The cavitation at <TEX>$15^{\circ}C$</TEX> and <TEX>$25^{\circ}C$</TEX> exposed the substrate in 12.5 hours and in 10 hours, respectively. Starting from 5 hours of cavitation, the coating layer continued to show damage by higher than 160% over time when the temperature of seawater was elevated from <TEX>$15^{\circ}C$</TEX> to <TEX>$25^{\circ}C$</TEX>. Under cavitation environment, although WC-10Co4Cr has good wear resistance and durability, increase in temperature may accelerate the damage rate of the coating layer mainly due to cavitation damage.

Sliding and Abrasive Wear Behavior of WC-CoCr Coatings with Different Carbide Sizes

This study examines the sliding and abrasive wear behaviors of high-velocity oxy-fuel (HVOF)-sprayed WC-CoCr coatings with different WC grain sizes. The HVOF coating deposition was assisted by in-flight particle temperature and velocity measurement system. The powder feedstocks and their corresponding coatings were characterized by means of XRD and Field Emission Scanning Electron Microscope analysis. Hardness, porosity, and indentation fracture toughness of these coatings were calculated and compared with each other. Sliding wear resistance of these coatings was calculated using pin-on-disk tribometer (ASTM G99-90). The two-body abrasion was quantified by sliding the samples over silicon carbide (SiC) abrasive paper bonded to a rotating flat disk of auto-polisher. The mechanism of materials' removal in both the sliding and abrasive wears was studied and discussed on microstructural investigations. It was observed that fine grain WC-CoCr cermet coating exhibits higher sliding and abrasive wear resistances as compared with conventional cermet coating.

Comparison of Structure and Wear Properties of Fine-Structured WC-CoCr Coatings Deposited by HVOF and HVAF Spraying Processes

The aim of the work was to study the microstructure and wear properties of fine-structured HVOF and HVAF sprayed WC-10Co-4Cr coatings prepared from powder having submicron-sized tungsten carbides. The coatings were deposited by HVOF (High Velocity Oxygen Fuel) and HVAF (High Velocity Air Fuel) using propane as a fuel gas in both processes, and using oxygen or air as oxidizing gas for combustion, respectively. Nitrogen was used as carrier gas for the powder. Commercially available agglomerated and sintered cermet powder with main carbide sizes under 500 nm was used in this study. Scanning electron microscopy (SEM) and X-ray diffraction were performed in order to characterize the powder and the microstructures formed during the spraying processes. The microhardness HV0.3 of the coatings was investigated and the pin on disk test was used to determine the sliding wear behaviour. The rubber wheel abrasion test was performed in order to determine the abrasion wear resistance of the coatings.

Wear Resistance of Internal WC-CoCr Coatings Produced by High Velocity Oxy-Fuel Spraying

Thermal spray coatings, and in particular those applied by HVOF process, are being used in a diverse range of engineering applications to extend component life by retarding wear and corrosion degradation [1]. Coatings performance is strongly dependent on microstructure [2]. The mechanical properties are a complex function of carbide size, shape and distribution, matrix hardness and toughness and the presence of various reaction products. This paper is focused on the wear resistance of WC-based coatings containing 10 mass% Co and 4 mass% Cr as the metallic binder, involving a heat-treated steel rings as substrate, a hot rolled product of structural steel in accordance with DIN EN 10113-2 for steel construction, detaining a tensile strength about 520-680 N/mm2. A feedstock powder of WC-CoCr 86 10 4 agglomerated/sintered was used for coatings production. It has good properties, such as: high resistance against oxidation and corrosion (due to the metallic component), good wear resistance (due to the ceramic component) and with a hardness in range of 1000-1300HV. The coatings have been produced by HVOF spraying of four different WC-CoCr grain size of powder particles, such as:sample 1 (-45+22µm), sample2 (-22+5µm), sample 3 (-10+2µm) and sample 4 (-2+0.5µm), using an ID CoolFlow mono gun developed especially for internal surfaces. This type of gun is a HP HVOF system for high pressure, but cold internal HVOF spraying. The morphology and the structure of the sprayed coatings were investigated by means of scanning electron microscopy (SEM/EDS) and the qualitative and quantitative phase composition of those coatings were determined applying the x-ray diffraction technique (XRD). In order to determine the sliding wear resistance of the coated samples, it was used the pin on ring test method under dry condition, with and without compressed air. This method consists into a stationary ball loaded against a ring sample in relative motion. The sliding wear rate of the tested samples was calculated from the volume of material lost during the test. The investigated samples were examined by means of stereo and light microscopy before and after exposure to wear tests. Based on the obtained results it was demonstrated that the coatings obtained from -10+2µm powder particles exhibit an optimal wear behavior when exposed to severe conditions.

Influence of mechanical properties of tungsten carbide–cobalt thermal spray coatings on their solid particle erosion behaviour

The present investigation has been carried out in order to study the erosion wear behaviour of WC–Co base thermal spray coatings. WC–12Co and WC–10Co–4Cr coatings were deposited by means of high velocity oxygen fuel (HVOF) thermal spraying. The erosion tests were conducted at impact angles of 30 and 90° using SiC particles of ∼50 μm in diameter as erodent, at a velocity of 83·4 m s−1. It has been found that the erosion rate for both coated systems was higher when the test was carried out at an angle of 90°. The through-thickness residual stresses of the coatings, as well as the microstructural characterisation, allowed an explanation of the results and the erosion mechanisms in each case. It has been found that, under the experimental conditions carried out in the present study, the WC–10Co–4Cr coating exhibited a higher erosive wear resistance as compared to the WC–12Co coating.

An investigation on erosion behavior of HVOF sprayed WC–CoCr coatings

Present work is an investigation of slurry erosion behavior of WC–CoCr cermet coatings deposited with two different WC grain sizes. HVOF thermal spray process was employed due to its high velocity and low flame temperature characteristics resulting in quality coating. HVOF spraying was assisted with in-flight particle temperature and velocity measurement system to control its heating. Slurry erosion testing was performed using a pot-type slurry erosion tester to evaluate slurry erosion resistance of the coatings. Two parameters were considered for testing viz. erodent particle size and slurry concentration. Surface morphology was examined using SEM images and phase identification was done by XRD. The erosion behavior and mechanism of material removal was studied and discussed based on microstructural examination. It was observed that WC–CoCr cermet coating deposited with fine grain WC exhibits higher slurry erosion resistance under all testing conditions as compared to conventional cermet coating.

Corrosion behavior in artificial seawater of thermal-sprayed WC-CoCr coatings on mild steel by electrochemical impedance spectroscopy

The corrosion behavior in artificial seawater of different as-sprayed ceramic-metallic (cermet) coatings applied on low-alloy steel was studied. Five conditions, associated to modifications of the composition of the powder or deposition parameters were evaluated. The degradation mechanisms were studied during extended immersion tests using conventional electrochemical measurement and electrochemical impedance spectroscopy. The extended immersion tests reveal that these as-thermal-sprayed coatings present a cathodic behavior compared with steel. During the first hours of immersion, the electrolyte infiltrates the defects of the coatings, which then result to the local degradation of the substrate accelerated by the galvanic coupling with the cermet coating. Optical observations and Raman analyses reveal the formation of calcium carbonates like aragonite on the cermet surface, very close to the appearance of local anodic sites. The cross-sectioned views reveal the infiltration of the corrosive solution, and the depth penetration of the degradation of steel substrate probably due to the acidification of the anodic sites.

Effect of oxygen/fuel ratio on the in-flight particle parameters and properties of HVOF WC-CoCr coatings

High Velocity Oxy-Fuel (HVOF) spray techniques can produce high performance alloy and cermet coatings for applications that require wear resistant surfaces. In HVOF process, the particle velocity and temperature determine the resultant coating properties and in many cases enables a better understanding of the process.The aim of this study is to investigate influences of different oxygen/fuel ratios on velocity and temperature of flying particles as well as properties of the HVOF thermal sprayed WC-CoCr coatings. Particle parameters were recorded just prior to impact on the substrate using in-flight particle diagnostic tool Accuraspray-g3®. Detailed correlation of particle parameters and the coating properties are evaluated in order to deduce particle parameter ranges providing coatings with optimum properties.

Sealing HVOF Thermally Sprayed WC-CoCr Coatings by Sol-Gel Methods

The offshore industry faces challenges with corrosion resistance in hydraulic cylinders used in marine environments. Thermal spray coating appears to be a promising technique for replacing electroplated coating in many applications, but the performance of thermal spray coatings in highly corrosive environments must be improved. In the present work, a new sealing method for HVOF WC-CoCr coatings for their application on hydraulic cylinders for marine environments has been tested. The method consists of applying sol-gel solution that can penetrate, fill, and thus seal the pores and cracks in the coating. The sealed coatings have been tested with and without posttreatment and compared with as-sprayed coatings. Open-circuit potential (OCP) and tribocorrosion tests in 3.4% NaCl were performed to evaluate the performance of the sealing method. Both tests showed that the sealed coatings had the best performance.

Investigation of wear behaviour of HVOF sprayed WC-Co coatings for automotive parts in different working conditions

Reinforced metal matrix composite (MMC) hard coatings which are produced by thermal spray processes are used for various automotive parts such as piston rings and cylinder liners. Generally, WC-CoCr coatings are deposited by HVOF process for improving surface resistance of low alloyed steels. In this study, we examined the wear characteristics of WC-CoCr coatings on low alloyed steel substrate under different test conditions, using a sliding wear test unit with abrasive ball. Experimental studies discussed the effects of test load, temperature, work medium and sliding distance on the wear characteristics of coatings. Weight loss, wear track depth and width were observed. It was found that the weight loss of coatings increased in dry medium conditions. Wear weight loss was significantly reduced in lubricated condition due to decreasing friction. The wear track surface profile changed with lubrication and increasing temperature in test conditions. Under the base oil lubricated conditions the WC-CoCr coating performed well.

Microstructure refinement and alloying of WC–CoCr coatings by electron beam treatment

The corrosion and wear behaviour of HVOF (high velocity oxygen fuel) sprayed WC–CoCr (cermet) coatings were investigated before and after electron beam (EB) remelting. In this regard, the coatings were deposited on INCONEL 617 substrate. The mentioned Ni-based alloy is well known for its good corrosion behaviour in chloride containing media but exhibits not enough good wear properties.The paper investigates the influence of EB-remelting process on sliding wear respectively on corrosion resistance in 1M NaCl solution of the alloyed surface. Scanning electron microscopy (SEM) and X-Ray Diffraction (XRD) were performed before respectively after the EB-treatment in order to investigate the coating morphology as well as the phase modification achieved through the alloying process. Tribological tests concerning the sliding wear behaviour of the tested materials revealed a significant decrease of the wear rate for both the as-sprayed coating respectively the alloyed surface in comparison with the base material. However, the as-sprayed cermet coating exhibits the lowest wear rate among the investigated samples.The microhardness of the alloyed surface was higher (1100HV03) in comparison with that of the as-sprayed cermet coating (905HV03) as a result of new phase formation (especially the η-Co4W2C). The corrosion behaviour in salt water of the EB remelted surface was also considerably improved in contrast to the as-sprayed cermet coating.

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.

Behavior of HVOF WC-10Co4Cr Coatings with Different Carbide Size in Fine and Coarse Particle Abrasion

A modified ASTM G 65 rubber wheel test was employed in wet and dry conditions using 220 nm titania particles and 368 μm sand particles, respectively. Both tests were conducted on WC-CoCr coatings produced with two powders with different carbide grain sizes (conventional and sub-micron) to address the effect of carbide size and abrasive medium characteristics on the wear performance. The same spot before and after the wet abrasion wear testing was analyzed in detail using SEM to visualize wear mechanisms. It was shown that the wear mechanism depends on the relative size of the carbide and abrasive particles. Wear mechanisms in dry sand abrasion were studied by analyzing the single scratches formed by individual abrasive particles. Interaction of surface open porosity with moving abrasive particles causes formation of single scratches. By tailoring the carbide size, the wear performance can be improved.

Wear and Corrosion Behavior of HVOF-Sprayed WC-CoCr Coatings on Al Alloys

WC-CoCr coatings were HVOF-sprayed onto an AA6082T6 substrate. Thickness values between 50 and 150 μm were produced by stepwise increase of the number of torch scans. This increase made the coatings not only thicker but also denser. This was due both to peening effects and by modifications to the splat formation mechanism, investigated by focused ion beam technique. Thanks to such densification, the hardness, the wear and impact resistance, and the corrosion protectiveness of the layers increased with the number of torch scans. The largest improvement occurred from 2 to 3 torch scans. These coatings were also compared to anodized films: cermets had superior wear and impact resistance but offered less corrosion protection.

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.