Sprayed WC-Co Coatings Research Articles

Role of carbide grit size and in-flight particle parameters on the microstructure, phases, and nanoscale properties of HVOF sprayed WC-12Co coatings

Effects of particle temperature and velocity on crystallinity and phase composition of WC-Co coating are successfully decoupled. The effect of grit size on the dissolution and decarburization process is established analytically. The nanostructured coatings were found to be less crystalline than their conventional counterpart for similar in-flight parameters. The cooling rate and the degree of carbide dissolution in the binder phase govern the crystallinity of the as-sprayed coatings. An increase in either of these factors results in a higher degree of amorphization of as-sprayed coatings. The phases present in the coating were quantified, and correlated with particle temperature and velocity independently. The degree of decarburization increased with an increase in particle temperature. The calculated change in Gibbs free energy justified these observations. The effect of particle velocity on crystallinity or phase composition was not significant. The nano hardness of the carbide grits was increased with particle temperature owing to the formation of a hard W2C phase. Similarly, the dissolution of carbides in the binder and the formation of the Co3W9C4 phase increased the binder hardness. The energy analysis revealed brittleness of the binder matrix increased significantly with deposition temperature owing to the formation of the Co3W9C4 phase.

A Comparative Study of the Life Cycle Inventory of Thermally Sprayed WC-12Co Coatings

In this research, a life cycle inventory (LCI) is developed for tungsten carbide–cobalt (WC-Co) coatings deposited via atmospheric plasma spray (APS), high-velocity oxy-fuel (HVOF), and cold gas spray (CGS) techniques. For the APS process, a mixture of Ar/H2 was used, while the HVOF process was fueled by H2. The carrier gas for CGS was N2. This study aims to determine and quantify the inputs (consumption of inputs and materials) and outputs (emissions to air, soil, water, and waste generation) that could be used in the life cycle analysis (LCA) of these processes. The dataset produced will allow users to estimate the environmental impacts of these processes using WC-Co feedstock powder. To obtain a complete and detailed LCI, measurements of electrical energy, gas, WC-CO powder, and alumina powder consumption were performed (the use of alumina was for sandblasting). Furthermore, emissions like carbon dioxide (CO2), carbon monoxide (CO), and noise were also measured. This practice allowed us to determine the input/output process quantities. For the first time, it was possible to obtain LCI data for the APS, HVOF, and CGS deposition processes using WC-12Co as a feedstock powder, allowing access to the LCI data to a broader audience. Comparisons were made between APS, HVOF, and CGS processes in terms of consumption and emissions. It was determined that the APS process consumes more electrical energy and that its deposition efficiency is higher than the other processes, while the HVOF process consumes a large amount of H2, which makes the process costlier. CGS has comparatively low electricity consumption, high N2 consumption, and low deposition efficiency. The APS, HVOF, and CGS processes analyzed in this study do not emit CO, and CO2 emissions are negligible.

Effect of inert environment on the sliding wear behaviour of the HVOF sprayed WC-12Co coating

The reciprocating sliding wear behaviour of high velocity oxy-fuel sprayed WC-12Co coatings in the inert environment at room temperature, and 300 °C temperature with varying load conditions was investigated. The worn-out coatings and the debris were analyzed by scanning electron microscopy and Raman spectroscopy. From the wear test data, the coefficient of friction was measured as lower in the air environment at both room temperature and 300 °C, compared with the argon environment due to the development of self-lubricating tribo-oxide layers. The development of CoWO4 phases at the surface decreases friction and wear and allows the coating to uphold the tribological properties at higher temperatures. Higher loads mechanically damage the coatings and quickly remove the protective oxide layer compared with the lower loads. In the protective environment, abrasive wear and adhesive wear mechanisms were observed to be played predominant role, whereas, in the air environment, the oxidative wear mechanism was found to be significant.

Effects of 3D Roughness Parameters of Sandblasted Surface on Bond Strength of HVOF Sprayed WC-12Co Coatings

This work aims to reveal the effects of 3D roughness parameters of sandblasted surfaces on bond strength between thermal spray coatings and substrates. The investigation was carried out on the surface of AISI 4140, which were pretreated with automatic-sandblasting system. 3D topography and roughness parameters were analyzed by a 3D optical profiler. The bond strength of WC-12Co coatings was measured using a pull-off test method. Scanning electron microscope revealed that the morphology of the surface after sandblasting was rough. Furthermore, the surface topography was characterized by several irregular peaks and pits with different directions and no fixed orientation randomly distributed on sandblasted surface. The average values for surface roughness Sa = 4.84 ± 0.34 μm and bond strength = 32.8 ± 2.8 MPa were obtained. In terms of 3D roughness parameters, Sa, Sdr, Sdq and Sq were found to have more significant impact on affecting the bond strength, showing a nonlinear regression relationship. Furthermore, bond strength was positively correlated with Sa, Sdr and Sdq, while inversely proportional to Sq. This confirmed that a greater surface roughness of a sandblasted surface was not more conducive to the improvement of bond strength. The influence mechanism of each parameter was discussed, which was consistent with the regression mathematical model.

Mechanical performance and residual stress of WC-Co coatings manufactured by Kinetic Metallization™

Electrolytic hard chrome (EHC) plating is a source of environmental pollution due to evolution of hexavalent chromium, which is a known carcinogen. Coating technology known as Kinetic Metallization™ (KM™) is an alternative to EHC for depositing wear resistant WC-Co coatings. Kinetic Metallization™ employs high kinetic energy of impacted feedstock particles and achieves a dense coating with good intersplat bonding. However, the high kinetic energy imparts peening and thermal stresses that are different from other thermal spray processes. This work characterizes the residual stress profile and room temperature wear behaviour of WC-Co coatings deposited under several input parameters. A combination of SEM imaging and etched optical micrographs revealed significant plastic deformation and particle penetration at the interface due to high energy impact of feedstock particles. No signs of substrate grain refinement were detected. The coating was composed primarily of WC and Co with minor compositional variations, as calculated by quantitative Rietveld analysis, with respect to the input parameters. Coating residual stresses were compressive in nature due to domination of thermal contraction stresses over peening stresses. The wear resistance assessed by the pin-on-disk method revealed a wear performance equivalent to conventionally sprayed WC-Co coatings.

Adhesion of HVOF-Sprayed WC-Co Coatings on 316L Substrates Processed by SLM

Different studies have been demonstrated that the surface integrity of substrate bulk materials to be coated has a significant impact on the adhesion of thermally sprayed coatings. It is known that the surface integrity of parts processed by selective laser melting (SLM) differs from those obtained from bulk materials. Although 316L stainless steel is among the most investigated material for SLM, the adhesion of thermally sprayed coatings on 316L stainless steel substrates processed by SLM has not been studied yet. This study aims at evaluating the effect of various mechanical pre-treatments onto 316L stainless steel substrates processed by SLM and their effect on the adhesion of high velocity oxy-fuel (HVOF)-sprayed WC-Co coatings. To differentiate between topographical effects and residual stress-related phenomena, a stress-relief heat treatment of the SLM substrates served as a reference throughout the investigations. The differently pre-treated SLM substrates were investigated with regard to the surface roughness and residual stresses. For the HVOF-sprayed SLM composites, Vickers interfacial indentation tests were conducted to assess the resulting coating adhesion. The findings demonstrated that the HVOF-sprayed WC-Co coatings predominantly exhibit good adhesion to the SLM 316L substrates. However, it was found that the stress state in the SLM 316L substrate surface is more likely to affect the adhesion of the WC-Co coating, while the substrate surface roughness showed a marginal effect.

Tribological investigation of surface structures processed by high-feed milling on HVOF sprayed WC-12Co coatings

Since properties of high-velocity oxy-fuel (HVOF) sprayed WC-12Co coatings are limited due to the inherent restrictions of the material, new approaches that involve tailored surfaces and designing friction properties of functional materials have attracted enormous interest. Within this scope, two different deterministic patterns are machined on as-sprayed and polished WC-12Co coatings by means of a high-feed milling (HFM) process, whereby different process parameters with respect to the feed per tooth as well as the lead angle are taken into account. The produced high-feed milled surfaces are investigated with respect to the resulting topography and tribological properties under dry sliding conditions.The tribological examination of the high-feed milled surfaces reveals a decreased coefficient of friction (COF) compared to the polished WC-12Co coating sliding against a 100Cr6 counter-body. The lower friction is attributed to the reduction of the contact area as well as the capability of the cavities to store the transferred material resulting from wear debris. With respect to the different surface patterns, higher values for the kurtosis Sku lead to a reduction of friction. In contrast to the results presented for 100Cr6, the WC-12Co surfaces sliding against Al2O3 counter-bodies exhibit an almost constant COF for all surfaces. Although the reduced contact area leads to increased tribological stresses, the processed surfaces show a high resistance to sliding wear. Finally, it is clarified that time- and cost-consuming mechanical surface treatments such as polishing or grinding prior to a subsequent HFM process do not have to be applied. Therefore, HFM enables a new manufacturing route to structure WC-12Co coatings as well as to modify the tribological properties.

Sliding wear resistance enhancement by controlling W2C precipitation in HVOF sprayed WC-based cermet coating

A unique method based on reactions between WC and Co-W-C phases was developed for controllable precipitation of W2C/W in thermal sprayed WC-Co coatings. With the increase in amount of the ultrafine precipitates, it is surprising to find that the wear rate of the prepared WC-based coatings drastically decreased by nearly 6 times before an obvious rise, and the wear mechanism gradually changes from abrasive wear in a ploughing mode to brittle fracture at surface microcracks because of the reduction in plasticity and fracture toughness. FEM modeling results indicate that inhomogeneous W2C distribution is more likely to cause localized stress concentration of coating, especially at the Co binder around which there are few W2C particles. Instead, an appropriate amount of evenly dispersed W2C can act as transitional sites to reduce the shear stress gradient between WC and Co binder, thus decreasing the plastic deformation, fatigue fracture and ploughing wear of the cermet coating.

Study of alumina sealing of HVOF thermally sprayed WC-Co coatings by sol-gel method

Tungsten carbide (WC) has been known as a hard and wear resistant material. Thermal spray coatings were used widely to deposit Ni and/or Co based WC. The high-velocity oxy-fuel (HVOF) spraying is one of the best methods among thermal spray processes that creates denser coatings with less pores for high corrosive environments applications. In this study, the effect of alumina sealing process on the HVOF WC-12Co coatings by the Sol-Gel method for corrosion and wear properties at high temperature applications was investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were utilized to characterize the morphology and properties of the coatings before and after the sealing process. High temperature wear properties of coatings before and after sealing were tested by ball-on-disk wear tester at 400 °C. The cross- section of the sealed coatings showed that open pores were filled after sol gel. The thickness of the sealed layer was about 14 μm. Results indicated that the sealing process increased the microhardness of coating slightly from 1160 ± 160 to 1250 ± 150 HV. Comparing the results of polarization in 3.5 wt% NaCl solution and wear test showed that the corrosion current density decreased and the wear resistance increased after sealing. A sealed sample with filled open porosity led to recording a higher corrosion resistance (2.9 ± 0.7 μA cm−2) compared to the as sprayed coating (8.90 ± 0.5 μA cm−2). The friction profile of the sealed coating showed reduction of the friction coefficient between 0.25 and 0.5 in comparison to the as- sprayed coating at room temperature (04–0.6). Also, the high temperature wear behavior of the sealed coatings was improved. The weight loss was much lower for the sealed coating (5 mg) than that for the as-sprayed coating (13 mg).

Microstructural characteristics of high-feed milled HVOF sprayed WC-Co coatings

Over the last decade, great efforts have been undertaken in science and industry to provide WC-Co feedstock with nano-sized WC particles that significantly improves tribo-mechanical coating properties. For tribologically stressed surfaces, superior surface characteristics can be achieved by applying tailored surfaces, using novel technologies in the field of production engineering such as High-Feed Milling (HFM).For the first time, textured surface patterns were produced onto HVOF sprayed WC-Co coatings with nano-sized WC particles by implementing a HFM post process. In dependence to two different textures resulting from the HFM, the microstructural characteristics of the produced surfaces are analyzed. Confocal microscopy revealed the machinability of textured patterns onto a HVOF-sprayed WC-12Co hard coating, which comprise of nano-sized WC particles, by means of HFM technology. X-ray diffraction analyses confirmed the insertion of macro- and micro-scale residual stresses. The experiments showed a significant insertion of compressive residual stresses transverse to the feed direction, whereas the insertion of compresses residual stresses with its feed direction being less pronounced. It was found that the HFM post process leads to a refinement of the WC crystallite size and a distinct increase of its internal strain, which both can be attributed to plastic deformations during HFM.

Influence of etching-pretreatment on nano-grained WC-Co surfaces and properties of PVD/HVOF duplex coatings

The deposition of coatings by means of Physical Vapor Deposition (PVD) is an established process to enhance the lifetime and performance of carbide bulk tools. Although the effect of surface pretreatments on conventional WC-Co surfaces is well known, this investigation examines for the first time, how different surface pretreatments affect the surface integrity of thermally sprayed WC-Co substrates prior a subsequent PVD layer deposition and its resulting properties. Therefore, a WC-12Co feedstock with an average WC particle size of 100 nm was thermally sprayed on AISI M3 steel substrates using High Velocity Oxy-Fuel (HVOF) technique. Hereinafter, the HVOF sprayed WC-Co coatings were grounded and polished, thus serving as substrates for further surface pretreatments and the deposition of a CrAlN PVD hard coating by means of magnetron sputtering. To evaluate the influence of various surface pretreatments on the HVOF sprayed WC-Co coatings, several sequences such as heating, inert gas ion etching, metal ion etching, and High Power Impulse Magnetron Sputtering (HIPIMS)-etching were carried out. With respect to the subsequent PVD layer deposition, the results show that the pretreatment does neither affect the hardness nor Young's modulus of the CrAlN top layer. Yet, different effects on the WC-Co surface and PVD coating adhesion are observed. Inert gas ion etching leads to a faster removal of the carbides than of the Co-binder matrix. In contrast, metal ion etching provides a “micro-blasting” effect and removes the binder matrix as verified by Atomic Force Microscope (AFM) measurements. As a result, a decrease of the compressive residual stress state and an increase of the surface free energy are observed. With respect to HIPIMS-etching, a Cr-nanolayer was applied onto the WC-Co surface, which enhances the adhesion of the CrAlN top layer. Nevertheless, HRC Rockwell adhesion and scratch tests reveal a superior adhesion for samples pretreated with the metal ion etching.

The effect of spraying temperature on the corrosion and wear behavior of HVOF thermal sprayed WC-Co coatings

Cobalt matrix carbides are favorite cases of wear and corrosion-resistant coatings. Among thermal spray processes the high-velocity oxy-fuel (HVOF) spraying is a popular choice because of its unique properties. In this paper the effect of particles’ temperature on the porosity, corrosion as well as wear behavior of high-velocity oxy-fuel (HVOF) thermally sprayed WC-12Co coating has been studied. The temperature of particles was measured using a Spray-Watch diagnostic system. Also the scanning electron microscopy (SEM), image analysis, and X-ray diffraction (XRD) were used for examining the powder and the coating. The corrosion potential and corrosion current density was evaluated using potentio-dynamic polarization test. Results show that the porosity is affected by thermal flow on the coating surface. Besides, a higher particle temperature resulted in achieving more amorphous phases and eventually the higher corrosion resistance. Sample A was coated with an impact temperature of 2012.4 °C. Although the porosity was higher than other sprayed samples (1.7%), the higher content of amorphous phase led to recording a higher corrosion resistance (64.3E-6A). Sample B was coated with an impact temperature of 1880 °C for which the Jcorr was 67.2E-6 A. Sample C was sprayed with a low impact temperature of 1702 °C for which a surface porosity of 1% and the low corrosion resistance of 79.5E-6A were recorded. Based on the experiments it was concluded that for the WC-12Co coating the corrosion resistant phases have the dominant influences on the corrosion resistance.

Tribological behaviour of low-pressure plasma sprayed WC-Co coatings at elevated temperatures

PurposeThis paper aims to investigate the effect of coating microstructure, mechanical and oxidation property on the tribological behaviour of low-pressure plasma spraying (LPPS) tungsten carbide/cobalt (WC-Co) coatings.Design/methodology/approachWC-12Co and WC-17Co coatings were deposited via the LPPS spraying method. Tribological tests on the coatings were performed using a high-temperature ball-on-disc tribometer at temperatures from room temperature (RT, approximately 25 °C) up to 800 °C in ambient air.FindingsWC-12Co coating contained brittle phases, pores and microcracks, which led to the low hardness, and finally promoted the splat delamination and the carbide debonding during wear. WC-17Co coating had higher cobalt content which benefited the coating to contain more WC particles, less brittle phases, pores and nearly no microcracks, and resulted in the high hardness and better wear resistance. Higher cobalt content also decelerated the oxidation rate of the coating and promoted the formation of cobalt oxides and CoWO4, which were able to maintain the load-bearing capacity and improve the tribological behaviour of the coating below 650°C. Above 650°C, the increase of oxidation degree and the decrease of mechanical property deteriorated the wear resistance of coatings.Originality/valueThe LPPS WC-Co coating with higher cobalt content had better tribological properties at different temperatures. The LPPS WC-Co coatings should not be used as wear-resistant coatings above 650 °C.

Effect of the Spray Distance on the Properties of High Velocity Oxygen-Fuel (HVOF) Sprayed WC-12Co Coatings.

In this study influence of spray distance on the properties of WC-12Co coatings deposited by HVOF was investigated. WC-12Co coating was sprayed at spray distance of 300, 385 and 450 mm. From microstructure observation, it is confirmed that the porosity of coatings increases with increasing the spray distance. The X-ray diffraction patterns indicate that the coatings consist of pure WC, W, and Co as well as W2C and Co6W6C phases. The increase of the spray distance accelerated the decarburization of coatings. From micro hardness tests, it was found that the hardness and the fracture toughness decreased with increasing spray distance. These mechanical properties would be related with not only porosity but also the degree of decarburization.

Wear behaviors of HVOF sprayed WC-12Co coatings by laser remelting under lubricated condition

A HVOF (high velocity oxygen fuel) sprayed WC-12Co coating was remelted with a CO2 laser. The surface-interface morphologies and phases were analyzed by means of SEM (scanning electron microscopy), and XRD (X-ray diffraction), respectively. The friction and wear behaviors of WC-12Co coating under the dry and lubricated conditions were investigated with a wear test. The morphologies and distributions of chemical elements on worn scar were analyzed with a SEM, and its configured EDS (energy diffusive spectrometer), respectively, and the effects of lubricated condition on COFs (coefficient of friction) and wear performance were also discussed. The results show that the adhesion between the coating and the substrate is stronger after laser remetling (LR), in which mechanical bonding, accompanying with metallurgical bonding, was found. At the load of 80N, the average COF under the dry and lubricated friction conditions is 0.069, and 0.052, respectively, the latter lowers by 23.3% than the former, and the wear rate under the lubricated condition decreases by 302.3% than that under the dry condition. The wear mechanism under the dry and lubrication conditions is primarily composed of abrasive wear, cracking, and fatigue failure.

Sliding Wear Behavior of High Velocity Oxy-Fuel Sprayed WC-CO Coatings

The Thermal spraying techniques are coating processes in which melted materials are sprayed onto a surface and the feedstock is heated by electrical or chemical means. High velocity oxy-fuel (HVOF) spraying is seen as the pre-eminent process for deposition of such coatings. The coating that is formed is not homogenous and typically contains a certain degree of porosity, and, in the case of sprayed metals, the coating will contain oxides of the metal. This paper examines the effect of WC–Co coatings on the wear behaviour, WC were thermally sprayed by a high-velocity oxy-fuel (HVOF) system from WC–12% Co powder. Sliding wear resistance of this coating was calculated using pin-on-disk tribometer (ASTM G99-90). The morphologies of WC-Co coating were observed by X-ray diffraction (XRD), Energy dispersive analysis and Scanning electron microscope (SEM). Micro hardness is calculated by using Vickers Hardness test. And it was found that WC-Co coatings show better wear resistant properties.

A Comparison between Hardness, Corrosion and Wear Performance of APS Sprayed WC-CoMo and WC-Co Coatings

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.

Laser surface remelting of WC–12Co coating: finite element simulations and experimental analyses

A three-dimensional finite element model taking into account interfacial topography, porosity of plasma sprayed coatings, temperature dependent thermophysical parameters and phase change has been developed to simulate multipass laser remelting process. Temperature evolution, temperature gradient, melting pool shape and dimensions are simulated. The laser remelting experiments are carried out on the plasma sprayed WC–12Co coatings. The simulated results are in good agreement with the experimental measurements. After laser remelting, a denser and more homogenous coating is obtained. The microhardness of the coating is significantly enhanced owing to the dispersion strengthening, the fine grain strengthening and the solution strengthening, which is increased by three times compared with that of the plasma sprayed coatings.

The Mechanical Properties and Wear Resistance of HVOF Sprayed WC-Co Coatings

In this work, the Woka 5810 powders (88% tungsten carbide–12% cobalt) were used to produce coating by high velocity oxy-fuel spraying. WC–Co is widely used as a tribological coating material providing a combination of high toughness, high hardness, and good strength. The treated samples were characterized by using optical micrograph, stereo microscope and scanning electron microscopy, X-ray diffractometry, and microhardness tests. Also the wear performance of the coatings was investigated. The results indicated that the coating shows slight higher microhardness and better abrasive wear resistance than the conventional counterpart. The friction coefficient of coating was low. The scanning electron microscopy and energy dispersive spectroscopy analyses were applied to worn surfaces.

Improving the wear resistance of HVOF sprayed WC-Co coatings by adding submicron-sized WC particles at the splats' interfaces

Abstract In this paper, the submicron-sized WC particles (~ 300 nm) with the content of 3 wt.% and 5 wt.% are incorporated into high velocity oxy-fuel (HVOF) sprayed WC-Co coatings with the aim of improving properties of the coatings. XRD analyses suggest a small amount of decarburization of the incorporated WC phase after the composite coating deposition. The SEM microstructure showed even distribution of WC particles at the interfaces of WC-Co splats, indicating significantly enhanced wear resistance of the coatings with the wear rate as much as ~ 10− 7 mm3/N·m. The content of submicron-sized WC particles plays an important role in determining the wear performances of the coatings. The increment of submicron-sized WC particles causes a decrease in wear rate from 6.09 × 10− 7 mm3/N·m to 5.15 × 10− 7 mm3/N·m. Also, the Vickers microhardness of the coatings enhances as the increasing of WC particle ratio (reaches 1365 HV with the content of the WC particles of 5 wt.%). The wear failure analysis gives further insight into the mechanism of the property enhancement. The change of stress state and crack initiation at splats' interfaces act as the predominant mechanism, which is caused by the presence of submicron-sized WC particles at splats' interfaces.