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.     

A comparative study of tribological behavior of plasma and D-gun sprayed coatings under different wear modes

In recent years, thermal sprayed protective coatings have gained widespread acceptance for a variety of industrial applications. A vast majority of these applications involve the use of thermal sprayed coatings to combat wear. While plasma spraying is the most versatile variant of all the thermal spray processes, the detonation gun (D-gun) coatings have been a novelty until recently because of their proprietary nature. The present study is aimed at comparing the tribological behavior of coatings deposited using the two above techniques by focusing on some popular coating materials that are widely adopted for wear resistant applications, namely, WC-12% Co, A1₂O₃, and Cr₃C₂-MCr. To enable a comprehensive comparison of the above indicated thermal spray techniques as well as coating materials, the deposited coatings were extensively characterized employing microstructural evaluation, microhardness measurements, and XRD analysis for phase constitution. The behavior of these coatings under different wear modes was also evaluated by determining their tribological performance when subjected to solid particle erosion tests, rubber wheel sand abrasion tests, and pin-on-disk sliding wear tests. The results from the above tests are discussed here. It is evident that the D-gun sprayed coatings consistently exhibit denser microstructures and higher hardness values than their plasma sprayed counterparts. The D-gun coatings are also found to unfailingly exhibit superior tribological performance superior to the corresponding plasma sprayed coatings in all wear tests. Among all the coating materials studied, D-gun sprayed WC-12%Co, in general, yields the best performance under different modes of wear, whereas plasma sprayed Al₂O₃ shows least wear resistance to every wear mode.     

Study of plasma- and detonation gun-sprayed alumina coatings using taguchi experimental design

Atmospheric plasma spraying (APS) is a most versatile thermal spray method for depositing alumina (Al₂O₃) coatings, and detonation gun (D-gun) spraying is an alternative thermal spray technology for depositing such coatings with extremely good wear characteristics. The present study is aimed at comparing the characteristics of Al₂O₃ coatings deposited using the above techniques by using Taguchi experimental design. Alumina coating experiments were conducted using a Taguchi fractional-factorial (L₈) design parametric study to optimize the spray process parameters for both APS and D-gun. The Taguchi design evaluated the effect of four APS and D-gun spray variables on the measured coating attributes. The coating qualities evaluated were surface roughness, porosity, microhardness, abrasion, and sliding wear. The results show that the coating quality is directly related to the corresponding coating microstructure, which is significantly influenced by the spray parameters employed. Though it is evident that the D-gun-sprayed coatings consistently exhibit dense and uniform microstructure, higher hardness, and superior tribological performance, the attainment of suitable plasma-sprayed coatings can be improved by employing the Taguchi analysis.     

Selection criteria for wear resistant powder coatings under extreme erosive wear conditions

Wear-resistant thermal spray coatings for sliding wear are hard but brittle (such as carbide and oxide based coatings), which makes them useless under impact loading conditions and sensitive to fatigue. Under extreme conditions of erosive wear (impact loading, high hardness of abrasives, and high velocity of abradant particles), composite coatings ensure optimal properties of hardness and toughness. The article describes tungsten carbide-cobalt (WC-Co) systems and self-fluxing alloys, containing tungsten carbide based hardmetal particles [NiCrSiB-(WC-Co)] deposited by the detonation gun, continuous detonation spraying, and spray fusion processes. Different powder compositions and processes were studied, and the effect of the coating structure and wear parameters on the wear resistance of coatings are evaluated. The dependence of the wear resistance of sprayed and fused coatings on their hardness is discussed, and hardness criteria for coating selection are proposed. The so-called “double cemented” structure of WC-Co based hardmetal or metal matrix composite coatings, as compared with a simple cobalt matrix containing particles of WC, was found optimal. Structural criteria for coating selection are provided. To assist the end user in selecting an optimal deposition method and materials, coating selection diagrams of wear resistance versus hardness are given. This paper also discusses the cost-effectiveness of coatings in the application areas that are more sensitive to cost, and composite coatings based on recycled materials are offered.     

Effect of nanostructuring and Al alloying on friction and wear behaviour of thermal sprayed WC–Co coatings

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.     

Effect of Feedstock Size and its Distribution on the Properties of Detonation Sprayed Coatings

The detonation spraying is one of the most promising thermal spray variants for depositing wear and corrosion resistant coatings. The ceramic (Al₂O₃), metallic (Ni-20 wt%Cr) , and cermets (WC-12 wt%Co) powders that are commercially available were separated into coarser and finer size ranges with relatively narrow size distribution by employing centrifugal air classifier. The coatings were deposited using detonation spray technique. The effect of particle size and its distribution on the coating properties were examined. The surface roughness and porosity increased with increasing powder particle size for all the coatings consistently. The feedstock size was also found to influence the phase composition of Al₂O₃ and WC-Co coatings; however does not influence the phase composition of Ni-Cr coatings. The associated phase change and %porosity of the coatings imparted considerable variation in the coating hardness, fracture toughness, and wear properties. The fine and narrow size range WC-Co coating exhibited superior wear resistance. The coarse and narrow size distribution Al₂O₃ coating exhibited better performance under abrasion and sliding wear modes however under erosion wear mode the as-received Al₂O₃ coating exhibited better performance. In the case of metallic (Ni-Cr) coatings, the coatings deposited using coarser powder exhibited marginally lower-wear rate under abrasion and sliding wear modes. However, under erosion wear mode, the coating deposited using finer particle size exhibited considerably lower-wear rate.     

Sliding wear evaluation of hot isostatically pressed thermal spray ceramet coatings

The principal aim of this study was to compare the sliding wear performance of as-sprayed and Hot Isostatically Pressed (HIPed) thermal spray cermet (WC-12Co) coatings. Results indicate that HIPing technique can be successfully applied to post-treat thermal spray cermet coatings for improved sliding wear performance, not only in terms of coating wear, but also in terms of the total volume loss for test couples. WC-12Co coatings sprayed by a HVOF system were deposited on SUJ-2 bearing steel substrate and then encapsulated and HIPed at 850 °C for one hour. A high frequency reciprocating ball on plate rig was used to measure the sliding wear resistance of these coatings in dry conditions under steel and ceramic contact configurations at two different loads. Results are discussed in terms of coating microstructure, microhardness, fracture toughness and residual stress evaluations. Microstructural investigations indicate fundamental changes in grain morphology, whereas x-ray diffraction revealed beneficial transformations in phase composition of these coatings during the HIPing post treatment. The effects of these microstructural changes on the physical properties and wear resistance are discussed.     

NiCrAlYSi/h-BN高温可磨耗封严涂层摩擦磨损性能研究

目的加入h-BN和团聚聚苯酯分别作为固体润滑剂和造孔剂,以提高NiCrAlYSi基封严涂层的高温摩擦磨损性能。方法采用真空熔炼雾化造粒和料浆喷雾造粒技术制备NiCrAlYSi/h-BN聚苯酯复合粉体,再利用大气等离子喷涂技术制备高温可磨耗封严涂层,通过扫描电子显微镜(SEM)、能谱仪(EDS)、CSM摩擦磨损试验机和三维轮廓仪等手段,研究NiCrAlYSi/h-BN封严涂层显微结构、元素组成以及室温和800℃下的摩擦磨损性能,探究涂层在室温和800℃下的磨损机理。结果等离子喷涂NiCrAlYSi/h-BN封严涂层组织比较均匀,涂层结合强度可达15MPa,孔隙率约为32%。室温下封严涂层与DD6镍基单晶高温合金球间的平均摩擦因数为0.897,涂层磨损体积为2.09×10~8μm~3;800℃高温下平均摩擦因数为0.425,涂层磨损体积为3.22×10~8μm~3;封严涂层800℃下有良好的可磨耗性,相应对偶件的磨损较轻;高温下,封严涂层金属基相软化、h-BN的润滑作用和部分金属基相高温下生成自润滑性氧化物,是摩擦因数降低和对偶件磨损较轻的主要原因。NiCrAlYSi/h-BN封严涂层室温的主要磨损机理为涂抹、磨粒磨损和轻微氧化,高温下的主要磨损机理为切削、塑性变形、氧化和粘着磨损。结论等离子喷涂NiCrAlYSi/h-BN高温封严涂层在高温下的可磨耗性能较好。     

Enhanced tribological properties of PECVD DLC coated thermally sprayed coatings

This research investigates the enhancement of the tribological properties of various thermally-sprayed coatings (APS Ni-50Cr, APS Al₂O₃-13%TiO₂ and HVOF WC-17Co) on steel substrate, achieved through the deposition of a thin DLC-based film. Higher adhesive strength between thin films and thermally-sprayed coatings compared to the simple thin film/carbon steel system was found by scratch testing. Dry sliding ball-on-disk tests performed under lower contact pressure conditions (5 N normal load, 6 mm diameter alumina ball) indicated a significant decrease in wear rates and friction coefficients of thermally-sprayed coatings when the thin DLC-based film is employed; little differences exist between the tribological behaviour of the various thin film/thermal spray coating systems and that of DLC-based film on carbon steel. Under higher contact pressure conditions (10 N normal load, 3 mm diameter alumina ball), the thin film/WC-Co system exhibited the best wear performance. These results indicate the superior tribological performance of DLC/thermal spray coating systems, especially under severe contact conditions.     

超音速火焰喷涂WC-10Co4Cr涂层的耐滑动磨损行为

采用超音速火焰喷涂(HVOF)工艺制备微米结构WC-10Co4Cr涂层,分别采用金相显微镜、扫描电镜(SEM)、X射线衍射(XRD)和滑动磨损设备分析涂层的微观结构和滑动磨损行为。结果表明:采用液体煤油燃料HVOF喷涂的微米结构WC-10Co4Cr涂层的脱碳程度较低,涂层中仅出现WC和W2C相,而无η相(Co3W3C、Co6W6C)以及软相W。涂层微观结构致密,孔隙率约为1%,平均显微硬度为1 322HV0.3;在相同试验条件下,WC-10Co4Cr涂层的摩擦因数(约0.8)高于不锈钢(1Cr18Ni9Ti)的摩擦因数(约0.5),其滑动体积损失量仅为不锈钢涂层的1/146,具有优异的抗滑动磨损性能。涂层在滑动磨损过程中首先是粘结相的脱落,然后是WC颗粒的磨损。     

Propylene flow,microstructure and performance of WC–12Co coatings using a gas-fuel HVOF spray process

Five WC–12Co coatings were deposited by a high velocity oxy-fuel (HVOF) system using constant oxygen flow and varying propylene flow. The phase composition, microstructure, as well as abrasive and sliding wear performance of the as-sprayed coatings were investigated. The degree of tungsten carbide (WC) decarburization in the as-sprayed coatings increases while the coating porosity decreases with the increase of the propylene flow. The coating hardness, fracture toughness, resistance to abrasive and sliding wear increases with the increase of the propylene flow, reaches maximum and then decreases. At the low flow of the propylene, relatively loose coating microstructure is formed, which leads to fracturing and pulling off the WC particles during abrasive and sliding wear process. Herewith, at the high flow of the propylene, the high degree of the WC decarburization and high brittleness of the coating leads to micro-cutting during abrasive wear as well as to cracking and delamination of the coating in the sliding wear process.     

Sliding and abrasive wear resistance of thermal-sprayed WC-CO coatings

We studied the resistance of the coatings to abrasive and unlubricated sliding wear of 40 WC/Co coatings applied by high velocity oxygen fuel (HVOF), high-energy plasma spray (HEPS), and high velocity plasma spray (HVPS), using commercial and nanostructured experimental powders. The hardness of the coatings varies from 3 to 13 GPA, which is much lower than that of sintered samples (10 to 23 GPA) because of the porosity of the coatings. Phase analysis by x-ray diffraction revealed various amounts of decarburization in the coatings, some of which contain WC, W₂C, W, and η phase. The abrasive and sliding wear resistance is limited by the hardness of the samples. For a given hardness, the wear resistance is lowered by decarburization, which produces a hard but brittle phase. Nanocarb powders have the shape of thin-walled hollow spheres that heat up rapidly in the gun and are more prone to decarburization than commercial materials. The work shows that, in order to obtain the performance of nanostructured coatings, the powder and spray techniques must be modified.     

热喷涂技术替代电镀硬铬的研究进展

从热喷涂技术的发展,高速火焰喷涂(HVOF)涂层的性能,包括耐磨性、耐蚀性、对基体耐疲劳性的影响,热喷涂涂层替代电镀硬铬镀层的成本以及应用4个方面综述了近年来热喷涂技术替代电镀硬铬的研究进展.指出热喷涂技术已经在航空、冶金、包装印刷和造纸等工业领域部分替代了电镀硬铬工艺;由于热喷涂技术对于大工件、厚涂层在性能和价格上的优势,热喷涂技术在其他工业领域的应用也将得到迅速发展.     

等离子喷涂硫化亚铁润滑层的摩擦学性能

利用等离子喷涂在 45钢表面制备了三种厚度不同的硫化亚铁固体润滑层。在QP 1 0 0摩擦磨损试验机上测定了硫化亚铁喷涂层在油润滑条件下的摩擦学性能。利用XRD分析了硫化亚铁喷涂层的相结构 ,用SEM观察了喷涂层的磨面形貌。结果表明 ,硫化亚铁喷涂层的减摩、耐磨、抗擦伤性能明显优于 45钢原始表面。     

火焰喷涂和等离子喷涂FeCrBSi涂层及其防滑和耐磨性能研究

目的采用大气等离子喷涂(APS)和火焰喷涂(FS)在304不锈钢基体上制备FeCrBSi涂层,并对比研究两种工艺制备涂层的防滑和耐磨性能。方法通过光学显微镜、场发射扫描电镜和X射线衍射仪对涂层的显微形貌和结构进行分析,通过维氏硬度计测试涂层的显微硬度。采用摩擦磨损试验机和三维光学显微镜,测量涂层在干摩擦条件下的摩擦系数和磨损量。结果两种喷涂方法制备的涂层多孔,在喷涂过程中极少发生氧化。与火焰喷涂涂层(749HV0.1)相比,大气等离子喷涂涂层(837HV0.1)具有更高的维氏硬度值。在摩擦试验中,火焰喷涂涂层的磨损率为(38.63±2.37)×10~(-6)m~3/(N·m),而大气等离子喷涂涂层的磨损率为(9.5±0.49)×10~(-6)m~3/(N·m),但两种涂层的摩擦系数区别较小,在频率2 Hz、载荷10 N的条件下的摩擦系数为0.6~0.7。结论两种涂层的磨损机制均为疲劳磨损,喷涂态FeCrBSi涂层具有较好的防滑耐磨性能,且大气等离子喷涂涂层性能优于火焰喷涂涂层。     

铝合金基体电热爆炸喷涂钼涂层的制备工艺与性能

采用电热爆炸喷涂技术在铝合金基体上制备钼涂层,用SEM、划痕仪、显微硬度计、表面粗糙度仪、微磨损试验机等表征了涂层性能。借助正交试验方法分析了钼涂层制备中各工艺参数对涂层质量的影响,确定了最佳工艺参数。结果表明:用电热爆炸喷涂方法制备的钼涂层均匀致密,孔隙率低;涂层与基体形成冶金结合,结合强度高;涂层硬度为基体的4.9倍,体积磨损量仅为基体的11.2%,耐磨性大幅增加。电爆炸喷涂的工艺参数中,对涂层质量和结合力影响最大的是喷涂距离,其次是喷涂次数,影响最小的是喷涂电压。     

Preparation and characterization of plasma sprayed Ni3Al-hBN composite coating

Ni₃Al-hBN composite powders were manufactured by spray drying technology, and then plasma sprayed to form the coatings. The influence of hBN addition amount on the flowability and apparent density of the composite powders, as well as the mechanical and tribological properties of the as-sprayed coatings was evaluated. The results indicate that the spherical powders with uniformly distributed compositions are successfully manufactured by spray drying technology. Both the flowability and apparent density of the Ni₃Al-hBN powders, as well as the bond strength and hardness of the composite coatings decrease with the increase of hBN content. Addition of hBN less than 10 wt.% reduces the friction coefficient and brittle fracture of the coatings, which is beneficial to improve the tribological properties of the Ni₃Al-hBN composite coatings. However, high hBN addition, damaging the bond in the coating, will deteriorate the wear resistance of the coating. The Ni₃Al-hBN composite coating containing 10%hBN shows the optimum properties combining strength and hardness with tribological properties.     

粉末原料粒径对WC-17Co涂层性能的影响

本文利用超音速火焰喷涂技术喷涂四种不同粒径的WC-17Co粉末,评价粉末粒径对涂层机械性能和抗磨粒磨损性能的影响。结果表明,粉末的粒径越小,在超音速焰流作用下获得的速度和温度越高,形成的涂层越致密,颗粒间的粘接强度越高,同时涂层的显微硬度也越高。WC-17Co粉末的粒径越小,获得涂层的孔隙直径越小,颗粒间的粘接缺陷越少,因此涂层的抗磨粒磨损性能越好。但是当WC-17Co粉末的粒径过于微小时,涂层的断裂韧性将受到影响。在本文研究的四种粒径分布的WC-17Co粉末中,中间粒径且分布范围集中的粉末制得的涂层兼具良好的机械性能和抗磨粒磨损性能。     

超音速等离子喷涂非晶涂层微观结构与摩擦学性能

采用新一代超音速高能等离子喷涂(SAPS)技术制备了Fe基和Mo基两种非晶涂层,对涂层的微观结构与摩擦学性能进行对比分析。结果表明,SAPS喷涂Fe基和Mo基非晶涂层内部孔洞少、结构致密性高。与Fe基涂层相比,具有更低孔隙率的Mo基涂层在摩擦过程中表现出更低的磨损率(1.1×10^-4 μm·N^-1·s^-1),耐磨性更优。涂层的磨损机理均以磨粒磨损和疲劳磨损为主,并伴随着磨屑氧化。