A study on powder mixing for high fracture toughness and wear resistance of WC-Co-Cr coatings sprayed by HVOF

The effects of mixing powders with various particle sizes on the fracture toughness and wear resistance of thermally sprayed WC-10Co-4Cr coating layers fabricated by the HVOF (High-Velocity Oxygen Fuel) process on a S45C steel substrate were investigated. In order to obtain a high fracture toughness and wear resistance, the powder size and powder mixing ratio were varied. The microstructure and chemical composition of the phases in the coatings were characterized by means of the SEM and XRD techniques. Image analysis was used for the evaluation of the porosity of the coatings. Indentations tests were carried out on the cross sections of the coatings to evaluate the hardness and fracture toughness. The wear properties of the coatings were assessed using a pin-on-disk wear tester at ambient temperature without lubrication. The mixing of a small amount of coarse powders with fine powders resulted in the highest fracture toughness and wear resistance, due to the formation of coating layers having the lowest porosity.     

AC-HVAF喷涂纳米结构WC-12Co耐磨涂层的微观结构和性能

本文采用含纳米WC颗粒的WC-12Co粉末,通过空气助燃超音速火焰喷涂系统（AC-HVAF）制备了耐磨涂层。研究了涂层相组成、微观结构、涂层硬度、断裂韧性和耐磨损性能。X射线衍射分析结果表明WC为涂层主相,未发现其他失碳分解产物。涂层孔隙率低于1%,晶粒尺寸为80-100nm,涂层磨光表面硬度平均值1940.3 HV0.3, 横截面平均硬度高达1662.1 HV0.3。使用WC硬质球为摩擦副,载荷1.5kg,工件转速1198r/min干磨条件下,纳米结构涂层的平均失重比微米结构涂层降低40%,且纳米结构涂层摩擦系数为0.26-0.28（微米结构涂层：0.25-0.4）,因此纳米结构涂层具有更加优异的耐磨性能。     

Microstructure and Sliding Wear Performance of Plasma-Sprayed TiB2-Ni Coating Deposited from Agglomerated and Sintered Powder

This work is aimed at developing a route for the deposition of TiB₂-Ni cermet coating. The feedstock was firstly prepared by agglomeration and sintering, which was subsequently subjected to plasma spraying. The microstructures and the phase composition of the powder, as well as the sprayed coating were analyzed by scanning electron microscopy and x-ray diffraction. The microhardness (Hv) and the fracture toughness (K _IC) of the coating were evaluated. A sliding wear test was also performed on the sprayed coating by SRV^® tribo-tester using GCr15 steel as a counterpart. The results showed that the phase of sprayed TiB₂-Ni coatings consisted of TiB₂, Ni, and Ni₂₀Ti₃B₆, whose amount varied depending on the powder calcination temperature and the TiB₂ content in the powder. Both the hardness and the fracture toughness of the coating were also changed with different powders. The Ni₂₀Ti₃B₆ brittle phase was the main factor affecting the fracture toughness of coating, which also had detrimental effect on the sliding wear performance. The 60TiB₂-40Ni coating deposited from the powder calcined at 1250 °C had better sliding wear performance as it presented more dense structure, higher TiB₂ content and less retained Ni₂₀Ti₃B₆ phase in the coating.     

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.     

Microstructure and properties of Al2O3-13%TiO2 coatings sprayed using nanostructured powders

The microstructure and wear performance of M203-13% TiO2 coatings prepared by plasma spraying of agglom- erated nanoparticle powders were investigated. SEM analysis showed that the as-sprayed Al2O3-TiO2 coatings comprise of two kinds of typical region： fully melted region and unmelted/partially melted nanostructured region, which is different than the conventional coating with lamellar structure. It is shown that the microhardness of the nanostructured coatings was about 15%-30% higher than that of the conventional coating and the wear resistance is significantly improved, especially under a high wear load. The nanostructured coating sprayed at a lower power shows a lower wear resistance than the coatings produced at a higher power, because of the presence of pores and microstructural defects which are detrimental to the fracture toughness of the coatings.     

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.     

Tribological behaviour of HVOF sprayed near-nanostructured and microstructured WC-17wt.%Co coatings

A ‘duplex cobalt coated’ near-nanostructured WC-17wt.%Co powder was used to produce nanostructured coatings. The tribological performance of this coating was compared with a commercial WC-17wt.%Co microstructured coating using a pin-on-plate method (ASTM G133-05 standard) with a data acquisition software to perform a real time analysis of the sliding wear process. The wear rate was studied using loads from 10 to 60 N and for various sliding distances. The metallurgical analysis of the coatings showed that the duplex Co coated powder could be sprayed to produce dense coating. Furthermore, the near-nanostructured coating showed better fracture toughness values and this corresponded to a difference in wear mechanism between the two types of coatings. The greater “plasticity” in the near-nanostructured coating was recorded as microgrooves in the wear tracks and, in comparison, brittle fracture was observed in the wear tracks produced on the microstructured coating.     

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.     

Influence of carbide grain size and crystal characteristics on the microstructure and mechanical properties of HVOF-sprayed WC-CoCr coatings

Nanostructured WC-CoCr coatings play a significant role in industrial sectors including petrochemical and aerospace fields. However, their fracture toughness requirements still need to be further improved because of more decarburization of conventional nano scale WC grains. In the study, three representative types of WC-CoCr powders were selected to illustrate the dependence of the microstructure and mechanical properties of HVOF-sprayed coatings on the carbide grain size and their crystal characteristics in the starting powders. It is demonstrated that the ultrafine powder causes excessive decomposition of the WC phase to W₂C phase when compared to submicrostructured and nanostructured powders, owing to the WC grains with a higher density dislocation. The present study implies the critical role of apparent twin characteristics within WC grains in enhancing the microhardness and fracture toughness of the nanostructured coating. The coating deposited with nanostructured powders exhibits the best comprehensive properties, which include low decarburization, superior microhardness and fracture toughness.     

爆炸喷涂WC-12%Co涂层的滑动磨损性能

采用爆炸喷涂技术制备纳米和普通WC-12%Co涂层,用往复试验机对涂层的干滑动磨损性能进行了研究,分析了涂层磨损前后的形貌、结构及成分变化.结果表明:相同的喷涂条件下,WC-12%Co纳米涂层比普通涂层结构均匀、致密,但碳化物分解严重.尽管纳米涂层与普通涂层具有相近的硬度,但普通涂层的耐磨性优于纳米涂层,尤其是在重载条件下.普通涂层的磨损机制为微切削;纳米涂层在轻载(10 N)下,以塑性变形为主要磨损机制,随载荷增加至30 N,纳米WC粒子不能起到阻抗陶瓷球对磨副的磨削作用,而是随粘结相一起被去除,同时由于纳米涂层脱碳导致的层间结合薄弱,在滑动磨损中易发生成片剥落,耐磨性大幅下降.     

喷涂工艺对超音速火焰喷涂Al-Cu-Fe-Si准晶合金涂层性能的影响

目的使用活性燃烧高速燃气喷涂(AC-HVAF)方法制备高质量的Al-Cu-Fe-Si准晶涂层,研究喷涂工艺对涂层性能的影响。方法采用气雾化Al-Cu-Fe-Si准晶合金粉末,利用AK02T型AC-HVAF喷涂系统制备Al-Cu-Fe-Si准晶涂层材料。通过X射线衍射及扫描电镜观察分析准晶合金粉末和涂层的组织与结构,通过电化学工作站、显微硬度计和接触角测试仪等手段分析准晶合金涂层的耐蚀性、显微硬度及抗粘性能。结果对气雾化准晶Al-Cu-Fe-Si合金粉末的研究发现,冷却速率显著影响准晶合金粉末的组织,在冷却速率较快的粉末中形成胞状晶组织,准晶I相含量较高。对准晶合金涂层进行热处理,高温退火显著提高了涂层的硬度,950℃退火12 h后,硬度值达到(724±153)HV0.1。分别对准晶合金涂层和基体45~#钢的接触角进行测量,准晶合金涂层的接触角最大为95°,而45~#钢的仅为79°。通过电化学工作站测试比较涂层的耐蚀性,发现在3.5%(质量分数)的Na Cl溶液中,喷涂在45~#钢和5052铝合金基体上的涂层腐蚀电流密度J_(corr)分别为6.8×10~(-6),2.0×10~(-7)A/cm~2。结论不同粒径的气雾化准晶合金粉末的相组成不同,选择合适的粒径是保证铝基准晶合金涂层质量的前提。对涂层进行合适的热处理可以有效地提高涂层的显微硬度,铝基准晶合金涂层的接触角较45~#钢的高,提高了基体的抗粘性。不同基体上制备的准晶合金涂层的耐蚀性有很大差异,5052铝合金基体上的准晶涂层耐蚀性优于喷涂在45~#钢基体上的涂层。     

Microstructure and Wear Resistance Evolution of HVOFSprayed WC-（W,Cr）2C-Ni Coating with Post Heat Treatment In Air Atmosphere

WC-(W,Cr)2C-Ni coating was prepared on 1Cr18Ni9Ti stainless steel and C-276 Ni-base Hastelloy by high velocity oxy-fuel(HVOF)spraying.The effect of post heat treatment in air atmosphere on the microstructure,phase composition,microhardness,fracture toughness,and wear resistance of HVOF-sprayed WC-(W,Cr)2C-Ni coating was investigated.The microstructure and phase composition of the coatings were analyzed by means of field emission scanning electron microscopy(FESEM)and X-ray diffraction(XRD).The microhardness and fracture toughness of the coatings were measured using a microhardness tester and a Vickers hardness tester.Moreover,dry friction and wear behavior of the coatings sliding against Si3N4 ball was investigated using an oscillating friction and wear tester;and the worn surfaces of the coatings were analyzed by means of scanning electron microscopy(SEM).It was found that heat treatment within 500-800°C resulted in crystallization of amorphous phase in as-sprayed coating,generating nanoscale new phases such as NiWO4,CrWO4 and Cr2WO6.Besides,heat treatment led to increase of the microhardness of as-sprayed coating,and the highest microhardness was obtained after heat treatment at 800°C.The fracture toughness and wear resistance of the as-sprayed coating increased with increasing heat treatment temperature up to 700°C but tended to decrease with further elevating temperature.In other words,the mechanical properties and wear resistance of the as-sprayed coatings were worsened owing to excessive growth of oxidation grains and depletion of ductile Ni binder after heat treatment above 700°C.Thus it was suggested that as-sprayed ceramic composite coating should be post heat treated in air at a moderate temperature of 700°C so as to achieve the optimized mechanical properties and wear resistance.     

Influence of WC addition in Co-Cr-W-Ni-C flame sprayed coatings on microstructure, microhardness and wear behaviour

In present paper the influence of the tungsten carbide (WC) particle addition on the microstructure, microhardness and abrasive wear behaviour of flame sprayed Co-Cr-W-Ni-C (EWAC 1006) coatings deposited on low carbon steel substrate has been reported. Coatings were deposited by oxy-acetylene flame spraying process. Wear behaviour of coatings was evaluated using pin on flat wear system against SiC abrasive medium. It was observed that the addition of WC particle in a commercial Co-Cr-W-Ni-C powder coating increases microhardness and wear resistance. Wear behaviour of these coatings is governed by the material parameters such as microstructure, hardness of coating and test parameters (abrasive grit size and normal load). Addition of WC in a commercial powder coating increased wear resistance about 4-9 folds. WC modified powder coatings showed better wear resistance at high load. Heat treatment of the unmodified powder coatings improved abrasive wear resistance while that of modified powder coating deteriorated the wear resistance. SEM study showed that wear of coatings largely takes place by microgroove, crater formation and scoring. Electron probe micro analysis (E.P.M.A.) of unmodified and WC modified powder coating was carried out for composition and phase analysis.     

The effect of Mo content in plasma-sprayed Mo-NiCrBSi coating on the tribological behavior

NiCrBSi is a material popularly used as a hard thermal sprayed coating. The coating performs well as a wear resistant coating under low stress. At higher stress in metal-to-metal sliding wear condition, however, the NiCrBSi starts to experience surface deformation, which will inevitably lead to seizure as the stress increases. In order to improve the tribological properties of the NiCrBSi plasma-sprayed coating, Mo is added to the coating to reduce the friction between the coating and other metal contacting surface, thus, improving its dry sliding wear resistance. In this study, various amounts of Mo were mixed with NiCrBSi at 0, 25, 50, 75 and 100 wt.%. The powders were sprayed using an air plasma spraying technique onto stainless steel samples to form coatings, which were ground to achieve flat surfaces and a thickness of 350-400 μm. The mechanical properties of the coatings were determined. The coating samples were then tested using a reciprocation ball-on-flat tribometer. It was found that as the Mo/NiCrBSi ratio increases, the wear mechanism changes. Coatings containing 75%Mo and 25%NiCrBSi exhibit the highest wear depths corresponding to the cracking of the thin NiCrBSi splats. On the other hand, coatings containing 25%Mo and 75%NiCrBSi possess the lowest wear depths with no surface cracks. The presence of Mo covering the coating surface hinders the metal seizure between NiCrBSi and steel counter surface.     

High-Power Diode Laser Surface Treated HVOF Coating to Combat High Energy Particle Impact Wear

High-velocity oxy-fuel (HVOF)-sprayed coatings have performed exceptionally well in low-energy particle impact wear and are accepted worldwide. However, their application for high-energy particle impact wear (HEPIW) requires a different approach and more efforts. HVOF-coating systems typically use WC-Co, WC-Co-Cr, WC-Ni-Cr, and FeCrAlY-Cr₃C₂ powders. WC-Co-Cr powders are preferred when there is a high demand for corrosion resistance. WC-10Co-4Cr coating powder has been selected in the current study. To improve coating properties such as microhardness, fracture toughness, and HEPIW resistance, a new approach of surface treatment with robotically controlled high-power diode laser (HPDL) is attempted. The robotically controlled HVOF-coating deposition and laser surface treatment were monitored using real-time diagnostic control. The HPDL-treated coating has been compared with “as-sprayed” HVOF coating for HEPIW resistance, fracture toughness, microhardness and microstructure. The coating characteristics and properties after laser surface treatment have improved many times compared with “as-sprayed” HVOF coating. This is due to the elimination of pores in the coating and formation of a metallurgical bond between coating and substrate. This new development opens up a possibility of using such laser treatments in specialized areas where HEPIW damages are acute. The fracture toughness and HEPIW resistance along with optical micrographs of HPDL-treated and untreated HVOF coatings are discussed and reported in this article. HEPIW resistance is observed to be proportional to the product of fracture toughness and microhardness of the HVOF coating.     

Thermal Spray of Cemented Carbide Coatings in Off-Angle Spraying: Correlations Between Process,Coating Features/Characteristics and Performance

Oxidation or decomposition of thermally sprayed coatings during deposition may noticably impair the coating features and characteristics, i.e. the surface integrity of coated components, including coating porosity, phase, microhardness, elastic modulus and toughness, etc., having a profound influence on the final performance of components. Further degradations along with enhanced oxidation of the coatings were observed for an off-angle spraying scheme in industrial practice, where inconsistent results were previously reported for spray angle-dependent degradation of various thermal spray coatings, with the mechanism not being fully understood with the less controlled surface integrity. An improved off-angle HVOF thermal spray has been developed for cemented carbide coatings, using pretreated WC-Ni feedstock powders covered with a nanoscale capsulizing nickel layer. A comprehensive study on the formation of multiple surface integrity parameters and the correlative interactions between them is thus facilitated with controllable surface integrity generation using the capsulized powders. Decarburization is mainly attributed to the exposure of pristine WC grains causing enhanced porosity in inter-splats regions, and then microhardness depends mainly on porosity and partly on phase composition, while the elastic modulus depends on intra-splats cohesion and the indentation fracture toughness on inter-splats cohesion, respectively. Abrasive wear from mild to severe regime transition could be interpreted by the inter-splats and intra-splats cohesion correlation. It has been demonstrated by the improved off-angle thermal spray that a manufacturing process could be designed and optimized on the identified unique correlations between the processes, surface integrity and the final performance.     

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。     

Surface hardening of die steel by superplastic deformation of thermo-sprayed powder alloy

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Improved Protection Properties by Using Nanostructured Ceramic Powders for HVOF Coatings

The potential of the high-velocity oxy-fuel (HVOF) thermal spray process for reduced porosity in coatings compared to those produced by other ambient thermal spray processes is well known. The ability to produce high-density ceramic coatings offers potential in high-performance applications in the field of wear, corrosion resistance, and dielectric coatings. However, due to operational limit of the HVOF process to effectively melt the ceramic particles, the process—structure relationship must be well optimized. It has been also demonstrated that benefits from HVOF ceramic coatings can be obtained only if particles are melted enough and good lamella adhesion is produced. One strategy to improve melting of ceramic particles in relative low-flame temperatures of HVOF process is to modify particle crystal structure and composition. In this paper the effect of the powder manufacturing method and the composition on deposition efficiency of spray process as well as on the mechanical properties of the HVOF sprayed are studied. Effect of fuel gas, hydrogen vs. propane, was also demonstrated. Studied materials were alumina-, chromia-, and titania-based agglomerated powders. Coating properties such as microstructure, hardness, abrasive wear resistance, and relative fracture toughness were compared to the coating manufactured by using conventional fused and crushed powders. It can be concluded that powder size distribution and microstructure should be optimized to fulfill process requirements very carefully to produce coatings with high deposition efficiency, dense structure, improved fracture toughness, and adhesion.     

The influence of HVAF powder feedstock characteristics on the sliding wear behaviour of WC-NiCr coatings

In this paper, the HVAF technique was used to deposit coatings with mechanically milled nano and micron blended WC-NiCr powders. The mechanically milled nano powder was produced using the high-energy ball milling process. Due to low particle heating and high particle velocity in the HVAF process, the fine and dense microstructure of the mechanically milled powder remains almost unchanged after spraying. The sliding wear resistance of the two coatings was evaluated by ball-on-disc tests. It was found that the sliding wear resistance of the coating produced using mechanically milled powders was greatly improved compared with the coating produced using micron blended powder. The sliding wear resistance of the coating produced using mechanically milled powder was about two hundred times than that of the coating produced using blended powder. The coefficient of friction for the coating produced using mechanically milled powder increased steadily from 0.22 to 0.51, while for the coating produced using blended powder it attained a steady value of 0.58. Both the coating produced using mechanically milled powder and blended powder were characterized in terms of their microstructure by X-ray diffraction, and scanning electron microscopy in the as-sprayed state as well as after sliding wear. The sliding wear mechanism was also discussed.