Comparative study of WC-cermet coatings sprayed via the HVOF and the HVAF Process

The high velocity air fuel (HVAF) system is a high-velocity combustion process that uses compressed air and kerosene for combustion. Two WC-cermet powders were sprayed by the HVAF and the high-velocity oxyfuel (HVOF) processes, using an AeroSpray gun (Browning Thermal Systems Inc., Enfield, New Hampshire) and a CDS-100 gun (Sulzer Plasma Technik, Wohlen, Switzerland) respectively. Several techniques, including x-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy, were used to characterize the microstructures and phase distribution of the powders and coatings. In addition, mechanical properties such as hardness and wear resistance (pin-on-disk) were investigated. A substantial amount of W₂C was found in the HVOF coatings, as well as a high concentration of tungsten in the binder phase, indicating that oxidation and dissolution processes change the composition and microstructure from powder to coating during spraying. This was in contrast to the HVAF coatings in which composition and microstructure were unchanged from that of the powder. Additionally, the wear resistance of the HVAF coatings was superior to that of the HVOF coatings.     

Heat treatment effects on the tribological performance of HVOF sprayed Co-Mo-Cr-Si coatings

The tribological behavior of high-velocity oxyfuel sprayed Co-28%Mo-17%Cr-3%Si coatings, both assprayed and after heat treatments at 200, 400, and 600 °C for 1 h, has been studied. The as-sprayed coating contains oxide stringers and is mostly amorphous. It has low hardness (∼6.7 GPa) and toughness and undergoes adhesive wear against 100Cr6 steel. The friction coefficient increases up to ∼0.9, so the flash temperature reaches a critical oxidation value; then, friction decreases and increases again. This phenomenon occurs periodically. Much adhesive wear occurs in the first stage. Abrasive wear prevails against alumina pin: the coating wear rate is lower because it possesses good plasticity. Thermal effects still occur. The 600 °C treatment causes formation of submicrometric crystals. Hardness increases (∼8.8 GPa), adhesive wear is prevented, the friction coefficient has no peaks. Against the alumina pin, wear rates remain similar to the as-sprayed case. Nevertheless, the friction coefficient has no peaks and its final value is lowered (from 0.84 to 0.75). This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

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.     

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。     

Fatigue cracks in HVOF thermally sprayed WC-Co coatings

Initiation and early growth of fatigue cracks of a medium carbon steel with HVOF thermally sprayed WC-Co coatings prepared from two types of commercially available powders with similar total chemical composition were investigated under rotating bending conditions. The morphology of the fatigue crack is divided into two type—linear cracks and net-like cracks—depending on the types of powders and the thickness of the coatings. The fatigue cracks in thinner coatings were closer to each other than those for the thick coatings.     

Reactive HVOF sprayed TiN-matrix composite coating and its corrosion and wear resistance properties

TiN-matrix composite coating was prepared on 45# steel by reactive high-velocity oxy-fuel (HVOF) spraying. Its microstructure, phase composition, micro-hardness, corrosion resistance in 3.5% NaCl solution and wear resistance were analyzed. The results suggest that the TiN-matrix composite coating is well bonded with the substrate. The micro-hardness measured decreases with the increase of applied test loads. And the micro-hardness of the coating under heavy loads is relatively high. The TiN-matrix composite coating exhibits an excellent corrosion resistance in 3.5% NaCl solution. The corrosion potential of coating is positive and the passivation zone is broad, which indicates that the TiN-matrix composite coating is stable in the electrolyte and provides excellent protection to the substrate. The wear coefficient of the coating under all loads maintains at 0.49–0.50. The wear mechanism of the coating is revealed to be three-body abrasive wear. Yet the failure forms of TiN-matrix composite coating under different loads have an obvious difference. The failure form of coating under light loads is particle spallation due to the stress concentration while that of coating under heavy loads is cracking between inter-lamellae.     

Solid particle erosion of HVOF sprayed NiCr and Stellite-6 coatings

Solid particle erosion behavior of the HVOF deposited NiCr and Stellite-6, coatings on boiler tube steels was evaluated. The study was conducted, using an air jet erosion test rig at a velocity of 26 m/s and impingement angle of 30° and 90°, on uncoated as well as HVOF spray coated boiler tube steel (GrA1) at 250 °C. The coatings were harder as compared to substrate steel. Scanning electron microscopy (SEM) technique was used to analyse the eroded surface. Mass losses of the coatings were found marginally higher than the boiler tube steel.     

粉末结构对HVOF金属陶瓷涂层腐蚀冲蚀磨损性能的影响

采用3种不同NiCr粘结相含量和两种不同尺度WC颗粒的金属陶瓷粉末,运用超音速火焰喷涂（HVOF）方法制备了Cr3 C2-NiCr、WC-12%Co涂层,在5%H2 SO4、15%棕刚玉和水混合介质条件下,运用腐蚀冲蚀磨损试验机测定了涂层在45°冲蚀角下的失重量变化规律,研究了喷涂粉末NiCr含量和WC颗粒尺度对所沉积金属陶瓷涂层耐腐蚀冲蚀磨损性能的影响。结果表明,HVOF喷涂Cr3C2-NiCr、WC-12%Co涂层在5%H2SO4、15%棕刚玉和水混合介质条件下的腐蚀冲蚀率均低于低碳钢,并且Cr3 C2-NiCr涂层的抗腐蚀冲蚀性能要优于WC-12%Co涂层。Cr3 C2-40%NiCr和Cr3 C2-25%NiCr涂层其腐蚀冲蚀率低于Cr3 C2-10%NiCr涂层,与纳米尺度WC颗粒相比,微米尺度WC颗粒所制备的WC-12%Co涂层的抗腐蚀冲蚀性能较好。     

Fabrication of conventional and nanostructured NiCrC coatings via HVAF technique

The conventional and nanostructured NiCrC （with chemical composition of 80%NiCr-20%CrC） coatings with high quality were fabricated via high velocity air-fuel（HVAF） spraying technique. The microstructures of these coatings were characterized by means of metallographic microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffractometry. A Vickers microhardness tester was used to determine the mechanical properties of the as-sprayed coatings. The single-line approximation（SLA） method was employed to calculate the grain size and microstrain of as-sprayed nanostructured coating based on the XRD data. The results show that nanostructured NiCrC coating possesses a more uniform and denser microstructure, much higher microhardness and better fracture toughness than its conventional counterpart. Both TEM observation and calculation results based on XRD profile show that as-sprayed nanostructured NiCrC coating has a homogeneous nanocrystalline microstructure with an average grain size of 40 nm.     

Relationships between in-flight particle characteristics and properties of HVOF sprayed WC-CoCr coatings

Commercially available WC10Co4Cr powder was thermally sprayed by HVOF process. The methane was used as the fuel gas and its flow rate was successively changed as well as the oxygen. The investigation was carried out to determine the influence of operating parameters on the evolution of velocity and temperature of in-flight particles in order to have a better understanding of the interaction between the particle and the flame jet. In relation to the particle characteristics, properties of the sprayed coatings were examined in terms of microstructure, porosity level and microhardness. The results show that the particle velocity and temperature depends strongly on the particle size. The variation of the methane flow rate has a more obvious influence on the velocity and temperature of particles than that of the oxygen. The changes of porosity and microhardness of deposited coatings are discussed corresponding to the variation of fuel and oxygen flow rates.     

活性燃烧高速燃气喷涂WC-CoCr涂层的微观组织及性能

利用活性燃烧高速燃气(AC-HVAF)喷涂技术在0Cr13Ni5Mo不锈钢表面制备了WC-CoCr涂层,并利用XRD、SEM、滑动磨损以及电化学试验分析了涂层的微观组织以及耐磨耐蚀性.结果表明,涂层具有优异的微观结构以及良好的耐磨耐蚀性.XRD分析未发现其他喷涂技术普遍存在的W2C以及W相,AC-HAVF喷涂技术可以有效抑制WC的分解;涂层致密且与基体结合良好,孔隙率仅为0.75%.滑动磨损试验表明,涂层具有很低的磨损率.其主要原因为涂层硬度极高、WC颗粒细小和没有W2C相.电化学试验表明,WC-CoCr涂层的耐蚀性优于基体0Cr13Ni5Mo不锈钢,Cr的加入、W的缺少以及孔隙率低是WC-CoCr涂层耐蚀性优异的重要原因.     

Characterization of Fe-Cr-B based coatings produced by HVOF and PTA processes

Two Fe-Cr-B based gas atomized powders, Armacor M and 16, were thermally sprayed on a low carbon steel substrate, using the HVOF (High Velocity Oxygen Fuel) process. Armacor M was also weld-surfaced with the PTA (Plasma Transferred Arc) process. The resultant deposits were subsequently characterized, using X-ray diffraction, scanning electron microscopy, and microhardness measurement. The effects of heat treatment were also studied for HVOF-sprayed coatings. The wear performance of the coatings was investigated by two-body abrasive wear tests. The results of microstructural analysis of as-sprayed deposits revealed oxide and boride phases such as Fe₃O₄ and Cr_1.65F_0.35B_0.96 in an α matrix for the HVOF-sprayed Armacor 16 coating, and only the boride phases (Cr_1.65F_0.35B_0.96 and Cr₂B) in an α matrix for the HVOFsprayed Armacor M coating. PTA weld-surfaced Armacor M coating contains needle-type long precipitates of Cr₂B) and Cr_1.65F_0.35B_0.96, in the α matrix. The hardness of the HVOF-sprayed Armacor 16 coating after heat treatment was substantially less than that of the as-sprayed coating due to the phase transformation from α to γ phase. Heat treatments of the HVOF-sprayed Armacor M coating did not produce changes in phase and its hardness decreased as compared to that of the as-sprayed coating. While HVOF-sprayed and PTA weld-surfaced Armacor M coatings have the same hardness, the latter shows better abrasive wear resistance because of the size and orientation of its boride phases. The broadening of the XRD patterns and the increase in hardness after wear testing suggest that the transformation from the crystalline to the amorphous structure occurred on the uppermost layer during wear testing.     

超音速火焰喷涂NiCrC涂层的滑动摩擦磨损行为

采用空气助燃超音速火焰(HVAF)喷涂技术,分别以相同成分的气雾化合金粉体和低温球磨纳米晶合金粉体作为喂料,制备出高质量的耐磨耐蚀粗晶和纳米晶NiCrC合金涂层。利用SRV摩擦磨损试验机、表面形貌仪和扫描电镜等设备对两种涂层和45钢试样的滑动摩擦磨损行为进行了对比研究。结果表明:在载荷为10～30 N的无润滑剂常温滑动摩擦条件下,两种涂层具有较为接近的滑动摩擦系数,但纳米晶NiCrC涂层的抗磨损性能远优于常规粗晶涂层。三种测试材料都呈现出磨粒磨损和粘着磨损的特征,随着载荷的增加,磨损程度逐渐加剧,同时粘着磨损的特征更加突出。此外,两种涂层材料在磨损过程中都出现了颗粒部分或整体剥落的现象,加剧了涂层的磨损程度。     

Examination of the wear properties of HVOF sprayed nanostructured and conventional WC-Co cermets with different binder phase contents

There has been an increase in interest of late regarding the properties of thermally sprayed WC-Co cermets with nanograin carbide particles. These powders have shown interesting properties in sintered components, giving high values of hardness (2200–2300 VHN) and improved wear properties. The method used for the processing for these materials—solution formation, spray drying and chemical conversion, rather than introduction of WC as solid particles to a molten binder—allows the formation of sub-100 nm WC particles as a hard second phase. The work presented here examined the effect of composition on the microstructure and wear properties of some nanostructured WC-Co materials. WC-Co cermets with 8, 10, 12, and 15% Co binder phase were deposited using a Sulzer Metco hybrid DJ HVOF thermal spray system. Optimization of deposition conditions was necessary because of the unique morphology of the powders (thick-shelled hollow spheres) to produce dense consolidated deposits. There is a higher degree of decarburization of the WC phase in the nanostructured materials compared with the conventional WC-Co. This dissolution of the hard phase is also noted to increase on decreasing binder phase content. The nanostructured WC-Co coatings have a lower wear resistance compared with the conventional WC-Co for abrasive wear and small particle erosion. The abrasive wear resistance of these nanostructured materials was found to increase on decreasing cobalt binder content. This trend in abrasive wear resistance is consistent with studies on conventional sized cermets and is believed to be more dependent upon proportion of binder phase content than degree of decarburization for the materials studied. The small particle erosion resistance of the nanostructured coatings was found to increase on increasing cobalt content.     

用欧拉法对薄壁件激光直接成形建模

采用欧拉方法建立了激光直接成形薄壁件的二维传热模型。分离模型中的参数和变量并对模型进行了无量纲化,对最终无量纲化模型中的6个无量纲参数进行了分析讨论。研究表明：针对此二维传热模型的无量纲化减少了参数,简化分析过程;在激光直接成形过程中,实际层高和光束移动速度的乘积共同对温度场起作用,而非单独影响温度场。     

NiCrBSiWCe合金粉末喷熔层滑动磨损特性研究

采用热喷熔方法在45钢表面制备了NiCrBSiWCe合金粉末喷熔层,在SRV磨损试验机上进行小振幅滑动磨损试验研究.结果表明,NiCrBSiWCe合金喷熔层的摩擦磨损性能明显高于SAE52100钢.在较低载荷和滑动速度较低下,小振幅滑动磨损机理为磨损表面的划痕、裂纹和疲劳脱层.而在较高滑动速度下,小振幅滑动磨损机理为磨损表面的氧化磨屑层的形成,含稀土的磨屑层阻碍了喷熔层小振幅滑动磨损.     

AZ31镁基高速火焰喷涂WC涂层的研究

材料的轻量化已经越来越成为一种趋势.镁合金因为更高的比强而作为铝合金的替代材料,在机械、电子和航空等工业有着广泛的应用前景.然而镁合金因硬度低,抗腐蚀性差而使应用受限制,因此需要进行表面处理以提高材料的使用寿命和拓宽应用领域.采用High-Velocity-Air-Fuel(高速火焰喷涂)技术在AZ31镁合金基体上制备了WC-CoCr涂层,采用XRD、SEM分别对涂层的相和显微结构进行了分析,同时研究了拉伸对偶件的弹性模量对涂层结合强度测试的影响,重点阐述了涂层的结合机理.此外,还研究了镁基高速火焰喷涂WC涂层的抗中性盐雾腐蚀性能.结果表明,采用HVAF技术,可在AZ31镁合金基体上获得致密,结合强度较高且具有良好抗腐蚀性能的WC-CoCr涂层.     

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.     

Formation and corrosion behavior of Fe-based amorphous metallic coatings by HVOF thermal spraying

Amorphous coatings with a composition of Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Y₂ were prepared by means of high velocity oxygen fuel (HVOF) thermal spraying under different conditions. Microstructural studies show that the coatings present dense layered structure and low porosity with a fraction of nanocrystals precipitated. The porosity and amorphous fraction of the coatings decrease as the kerosene and oxygen flow increase within the parameter range examined. Corrosion behavior of the amorphous coatings was investigated by electrochemical measurement. The results show that the coatings are spontaneously passivated with wide passive region and low passive current density in 3.5% NaCl, 1 N HCl and 1 N H₂SO₄ solutions, and exhibit an excellent ability to resist localized corrosion. However, the corrosion resistance of the coatings decreases in 1 N NaOH solution with lower transpassive potential and passive region. In addition, the optimal spraying parameter improves the corrosion resistance of the amorphous coatings obviously due to the proper proportion of porosity and amorphous fraction.     

Microstructure and stresses in HVOF sprayed iron aluminide coatings

The microstructure and state of stress present in Fe₃Al coatings produced by high velocity oxygen fuel (HVOF) thermal spraying in air at varying particle velocities were characterized using metallography, curvature measurements, x-ray analysis, and microhardness measurements. Sound coatings were produced for all conditions. The microstructures of coatings prepared at higher velocities showed fewer unmelted particles and a greater extent of deformation. Residual stresses in the coatings were compressive and varied from nearly zero at the lowest velocity to approximately −450 MPa at the highest velocity. X-ray line broadening analyses revealed a corresponding increase in the extent of cold work present in the coating, which was also reflected in increased microhardness. Values of mean coefficient of thermal expansion obtained for assprayed coatings using x-ray analysis were significantly lower than those for powder and bulk alloy.