Dynamic hardness of detonation sprayed WC-Co coatings

The objective of the present work was to determine the dynamic hardness of WC-Co coatings from the dynamic hardness of the coating substrate system. It was also the purpose of this work to evaluate the influence of coating composition, coating thickness, and substrate materials on the dynamic hardness of the coating. To achieve the above-mentioned objectives, WC-12%Co and WC-17%Co coatings were deposited by detonation spraying on three different substrate materials: mild steel, commercially pure (CP) aluminum, and CP titanium. The dynamic hardness of the coating/substrate composite was evaluated by a drop weight system. The dynamic hardness of the coating independent of the substrate was determined from the dynamic hardness of the coating/substrate composite.     

Characterization of thermal sprayed nanostructured WC-Co coatings derived from nanocrystalline WC-18wt.%Co powders

Nanostructured WC-Co coatings were synthesized using high velocity oxygen fuel (HVOF) thermal spray. The nanocrystalline feedstock powder with a nominal composition of WC-18 wt.%Co was prepared using the novel integrated mechanical and thermal activation (IMTA) process. The effects of HVOF thermal spray conditions and powder characteristics on the microstructure and mechanical properties of the as-sprayed WC-Co coatings were studied. It was found that the ratio of oxygen-to-hydrogen flow rate (ROHFR) and the starting powder microstructures had strong effects on decarburization of the nano-coatings. Decarburization was significantly suppressed at low ROHFR and with the presence of free carbon in the powder. The level of porosity in the coatings was correlated with the powder microstructure and spray process conditions. The coating sprayed at ROHFR=0.5 exhibited the highest microhardness value (HV_300g=1077), which is comparable to that of conventional coarse-grained coatings.     

Deposition of metallic coatings on polymer surfaces using cold spray

Current coating technologies such as plasma spray, High Velocity Oxygen Fuel (HVOF) or laser cladding involve the delivery of molten materials during the deposition process. However, such techniques are not well suited to the deposition of metallic coatings on polymers and composites. Cold spray (CS) has attracted much industrial interest over the past two decades. In this method, a material in powder form is accelerated on passage through a converging-diverging nozzle to high speeds via a high pressure coaxial carrier gas jet. The high impact kinetic energy deforms the particles, which creates effective bonding to the substrate.This paper presents the results of an initial study on the potential of the CS process to produce metallic coatings on non-metallic surfaces such as polymers and composites for engineering applications. Experimental and Computational Fluid Dynamics (CFD) results when spraying copper, aluminium and tin powder on a range of substrates such as PC/ABS, polyamide-6, polypropylene, polystyrene and a glass-fibre composite material are presented and analyzed.     

超音速喷涂纳米WC复合涂层与电镀铬层的组织及性能

采用超音速火焰喷涂(HVOF)技术和电镀技术分别制备了纳米WC-10Co-4Cr涂层及电镀铬层,通过扫描电镜(SEM)、X射线衍射仪(XRD)、显微硬度计、摩擦磨损试验机、电化学工作站、磨损腐蚀加速试验机等手段对比分析了这两种涂层的性能以及使用寿命。结果表明,纳米WC复合涂层具有远高于电镀铬层的显微硬度、耐磨损性能及结合强度等,并且该涂层具有较高的致密度,而耐腐蚀性能略低于电镀铬层但也远高于基材,磨损腐蚀加速试验结果显示该涂层的使用寿命高达电镀铬层的5倍以上。     

Effect of heat treatment on the intermetallic layer of cold sprayed aluminum coatings on magnesium alloy

Dense and thick pure aluminum coatings were deposited on AZ91D-T4 magnesium substrates using the cold spray process. Heat treatments of the as-sprayed samples were carried out at 400 °C using different holding times. The feedstock powder, substrate and coating microstructures were examined using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) as well as Vickers microhardness analysis. The results demonstrate that aluminum coatings having dense and uniform microstructure can be deposited successfully using a relatively large feedstock powder. It has been identified that the intermetallics Al₃Mg₂ (γ phase) and Mg₁₇Al₁₂ (β phase) were formed at the coating/substrate interface during heat treatment. The growth rate of these intermetallics follows the parabolic law and the γ phase has a higher growth rate than the β phase. The thickness of the Mg₁₇Al₁₂ and Al₃Mg₂ intermetallic layers has reached 83 μm and 149 μm, respectively. This result is almost 45% higher than what has been reported in the literature so far. This is attributed to the fact that T4 instead of as cast Mg alloy was used as substrate. In the T4 state, the Al concentration in the Mg matrix is higher, and thus intermetallic growth is faster as less enrichment is required to reach the critical level for intermetallic formation in the substrate. The AZ91D-T4 magnesium substrate contains single α phase with fine clusters/GP-zones which is considered beneficial for the intermetallic formation as well as the intimate contact between the coating/substrate interface and the deformed particles within the coating.     

WC粉末X射线衍射的粒度效应

用X射线衍射研究粒度 10 .86～ 0 .12 4μm系列WC粉末的粒度效应。随粒度变小 ,衍射线线形发生明显变化 ,从敏锐到极其漫散。亚微米级粒度的WC粉末谱线半高宽随粒度变细显著变化 ,半高宽是度量粒度效应的主要指标 ,而对于微米级粒度的粒度效应则可应用谱线背底宽度来衡量。极细和超细粒度WC粉末衍射谱线的2θ位置明显偏离平衡位置 ,这与临近纳米粒度有关。WC粉末衍射强度也随粒度产生明显变化 ,衍射强度随粒度变细的变化规律与半高宽的规律呈反转对应关系。应用WC粉末X射线衍射的粒度效应 ,尤其是谱线宽化的规律作为评定亚微米级WC粉末粒度尤其是极细和超细粒度 ,是有应用前景的。     

Characterization and residual stresses of WC-Co thermally sprayed coatings

The present investigation has been conducted in order to determine the residual stresses of an as-ground WC-12Co coating of two different thicknesses, by means of two different methods. Firstly, X-ray diffraction techniques, which allowed the determination of the surface residual stresses of the coating by means of the method called “sin²ψ” method. Secondly, an incremental hole drilling technique together with the integral method, which allowed the analysis of the non-uniform through-thickness residual stresses present in the coatings. It has been determined that the surface residual stresses are of a compressive nature, which could be due to the grinding that was applied to the coatings in order to achieve the desired thicknesses. On the contrary, the results of the incremental hole drilling tests indicated that the through-thickness residual stress distributions are not uniform and are characterized by the presence of tensile peak stresses, at depths in the range of ~ 50-125 μm. Such stresses were observed to decrease towards the coating-substrate interface where the compressive component of the stress state becomes greater than the tensile component. It has been found that the mean residual von Mises stress is higher in the thinner coating than in the thicker one, of approximately 180 and 107 MPa, respectively.     

Influence of HVOF parameters on the corrosion and wear resistance of WC-Co coatings sprayed on AA7050 T7

Thermal spray WC-based coatings are widely used in the aircraft industry mainly for their resistance to wear, reworking and rebuilding operations and repair of worn components on landing gear, hydraulic cylinders, actuators, propeller hub assemblies, gas turbine engines, and so on. The aircraft industry is also trying to use thermal spray technology to replace electroplating coatings such as hard chromium. In the present work, WC-Co coatings were built up on an AA 7050 aluminum alloy using high velocity oxygen fuel (HVOF) technology and a liquid nitrogen cooling prototype system. The influence of the spray parameters (standard conditions, W19S, increasing the oxygen flux, W19H, and also increasing the carrier gas flux, W19F) on corrosion, friction, and abrasive wear resistance were also studied. The coatings were characterized using optical (OM) and scanning electron (SEM) microscopy, and X-ray diffraction (XRD). The friction and abrasive wear resistance of the coatings were studied using Rubber Wheel and Ball on Disk tests. The electrochemical studies were conducted using open-circuit potential (E_OC) measurements and electrochemical impedance spectroscopy (EIS). Differences among coated samples were mainly related to the variation of the thermal spray parameters used during the spray process. No significant differences were observed in the wear resistance for the coatings studied, and all of them showed a wear rate around 10 times lower than that of the aluminum alloy. The results of mass loss and wear rate were interpreted considering different mechanisms. Comparing the different spray parameters, the oxygen flux (higher flame temperature) produced the sample which showed the highest corrosion resistance in aerated and unstirred 3.5% NaCl solution. Aluminum ions were detected on the surface almost immediately after the immersion of samples W19S and W19F in chloride solution, showing that the electrolyte reached the substrate and galvanic corrosion probably occurred. For sample W19H, aluminum ions were not detected even after 120 min of immersion in NaCl solution.     

Ni3Al添加量对WC-Co硬质合金中WC晶粒形状的影响

采用粉末冶金制备技术,以粗WC粉末、Co粉和WC＋Ni3Al预合金粉末为原料制备出WC-40vol％（Co—Ni,Al）硬质合金。利用扫描电镜和透射电镜研究了不同NbAl含量对WC-40vol％（Co—Ni3Al）硬质合金中WC晶粒形状的影响规律。结果表明：W在Co粘结相中的固溶度接近25．4wt％,而W在Ni,Al粘结相中的固溶度接近9．5wt％,随着NbAl含量的增加,粘结相对W的固溶度减小,合金中的WC晶粒圆钝和细小;WC晶粒表面上出现明显的台阶。相应的,延长烧结时间,WC—Co—Ni3Al硬质合金具有与WC—Co硬质合金相同的WC生长行为,WC-40vol％（Co—Ni3Al）硬质合金中的WC晶粒表面上的台阶处出现明显的刻面。     

几种金属基板上冷喷涂铜涂层的试验与模拟

采用自主研制的冷喷涂设备在三种典型基板上进行喷涂试验,相同的工艺参数下,在铜和铝基板上得到良好的铜涂层,而在钢基板上则没有沉积.实验结果表明:涂层与基板界面、涂层内部颗粒界面结合良好,铜涂层组织致密,显微硬度高达150HV0.1;从涂层表面形貌扫描电镜(SEM)照片中可以观察到射流状的金属,说明颗粒发生了巨大变形,经计算知颗粒在碰撞中压缩率达69%;粉末和涂层的X射线衍射(XRD)结果表明铜粉末在冷喷涂过程中没有发生氧化.同时,数值模拟了铜颗粒与三种基板的碰撞过程,讨论了形成有效结合的判断准则,根据该准则,计算出铜颗粒在铜、铝、钢基板上的临界沉积速度分别为600m/s,500m/s,800m/s,从而解释了铜颗粒在三种基板上不同的沉积行为.     

亚微米WC添加对超音速火焰喷涂WC-Co涂层磨损性能影响

采用超音速火焰喷涂方法(HVOF)在304不锈钢基体表面制备WC和WC-12Co的复合涂层WC-Co,研究亚微米WC的添加对涂层相组成、显微硬度、耐磨性能和表面形貌的影响。利用X射线衍射、压痕法、往复式摩擦磨损实验和扫描电子显微镜(SEM)分别对涂层的相组成、显微硬度、磨损性能和表面形貌进行分析测试,并分析涂层的磨损过程和机制。结果表明,添加质量分数5%的亚微米WC颗粒显著提高了涂层的显微硬度(16.3%);增强了涂层的耐磨性,磨损率从6.09×10-7 mm3/Nm减小到5.15×10-7 mm3/Nm(减小13.8%);亚微米WC颗粒喷涂后在涂层中保持了WC相,并主要存在于WC-Co扁平粒子界面和孔隙。基于涂层中扁平粒子的结合特性与磨损失效特征,建立强化模型,分析亚微米WC颗粒对涂层扁平粒子界面的强化机制。     

Development of multimaterial coatings by cold spray and gas detonation spraying

The basic objective is the development of multifunctional multimaterial protective coatings using cold spraying (CS) and computer controlled detonation spraying (CCDS).As far as CS is concerned, the separate injection of each powder into different zones of the carrier gas stream is applied. Cu-Al, Cu-SiC, Al-Al₂O₃, Cu-Al₂O₃, Al-SiC, Al-Ti and Ti-SiC coatings are successfully sprayed. As to CCDS, powders are sprayed with a recently developed apparatus that is characterized by a high-precision gas supply system and a fine-dosed twin powder feeding system. Computer control provides a flexible programmed readjustment of the detonation gases energy impact on powder thus allowing selecting the optimal for each component spraying parameters to form composite and multilayered coatings. Several powders are sprayed to obtain composite coatings, specifically, among others, WC-Co-Cr + Al₂O₃, Cu + Al₂O₃, and Al₂O₃ + ZrO₂.     

Influence of annealing treatment on microstructure and properties of cold sprayed stainless steel coatings

304 stainless steel coatings had been deposited on carbon-steel substrate by cold spray technique, vacuum annealing treatment was applied to the coatings with different temperatures, and the influence of annealing treatment on the microstructure and electrochemical behavior of the coatings in 3.5% NaCl were analyzed. The results indicated that, the cold sprayed coating was constituted by the flattened particles, and the interfaces were clearly observed between the deposited particles. It was also found that...     

The influence of process parameters and heat treatment on the properties of cold sprayed silver coatings

Cold spray coating technology is a promising low temperature variant of thermal spray technology which can deposit pure, dense and thick coatings at a rapid rate. Unlike the other thermal spray coating techniques, cold spray is especially suitable for depositing coatings with high electrical and thermal conductivity as the integrity of the feedstock is maintained during the coating process. In the present study, the effect of process parameters and heat treatment on the properties of silver coatings has been investigated. An attempt has been made to correlate the powder particle velocity with the properties and microstructure of the coating. The effect of heat treatment temperature and atmosphere on the properties of the coatings, especially electrical conductivity, has been studied in detail in conjunction with a thorough analysis of the evolution of microstructure of the coatings.     

Study of oxidation behavior of nanostructured NiCrAlY bond coatings deposited by cold spraying

Nanostructured NiCrAlY bond coating was deposited using a milled powder by cold spraying. A shot-peening treatment was then applied to the as-sprayed coating to modify the coating surface morphology. The oxidation behavior of the coating with the as-sprayed surface and shot-peened surface was investigated under isothermal oxidation at 900 °C and 1000 °C for different times. The oxidation behavior of the coating was characterized through surface morphology and cross-sectional microstructure by scanning electron microscopy. It was found that a uniform oxide layer was formed on the surface of the shot-peened nanostructured NiCrAlY coating during oxidation at temperatures of 900 °C and 1000 °C. The surface morphology of the coating has significant effect on the morphology of the oxide. The surface geometry of the cold-sprayed MCrAlY coating must be modified to promote formation of a protective oxide film during oxidation, through application of a post-treatment process such as shot-peening.     

Indentation size effect on mechanical properties of HVOF sprayed WC based cermet coatings for a roller cylinder

The present paper concerns the determination of mechanical properties such as hardness, elastic modulus and yield strength of WC-based cermet coatings for a roller cylinder. With this regard, Co and Ni containing WC-based coatings were sprayed on Ni-Al deposited 316 L stainless steel substrates by using High Velocity Oxygen Fuel (HVOF) technique. These HVOF sprayed coatings were analyzed by Scanning Electron Microscopy (SEM) with an Energy Dispersive Spectroscopy (EDS) system attachment. Mechanical properties of the coatings were examined by Shimadzu Dynamic Ultra-micro hardness test machine in order to determine the Young's modulus through load-unload sensing analysis. In addition to mechanical investigation, hardness-depth and hardness-force curves of WC-based coatings were investigated. It was found that both of these characteristics exhibit significant peak load dependency. Experimental indentation studies were carried out to determine load-unload curves of WC-Co and WC-Ni based coatings under 300 mN, 350 mN, 400 mN and 450 mN applied peak loads. Hardness and Young's modulus of WC-based coatings were calculated from experimental indentation test data of samples. It has been observed that the hardness and Young's modulus of the coating depends on the contact area and indentation size. The originality of this study is to determine the indentation size effect and contact area variations on mechanical properties of HVOF sprayed WC-based coatings.     

Nanoindentation on the surface of thermally sprayed coatings

The ability to quantify surface mechanical properties is valuable for assessing the quality of thermal spray coatings. This is especially important for prostheses where loading is placed directly on the surface. Hydroxyapatite was classified to small (20-40 μm), medium (40-60 μm) and large (60-80 μm) particle sizes and thermal sprayed to produce a coating from spread solidified hydroxyapatite droplets. It was revealed for the first time, that nanoindentation can be successfully used to determine the hardness and elastic modulus on the surface of well spread solidified droplets at the hydroxyapatite coating surface. Comparison with indentation results from polished cross-section exhibited comparable values and statistical variations. The hardness was 5.8 ± 0.6, 5.4 ± 0.5 and 5.0 ± 0.6 GPa on coatings produced from small, medium and large sized powder. Similarly, the elastic modulus decreased from 121 ± 7, 118 ± 7 to 114 ± 7 GPa, respectively. Use of several indentation loads gave comparable results with sintered hydroxyapatite suggesting good inter-splat bonding within the coating. MicroRaman spectroscopy and X-ray diffraction confirmed a larger degree of dehydroxylation for the smaller particles also revealing a lower elastic modulus. This shows the influence of particle size and possibly dehydroxylation of hydroxyapatite on the mechanical properties of the coating surface.     

Towards engineering isotropic behaviour of mechanical properties in thermally sprayed ceramic coatings

It is widely recognized by the scientific community that thermal spray coatings exhibit anisotropic behaviour of mechanical properties, e.g., the elastic modulus values of the coating in-plane (i.e., parallel to the substrate surface) or through-thickness (i.e., perpendicular to the substrate surface) will tend to be significantly different due to their anisotropic microstructures. This work shows that thermally sprayed ceramic coatings may exhibit isotropic mechanical behaviour similar to that of bulk materials even when exhibiting the typical anisotropic coating microstructure. Elastic modulus values on the in-plane and through-thickness directions were measured via Knoop indention and laser-ultrasonic techniques on a coating produced via flame spray (FS) using a nanostructured titania (TiO₂) powder. No significant differences were found between the coating directions. In addition, four major cracks with similar lengths were observed originating near or at the corners of Vickers indentation impressions on the coating cross-section (i.e., a typical characteristic of bulk ceramics), instead of two major cracks propagating parallel to the substrate surface, which is normally the case for these types of coatings. It was observed by scanning electron microscopy (SEM) that coatings tended to exhibit an isotropic behaviour when the average length of microcracks within the coating structure oriented perpendicular to the substrate surface was about twice that of the microcracks aligned parallel to the substrate surface. Modelling, based on scalar crack densities of horizontal and vertical cracks, was also used to estimate when thermal spray coatings tend to exhibit isotropic behaviour.     

Effects of the powder manufacturing method on microstructure and wear performance of plasma sprayed alumina-titania coatings

Three Al₂O₃-13wt.% TiO₂ powders, with the same chemical composition but different Al₂O₃-TiO₂ distribution patterns, are plasma sprayed and the resulting coatings are compared in terms of their phase composition, microstructure, hardness, crack growth resistance, and abrasive wear performance. It is demonstrated that the degree of mixing of the Al₂O₃ and TiO₂ ingredients in the feed powder has immense impact on the phase composition, microstructure, hardness, crack growth resistance, and abrasive wear performance of the coatings. A high degree of mixing of Al₂O₃ and TiO₂ in the powder state results in more uniform microstructure, higher hardness, higher crack growth resistance, and consequently better abrasive wear resistance of the coating.     

等离子熔覆添加和内生联合WC颗粒增强铁基涂层的组织和性能

采用等离子熔覆技术,以铸造碳化钨、钨铁粉、镍包石墨和铁基合金粉为原材料,在Q235钢基体上制备了外加和内生联合WC颗粒增强铁基复合涂层,通过扫描电镜和能谱分析、X射线衍射、硬度测试和磨料磨损试验对其微观组织、物相组成、硬度和耐磨性能进行了表征。结果表明,在优化的工艺参数下,可以获得与基体冶金结合良好的涂层,硬质相除外加的WC颗粒,还有内生的WC、W₂C、W₃C、Fe₃W₃C和Fe₂W₂C等;随着混合粉末中除外加WC之外的W含量增加,熔池中合金液密度增大,可以减弱外加WC颗粒下沉;当W含量达到15%时,外加WC颗粒均匀分布在涂层中,没有团聚现象发生,且在外加WC颗粒周围有细小的原位WC颗粒生成,涂层的显微硬度和耐磨损性能显著提高,涂层的平均硬度约为1300 HV0.2,耐磨性为Q235钢基体的10倍。