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.     

Evaluation of four high velocity thermal spray guns using WC-10% Co-4% Cr cermets

Four high velocity thermal spray guns were evaluated in the production of 10% Co-4% Cr tungsten carbide (WC) cermets. Three high velocity oxygen fuel guns (JP-5000, JP-5000ST, and Diamond Jet [DJ]-2700) and one plasma gun (Axial III) were used to spray the same angular, agglomerated, and crushed WC-10Co-4Cr powder. The DPV-2000 was used to monitor the in-flight velocity and temperature of the WC cermet-sprayed particles. From those values, spray conditions were selected to produce coatings that were evaluated in terms of porosity, hardness, and deposition efficiency. Results show that the plasma Axial III provides the highest particle temperature, between 2000 °C and 2600 °C, depending on the spray conditions. The JP-5000 imparts the highest velocity to the particles, between 550 and 700 m/s, depending on the spray conditions. The ST version of the JP-5000 provides the same velocity as the standard version but with lower particle temperature. The DJ-2700 sprays particles with temperature and velocity between those of the JP-5000 and the Axial III. Minimum porosity values of 2.1%, 3.7%, and 5.3%, respectively, were obtained for the JP-5000, the DJ-2700, and the Axial III guns. The porosity and carbide degradation are found to depend mostly on the particle velocity and temperature, respectively. The values for the Vickers microhardness number (200g) ranged from 950 to 1250. Measurements of the deposition efficiency indicated a variation between 10% and 80%, depending on the spray conditions and the gun used.     

Microstructure and properties of flame sprayed tungsten carbide coatings

This article reports on feasibility experiments carried out with oxy-acetylene spray system with various oxygen to fuel ratios using two different tungsten carbide powders and powder feeding methods, to evaluate the newly developed fused WC, synthesised by transferred arc thermal plasma method. Transferred arc thermal plasma method is more economical and less energy intensive than the conventional arc method and results in a fused carbide powder with higher hardness. The microstructure and phase composition of powders and coatings were analysed by optical and scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Carbon content of the powders and coatings were determined to study the decarburisation of the material during spraying process. Coatings were also characterised by their hardness and abrasive wear. The effects of metallurgical transformation and phase content are related to wear performance. The results demonstrate that the powders exhibit various degree of phase transformation during the spray process depending on the type of powder, powder feeding and spray parameters. The carbon loss during the spray process in excess of 45% resulted in reduced hardness and wear resistance of the coatings. Coatings with high amount of WC and W₂C along with FeW₃C showed higher wear resistance. Thus, coatings of high wear resistance can be produced using fused tungsten carbide powder with WC and W₂C phases, which can be economically synthesised by thermal plasma transferred arc method.     

面向等离子体材料钨厚涂层的制备及表征

等离子体喷涂技术在面向等离子体材料钨涂层的制备中占据主导地位,本实验采用CuMo/MoW作为涂层的中间过渡层,分别以结晶钨粉和羰基钨粉为原料,用大气等离子体喷涂技术在CuCrZr合金基体(110 mm×130mm)上制备了3-4 mm厚的3种钨涂层.对钨涂层微观组织、力学性能和热学性能研究表明,羰基钨粉制备的钨涂层的综合性能优于结晶钨粉,且薄涂层的结合强度优于厚涂层.优化喷涂工艺后,金相法测得钨涂层孔隙率＜2％,涂层的结合强度最大值为10 MPa,EDS测得氧含量为6％左右,纯钨层热导率最大值为12.52 W/(m.K),涂层氧含量过高导致涂层热导率显著降低.研究表明采用大气等离子体喷涂技术在铜合金上制备3～4mm厚的钨涂层是可行的,该技术可为下一步低成本、高性能厚钨涂层的制备奠定基础.     

A design of experiment study of plasma-sprayed alumina-titania coatings

This article presents an experimental study of the plasma spraying of alumina- titania powder. This powder system is being used to fabricate heater tubes that emulate nuclear fuel tubes for use in thermal-hydraulic testing. Coating experiments were conducted using a Taguchi fractional- factorial design parametric study. Operating parameters were varied around the typical spray parameters in a systematic design of experiments to display the range of plasma processing conditions and their effect on the resultant coating. The coatings were characterized by hardness and electrical tests, image analysis, and optical metallography. Coating qualities are discussed with respect to dielectric strength, hardness, porosity, surface roughness, deposition efficiency, and microstructure. The attributes of the coatings are correlated with the changes in operating parameters.     

WC-based composite coatings prepared by the pulsed gas dynamic spraying process: Effect of the feedstock powders

WC-based cermet coatings are typically produced using the HVOF process, due to high particle velocity and the lower heating characteristic of this technique. Despite the effort of optimisation of the coating process, degradation of the feedstock materials such as decarburisation of WC and amorphization of the metallic phase still occurs. It is known that the coating properties do not depend only on the spray process and its parameters, but also on the feedstock powder characteristics such as its chemistry, carbide size, particle morphology and production method. The work presented here is part of a research program aimed at exploring the possible advantages of the Pulsed Gas Dynamic Spray (PGDS) process, as an alternative technique for the preparation WC-based cermet coatings. In this paper, WC-based coatings have been prepared using six different types of cermets powders. In order to study the effects of the feedstock powder on the coatings microstructures and hardness, the selected starting powders differed not only in microstructural features such as size and morphology but also in the chemistry and phases. Using different analysis technique (OM, SEM, XRD, and HV), a detailed comparison of powders and coatings microstructures, phase compositions, and hardness are presented and discussed in detail. It was found that the PGDS process preserves the microstructure of the starting cermet powders in such a way that no significant degradation of the phase composition, even those that show the pre-existence of complex carbides, has been observed. Furthermore, although the same spray parameters were used, the thickness, deposition efficiency, porosity, and micro-cracks within the coatings are different from one type of cermet to another, suggesting that PGDS optimum process parameters are material dependant.     

等离子喷涂参数对Ta／W／Sn复合涂层组织性能的影响

采用等离子喷涂方法在LY12铝上制备了Ta/W/Sn复合涂层。研究了各种工艺参数对涂层材料熔化程度和沉积效率的影响。采用扫描电子显微镜观察了复合涂层的表面及界面，精密机械天平称量涂层的增重，力学拉伸机测量涂层的抗拉结合强度，能谱和化学方法分析涂层的化学成分。结果表明，等离子喷涂工艺可以制作组织致密，厚度均匀的Ta/W/Sn复合涂层；喷涂功率和送粉速率对材料的熔化状态影响较大，在特定喷距范围内喷距的影响则较小。     

WC-based cermet coatings produced by cold gas dynamic and pulsed gas dynamic spraying processes

Due to their mechanical properties, WC-based cermet coatings are extensively used in wear-resistant applications. These coatings are usually produced using thermal spray processes. However, due to the nature and the environment of these spraying processes, the feedstock powder structure and properties suffer from decomposition, which subsequently degrade the performance of the coatings produced. The cold gas dynamic spraying process appears to be a promising alternative technique to preserve the properties of the feedstock powder during the coating preparation. Although the latter technique can minimize or eliminate the degradation of the sprayed material, the deposition of cermet using this technique is a difficult task. In this study, two types of cermet powders, the nanocrystalline (WC-15Co) and the conventional (WC-10Co4Cr) powders were deposited using the cold gas dynamic spraying and the pulsed gas dynamic spraying processes. The feedstock powders and coatings microstructures were investigated by OM, SEM and XRD, as well as their hardness. The results revealed the possibility of depositing cermet coatings onto aluminum substrates using both processes without any degradation of the carbide phase of the feedstock powder. The cold gas dynamic spraying process experienced difficulty in depositing and building up dense coatings without major defects. The pulsed gas dynamic process produced thick cermet (conventional and nanocrystalline) coatings with low porosity as long as the feedstock powder was preheated above 573 K.     

Cold spraying of Al-Sn binary alloy: Coating characteristics and particle bonding features

In this study, Al-Sn binary alloy coatings were prepared with Al-10 wt.% Sn (Al-10Sn) and Al-20 wt.%Sn (Al-20Sn) gas atomized alloy powders by low pressure cold spray process. The microstructure and microhardness of the coatings were characterized. The deposition efficiency of Al-Sn and pure aluminum powder were tested on sand blasted substrates of Al6061, copper and SUS304. To investigate the particle bonding features of Al-10Sn powder, the wipe test was carried out on polished substrate surfaces. The average critical velocity of Al-Sn alloy powders was estimated combining with the deposition efficiency. The results proved that Al-Sn coatings with dense and uniform structure can be deposited successfully by low pressure cold spray with helium as the propellant gas. The weight content of tin in Al-Sn coating is 12% and 22% for Al-10Sn and Al-20Sn coating, respectively. With the increase of tin content in feedstock powder, the as-sprayed coatings present similar microhardness of 73 and 74 for Al-10Sn and Al-20Sn while, on the other hand, the deposition efficiency decreased. Bonding features of Al-10Sn particles indicated that the melting of tin phase occurred on the contact interface during particle impacting. This melting of tin phase and low strength of tin may affect the particle bonding process and hence increase the critical velocity for Al-Sn binary alloy powders.     

Plasma spraying of combustion flame spheroidized hydroxyapatite (HA) powders

Tailoring powder characteristics to suit the plasma spray process can alleviate difficulties associated with the preparation of hydroxyapatite (HA) coatings. Commercial HA feedstock normally exhibit an angular morphology and a wide particle size range that present difficulties in powder transport from the powder hopper to the plasma spray gun and in nonuniform melting of the powders in the plasma flame. Hence, combustion flame spheroidized hydroxyapatite (SHA) was used as the feedstock for plasma spraying. Spherical particles within a narrow particle size range are found to be more effective for the plasma spray processes. Results show coatings generated from spheroidized HA powders have unique surface and microstructure characteristics. Scanning electron microscope (SEM) observation of the coating surface revealed well-formed splats that spread and flatten into disc configurations with no disintegration, reflecting adequate melting of the HA in the plasma and subsequent deposition consistency. The surface topography is generally flat with good overlapping of subsequent spreading droplets. Porosity in the form of macropores is substantially reduced. The cross-section microstructure reveals a dense coating comprised of randomly stacked lamellae. The tensile bond strengths of the SHA coatings, phase composition, and characteristics of the coatings generated with different particle sizes (125 to 75 μm, 45 to 75 μm, 20 to 45 μm, and 5 to 20 μm) showed that a high bond strength of ∼16 MPa can be obtained with SHA in the size range from 20 to 45 μm. This can be improved further by a postspray treatment by hot isostatic pressing (HIP). However, larger particle size ranges exhibited higher degrees of crystallinity and relatively higher HA content among the various calcium phosphate phases found in the coatings.     

Effect of Helmholtz Oscillation on Auto-shroud for APS Tungsten Carbide Coating

The atmospheric-pressure plasma spray (APS) of tungsten coating was performed using tungsten carbide (WC) powder by means of DC plasma torch equipped with a stepped anode nozzle as a potential method of W coating on graphite plasma-facing component of fusion reactors. This nozzle configuration allows Helmholtz oscillation mode dominating in APS arc fluctuation, and the variation of auto-shroud effect with Helmholtz oscillation characteristics can be investigated. Tungsten coating made from WC powder has lower porosity and higher tungsten purity than that made from pure tungsten powder. The porosity and chemical composition of coatings were investigated by mercury intrusion porosimetry and x-ray photoelectron spectroscopy, respectively. The purity of tungsten coating layer is increased with the increasing frequency of Helmholtz oscillation and the increasing arc current. The modulation of Helmholtz oscillation frequency and magnitude may enhance the decarburization of WC to deposit tungsten coating without W-C and W-O bond from WC powder.     

Influence of Powder Porous Structure on the Deposition Behavior of Cold-Sprayed WC-12Co Coatings

The limited deformation of hard cermet particles and impacted coating makes it difficult for conventional thermal spray powders to continuously build up on impact in cold spraying. In this study, three nanostructured WC-12Co powders with different porous structure and apparent hardness were employed to deposit WC-Co coatings on stainless steel substrate by cold spraying. The deposition characteristics of three powders of porosity from 44 to 5% were investigated. It was found that WC-Co coating is easily built-up using porous powders with WC particles bonded loosely and a low hardness. The microhardness of WC-12Co coatings varied from 400 to 1790 Hv with powders and spray conditions, which depends on the densification effects by impacting particles. With porous WC-Co powders, the fracture of particles on impact may occur and low deposition efficiency during cold spraying. The successful building up of coating at high deposition efficiency depends on the design of powder porous structure.     

HVOF Combustion spraying of inconel powder

A major trend in the thermal spray industry has been to increase the gas jet velocity to obtain better coating attributes. One emerging technology now used in industry is the high-velocity oxygen fuel process (HVOF). High-velocity spray guns combine oxygen and a fuel gas to generate heat and extremely high particle velocities. In this study, Inconel 718 powder was deposited on steel substrates. The primary coating function was electrical resistivity for a heater application. Experiments were conducted using a Taguchi L8 statistical fractional/factorial design parametric study. The Taguchi experiment evaluated the effect of six HVOF processing variables on the measured responses. The parameters were oxygen flow, fuel flow, air envelope gas flow, powder feed rate, spray distance, and nozzle configuration. The coatings were characterized by hardness tests, surface profilometry, optical metallography, and image analysis. This article investigates coating hardness, porosity, surface roughness, deposition efficiency, and microstructure with respect to the influence of the processing parameters. Analytical studies were conducted to investigate gas, particle, and coating dynamics for two of the HVOF thermal spray experiments.     

Effects of Anode Arc Root Fluctuation on Coating Quality During Plasma Spraying

To obtain a coating of high quality, a new type of plasma torch was designed and constructed to increase the stability of the plasma arc and reduce the air entrainment into the plasma jet. The torch, called bi-anode torch, generates an elongated arc with comparatively high arc voltage and low arc fluctuation. Spraying experiments were carried out to compare the quality of coatings deposited by a conventional torch and a bi-anode torch. Alumina coatings and tungsten carbide coatings were prepared to appraise the heating of the sprayed particles in the plasma jets and the entrainment of the surrounding air into the plasma jets, respectively. The results show that anode arc root fluctuation has only a small effect on the melting rate of alumina particles. On the other hand, reduced air entrainment into the plasma jet of the bi-anode torch will drastically reduce the decarbonization of tungsten carbide coatings.     

Desirability-Based Multi-Criteria Optimization of HVOF Spray Experiments to Manufacture Fine Structured Wear-Resistant 75Cr3C2-25(NiCr20) Coatings

Thermal spraying of fine feedstock powders allow the deposition of cermet coatings with significantly improved characteristics and is currently of great interest in science and industry. However, due to the high surface to volume ratio and the low specific weight, fine particles are not only difficult to spray but also show a poor flowability in the feeding process. In order to process fine powders reliably and to preserve the fine structure of the feedstock material in the final coating morphology, the use of novel thermal spray equipment as well as a thorough selection and optimization of the process parameters are fundamentally required. In this study, HVOF spray experiments have been conducted to manufacture fine structured, wear-resistant cermet coatings using fine 75Cr₃C₂-25(Ni20Cr) powders (?8 + 2 μm). Statistical design of experiments (DOE) has been utilized to identify the most relevant process parameters with their linear, quadratic and interaction effects using Plackett-Burman, Fractional-Factorial and Central Composite designs to model the deposition efficiency of the process and the majorly important coating properties: roughness, hardness and porosity. The concept of desirability functions and the desirability index have been applied to combine these response variables in order to find a process parameter combination that yields either optimum results for all responses, or at least the best possible compromise. Verification experiments in the so found optimum obtained very satisfying or even excellent results. The coatings featured an average microhardness of 1004 HV 0.1, a roughness Ra = 1.9 μm and a porosity of 1.7%. In addition, a high deposition efficiency of 71% could be obtained.     

新型大气长层流等离子体喷涂方法和研究进展

介绍了一种新型大气等离子喷涂方法,该方法采用特殊内通道结构的直流非转移电弧等离子发生器,可以直接在大气条件下获得长度100～1000 mm之间变化的等离子射流。在大气条件下,等离子射流的流动特性具有"长、直、准"的层流或类层流状态,工作时噪音小于80 dB。在工作参数范围内,等离子射流的长度在固定总气流量条件下可以随输出功率的增加而增长;射流的长度在固定输出功率的条件下随总气流量的增加而减小。当使用在大气等离子喷涂技术中时,会为飞行粉末颗粒带来超长的加热和加速过程。文中详细介绍了大气层流等离子喷涂技术的研究历史和研究现状,以及研究团队利用该新型技术制备的6种涂层的显微结构、颗粒的飞行和加热特点,并对比了目前其他大气等离子喷涂技术的结果。结果表明,文中介绍的方法在最低的输出功率和气流量条件下,为金属和陶瓷颗粒提供了超长的飞行和加热条件,表现为较低的颗粒飞行速度和超高的颗粒表面温度。可以在不同的射流长度或喷涂距离下,获得不同的颗粒熔化状态或涂层结构,并发现可以直接在大气条件下获得大规模气液共沉积的涂层。     

节能,高效等离子喷涂设备的开发和喷涂组织

分析了现在常用的４０～１００ｋｗ等离子体喷涂设备的特征,从提高等离子喷涂效率和节能出发,开发了一种内部粉末供给,电能消耗为２－１３ｋｗ的Ｐｌａｓｍａ－ＬＥ１５等离子体喷涂系统,利用该系统对陶瓷,金属碳化物和金属合金粉末进行了喷涂实验,其中金属碳化物和金属合金粉末的喷涂沉积效率８０％以上,对几种喷涂组织进行了测试和调查,用该系统喷涂得到的组织性能高于或者等于通常大气条件下外部送粉式４０ＫＷ等离子喷涂     

瓷罗拉等离子喷涂防静电耐磨损涂层工艺

目的 提高纺织行业瓷罗拉转动轴的防静电和耐磨损性能。 方法 采用等离子喷涂技术,在瓷罗拉表面先喷涂 Ni 包 Al 过渡层,再喷涂 Al₂O₃-TiO₂ 陶瓷层,获得防静电耐磨复合涂层,并优化等离子喷涂工艺。 结果 获得了优化的等离子喷涂工艺,在后续的磨削加工中,宜选用碳化硅砂轮。 结论 制备的防静电耐磨涂层性能较好,在瓷罗拉上得到了成功应用。     

Plasma spray forming of tungsten coatings on copper electrodes

Both direct current dc plasma and radio frequency induction plasma were used to deposit tungsten coatings on copper electrodes. Fine tungsten powder with mean particle size of 5 um and coarse tungsten powder with particle size in the range from 45 um to 75 um were used as plasma spray feedstock. It is found that dc plasma is only applicable to spray the fine tungsten powder and induction plasma can be used to spray both the coarse powder and the fine powder. The tungsten coating deposited by the induction plasma spraying of the coarse powder is extremely dense. Such a coating with an interlocking structure and an integral interface with the copper substrate demonstrates high cohesion strength and adhesion strength.     

火焰喷涂合成TiC-Fe涂层的热力学分析

火焰喷涂合成技术是自蔓延高温合成（ＳＨＳ）与氧乙炔火焰喷涂技术结合而发展起来的,已成功制备了ＴｉＣ－Ｆｅ复合涂层,利用计算相图技术,确定Ｆｅ－Ｔｉ－Ｃ系的平衡相组成,并结合火焰喷涂合成工艺,重点讨论了原材料配比对体系熔化,凝固行为及涂层形成的影响,并讨论了燃烧合成ＴｉＣ－Ｆｅ的反应机理。