Parametric study of an HVOF process for the deposition of nanostructured WC-Co coatings

Nanocrystalline WC-Co coatings were deposited by high velocity oxyfuel from commercial nanostructured composite powders. Processing parameters were optimized for maximal retention of the nanocrystalline size and for minimal decarburation of the ceramic reinforcement. Thermochemical and gas-dynamical properties of gas and particle flows within the combustion flame were identified in various operating conditions by computational fluid-dynamics (CFD) simulation. Significant improvements of the mechanical properties of the coatings were obtained: a decrease of the friction coefficient was measured for the nanostructured coatings, together with an increase of microhardness and fracture toughness.     

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。     

Effect of flame conditions on abrasive wear performance of HVOF sprayed nanostructured WC-12Co coatings

Nanostructured WC-12Co coatings were deposited by high velocity oxy-fuel (HVOF) spraying with an agglomerated powder. The effect of flame conditions on the microstructure of the nanostructured coatings was investigated. The wear properties of the coatings were characterized using a dry rubber-wheel wear test. The results show that the nanostructured WC-Co coatings consist of WC, W2C, W and an amorphous binder phase. The microstructure of the coating is significantly influenced by the ratio of oxygen flow to fuel flow. Under the lower ratio of oxygen/fuel flow, the nanostructured coating presents a relative dense microstructure and severe decarburization of WC phase occurs during spraying. With increasing ratio of oxygen/fuel flow, the bonding of WC particles in the coating becomes loose resulting from the original structure of feedstock and the decarburization of WC becomes less owing to limited heating to the powder. Both the decarburization of WC particles in spraying and the bonding among WC particles in the coatings affect the wear performance. The examination of the worn surfaces of the nanostructured coatings reveals that the dominant wear mechanisms would be spalling from the interface of WCCo splats when spray particles undergo a limited melting. While the melting state of the spray particles is improved,the dominant wear mechanisms become the plastic deformation and plowing of the matrix and spalling of WC particles from the matrix.     

Thermally sprayed wear resistant coatings with nanostructured hard phases

The aim of this investigation was the development of a new quality of thermally sprayed coatings with high resistance against wear and corrosion and to evaluate the application potential of nano-sized hard phases in thermally sprayed layers. The newly developed material consists of a highly corrosion-resistant matrix of stainless steel (even without nickel) combined with nano-structured hard phases of vanadium nitrides (VN). On the other hand, matrices consisting of cobalt-chromium (CoCr) with submicron hard phases of tungstencarbides (WC) were investigated with respect to microstructure and wear resistance compared with conventional ones.     

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.     

Formation of an amorphous phase in thermally sprayed WC-Co

A WC-Co coating was sprayed by the high-velocity oxyfuel process using a feedstock of tungsten carbide clad with cobalt. The structure of the sprayed coating was characterized by x-ray diffraction (XRD), differential scanning calorimetry (DSC), and differential thermal analysis (DTA). It was found that an amorphous phase of Co-W-C ternary alloy observed as a large, broad peak in the XRD pattern can be formed in the as-sprayed WC-Co coating. The DSC, DTA, and XRD analyses revealed that the amorphous phase crystallized at a temperature of around 873 K to metallic cobalt, Co₆W₆C, and tungsten with appreciable precipitation of free carbon. The heat treatment of as-sprayed WC-Co coating at a high temperature of 1173 K suggests that annealing at a temperature higher than about 1104 K will promote the reaction of tungsten and cobalt with carbon to form the complex carbide C₀₆W₆C.     

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。     

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.     

超音速火焰喷涂碳化钨-钴涂层磨粒磨损行为

采用超音速火焰喷涂工艺在16Mn钢上制备了WC-12Co涂层,并测试了该涂层的力学性能特别是其抗磨粒磨损性能。结果表明:WC-12Co涂层的主相为碳化钨,显微硬度为(1341.0±134.3)HV,孔隙率为0.21%±0.04%。该涂层的磨损率随着磨粒硬度、磨粒粒度和加载载荷的增加而增加。当磨粒的硬度低于涂层硬度时,涂层的磨损机制以磨耗磨损为主,磨损率低;当磨粒的硬度超过涂层的硬度时,涂层的磨损以微切削为主,磨损率高。另外,WC-12Co涂层的耐磨性相对于16Mn钢也是随着磨粒的硬度变化而变化。     

粉末结构对HVOF喷涂WC-Co涂层组织性能的影响

选用4种不同WC尺度的WC-12Co粉末作为初始喂料,通过超音速火焰喷涂系统(HVOF)制备了涂层。考察了不同粉末结构对涂层沉积过程的脱碳行为和涂层组织性能的影响。结果表明:WC颗粒尺寸减小加剧了涂层脱碳行为,涂层中W2C含量增加,粘结相非晶化现象明显,涂层硬度增加,但是当WC颗粒尺寸减小到纳米尺度时,韧性下降。双峰结构涂层表现出最好的韧性同时兼备较高的硬度。     

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.     

Microstructure and sliding wear behavior of nanostructured Ni60-TiB2 composite coating sprayed by HVOF technique

A nanostructured Ni60-TiB₂ composite coating (Ni60 is a brand of Ni-based self-fluxing alloy with a hardness of HRC60) was sprayed on steel substrate by high velocity oxy-fuel (HVOF) process using high energy ball milled powders. Its sliding wear resistance at room-temperature was evaluated by ball-on-disc testing. For comparison, conventional Ni60-TiB₂ composite coating was prepared by HVOF using mechanically mixed Ni60 and TiB₂ powders and tested under the same conditions. The results show that the nanostructured composite coating has excellent mechanical properties and sliding wear resistance due to the microstructural homogenization and the well preserved nanostructure characteristic of the ball milled powders. Adhesive and abrasive wears are found to be responsible for the wear down mechanisms of the nanostructured Ni60-TiB₂ composite coating.     

Study of the influence of microstructural properties on the sliding-wear behavior of HVOF and HVAF sprayed WC-cermet coatings

The microstructural properties of WC-Co-Cr and WC-Co coatings deposited by high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) processes were investigated. The tribological behavior of the coatings was studied by means of pin-on-disk tests. Microcracking of the HVOF sprayed WC-Co coatings did not allow preparation of suitable disks for wear tests. The wear rates of the remaining coatings were determined, and wear tracks on the coatings and counterbodies were investigated by SEM. The HVAF sprayed coatings showed greater sliding-wear resistance compared to the HVOF coatings. The prime wear mechanism in the WC-Co HVAF coatings was adhesive wear. The cobalt matrix is lubricious, resulting in very low wear rates and low debris generation. The main wear mechanisms in the WC-Co-Cr coatings were adhesive and abrasive wear. Adhesive wear results in coating material dislodgments (i.e., “pullouts”) that become trapped in the contact zone and act as a third-body abrasive. Particle pullout from the coating significantly increases the wear rate of the coated specimen. The HVAF/WC-Co-Cr coatings exhibited better resistance to particle pullout, resulting in a considerably lower wear rate than the HVOF/WC-Co-Cr coatings.     

超细WC-Co硬质合金及其磨损性能研究

采用低温化学镀方法在超细WC颗粒表面进行金属钴包覆,烧结包覆后的复合粉体制备新型硬质合金NYG（WC-3%Co）.研究了超细WC-Co硬质合金的力学性能、断口形貌和显微结构,在销盘式磨损试验机上进行干滑动磨损实验.结果表明,在硬质合金烧结过程中,沿WC晶界均匀分布的金属钴不仅起粘结剂作用,也起抑制剂作用阻碍晶粒的长大;新型硬质合金的抗弯强度、断裂韧性、硬度和耐磨性能均得到较大提高;在干滑动摩擦条件下,新型WC-Co硬质合金的失效以塑性变形及细小碳化钨相颗粒脱落为特征.     

Duplex coating of electroless nickel and HVOF (high-velocity oxygen fuel) sprayed WC-Co

The porosity of thermal sprayed coatings is usually a problem when coatings are used in wet corrosion application. The porosity allows media to penetrate to the surface of the base material. Corrosion spreads rapidly and the coating loses contact with the substrate and delaminates. This problem can be initiated by impregnating different polymers into the pores. An alternative approach has been tested in the present work to prevent corrosion of cemented carbide coated carbon steel in wet corrosion environments. Carbon steel substrates were coated with a thin film of electroless nickel (electroless nickel plating) and then HVOF (High-Velocity Oxygen Fuel) sprayed with cemented carbide. Reference specimens without electroless nickel were sprayed at the same time. The microhardness of the specimens was measured and the coating structure examined using optical microscopy and X-ray diffractometry (XRD). The bond between the layers and the base material was examined by means of a bend test. A salt chamber test was also performed for the specimens. The structure of the electroless nickel layer was crystalline as a result of the HVOF spraying. There were no cracks in the nickel layer, if the layer was about 20 μm thick. According to the results of the bend test, the adhesion between coatings and substrate was good, and there was no difference between the duplex-coated specimen and the reference specimen. A sample with a thin nickel layer under an HVOF sprayed cemented carbide did not exhibit corrosion after 8 h in the salt chamber test.     

NiCoCrAlYTa/WC-Co复合涂层的制备及摩擦学性能研究

目的 为了改善MCrAlY涂层的耐磨损性能,通过在NiCoCrAlYTa粉末中添加不同比例的硬质相WC-Co粉末（质量分数为25%、50%、75%）,将2种粉末充分地机械混合、振荡均匀后,采用超音速火焰喷涂（HVOF）技术,制备不同配比的NiCoCrAlYTa/WC-Co复合涂层。方法 利用SEM、XRD、EDS等分析了复合涂层的微观形貌、物相组成和元素分布规律等;研究该复合涂层的力学性能、摩擦学性能以及摩擦磨损机理等。结果 采用HVOF技术制备的Ni CoCrAlYTa/WC-Co复合涂层结构致密,各元素及物相分布均匀;硬质相WC-Co的添加提高了涂层的显微硬度,同时也可显著改善复合涂层的耐磨损性能;复合涂层的摩擦因数随着WC-Co含量的增加逐渐增大,而磨损率逐渐减小。当WC-Co的添加量为75%时,复合涂层的摩擦因数最大,约为0.84;磨损率最小,约为9.28×10^-6 mm³/（N·m）。结论 在金属基涂层中引入硬质相WC-Co可有效提高涂层的硬度,并且提升该涂层的耐磨损性能,为金属基涂层发挥优异的摩擦学性能提供理论基础。     

Effect of ion nitriding on the abrasive wear resistance of ultrahigh-strength steels with different silicon contents

This article studies the effect of silicon (Si) on ultrahigh-strength AISI 4340 steels in connection with the thermal treatment, as well as the influence of this element on nitriding and, consequently, abrasive wear. Four alloys with different Si contents were nitrided at 350 °C (4 and 8 h) and 500 and 550 °C (2 and 4 h) in a gas mixture of 80 vol.% H₂ and 20 vol.% N₂. The nitrided layers were characterized by microhardness and pin-on-disk tests, optical microscopy, scanning electron microscopy with energy-dispersive x-ray spectrometry, and x-ray diffraction (XRD). The increase in Si enhanced the tempering resistance of the steels and also improved considerably the hardness of the nitrided layers. The increase in Si produced thinner compound layers with better hardness quality and high abrasive wear resistance. XRD analysis detected a mixture of nitrides in the layers γ′-Fe₄N, ε-Fe_2–3N, CrN, MoN, and Si₃N₄ with their proportions varying with the nitriding conditions.     

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.     

Influences of the compositions and mechanical properties of HVOF sprayed bimodal WC-Co coating on its high temperature wear performance

In this work, the bimodal WC-Co coatings were sprayed by high-velocity oxygen-fuel (HVOF), and the conventional WC-Co coatings were also fabricated for comparison. The microstructure, mechanical properties and high temperature wear performance were investigated. The bimodal WC-Co coating presented denser structure (porosity lower than 1.0%), higher average hardness (1164 HV0.1) and fracture toughness (11.5 ± 1.4 MPa·m^1/2) than that of conventional coating. The Weibull analysis of microhardness data of the bimodal coating presents a mono-modal distribution. The friction coefficient and wear rate of the bimodal coating were 0.61 and 2.96 × 10^− 6 mm³·N^− 1·m^− 1, respectively, which is lower than that of conventional coating at the test temperature of 450 °C. The tribofilm could be formed on the worn surface of bimodal WC-Co coating, which is composed of WO₃ and CoWO₄. The formation of tribofilm could reduce friction and wear.     

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.