目的 以超音速火焰喷涂WC-Ni涂层的氧气/煤油化学计量比λ为特征工艺参数,研究工艺参数对涂层组织结构与力学特性的影响机理。方法 固定λ值为1.05,调节煤油与氧气流量,在相同喷距下实现WC-Ni粒子沉积温度相近、沉积速度不同的条件,制备涂层并分析其孔隙率、相结构及显微硬度、弹性模量、断裂韧性的变化规律。结果 当煤油流量从19.3 L/h增至23.8 L/h,喷枪燃烧室压力从0.66 MPa增至0.76 MPa时,在喷距350 mm处粒子温度为(1 682±6) ℃,粒子平均速度从847 m/s增至916 m/s。随着粒子沉积速度的增加,涂层表面和横截面的微观孔隙率呈下降趋势,且表面孔隙率低于横截面;WC相保留率呈上升趋势,从71.4%增至82.3%,其氧化分解产物金属W含量（质量分数）从9.5%降至3.4%。涂层表面与横截面显微硬度值均随粒子沉积速度的增加而增大,范围为7.8~10.3 GPa,且横截面与表面硬度值逐渐趋于一致,弹性模量从313 GPa增至405 GPa。此外,采用大载荷维氏硬度计测量涂层表面断裂韧性,结合压痕尖端与棱边裂纹特征,分析了喷涂粒子沉积速度与涂层断裂力学行为的相关性。结论 在该WC-Ni粒子沉积温度与速度区间内,更高的粒子沉积速度增加了沉积撞击动能,使沉积粒子扁平化变形更加充分,显著减少了扁平粒子间界面微观缺陷,提高了涂层致密度及粒子间结合强度;飞行粒子速度的增加缩短了其在焰流中的加热时间,减少了WC相的氧化分解,使涂层中WC相保留率升高;这些因素共同促进了涂层显微硬度和弹性模量的提升。
Abstract
The thermal spraying process of WC-Ni coatings is investigated with the Ni bonding phase instead of Co, considering the application requirements of cemented carbide coatings in corrosive medium conditions of hydraulic mechanical equipment such as pumps and valves. To solve the complex problem of multi-objective optimization caused by the coupling of high-velocity oxy-fuel (HVOF) spraying process parameters and spraying materials, particle online monitoring equipment is used to derive the correlation between the high-temperature particle deposition velocity and the microstructure and mechanical properties of the coatings. The oxygen/kerosene stoichiometric ratio λ of HVOF spraying WC-Ni coatings is taken as the characteristic process quantity to study the effect of the process parameters on the microstructure and mechanical properties of the coatings. The kerosene and oxygen flow rate parameters are adjusted while keeping λ at a constant value of 1.05 to obtain coatings under different deposition velocities with similar deposition temperature of WC-Ni particles, and to analyze the change rule of porosity, phase structure, microhardness, elastic modulus and fracture toughness of the coatings. As the kerosene flow rate increases from 19.3 L/h to 23.8 L/h, the combustion chamber pressure of the spray gun increases from 0.66 MPa to 0.76 MPa, the average particle velocity increases from 847 m/s to 916 m/s with an average particle temperature about (1 682± 6) ℃ under the same spray distance of 350 mm. The particle velocity enhancement leads to reduction in the porosity of the coating surface and cross section, with the lower surface porosity values to the corresponding cross-sectional ones. The retention ratio of the WC phase increases from 71.4% to 82.3%, and the oxidation decomposition product metal W decreases from 9.5% to 3.4%. Accordingly, the microhardness values in the range of 7.8-10.3 GPa obtained from both the coating's surface and cross section increase as the particle velocity rises with a convergence tendency of the hardness values at higher velocities; while the elastic modulus of the coatings increases from 313 GPa to 405 GPa. In addition, the fracture toughness of the coating is tested with a large load Vickers hardness tester, and the correlation between the sprayed particle velocity and the fracture mechanical behavior of the coating is analyzed in conjunction with the different cracking characteristics of the indentation tips and edges. In the range of WC-Ni particle deposition velocity and temperature of this study, the faster particle has the higher impact kinetic energy for the deposition, which causes a sufficient flattening deformation of deposited particles and thus significantly reduces the interfacial defects between the flattened particles, increases the density of the coating, and improves the bonding strength between the deposited particles. At the same time, the increase in the velocity of in-flight particles shortens its heating time in the flame, reducing the oxidative decomposition of the WC phase and resulting in higher retention of the WC phase in the deposited coating. These two aspects contribute to the enhancement of the microhardness and elastic modulus of the coatings. The intrinsic correlation between the spraying process parameters and the surface integrity parameters of coated parts established in this study can provide a basis for process optimization and online monitoring for high-performance manufacturing of cemented carbide coated parts.
关键词
超音速火焰喷涂 /
WC-Ni涂层 /
喷涂粒子速度 /
表面完整性 /
涂层力学性能
Key words
high-velocity oxy-fuel spraying /
WC-Ni coating /
spray particle velocity /
surface integrity /
coating mechanical properties
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基金
国家自然科学基金面上项目（52371054）;国家自然科学基金重点项目（U21B2078）
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