【作者】 周小平；

【导师】 华林；

【作者基本信息】 武汉理工大学 ， 车辆工程， 2006， 博士

【摘要】 采用真空粉末烧结的方法，在钢铁材料表面制备硬质覆层是近年来发展的一项表面处理和材料复合新技术。这种方法是将混合后的粉末，涂敷在钢铁材料的表面，经过真空烧结后获得硬质覆层。它的显著优点是制备的覆层厚度较大，结合强度高，生产成本低。本文研究了在45钢、H13钢和W18Cr4V钢表面制备的三元硼化物（Mo₂FeB₂）基金属陶瓷和硬质合金覆层的组织和性能。用扫描电镜、能谱仪、电子探针、X射线衍射仪和硬度计分析了不同烧结温度对覆层微观组织和界面结构的影响，研究了覆层的形成机制。用特定的拉伸试验方法，精确地测量了覆层和钢基体间的结合强度。根据覆层硬度（HRA）的测定和试样断面上显微硬度（HV）的分布，分析了覆层的强韧性特点。比较了覆层试样和H13钢、W18Cr4V钢的耐磨性，并分析覆层的耐磨机理。测定了H13钢表面覆层试样300℃、500℃和700℃时的高温显微硬度（HV），比较覆层试样和无覆层试样之间热强性的差别。用Uddehole法对H13钢表面覆层试样进行热疲劳试验，分析覆层试样和无覆层试样表面热疲劳裂纹形态、截面上热疲劳裂纹分布及热循环前后表面硬度（HV）的变化，探讨覆层的热疲劳机理。从上述试验结果得出以下结论：三元硼化物基金属陶瓷覆层适宜的烧结温度范围是1220℃～1240℃，组织为Mo₂FeB₂+α-Fe；硬质合金覆层适宜的烧结温度范围是1280℃～1300℃，组织为WC+α-Fe；覆层和钢基体间产生冶金结合和扩散结合界面，结合强度达到400MPa以上。覆层的HRA值和HV值分别超过80及1000。断面上微观硬度分布在界面两侧变化平缓。覆层的耐磨性高于H13钢、W18Cr4V钢，其磨损的机理是微观切削和塑性变形机理所产生的综合磨损。H13钢表面三元硼化物基金属陶瓷覆层在700℃时，HV值保持1000左右，具有很高的热强性。H13钢表面三元硼化物基金属陶瓷覆层的热疲劳抗力大于H13钢，而H13钢表面硬质合金覆层的热疲劳抗力小于H13钢。前者基于马氏体基体上分布着大量的细小硬质相Mo₂FeB₂，晶界大量增多，抵抗裂纹扩展的能力得到提高，而后者是因为WC颗粒呈粗大的多角形，容易产生应力集中，裂纹容易形成并扩展。更多还原

【Abstract】 Recently developed surface treatment and composite technology by vacuum powder sintering have been widely used for preparation of coating from hard materials on steel surface. It was coated with prepared mixed powder on surface of steel, sintered in vacuum furnace. The major excellences exist in high thickness, high bonding strength and low quality cost in coating.In this dissertation, microstructures and properties of the preparated coating from ternary boride based cermet and cemented carbide by vacuum powder sintering on substrate of 45 steel、H13 steel and W18Cr4V steel were studied. The effect of different sintering temperatures on microstructures of the coating and the interface morphology by SEM、EDS、EMPS、XRD and hardness tester was analysed. The mechanism of their formation was studied. Using specially designed tensile test method the bonding strength between coating and substrate was precisely measured. Based on hardness（HRA） measurement of the coating and microhardness（HV） distribution on cross-section of the sample the ductility-strength characteristic of the coating was analysed. Wear resistance of coated sample was compared with that of H13 steel and W18Cr4V steel and wear mechanism of the coating was analysed. Thermal microhardness（HV） at temperatures of 300℃,500℃and 700℃on the surface of coated H13 steel sample was measured .The difference of the thermal strength between coated and uncoated sample was compared. Thermal fatigue test of coated H13 steel sample was conducted by Uddeholm method. The morphologies of thermal fatigue crack on coated and uncoated samples, their distribution on cross section and surface microhardness（HV） variation before and after thermal cycle were analysed. An approach to mechanism of the thermal fatigue formation of the coating was performed. From above mentioned experimental results the fallowing can be concluded:The coating from ternary boride cermet has microstructure of Mo₂FeB₂+α-Fe at pertinent sintering temperatures ranging from 1220℃to 1240℃. While the coating from cemented carbide has microstructure of WC+α-Fe at pertinent temperature ranging from 1280℃to 1300℃. The bonding strength on the metallurgically bonded interface between coating and substrate exceeded 400MPa.HRA and HV values on the coated exceeded 80 and 1000 respectively. The microhardness distribution on cross section at both sides of the interface changed smoothly. The wear resistance for the coating was higher than for H13 steel and W18Cr4V steel. Wear mechanism could be expounded by the comprehensive wear both micro-cutting and plastic deformation.The coated H13 steel from ternary boride cermet with HV value of about 1000 at temperature of 700℃exhibited very high thermal strength. The coated H13 steel from ternary boride cermet exhibited higher thermal fatigue resistance than uncoated H13 steel while the coated H13 steel from cemented carbide possessed less thermal fatigue resistance than uncoated H13 steel. The higher thermal fatigue resistance of coated H13 from ternary boride cermet was attributed to great number of fine hard phase Mo₂FeB₂ distributed in martensite matrix and great increase of boundary so as to increase resistance again crack propagation while the lower thermal fatigue resistance of coated H13 steel from cemented carbide appeared due to coarse and polygonal WC grains which were susceptible to stress concentration so as to originate and propagate crack easily.更多还原