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研究了预氧化对42CrMo钢离子渗氮的催渗作用及机理。采用光学显微镜、显微硬度计、XRD、SEM和接触角测量仪研究了渗氮层厚度、渗氮层物相、预氧化后表面形貌和表面自由能。结果表明,预氧化对离子渗氮具有明显的催渗作用,在300℃预氧化30 min后进行离子渗氮(500℃、4 h),化合物层厚度达到15μm,是不经预氧化处理的传统离子渗氮化合物层厚度的2倍以上;有效扩散层厚度达到最大值570μm,明显高于传统离子渗氮的有效扩散层厚度。研究还表明,300℃预氧化30 min后表面产生了大量纳米级氧化物颗粒和微裂纹、孔洞,同时接触角最小、表面自由能最大,离子渗氮阶段氧化物可以有效地转化为氮化物。由此推测预氧化催渗机理可能是表面纳米级氧化物颗粒和微裂纹、孔洞的形成,一方面有利于活性氮原子的吸附,从而促进化合层的形成,另一方面为氮原子提供的扩散通道,有利于扩散层的增加。
The effect of pre-oxidation on ion nitriding of 42CrMo steel was studied. The thickness of nitrided layer, phase of nitrided layer, surface morphology after preoxidation and surface free energy were studied by optical microscope, microhardness tester, XRD, SEM and contact angle measuring instrument. The results showed that the pre-oxidation had obvious catalytic effect on ion nitriding. After pre-oxidation at 300 ℃ for 30 min, it was subjected to ion nitriding (500 ℃, 4 h) and the thickness of compound layer reached 15μm, Ion nitriding compound layer thickness more than 2 times; effective diffusion layer thickness reaches the maximum 570μm, significantly higher than the traditional ion nitriding effective diffusion layer thickness. The study also showed that a large amount of nano-scale oxide particles and micro-cracks and voids were formed on the surface after pre-oxidation at 300 ℃ for 30 min, with the smallest contact angle and the largest surface free energy. The oxide at the stage of ion nitriding can be converted into nitride efficiently. It is speculated that the mechanism of pre-oxidation and catalysis may be surface nanoscale oxide particles and micro-cracks, the formation of holes, on the one hand, is conducive to the adsorption of active nitrogen atoms, thereby promoting the formation of chemical compounds, on the other hand, the diffusion of nitrogen atoms provided Channel, is conducive to the proliferation of layers.