Fundamental Study On Lapping Process For Micro Silicon Nitride Balls

Silicon nitride (Si3N4) balls (hot pressed or hot isopiestic pressed) are being actively considered for high speed/high temperature precision bearing because of many superior properties, including lower density, higher hardness, higher elastic modulus (higher stiffness), lower thermal expansion coefficient, higher thermal and chemical stability and non-magnetic characteristics compared to the traditional steel balls.Fundamental lapping process for micro Si3N4 balls is studied is this dissertation. Si3N4 balls are difficult to machine owing to their hardness and inherent brittleness, and the higher requirement of micro balls for absolute dimension accuracy place more difficulty on the lapping process. The lapping efficiency and lapping quality including surface quality and sphericity are the two critical indicators for the lapping process, for the low efficiency and high cost have being hinder the application of ceramic balls, and the surface quality and sphericity are the key factors affecting the performance and reliability of ceramic ball bearings. The lapping process for Si3N4 balls are studied from three aspects in this dissertation, including lapping methods, the effects of different factors on lapping efficiency and quality, and chemical mechanical polishing (CMP) for finishing, whichexert most important effects on the lapping efficiency and quality.By the combination of theoretic analysis and experiments, a new lapping method for miniature Si3N4 balls, which can improve the sphericity and lapping efficiency are put forward, and reasonable lapping process and parameters are fixed to balance the lapping efficiency an quality. Finally, the fundamental mechanism of chemical mechanical polishing for finishing the micro SisN4 balls are discussed, and an extremely smooth and damage-free surface with a surface finish Ra of 5nm and sphericity of 0.2 um are obtained by CMP experiments (03.147mm ) .The application of ceramic ball bearings, shaping process for green SisN4 balls, basic theories of lapping process and the research done by others are also introduced in the dissertation.