Influences of surface quality on the rolling contact fatique behaviour of ceramics.

Abstract

A novel eccentric lapping machine was designed by the author and manufactured in-house, to serve as a test bench to investigate the finishing process parameters in relation to surface quality, as well as the feasibility of accelerating the finishing process of HIPed silicon nitride balls. The kinematics of the eccentric lapping were analysed and discussed. Taguchi Methods were used to optimise the finishing parameters to achieve maximum material removal rate in lapping and to achieve minimum surface roughness value Ra in polishing. Two kinds of HIPed silicon nitride ball blanks were finished by this machine. A finishing rate of 68 µm/hour was achieved which is 15 times higher than the conventional concentric lapping (normally 3'-4µm/hour). The surface roughness and roundness of the polished ball were above grade 5, close to grade 3 precision bearing ball specification. The upper limits of lapping load and lapping speed were determined by aggressive lapping tests. The effects of various finishing parameters on the surface quality generated were studied by detailed surface analysis, including X-ray diffraction residual stress measurement. As a result, the relationship between the finishing process and surface quality has been established. Accelerated rolling contact fatigue tests were performed both under a standard 4-ball and a modified 5-ball rolling configuration, with a ceramic ball as the upper ball and steel balls as lower balls. The tests were conducted at high speed and lubricated conditions under different loads and were run for up to 135-200 million stress cycles. Tests were conducted on commercially finished balls with different surface roughness and with different surface integrity (heterogeneous porosity, star defect, grinding defect and C-cracks). Tests were also conducted on self-finished balls with different finishing parameters and with different surface roughness. After tests, the rolling tracks and failure areas were examined by detailed surface analysis. The residual stresses on the rolling tracks were measured. Finite Element Approaches were also employed to describe the contact stress status. Failure modes in relation to surface quality were discussed. The research has provided an understanding of the finishing process, surface quality and rolling contact fatigue behaviour of HIPed silicon nitride balls.