A 'gentle' method for finishing Si sub 3 N sub 4 balls for hybrid bearing applications

This paper is an overview of the work carried out in the finishing of Si sub 3 N sub 4 balls for bearing applications using a novel process known as the magnetic float polishing (MFP). It is a 'gentle' process in which very low levels of controlled force (approx =0 5-2 N) is applied. The process can be divided into the following three stages: (1) an initial high material removal stage where the objective is to remove material as rapidly as possible with minimal damage to the surface, (2) an intermediate stage of semifinishing where size and sphericity are carefully monitored, and (3) final finishing stage where size, sphericity, and finish are closely controlled to meet the final requirements. The first process predominantly involves mechanical removal of material with harder and coarser abrasives e.g., coarse B sub 4 C; the second process involves the use of 'less' harder and finer abrasives, e.g., medium to fine B sub 4 C or SiC; and the third process involves the use of 'much' softer abrasives, e.g., CeO sub 2 , ZrO sub 2 , and Cr sub 2 O sub 3 that can chemo-mechanically react with the work material (Si sub 3 N sub 4 ) and/or the environment (air or water) to generate a smooth surface. Chemo-mechanical polishing of Si sub 3 N sub 4 balls with CeO sub 2 abrasive yields an extremely smooth and damage-free surface with a surface finish R sub a of 4 nm and an R sub t of 40 nm and a sphericity of 0.15- 0.25 mu M.