A "gentle" method for finishing Si3N4 balls for hybrid bearing applications

This paper is an overview of the work carried out in the finishing of Si3N4 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 (similar to 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 B4C; the second process involves the use of "less" harder and finer abrasives, e.g., medium to fine B4C or SiC; and the third process involves the use of "much" softer abrasives, e.g., CeO2, ZrO2, and Cr2O3 that can chemo-mechanically react with the workmaterial (Si3N4) and/or the environment (air or water) to generate a smooth surface. Chemo-mechanical polishing of Si3N4 balls with CeO2 abrasive yields an extremely smooth and damage-free surface with a surface finish R-a of 4 nm and an R-t of 40 nm and a sphericity of 0.15-0.25 mu m.