Finishing of Silicon Nitride Balls for High-Speed Bearing Applications

A batch of 12, HIP’ed silicon nitride (Si₃N₄) balls (NBD 200 from CERBEC) of 13.4 mm nominal diameter were finished by magnetic float polishing (MFP) process. These balls contained a large rim of material (about 200 μm thick × 5 mm wide) around the periphery at the parting line originating from the HIP’ing process. The finishing process consisted of three stages–1. semi-finishing using a 17 μm B₄C abrasive at 4000 rpm spindle speed to remove the rim on the Si₃N₄ balls, 2. intermediate finishing using a 17 μm B₄C abrasive at 2000 rpm spindle speed to reduce the diameter and the sphericity, and, 3. final finishing stage using a 3 μm SiC abrasive, followed by 1 μm SiC and 1–5 μm Cr₂O₃. The balls were finished to a final diameter of 12.7 mm with an average sphericity of 1.3 μm (0.5–2.0 μm) and an average surface finish, R_a, of 24.6 nm (16.6–39.8 nm). The total time for finishing was about 21 hours. This, however, included an intermediate stage of finishing with Cr₂O₃ in order to address some problems experienced earlier in attempting to obtain good finish. As this stage is not required under normal circumstances, some 4-1/2 hours of polishing time can be saved resulting in total polishing time of about 16-1/2 hours. This time should be compared to some 4–6 weeks required for conventional grinding and lapping of ceramic balls. The technology is easy to implement in the manufacturing shop floor requiring very little capital investment.