On the finishing of Si 3N 4 balls for bearing applications

Abstract

The conventional method of producing of Si 3N 4 balls for bearing applications by grinding and lapping using diamond abrasive at low speeds ( < a few hundred rpm) and higher loads (several tens of N/ball) is generally an expensive and time-consuming operation (several weeks). It also leads to the formation of scratches, microcracks, and pits on the finished balls resulting from large radial and circumferential cracks and dislodgement of grains. Since failure of ceramics initiates from such defects, the reliability of Si 3N 4 halls in service is of prime concern. This paper deals with an alternate technology for finishing Si 3N 4 balls for hybrid bearing applications using magnetic float polishing (MFP) process that overcomes some of these limitations. A methodology for finishing of HIP'ed Si 3N 4 balls from the as-received condition by MFP is presented. It involves the mechanical removal of material initially using harder abrasives with respect to the workmaterial (of different materials of progressively lower hardnesses and finer grain sizes) followed by final chemo-mechanical polishing (CMP) using preferably a softer abrasive for obtaining superior finish with minimal surface or subsurface defects, such as scratches, microcracks, or pits on the Si 3N 4 balls. High material removal rates (1 μm/min) with minimal subsurface damage is obtained with harder abrasives, such as B 4C or SiC (relative to Si 3N 4) due to the use of a flexible support system, small polishing loads (≈ 1 N/ball), and fine abrasives but high polishing speeds (compared to conventional polishing) by rapid accumulation of minute amounts of material removed by microfracture. Final polishing of the Si 3N 4 balls using a softer abrasive, such as CeO 2 (that chemo-mechanically react with the Si 3N 4 workmaterial) results in high quality Si 3N 4 balls of bearing quality with superior surface finish ( R a < 4nm, R 1 < 0.04 μm) and damage-free surface. It is found that CMP is very effective for obtaining excellent surface finish ( R 4 ≈ 4nm andR 1 ≈ 40nm) on Si 3N 4 ceramic material and CeO 2 in particular is one of most suitable material for this application.