Development of magnetically assisted lapping process for nano-finishing of alumina balls

Abstract

The advanced ceramics have emerged as a successful alternative to the conventional materials used in ball bearing industry. However, extreme hardness and lower toughness of the ceramic balls make their finishing a challenging task. To address this problem, a novel process is developed and results are presented in this article. The process is termed as magnetically assisted lapping (MAL) wherein the lapping and the polishing action are supported by magnetic levitation. The process parameters are identified. The capability of the process in terms of surface finish, roundness and material removal rate of alumina balls is assessed. The process is carried out in two stages. In the first stage, the main focus is on material removal and sphericity while the second stage focuses on achieving nano level surface finish. A very high material removal rate of 2.5 µm/min is achieved in the first stage. In the second stage, diamond abrasive powder (0.25–1 µm) mixed with silicone oil is used as a polishing medium for fine finishing of balls. The final surface finish of 20 nm and roundness of 0.23 µm is achieved which meets the requirement of G10 grade bearings (as per ISO3290). Atomic force microscope images show remarkable improvement of the surface up to 8 nm. The developed process is capable of producing nanometric finish in quite lesser time as compared to conventional and eccentric lapping processes. The underlying mechanism of material removal is proposed with the help of scanning electron microscope and atomic force microscope images.