Research on abrasives in the chemical–mechanical polishing process for silicon nitride balls

Silicon nitride (Si3N4) has been the main material for balls in ceramic ball bearings, on account of its lower density, high strength, high hardness, fine thermal stability and anticorrosive qualities, and it is used widely in various fields, such as high speed and high temperature areojet engines, precision machine tools and chemical engineering machines. Silicon nitride ceramics is a kind of brittle and hard material that is difficult to machining. In the traditional finishing process of silicon nitride balls, the balls are lapped by expensive diamond abrasive. The machining is inefficient and the cost is high, but also numerous pits, scratched subsurface micro-crazes and dislocations are produced on the surface of the balls, their performance then being seriously impaired. In recent years, a kind of new technology, known as chemical–mechanical polishing has been introduced for the ultraprecision machining process of ceramic balls. In this technology, abrasives such as ZrO2, CeO2 having a hardness that is close to or lower than that of the work material (Si3N4) are used to polish the balls. In a special slurry, these abrasives can chemo-mechanically react with the work material and the environment (air or water) to generate a softer material (SiO2), and the resultants can be removed easily at the 0.1 nm level. Thus surface defects can be minimized, and a very smooth surface (Ra=4 nm) and fine sphericity (0.15–0.25 μm) can be obtained, and the machining efficiency is also improved. The active mechanism of the abrasives in the chemical–mechanical process in the finishing of the silicon nitride balls in introduced in this paper.