Chemical mechanical polishing (CMP) in magnetic float polishing (MFP) of advanced ceramic (silicon nitride) and glass (silicon dioxide)

A novel process known as the magnetic float polishing (MFP) is introduced for the finishing of Si3N4 balls for high speed bearing applications. It is a 'gentle' process in that very low level of controlled force (1 N/ball) is applied. The equipment used in this process is simple and inexpensive. Consequently the capital costs associated with this technology are low. Also, conventional abrasives (instead of expensive diamond abrasive) can be used for finishing almost any material. This process can be used to finish a range of advanced ceramics and glasses. In order to obtain very smooth surfaces (4 nm Ra) and good sphericity (0.15-0.25 lam), the process may be divided conveniently into 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 stage involves predominantly mechanical removal of material with harder and coarser abrasives, e. g. coarse B4C; the second stage involves the use of 'less' harder and finer abrasives, e. g. medium to fine B4C or SiC; and the third stage, chemical mechanical polishing, (CMP), involves the use of abrasives about equal or much softer than the workmaterial, e.g. CeO2, ZrO2, Fe2O3, and Cr2O3 that can chemo-mechanically react with the workmaterial (Si3N4) and the environment (air or water) to generate a smooth surface. In this investigation various abrasives, some harder and some softer than the workmaterial were used to delineate the mechanical and chemo-mechanical mechanisms of material removal. Surface defects during mechanical polishing (in which the abrasive is harder than the work material) can be minimized when successively fine abrasives are used but could not be eliminated altogether. For example, the best surface finish obtained with a fine B4C abrasive (1 mum) (harder abrasive) by MFP on uniaxially hot pressed Si3N4 balls was 20 nm Ra and 200 nm Rt (where Ra is the arithmetic average surface roughness and Rt is the peak to valley distance). Similarly, the best surface finish obtained using, a fine SiC abrasive (I lam) (also a harder abrasive than Si3N4) was 15 nm for Ra and 150 nm for Rt. Besides fine mechanical polishing, chemical mechanical polishing (CMP) method can be applied to improve surface finish. CeO2 and ZrO2 are found to be most effective abrasives followed by Fe2O3 and Cr2O3 for chemical mechanical polishing (CMP) of Si3N4. CMP is found to be particularly effective in a water-based environment. There are similarities between polishing SiO2 glass and polishing Si3N4 workmaterial including the role of water, polishing environment, and chemical effectiveness and mechanical hardness of abrasive for effective polishing. Fundamental mechanisms of chemical mechanical polishing (CMP) have been studied based on thermodynamic and kinetic analysis. The process, involved mechanical polishing followed by chemical mechanical polishing (CMP) Chemical mechanical polishing (CMP) of Si3N4 balls yields an extremely smooth and damage-free surface with a surface finish Ra of 4 nm and an Rt of 40 nm and a sphericity of 0.15-0.25 mum.