ON THE MECHANISMS OF MATERIAL REMOVAL IN FINE GRINDING AND POLISHING OF ADVANCED CERAMICS AND GLASSES

This paper deals with the fundamental considerations on the interactions between the abrasive and the work material as well as the micro-mechanisms of material removal and surface generation process in ‘gentle’ grinding and polishing (surface roughness in the range of a few nanometers rms) of advanced ceramics and glasses. In developing plausible mechanisms, an attempt is made to delineate the failure mechanisms operable in polycrystalline ceramics and glasses versus the metals. This is necessary because of the significant differences in the nature of bonding, microstructure and the tribochemistry, and, flaws generated during processing of these materials and their consequent effect on the failure mechanisms associated. For example, metals are fully dense crystalline materials with an orderly arrangement (both long and short range) of atoms in all directions. Plastic deformation, instead of brittle fracture, is by far the predominant mode of failure in these materials. Glasses, in contrast, are non-crystalline (or amorphous) and respond intermediate between a liquid and a solid, i.e. at room temperature they behave in a brittle manner but above the glass transition temperature in a viscous manner. Ceramics, though mostly crystalline, are different in the nature of bonding. For example, metallic bonding in the case of metals, no long range order in the case of glasses, and ionic and/or covalent bonding in ceramics. Since ceramics are processed from powders using sintering, hot-pressing, or hot-isostatic pressing, they are less than theoretically dense. This results in some inherent porosity in the microstructure. Also, the grain boundaries generally consist of a weak, glassy phase. All these factors affect the strength and failure (deformation/fracture) behavior of these materials. Grain dislodgement, viscous flow at the grain boundaries, and microfracture of the crystals may be the predominant modes of failure than plastic deformation in this case. Also, under the high pressures and temperatures generated at the contacting points during polishing, mechano-chemical effects can play an important role in the material removal. In this investigation, effect of these factors are carefully considered in the development of a mechanistic model of the process. While plastic deformation is feasible at high temperatures and/or under high hydrostatic pressures, the wide range of defect structures present at various levels in most ceramics tend to favor microfracture, grain dislodgement etc. Further, in ceramics containing a glassy phase at temperatures higher than the glass transition temperature, viscous flow of the glassy phase takes place. Since the glassy phase is present at the grain boundaries, this can cause grain boundary sliding.