Removal mechanism of the chemical products layer on single-crystal sapphire surface in magneto-rheological polishing with Fe3O4/SiO2 core-shell abrasives

Chemical reactions will occur on sapphire surface in the magneto-rheological polishing (MRP) process with Fe3O4/SiO2 core–shell abrasives, resulting in a chemical products layer, which is easily removed due to its low hardness. In this paper, the specific mechanical removal mechanism of this chemical products layer was investigated by analyzing scratch morphology and scribing forces obtained from computer simulation and scratching experiment. The results showed that the tangential force plays a major role in the material removal process, the shearing action of abrasives dominates the material removal of the chemical products layer on sapphire surface. During scribing process, the chemical products layer is plastically deformed firstly and then removed when the shear stress exceeds its material yield strength. The materials on both sides of scratch flow outward plastically cause ridges under the continuous extrusion process. The material on the bottom of scratch is destroyed and removed under the continuous severe extrusion and shearing process, resulting in an uneven “jag-like” morphology. Both normal and tangential forces increase fluctuatingly with scribing distance, and the average depth and width of scratch increase with scribing velocity. Therefore, the material removal mechanism of sapphire in MRP process is the combination of chemical reaction of surface material and the synthetic action of plastic deformation and shearing removal of chemical products layer.