Study on the finishing capability and abrasives-sapphire interaction in dry chemo-mechanical-grinding (CMG) process

Chemo-mechanical-grinding (CMG) is a fixed abrasive process by integrating both chemical reaction and mechanical grinding. This paper targets on studying the finishing ability of dry CMG processes for mono-crystal sapphire and understanding the underlying abrasives-sapphire interaction. The experiment results suggest that dry CMG processes by applying either Cr2O3 or SiO2 are able to produce sapphire substrate with an excellent surface quality and subsurface integrity, meanwhile, maintain the geometric accuracy. The underlying abrasives-sapphire interaction termed as solid phase reaction is demonstrated by X-ray photoelectron spectroscope (XPS) analysis. It is suggested that Cr2O3 abrasives is more chemically active against sapphire (Al2O3) when compared with SiO2 abrasives. Combined with a higher mechanical effect provided by Cr2O3 abrasives, the finishing efficiency of Cr2O3 abrasives is almost 2 times higher than traditional SiO2 abrasives. The subsurface integrity characterization by using Raman microscope and transmission electron microscope reveals that Cr2O3 abrasives introduces a shallow damage layer (about 100 nm) on the top surface of sapphire substrate since the chemical effect is not sufficient enough to meet the balance with mechanical effect. With a better mechanical-chemical balance achieved by SiO2 abrasives, a damage free subsurface is obtained. Therefore, besides of traditional SiO2 abrasives, Cr2O3 abrasives is suggested to be another potential candidate for finishing sapphire substrate.