Conventional colloidal silica (CS) chemi-mechanical polishing (CMP) of Si-face SiC substrates typically results in a low material removal rate (MRR<0.1mum/h). In this study, the chemical surface oxidation and mechanical removal of the oxide layer during CMP of on-axis semi-insulating (SI) 6H SiC substrates, and the effect on material removal rate and surface roughness were investigated separately by addition of (a) compatible oxidizers, (b) abrasives and (c) both oxidizers and abrasives to the CS slurry. Neither oxidizer nor soft abrasive addition individually resulted in a significant MRR increase, yet both increased the substrate surface roughness. The addition of nano-size diamond abrasive (0.1 mum grain size) to the CS slurry resulted in a MRR of 0.60 mum/h, a 10x increase over CS CMP, and produced substrates with a Ra surface roughness of 5.5Angstrom. The addition of 0.1 mum diamond abrasive and sodium hypochlorite oxidizer to the CS slurry resulted in a MRR increase to 0.92 mum/h and produced substrates with a Ra surface roughness of 5.2Angstrom.