Potassium persulfate as an effective oxidizer for chemical mechanical polishing of GCr15 bearing steel

Potassium persulfate was used as an effective oxidizer for chemical mechanical polishing (CMP) of GCr15 steel. The effects of pH, potassium persulfate content and chelating agents on the CMP performance of GCr15 steel were investigated, and the surface films formed in the potassium persulfate-based slurries were characterized accordingly. It is revealed experimentally that, with pH increasing from 2.0 to 9.0, the material removal rate (MRR) is independent of pH while the surface roughness Ra decreases. At pH 8.0, as the potassium persulfate content increases, the MRR linearly increases with a slope of 48 nm min−1 mM−1, but the surface quality continues to deteriorate accompanied with the black Fe3O4 residue due to the severe corrosion by persulfate. In particular, Fe2+ and Fe3+ ions are continuously produced by oxidation of persulfate, and a significant part of Fe2+ and Fe3+ further reacts with OH− to form a non-uniform and rough surface film which primarily consists of Fe3O4. After adding an appropriate chelating agent EDTA, the Ra reaches 1.7 nm without any black residue due to the chelation reactions between iron ions and EDTA. The findings suggest that a nano-precision surface finish of bearing steel could be achieved at weakly alkaline pH with persulfate-based slurries.