Characterization of wear-resistant coatings on 304 stainless steel fabricated by cathodic plasma electrolytic oxidation

In this paper, a novel rapid surface hardening technology on steels, namely cathodic plasma electrolytic oxidation (CPEO), was introduced. The oxide coatings of 35 μm-180 μm thick on AISI 304 stainless steel were prepared by cathodic plasma electrolytic oxidation in 15% borax electrolyte with additive at 260 V. The surface morphology, cross-sectional microstructure, composition and phase constituents of CPEO coatings under different treatment time were characterized. The hardness profiles of coatings were determined. Their tribological behavior was evaluated. It was found that the oxide coatings contained two layers: one loose outer layer which mainly consisted of Fe3O4 phase and one compact inner layer with Cr and Ni aggregation where FeCr2O4, NiCr2O4, Fe3O4 and FeO phases were detected. The microhardness of CPEO coating increased with increasing coating thickness, and its maximum microhardness was up to 1335 HV. Pin-disk wear test showed that the CPEO coatings on stainless steel had a low friction coefficient close to 0.1 under dry sliding against ZrO2 ball. The wear rate of the thickest CPEO coating is two orders of magnitude lower than that of stainless steel substrate. So the CPEO coating had better wear resistance than the bare stainless steel substrate.