Revealing the hidden world of fretting wear processes of surface coatings by X-ray imaging

Detailed wear and friction phenomena are usually hidden between the two contacted surfaces. Direct or in-situ observation of wear process is supposed to obtain some time-dependent changes and shed more light on the real wear and friction mechanisms. In this paper, X-ray imaging, one of the advanced tools for modern non-contact and non-destructive inspection, was applied to observe in-situ the fretting wear processes of a laser-alloyed coatings and plasma-sprayed hydroxyapatite (HA) coatings against stainless steel under unlubricated conditions. X-ray images clearly showed the contact nature, the wear process, the formation and compaction of wear debris, spallation of the coatings, large-scale removal of materials, crack formation and propagation, etc. According to the analysis of X-ray images, the fretting wear process of laser-treated specimens was a combination of adhesive, abrasive, oxidative and delamination wear; and the fretting wear mechanisms for the plasma-sprayed HA coating were mainly delamination and abrasive wear. It can be concluded that X-ray imaging is a promising method for the in-situ observation of wear processes and deterioration mechanisms of coatings under fretting conditions.