Effect of surface treatment on the adhesion of DLC film on 316L stainless steel

Stainless steel type 316L is a material widely used for medical implant purposes, for example, for artificial hips. To improve their wear resistance and biocompatibility, these implants are coated with various materials, in particular diamond-like carbon (DLC). It is known that numerous parameters have an influence on coating adhesion including stress in the film, contamination and chemical bonding between the film and the substrate, and the physical properties and roughness of the substrate. DLC films have been deposited on to substrates of 316L stainless steel using a saddle field neutral beam deposition system with acetylene as the process gas. The adhesion of the films has been measured as a function of the duration of in situ etching by a neutral argon beam on DLC films with thickness ∼0.4 μm. It was measured using pull-off and Rockwell C tests. Argon pre-etching for 15 min is recommended to guarantee an optimal adhesion. The relationship between etching time and film adhesion, structure and stress was investigated. It was found that the adhesion was maximised with an etching period of 15 min and that the etching process also influenced the film structure in terms of the sp2/sp3 bonding ratio and stress. The nature of the surface oxide after etching was investigated by Fourier transform infrared spectroscopy and it was found that the adhesion is correlated with a change in the structure and thickness of the native oxide layer on the surface of the steel, being greatest when the surface oxide was nickel-rich.