Effect of carbonitriding temperature process on the adhesion properties of diamond like-carbon coatings deposited by PECVD on austenitic stainless steel

• It has been difficult to deposit thick diamond-like carbon (DLC) coatings directly onto steel substrates because of high compressive stress and large thermal expansion mismatch.
• The DLC films were analyzed for their microstructure and mechanical properties of adhesion as a function of temperature.
• In this work, we demonstrated an in situ low-temperature plasma carbonitriding process by which the carbon concentration in the steel substrate was varied gradually over a thickness of about 80 microns.
• When followed by DLC deposition, we obtained a one-micron thick coating and adhesion comparable to that obtained by a more elaborate process. This improved adhesion is due to the lower interfacial energy resulting from the gradual change of composition and treatment temperature.

The deposition of adherent coatings such as diamond-like carbon (DLC) on substrates of iron-based materials is difficult to obtain for two reasons: high residual compressive stress occurs in the inner film formation, and the mismatch of thermal expansion coefficient between steel and DLC film generates delamination effects. In order to determine the carbonitriding temperature prior to film deposition, the steel substrate and the DLC films were analyzed for their microstructure and mechanical properties of adhesion as a function of temperature. The technique used to deposit the coating was DC-pulsed plasma enhanced chemical vapor deposition. The delamination distances and the critical load of the film were obtained by scratch testing. The surface analysis by X-ray diffraction indicated the formation of nitride phases on the steel. Raman spectroscopy showed the fraction of sp3 carbon bonds in DLC films. Hardness profiling was used to verify the extent of the interface modified by carbonitriding along the cross section. For this, the steel sample with the appropriate surface modification to have high adhesion of the DLC film was used.