Tribological properties of silicon nitride ceramics coated with DLC and DLC-Si against 316L stainless steel

Diamond-like carbon (DLC) is an adequate coating on a large variety of materials for tribological purposes, namely against iron alloys in automotive parts, bearings and forming tools. Herein, the tribological properties of DLC-stainless steel couples were assessed by unlubricated pin-on-disc experiments. Plasma enhanced chemical vapour deposition (PECVD) of DLC-Si or pure DLC coatings were performed, respectively, by conventional rf glow discharge from gaseous mixtures of methane and silane or taking only pure methane, with self-bias voltages varying from −200 to −800 V. Silicon nitride (Si3N4) was used as DLC substrate aiming the minimisation of adhesion problems, usually found when some metallic substrates are employed. An improved tribological response was obtained with pure DLC coated Si3N4 discs sliding against the stainless steel pins. This system almost instantaneously attains a steady-state friction regime with friction coefficients in the range 0.20–0.30 and a wear coefficient value of about 10−6 mm3 N−1 m−1, one order of magnitude lower than that of the DLC-Si coated ones. The application of a distinct self-bias during the PECVD process only marginally affected the tribological properties of the pure DLC coatings.