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.