Tribological evaluation of AISI 304 stainless steel duplex treated by plasma electrolytic nitrocarburising and diamond-like carbon coating

In previous studies we have demonstrated that the tribological performance of stainless steels can be improved by applying a duplex surface treatment of plasma electrolytic nitrocarburising (PEN/C) and plasma-immersion ion-assisted deposition (PIAD) of a diamond-like carbon (DLC) coating. Diffusion-hardened layers between 15 and 60 μm thick, consisting primarily of expanded austenite, provide improved wear resistance and enhanced load-bearing capacity for the PIAD DLC low-friction coating. In this paper, further investigations are reported regarding the tribological and mechanical properties of these duplex treatments in wear tests against sintered WC–Co and SAE 52100 chromium–steel counterfaces using a low frequency (5 Hz) reciprocating-sliding ball-on-plate geometry with a ball diameter of 10 mm loads between 10 and 25 N. The friction coefficients remained remarkably similar for this range of loads (i.e. μ=0.09–0.13 and 0.24–0.32 for WC–Co and SAE 52100 counterface materials, respectively). The primary wear mechanism observed is one of mild asperity deformation and polishing and, in extreme cases, localised delamination of the DLC coating. The volumetric wear coefficient was almost constant for loads up to 15 N, particularly when sliding against WC–Co, where material loss from the ball counterface is less severe than with SAE 52100, despite the higher contact pressure at an equivalent load. No failure of the DLC coating was observed for 10 and 15 N loads against WC–Co and at up to 25 N against SAE 52100 steel. Scanning electron microscopy was employed to assist in the evaluation of the wear behaviour and failure mechanisms, with energy dispersive X-ray (EDX) analysis used to investigate material transfer to the coated surface.