Duplex surface treatments combining plasma electrolytic nitrocarburising and plasma-immersion ion-assisted deposition

This paper discusses the feasibility of improving the tribological performance of AISI 304 stainless steel by applying a duplex surface treatment which combines the methods of plasma electrolytic nitrocarburising (PEN/C) and plasma-immersion ion-assisted deposition (PIAD) of a diamond-like carbon (DLC) coating. The effects of different regimes of plasma electrolytic diffusion treatment on the surface microstructural, mechanical and tribological characteristics of the stainless steel substrate are studied. The major advantages of the PEN/C technique are shown to be extremely high growth rate of the N/C diffusion layer (typically 3–6 μm/min) and the near-ambient (bulk) treatment temperature; this combination of characteristics far exceeds the capabilities of most conventional methods of diffusion treatment. Diffusion hardened layers 15–60 μm thick were produced after 5–10 min treatment, consisting predominantly of expanded austenite (γN,C) with a Knoop microhardness of 800–1400 HK10, providing significant improvement in the substrate load bearing capacity for subsequent deposition of a PIAD DLC coating. Reciprocating-sliding friction and wear tests against a WC–Co sintered-carbide or SAE 52100 chromium-steel ball counterface have shown that the application of a duplex treatment promotes a change in wear mechanism of the friction pair from relatively severe adhesion/abrasion to mild asperity deformation and ‘polishing’. This results in simultaneous reduction of the volumetric wear rate from ∼10−4 to ∼10−7 mm3/Nm and of the friction coefficient from (0.59–0.63) and (0.68–0.74) to (0.09–0.13) and (0.24–0.32) for WC–Co and chromium–steel counterfaces, respectively. It is also shown that the ball-on-plate impact wear resistance of the PIAD DLC coating can be considerably increased by application of a nitrocarburised load-supporting sublayer.