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

This paper discusses the feasibility of improving the tribological performa nce of AISI 304 stainless steel by applying a duplex surface treatment whic h combines the methods of plasma electrolytic nitrocarburising (PEN/C) and plasma-immersion ion-assisted deposition (PIAD) of a diamond-like carbon (D LC) coating. The effects of different regimes of plasma electrolytic diffus ion treatment on the surface microstructural, mechanical and tribological c haracteristics of the stainless steel substrate are studied. The major adva ntages of the PEN/C technique are shown to be extremely high growth rate of the N/C diffusion layer (typically 3-6 mum/min) and the near-ambient (bulk ) treatment temperature; this combination of characteristics far exceeds th e capabilities of most conventional methods of diffusion treatment. Diffusi on hardened layers 15-60 mum thick were produced after 5-10 min treatment, consisting predominantly of expanded austenite (gamma (N,C)) with a Knoop m icrohardness of 800-1400 HK10, providing significant improvement in the sub strate load bearing capacity for subsequent deposition of a PIAD DLC coatin g. Reciprocating-sliding friction and wear tests against a WC-Co sintered-c arbide or SAE 52100 chromium-steel ball counterface have shown that the app lication of a duplex treatment promotes a change in wear mechanism of the f riction pair from relatively severe adhesion/abrasion to mild asperity defo rmation and 'polishing'. This results in simultaneous reduction of the volu metric wear rate from similar to 10(-4) to similar to 10(-7) mm(3)/Nm and o f 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 DL C coating can be considerably increased by application of a nitrocarburised load-supporting sublayer. (C) 2001 Elsevier Science B.V. All rights reserv ed.