Friction and wear properties of DLC-coated plasma nitrided steel in unidirectional and reciprocating sliding

In this study samples of AISI 4140 steel were surface treated by plasma nit riding and coated with a hydrogen-free hard carbon coating, also known as t etrahedral amorphous carbon coating (ta-C). The specimens were investigated with respect to their microhardness, residual stress, coating adhesion and dry sliding wear resistance. Wear tests in which duplex-treated specimens were mated to hardened ball bearing steel were performed under unidirection al and reciprocating sliding. To examine the influence of the nitriding con ditions on the performance of duplex-treated steel,; the coating was deposi ted on hardened as well as on plasma nitrided samples, nitrided under two d ifferent nitriding conditions. The results show greatly improved sliding we ar resistance of plasma-nitrided hard-coated specimens compared with un-coa ted and pre-hardened ones, observed for both unidirectional and reciprocati ng sliding. Furthermore, the hydrogen-free hard carbon coating produced a r eduction in the coefficient of friction of approximately one order of magni tude. Although previous investigations showed the negative effect of a comp ound layer, it was found that the compound layer may act as an intermediate hard layer, leading to superior tribological properties of the composite. (C) 2000 Elsevier Science B.V. All rights reserved.