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

Abstract

In this study samples of AISI 4140 steel were surface treated by plasma nitriding and coated with a hydrogen-free hard carbon coating, also known as tetrahedral 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 unidirectional and reciprocating sliding. To examine the influence of the nitriding conditions on the performance of duplex-treated steel, the coating was deposited on hardened as well as on plasma nitrided samples, nitrided under two different nitriding conditions. The results show greatly improved sliding wear resistance of plasma-nitrided hard-coated specimens compared with un-coated and pre-hardened ones, observed for both unidirectional and reciprocating sliding. Furthermore, the hydrogen-free hard carbon coating produced a reduction in the coefficient of friction of approximately one order of magnitude. Although previous investigations showed the negative effect of a compound layer, it was found that the compound layer may act as an intermediate hard layer, leading to superior tribological properties of the composite.