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.