Nitrogen-induced microstructural evolution and the resultant tribological enhancement in RF magnetron-sputtered Zr-B-N films

Transition metal borides offer high hardness but suffer from high friction and brittle wear. Nitrogen doping is a promising strategy, yet its impact on the underlying tribological mechanisms remains unclear. In this paper, the effects of nitrogen doping on the microstructural evolution, mechanical properties, and tribological behaviors of RF magnetron-sputtered Zr–B–N composite films were systematically investigated. Nitrogen incorporation induced a pronounced structural transition from a crystalline, columnar ZrB x architecture to a dense, amorphous-like network through the formation of Zr–N and B–N bonds. This structural transformation was accompanied by a reduction in hardness and elastic modulus, but simultaneously enabled a remarkable improvement in tribological performance. At an optimal nitrogen content of 29.7 at.%, the specimen exhibited an ultralow friction coefficient (∼0.05) and exceptional wear resistance at the three tested loads of 2, 4, and 6 N. Detailed surface analysis revealed that this behavior originated from a synergistic mechanism involving enhanced structural compliance and the in-situ formation of a self-adaptive lubricious tribofilm, primarily composed of boron oxide (B 2 O 3 /H 3 BO 3 ) and zirconium oxynitride species, generated via tribo-oxidation under sliding conditions. This study established a N-doping-dependent relationship of structure and properties in Zr–B–N films and provided critical insights for the rational design of low-friction, wear-resistant boride-based films for the real self-lubricating applications.