Strategy for preparing nanocrystalline Ta-N gradient layer with enhanced mechanical and tribological performance via microwave plasma nitriding

Abstract

Herein, microwave plasma nitriding (MPN) was employed to fabricate a nanocrystalline tantalum nitride (Ta–N) layer with enhanced mechanical and tribological performance. The results revealed that the nitrogen content in the Ta–N layer appeared as a gradient distribution, resulting in a component gradient structure comprising Ta2N, a solid solution of N in Ta and superlattice TaN0.04 from the top surface to the matrix, which was advantageous for relaxing the stress between the top Ta–N layer and the matrix. Nanocrystalline Ta–N layers (D(avg) = 45–55 nm) with dense and smooth surfaces were obtained at ≤950 °C, while grains grew and pores appeared on the surface of the Ta–N layers (D(avg) = 195–205 nm) at >950 °C. Consequently, the nanocrystalline Ta–N layer with gradient structure and desired thickness exhibited robust hardness and modulus of 26.89 and 247.64 GPa, respectively, which were considerably higher than those prepared by traditional gas/ion/liquid nitriding. The friction and wear test results showed that compared with pure Ta, the MPN-treated Ta matrix exhibited a shallower wear track, lower wear volume, and smaller superficial area, indicating that the Ta–N layer offered excellent surface protection for the matrix.