Investigation on the high temperature tribological behaviors of pristine and plasma-based Mo-Si-Ti coated γ-TiAl

Abstract

A Mo-Si-Ti coated γ-TiAl substrate was fabricated using plasma alloying technology to enhance its high temperature wear resistance. The coated substrate was composed of a deposition layer and a diffusion layer, with the grain size decreasing from the substrate toward the coating, forming a gradient structure. XRD and TEM analysis revealed that the deposition layer included the (Ti, Mo)₅Si₃, TiSi and MoSi₂, while the diffusion layer consisted of the γ-TiAl, TiSi and Al₈Mo₃. Nanoindentation results showed that the coated substrate exhibited high hardness (19.6 GPa), as well as high plastic deformation resistance and load-bearing capacity. Furthermore, the presence of residual compressive stress (−1255.9 MPa), stress concentrations at interfaces between different phases and gradient structure contributed to the high surface fracture toughness of the coated substrate. Wear testing indicated that the lower nanomechanical properties of pristine substrate combined with the dynamic cyclic generation of oxide film during high temperature friction caused to an increase in specific wear rate at loads of 4.2 N and 6.2 N. However, post-oxidation hardness elevation as well as the lubrication and supportive effect of extensively covered oxide film reduced the wear rate as load increased to 8.2 N. The transition from residual compressive stress to tensile stress along with the formation of oxides at grain boundaries reduced the surface fracture toughness of the coated substrate. Meanwhile, the rapid formation and spalling of oxide film resulted in an increase in the specific wear rate of the coated substrate with increasing load. Nevertheless, coated substrate exhibited better wear resistance than pristine substrate owing to its higher surface mechanical properties. The specific wear rates of the coated substrate were 3.7, 6.0 and 19.5 × 10⁻⁵ mm³N⁻¹ m⁻¹ at loads of 4.2, 6.2 and 8.2 N, respectively, reflecting reductions of 88.9 %, 84.3 %, and 34.6 % compared to the pristine substrate.