Effect of PVD coatings on the strain and low-cycle fatigue resistance of stainless steel and titanium alloys

Abstract

Introduction. The fracture of structural components is known to be often initiated by the local damage of surface layers of the material. The mechanical properties of the surface layer are built up in manufacturing the structural components and affected in service. The major contributors to the fracture of surface layers are their roughness, stress concentrations, and nonuniform distribution of mechanical properties over the surface and in depth of the material [1]. Experimental investigations, e.g., [2], demonstrated that in a homogeneous stressed state, the yield limit of the surface layer of low-carbon steel was 25% lower than that of the base material. As evidenced by some other findings [3], the yield limit of the thinnest specimens is only 20% of that of the thick ones. Therefore, plastic flow on the surface sets in earlier than in the middle layers. Engineering practice employs different treatments, including the application of metallic and nonmetallic films and coatings, to enhance the strength and protective properties of item surfaces. As early as 1926, Rosko [4] began studying the effect of surface oxide films on the mechanical properties of metals in evaluation of the microhardness of cadmium monocrystals. It was revealed that the oxide film less than 20 atoms thick increased the critical stress of cadmium monocrystals by about 50%. It also interferes with plastic strains in the crystal, which contributes to an increase in the hardness of the surface layer. Investigations [5–9] showed that not only oxide films but also PVD coatings (physical vapor deposition) enhanced the tensile and cyclic load resistance, mainly due to the restriction on plastic strains of the base by a hard surface layer with higher mechanical strength, large residual compression stresses, and good adhesion of the coating material to the base [7–9]. An increase in the tensile and cyclic load resistance is associated with the thickness and hardness of PVD coatings and their nitrogen contents [7, 8, 10].