Deposition and nanoindentation study of a novel TiNi/Ag bi-layer shape memory film

Abstract

Compositions, surface morphologies and mechanical properties of a novel designed TiNi/Ag bi-layer film grown by DC magnetron sputtering at various processes have been investigated. Ar pressure and sputtering power had significant effects on quality and composition of TiNi/Ag bi-layer films. The TiNi/Ag bi-layer film processed by sputtering under high Ar pressure (0.36 Pa) was porous, while better density films can only be obtained at low pressure (0.12 Pa). The composition of TiNi/Ag bi-layer films can be adjusted by sputtering power. The load-displacement curves show the multiple “pop-ins”along nanoindentation loading, as indentation depth does not exceed the pure Ag film thickness (∼340 nm). At the maximum loading of 140 mN, martensitic transformation induced by nanoindentation in TiNi/Ag bi-layer films was not found, and depth recovery ratio was 39%. Moreover, based on the load displacement curve analysis, the hardness and elastic modulus values of TiNi/Ag bi-layer films were 4 ± 0.2 Gpa and 79 ± 2 Gpa, respectively. Comparing with martensitic phase of single-layer TiNi thin films at room temperature, the coexistence of parent phase and martensitic phase of TiNi/Ag bi-layer films were mainly due to strong confinement effect of Ag layer deposition on TiNi thin film. The residual stress calculated by using conventional sin2ψ method was around −0.458 GPa. The present study provided an important theoretical basis for the preparation of TiNi/Ag bi-layer film with higher quality and performance.