Achieving ultrahigh microhardness and superior wear resistances in 60NiTi alloys towards bearing applications

The external hard TiN ceramic layer and an internal reinforced NiTi matrix were controlled by combining nitriding and solution treatment to achieve ultra-high micro-hardness and excellent wear performance. In addition, the effects of nitriding and solution treatment on the microstructure evolution, martensitic transformation, and mechanical and functional properties of 60NiTi alloys were investigated. Compared with 60NiTi alloy, a harder TiN layer appeared at the surface of 60NiTi alloy with nitriding treatment. Meanwhile, a Ni₃Ti layer formed under the bottom of TiN layer owing to the consumption of Ti in matrix induced by formation of TiN layer. Upon undergoing solution treatment, the precipitates evolved from the coexistence of Ni₃Ti and Ni₃Ti₂ phase to Ni₄Ti₃ phase. The introduction of the outer harder TiN ceramic layer had a constraint of martensitic transformation of 60NiTi alloy, which led to the reduction of martensitic transformation temperature (Ms) and the suppression of martensitic transformation. Importantly, the construction of outermost harder TiN ceramic layer at the surface of B2-NiTi matrix enhanced Ni₄Ti₃ phase improved significantly the microhardness and wear performances by the nitriding and followed solution treatment. Meanwhile, the high compressive fracture strength, fracture strain and superior superelasticity of 60NiTi alloy can be maintained.