Structural and mechanical characterisation of boron and nitrogen implanted NiTi shape memory alloy

Super-elastic NiTi alloy is nowadays widely used for several applications, such as surgical staples, endoscopic and endodontic instruments. These last devices are useful in dentistry for restoration procedures. In spite of their elastic properties (high elasticity and ultimate tensile stress) NiTi alloys suffer from a lack of wear and bending fatigue resistance, yielding to brittle rupture inside the tooth of endodontic instruments. Plane surfaces of NiTi (nominal composition in wt.% Ni-46, Ti-54) were mechanically prepared from orthodontic wires of 1.5 mm diameter. In order to improve mechanical properties of the shape memory alloy and then increase the life of endodontic instruments, these surfaces were implanted at low energy (150 keV) with boron and nitrogen ions at several doses (1×1016, 5×1016 and 1×1017 at. cm−2). As a function of both ion species and fluence, we follow the different phases and microstructure using grazing incidence X-ray diffraction (GIXRD). Mechanical properties such as hardness and elastic modulus have been studied with ultra-low load indentation testing. Hardness vs. indentation depth curves have been recorded and correlated with micro-structural transformations observed with GIXRD. Finally, elastic recovery and plastic index were deduced from the load–displacement curves, showing an increase in yield stress of the implanted NiTi surface.