Indentation and tribological behavior of nickel titanium alloys and study of instrumented spherical indentation

In this work, microscopic shape memory (SME) and superelastic (SE) effects in martensitic and austenitic NiTi alloys were probed by instrumented indentation techniques. Both pyramidal and spherical indenters were used to study the mechanical response of the NiTi alloys. It was found that deformation due to indentation was recoverable by the shape memory or superelastic effect and that the magnitude of indent recovery can be rationalized using the concept of the representative strain and maximum strain. Instrumented indentation techniques, especially using spherical indenters, are shown to be useful in quantifying shape memory and superelastic effects at micron and nanometer length scales.; Novel tribological applications of superelastic NiTi thin films suggested by these results were also studied. A novel composite coating, with a superelastic NiTi interlayer between soft aluminum substrate and hard CrN coating, was studied using instrumented indentation, scratch, and pin-on-disk wear tests. It was found that a superelastic NiTi interlayer can dramatically improve hard-coating adhesion and wear resistance. The improved coating performance is attributed to the large elastic recovery ratio and strain tolerance of the superelastic NiTi interlayer.; It was demonstrated that spherical indentation is very useful in the characterization of the mechanical properties of NiTi shape memory alloys. To further understand the spherical indentation process, a general study of spherical indentation in elastic-plastic materials was carried out. Two previously unknown relationships between hardness, reduced modulus, indentation depth, indenter radius, and work of indentation were found. Based on these relationships a novel energy-based method for determining contact area, reduced modulus, and hardness of materials from instrumented spherical indentation measurements was proposed. This method also provides a new way of calibrating the effective radius of imperfectly shaped spherical indenters.