Nanoindentaion and Wear Behavior of Nickel-Titanium Alloys and Thin Films
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Nanoindentaion and Wear Behavior of Nickel-Titanium Alloys and Thin Films Wangyang Ni,1, 2 and David S. Grummon1 1 Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48823, USA 2 Materials and Processes Laboratory, General Motors Research and Development Center, Warren, MI 48090, USA ABSTRACT The unusual constitutive behavior of NiTi alloys that display shape memory and transformational superelasticity may impart useful tribological properties. This paper presents some preliminary results on nanoindentation and pin-on-disk wear experiments that suggest potential tribological applications of NiTi alloys. It is shown that high levels of spring-back reversibility together with high hardness, as measured by nanoindentation experiments, correlate with improved wear resistance in bulk Nitinol alloys. Amorphous thin films of equiatomic NiTi, which are readily produced by physical vapor deposition, were found to be especially hard and wear resistant. Finally, stress induced B2-B19’ transformation is shown to occur during wear-loading of martensitic NiTi, indicating that wear processes are capable of inducing superelastic effects in B2 NiTi alloys. INTRODUCTION Superelastic titanium-nickel and NiTi shape-memory alloys have become technically and economically important in a variety of industries [1-2]. The salient feature of shape-recovery and/or superelastic behavior, with respect to tribological phenomena, is that large reversible nonlinear strains can occur without generating the large numbers of lattice defects that normally accompany deformation by dislocation-mediated slip. It has been reported, for example, that TiNi alloys have better wear resistance than 38CrMoAl and 304 stainless steels [4-7], in some cases by as much as two orders of magnitude [5]. In the following sections we describe the results of experiments using nanoindentation methods to determine elastic modulii, hardness, and springback reversibility characteristics for two distinct commercial melt-solidified nitinol alloys, as well as a sputtered amorphous thin film of near-equiatoimic binary NiTi. We also present data from investigation of pin-on-disk wear performance, and wear-induced phase transformation behavior for these materials. The results suggest that NiTi based shape-memory alloys (especially in thin film form) may function as novel and useful tribological materials. EXPERIMENTAL Two commercial wrought Nitinol alloys, designed for shape memory and superelasticity applications respectively, were obtained from Shape Memory Applications Inc. Specimens were mechanically polished, finishing with 0.25µm diamond paste, to give an average surface roughness of
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