Novel tribological systems using shape memory alloys and thin films

Thin film and bulk shape memory alloys are shown to have robust indentation-induced shape memory and superelastic effects, where indents can recover upon martensite-to-austenite phase transformations. Since many tribological contact conditions are similar to those under indentations, we have applied the micro- and nano-scale shape memory and superelastic effects in thin films and bulk NiTi shape memory alloys into novel tribological systems. First, we show that superelastic NiTi thin films as interlayers between hard coatings and aluminum substrates can improve coating adhesion and wear resistance. The NiTi interlayers are sputter-deposited onto 6061 T6 aluminum substrates. Chromium nitride (CrN) and diamond-like carbon (DLC) hard coatings are deposited by unbalanced magnetron sputter deposition. X-ray diffraction and nanoindentation are used to characterize NiTi interlayers. Wear and scratch tests show that superelastic (austenitic) NiTi interlayers significantly improve the tribological performance of hard coatings on aluminum substrates. Second, we show that the indentation-induced two-way shape memory effect can be realized in thin film and bulk NiTi alloys, where spherical indents exhibit two-way reversible depth change with heating and cooling. Reversible surface protrusions are realized after the indents are planarized (a polishing procedure to restore a flat surface). Micro- and nano- scale circular protrusions arise from planarized spherical indents in thin film and bulk NiTi alloy; line protrusions appear from planarized scratch tracks. Surfaces with patterned reversible indentations and protrusions can exhibit unusual tribological behavior that may be used for controlling friction in tribological systems.