Microstructural and nanomechanical characterisation of Ni-Ti(-Cu) shape memory alloy thin films for tribology

Abstract

Protective and functional coatings have been undergoing development for decades and further improvement to their mechanical and tribological properties are more and more challenging. Nowadays most research is aimed at improving the tribological behaviour of hard and functional coatings through the optimisation of their microstructure on the nanoscale. Over time significant breakthroughs have been achieved, however improving material performance is becoming harder. Combining different layers is another possible way of improving the tribological performance of functional coatings. The use of bonding layers can mitigate the differences in mechanical and thermal properties between coating and substrate and might change the behaviour of the coating system in a tribological scenario. As a consequence, the nature of this bonding interlayer plays an important role in the response of the coatings to the complex stress conditions taking place in a tribological system. Among the possible interlayer candidates a layer with the capability of accommodating large deformation, thus protecting the substrate from plastic deformation as well as improving the adhesion of the top layer to the substrate, could represent a suitable choice.

One of the potential classes of materials satisfying the above requirements are the Ni-Ti based alloys which are known to exhibit superelastic properties also when sputter deposited. In this study we first focus our investigation on the characterisation of sputter-deposited Ni-Ti based thin films. In particular, the Ni-Ti system is doped by a third element, Cu through which mechanical and microstructural properties can be changed without detrimental effects on the typical functional properties of Ni-Ti alloys. The effects of Cu, in the range 0 – 20 at.%, and of the post-deposition heat treatments, with particular regard to annealing temperature, on mechanical and microstructural properties of sputter-deposited Ni-Ti(-Cu) thin films are investigated by nanoindentation, X-ray diffraction and transmission electron microscopy. Based on the objective of using Ni-Ti(-Cu) thin films as the interlayer in tribological coatings, some of the Ni-Ti(-Cu) films are selected and integrated in a bilayer design. Among the tribological coatings self-lubricant W-S-C coatings are known for their excellent non-Amonton frictional behaviour with friction decreasing with increasing contact pressure. The low friction of W-S-C coatings is associated with the formation of a WS2 tribolayer on the sliding surface. When the basal plane of the WS2 tribolayer is aligned with the sliding direction, the friction coefficient drops to very low values owing to the weak bonding between chalcogenide atomic planes. The formation of the low-shear surface layer is directly related to contact pressure; therefore, a W-S-C coating is an ideal functional layer with which to study the effect of Ni-Ti(-Cu) interlayers on sliding properties. W-S-C/Ni-Ti(-Cu) bilayer coatings are fabricated following a three-step process consisting of deposition and annealing of the Ni-Ti(-Cu) layers and subsequent deposition of the top functional layer. Mechanical and nano scratch behaviour of these bilayers is investigated in order to study the functional role of different Ni-Ti(-Cu) interlayers on the response of the bilayers to nanoindentation and nano-scratch tests.

The tribological performance of W-S-C single layer and selected W-S-C/Ni-Ti(-Cu) bilayers are investigated by sliding tests in humid air under different test conditions in order to assess the potential beneficial effects of the interlayer on the tribological properties of W-S-C. Correlation between the tribological properties measured and microstructural changes induced by sliding is achieved by investigating the tested coatings by focused ion beam and transmission electron microscopy. Chemical changes on the sliding surfaces are also investigated by Raman spectroscopy in order to highlight possible differences in the tribolayer formation under different test conditions and for different Ni-Ti(-Cu) interlayers. The study is also aimed at understanding how the stress-induced martensitic transformation is activated in the interlayers during sliding.

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ORCID for Tomas Polcar: orcid.org/0000-0002-0863-6287

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