Stress-induced martensitic transformation in Ni–Ti(–Cu) interlayers controlling stress distribution in functional coatings during sliding

Abstract

The stress-induced martensitic transformation occurring in sputter-deposited Ni48.1Ti51.9 and Ni43.4Ti49.6Cu7 interlayers, integrated in a W-S-C/Ni–Ti(–Cu) bilayer design, was investigated by transmission electron microscopy, after these bilayers were subjected to different sliding conditions. Martensitic bands across the interlayers were formed depending on the sliding direction with their shape and distribution a function primarily of both applied normal load and grain size.The Ni48.1Ti51.9 interlayer (lateral grain size of ∼3μm) showed well oriented and ordered martensitic bands extended through the interlayer thickness under low load (5N). At a higher load (18N) the growth of these bands was limited by the stabilised martensite formed as a consequence of the high compressive stress, at the interface with the substrate.The Ni43.4Ti49.6Cu7 interlayer (lateral grain size of ∼650nm) exhibited no significant evidence of stabilised martensite under different loading conditions. The martensitic transformation was limited by the smaller grain size and most of the stress was relaxed by elastic and, to some extent, pseudo-elastic deformation of the austenitic phase. Grain boundaries were found to stop the growth of martensitic bands, thus limiting the activation of the martensitic transformation into the neighbouring grains during sliding.The grain refinement caused a change in the capability of the interlayer to relax shear and compressive stresses. Such a change was found to affect the formation of the WS2-rich tribolayer on the W-S-C sliding surface, and consequently the shear stress transmitted down throughout the bilayers thickness. Accordingly, different levels of deformation were observed on the top layer.