Structure characterization and wear performance of NiTi thermal sprayed coatings

Abstract

NiTi shape memory alloy (SMA) has been studied for many years for its shape memoryand pseudoelastic properties, as well as its biocompatibility, which make it suitable formany biomedical applications. However, SMA NiTi is also interesting for relevant wearresistance near the transition temperature which, along with its high oxidation andcorrosion resistance, suggests its use as a coating to increase the lifetime of somecomponents. Also, whereas bulk material properties have been characterized inrespect of the nominal composition, manufacturing methods and thermo-mechanicaltreatments, NiTi overlays have been investigated much less. Most existent works in thisfield specifically deal with magnetron sputtering technology for thin films andits use in micro-devices (micro-electro-mechanical systems, MEMS), just someworks refer to vacuum plasma spraying (VPS) for thicker coatings. The presentpaper explores and compares the microstructure and wear-related properties ofcoatings obtained from atomized NiTi powders, by VPS as well as by atmosphericplasma spraying (APS) and high velocity oxygen fuel (HVOF) techniques. Inthe present case, the wear behaviour of the NiTi deposits has been studied byrubber-wheel equipment and ball-on-disk tests. The results obtained at roomtemperature show that the APS-quenched coatings exhibit a preferential dry sliding wearmechanism, while the VPS and HVOF coatings show an abrasive mechanism.