Tuning Ta coating properties through chemical and plasma etching pre-treatment of NiTi wire substrates

Abstract

Tantalum coatings have wide applications including as corrosion resistant coatings and radiopaque films for biomedical implants. However, producing coatings of sufficient thickness with the desired mechanical properties remains a substantial challenge. In this study, we show that the microstructure and properties of thick Ta films deposited on NiTi wires can be controlled significantly by pre-treating the substrate surface via ultrasonication followed by Aqua Regia etchant, or by plasma-based etching alone, while maintaining overall adhesive strength. Films from plasma and chemically etched samples exhibited substantially greater bulk texturing in the <111> direction than the control film, also yielding more elongated and columnar grains. The extent of <111> texturing in each sample was reflected in differences in Σ3 CSL boundary density. Additionally, only plasma etching yielded a β-Ta secondary phase. These microstructural observations demonstrate that film phase composition, texturing, grain shape distribution and grain boundary characteristics can be optimised for particular applications, simply by applying different NiTi substrate pretreatment regimes. It is suggested that this bears significant implications for influencing bulk properties such as cohesive and shear strength, as well as ductility, conductivity and cohesion in thick tantalum coatings.