Influence of Si alloying on the high-temperature mechanical properties of CrB2 based thin films

Abstract

Among transition-metal diboride based thin films CrB2 is an exciting prospect due to a unique portfolio of materials properties, especially through offering high fracture resistance [1]. However, this class of PVD coatings suffers from an inherently low oxidation resistance. Therefore, establishing pathways to improve the oxidation resistance and thus maintain the mechanical properties to higher temperatures (i.e., above 1000 °C) for these hard-protective coatings is an integral aspect for future high-performance components in thermo-mechanically demanding environments. In this work, the influence of Si alloying on the high-temperature mechanical properties and fracture toughness of magnetron sputtered CrB2 thin films is studied for various concentrations. As indicated by thermogravimetric analysis, adding Si to the hexagonal α-AlB2 structured coatings improves the onset of oxidation by at least 600 °C in lab-air conditions. This drastic improvement in the formation of a stable oxide scale is further correlated with detailed analysis on the thermal stability and phase formation as obtained by non-ambient X-ray diffraction in both vacuum and lab-air atmospheres. The mechanical properties of the synthesised Cr-Si-B2±z thin films – including hardness, Young’s modulus and fracture toughness – are obtained by nanoindentation and bending of pre-notched, unstrained micro-cantilever beams in the as-deposited as well as the annealed state. Complementary in-situ characterisation of the aforementioned mechanical properties on coated high temperature bulk materials up to 800 °C allowed for a direct identification of changes in the deformation and fracture mechanisms across the investigated temperature regime [2]. Hence, this study showcases the potential of transition-metal diboride thin films – a class previously deemed unsuitable for high-temperature applications – by utilising Si-alloying to preserve the CrB2 thin films.

[1] Gu, Xinlei, et al. "Sorting transition-metal diborides: New descriptor for mechanical properties." Acta Materialia 207 (2021): 116685. [2] Buchinger, J., et al. "Fracture properties of thin film TiN at elevated temperatures." Materials & Design 194 (2020): 108885.