Bipolar HiPIMS kick‐pulse for high hardness in high‐entropy boride thin films

Abstract We report a microhardness indentation study for multicomponent refractory metal boride thin films that belong to the family of high‐entropy ceramics exhibiting superior hardness and high temperature properties. We focus on the nominally equimolar composition (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2 )B 2 , which we refer to as HEB3, on c ‐plane sapphire substrates. Thin films are prepared using bipolar high power impulse magnetron sputtering (HiPIMS), where the positive kick pulse is optimized to produce films with high density, high crystallinity, and a microstructure that is isometric in nature as revealed by cross‐sectional fracture surface imaging, X‐ray diffraction, and X‐ray reflectometry. Low‐load Knoop indentation is used to measure microhardness. We find strong trends linking kick‐pulse magnitude, microstructure uniformity, high density, and Knoop hardness. Optimized conditions can produce films with low‐load Knoop hardness exceeding 30 GPa. To validate these trends, we present a rigorous indent characterization campaign and discussion considering possible artifacts from roughness, thickness, and indent loading. The key finding of our study is the ability of engineering bombardment by using a low duty‐cycle pulsed plasma to manipulate crystal structure without sacrificing crystallinity, and in turn boost material hardness.