Impact of the B/Ti-ratio on microstructure, mechanical properties, and thermal stability of DCMS and HiPIMS TiB2 thin films

Abstract

Titanium diboride is widely known as a superhard material achieving an indentation hardness of 40 GPa and beyond. Coatings of TiB2 produced by DC magnetron sputtering (DCMS) are typically in the superstoichiometric regime regarding the B/Ti-ratio in a range of TiB2.4 to TiB3.5. This surplus in Boron is attributed to a different radial distribution of Boron and Titanium during the deposition process and forms a tissue phase which is highly relevant for the material’s mechanical properties. A reduction of Boron content in the coating would thus mean a reduction of tissue phase formed and consequently a clear change in e.g., hardness or fracture toughness. Earlier works have already shown an increase in fracture toughness (measured by microcantilever bending) of DC-sputtered TiB2 when reducing the B/Ti-ratio from 4.4 to 2.1. There have also been reports of even higher fracture toughness in understoichiometric TiB1.43 produced through high power impulse magnetron sputtering (HiPIMS) and measured by cube corner indentation, while films grown by DCMS showed a decrease in KC with decreasing B-content. Here we address this discrepancy, via detailed studies of DCMS as well as HiPIMS developed TiB2±z thin films with B/Ti-ratios varying between 1.5 and 3.2. The stoichiometry of DCMS-grown films is adjusted by placing Ti-pieces at the target race track, while that of HiPIMS-grown films is adjusted by varying the pulse on-time. The B/Ti-ratios are measured by ICP-OES and the mechanical properties are characterized by nanoindentation (Hardness, Young’s Modulus) as well as cube corner indentation for KIC. Vacuum annealing treatments with subsequent detailed transmission electron microscopy studies as well as nanoindentation experiments clarify the impact of the B/Ti-ratio on these important characteristics.