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 are typically in the superstoichiometric regime regarding the B/Ti-ratio in a range of TiB_2.4 to TiB_3.5. 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 [1]. There have also been reports of even higher fracture toughness in understoichiometric TiB_1.43 produced through HiPIMS and measured by cube corner indentation, although films grown by DC showed a decrease in K_C with decreasing B-content [2].
In the current work, we aim to show how the mechanical properties of TiB₂ change when crossing the stoichiometric point by DC and HiPIMS sputter deposition. The stoichiometry of DC-grown films is controlled by placing pieces of pure Ti along the race track of the target, while in HiPIMS mode it is controlled by varying the pulse on-time. B/Ti-ratios are measured by XRF and ICP-OES for cross-validation and mechanical properties are measured by nanoindentation (Hardness, Young’s Modulus) as well as cube corner indentation and microcantilever bending for K_IC to rule out influences of stresses.

References
[1] C.Fuger Surface and Coatings technology, 2022, 446, 128806.
[2] J. Thörnberg Surface and Coatings technology, 2020, 404, 126537.