Tuning microstructure and mechanical properties in superstoichiometric to substoichiometric TiB2 thin films: a DCMS and HiPIMS study

Controlling the mechanical performance of TiB 2 -based coatings is challenging due to the complex interplay between chemical composition, texture, microstructure, and residual stress. To decode this, we synthesized TiB 2±z coatings with B/Ti ratios from 1.46 to 3.16 using non-reactive Direct Current Magnetron Sputtering (DCMS) and High-Power Impulse Magnetron Sputtering (HiPIMS), varying composition systematically within each method by Ti addition (DCMS and HiPIMS) and pulse on-time (HiPIMS). This approach mostly avoided changes to the films' morphology, allowing a focused investigation of compositional effects. B-rich coatings showed strong (0001) texture and B-rich tissue phase; both declining by decreasing the B-content, but the texturization returning at strongly substoichiometric compositions. Despite this, most coatings featured dense columnar grains ≈10 nm wide, with only TiB 1.46 displaying a more homogeneous morphology. Fracture toughness from cube corner indentation varied strongly with residual stress: from ≈0.3 MPa√m (DCMS TiB 2.12 ) to ≈5.0 MPa√m (HiPIMS TiB 2.11 ). Cantilever bending – performed on freestanding beams that are free from deposition-induced residual macrostress – revealed consistent intrinsic toughness (≈2.5–3.5 MPa√m) across all compositions, independent of B/Ti ratio or the presence of a tissue phase. Hardness and modulus ranged from 35 to 40 GPa and 380–430 GPa, again dominated by stress rather than stoichiometry. HiPIMS TiB 2.11 combined superhardness (40.4 GPa), high stiffness (425 GPa), and high fracture toughness (stressed and stress-less) – demonstrating that optimized HiPIMS processing enables coatings with exceptional mechanical performance. • Simple but effective way to tune stoichiometry in DCMS and HiPIMS TiB 2 thin films without altering microstructure • Cube corner indentation reflects stress states; cantilever bending reveals intrinsic toughness. • Residual stresses dominate fracture toughness more than B/Ti-ratio or tissue phase. • Superhardness and high fracture toughness can be achieved at the same time through fine-tuning of HiPIMS-parameters.