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

Abstract

Controlling the mechanical performance of TiB₂-based coatings is challenging due to the complex interplay between chemical composition, texture, microstructure, and residual stress. To decode this, we synthesized TiB2±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 TiB1.46 displaying a more homogeneous morphology. Fracture toughness from cube corner indentation varied strongly with residual stress: from ≈0.3 MPa√m (DCMS TiB₂.₁₂) to ≈5.0 MPa√m (HiPIMS TiB₂.₁₁). 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 35–40 GPa and 380–430 GPa, again dominated by stress rather than stoichiometry. HiPIMS TiB₂.₁₁ 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.