New insights on fracture tolerant and superhard hexagonal TMB2 thin films

Abstract

In the progression of novel protective thin film materials, the attention for transition metal diborides (TMB2) substantially increased during the last years. The unique strength of their hybridized covalent bonds combined with their hexagonal close-packed (hcp) structures is a big advantage and a limiting factor. The related brittleness, variety of crystal structures, and stoichiometries depict significant challenges for a broad usage of physical vapor deposited TMB2 coatings in diverse applications. This study focuses on a deeper understanding of elasto-plastic properties, including fracture mechanics and synthesis-structure-property relationships of various binary and ternary TMB2-based thin films.The thermodynamic stability of AlB2 (α) and W2B5 type (ω) structured WB2±z is investigated by ab-initio density functional theory calculations. While ω-WB2 reveals the energetic minimum near the stoichiometric composition (hence, a perfectly arranged crystal structure), the α-phase is preferentially stable in the sub-stoichiometric regime stabilized by Boron vacancies. Theoretical calculations prove α-WB1.5 as the most energetically (energy of formation) and mechanically (c44) stable stoichiometry. Nanoindentation experiments revealed a pronounced anisotropy for super-hard 0001 WB2-z textured films. Increasing 10-11 orientation significantly reduces the film hardness, attributed to easier to activate basal slip planes. Still, no texture-related fracture behavior is observed by micro-cantilever bending. Interestingly, the addition of Ta, forming α-W1-xTaxB2-z with x = 0, 0.07, 0.14, 0.26, 0.43, 0.70, and 1, maintains a single-phase structure up to x = 0.26 Ta on the metal sublattice. Solid solution and grain refinement strengthening are the main effects of increasing hardness from 40.8 ± 1.5 GPa to 45.0 ± 2.0 GPa. In contrast, the intrinsic fracture toughness (KIC) generally decreases with rising Ta but reveals a maximum of KIC = 3.8 ± 0.5 MPa√m for α-W0.93Ta0.07B1.76, being in good correlation to theoretical predictions. In addition to the enhanced mechanical properties, low amounts of Ta also improve the oxidation resistance. The scale thickness decreases from 8.0 μm for pure WB2-z to 1.6 μm for TaB2-z after oxidizing in ambient air at 600°C for 1000 min. Moreover, the growth mode changes from a paralinear for W-rich to a linear – but retarded – mode for Ta-rich films. Hence, an optimum composition for α-W1-xTaxB2-z coatings in the range of x = 0.2 to 0.3, combining fracture resistance, super-hardness, and decelerated oxidation kinetics owing to the formation of denser scales.The second part of the thesis investigates the sputter growth conditions (DC-mode) and related elasto-plastic properties of TiB2+z based thin films. The focus is on the influence of structural mannerisms – such as the anisotropic behavior of hexagonal crystals or the formation of nano-columnar tissue phases – on the mechanical properties of TiB2+z. By systematically varying the target-substrate distance and the deposition pressure, a broad compositional variation from nearly stoichiometric α-TiB2.07 up to super-stoichiometric α-TiB4.42 is achieved. A significant deviation in the angular distribution of sputtered B and Ti species is evident, where B atoms are preferentially emitted along the target normal. Thorough structure-mechanical analysis revealed a deposition pressure related to 0001 film texture, which is essential to achieve super-hardness. Higher pressure (> 1.2 Pa) arising 10-11 and 1000 orientations contribute to a hardness drop of more than 10 GPa. A similar slope of H with varying 0001 fraction is observed for WB2-z, suggesting a relation in anisotropic behavior for hexagonal structured TMB2 films. HR-TEM analysis gains insights into the formation of the B-rich tissue phases and the related morphology, based on the broad stoichiometric variation. B to Ti ratios above 2.5 force the formation of smaller nano-columns (i.e., decreased column size from 10 to 2 nm) to distribute excess B on the surplus of tissue phase fraction. Consequently, this reduces fracture toughness from about 3.0 to 2.5 MPa√m, related to the increased B-rich tissue phase fraction, providing the weakest pathways for crack propagation. The small nano-columns also promote grain boundary sliding events leading to a declining hardness of about 5 GPa for 0001 textured TiB2+z films.