Thermally stable superhard diborides: An ab initio guided case study for V-W-diboride thin films

Abstract

Recent investigations on ternary diborides revealed their enormous potential for the use as protective thin films. Within this study we investigate DC magnetron sputtered V1−xWxB2 thin films (with x = 0, 0.05, 0.13, 0.21) concerning their morphology, structure, mechanical properties, and thermal stability. Ab−initio calculations revealed, that while the equilibrium state of VB2 is the AlB2-prototype (α-type), that of WB2 is the W2B5−x-prototype (ω-type). However, when considering the formation of point defects such as vacancies, the preferred structure of WB2 can even be α, while that of VB2 remains with α. This allows for the formation of metastable α-WB2 – and especially ternary α-V1−xWxB2, which are stable with respect to their constituting α-VB2 and α-WB2 phases (as suggested by DFT and vacuum annealing for 1h at 1400∘ C)even without considering point defects – when using physical vapor deposition, as thereby the point defect concentration is typically high. Our results further show that the highest W-containing ternary coating, V0.69W0.21B2 (which is still single-phase α-structured with a hardness of ∼ 40 GPa combined with low tensile stresses of 0.3 Pa in the as deposited state) exhibits the highest hardness of ∼ 43 GPa and ∼ 40 GPa after 1-h-vacuum annealing at 1000 and 1400∘ C, respectively, among all coatings studied. Their indentation modulus ( ∼ 500 GPa) and also their microstructure (single-phase α-structure, pronounced 0001-orientation, column diameter of 14.7 nm) showed no significant changes up to 1000∘ C.