Substoichiometry and tantalum dependent thermal stability of α-structured W-Ta-B thin films

Abstract

Physical vapor deposited (PVD) WB2 thin films crystallize in the α-AlB2-prototype structure rather than in their thermodynamically stable ω-W2B5-z-prototype structure. Contrary to the majority of α-AlB2-type transition metal diborides (TMB2), α-WB2 exhibits a more ductile character. Combining density functional theory and sophisticated experiments, we show that the stability of α-WB2 thin films is basically influenced by point defects such as vacancies present in PVD materials. With the help of α-TaB2 (one of the most ductile TMB2 with high preference for α-AlB2-type), the thermally driven decomposition and phase transformation of α-W1-xTaxB2-z to the ω-W2B5-z-type can be shifted to temperatures above 1200 °C.