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

Physical vapor deposited (PVD) WB2 thin films crystallize in the alpha-AIB(2)-prototype structure rather than in their thermodynamically stable (omega-W2B5-z-prototype structure. Contrary to the majority of alpha-AlB2-type transition metal diborides (TMB2), alpha-WB2 exhibits a more ductile character. Combining density functional theory and sophisticated experiments, we show that the stability of alpha-WB2 thin films is basically influenced by point defects such as vacancies present in PVD materials. With the help of alpha-TaB2 (one of the most ductile TMB2 with high preference for alpha-AlB2-type), the thermally driven decomposition and phase transformation of alpha-W1-xTaxB2-z to the omega-W2B5-z-type can be shifted to temperatures above 1200 degrees C. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.