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C. Fuger, V. Moraes, R Hahn, H. Bolvardi, P. Polcik, P.H. Mayrhofer, H. Riedl:
"Thermomechanical properties and oxidation resistance of ternary W1-xTaxB2-z coatings";
Talk: 47th International Conference on Metallurgical Coatings and Thin Films, ICMCTF, San Diego (USA); 2021-04-26 - 2021-04-30.

Future tasks in many different fields of academia and industry are directed towards environmental sustainability, asking also for new advance in the field of protective coating materials. Especially, transition metal diboride based thin films exhibit a great potential to be applied in various applications because of their extreme refractory character as well as interesting electrical properties. Latest studies on various diborides emphasized their strong and weak points being on the one hand high hardness and phase stability in a wide range but on the other limited fracture and oxidation resistance. Alloying concepts involving transition metals (TM) such as W, Ta, or Zr forming ternary diborides (TMI1-xTMIIxB2-z) suggest to be a proper solution to overcome these restriction [1-3].
Therefore, within this study we provide a detailed insight on the thermomechanical properties and oxidation resistance of ternary W1-xTaxB2-z thin films prepared by DC magnetron sputtering utilizing diboride based target materials. Based on theoretical investigations using density functional theory, we classified the two binary systems α-WB2-z and α-TaB2-z as highly ductile materials according to semi-empirical criteria e.g. Pettifor and Pugh. On behalf of structural investigations, we could confirm that single phased structured α-W1-xTaxB2-z thin films are formed up to Ta contents of x=0.26. The addition of Ta also clearly enhances the phase stability in inert atmosphere shifting the decomposition of the metastable α- into the thermodynamically more stable ω-structure from 800 to 1400 °C for x=0.26. During oxidation treatments up to 800 °C the high Ta alloyed coatings outperform its counterparts, forming dense and stable oxide scale also after long exposure times (1000 min). In addition, these films were investigated by nanoindentation and in-situ micromechanical bending tests to evaluate the mechanical properties. The fracture toughness only slightly decreases with increasing Ta content from 3.7 to 3.0 MPam1/2. In summary, the study highlights the potential of Ta alloyed WB2-z coatings and confirms the strategy to form ternary or multinary diborides to expand the limitations in specific material properties.