Ternary W 1−x Ta x B 2−z is a promising protective coating material possessing enhanced ductile character and phase stability compared to closely related binaries. Here, the oxidation resistance of W 1−x Ta x B 2−z thin films was experimentally investigated at temperatures up to 700 °C. Ta alloying in sputter deposited WB 2−z coatings led to decelerated oxide scale growth and a changed growth mode from paralinear to a more linear (but retarded) behavior with increasing Ta content. The corresponding rate constants decrease from k* p = 6.3 ⋅ 10 −4 µm 2 /s for WB 2−z , to k* p = 1.1 ⋅ 10 −4 µm 2 /s for W 0.66 Ta 0.34 B 2−z as well as k l = 2.6 ⋅ 10 −5 µm/s for TaB 2−z , underlined by decreasing scale thicknesses ranging from 1170 nm (WB 2−z), over 610 nm (W 0.66 Ta 0.34 B 2−z) to 320 nm (TaB 2−z) after 10 min at 700 °C. Dense and adherent scales exhibit an increased tantalum content (columnar oxides), which suppresses the volatile character of tungsten-rich as well as boron oxides, hence being a key-factor for enhanced oxidation resistance. Thus, adding Ta (in the range of x = 0.2-0.3) to α-structured WB 2−z does not only positively influence the ductile character and thermal stability but also drastically increases the oxidation resistance.
C. Fuger, B. Schwartz, T. Wojcik, V. Moraes, M. Weiss, A. Limbeck, C.A. Macauley, O. Hunold, P. Polcik, D. Primetzhofer, P. Felfer, P.H. Mayrhofer, and H. Riedl
Journal of Alloys and Compounds