Publications

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.