Influence of Ta on the oxidation resistance of WB2−z coatings

Abstract

Ternary W1−xTaxB2−z is a promising protective coating material possessing enhanced ductile character and phase stability compared to closely related binaries. Here, the oxidation resistance of W1−xTaxB2−z thin films was experimentally investigated at temperatures up to 700 °C. Ta alloying in sputter deposited WB2−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 kp* = 6.3 ⋅ 10−4 µm2/s for WB2−z, to kp* = 1.1 ⋅ 10−4 µm2/s for W0.66Ta0.34B2−z as well as kl = 2.6 ⋅ 10−5 µm/s for TaB2−z, underlined by decreasing scale thicknesses ranging from 1170 nm (WB2−z), over 610 nm (W0.66Ta0.34B2−z) to 320 nm (TaB2−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 WB2−z does not only positively influence the ductile character and thermal stability but also drastically increases the oxidation resistance.