Thermal stability of reactive sputtered tungsten oxide coatings

The thermal stability of different W–O coatings, W100, W90O10, W54O46, W30O70 and W25O75, were studied by in-situ X-ray diffraction at elevated temperatures up to 900 °C. The coatings were deposited by DC reactive magnetron sputtering from a pure tungsten target in an Ar + O2 atmosphere onto Fecralloy alloy. The evolution of the structure of the coatings was studied in both protective and oxidant atmospheres. Three groups of films were identified: (1) W25O75 which showed structural evolution following the W–O phase diagram; (2) amorphous O-deficient WO3 whose structure followed the W–O phase diagram either as WO3 or as its chemical composition depending on annealing in oxidant or protective atmospheres, respectively; and (3) low O-content crystalline films that oxidized from 500 °C.