Influence of GdO coatings on the oxidation behavior of Zirconium Diboride

Kurzfassung

ZrB2 combines properties such as high melting point above 3000°C and a low relative density of 6.09 g/cm³ to project itself as a suitable candidate for leading edges or Thermal Protective Systems of hypersonic vehicles. ZrB2 forms a thick and porous oxide scale at temperatures above 1500°C in oxidizing atmosphere and offers no protection against further oxidation. Oxidation protective overlay coatings such as Nb and HfO2 on ZrB2 have shown an improvement in oxidation resistance of ZrB2. These studies have shown that a suitable material as an overlay coating can further enhance the oxidation protection. Therefore, GdO coatings have been investigated as a potential new candidate as an overlay coating in this study. The coatings were applied by means of reactive magnetron sputtering (see Figure 1). Subsequently the coated specimens were exposed at 1500°C, 1600°C, and 1700°C for 15 min, 30 min, and 60 min in a tube furnace. SEM, EDS, XRD, and TEM were used to understand the oxidation mechanisms and kinetics of GdO coated ZrB2. GdO coatings have performed much better than previous Nb and HfO2 coating in terms of oxidation resistance. Two protective mechanisms were identified for such behavior. On one hand, the coating reacts with the liquid B2O3 and forms a solid/liquid solution at the surface which delays the boria evaporation. This newly formed glass prevents the oxygen diffusion to the reaction front of ZrB2 during oxidation as long as it remains at the surface. On the other hand, the reacted glass infiltrates the porous ZrO2 and forms a dense oxide scale due to liquid phase sintering. The dense oxide zone provides further protection against oxygen diffusion to the reaction front of ZrB2 even when the protective glass evaporates completely.