Application of Oxidation Protective Coatings on Diboride Based Ultra-High Temperature Ceramics: Fundamental Studies on the Oxidation Behavior

Kurzfassung

The aerospace industry is currently undergoing a renaissance due to visions such as space tourism or the colonization of Mars. As a result, the demand for high-performance materials for extreme temperatures is ever-increasing. The re-entry operation of a spacecraft or a hypersonic flight within the Earth's atmosphere induces extreme thermal loads on certain components such as leading edges or tiles of the thermal protective system. No current state-of-the-art material is suitable to face these harsh environmental requirements and to ensure the reusability of such kind of components, which is mandatory to accommodate the huge economic costs. Potential candidates for such kind of reusable components are the group of ultra-high temperature ceramics, in particular diboride-based transition metals. These materials combine a high thermal conductivity κT, high oxidation resistance up to ~1200°C through self-passivation, low density, and extremely high melting points (TM >3000°C). Especially ZrB2 offers a high potential for corresponding applications in hypersonic missiles due to its low density (~6,085 g/cm³). Unfortunately, the insufficient oxidation resistance at the desired temperatures between 1500°C and 1700°C is the current bottleneck for ZrB2, forming a porous oxide scale of ZrO2 and volatile B2O3 glass at the surface. For this reason, a novel approach to improve the oxidation resistance of diboride-based UHTCs is proposed in this study and is investigated extensively. In a first attempt, it has been shown that protective overlay coatings of different material classes (metallic, ceramic) can be deposited on ZrB2 surfaces by means of (reactive) magnetron sputtering. The oxidation experiments have clearly shown, that the applied coatings are thermal shock resistant up to 1700°C and improve the oxidation resistance of the ZrB2 substrate. Protective mechanisms were found to be the dense microstructure of the applied solid coating, the formation of a protective liquid layer at the surface (glass stabilizer), or the densification of the uppermost section of the formed oxide scale (solid scale densifier). Tested coatings including gadolinium-based oxide coatings (GdO), metallic niobium coatings, and ceramic hafnia coatings (HfO2). Compared to uncoated baseline ZrB2, the GdO coatings, in particular, have contributed a significant reduction in the oxide scale formation and reduced the scale thickness by approximately -60% after an exposure for 30 min at 1700°C. The protection mechanism is based on the formation of a protective liquid layer (Gd,B)O at the surface, which provoked the permeation of oxygen molecules to the oxidation front of the underlying ZrB2 substrate. Simultaneously, the protective liquid induced the densification of the uppermost section of the oxide scale by liquid phase sintering. The densification reduced the Knudsen diffusion of oxygen molecules to the oxidation front of the ZrB2. Similar protection mechanisms were found for metallic Nb coatings. The coating reduced the oxide scale thickness by -55% after 30 min at 1700°C. The metallic Nb coatings were found to be less protective than the GdO coatings due to the more volatile nature of the formed (Nb,B)O liquid solution. However, the densification of the scale was found to be sufficient to ensure the precipitation of secondary ZrO2 (dissolved in liquid B2O3) at the surface during the evaporation of the protective liquid B2O3. A subsequent densification of the oxide scale was observed at 1700°C or extended exposure times of 60 min. In contrast to the glass stabilizing coatings, the dense and solid HfO2 coating protected the substrate and induced the reduction of the Knudsen diffusion of oxygen molecules to the oxidation front of ZrB2. As a result, the protective coating reduced the oxide scale thickness by -48% after 60 min at 1700°C. The inevitable formation of liquid B2O3 beneath the dense HfO2 coating detached the coating from the formed oxide scale and induced the formation of cracks across the coating thickness. .The ruptured coating has been densified due to the precipitation of secondary ZrO2 at the surface and improved the oxidation resistance at 1700°C or extended exposure times of 240 min. The comparison of the performance of uncoated ZrB2 with coated ZrB2 proved the oxidation protection of the substrate and validated the approach of the application of protective overlay coatings on diboride-based UHTCs. The solution to improve the oxidation behavior of UHTCs by overlay coatings represents a new and highly promising research field, which has the potential to be established in the UHTC community.