Oxidation protection at 1700 °C and oxygen barrier reinforcement of HfB2-HfSi2 coating modified by Zr-based borosilicate

To enhance the anti-oxidation performance of ultra-high temperature ceramic (UHTC) coatings applied on carbon-based materials under extremely high-temperature conditions, HfB 2 -HfSi 2 powders modified with a Zr-based borosilicate glass (BSG) were employed to fabricate HfB 2 -HfSi 2 -BSG(ZrO 2 ) composite coatings via self-propagating high-temperature synthesis (SHS) and spark plasma sintering (SPS). A systematic investigation was conducted into the microstructural changes and the oxygen-barrier strengthening mechanism of the Zr-based borosilicate-modified HfB 2 -HfSi 2 coating when exposed to 1700 °C. The findings indicated that the introduction of ZrO 2 facilitated the in-situ development of an oxidized Zr-Hf-Si-O multiphase glassy layer on the surface, which significantly reinforced the blocking of inward oxygen diffusion and the retention of volatile reaction products. The optimal overall oxidation resistance of the coating was achieved when the BSG:ZrO 2 proportion was set to 1:1. The average oxidation rate and the average oxygen permeability were 0.41 × 10 −7  g cm −2  s −1 and 0.09%, which were 61.68% and 82.0% lower than those of the coating without ZrO 2 addition. However, doping with an excess of ZrO 2 caused the glass layer to exhibit an overly high viscosity and induced dendritic precipitation, which consequently compromised self-healing sealing capability. This study reveals the oxygen-barrier mechanism of HfB 2 -HfSi 2 -BSG(ZrO 2 ) coatings, offering a new strategy for long-life oxidation-resistant coatings in extreme environments.