Functionally Graded Oxide Scale on (Hf,Zr,Ti)B2 Coating with Exceptional Ablation Resistance Induced by Unique Ti Dissolving.

Multicomponent Ti-containing ultra-high temperature ceramics (UHTCs) have emerged as more promising ablation-resistant materials than typical UHTCs for applications above 2000 °C. However, the underlying mechanism of Ti improving the ablation performance is still obscure. Here, (Hf,Zr,Ti)B₂ coatings are fabricated by supersonic atmospheric plasma spraying, and the effects of Ti content on the ablation performance under an oxyacetylene flame are investigated. The (Hf_0.45Zr_0.45Ti_0.10)B₂ coating shows superior ablation resistance and cycling reliability at ≈2200°C. A functionally graded oxide scale comprising an outer dense layer and an underlying fine granular layer formed. The former is a better oxygen barrier owing to fewer cracks and the latter has high strain tolerance due to finer grain size. The uniform dissolving of ≈4 mol% Ti in the inner layer results in grain refinement via sluggish diffusion and thus stress release. For the outer layer, Ti segregation at the nanoscale leads to a metastable cubic (Hf,Zr,Ti)O₂ and local severe lattice distortion, inhibiting the propagation of cracks. Ti ions' unique dissolving in the oxide scale enables a strong oxygen diffusion barrier with high strain tolerance, which is responsible for superior performance. This study provides new insights into the ablation behavior of Ti-containing multicomponent UHTCs.