Ablation Resistance and Mechanisms of Non-Equimolar (HfZrTaTiY)B₂ Under Cyclic Oxyacetylene Flame Conditions

To explore ultra-high temperature ceramics (UHTCs) with improved ablation resistance, a novel non-equimolar high-entropy diboride (HfZrTaTiY)B₂ and equimolar (HfZrTaTi)B₂ ceramic bulks were fabricated by spark plasma sintering (SPS). Their ablation behaviors were evaluated under cyclic oxyacetylene flame conditions at a heat flux of 4.18 MW/m² (maximum temperature ∼2100°C). By comparison, the non-equimolar design and the introduction of element Y further enhance the ablation resistance of the bulk material as well as the thermal stability of its surface oxide layer, resulting in a mass ablation rate of −0.039 mg/s and a linear ablation rate of −0.088 µm/s. Notably, a pronounced eutectic-like structure is observed in the oxide scale during cyclic ablation. The clarification of this eutectic precipitation behavior may aid in interpreting ablation morphologies in other multi-component UHTCs.