Solubility, crystal growth, and film-formation mechanism of NbC-modified ZrC coating under oxyacetylene flame

Herein, the ablation resistance of plasma-sprayed NbC-modified ZrC (ZrC-NbC) coatings was firstly tested using an oxyacetylene flame. Next, the phase composition of the ZrO₂-Nb₂O₅ (NbO_2.5) plates heat-treated at 2100 °C in an argon atmosphere was determined. The results indicated that, the ZrC-20 mol% NbC coating formed a dense oxide layer after ablation for 120 s and a composite film consisting of self-bridged ZrO₂ columnar crystals up to 300 s, achieving the best ablation property. At 2100 °C, the solubility (solid solubility and adsorption capacity) of niobium suboxide in ZrO₂ was 30 mol%. The stable phases were t-ZrO₂ and Nb as the NbO_2.5-addition was no more than 20 mol%, and t-ZrO₂, Nb, and NbO when it ranged from 25 to 40 mol%. Furthermore, ZrO₂ increased the viscosity of Nb-O liquids and ensured the transformation of NbO_2.5 into Nb and NbO phases, improving the thermal stability. Nb-O liquids improved the transfer of Zr and formed gradients of temperature and oxygen concentration, strengthening the film-forming ability of the ZrC coating under an oxyacetylene flame.