Ablation behavior of PEO coatings on niobium alloy

Abstract

To improve the ablation resistance of niobium alloy, plasma electrolytic oxidation (PEO) was introduced to fabricate coatings with different pore structures on niobium alloy by adjusting the applied voltage. The pore volume and shape were investigated by X-ray computed tomography, and the ablation behavior of PEO coatings was also thoroughly discussed. It was found that the ablation behavior of PEO coatings could be divided into the crack propagation stage and the coating peeling stage, which are related to the specific pore structure and ablative oxidation. The stress concentration in the pore tip increased by 60.61% as the pore shape transforms from isolated pores to connected pores, accelerating crack initiation and propagation. Subsequently, oxygen erodes the substrate through connected pores and cracks, which is accelerated by high overall porosity (23.4%) and severe crack propagation, followed by ablative oxidation of the substrate leading to the eventual peeling of the PEO coating. After 15 min of ablation, no coating peeling occurred for the isolated pore structure, while the coating peeling area of the connected pore structure reached 82.1%. Thus, the isolated pores could transfer stress and inhibit further diffusion of oxygen into the coating, improving the ablation resistance of PEO coatings. It is expected that this research would provide feasible ideas for the design and fabrication of ablation-resistant coatings on niobium alloy.