Structural Characteristics And Ablation Resistance Of MAO Coatings On Niobium Alloy

Niobium alloys are widely used in the aerospace industry due to their high melting point,low density,and good machinability.However,the high affinity of niobium alloys for oxygen leads to severe ablation damage of hot end components during high-temperature oxygen-rich flame flushing,which greatly affects the service performance and lifetime of the material.Aiming at the problem of ablation damage of niobium alloy under high-temperature service environment,micro-arc oxidation（MAO）technology was introduced to prepare ablation-resistant ceramic coating on the niobium alloy,which provides feasible ideas for the design and fabrication of niobium alloy ablation-resistant coating.The phase composition of the MAO coatings was examined by using XRD and XPS analysis methods,and the pore distribution of ceramic coatings was obtained by SEM and XCT microscopic characterization.To investigate the effects of applied voltage and Na₂SiO₃ concentration on the pore structure of MAO coatings,elucidating the pore structure transformation pattern.High-temperature ablation tests were used to investigate the high-temperature protection performance of the ceramic coating,and numerical simulations were performed for the stress distribution of the coating under ablation conditions.To clarify the ablation behavior of ceramic coating with different pore structures and to establish the regular relationship between pore structure and ablation resistance performance.The results showed that the increase in applied voltage leads to a significant improvement in coating thickness,where dielectric breakdown appears difficult.The discharge frequency decreases at high voltages,but the energy carried by a single strong discharge increase.The intense discharge processes from the coating surface to the substrate,and the pore shape of the coating changes from isolated pores to connected pores,where the transformation of the pore shape is accompanied by an increase in the pore volume.Pores of 100～500μm³ appear in the Nb₄₄₀ coating,reaching 7%.As the concentration of sodium silicate increases,a large amount of SiO₃^2-enters the interior of the coating and produces more molten oxides.The refilling of the pores by molten oxides makes the pores decrease substantially,while the pore shape changes to isolated pores.The S0 coating has the highest overall porosity of 27.5%.In contrast,the S10 coating has the lowest overall porosity of 10.7%.However,the excessive sodium silicate concentration leads to more intense discharge at the defects of the coating,resulting in localized ablation.After high-temperature ablation tests,it was found that the ablation behavior of MAO 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.As the pore shape changes from isolated pores to connected pores,the stress concentration in the pore tip increases substantially,accelerating the initiation and propagation of cracks.Subsequently,oxygen erodes the substrate through connected pores and cracks,which is accelerated by high overall porosity and severe crack propagation,followed by ablative oxidation of the substrate leading to the eventual coating peeling with a peeling area of up to 82.1%.In contrast,the coatings with isolated pore structures showed no significant damage and peeling after 15 minutes of ablation,which could transfer stress and inhibit further diffusion of oxygen into the coating during the ablation process,thus improving the ablation resistance of the MAO coating.