Thermal protection mechanism of novel high-entropy rare-earth niobate coating deposited by atmospheric plasma spraying

Abstract

As aero-engines face more severe high-temperature environments, the choice of thermal protection coating materials becomes very limited. In this study, high-entropy rare-earth niobate (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)NbO₄ (HE-RENbO₄) was synthesized by a solid-phase reaction. HE-RENbO₄ has excellent thermal physical properties, with a low thermal conductivity (1.452 W·m⁻¹K⁻¹, 600 ℃) and high thermal expansion coefficient (11.04 × 10^-6 K⁻¹, 1200 ℃). By optimizing the process parameters of atmospheric plasma spraying, the HE-RENbO₄ coating with excellent mechanical properties, a high density and good spreading was fabricated. During the spraying process, the reversible phase transformation of HE-RENbO₄ coating occurred, resulting in the existence of t and m phases. Using plasma spraying flame to investigate thermal protection performance and ablation mechanism of the coating. During the plasma thermal shock process, the crystallinity of the coating was enhanced, and a large number of closely packed grains were formed on the surface. With the progress of the test, cracks increased and expanded, and finally led to the failure and spalling. Due to the high fracture toughness, the coating cracked mainly in the form of intergranular fracture, forming the “rock candy” fracture surface. In addition, when the coating surface temperatures were 1000 ℃ and 1200 ℃, the corresponding heat insulation temperatures were approximately 270 ℃ and 210 ℃ respectively, which proved that the HE-RENbO₄ coating has good thermal protection performance. This study demonstrates the application feasibility of the high-entropy rare-earth niobate as a thermal protection coating material and elucidates its ablation mechanism and thermal protection performance.