Thermal Cycling Performance And CMAS Corrosion Behavior Of GdPO₄ Thermal Barrier Coatings

Thermal barrier coatings?TBCs?are one of key techniques for advanced turbine engine blades.The extensively used TBCs are made of 7⁸ wt.%yttria partially stabilized zirconia?YSZ?.However,when the service temperature exceeds 1200°C,it suffers fatal failures resulted from phase transformation,accelerated sintering and serious corrosion from calcium-magnesium-alumina-silicate?CMAS?attack.Therefore,YSZ TBCs cannot meet the requirements of increasing thrust-weight ratio of the new-generation aero engine.This study aims to explore a novel GdPO₄ TBC for ultrahigh temperature applications.The phase composition,thermal conductivity and mechanical properties of GdPO₄ were investigated.GdPO₄/YSZ double-ceramic-layer?DCL?TBCs were produced by atmosphere plasma spraying?APS?.The microstructure,sintering behavior,thermal cycling duration and CMAS corrosion behavior of the TBCs were studied.The results could provide theoretical foundation for developing new-generation TBCs for ultrahigh temperature applications.GdPO₄ powders were synthesized through a chemical coprecipitation and calcination method,followed by cold pressing to obtain bulk materials.The phase composition,thermal conductivity and fracture toughness of GdPO₄ were studied.GdPO₄ powders and bulks all exhibited monoclinic crystal structure.GdPO₄ has ultralow thermal conductivities,with a value of⁰.98 W/m·K at1000°C,much lower than that of YSZ.The fracture toughness of GdPO₄ was measured to be 1.16±0.2 MPa·m^-1/2.A GdPO₄/YSZ double-ceramic-layer?DCL?TBC was designed and produced by APS,and its sintering behavior and thermal cycling performance were investigated.The GdPO₄ coating revealed a nanostructure,and kept phase stability during heat treatment at 1400°C for 50h.The thermal cycling test was conducted as follows:heating the coatings at 1050°C for 5 min,followed by cooling to room temperature by water quenching.After 70 thermal cycles,GdPO₄/YSZ began to spall.The spallation mainly occurred in the GdPO₄ ceramic layer,possibly attributed to the lower facture toughness of GdPO₄.CMAS corrosion behavior of GdPO₄ coatings were investigated.At 1250?,GdPO₄ reacted with CMAS melts rapidly,forming a dense reaction layer,which has two sub-layers with different chemical compositions.The upper sub-layer primarily consisted of acicular P-Si apatite,while the lower sub-layer was a mixture of P-Si apatite,anorthite and spinel phases.When the corrosion time was longer than 4 h,the chemical composition and thickness of the reaction layer kept stable,indicating that layer can effectively prevent further infiltration of the molten CMAS.