THE THERMODYNAMIC AND KINETIC STUDY OF CMAS - TBC MATERIAL INTERACTIONS

Abstract

The effects of rare earth (RE) elements on the resistance of zirconia thermal barrier coating materials to corrosion by calcium, magnesium, aluminium and silicon oxide glass (CMAS), were systematically investigated using ceramic pellets. Yb, Er, Gd and Sm elements were assessed as stabilising agents of zirconia at increasing concentrations for CMAS reactions at 1300 C across timeframes of 1 to 60-minutes. Two distinct microstructures of the ceramic pellet-CMAS reaction layer were observed, identified as a dense layer microstructure and a porous layer microstructure. The presence of each microstructure was dependent on the RE ionic radius and concentration. The thickness of the reaction layer and overall volume of precipitated reaction products increased with increasing RE ionic radii. Therefore, an optimal RE element would exhibit dense layer forming microstructure with the lowest overall infiltration depth. CMAS loading volume significantly impacted the rate of reaction product precipitation. The volume of apatite precipitate was inversely proportional to the CMAS loading quantity. Furthermore, 4D scanning transmission electron microscopy (4D-STEM) and energy dispersive x-ray-transmission electron microscopy (EDX -TEM) analysis was conducted to better understand the initial reaction layer microstructure that forms between Gadolinium Zirconate (GdZO) or Samarium Zirconate (SmZO) and CMAS at 1300 C. An alternating columnar microstructure was observed across the reaction layer interface comprising of both RE oxyapatite and Zr fluorite phases. With a Zr rich diffusion layer forming directly below the reaction layer interface. This microstructure suggests a eutectic solidification mechanism where the apatite and fluorite phases precipitate simultaneously. These techniques also highlighted the presence of an RE rich interaction volume around the ceramic grain boundaries close to the reaction layer. It is proposed that the larger diffusion coefficient of RE ions through the bulk ceramic in comparison to Zr is the leading cause for this observation, while the driving force would be related to the concentration gradient of these elements at the reaction layer. The Young's modulus, fracture toughness and hardness of the REZO materials was measured and calculated to observe how these properties are influenced by the reaction layer. A novel approach for accurately measuring reaction layer thickness without the reliance on image analysis was proposed and conducted. In addition, it was observed how the reaction layer affects the crack propagation around the local area. This led to the understanding that the barrier layer did not negatively influence the fracture toughness of the ceramic close to the reaction layer interface.

Date of Award	15 Mar 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Ping Xiao (Main Supervisor) & Ying Chen (Co Supervisor)