<p> Environmental Barrier Coatings (EBCs) protect ceramic gas turbine engine components from corrosion by high temperature water vapor, but the coatings often form complex metastable microstructures upon plasma spray deposition. In ytterbium disilicate (YbDS) and its yttrium-doped counterpart (Y/YbDS), two coatings compatible with SiC/SiC parts, plasma spray forms a largely cracked, mechanically weak amorphous phase comprising up to ~80% of the coating’s volume. Therefore, the coatings must undergo a pre-service heat treatment to crystallize into stable phases and heal cracks. During the treatment, however, interplay between thermal expansion and crystallization contraction can cause vertical cracks which expose the component to the corrosive atmosphere. Remedial treatments with long, high temperature holds (~1300 ºC) can both crystallize the coating and heal existing cracks. However, these temperatures cause unnecessary grain growth that reduces the structural integrity of the coating over its lifetime.</p> <p>Here we propose an alternate heat treatment informed by experiments and modelling that removes metastable phases, heals cracks, and reduces time at temperature to prevent significant grain growth. First, we determine crystallization and phase change kinetics by applying the Ozawa-Flynn-Wall and Vyazovkin kinetic methods to differential scanning calorimetry (DSC) data. Next, we track locations and microstructural effects of phase evolution using correlative Raman spectroscopic mapping, scanning electron microscopy (SEM), and X-Ray diffraction (XRD). We interpret the formation of three distinct phases – a major phase of stable β-YbDS, and minor phases of stable Χ2-YbMS and metastable α-YbDS – within the existing framework of kinetic theory and quantify differences in their transformations between YbDS and Y/YbDS. We find that cracks in the coating heal through the crystallization of the amorphous phase and the transformation of the metastable phase although the mechanisms remain unclear. Each phase transformation causes a bulk volumetric change which we measure using dilatometry and use to calculate delamination stresses during a simulated heat treatment. Lastly, we determine the viability of our heat treatment compared to the industry standard.</p>

<p dir="ltr">A current engineering challenge is to understand and validate material systems capable of maintaining structural viability under the elevated temperature and environmental conditions of hypersonic flight. One aspect of this challenge is the joining of multiple materials with thermal expansion mismatch, which can lead to residual stress, resulting in debits in component lifetime under in-service loading. The focus of this work is a series of studies focused on a ceramic-metal composite (WC/Cu), a zirconia coating applied to a carboncarbon (C/C) composite, and a silicide (R512E) coating applied to a Nb-based alloy (C103). Each of these material systems are candidates for elevated temperature applications in which dissimilar constituents result in residual stress in the material. Each study leveraged experimental residual strain measurements, with the primary focus on the use of synchrotron X-ray diffraction, in conjunction with representative models, and microscopy to illuminate the active mechanisms in the development and evolution of residual stress in the bulk material. The combination of experimental and modeling predictions provides a framework to inform the viability and lifing of material systems exhibiting dissimilar expansion properties.</p>