Melting temperature, emissivity, and thermal conductivity of rare-earth silicates for thermal and environmental barrier coatings
source:SCRIPTA MATERIALIA
In recent years, rare-earth silicates have become the industry standard for coating state-of-the-art SiC ceramic matrix composite (CMC) gas turbine engine components, due to their low volatility, high melting point, and thermal shock resistance. Current research is focused on designing rare-earth silicate based thermal- environmental barrier coatings (T/EBCs) with improved resistance to CMAS (CaO-MgO-Al2O3-SiO2), steam, and crack formation, while maintaining high temperature performance and stability. In this work we compare the high temperature performance of a variety of single and multi-component rare-earth mono- and disilicates (MS, DS) and rare earth apatites by measuring their melting points and spectrally averaged visible emissivities using laser heating and radiation pyrometry. We also report room temperature thermal conductivity measured by time- domain thermoreflectance (TDTR).
In order to achieve greater efficiency, gas turbine engines must be able to operate at higher temperatures. Silicon carbide ceramic matrix composites (CMCs) are promising alternatives to current industry standard nickel superalloys due to their high temperature performance and high strength-to-weight ratio. However, the introduction of SiC in engine components brings about a new set of challenges to overcome. SiC is highly susceptible to steam corrosion and reaction with deposits of molten atmospheric debris. A suitable barrier coating for SiC components must therefore not only have a low thermal conductivity and match the coefficient of thermal expansion (CTE) of SiC, but also limit the diffusion of steam and molten debris to the SiC surface. The class of materials collectively referred to as rare earth silicates has emerged as a strong candidate for the next generation of environmental barrier coatings.
The materials studied in this work include a variety of rare earth mono- and disilicates and rare earth apatites, as well as several compositionally complex disilicates containing multiple rare earth metals. Compositionally complex materials have received much attention in recent years, especially for high temperature applications, where their high configurational entropy offers the potential for improved stability. Multiple principle component barrier coating materials have been shown to have advantageous thermal and mechanical properties in a number of high temperature applications throughout literature, including low thermal conductivity and high hardness and modulus. Although rare earth silicates have been well studied up to 1500 °C , the target surface component temperature for CMC combustors and vanes is >1482 °C, and few studies have considered these materials at temperatures near melting since the work of Toropov and Bondar in the 1960s. In this work we use a high-power infrared laser to locally heat the silicate samples beyond their melting point and employ a spectrally-resolved radiation pyrometry technique to simultaneously measure melting point and spectrally-averaged visible emittance. We report the resulting melting point and high temperature thermal emittance, hereafter referred to as “emissivity”, for a variety of single and multiple principle component rare earth silicates. We also measure room temperature thermal conductivity using the optical pump-probe technique, time-domain thermoreflectance. Finally, we investigate the effects of high temperature laser heating and melting on the microstructure of select rare earth silicates through energy dispersive spectroscopy (EDS), and microfocus-Xray diffraction (XRD), reported in Supplemental Materials. These measurements are essential to the evaluation of new coating materials that would facilitate increases in gas turbine inlet temperature and improve efficiency in next-generation engines.
Room temperature thermal conductivity was measured via time-domain thermoreflectance (TDTR), as described in Supplemental Materials, and the results are plotted in Fig. 1. We find that the single-component disilicates collectively demonstrate substantially higher thermal conductivity than the single-component monosilicates or apatites, with Sc2Si2O7 exhibiting a notably high thermal conductivity exceeding 10 Wm−1K−1. Although the monosilicates measured have larger rare earth (RE) crystal radii than their disilicate counterparts, this appears to not be the only factor in play. When considering Yb2Si2O7 and Yb2SiO5, the Yb coordination numbers are very similar, resulting in nearly identical crystal radii. However, the two systems exhibit drastically different thermal conductivities. We hypothesize this is due to greater distortion in the monosilicate lattice [28], leading to increased phonon scattering.