Abradable environmental barrier coatings (EBCs) are essential for improving gas turbine efficiency with ceramic matrix composites (CMCs). This study examined ytterbium disilicate (Yb2Si2O7 or YbDS) abradable EBCs with 8 %, 15 %, and 22 % porosity, deposited via atmospheric plasma spraying. Coatings were exposed to steam, CMAS, and combined steam-CMAS at 1350 degrees C for 100 hr. Results showed that porosity had minimal impact on corrosion resistance. Steam exposure formed a thin ytterbium monosilicate (Yb2SiO5 or YbMS) layer, while CMAS caused dissolution-precipitation, forming reprecipitated Yb2Si2O7 and Yb-apatite (Ca2Yb8(SiO4)6O2). Combined exposure increased CMAS penetration depth by 25 %, enhanced Yb-apatite formation, and introduced ytterbium aluminium garnet (Yb3Al5O12, YbAG). These findings demonstrate that the interaction of CMAS and steam accelerates coating degradation, highlighting the need for optimized phase composition and microstructure to enhance abradable EBC durability under turbine-relevant conditions.

The first-of-its-kind use of the active learning (AL) framework in thermal spray is adapted to enhance the prediction accuracy of the in-flight particle characteristics. The successful AL framework implementation via Bayesian Optimisation is beneficial in, first, reducing the maximum uncertainty, which greatly improves the prediction accuracy and informativeness of the existing database. Second, it reduces local uncertainty around a contrived test point that offers the capability to find improvement in a limited search area, allowing an accurate prediction around a desired test point. The dataset for Machine Learning (ML) training consists of 26 atmospheric plasma spray (APS) parameters of silicon and a further six AL-guided spray runs carried out to reduce maximum uncertainty in the initial database. On average, a 52.9% improvement (error reduction) of RMSE and an R2 increase of 8.5% were reported on the predicted in-flight particle velocities and temperatures after the AL-driven optimisation. Furthermore, the contrived test point optimisation to predict the best possible characteristics in a limited search space resulted in a three-fold increase in prediction accuracy compared to the non-optimised prediction. The AL-driven optimisation proved to be greatly beneficial for resource-intensive thermal spraying, as the framework not only allowed an accurate prediction of the in-flight particle characteristics but also found expected improvement around a desired in-flight characteristic. Furthermore, the framework uses the Gaussian Process (GP) ML model as a surrogate that generalises a global solution without necessarily involving physical and underlying mechanisms, thus extending the framework to other thermal spraying methods.

Suspension plasma spray (SPS) is a promising technique to deposit coatings with controlled microstructure. In this work, we explored this capability to produce ytterbium disilicate environmental barrier coatings with enhanced performance against steam and molten calcium-magnesium-aluminium-silicate (CMAS) corrosion. Steam exposure tests on the samples using steams with velocities of 1 m/s and 5 m/s at 1350 degrees C for 96 h revealed that low-velocity steam caused silica volatilisation whereas the high-velocity steam caused both silica volatilisation and material erosion, yet no new cracks formed in the coating. CMAS degradation test for 5 and 240 mins at 1300 degrees C on the coatings showed that the large aspect ratio of the splats facilitated reaction between CMAS and ytterbium monosilicate to form a dense layer that slowed further CMAS infiltration. These outcomes suggest that SPS has the proclivity to manufacture EBCs with improved resistance to steam and CMAS degradation.

Rare-earth disilicates are promising candidates for thermal and environmental barrier coatings (TEBC) in gas turbines that safeguard SiCf/SiC ceramic matrix composites (CMCs) from thermal degradation and environmental attacks. Here, we report a systematic investigation on novel TEBC material, γ-Y1.5Yb0.5Si2O7. The γ-phase quarter molar ytterbium–doped yttrium disilicate exhibited low thermal conductivity (1.72 W·m−1·K−1 at 1200 °C) and reduced intrinsic thermal expansion (3.17 ± 0.22 × 10−6 K−1 up to 1000 °C), ensuring promisingly effective thermal insulation and minimized thermal stress with CMC substrates. Using density functional theory (DFT), the heat capacity of γ-Y1.5Yb0.5Si2O7 was predicted higher than that of undoped γ-Y2Si2O7. Comparing these predictions to results calculated using the Neumann–Kopp (NK) rule revealed only minor variations. A metastable CMAS interaction byproduct, cyclosilicate phase Ca3RE2(Si3O9)2, was identified based on energy dispersive X-ray spectrometer (EDS) and electron backscatter diffraction (EBSD) techniques, appearing at 1300 °C but disappearing at 1400 °C. The γ-Y1.5Yb0.5Si2O7 exhibited good CMAS resistance on both dense pellets and sprayed coatings, forming a protective apatite (Ca2RE8(SiO4)6O2) interlayer that effectively hindered CMAS infiltration at evaluated temperatures. The relatively higher Y:Yb atomic ratio (> 3) in the apatite grains indicate differential reactivity with molten CMAS and provides crucial insights into the CMAS corrosion mechanism. These findings highlight the potential of γ-Y1.5Yb0.5Si2O7 as a CMC coating material, emphasizing the need for tailored microstructural optimization as a thermal sprayed coating to enhance long-term performance in extreme gas turbine environments.