Gas turbine engines for fixed-wing or rotary-wing aircraft are operated in a variety of harsh weather environments ranging from arctic, volcanic zones, to desert conditions. Operation under these degraded conditions leads to the undesired entrainment of complex particulates resulting in drastic performance losses. Hence, there is a critical need to understand the governing mechanisms to inform the development of durable thermal and environmental barrier coatings. The objective of the current work is to present a novel multiscale physics-based approach to study two-phase flows that take into account the underpinning particle transport and deposition dynamics. Sessile droplet models are presented and used to compute the contact angle at high temperatures and compared with experiments. The study also investigates the sensitivity of deposition patterns to the Stokes number and the results identify local vulnerability regions. The analysis suggests that particle size distributions and the initial trajectories of the particles are critically important in predicting the final deposition pattern.

This study focuses on binary droplet collisions of equal calcium–magnesium–aluminosilicate (CMAS) droplets formed by the melting of dust and sand ingested by gas turbine engines. Head-on, off-center, and grazing collision of 1 mm CMAS droplets traveling toward each other at a relative velocity of 100 m/s are numerically investigated using a volume-of-fluid-based direct numerical simulation approach at operating pressure and temperature of 20 atm and 1548 K, respectively. It is found that head-on and off-center collisions lead to droplet coalescence, whereas stretching behavior is observed for the grazing configuration. To elucidate the effect of viscosity, a fictitious fluid with all properties the same as CMAS except for viscosity (1/10 of CMAS) is also studied. It is found that the lower viscosity liquid deforms significantly as compared to CMAS for the head-on and off-center cases. These differences are quantified using the budgets of kinetic, surface, and dissipation energies. This paper represents the first study of its kind on the binary collision of CMAS droplets.

The analysis presented in Part I of this study on the binary collision of equal molten calcium–magnesium–alumino–silicate (CMAS) droplets is extended to investigate the flow and interfacial dynamics of unequal CMAS droplet collision. Numerical investigations of head-on, off-center, and grazing collisions of two CMAS droplets of size 1 and 2 mm are conducted at pressure and temperature of 20 atm and 1548 K, respectively, that are representative of a gas-turbine combustor. At these conditions, the physical properties of CMAS are density, ρCMAS = 2690 kg/m3, surface tension between CMAS/air, σCMAS = 0.40 N/m, and viscosity, μCMAS = 11.0 N-s/m2. The primary difference between the CMAS and a fictitious fluid with viscosity 1/10 of CMAS was higher deformation for the lower viscosity case, leading to stretching and subsequent breakup of the liquid structure. These mechanisms are supported by the time evolution of surface, kinetic, and viscous dissipation energies.

This study presents a dual Eulerian–Lagrangian particle approach for time-accurate computational fluid dynamics (CFD) modeling of volcanic ash in gas turbine engines and initial results. The objective is to enable high-fidelity simulations of calcia–magnesia–alumina-silica (CMAS) particles in gas turbine engines to better predict deposition and particle paths to rapidly test mitigation solutions. The approach uses a primarily first principles framework to account for the various physical phenomena in the system. Particles are modeled using Lagrangian methods which track individual particles and Equilibrium Eulerian methods which track particles in terms of concentration densities. Lagrangian methods become prohibitively expensive for fine particles. Eulerian methods are physically appropriate for fine particles but become inaccurate for large particle sizes. A dual approach using both Eulerian and Lagrangian methods allows for optimal computational cost with maximum accuracy. Simulation results using the proposed approach are compared against experimental data for a representative gas turbine engine blade.

Calcium–magnesium–alumino-silicate (CMAS) particulates enter the aero-engine in a sandy environment, melt and infiltrate into 7 wt% yttria-stabilized zirconia (7YSZ) thermal barrier coatings (TBCs), reducing their lifetime. This leads to chemical degradation in 7YSZ accompanied by tetragonal to monoclinic phase transformation upon cooling. In this work, electron-beam physical vapor deposition coatings were infiltrated with a synthetic CMAS. Synchrotron X-ray diffraction measurements show that CMAS infiltration at 1250 °C has about 43% higher monoclinic phase volume fraction (PVF) at the coating surface compared to 1225 °C and remains consistently higher throughout the coating depth. Additionally, the increase in annealing time from 1 to 10 h results in a 31% higher monoclinic phase at the surface. Scanning electron microscopy revealed the presence of globular monoclinic phases corresponding spatially with the above findings. These results resolve the impact of time and temperature on CMAS infiltration kinetics which is important for mitigation.

The mitigation of CMAS (calcium–magnesium–aluminum–silicon oxide) infiltration is a major requirement for the stability of thermal barrier coatings. In this study, yttria-stabilized zirconia (YSZ)–Al2O3–SiC, YSZ–Al2O3–Ta2O5, and YSZ–Al2O3–Nb2O5 self-healing composites produced by uniaxially pressing powders were investigated as an alternative to YSZ. CMAS infiltration in these materials was tested at 1250 °C for 10 h. Comparing the depth of CMAS infiltration using scanning electron microscope (SEM) in tandem with electron-dispersive X-ray spectroscopy (EDS), all self-healing materials were found to perform better than the reference materials. While standard YSZ shows massive CMAS infiltration, SEM micrographs and EDS maps revealed a 33-fold improvement in CMAS resistance for the YSZ–Al2O3–Nb2O5 system, which exhibited the best performance among the selected self-repairing materials. X-ray diffraction and high-resolution SEM micrographs taken 10 μm below the surface revealed that CMAS only infiltrated pores in the topmost region of the samples. Both YSZ–Al2O3–Ta2O5 and YSZ–Al2O3–Nb2O5 systems showed no signs of chemical reaction with CMAS.

Air plasma sprayed thermal barrier coatings (TBCs) are used extensively throughout the gas turbine industry for both power and propulsion. As these engines push to higher temperatures, concern for failure from the melt infiltration of ingested siliceous debris [commonly called calcium–magnesium–alumino-silicate (CMAS)] arises, especially in aeroengines. 7 wt% yttria-stabilized zirconia is particularly prone to melt infiltration and stiffening-induced premature failure. Novel TBC materials such as gadolinium zirconate have been introduced for their infiltration-inhibiting CMAS reactions. Past academic work has utilized ideal laboratory furnace environments to study these phenomena. In this work, the influence of TBC microstructure and chemistry on impinging molten CMAS injected via a burner rig is studied. An observational study of the impacted surfaces and location-specific cross-sectional analysis is reported. Results point toward the critical role of surface microstructure on the mobility and reactivity of the molten CMAS.

Military operations occurring in particle-laden environments have resulted in aircraft incidents and loss of life due to sand ingestion into the engine. Sand melts in the hot combustion environment and deposits as glassy calcia–magnesia–alumino–silicates (CMAS) which leads to rapid performance degradation due to clogged air pathways in the engine. A novel, composite thermal barrier coating (TBC) consisting of yttria-stabilized zirconia (YSZ) blended with gadolinia is proposed that combines the excellent thermo-mechanical properties of YSZ together with the CMAS resistance of rare-earth oxides. YSZ was blended with 2, 8, 17, and 32 vol% gadolinia and tested under simulated engine-relevant conditions. The presence of gadolinia in the composite coating reduced the adhesion of the CMAS, and at 32 vol% gadolinia addition, the CMAS was completely delaminated. A possible CMAS adhesion mitigating mechanism is discussed. This work demonstrated the capability of a new composite TBC to significantly reduce CMAS adhesion.

Thermochemical interactions between calcium–magnesium–aluminosilicate (CMAS) glass and an environmental barrier coating of ytterbium disilicate (Yb2Si2O7) and ytterbium monosilicate (Yb2SiO5) were investigated. Top coats were deposited by plasma spray-physical vapor deposition onto silicon carbide substrates. CMAS powder was prepared as a glass and cast into a tape to yield a CMAS loading of ~29 mg/cm2. Samples were heat treated with CMAS at 1300 °C for 1–10 h or at 1400 °C for 1 h in air. Polished specimen cross-sections were characterized using scanning electron microscopy, X-ray diffraction, X-ray energy-dispersive spectroscopy, and transmission electron microscopy to evaluate resulting microstructures, phases, and compositions at CMAS/Yb2Si2O7 interfaces. Coatings exposed at 1300 °C—10 h and 1400 °C—1 h were fully infiltrated and compromised by CMAS. Dissolution of ytterbium silicate into molten CMAS followed by precipitation of cyclosilicate, silicocarnotite, and Yb2Si2O7 at 1300 °C and Yb2Si2O7 at 1400 °C enabled CMAS to effectively infiltrate top coats, rendering the predominantly Yb2Si2O7 coating ineffective at arresting molten CMAS degradation.

Calcium–magnesium–alumino-silicate (CMAS) reaction and infiltration behavior were studied in phase pure and mixed phase ytterbium silicate environmental-barrier coating (EBC) materials at 1300 °C. Phase pure Yb2Si2O7 (YbDS) was infiltrated by CMAS via grain boundaries/pores, resulting in loss of its structural integrity. Phase pure Yb2SiO5 (YbMS) reacted with CMAS to form either apatite (Ca2Yb8(SiO4)6O2) or YbDS, depending on the initial glass composition. Both reactions in YbMS slowed infiltration kinetics considerably compared to YbDS. Samples having a YbDS matrix with controlled amounts and dispersions of YbMS were also investigated as a model for air plasma spray coatings. Samples containing ≥20 vol% coarse YbMS showed dramatically improved infiltration behavior compared to phase pure YbDS. YbDS samples containing a fine dispersion of YbMS displayed a new mode of CMAS attack in which glass spread on the sample surfaces. The results of this study suggest that EBC phase compositions and microstructures may be tailored for optimized CMAS resistance.

High-temperature (1500 °C) interactions of promising environmental-barrier coating (EBC) ceramics in the rare-earth (RE) pyrosilicate system, Yb(2-x)YxSi2O7 (x = 0, 0.2, 1, or 2), with three different calcia–magnesia–aluminosiliate (CMAS) glass compositions, are explored. Only the Ca/Si ratio is varied in the CMAS: 0.76, 0.44, or 0.10. Interaction between the highest Ca/Si CMAS and the EBC ceramic with the lowest x (=0, Yb2Si2O7) promotes no reaction but the formation of “blister” cracks. In contrast, the highest x (=2, Y2Si2O7) promotes the formation of an apatite reaction product, but no “blister” cracks. Observationally, it is found that a decrease in the CMAS Ca/Si ratio (0.76–0.10) and a decrease in Y-content decreases the propensity for reaction crystallization (apatite formation) and “blister” cracks. These results are rationalized based on the relative affinities between Ca2+ in the CMAS and Y3+ or Yb3+ in the EBC ceramics, suggesting a way to tune the CMAS interactions in RE pyrosilicate solid solutions.