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Degradation caused by silicate deposits limits the lifetime of protective coatings in gas turbine engines. In order to design more durable coating materials and architectures it is important to understand the behavior of ingested debris leading to the formation of deposits on component surfaces, and the subsequent reaction of those deposits with the coating materials. This article reviews recent advances in thermodynamic models and databases capable of predicting these reactions. It also describes the development of modeling approaches to capture key features associated with the intrinsic behavior of debris and deposits, their interaction with dense environmental barrier coatings, and reactions occurring during infiltration of molten deposits into porous or segmented thermal barrier coatings.

Metal-ceramic nanocomposites exhibit exceptional mechanical properties with a combination of high strength, toughness, and hardness that are not achievable in monolithic metals or ceramics, which make them valuable for applications in fields such as the aerospace and automotive industries. In this study, interpenetrating nanocomposites of three-dimensionally ordered macroporous (3DOM) tungsten-silicon oxycarbide (W-SiOC) were prepared, and their mechanical properties were investigated. In these nanocomposites, the crystalline tungsten and amorphous silicon oxycarbide phases both form continuous and interpenetrating networks, with some discrete free carbon nanodomains. The W-SiOC material inherits the periodic structure from its 3DOM W matrix, and this periodic structure can be maintained up to 1000 °C. In situ SEM micropillar compression tests demonstrated that the 3DOM W-SiOC material could sustain a maximum average stress of 1.1 GPa, a factor of 22 greater than that of the 3DOM W matrix, resulting in a specific strength of 640 MPa/(Mg/m3) at 30 °C. Deformation behavior of the developed 3DOM nanocomposite in a wide temperature range (30-575 °C) was investigated. The deformation mode of 3DOM W-SiOC exhibited a transition from fracture-dominated deformation at low temperatures to plastic deformation above 425 °C.

The degradation of thermal barrier coatings (TBCs) by molten silicates (CMAS) represents a fundamental barrier to progress in gas turbine technology, requiring a mechanistic understanding of the problem to guide the development of improved coatings. This article investigates the dissolution of yttria-stabilized zirconia (7YSZ and 20YSZ) into two model silicate melts at 1300–1400 °C. The approach involves the 1D dissolution of YSZ into a semi-infinite melt, characterizing the dissolution rates of YSZ and the diffusion rates of Zr4+ and Y3+ therein. The assessed kinetics of YSZ dissolution and diffusion were then applied to modeling the same phenomena on TBC-relevant length scales. These findings provide fundamental insight into (i) the dissolution mechanism of YSZ, (ii) the subsequent reprecipitation upon saturation, (iii) the quantitative effects of temperature and melt composition on the dissolution and diffusion kinetics, and (iv) how the measured kinetics manifests on the scale of flow channels present in TBCs.

Porous metals represent a class of materials where the interplay of ligament length, width, node structure, and local geometry/curvature offers a rich parameter space for the study of critical length scales on mechanical behavior. Colloidal crystal templating of three-dimensionally ordered macroporous (3DOM, i.e., inverse opal) tungsten provides a unique structure to investigate the mechanical behavior at small length scales across the brittle–ductile transition. Micropillar compression tests show failure at 50 MPa contact pressure at 30 °C, implying a ligament yield strength of approximately 6.1 GPa for a structure with 5% relative density. In situ SEM frustum indentation tests with in-plane strain maps perpendicular to loading indicate local compressive strains of approximately 2% at failure at 30 °C. Increased sustained contact pressure is observed at 225 °C, although large (20%) nonlocal strains appear at 125 °C. The elevated-temperature mechanical performance is limited by cracks that initiate on planes of greatest shear under the indenter.

Reactions between silicate deposits and environmental barrier coatings (EBCs) for SiC/SiC composites can severely degrade coating performance. The goal of the present study is to elucidate effects of deposit composition and exposure temperature and time on the recession of yttrium disilicate (YDS), a candidate EBC material. Phase equilibrium calculations are used to predict reactions of YDS with deposits of twelve different compositions at 1300 °C and 1400 °C; experimental observations are reported for three exemplary compositions at both temperatures and compared with the thermodynamic predictions. For initial deposit thicknesses of about 100 μm, recession depths reach terminal values after exposures of about 100 h. Terminal recession depths are sensitive to deposit composition but are only weakly affected by temperature. Deposits with high initial Ca:Si ratios react most severely with YDS, forming thick layers containing apatite interpenetrated by residual melt, whereas reactions with deposits that have Ca:Si ratios below a threshold for apatite formation are more benign, albeit still significant.

Phase equilibria in ZrO2-YO1.5-SiO2 (ZYS) and ZrO2-GdO1.5-SiO2 (ZGS) were experimentally assessed at 1400 °C and 1600 °C as they can offer insight on reactions between thermal barrier coatings (TBCs) based on ZrO2-YO1.5/GdO1.5 and molten silicate deposits in gas turbine engines. Features shared in both systems include the absence of ternary compounds and no ternary solubility in the binary phases. In ZYS however, a quaternary invariant reaction was observed that eliminates the zircon-disilicate equilibrium at higher temperatures. The results suggest no appreciable difference in the reactions between silica and thermal barrier oxides based on ZrO2-YO1.5 or ZrO2-GdO1.5, or environmental barrier coatings based on the corresponding Y/Gd silicates. The phase diagrams derived from these experiments are part of a broader effort to develop thermodynamic databases that can help guide the design of next-generation TBCs.

To facilitate the development of matrix processing protocols for SiC-based ceramic matrix composites (CMC) using the polymer infiltration and pyrolysis (PIP), the crystallization behavior of a commercial Si-(O)-C polymer derived ceramic (PDC) was studied using fiberless matrix monoliths replicating the microstructure features common in PIP CMC matrices. The study examined how the processing time and temperature impact the apparent bulk (average) crystallinity as well as how microstructure features lead to variations in the local crystallization rates. The first stage of crystallization involves rapid β-SiC precipitation throughout the bulk of the material. This stage appears to be insensitive to the local microstructure. Subsequent crystallization stages involve partial decomposition of PDC, producing CO and/or SiO. Because these processes involve gaseous exchange with the atmosphere, they are more directly influenced by the local microstructure. Therefore, although the bulk crystallinity quickly reaches an intermediate plateau corresponding to partial phase separation, the near-surface regions are fully crystallized more quickly than the interior regions. The implications of these effects are discussed in the context of the processing and performance of SiC-CMCs.

Reactions between molten calcium-magnesium-iron alumino-silicate (CMFAS) deposits and yttrium disilicate (Y2Si2O7, YDS) based environmental barrier coatings (EBC) on SiC/SiC ceramic matrix composites (CMCs) were investigated at 1300 °C. The coating readily dissolves into the melt from which an apatite phase, nominally Ca2Y8(SiO4)6O2, precipitates. These reactions are sufficiently fast to consume the majority of the approximately 275 μm thick coating in 24 h. Liquid phase separation, producing an essentially pure SiO2 second phase, occurs near the reaction front suggesting dissimilar rates of CaO and SiO2 exchange with the overlaying deposit. The rise of large bubbles through the melt above the coatings appears to disrupt the reaction layer and distributes apatite throughout the residual deposit. Channel cracks were found in the deposits and the reaction layers; after longer exposures, the cracks branch and extend laterally through the Si bond coat and into the underlying CMC. Complementary experiments performed on monolithic YDS pellets yielded long-term recession rates similar to those of the coatings, although some differences were evident in recession rates and reaction layer morphologies in the early stages. Thermodynamic calculations were used to understand the evolving driving force for the YDS-to-apatite conversion. The agreement between the simulated and experimentally observed behaviors suggests that such calculations could be used to predict the influence of temperature and deposit composition on EBC degradation.

Modern gas turbines rely on ceramic coatings to protect structural components along the hot gas path. These coatings are susceptible to accelerated degradation caused by silicate deposits formed when ingested environmental debris (dust, sand, ash) adheres to the coatings. This article reviews the current understanding of the deposit-induced failure mechanisms for zirconia-based thermal barrier coatings and silicate environmental barrier coatings. Details of the debris melting and crystallization behavior, the nature of the chemical reactions occurring between the deposits and coatings, and the implications for the thermocyclic durability of the coatings are described. Given the challenges posed in understanding how prospective coating materials and architectures will respond to a broad range of deposit compositions, it is proposed to develop an integrated framework linking thermochemical and thermomechanical models to predict coating durability. Initial progress toward developing this framework, and the requisite research needs, are discussed.

The phase equilibria in the CaO-Gd2O3-SiO2 system were investigated by heat treating precursor-derived powders at 1400 °C and 1600 °C, determining the resulting phase assemblages, and measuring the compositions of the liquid and solid solution phases. The stable ternary phases at 1400 °C include silico-carnotite (Ca3Gd2Si3O12), Ca/Gd-cyclosilicate (Ca3Gd2Si6O26), and the apatite solid solution field extending from Gd9.33(SiO4)6O2 to Ca2.75Gd7.25(SiO4)6O1.625. Of these, only apatite remains stable to 1600 °C. The high-temperature α- and α′-Ca2SiO4 polymorphs exhibit moderate Gd2O3 solubility. The results are compared to the equilibria for the related CaO-Y2O3-SiO2 system. The implications for understanding the interaction between rare earth containing thermal barrier coatings (TBCs) and silicate deposits (CMAS) are discussed.

The oxidation behavior of an ultra-high temperature ceramic (UHTC) based on HfB 2 with 20vol% SiC and was studied following two 10 min arc-jet test cycles with nominal heat flux of 350 W cm −2 , stagnation pressure of 7 kPa, and a sustained peak surface temperature of 2360 °C. Microstructure characterization revealed a modified, layered structure comprising ∼390 μm of porous HfO 2 at the surface and an underlying ∼740 μm porous region containing un-oxidized HfB 2 over the bulk UHTC, unaffected below the oxidation front. The SiC presumably undergoes active oxidation, as commonly reported for temperatures above ∼1600 ± 100 °C. However, unlike typical of exposures below ∼2000 °C no molten silicate phase was present at the surface to mediate the exchange of oxidant and gaseous reaction products. Additionally, a HfC impurity phase oxidizes concurrently with SiC rather than HfB 2 . A thermodynamic analysis is provided to explain the observed behavior and the differences with lower temperature scenarios in the literature.

The phase equilibria between thermal barrier coating (TBC) materials and calcium–magnesium–aluminosilicate melts (CMAS, representing deposits formed when siliceous debris is ingested into modern turbine engines), were investigated at 1300 °C. The primary goal was to understand the influence of the deposit and thermal barrier oxide compositions on the melt solubility limits and reaction product constitution. Model deposit compositions with SiO 2 to CaO ratios from 1.4 to 4.7 and with or without MgO were reacted with yttria- and gadolinia-zirconia thermal barrier oxides ranging from pure ZrO 2 to the pure yttria and gadolinia. The reactions formed various crystalline silicate phases; rare earth–calcium–silicate apatite and zircon (ZrSiO 4 ) were observed most frequently. Following the reactions, the residual melts were depleted in SiO 2 and generally enriched in CaO, MgO, and Al 2 O 3 . The implications of the anticipated changes in the melt viscosity and the cation partitioning between the melt and various solid solution phases on the efficacy of degradation-mitigating crystallization reactions are discussed.

The phase equilibria in the CaO–SiO2–Y2O3 system have been assessed at 1400 °C and 1600 °C using powders prepared by precipitation from precursors. Four ternary phases were identified: Ca/Y-cyclosilicate (Ca3Y2Si6O26), silico-carnotite (Ca3Y2Si3O12), cuspidine (nominally Ca2Y2Si2O9), and apatite (solid solution based on Ca2Y8Si6O26). The melting temperature of each ternary phase was measured and the shape of the liquid field at 1600 °C was assessed. The study emphasizes understanding the extent and crystallographic origin of the solid solution homogeneity range for apatite between Ca0.5Y9(SiO4)6O2 and Ca2.5Y7.5(SiO4)6O1.75, deviations from the nominal cuspidine stoichiometry, and solubility of Y2O3 in the high-temperature α-Ca2SiO4 polymorph. The results are discussed in the context of molten silicate (CMAS) degradation of thermal and environmental barrier coatings (T/EBCs).

SiC-based composites exhibit oxidative embrittlement at intermediate temperatures. Although the mechanisms of internal oxidation in composites with initially hermetic matrices have been studied extensively, comparable studies on composites with semipermeable matrices, such as those produced by polymer infiltration and pyrolysis, have not been reported. The present article focuses on the latter class of composites, specifically a SiCf /SiCNm with a dual BN/Si3N4 fiber coating. It describes detailed SEM and TEM analyses of the microstructure before and after oxidation in dry air or water vapor at 800°C. The results show that internal oxidation is more aggressive in water vapor and occurs appreciably even in the absence of an applied stress. The sequence of oxidation of the constituent phases appears to be consistent with the underlying thermodynamic hierarchy for the respective oxidation reactions. Notably, contrary to existing models based on preferential oxidation of BN coatings, oxidation occurs first on the SiC fiber surfaces and the Si3N4 overcoat; crystalline BN remains even after significant fiber and matrix oxidation has occurred. The results are discussed in terms of rate-controlling kinetic processes, the effect of oxidant type, and applied stress.

This study investigated the influence of YbO1.5 concentration on the reactivity between YbO1.5–HfO2 thermal barrier coating (TBC) materials and silicate melts [calcium–magnesium aluminosilicate (CMAS), specifically 33CaO–9MgO–13AlO1.5–45SiO2, all in mol%]. Three thermal barrier oxides (TBO) with YbO1.5 concentration between 30 and 80 mol% were tested at 1300°C and 1500°C. Porous pellets were used to study reaction layer growth and the behavior within infiltrated porosity. Complementary experiments examined the phase equilibria in melts containing varying quantities of the candidate TBOs. The effectiveness of reactive crystallization to limit interaction depth increased with the YbO1.5 concentration in the TBO. The results indicate that the TBO must contain sufficient YbO1.5 to satisfy the equilibrium between the melt and HfO2-based fluorite phase before YbO1.5-bearing silicates can precipitate. Increasing the YbO1.5 availability leads to the crystallization of apatite, garnet, silicocarnotite, and/or cuspidine (alumino)silicates. Apatite was observed at 1300°C and 1500°C, garnet and silicocarnotite only appeared at 1300°C, and cuspidine was only evident at 1500°C. The implications for the design of CMAS-resistant coatings are discussed in the context of the experimental results.

This paper examines the effect of temperature gradients on the interaction between silicate deposits and thermal barrier coating (TBC) systems. A dedicated test facility, in which a CO2 laser is employed to impose a controllable thermal gradient through the coating and underlying substrate, is used to investigate the interaction between two silicate compositions with state-of-the-art 7YSZ EB-PVD TBCs. The experimental results are then used to guide the development of expressions that describe the nature of silicate infiltration in the TBCs, the evolution of coating elastic modulus, and the generation and release of stresses.

The effectiveness of candidate rare earth (RE) bearing oxides to mitigate degradation of thermal barrier coatings by molten silicates is determined by the constitution and crystallization kinetics of the reaction products. The relationships between the oxide composition, test temperature, and reaction product constitution were investigated using sintered pellets of hafnates or zirconates containing YbO1.5, GdO1.5, or LaO1.5. The results suggest that the composition of the reprecipitated ZrO2- or HfO2-based phase, typically fluorite, is a useful indicator of the effectiveness of the silicate crystallization reactions. Meanwhile, the composition of the primary crystalline reaction product, typically a RE oxy-apatite, is relatively insensitive to the experimental variables considered herein. The observed trends indicate that the larger RE cations are more potent in rapidly crystallizing the silicate melt and that their relative effectiveness increases with the REO1.5 concentration in the coating material.

Environmental barrier coating (EBC) systems consisting of multiple layers tailored to address individual protection needs may offer improved performance relative to conventional architectures. If the requirements of thermochemical and thermomechanical compatibility are met, the deposition of a segmented thermal barrier coating on a dense rare earth silicate EBC could provide additional thermal protection and resistance to attack by molten deposits. The thermochemical compatibility between silicates in the YbO 1.5 –SiO 2 system and phases in the YbO 1.5 –HfO 2 system was investigated by equilibrating powder compacts of selected ternary compositions; diffusion couples were used to simulate interactions at the layer interfaces in the proposed architectures. The deduced 1500 °C ternary isothermal section reveals that the ordered δ-Yb 4 Hf 3 O 12 and H 3 –Yb 6 HfO 11 phases are only compatible with ytterbium monosilicate (Yb 2 SiO 5 ) EBC. Implementation of these hafnates in contact with ytterbium disilicate (Yb 2 Si 2 O 7 ) leads to interfacial reactions that facilitate layer debonding. The results provide criteria to guide the design of future thermal/environmental barrier coating architectures.

Application of SiC-based ceramic matrix composites (CMCs) in combustion environments demands the use of an environmental barrier coating (EBC) to prevent volatilization of the protective SiO2 scale in flowing water vapor. The EBC only provides protection while present on the surface; cracking and spallation of the coating leaves the underlying SiC vulnerable to the oxidation–volatilization processes. A robust matrix material chemically tailored to regrow a yttrium silicate scale in the event of EBC loss has been developed by incorporating yttrium bearing species including YB2, Y2O3, and Y5Si3 into the SiC. During oxidation a borosilicate glass helps seal cracks while Y2O3 and SiO2 react to form Y2Si2O7 for environmental protection. Candidate compositions were oxidized for 10 min to 100 h at 1400°C and for 24 h at 1500°C to understand the scale growth. The prospects for effectively applying this approach in CMCs are discussed.

Thermal barrier coatings (TBC) employed in the hot section of modern gas turbine engines are susceptible to degradation by molten deposits formed when siliceous debris is ingested into the engine [1]. These deposits, generically known as CMAS, may contain SiO2, CaO, MgO, AlO1.5, FeOx, and minor fractions of other oxides; the compositions can vary widely based on the geological source and engine service history [2]. Regardless of the specific deposit composition, the melt wicks into the open features in the TBC microstructure and solidifies under the imposed thermal gradient, leading to stiffening of the coating material and, upon cycling, elevated thermal stresses in the infiltrated region may produce cracking and/or spallation. Thermochemical attack may compound the damage, especially if the CMAS interacts with the thermally grown aluminum oxide that protects the alloy from oxidation. These degradation mechanisms can be mitigated, in part, with coatings designed to minimize the depth of melt penetration for a given exposure condition. In this context, the most effective coatings react readily with the deposit, converting the penetrating melt to a combination of stable crystalline phases that block the open porosity near the coating surface. The desired behavior is most clearly demonstrated by rare earth (RE) zirconate TBCs, notably Gd2Zr2O7, which react with CMAS to form a combination of fluorite and apatite (~Ca2Gd8Si6O26)[3]. Work with one model CMAS composition (33CaO-9MgO-13AlO1.5-45SiO2 in mol%, or Ca33Mg9Al13Si45) has shown that related RE zirconate and hafnate phases also form fluorite and apatite, among other reaction products. The efficacy of the crystallization reactions, however, varies with the RE cation radius, with the larger RE’s (i.e. La) acting as more potent apatite formers [4]. Efforts to design new TBC compositions would benefit from improved understanding of the role of the silicate deposit composition on the reaction processes. Elucidating these effects is the focus of this study. This work examined the effect of melt composition on the phase equilibria between the melts and candidate TBC materials such yttria stabilized zirconia (YSZ) and yttrium zirconate (Y4Zr3O12, YZO). The silicate melts were selected to span a relevant range of compositions while leveraging existing knowledge about the intrinsic melting and crystallization behaviors [5]. Initial experiments were performed in the simple, well-characterized CaO-AlO1.5-SiO2 (CAS) system with additional work in more complex silicate melts. Pre-reacted crystalline silicate powders were mixed with thermal barrier oxide (TBO) particles and annealed to identify the equilibrium crystalline phases and determine how the solubility limit of the TBO constituents in the melt changes with the melt composition. The results reveal that the melt composition plays a significant role in determining the crystallization product identity and volume fraction. Figure 1 highlights the effect of systematic changes in melt composition on the phase equilibria with YZO at 1300ºC. Figure 1(a,b) show melt compositions in the CAS system. Both systems form apatite while the primary ZrO2-bearing phase changes from zircon (ZrSiO4) to fluorite with a moderate shift in the SiO2 concentration. Meanwhile Ca33Mg9Al13Si45, the CMAS composition used in many past laboratory studies and shown in Figure 1(c), forms only fluorite. The latter behavior is related to the relatively high solubility of YO1.5 in both the melt and precipitated fluorite so the melt is not sufficiently saturated in YO1.5 to promote apatite crystallization. The findings shed new light on observations from ex-service TBCs that exhibit a range of deposit compositions and corresponding crystallization products and offer guidance for improved laboratory tests. The conclusions underscore the importance of considering a variety of factors related to melt chemistry when assessing the CMAS degradation of existing TBC materials and designing novel TBCs for improved CMAS resistance. Figure 1: Reaction between Y4Zr3O12 TBC material and selected silicate melt compositions (initially 25mol% YZO in the melt) produced a variety of crystallization products after 50h/1300ºC equilibration. Compositions (a) and (b) represent changes in the Ca:Si ratio but similar AlO1.5 concentration in the CAS system; (a) has sufficient SiO2 activity to form zircon (ZS) while (b) forms fluorite (F) as the only ZrO2-containing reaction product. Melt composition (c) has lower SiO2 activity and higher YO1.5 solubility; as such, no apatite (Ap) crystallization is observed even with the higher CaO concentration than (a) or (b). [1] M.P. Borom, C.A. Johnson, L.A. Peluso, Surf. Coat. Technol, 86 (1996) 116-126. [2] C.G. Levi, J.W. Hutchinson, M.-H. Vidal-Sétif, C.A. Johnson, MRS Bulletin, 37 (2012) 932-941. [3] S. Krämer, J. Yang, C.G. Levi, J.Am. Ceram. Soc., 91 (2008) 576-583. [4] D.L. Poerschke, C.G. Levi, J.Eur. Ceram. Soc., 35 (2015) 681-691. [5] E.M. Zaleski, C. Ensslen, C.G. Levi, Submitted (2014) Figure 1