Thermochemical interactions between CMAS and Ca2Y8(SiO4)6O2 apatite environmental barrier coating material

Thermochemical interactions between Ca₂Y₈(SiO₄)₆O₂ apatite, a potential environmental barrier coating (EBC) material, and a synthetic CMAS having the composition 23.3 CaO - 6.4 MgO - 3.1 Al₂O₃ - 62.5 SiO₂ - 4.1 Na₂O - 0.5 K₂O - 0.04 Fe₂O₃ mole % were investigated. Pellets of apatite + CMAS powder and hot-pressed apatite disc-CMAS couples were annealed at 1200–1500 °C for 1–50 hours in air. Powder X-ray diffraction (XRD) was used to identify the phases present. Polished cross-sections of the heat treated pellets and diffusion couples were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high angle annular dark field (HAADF) imaging, selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS). Ca₃Y₂(Si₃O₉)₂ cyclosilicate, apatite, and amorphous phases were present in the samples heat treated at 1200 and 1300 °C, whereas no cyclosilicate was detected in samples annealed at 1400 and 1500 °C. A distinct cyclosilicate layer was observed at the apatite-CMAS interface in the diffusion couples heat treated at 1200 and 1300 °C. However, at 1400 and 1500 °C, due to its much lower viscosity, CMAS quickly infiltrated the apatite substrate through pores and along the grain boundaries and no cyclosilicate was observed; the apatite grains dissolved in molten CMAS followed by re-precipitation of apatite needles within an amorphous phase on cooling.     

High-Temperature thermochemical interactions of molten silicates with Yb2Si2O7 and Y2Si2O7 environmental barrier coating materials

The thermochemical behavior of EBC candidate materials yttrium disilicate (Y₂Si₂O₇) and ytterbium disilicate (Yb₂Si₂O₇) was evaluated with three calcium-magnesium-aluminosilicate (CMAS) glasses possessing CaO:SiO₂ ratios relevant to gas turbine systems. Pellet mixtures of 50 mol% EBC powder to 50 mol% CMAS glass powder were heat treated at 1200°C, 1300°C, and 1400°C. The products of these interactions were evaluated using X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. Above glass melting temperatures, exposure of the disilicates primarily resulted in dissolution into the molten glass followed by precipitation of a Ca₂RE₈(SiO₄)₆O₂ (RE = Yb³⁺, Y³⁺) apatite-type silicate and/or rare earth disilicate. In glasses with high CaO concentrations, apatite readily forms while the disilicate material is consumed by the reaction. As CaO content decreases, the disilicate phase becomes the main reaction product. Overall, reactions with yttrium disilicate favored more apatite crystallization than ytterbium disilicate. The viability of using these disilicates in various operating environments is discussed.     

Interactions between Ca2Gd8(SiO4)6O2 apatite and calcium-magnesium-aluminosilicate (CMAS) at elevated temperatures

High temperature corrosion behavior of Ca₂Gd₈(SiO₄)₆O₂ (CGdS) apatite has been investigated in the presence of molten calcium-magnesium-aluminosilicate (CMAS) glass having the composition 21.9 CaO - 4.3 MgO - 5.4 Al₂O₃ - 63.0 SiO₂ - 4.3 Na₂O - 0.8K₂O - 0.1 Fe₂O₃ (weight %). CGdS apatite powder was prepared by solid state synthesis from constituent oxides. Pellets of CGdS apatite + CMAS mixed powder and CGdS-CMAS diffusion couples were annealed at 1200, 1300, 1400, and 1500 °C for 1 and 20 h in ambient atmosphere. Development of phases in heat treated specimens was characterized using various analytical techniques as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high angle annular dark field imaging, selected area electron diffraction and energy dispersive X-ray spectroscopy. In both pellets and diffusion couples, monoclinic cyclosilicate Ca₃Gd₂(Si₃O₉)₂ formed from reaction of apatite with CaO in the CMAS melt only in samples heat treated at 1200 °C for 1 and 20 h or at 1300 °C for 1 h. Triclinic CaSiO₃ and monoclinic diopside MgCaSi₂O₆ were also observed in samples annealed at 1200 and 1300 °C. At 1400 and 1500 °C, because of its low viscosity, CMAS infiltrated along the pores and grain boundaries of the apatite substrates in diffusion couples. Phase compositions predicted from thermochemical computation were in good agreement with those observed experimentally. Ca₂Gd₈(SiO₄)₆O₂ apatite has the potential for being an effective T/EBC in circumventing the penetration of molten CMAS up to about 1300 °C but not at higher temperatures.     

Thermochemical degradation of HfSiO4 by molten CMAS

The thermochemical degradation of hafnium silicate (HfSiO₄) was investigated with a molten calcium-magnesium-aluminosilicate (CMAS) glass relevant to gas turbine engine applications. Sintered HfSiO₄ coupons were fabricated, within which wells were drilled and filled with CMAS glass powder at a loading of ～35 mg/cm². Samples were heat treated at 1200°C, 1300°C, 1400°C, and 1500°C for 1 h, 10 h, and 50 h. At 1200°C and 1300°C, slow formation of a Ca₂HfSi₄O₁₂ cyclosilicate phase was observed at the HfSiO₄-CMAS interface. At 1300°C and higher, rapid infiltration of CMAS into the material along the grain boundaries was observed. Initial conjecture into CMAS degradation mechanisms of HfSiO₄ are presented herein.     

Corrosion of RE2Si2O7 (RE=Y,Yb, and Lu) environmental barrier coating materials by molten calcium-magnesium-alumino-silicate glass at high temperatures

With the increased demand for high operating temperature of gas turbine engines, corrosion by molten calcium-magnesium-alumino-silicate (CMAS) exhibits a significant challenge to the development of durable environmental barrier coatings (EBCs). EBC candidates, γ-Y₂Si₂O₇, β-Yb₂Si₂O₇, and β-Lu₂Si₂O₇ were explored on their corrosion resistance to CMAS melts at 1300 °C and 1500 °C for 50 h. Interaction and degradation mechanisms were investigated and the corrosion behaviors showed different trends at high temperatures. At 1300 °C, RE₂Si₂O₇ dissolves into CMAS melts and apatite phases reprecipitate forming a thick recession layer. However, when the temperature increases to 1500 °C, CMAS melts vigorously penetrate through the grain boundary of RE₂Si₂O₇ and ‘blister’ cracks form throughout the samples. The reduced grain boundary stability at 1500 °C promotes the penetration of CMAS melts in RE₂Si₂O₇. Grain boundary engineering is critically demanded to optimize CMAS corrosion at high temperatures.     

CaO–MgO–Al2O3–SiO2 (CMAS) corrosion behaviour of LaMgAl11O19/GdPO4 thermal barrier coating materials

CaO–MgO–Al₂O₃–SiO₂ (CMAS) corrosion poses serious hidden dangers for the application of thermal barrier coatings (TBCs). In this study, LaMgAl₁₁O₁₉ (LMA) and GdPO₄ were mixed at molar ratios of 2:1, 1:1 and 1:2 to prepare LMA/GdPO₄ materials, and the CMAS corrosion behaviours of these materials were investigated at 1300°C–1500 °C for 20 h and 40 h. It was demonstrated that temperature was the main factor influencing the corrosion behaviours and products. The materials were damaged at 1300 °C by the crystallization of CMAS melts to form CaAl₂Si₂O₈. In contrast, the materials were corroded by CMAS melts via the reaction between CMAS and GdPO₄ at 1500 °C. These results indicate that the addition of GdPO₄ to LMA can improve the resistance of the LMA material to CMAS corrosion.     

Interaction of Gd2Si2O7 with CMAS melts for environmental barrier coatings

Environmental barrier coatings (EBCs) prevent the oxidation of ceramic matrix composites (CMC), which are used as components in gas turbines. However, EBCs deteriorate more rapidly in real environments, molten silicate deposits accelerate the deterioration of EBCs. In this study, high-temperature behavior sintered Gd₂Si₂O₇ with calcia-magnesia-alumina-silica (CMAS) melt at 1400 °C for 0.5, 2, 12, 48, and 100 h was investigated. HT-XRD results showed that at 1300 °C, CMAS and Gd₂Si₂O₇ chemically reacted to form Ca₂Gd₈(SiO₄)₆O₂ (apatite). The reaction layer became thicker as the heat-treatment time increased, and the thickness of the reaction layer has increased following a parabolic curve. With the extension of the reaction time from 0.5 to 100 h, the thickness of the reaction layer increased from approximately 98 to 315 µm. It was confirmed that Ca₂Gd₈(SiO₄)₆O₂ grew vertically on the Gd₂Si₂O₇ surface. Vertical and horizontal cracks were found after reacting at 1400 °C for 100 h, but no interfacial delamination occurred in this study. In addition, the effects of CaO:SiO₂ molar ratios, monosilicates (RE₂SiO₅) and disilicates (RE₂Si₂O₇), heat-treatment time, and cation size were determined and compared with the results of previous studies (Gd₂SiO_5, Yb₂SiO_5, and Er₂Si₂O₇).     

Effects of crystal structure and cation size on molten silicate reactivity with environmental barrier coating materials

Rare earth (RE) disilicates are utilized in environmental barrier coatings to protect Si-based engine components from destructive reactions with water vapor and other combustion species. These coating materials, however, degrade when exposed to molten silicate deposits in the engine. Four RE-disilicates (RE₂Si₂O₇, RE = Er, Dy, Gd, Nd) are analyzed herein in thermochemical interactions with glassy calcium-magnesium-aluminosilicate (CMAS) compositions at 1400°C. Crystalline reaction products included RE₂Si₂O₇, SiO₂, and a Ca_2+yRE_8+x(SiO₄)₆O_2+3x/2+y apatite-type silicate. RE₂Si₂O₇ formation was favored in interactions with CMAS having low CaO:SiO₂ ratios. Increased reactivity was observed for higher CaO:SiO₂ ratios in CMAS combined with larger RE³⁺ cation size, resulting in apatite formation of varying stoichiometry and changes in lattice parameters. The crystallization of SiO₂ was dependent on both thermodynamic equilibrium at low CaO:SiO₂ ratios and sequestration of silicate modifiers at higher CaO:SiO₂ ratios, although residual amorphous content after CMAS exposure in both cases was still substantial.     

Hot corrosion behavior of Yb2Si2O7 ceramic under NaVO3 salt attack

Hot corrosion behavior of Yb₂Si₂O₇ bulk exposed to NaVO₃ molten salt was investigated at 1000–1500 °C for 2 h. Results showed that YbVO₄ was produced on the bulk surface at 1000–1200 °C because of the acidity of the corrosion medium. The corrosion product of Yb₂SiO₅ was yielded at 1300–1500 °C due to the transformation of the corrosion medium from acidity to alkalinity. The content of YbVO₄ in the products decreased as the temperature increased, while that of Yb₂SiO₅ increased with an increase in temperature. In this paper, various hot corrosion mechanisms of Yb₂Si₂O₇ bulk exposed to NaVO₃ molten salt are discussed based on the formation of YbVO₄ and Yb₂SiO₅.     

Resistance of pure and mixed rare earth silicates against calcium-magnesium-aluminosilicate (CMAS): A comparative study

Rare earth silicate environmental barrier coatings (EBCs) are state of the art for protecting SiC ceramic matrix composites (CMCs) against corrosive media. The interaction of four pure rare earth silicate EBC materials Yb₂SiO₅, Yb₂Si₂O₇, Y₂SiO₅, Y₂Si₂O₇ and three ytterbium silicate mixtures with molten calcium-magnesium-aluminosilicate (CMAS) were studied at high temperature (1400°C). The samples were characterized by SEM and XRD in order to evaluate the recession of the different materials after a reaction time of 8 hours. Additionally, the coefficient of thermal expansion (CTE) was determined to evaluate the suitability of Yb silicate mixtures as EBC materials for SiC CMCs. Results show that monosilicates exhibit a lower recession in contact with CMAS than their disilicate counterparts. The recession of the ytterbium silicates is far lower than the recession of the yttrium silicates under CMAS attack. Investigation of the ytterbium silicate mixtures exposes their superior resistance to CMAS, which is even higher than the resistance of the pure monosilicate. Also their decreased CTE suggests they will display better performance than the pure monosilicate.     

Crystallization behavior of CMAS and NaVO3+CMAS mixture and its potential effect to thermal barrier coatings corrosion

Calcium-magnesium-alumina-silicate (CMAS) and molten salt corrosion pose great threats to thermal barrier coatings (TBCs), and recently, a coupling effect of CMAS and molten salt has been found to cause even severer corrosion to TBCs. In this study, the crystallization behavior of CMAS and CMAS+NaVO₃ is investigated for potentially clarifying their corrosion mechanisms to TBCs. Results indicated that at 1000 °C and 1100 °C, CMAS was crystallized to form CaMgSi₂O₆, while at 1200 °C, the crystallization products were CaMgSi₂O₆, CaSiO₃ and CaAl₂Si₂O₈. The introduction of NaVO₃ in CMAS reduced the crystallization ability, and as the NaVO₃ content increased, glass crystallization occurred at a lower temperature, with crystallization products mainly consisting of CaAl₂Si₂O₈ and CaMgSi₂O₆. At 1200 °C, CMAS+10 wt% NaVO₃ was in a molten state without any crystallization, which suggested that NaVO₃ addition in CMAS could reduce its melting point, indicating enhanced penetration ability in TBCs and thus increased corrosiveness.     

Microstructure and properties of SiCf/SiC composite joints with CaO–Y2O3–Al2O3–SiO2 interlayer

Using CaO, Y₂O₃, Al₂O₃, and SiO₂ micron-powders as raw materials, CaO–Y₂O₃–Al₂O₃–SiO₂ (CYAS) glass was prepared using water cooling method. The coefficient of thermal expansion (CTE) of CYAS glass was found to be 4.3 × 10^?6/K, which was similar to that of SiC_f/SiC composites. The glass transition temperature of CYAS glass was determined to be 723.1 °C. With the increase of temperature, CYAS glass powder exhibited crystallization and sintering behaviors. Below 1300 °C, yttrium disilicate, mullite and cristobalite crystals gradually precipitated out. However, above 1300 °C, the crystals started diminishing, eventually disappearing after heat treatment at 1400 °C. CYAS glass powder was used to join SiC_f/SiC composites. The results showed that the joint gradually densified as brazing temperature increased, while the phase in the interlayer was consistent with that of glass powder heated at the same temperature. The holding time had little effect on phase composition of the joint, while longer holding time was more beneficial to the elimination of residual bubbles in the interlayer and promoted the infiltration of glass solder into SiC_f/SiC composites. The joint brazed at 1400 °C/30 min was dense and defect-free with the highest shear strength of about 57.1 MPa.     

Significantly improved corrosion resistance of high-entropy rare-earth silicate multiphase ceramics against molten CMAS

To improve the ability of rare-earth (RE) silicates to resist molten calcium–magnesium–aluminosilicate (CMAS) at high temperature, a novel high-entropy (4RE_0.25)₂Si₂O₇/(4RE_0.25)₂SiO₅ (RE = Y, Yb, Er, and Sc) multiphase ceramic was prepared by a two-step process. During sintering, (4RE_0.25)₂SiO₅ can react with SiO₂ at the grain boundaries of (4RE_0.25)₂Si₂O₇, which can not only purify the grain boundary but also promote the growth of the original (4RE_0.25)₂Si₂O₇ grains, thereby significantly improving the ability to resist molten CMAS corrosion at high temperature. After corroding at 1500°C for 48 h, the reaction layer of the multiphase ceramic was only 55 μm thick. Our results confirm that the high-entropy RE silicate multiphase ceramics represent an effective way to improve the ability to resist molten CMAS corrosion at high temperature.     

Thermochemical stability and microstructural evolution of Yb2Si2O7 in high-velocity high-temperature water vapor

Thermochemical stability and microstructural evolution of Yb₂Si₂O₇ was studied in high-temperature high-velocity water vapor at temperatures between 1200–1400 °C. Two reactions were shown to occur in the steam environment: Yb₂Si₂O₇ reaction to form Yb₂SiO₅, and further Yb₂SiO₅ reaction to form Yb₂O₃. Parabolic rates of both reactions were observed, and similar reaction enthalpies were determined for each reaction; 207 kJ/mol and 205 kJ/mol, respectively. Densification of the product phase Yb₂SiO₅ shut off pore connectivity for gas transport to the reaction interface at gas velocities exceeding 115?125 m/s and for temperatures of 1300 °C and 1400 °C, resulting in reduced reaction rates at higher velocities. Outward gas diffusion by a silicon hydroxide species is predicted to govern ytterbium silicate reactions with high temperature water vapor. Microstructure changes at high temperatures and velocities were shown to greatly impact the long-term stability of Yb₂Si₂O₇.     

Silicothermal synthesis and densification of x-sialon in the presence of metal oxide additives

The effect is reported of seven inorganic oxide additives on both the formation mechanism and the densification of X-sialon prepared by a silicothermal process. The oxides were added to the starting mixture of halloysite clay, alumina and elemental silicon at a level of 1 wt% of the calculated final product, and fired in nitrogen at 1200–1500°C. The formation of X-sialon was monitored by thermal analysis, powder XRD and ²⁷Al and ²⁹Si solid state MAS NMR. The effects of the additives are temperature dependent, and influence the various stages of the reaction by differing degrees. The oxides which best promote the formation of crystalline X-sialon (Y₂O₃, CaO and MgO) are also those which facilitate the conversion of initially-formed Si₃N₄ to SiO₂N₂ and SiO₃N units, the latter being particularly enhanced by Y₂O₃, Fe₂O₃ enhances the initial nitridation of Si but suppresses X-sialon formation by stabilising the preceding mullite phase. Densification is most enhanced by Y₂O₃, CaO and CeO₂; MgO exerts its maximum effect on sintering at lower temperatures. The beneficial influence of MgO and Y₂O₃ on both X-sialon formation and sintering is due to the formation of liquid phases.     

Effect of Y2O3 on the oxidation properties of ZrSi2/SiC coating prepared by SAPS on the carbon-carbon composites

In order to improve the oxidation resistance of carbon-carbon (C/C) composites at high temperature, different content of Y₂O₃ modified ZrSi₂/SiC coating for C/C composites were prepared by pack cementation and supersonic atmosphere plasma spraying (SAPS). Microstructure observation and phase identification of the coatings were analyzed by SEM, XRD, DSC/TG and EDS. Experimental results shown that the coating with 10?wt% Y₂O₃ effectively protected C/C composites from oxidation at 1500?°C in air for 301?h with a mass loss of 0.13% and experienced 18 thermal shock times from room temperature (RT) to 1500?°C. First, Y₂O₃ could restrain the phase transition of ZrO₂ to reduce the formation of thermal stresses of the coating; second, the random distribution of ZrO₂ ceramic particles and the formation of ZrSiO₄ enhanced the stability of the SiO₂; third, the formation of Y₂Si₂O₇ and Y₂SiO₅ could relieve the thermal mismatch between ZrSi₂-Y₂O₃ outer layer and the inner layer.     

Corrosion behavior of calcium–magnesium–aluminosilicate (CMAS) on sintered Gd2SiO5 for environmental barrier coatings

The Gd₂SiO₅ performed high-temperature corrosion behavior on calcium–magnesium– aluminosilicate (CMAS) for environmental barrier coatings (EBCs). The synthesized Gd₂O₃-SiO₂ powder was prepared to fabricate a sintered Gd₂SiO₅ by spark plasma sintering (SPS) at 1400°C for 20 min. CMAS was sprinkled on the sintered Gd₂SiO₅ surface and exposed for 2, 12, and 48 h at 1400°C by isothermal heat treatment. The main corrosion factor is Ca, and Ca₂Gd₈(SiO₄)₆O₂ phase is formed by reacting with Gd₂SiO₅. Extended morphology of Ca₂Gd₈(SiO₄)₆O₂ particles observed in the reaction area become thicker as the heat treatment time increases as the CMAS is dissolved. According to the results of high-temperature X-ray diffraction (HT-XRD) and differential scanning calorimetry (DSC), CMAS melted at 1243°C or a higher temperature formed the reaction area. The Ca₂Gd₈(SiO₄)₆O₂ phase was recrystallized and grown due to the reaction of Gd₂SiO₅ and Ca of the CMAS components.     

Enhanced mechanical properties of 3 Y2O3 stabilized tetragonal ZrO2 incorporating tourmaline particles

The current study reports on the improvement of mechanical properties of 3?mol% Y₂O₃ stabilized tetragonal ZrO₂ (3Y-TZP) by introduction of tourmaline through ball milling and subsequent densification by pressureless sintering at 800, 1200, 1300, 1400?°C. Findings demonstrate that no matter which sintering temperature the 3Y-TZP ceramic containing 2?wt% tourmaline reach a maximum value in flexural strength and fracture toughness as compared to other composite ceramics. As the tourmaline content is 2?wt% and the sintering temperature is 1300?°C, the flexural strength and fracture toughness of the composite ceramics are the highest, increases of 36.2% and 36.6% over plain 3Y-TZP ceramic respectively. The unique microstructure was systematically investigated through X-ray diffraction, scanning electron microscopy, energy dispersive spectrum, and flourier transform-infrared. The strengthening and toughening mechanism of tourmaline in 3Y-TZP ceramic were also discussed.     

Formation peculiarities and optical properties of highly-doped (Y0.86La0.09Yb0.05)2O3 transparent ceramics

Formation peculiarities of highly-doped (Y_0.86La_0.09Yb_0.05)₂O₃ transparent ceramics have been studied by X-ray diffraction and electron microscopy methods. The phase composition evolution of 1.81Y₂O₃∙0.18La₂O₃∙0.01Yb₂O₃ powder mixtures annealed at the temperatures of 1100, 1200, 1300, and 1400 °C has been studied by XRD. It has been shown that Yb₂O₃ phase dissolves in Y₂O₃ matrix in the calcination temperature range of 1300–1400 °C. Complete dissolution of La₂O₃ in Y₂O₃ matrix occurs at temperatures above 1400 °C. La³⁺ ions enter in Y₂O₃ and Yb₂O₃ crystal structures simultaneously in the 1200–1300 °C range, which leads to a remarkable increase in the volume of the corresponding crystal lattices. The possible reasons for suppressing the crystalline growth of Y₂O₃ and Yb₂O₃ cubic phases have been discussed. Finally, (Y_0.86La_0.09Yb_0.05)₂O₃ transparent ceramics have been obtained by solid-state vacuum sintering at 1650–1750 °C. Ceramics synthesized at a temperature of 1750 °C have been characterized by an in-line optical transmittance of 60% and a homogeneous distribution of constituent components within the volume and along the grain boundaries.     

Environmental-barrier coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass: Part II, β-Yb2Si2O7 and β-Sc2Si2O7

The high-temperature (1500?°C) interactions of two promising dense, polycrystalline EBC ceramics, β-Yb₂Si₂O₇ and β-Sc₂Si₂O₇, with a calcia-magnesia-aluminosilicate (CMAS) glass have been explored as part of a model study. Unlike YAlO₃ and γ-Y₂Si₂O₇ in the accompanying Part I paper, little or no reaction is found between the Y-free EBC ceramics and the CMAS. In the case of β-Yb₂Si₂O₇, a small amount of reaction-crystallization product Yb-Ca-Si apatite solid solution (ss) forms, whereas none is detected in the case of β-Sc₂Si₂O₇. The CMAS glass penetrates into the grain boundaries of both EBC ceramics, and they both suffer from a new type of ‘blister’ cracking damage. This is attributed to the through-thickness dilatation-gradient caused by the slow grain-boundary-penetration of the CMAS glass. The success of a ‘blister’-damage-mitigation approach is demonstrated, where 1?vol% CMAS glass is mixed into the β-Yb₂Si₂O₇ powder prior to sintering. The CMAS-glass phase at the grain boundaries promotes rapid CMAS-glass penetration, thereby avoiding the dilatation-gradient.