Calcium-magnesium-alumino-silicate Attack Research Articles

Thermochemical interactions between Ca2Sm8(SiO4)6O2 apatite and molten calcium–magnesium-aluminosilicate (CMAS)

Thermochemical interactions between Ca2Sm8(SiO4)6O2 (CSmS) apatite and molten calcium-magnesium-aluminosilicate (CMAS) glass having CaO/SiO2 mole ratio 0.37 have been evaluated. CSmS apatite-CMAS diffusion couples were annealed at 1200, 1300, and 1400 °C for 1–50 h. 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 were used to characterize the phases formed during the reaction. Formation of a distinct cyclosilicate [Ca3Sm2(Si3O9)2] layer was observed at apatite-CMAS interface in couples heat treated at 1200 °C but not at higher temperatures. The cyclosilicate layer nucleated at the apatite-CMAS reaction front, continued to grow into the residual melt and eventually detached from the apatite substrate. Residual CMAS melt became Ca-lean when cyclosilicate was present. Formation of MgCaSi2O6 diopside and dendrites of CaSiO3 wollastonite was also observed within the residual CMAS at 1200 °C. At 1300 and 1400 °C molten CMAS, due to its much lower viscosity, infiltrated the apatite substrate through open pores and along the grain boundaries. Ca2Sm8(SiO4)6O2 apatite thermal/environmental barrier coating has the potential to provide protection against CMAS attack up to about 1200 °C but not at higher temperatures.

The corrosion behavior of Sc2O3-Y2O3 co-doped ZrO2 influenced by Sc2O3 content in CMAS at 1300 °C

The effect of Sc2O3 content on the corrosion behaviors of xSc2O3-3.5Y2O3 co-doped ZrO2 (ScYSZ, x = 4.5, 5.5 and 6.5 mol.%) in calcium-magnesium-alumino-silicate (CMAS) at 1300 °C was investigated in this paper. It is shown that the ScYSZ ceramic exhibits better resistance to molten CMAS attack when doping with more Sc2O3. The CMAS penetration depth in the ScYSZ ceramic decreases with increasing Sc2O3. It is about 18 µm of CMAS penetrated depth in the 6.5Sc3.5YSZ after corrosion for 4 h at 1300 °C, including an 11 µm and 7 µm for the reaction-reprecipitation and grain-boundary infiltration layers, respectively. High thermochemical stability and weak reactivity of the ScYSZ to the glassy CMAS melt are beneficial to enhance the high temperature corrosion resistance, attributing to the low solubility in molten CMAS and the weak affinity to Ca2+ for small ionic radius Sc3+. The mechanism of enhancing the resistance to CMAS is discussed.

Calcium magnesium aluminosilicate (CMAS) corrosion behaviors of apatite Ca2La8(SiO4)6O2 thermal barrier coating material

The calcium magnesium aluminosilicate (CMAS) attack is becoming a critical obstacle to the application of thermal barrier coatings (TBCs). In this work, we prove that Ca2La8(SiO4)6O2 is an effective TBCs material for resisting CMAS attack, and investigates its CMAS corrosion behaviors. The results indicate that no evidence of chemical reactions was found between CMAS and Ca2La8(SiO4)6O2 heat-treated at 1300 °C and 1500 °C. The corrosion of apatite by CMAS occurs via dissolution and re-precipitation processes. At 1300 °C, the apatite Ca2La8(SiO4)6O2 substrates have little evidence of damage. Moreover, the apatite can also effectively block the infiltration of CMAS up to 1500 °C.

Corrosion behaviour of rare-earth monosilicates in CMAS exposure

The thermochemical reactions between calcium-magnesium-aluminosilicate (CMAS) and three different rare earth (RE)-based (Yb, Lu and Er) environmental barrier coatings (EBCs) were studied at 1300 °C for 1–48 h. The EBCs were applied on a reaction bonded SiC-B4C substrate using dip coating technique. The coating’s layout consists of a dense RE2SiO5 bottom coat and a protective RE2O3 topcoat. The microstructure and phase composition of the CMAS-EBC specimens were examined using scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results showed that CMAS dissolved in RE2O3 to form protective Ca-RE garnet phase in the case of Yb and Lu-based coatings and unprotective apatite in the case of Er-based coating. CMAS melts penetrated the RE2O3 topcoat after 6 h. Yb and Lu-silicate EBCs offer higher resistance to CMAS attack compared to Er-silicate coat. The use of RE2O3 topcoat proves beneficial strategy to limit CMAS corrosion to the RE2SiO5 coating.

The Effect of Thermal Barrier Coating Surface Temperature on the Adhesion Behavior of CMAS Deposits

Abstract The interaction of thermal barrier coating’s surface temperature with calcium magnesium aluminosilicate (CMAS) like deposits in gas turbine hot flowpath hardware is investigated. Small Hastelloy X coupons were coated in thermal barrier coatings (TBC) using the air plasma spray (APS) method and then subjected to a thermal gradient via backside impingement cooling and frontside impingement heating using the High-Temperature Deposition Facility (HTDF) at The Ohio State University (OSU). A one-dimensional (1D) heat transfer model was used to estimate TBC surface temperatures and correlate them to intensity values taken from infrared (IR) images of the TBC surface. TBC frontside surface temperatures were varied by changing backside mass flow (kept at a constant temperature) while maintaining a constant hot-side gas temperature and jet velocity representative of modern commercial turbofan high-pressure turbine (HPT) inlet conditions (approximately 1600 K and 200 m/s, or Mach 0.25). In this study, Arizona Road Dust (ARD) was utilized to mimic the behavior of CMAS attack on TBC. To identify the minimum temperature at which particles adhere, the backside cooling mass flow was set to the maximum amount allowed by the test setup, and trace amounts of 0–10 µm ARD particles were injected into the hot-side flow to impinge on the TBC surface. The TBC surface temperature was increased through coolant reduction until noticeable deposits formed, as evaluated through an IR camera. Accelerated deposition tests were then performed where approximately 1 g of ARD was injected into the hot side flow while the TBC surface temperature was held at various points above the minimum observed deposition temperature. Surface deposition on the TBC coupons was evaluated using an infrared camera and a backside thermocouple. Coupon cross-sections were also evaluated under a scanning electron microscope for any potential CMAS ingress into the TBC. Experimental results of the impact of surface temperature on CMAS deposition, and deposit evolution and morphology are presented. In addition, an Eulerian–Lagrangian solver was used to model the hot-side impinging jet with particles at four TBC surface temperatures and deposition was predicted using the OSU deposition model. Comparisons to experimental results highlight the need for more sophisticated modeling of deposit development through a conjugate heat transfer and mesh morphing of the target surface. These results can be used to improve physics-based deposition models by providing valuable data relative to CMAS deposition characteristics on TBC surfaces, which modern commercial turbofan high-pressure turbines use almost exclusively.

Experimental and numerical investigation on the thermal and mechanical behaviours of thermal barrier coatings exposed to CMAS corrosion

Calcium-magnesium-alumino-silicate (CMAS) corrosion is a critical factor which causes the failure of thermal barrier coating (TBC). CMAS attack significantly alters the temperature and stress fields in TBC, resulting in their delamination or spallation. In this work, the evolution process of TBC prepared by suspension plasma spraying (SPS) under CMAS attack is investigated. The CMAS corrosion leads to the formation of the reaction layer and subsequent bending of TBC. Based on the observations, a corrosion model is proposed to describe the generation and evolution of the reaction layer and bending of TBC. Then, numerical simulations are performed to investigate the corrosion process of free-standing TBC and the complete TBC system under CMAS attack. The corrosion model constructs a bridge for connecting two numerical models. The results show that the CMAS corrosion has a significant influence on the stress field, such as the peak stress, whereas it has little influence on the steady-state temperature field. The peak of stress increases with holding time, which increases the risk of the rupture of TBC. The Mises stress increases nonlinearly along the thick direction of the reaction layer. Furthermore, in the traditional failure zone, such as the interface of the top coat and bond coat, the stress obviously changes during CMAS corrosion.

Investigation of calcium–magnesium-alumino-silicate (CMAS) resistance and hot corrosion behavior of YSZ and La2Zr2O7/YSZ thermal barrier coatings (TBCs) produced with CGDS method

Thermal barrier coatings (TBCs) commonly expose to oxidation, hot corrosion, and CaO-MgO-Al2O3-SiO2 (CMAS) attacks during the service condition. All of these attacks cause the spallation or delamination of TBCs from the bond coat. In this study, the effect of these attacks on YSZ and La2Zr2O7/YSZ TBC systems was investigated. YSZ and double layer La2Zr2O7/YSZ topcoats were deposited using electron beam physical technique (EB-PVD) on CoNiCrAlY bond coat produced by cold gas dynamic spray (CGDS) technique. CMAS tests at 1225 °C for 4, 8, 12, 16, 20, 24, 28, and 32 h and 5 h cyclic hot corrosion tests at 1000 °C were carried out on TBCs. Before and after the tests, TBCs were characterized using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), x-ray diffraction (XRD) and image analysis software program. At the end of the tests, the formed phases, microstructural changes, and general failure mechanisms were investigated in detail for each test. The general result shows that double-layered TBCs provide significant contributions and durability against high-temperature corrosive attacks of conventional YSZ TBC. In addition, in this study, hot corrosion damage mechanism and CMAS attack significantly damaged the column morphology of the EB-PVD method, reducing the thermal insulation properties and strain tolerance of TBCs. As a result of the tests, spallation and phase changes occurred in the damaged TBC systems.

Mixed phase ytterbium silicate environmental-barrier coating materials for improved calcium–magnesium–alumino-silicate resistance

Abstract

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

AbstractRare 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.

Pore filling behavior of air plasma spray thermal barrier coatings under CMAS attack

The pore filling behavior in yttria-stabilized zirconia air plasma sprayed (APS) thermal barrier coatings (TBCs) during calcium-magnesium-alumino-silicate (CMAS) infiltration process was investigated. After fully infiltration, the total porosity of the coating decreased. However, the total porosity of the top region increased due to severe microstructural change. For the bottom region, whose microstructure did not change significantly, its total porosity dropped and almost all crack network disappeared, but 48 vol. % of globular pores still existed and larger globular pores were more likely to remain. This study indicates that introducing relatively large globular pores into APS TBCs may mitigate CMAS damage.

Interactions between zirconia–yttria–tantala thermal barrier oxides and silicate melts

The effects of TaO2.5 additions to Y4Zr3O12 (YZO) on its interactions with calcium-magnesium alumino-silicate (CMAS) melts are investigated. YZO is a candidate material for the protection of novel ZrO2-YO1.5-TaO2.5 based thermal barrier coatings (TBCs) against CMAS attack, but thermochemical considerations dictate that YZO contain TaO2.5 for interfacial compatibility. Oxides based on Y4Zr3-xTaxO12+0.5x (x = =0, 0.7, 1.4) were mixed with various ternary and quinary silicate compositions ranging in Ca:Si ratio from 0.35 to 1.13 and equilibrated at 1300 °C. The results show that all combinations form the desired apatite phase except for the mixture of the higher Ta5+ content with the lowest Ca:Si melt. It is found that the addition of Ta5+ increases the capture of Y3+ by the reprecipitated fluorite, decreasing the amount available to form apatite and eventually suppressing it at low Ca:Si. The effect is ascribed to the attractive interactions between Y3+ and Ta5+ in solid solution, similar to that found in tetragonal zirconia, but with concurrent incorporation of anion vacancies associated with excess Y3+ that stabilize the cubic form. As the Ca2+ content of the melt increases, so does the amount of Ca2+ incorporated in fluorite. The latter also interacts attractively with Ta5+ and leads to a reduction of captured Y3+ in fluorite as the Ca content in the reacting deposit increases. This serves to increase the amount of Y3+available in the melt to form apatite in TBC-CMAS interactions.

Degradation mechanisms of air plasma sprayed free-standing yttria-stabilized zirconia thermal barrier coatings exposed to volcanic ash

Due to the serious consequences that calcium‑magnesium-alumino-silicate (CMAS) attack has on reducing the service life of thermal barrier coatings (TBCs), it is important to understand the degradation mechanisms of CMAS attack on TBCs. Air plasma sprayed yttria-stabilized zirconia (YSZ) coatings were tested with 20 mg/cm2 real volcanic ash (VA) deposits at 1150 °C and 1250 °C for 8 h. The thermo-physical and mechanical properties of the coatings subjected to VA attack were investigated. It was found that VA reacted top coat exhibited apparently higher coefficient of thermal expansion, thermal conductivity, hardness and Young's modulus values than the as-sprayed condition. The microstructure and phase evolution of the coatings were characterized by X-ray diffraction, scanning electron microscope, Raman and transmission electron microscope. The results indicated that Y was leached from YSZ and into the VA melt, which interacted with VA to form yttrium iron garnet, resulting in the detrimental phase transformation from tetragonal to monoclinic.

Molten silicate interactions with plasma sprayed thermal barrier coatings: Role of materials and microstructure

Ingestion of siliceous particulate debris into both propulsion and energy turbines has introduced significant challenges in harnessing the benefits of enhanced operation efficiencies through the use of higher temperatures and thermal barrier coatings (TBCs). The so-called CMAS (for calcium-magnesium alumino-silicate) particles can melt in the gas path at temperatures greater than 1200C, where they will subsequently impact the coating surface and infiltrate through the carefully engineered porosity or cracks in a TBC. Ultimately, this CMAS attack causes premature spallation through its solidification and stiffening the ceramic during cooling. It has been noted in recent years, that TBCs based on yttria stabilized zirconia (YSZ) are completely non-resistant to CMAS attack due to their lack of reactivity with infiltrant liquid. New TBC ceramics such as Gadolinium Zirconate (GZO) show promise of CMAS resistance through rapid reaction-induced crystallization and solidification of the infiltrant, leading to its arrested infiltration. In both situations, the microstructure (porosity, micro and macro cracks) can be important differentiators in terms of the infiltration and subsequent failure mechanisms. This paper seeks to examine the interplay among microstructure, material, and CMAS attack in different scenarios. To do so, different types of YSZ & GZO single and multilayer coatings were fabricated using Air Plasma Spray (APS) and exposed to CMAS through isothermal and gradient mechanisms. In each of the cases, beyond their unique interactions with CMAS, it was observed the inherent microstructure and character of the porosity of the coating will have an additional role on the movement of the melt. For instance, vertical cracks can provide pathways for accelerated capillaric flow of the melt into both YSZ and GZO coatings. Based on these observations multilayer coatings have been proposed and realized toward potentially reducing complete coating failure and supporting multiple CMAS attack scenarios.

General trend on the phase stability and corrosion resistance of rare earth monosilicates to molten calcium–magnesium–aluminosilicate at 1300 oC

Thermochemical reactions between rare earth (RE) monosilicates (RE2SiO5) and molten calcium-magnesium-aluminosilicate (CMAS) at 1300 °C were investigated. Some RE2SiO5 (RE = Tb, Dy, and Ho) dissolve readily into CMAS melts and thereby, crystalline Ca2RE8(SiO4)6O2 reprecipitate. Other RE2SiO5 (RE = Tm, Yb, and Lu) resist CMAS attack, and a continuous layer of Ca2RE8(SiO4)6O2 establishes at the interface. The recession of RE2SiO5 is related to RE ionic radius. RE2SiO5 with small RE ionic radius exhibits good CMAS corrosion resistance due to its low reactivity with CMAS. This result is consistent with the same trend of difference in the optical basicity of RE2SiO5/CMAS.

Wetting, infiltration and interaction behavior of CMAS towards columnar YSZ coatings deposited by plasma spray physical vapor

Thermal barrier coatings (TBCs) produced by electron beam physical vapor deposition (EB-PVD) or plasma spray (PS) usually suffer from molten calcium-magnesium-alumino-silicate (CMAS) attack. In this study, columnar structured YSZ coatings were fabricated by plasma spray physical vapor deposition (PS-PVD). The coatings were CMAS-infiltrated at 1250 °C for short terms (1, 5, 30 min). The wetting and spreading dynamics of CMAS melt on the coating surface was in-situ investigated using a heating microscope. The results indicate that the spreading evolution of CMAS melt can be described in terms of two stages with varied time intervals and spreading velocities. Besides, the PS-PVD columnar coating (∼100 μm thick) was fully penetrated by CMAS melt within 1 min. After the CMAS attack for 30 min, the original feathered-YSZ grains (tetragonal phase) in both PS-PVD and EB-PVD coatings were replaced by globular shaped monoclinic ZrO2 grains in the interaction regions.

Calcium-magnesium-alumino-silicate induced degradation and failure of La2(Zr0.7Ce0.3)2O7/YSZ double-ceramic–layer thermal barrier coatings prepared by electron beam-physical vapor deposition

Calcium-magnesium-alumino-silicate (CMAS) induced degradation of the La2(Zr0.7Ce0.3)2O7/YSZ double-ceramic-layer thermal barrier coatings (TBCs) prepared by electron beam-physical vapor deposition (EB-PVD) was investigated in this study. CMAS reacted with La2(Zr0.7Ce0.3)2O7 to form a dense crystalline layer composed of apatite and fluorite phases, while the penetration was not arrested in the double-ceramic-layer coating due to the formation of vertical cracks during heat-treatment. In fact, the vertical cracks in the LZ7C3 layer inevitably provided efficient infiltration paths for molten CMAS to attack the bottom YSZ layer. Since the formation of Ca2(La1-xCex)8(SiO4)6O26 apatite phase during CMAS reactions led to CaO enrichment in the melt, the YSZ buffer layer of the double-ceramic-layer coating even suffered a severer CMAS attack than the single YSZ coating.

CMAS behavior of yttrium aluminum garnet (YAG) and yttria-stabilized zirconia (YSZ) thermal barrier coatings

Calcium magnesium aluminosilicate (CMAS) that is formed from the ingested deposits in gas turbines degrades thermal barrier coatings (TBCs), especially for the most widely used material; yttria-stabilized zirconia (YSZ). In the present work, we examine the behavior of yttrium aluminum garnet (YAG) as an alternative material for TBCs. CMAS interaction studies were conducted by making composite pellets of YAG-CMAS and YSZ-CMAS powders. These pellets, after being subjected to heat treatment between 1100°C and 1500°C were characterized by X-ray Diffraction (XRD), scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS), which showed YAG to be almost inert to CMAS whereas YSZ exhibited significant phase changes. To test the behavior of TBCs with YAG and 8YSZ as the topcoat material in a CMAS environment, cyclic furnace tests were conducted in which a controlled amount of CMAS was applied and then the samples were cycled to failure. In addition, to simulate the continuous accumulation of CMAS expected in service, a cyclic furnace test was devised in which a small dose of aqueous solution of CMAS was applied on TBC specimens at the start of every cycle until the samples were cycled to failure. In all these tests YAG TBCs outperformed YSZ in terms of durability. The mechanisms of CMAS attack are described and the relative resistance of YAG and YSZ is shown to be consistent with the Optical Basicity (OB) theory.

Lifetime and failure modes of plasma sprayed thermal barrier coatings in thermal gradient rig tests with simultaneous CMAS injection

Degradation of thermal barrier coatings (TBCs) in gas-turbine engines due to calcium–magnesium–aluminosilicate (CMAS) glassy deposits from various sources such as sand, volcanic ash, fly ash, or variable quality fuels has been a tenacious issue during the recent years. This follows from the fact that engines are required to operate under increasingly harsh conditions in all kind of gas turbine applications following the demands for higher efficiency and operational flexibility. While the understanding of the mechanism of CMAS induced degradation of TBCs as well as approaches for mitigation of CMAS attack by means of advanced TBC compositions have grown remarkably, most of the reported results have been obtained from lab testing at isothermal conditions or from evaluation of ex-service components, either. The isothermal tests are not reproducing important thermomechanical effects from service conditions, and it may be hard to figure out the thermal history of the ex-service examples.In this study a burner rig facility has been used for the evaluation of TBC performance, where a thermal gradient is applied across the TBC at cyclic load and CMAS is continuously injected to the combustion environment to simulate the conditions closer to actual service in an engine while allowing control of important degradation limiting parameters. Tests have been performed on the state of the art material YSZ while loading parameters as high temperature dwell time and CMAS deposition rate have been varied. Tests have been evaluated in terms of chemical degradation, failure mode and TBC spallation lifetime by means of SEM, EDS and XRD. Test conditions and relevance to in-service operating conditions are discussed. Major impacts on thermal gradient cycling lifetime and similarities and discrepancies related to reported failure mechanism are reviewed.

Hot corrosion of RE 2 SiO 5 with different cation substitution under calcium–magnesium–aluminosilicate attack

Rare earth (RE) silicates have been applied as advanced environmental barrier coatings (EBCs) to protect silicon carbide fibers reinforced silicon carbide ceramic matrix from water vapor and molten salt corrosion in engines. This process, however, is limited by volcanic ash corrosion as assessment of ash-induced corrosion is anecdotal and quantitative data are insufficient. In this account, the corrosion behavior of RE monosilicates (RE2SiO5, RE = Y, Lu, Yb, Eu, Gd, and La) by calcium–magnesium–aluminosilicate (CMAS), with similar composition as volcanic ash, was comprehensively investigated. Results indicated that RE2SiO5 could react with CMAS at 1200 °C at the interface, where the products crystallized in CMAS glass. RE2Si2O7 was formed by the reaction between RE2SiO5 and silica (SiO2) in CMAS, which was followed by corrosion of RE2Si2O7 by CMAS. RE2SiO5 with Type B structure showed better resistance toward CMAS than RE2SiO5 with Type A structure. Moreover, RE2SiO5 with larger radii of RE3+ cations led to easy formation of oxyapatite phase; however, RE2SiO5 with smaller radii of RE3+ cations easily formed garnet phase. Besides, smaller radii RE3+ cations induced slower reactions. These findings can contribute to identifying, preventing, and minimizing the damage to matrix components with EBCs caused by volcanic ash.

Effect of CMAS Composition on Hot Corrosion Behavior of Gadolinium Zirconate Thermal Barrier Coating Materials

The resistance of synthesized pyrochlore-type Gd2Zr2O7 bulk specimens to four calcium-magnesium aluminosilicate (CMAS) compositions at different temperatures was investigated. The reaction products were identified by X-ray diffraction and penetration depths were examined using scanning electron microscopy. A dense reaction layer is comprised mainly of Ca2Gd8(SiO4)6O2 and a cubic fluorite phase formed during the CMAS attack, and some unreacted CMAS was found in a transition layer below the reaction layer. The overall infiltration depth changed slightly with temperature, however, the thickness of the reaction layer and the morphology of the transition layer varied distinctly with temperature. The sintered sample underwent the most severe degradation by the CaO-lean CMAS, whereas the effect of CaSO4 and CaCO3 was not significant. Moreover, the Gd content of the ZrO2-based cubic fluorite phase depends on the temperature and the molar ratio of Ca:Si in the CMAS.