Molten Calcium-magnesium-alumino-silicate Research Articles

CaO–MgO–Al2O3–SiO2 corrosion resistance of multi-rare-earth-oxide-doped zirconia thermal barrier coating

As a thermal barrier coating (TBC) material, 8 wt% yttria-stabilized zirconia (8YSZ) exhibits poor resistance against calcium–magnesium–aluminosilicate (CMAS) corrosion. This is caused by the penetration of molten CMAS glass and the depletion of Y3+, which limits the lifetime of 8YSZ. In this study, a multi-rare-earth-oxide-doped zirconia (15 wt% (Lu0.2Yb0.2Dy0.2Gd0.2Y0.2)–ZrO2, denoted as 15LYDGY–SZ) ceramic was prepared to examine its interaction with CMAS. The sluggish diffusion effect and the competition between various dopant elements toward reaction with CMAS effectively slow down the atomic diffusion and limit the transformation of tetragonal phase into monoclinic phase in 15LYDGY–SZ ceramic. Compared with the currently used 8YSZ ceramic, the corrosion resistance of 15LYDGY–SZ to CMAS and its retention rate of fracture toughness after CMAS corrosion are improved. The results show that the multi-rare-earth-oxide-doped zirconia ceramic prepared in this study is a promising candidate material for the development of future TBCs.

Corrosion behavior of Al4SiC4/SiC ceramics exposed to molten calcium‐magnesium‐alumino‐silicate at 1350°C in air

AbstractAl4SiC4 shows excellent heat resistance, thermal shock resistance, machinability, and oxidation resistance. We focused on Al4SiC4‐based ceramics with SiC as a non‐oxide matrix for ceramic matrix composites for aircraft jet engines. In this study, monolithic Al4SiC4 and Al4SiC4/SiC ceramics were fabricated by hot‐pressing, and a corrosion test against molten calcium‐magnesium‐alumino‐silicate (CMAS) was conducted at 1350°C for 12–100 h in air, and their corrosion behavior was investigated. Scanning electron microscopy and energy‐dispersive X‐ray spectroscopy results revealed that severe damage was not observed at the interface between CMAS and the samples after the CMAS corrosion test. The recession of Al4SiC4‐100, ‐10, and SiC‐100 after corrosion for 100 h was 80–90 µm, and that of Al4SiC4‐50 was the highest of all samples and the value was 130 µm. The dissolution behavior of the oxidation layer into molten CMAS via a corrosion reaction was dependent on the composition of both the sample and the oxidation layer, the thickness, and the microstructure of the oxidation layers. The dominant mechanism of reaction between CMAS and Al4SiC4‐100, ‐90, and ‐50 samples was concluded to be the dissolution of the oxidation products, while in SiC‐100 and Al4SiC4‐10 samples, the dominant reaction was determined to be direct corrosion of the surface with CMAS.

Thermal properties and calcium-magnesium-alumino-silicate (CMAS) interaction of novel γ-phase ytterbium-doped yttrium disilicate (γ-Y1.5Yb0.5Si2O7) environmental barrier coating material

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.

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.

Corrosion behaviour of La2Hf2O7 thermal barrier coatings against molten CMAS melts at 1300 and 1500 °C

Thermal barrier coatings (TBCs) are susceptible to accelerated degradation caused by molten calcium-magnesium-alumino-silicate (CMAS), which exhibits a significant challenge to design and develop durable TBCs. Rare-earth hafnates are regarded as promising TBCs candidates. In this study, the corrosion behaviour of plasma-sprayed La2Hf2O7 coating at 1300 and 1500 °C was investigated and compared with the traditional YSZ coating. The study indicated that the La2Hf2O7 exhibited better corrosion resistance than the YSZ. During the corrosion process, La2Hf2O7 coating dissolved into CMAS melts and reprecipitated m-HfO2 and apatite which effectively mitigate the corrosion, while the YSZ coating only produced ZrO2 with a transition from tetragonal to monoclinic phase. The recession depth of the coatings increases noticeably when the reaction temperature raised from 1300 to 1500 °C. The YSZ coating failed at 1500 °C, while the La2Hf2O7 coating still remained intact. This study may provide a theoretical basis for selecting and design of rare-earth hafnates TBCs with excellent corrosion resistance.

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.

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

AbstractTo improve the ability of rare‐earth (RE) silicates to resist molten calcium–magnesium–aluminosilicate (CMAS) at high temperature, a novel high‐entropy (4RE0.25)2Si2O7/(4RE0.25)2SiO5 (RE = Y, Yb, Er, and Sc) multiphase ceramic was prepared by a two‐step process. During sintering, (4RE0.25)2SiO5 can react with SiO2 at the grain boundaries of (4RE0.25)2Si2O7, which can not only purify the grain boundary but also promote the growth of the original (4RE0.25)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.

Hot corrosion of high-entropy hafnate for thermal barrier coating material subjected to molten CMAS

The calcium magnesium aluminosilicate (CMAS) corrosion behavior of high entropy (La0.2Ce0.2Pr0.2Sm0.2Eu0.2)2Hf2O7 (HEF) thermal barrier coatings (TBCs) material heated at 1300 ℃ was first investigated. The results show that CMAS can quickly dissolve HEF, and then re-precipitate multicomponent apatite (Ap) solid solutions and HfO2 stabilized by rare earth (RE). Moreover, the re-precipitation rate of the Ap phase has a positive relation with RE cation size. The gradient re-precipitation of the Ap phase avoids the initiation of cracks in the reaction layer. It reveals that HEF exhibits good resistance to CMAS corrosion, which is regarded as a novel TBCs material.

Synthesis and characterization of ytterbium oxide: A novel CMAS‐resistant environmental barrier coating material

AbstractDemand for more powerful aircraft promotes development of ceramic matrix composites and environmental barrier coating (EBC). A promising EBC material, ytterbium oxide (Yb2O3), was fabricated by hot pressing, and its properties were systemically investigated. The evaluation of thermal properties provides a baseline for the application of Yb2O3 on SiCf/SiC or Al2O3f/Al2O3 composites. The performance in water vapor and molten calcium–magnesium–aluminosilicate (CMAS) environments indicates its excellent durability in harsh environment. Compared with rare‐earth silicates, the thermochemical interactions between ytterbium oxide and CMAS changed greatly with the absence of silicon oxide. Reactions of ytterbium oxide with CMAS form several reaction products, including apatite, garnet, and silicocarnotite. The crystallization of garnet and silicocarnotite could effectively consume and solidify the CMAS melt, which prevents the melt infiltration and mitigates the further corrosion.

Thermochemical reaction behavior between Y2O3 and calcium-magnesium-aluminosilicate (CMAS) at elevated temperature

The thermochemical reaction behaviors between Y2O3 and calcium-magnesium-aluminosilicate (CMAS) at 1250 °C∼1400 °C were investigated. A continuous-dense reaction layer mainly composed of Y-apatite phase quickly forms within 15 min, and its slow increasing thickness depends on the molten CMAS infiltration along the Y-apatite grain boundaries, the dissolution of the Y-apatite phase into the melt, the inhibiting effect on the CMAS penetration, and the reaction time and temperature. The CMAS crystallization occurs due to the change of chemical composition and the formation of other reaction products. The dense reaction layer shows good resistance to the molten CMAS infiltration.

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.

High-temperature corrosion of HfSiO4 environmental barrier coatings exposed to water vapor/oxygen atmosphere and molten calcium magnesium aluminosilicate

The hot corrosion behavior of plasma-sprayed HfSiO4 coatings exposed to molten calcium magnesium aluminosilicate (CMAS) and steam was investigated. No reactive crystallization was observed in the CMAS-attacked HfSiO4 coating which consist of tetragonal HfSiO4 and monoclinic HfO2. The penetration did not bring obvious volume expansion thanks to the buffering effect of the pores/microcracks. The bilayer environmental barrier coating (EBC) systems using HfSiO4 as top coat and Si as bond coat have the great resistance against steam corrosion. The coating degradation at high temperatures was associated with TGO volatilization and phase transformation as well as thermomechanical incompatibility.

A case study on CMAS corrosion of an in-situ microstructure modification YSZ TBCs fabricated by EB-PVD

At elevated temperature, the inter-columnar pores of electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) may act as infiltration channels for the molten calcium–magnesium–alumino-silicate (CMAS) and, therefore, makes it susceptible to be attacked by CMAS. To enhance the CMAS corrosion resistance of EB-PVD TBCs, a microstructure modification yttria-stabilized zirconia (YSZ) top coat consisting of a low porosity fibrous structure layer on the conventional columnar structure layer was fabricated by in-situ parameter modification. The anti-CMAS corrosion performance of the in-situ modified YSZ coating was evaluated at 1250 °C and the evolution of the microstructure was investigated in detail. The results show that the inter-columnar pores were sealed by the low-porosity and smooth fibrous structure layer. Due to the porosity and the size of the pores of the fibrous structure layer are dramatically reduced, the penetration rate of CMAS is significantly lower in the in-situ microstructure modification coating than in the conventional columnar structure coating, and the interaction between CMAS and YSZ coating was also reduced.

Preparation and corrosion resistance of high-entropy disilicate (Y0.25Yb0.25Er0.25Sc0.25)2Si2O7 ceramics

With the increasing demand for high thrust-weight ratio aero-engines, the development of environmental barrier coatings (EBCs) for SiC-based ceramics is an enormous challenge, which requires both high temperature stability and resistance to molten calcium-magnesium-aluminosilicate (CMAS) corrosion. In this work, a novel high-entropy disilicate (Y0.25Yb0.25Er0.25Sc0.25)2Si2O7 ((4RE0.25)2Si2O7) EBC materials was prepared by a two-step process. The high-entropy disilicate powder was synthesized by sol-gel method, and then Y-Si-Al-O silicate glass was used as sintering aids to facilitate densification of the (4RE0.25)2Si2O7 ceramic by liquid sintering. The results of XRD, SEM and EDS showed that as-synthesized powders were pure high-entropy disilicate (4RE0.25)2Si2O7, and bulk (4RE0.25)2Si2O7 ceramic exhibited a dense structure, in which (4RE0.25)2Si2O7 grains were uniformly distributed into Y-Si-Al-O glass. Compared with single disilicate RE2Si2O7 (RE = Y and Er) ceramics, the high entropy (4RE0.25)2Si2O7 ceramic showed a good phase stability and almost no change in grain size during sintering at 1600 °C for in the holding time range of 5–15 h, suggesting a good high temperature stability. During molten CMAS corrosion, Ca2+ ions can diffuse into the Y-Si-Al-O glass phase wrapped on the surface of (4RE0.25)2Si2O7 grains, and then corrode (4RE0.25)2Si2O7 grains, thereby forming the corrosion zone consisting of reaction layer and diffusion layer. The Y-Si-Al-O glass can play a blocking layer role to suppress Ca2+ diffusion and mitigate molten CMAS corrosion, so that as-prepared (4RE0.25)2Si2O7 ceramic showed a relatively good ability for resisting molten CMAS corrosion. After molten CMAS corrosion at 1500 °C for 48 h, thickness of the reaction layer was only 73 µm. The results will be a solid foundation for the application of environmental barrier coating materials in long-period high temperature molten silicate environment.

A potential ZrO2 doped scandium tantalite ceramics for thermal/environmental barrier coating material

Despite scandium tantalite (ScTaO4) has great potential application for thermal/environmental barrier coating system, it is necessary to reduce the thermal expansion coefficient difference between them to lower the thermal stress in practical applications. In this paper, (ZrO2)x-(ScTaO4)1-x (x = 0.1, 0.2) material was prepared by doping ZrO2 into scandium tantalite ceramics, and its thermal expansion coefficient and calcium–magnesium–alumino-silicate (CMAS) corrosion behavior were studied. After the CMAS corrosion experiments of (ZrO2)x-(ScTaO4)1-x material, the phase composition and microstructures of samples were detected by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis. The main results show that the alloying effect of ZrO2 doping can reduce the thermal expansion coefficient of ScTaO4 and then alleviate the thermal expansion coefficient difference in thermal/environmental barrier coating system. Besides, Zr4+ ions are separated from (ZrO2)x-(ScTaO4)1-x lattice by molten CMAS under high-temperature corrosion condition and (ZrO2)x-(ScTaO4)1-x material is decomposed into ScTaO4 and t-ZrO2 finally. In addition, only t-ZrO2 is defected in the corrosion products without other crystal type of ZrO2 in the XRD results of the corrosion products, which indicate that ScTaO4 can increase high-temperature phase stability of t-ZrO2 and prevent phase transformation from t-ZrO2 to m-ZrO2.

Wetting and spreading behaviour of molten CMAS towards thermal barrier coatings and its influencing factors

Thermal barrier coatings (TBCs) are inevitably subject to corrosion caused by molten calcium-magnesium-alumino-silicate (CMAS), which leads to severe degradation in the properties of the coatings. In this study, a novel device for evaluating the wetting performance of molten CMAS was developed, which solved the challenging problem of measuring the contact angle of CMAS on TBCs at high temperatures. The wetting and spreading behaviour of CMAS under various conditions was studied, and the effect of various factors such as temperature, surface roughness, and the coating preparation process on the wetting behaviour was systematically analysed using this new method. The results demonstrate that the spreading distance can decrease and the contact angle can increase on rough surfaces, and the contact angle is proportional to the square of the roughness at the stable stage. The wettability is also determined by the viscosity, which in turn is governed by the temperature and the reactive production of CMAS and the coatings.

Thermal Analysis of Calcium–Magnesium–Alumino–Silicate Infiltration Dynamics in Thermal Barrier Coatings

Molten calcium–magnesium–alumino–silicate (CMAS) infiltration into thermal barrier coatings (TBCs) of gas turbines causes loss of strain tolerance and delamination of the ceramic topcoat. To develop efficient mitigation strategies, it is crucial to understand CMAS infiltration dynamics into the porous topcoat. This study introduces an integrated model, incorporating liquid flow in unsaturated porous structures, heat transfer, and temperature-dependent viscosities, to study CMAS infiltration through TBCs grown by the electron beam physical vapor deposition (EB-PVD) method. The effects of different CMAS compositions, temperature gradients across the topcoat, and coating microstructures are investigated. Our simulation shows that CMAS infiltration exhibits significantly nonlinear dynamics with a fast infiltration rate at the early stage due to high temperature, high pressure gradients, and low viscosity. Neglecting heat transfer enhancement from CMAS by approximating the temperature distribution as linear underestimates the infiltration rate. Fine porous microstructures slow infiltration, and bilayer or multilayer structures, consisting of variable column and pore sizes, combine the advantages of an increased hydraulic resistance to infiltration and lower capillary pressures. Such heterogeneous structures can delay early-stage infiltration by manipulating the layer thickness and arrangement. It is anticipated that the quantitative information and advanced understanding obtained would benefit the development of CMAS-resistant EB-PVD TBC topcoats.

High-temperature corrosion mechanism of a promising scandium tantalate ceramic for next generation thermal barrier coating under molten calcium–magnesium-aluminosilicate (CMAS)

Hot corrosion caused by molten calcium–magnesium-aluminosilicate (CMAS) is one of the significant failure factors of thermal barrier coating (TBC). Scandium tantalite (ScTaO4) is expected to be a next generation of TBC material because of its great thermophysical characteristics. This study focused on high-temperature corrosion behaviors of ScTaO4 ceramic exposure to molten CMAS at 1300 °C. During corrosion experiments, a clear but discontinuous reaction layer containing CaTa2O6 and Sc2SiO5 was observed, which was formed by dissolution reprecipitation. In addition, ScTaO4 showed good phase stability and maintained monoclinic wolframite structure with P2/c (13) space group in the corrosion tests. Moreover, Ca, Mg and Al were all kept away from the ScTaO4 ceramic due to low solubility of Sc3+ in the CMAS. Therefore, ScTaO4 ceramic owns great ability to resist the corrosion of CMAS. Good phase stability enables ScTaO4 to stably exist at high temperature, reducing volume change, thus extending the service life of the coating. Meanwhile, great corrosion resistance of CMAS reduces the consumption of ScTaO4 under adverse environmental conditions, further extending the service life of the coating. In summary, ScTaO4 is a new kind of potential thermal barrier material to replace YSZ ceramic.

Molten calcium–magnesium–aluminosilicate interactions with ytterbium disilicate environmental barrier coating

Abstract