CMAS Corrosion Research Articles

CMAS corrosion resistance of rare earth phosphates at high temperatures for environmental barrier coatings

CMAS corrosion resistance of rare earth phosphates at high temperatures for environmental barrier coatings

Composition engineering of high-entropy rare-earth monosilicates enables remarkable CMAS corrosion resistance

Composition engineering of high-entropy rare-earth monosilicates enables remarkable CMAS corrosion resistance

High‐entropy (Ho1/4Er1/4Dy1/4Gd1/4)2Si2O7: A promising thermal environmental barrier coating material

AbstractThis work presents the exploration of a promising thermal environmental barrier coating material by engineering the compositions of (Ho1/4Er1/4Dy1/4X1/4)2Si2O7 high‐entropy rare‐earth disilicates (HERED‐X, X = Gd, Tm, Yb, Lu, Sc, Y). Among all the samples, the as‐fabricated HERED‐Gd samples possess the best CMAS corrosion resistance with a corrosion rate of 8.2 ± 0.14 µm h−1 at 1673 K, which is attributed to the synergistic effects of high chemical reactivity of HERED‐Gd and good stability of the formed apatite in CMAS melt. Moreover, they demonstrate a matched coefficient of thermal expansion (4.6 × 10−6 K−1) with SiCf/SiC composites (4.5–5.5 × 10−6 K−1) and a low thermal conductivity (1.89 W m−1 K−1 at room temperature).

Influence of annealing on the CMAS corrosion behaviour of ytterbium disilicate environmental barrier coatings: A Raman imaging study

The study examined the CMAS corrosion behaviour of ytterbium disilicate EBCs using Raman imaging before and after annealing. Annealing transformed the coatings to crystalline, healing pores and cracks, and reducing corrosion depth and rate. Raman spectroscopy showed increased residual stresses with corrosion time, while sprayed coatings had lower stresses and higher susceptibility to damage. This research enhances the use of Raman spectroscopy in non-destructively characterizing EBCs degradation under CMAS conditions, offering insights for aeroengine combustion chamber design improvements.

Improved comprehensive performance of high entropy rare earth aluminate as T/EBC materials by phase structure regulation

To optimize the comprehensive performance of rare earth (RE) aluminates as thermal/environmental barrier coatings (T/EBCs), we engineered six kinds of novel high-entropy RE aluminate multiphase ceramics with the coexisting garnet and perovskite phases by regulating equivalent ionic radius of RE3+, and the relationship between the microstructure and thermal properties, as well as resistance to CMAS corrosion was investigated in-detail. The findings revealed that the garnet (5RE0.2)3Al5O12 content gradually decreased in multiphase ceramics with the increasing equivalent ionic radius of RE3+. The interaction between garnet and perovskite grains can decrease grain size, which in turn enhanced phonon scattering, thereby leading to the decrease in thermal conductivity. Moreover, the synergistic effect in the different dissolution rates of garnet and perovskite phases can form a dense garnet/apatite barrier layer at corrosion front, thereby contributing to CMAS corrosion resistance. These results provide a robust theoretical foundation for the advancement of next-generation T/EBCs.

Effect of co-doping of CeO2 and Sc2O3 on mechanical properties and CMAS corrosion resistance of YSZ ceramics

Effect of co-doping of CeO2 and Sc2O3 on mechanical properties and CMAS corrosion resistance of YSZ ceramics

Effect of heterogeneous phases on thermal-mechanical properties and CMAS corrosion resistance of (Sc0.33Er0.33Yb0.34)2Si2O7 ceramics

Effect of heterogeneous phases on thermal-mechanical properties and CMAS corrosion resistance of (Sc0.33Er0.33Yb0.34)2Si2O7 ceramics

(Ba0.5Sr0.5)(Ti0.25Zr0.25Hf0.25Sn0.25)O3: A novel thermal barrier material with low thermal conductivity and excellent resistance to CMAS corrosion

High entropy ceramics are considered as promising materials for thermal barrier coatings due to phonon scattering caused by lattice distortion. In this study, perovskite high-entropy ceramics (Ba0.5Sr0.5)(Ti0.25Zr0.25Hf0.25Sn0.25)O3 (BSTZHS) were successfully prepared by dramatically increasing the conformational entropy of ceramics with complex crystal structures. The results showed that the sample exhibits a single-phase perovskite crystal structure with a dense surface, each element of the sample was uniformly distributed. Moreover, the thermal conductivity of (Ba0.5Sr0.5)(Ti0.25Zr0.25Hf0.25Sn0.25)O3 decreased significantly compared to SrZrO3 and BaZrO3 (the minimum thermal conductivity is 1.37 W/(m·K) at 273–1273 K). This study also identified descriptors for predicting the decrease in thermal conductivity of perovskite high-entropy ceramics. As well, the samples not only exhibit outstanding mechanical properties (Young’s modulus E = 255.7 ± 8.6 GPa、flexural strength Fb = 142.1 ± 4.8 MPa), but also good resistance to CMAS corrosion. Various results support that (Ba0.5Sr0.5)(Ti0.25Zr0.25Hf0.25Sn0.25)O3 is potential to be used for thermal barrier coatings.

In-situ observation and directional growth of Ca2Lu8(SiO4)6O2 in Lu2SiO5 attacked by CMAS at 1500 °C

Excellent resistance to CMAS is one of the key parameters for environmental barrier coating (EBC) operated in gas turbine engines. The interaction between CMAS and EBC was usually investigated by the characterization of reaction products after corrosion. The lack of direct observation of the high-temperature CMAS corrosion processes hinders the understanding of CMAS-induced degradation of EBC. In this work, the interaction between CMAS and an EBC candidate (Lu2SiO5) was investigated by an in-situ observation method, where the melting of CMAS and the growth of reaction products were first observed. It was found that the reaction product Ca2Lu8(SiO4)6O2 tends to precipitate vertically to form a textured structure. In addition, Lu2SiO5 tends to decompose into Lu2Si2O7 and Lu2O3 at 1500 °C, which induces an intergranular infiltration of CMAS and leads to the formation of Ca2Lu8(SiO4)6O2 fringes. Compared with the corrosion behavior at 1300 °C, Lu2SiO5 exhibits fast interaction with CMAS and poor CMAS resistance at 1500 °C. The results provide important guidelines on the design of rare earth silicate EBC.

Investigating the CMAS corrosion resistance and thermal cycling failure behavior of porous thermal barrier coatings with different morphologies prepared by plasma spraying

This study investigates the influence of two-phase powder feeding spacing on the thermal cycling and CMAS corrosion resistance of the thermal barrier coatings (TBCs). Porous thermal barrier coatings with varying microstructures were prepared using the embedded micro agglomerated particle (EMAP) coating preparation method, which is based on conventional yttrium stabilized zirconia (YSZ) coatings. The method involved changing the powder feeding spacing. TBCs' failure mode and microstructure are investigated under complex service conditions. The corrosion resistance, thermal cycling life, hardness, and elastic modulus of TBCs are investigated by thermal cycling-CMAS experiments and isothermal corrosion tests. The EMAP coating has a good service life in the thermal cycling-CMAS tests, with a lower hardness, lower elasticit modulus and larger surface roughness than conventional coatings. This research aims to provide a strategy to optimize structure for TBCs with excellent comprehensive performance.

Microstructural, thermal characterization and CMAS corrosion resistance of novel quaternary (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 high entropy disilicate material

A novel (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 high entropy disilicate quaternary composition was synthesized from commercial oxide powders using ball milling and sintering processes as a candidate material for environmental barrier coatings (EBC). As-synthesized high entropy disilicate powders were sintered at different durations (12, 18, and 24 h) at 1600 °C in a muffle furnace before characterization. The XRD and SEM analyses revealed the single-phase monoclinic structure (β-type) with homogeneous elemental distribution for the sintered samples. The (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 samples exhibited low thermal diffusivity coefficient, low thermal conductivity, a close coefficient of thermal expansion (CTE) to SiC and a high temperature stability. The (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 samples were subjected to CMAS corrosion tests at 1300 °C with different durations (2, 12, and 24 h) to evaluate CMAS corrosion resistance. Additionally, Yb2Si2O7 samples were prepared and subjected to molten CMAS in the same way for comparison. Based on the results, the CMAS corrosion resistance was improved with (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 composition.

Highly-durable plasma-sprayed Al2O3-YSZ/YSZ double ceramic layer TBCs against CMAS corrosion

The addition of Al2O3 inside thermal barrier coatings (TBCs) is highlighted effective on mitigating TBCs degradation induced by CMAS corrosion. In this study, the cost-effective 20 wt.%Al2O3-7YSZ/7YSZ double ceramic layer (DCL) TBCs were fabricated and their long-term performance resisting CMAS corrosion was investigated. Significant bending and CMAS infiltration occurred readily for the single layer 7YSZ coatings just after 5 h duration. The top sacrificial Al2O3-7YSZ layer of DCL coatings greatly constrained coating deflection and CMAS infiltration. This is due to the chemical reaction between alumina splats and CMAS, generating interconnected anorthites for effectively blocking CMAS infiltration and improving resistance to CMAS attack. However, it is highlighted that the role of alumina splats on mitigating CMAS degradation was found different between the short-term corrosion and long-term corrosion. In addition, interfacial delamination/cracking was not observed between the top Al2O3-7YSZ layer and bottom 7YSZ layer after 100 h CMAS corrosion, demonstrating superior interface integrity and durability.

Grain boundary infiltration and corrosion mechanism of CMAS attack on M-YTaO4 thermal barrier coating ceramics at 1300 °C

M-YTaO4 is one of the most promising candidates for the next-generation thermal barrier coatings. However, the CMAS-induced degradation of M-YTaO4 is still in dispute. This work comprehensively investigates the CMAS corrosion of M-YTaO4 at 1300 °C which provides new insights into the corrosion mechanism. CMAS attacks M-YTaO4 forming a thick recession layer and the main corrosion product is accurately confirmed to be (Ca2-xYx)(Ta2-y-zMgyAlz)O7 solid solution. The recession layer presented a small difference in Young's modulus with M-YTaO4. No delamination cracking occurred during the thermal cycling process indicating good reliability. In addition, grain boundary infiltration of CMAS is first revealed in M-YTaO4. The results have corrected the misjudgment of the corrosion mechanism and CMAS infiltration process in previous studies which provides guidelines for the design of M-YTaO4 thermal barrier coatings.

(Gd0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 and (Sc0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 high-entropy rare-earth disilicates as promising materials for environmental barrier coatings

The novel potential environmental barrier coating materials, two high-entropy rare earth disilicates (HEDs) (Gd0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 and (Sc0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 were prepared by the solid-phase method. Both the HEDs showed remarkable high-temperature phase stability and an appropriate thermal expansion coefficient (CTE) that matched that of SiCf/SiC. In addition, both the HEDs possessed above 20% lower thermal conductivity at 1000 °C (1.37 and 1.42 W·m−1·K−1, respectively) compared to traditional single-component rare earth disilicate material YbDS, and they also exhibited higher mechanical properties and good resistance to water vapour corrosion at 1300 °C. Moreover, the corresponding free-standing coatings obtained after deposition by atmospheric plasma spraying (APS) showed excellent resistance to CMAS corrosion, primarily because of the formation of an apatite layer.

Enhanced CMAS and hot corrosion degradation of YSZ thermal barrier coating with nano powders

Thermal barrier coatings are used to protect hot section parts of gas turbine engines. These coatings are subject to corrosion due to impurity elements in the fuel and dust absorbed from the atmosphere into the engine. The rough and porous surface nature of plasma-sprayed coatings facilitates the penetration of these factors into the coating. Coatings are damaged prematurely as a result of corrosion. In this study, nano-structure YSZ, YSZ + Al2O3, YSZ + TiO2, and YSZ + Al2O3 + TiO2 coatings were deposited on plasma sprayed YSZ coating by electrophoretic deposition method. Hot corrosion and CMAS resistance were investigated. The coatings were characterized by SEM, XRD, AFM, and microhardness before corrosion tests. The changes in the coatings after corrosion tests were characterized by SEM and XRD. The penetration behavior of corrosive factors was examined by elemental mapping with EDS. Nanostructure coatings deposited on APS YSZ made the surfaces of as-sprayed coatings more stable. The ability of these nanolayers to prevent pure penetration into coatings during hot corrosion and CMAS is limited. However, the Al2O3 + TiO2-doped nanostructure YSZ reduced the bond-layer oxidation. In addition, the addition of Al2O3 + TiO2 to the EPD deposited layer also reduced the tetragonal-monoclinic phase transformation that occurred in YSZ as a result of corrosion. Al2O3 and TiO2 additives alone are not effective in improving the corrosion behavior of coatings. Nanostructured coatings deposited with the EPD technique can help protect plasma-sprayed thermal barrier coatings against corrosion.

Hot corrosion of Gd2M2O7 (M = Zr, Hf) thermal/environmental barrier coatings subjected to calcium-magnesium-alumina-silicate melts at 1300 and 1500 °C

The new generation of T/EBCs requires their top layer materials with good resistance to severe corrosion by CMAS. Rare-earth zirconates/hafnates with pyrochlore structure are regarded as promising candidates for T/EBC. In this work, the corrosion behaviours and mechanisms of plasma-sprayed Gd2M2O7 (M = Zr, Hf) coatings at 1300 and 1500 °C for 4–25 h were investigated. The Gd2M2O7 coating showed good corrosion resistance under long-term corrosion at 1300 °C. The Gd2M2O7 coatings reacted with CMAS and precipitated apatite and fluorite, forming a dense layer that can prevent further penetration of CMAS. However, the CMAS penetrated into the Gd2M2O7 coatings because of the sharply decreased viscosity of the CMAS as the temperature raised to 1500°C, forming another infiltration layer. The results also suggested that the Gd2Hf2O7 possessed better resistance to CMAS corrosion than Gd2Zr2O7.

Improved thermal properties and CMAS corrosion resistance of high-entropy RE zirconates by tuning fluorite-pyrochlore structure

The order-disorder transition (ODT) of RE zirconate (RE2Zr2O7) has received much attention due to its potential applications in the design of thermal barrier coatings (TBCs). A series of high entropy RE zirconates (RE = La, Sm, Gd, Er and Lu) were designed and synthesized in order to regulate the fluorite-pyrochlore phases, and to investigate the effect of phase composition on the microstructures and performances. The results showed that the sintering temperature, as well as equivalent ion radius, played an important role in regulating the phase transformation of fluorite-pyrochlore. The coexistence of pyrochlore-fluorite phases not only can result in the decrease in grain size but also increased the lattice strain of each phase, thereby enhancing the phonon scattering and contributing to the reduction in the thermal conductivity. The CMAS corrosion resistance was mainly related to phase compositions, and the thicknesses of reaction layer decreased gradually with the increasing fluorite phase content. The results will provide theoretical guidance for the microstructural design of high entropy RE zirconates as next generation TBC materials in future.

Unraveling the CMAS corrosion mechanism of APS high-yttria-stabilized zirconia thermal barrier coatings

In this study, the CMAS corrosion behavior of two high-yttria-stabilized zirconia thermal barrier coatings (38YSZ and 55YSZ) deposited by air plasma spraying is investigated and compared with a typical 8YSZ coating at 1250 °C to unravel the underlying corrosion mechanism. The 8YSZ coating undergoes severe CMAS attack driven by dissolution-reprecipitation and intergranular corrosion. However, the 38YSZ and 55YSZ coatings can react with CMAS melt vigorously to form a dense reaction layer with CMAS-resistant apatite phase at the CMAS/coating interface, thereby resisting the CMAS infiltration. The apatite phase can be formed only when the yttria content in c-ZrO2 precipitated from CMAS attains the critical value (∼20 at%). Moreover, compared to the 38YSZ coating, apart from the high yttria content, the low zirconia content in the 55YSZ coating also allows higher free Y3+ available to form apatite phase via inhibiting the formation of c-ZrO2, thus facilitating the formation of more apatite phase.

A new high entropy rare earth disilicate ceramic (Lu1/8Yb1/8Sc1/8Er1/8Y1/8Ho1/8Dy1/8Tb1/8)2Si2O7: High temperature corrosion of water vapor and molten calcium–magnesium–aluminosilicate (CMAS)

A new β-(Lu1/8Yb1/8Sc1/8Er1/8Y1/8Ho1/8Dy1/8Tb1/8)2Si2O7 ((8RE1/8)2Si2O7) disilicate was prepared using a solid reaction process to protect SiCf/SiC CMC component materials. The thermal properties and corrosion resistance of (8RE1/8)2Si2O7 were systematically studied. Experimental results showed that (8RE1/8)2Si2O7 exhibited lower thermal conductivity than that of one-component silicate materials, only 1.80 (W•m−1•K−1) at 1000 °C. Meanwhile, (8RE1/8)2Si2O7 also exhibited superior water vapor and CMAS corrosion resistance. The weight loss was 1.05 × 10−4 g/cm2 after 200 h corrosion at 1400 °C. When it interacted with CMAS at 1300 ℃ for 48 h, the reaction layer was only 16 µm. Therefore, (8RE1/8)2Si2O7 could be regarded as a remarkable candidate for T/EBC.

CMAS corrosion resistance of Y2O3–Al2O3 binary ceramic materials

Alumina based rare earth oxides ceramic materials, with the good performance against the CMAS corrosion, were widely used as the thermal/environmental barrier coatings. In this work, three Y2O3–Al2O3 binary ceramic bulks, which were the Y3Al5O12, YAlO3 and Y4Al2O9, were prepared by high temperature solid state reaction using the Y2O3 and Al2O3 as the raw materials. The resistances to CMAS corrosion of these materials at 1250 °C and 1350 °C were compared fully. The compositions and structures of the samples before and after corrosion were analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results showed that the different chemical compositions of the materials resulted in the different optical basicity (OB) values which can decide the reactivity of the material with the CMAS melts. The Y3Al5O12 ceramic material was inert to the CMAS for its low OB value of 0.75 and damaged seriously by the CMAS through physical penetration. The YAlO3 and Y4Al2O9 with the high OB values of 0.80 and 0.87 reacted with the CMAS to form the Ca4Y6(SiO4)6O. The YAlO3 ceramic bulk exhibited the good performance against the CMAS corrosion benefitting from the formation of the Ca4Y6(SiO4)6O and the CaAl2Si2O8.