CMAS Infiltration Research Articles

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.

Reliability analysis of thermal barrier coatings under CMAS deposition and penetration

To address the issue of thermal barrier coating (TBC) failure and spalling caused by the deposition and infiltration of environmental particulate matter, a thermal-mechanical coupling model for EB-PVD TBC considering CMAS deposition failure was developed. Based on this model, the effects of CMAS deposition thickness and infiltration depth on TBC spalling failure were numerically studied. First, the deposition characteristics of CMAS were obtained using a critical velocity model. Second, the corresponding TBC failure criteria were established. Then, the effects of CMAS deposition thickness and infiltration depth on TBC spalling failure were specifically studied under different CMAS particle sizes and service times. The results show that the CMAS deposition thickness is the greatest at the leading edge and the pressure surface near the trailing edge of the blade. The CMAS infiltration depth is influenced by the temperature field, with higher temperatures leading to greater infiltration depth. With the simultaneous increase of service time and particle size, the average damage value of the coating surface (D¯) initially increases slightly to 0.2, then rapidly to 0.9, and finally slowly to 0.97, resulting in almost complete failure and spalling of the coating.

Influence of Yttria content in YSZ: An evaluation of water-based SPS coatings and spark plasma sintered pellets

The inherent phase instability of the state-of-the-art 7–8 mol% partially stabilised Yttria-Stabilised Zirconia (YO1.5, 8 YSZ, tetragonal) under a molten CMAS attack lacks the technological readiness needed to increase the gas inlet temperatures for more thermally efficient gas turbine engines. In this study, the concentration of Yttria in YSZ was systemically increased to impart CMAS resistance and their applicability via emerging Suspension Plasma Sprayed (SPS) coatings and Spark Plasma Sintered (SpPS) pellets under simulated conditions was evaluated. The fully stabilised higher Yttria YSZ compositions (21.4 mol% and 50.2 mol%, cubic) severely restricted the CMAS infiltration and interaction areas, with the later composition forming an apatite layer and the trans-granular cracking and chipping of YSZ into layers reduced with an increase in Yttria content. However, their application into water-based SPS coatings resulted in a complete coating failure following the high-temperature CMAS tests. During Furnace Cycling Test (FCT) tests, the 8 YSZ coating survived 34 thermal cycles compared to just one on the 50.2 mol% YSZ coatings. The premature delamination of the higher Yttria coating seems to have been caused by the spallation associated with horizontal cracks within the coating. The YSZ composition could be modified into CMAS-resistant chemistry; however, their applicability as a functioning TBC with inherent microstructural features, such as the Dense Vertically Cracked (DVC) coating strategy examined in this study, requires new coating design strategies that impart sustainable thermal cycling performance.

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.

Wetting kinetics and corrosion of CMAS and CMAS-NaCl to plasma-sprayed YSZ and Al2O3-YSZ thermal barrier coatings

Transient wetting and corrosion behavior of CMAS and CMAS-NaCl on 7YSZ and Al2O3-YSZ TBCs were investigated at 1250 ℃. NaCl addition decreased the melting point and viscosity of CMAS while improving the dynamic wettability during melt spreading. This intensified melt interactions with TBCs, facilitating CMAS infiltration and coating degradation. The Al2O3 addition to coatings promoted interfacial reactions between glassy melts and TBCs, generating anorthite products. Such reactive wetting decreased spreading velocity and increased contact angles of glassy melts at early interaction stages, but the densely stacked anorthite layer mitigated CMAS and CMAS-NaCl attacks for a longer duration of 10 h.

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.

Investigation of CMAS resistance in the (NdSmEuGd)(1-x)/2Dy2xZr2O7 series

The study examines the corrosion behavior of the (NdSmEuGd)(1-x)/2Dy2xZr2O7 series in 1300 °C molten salt CMAS, demonstrating that the dense layer formed by ZrO2 and apatite could effectively inhibit the penetration of CMAS. Smaller rare-earth ion radii tend to form larger-sized ZrO2, while larger rare-earth ion radii favor apatite formation. Therefore, by carefully balancing the rare earth ion radii in the composition design of high-entropy samples, the levels of ZrO2 and apatite formation can be adjusted to prevent CMAS infiltration. This approach helps identify high-entropy zirconate samples with superior corrosion resistance to CMAS.

Unveiling the protection mechanism of LaPO4 against CMAS attack

This work evaluated the resistance of LaPO4 ceramics to CMAS attack at 1250 °C and explored their interaction mechanisms. The chemical reaction between molten CMAS and LaPO4 involves two stages leading to formation of a continuous inner layer and outer polyhedron grains, which effectively prevents the infiltration of CMAS. The reaction layer in this study is formed by inter-diffusion between LaPO4 and CMAS melt in the interface region, which involves a series of events including solution, substitution and recrystallization. The mitigating mechanism differs significantly from the typical 'dissolution-precipitation' process observed in rare-earth zirconate or alumina-based materials during CMAS attack. Because of this novel mechanism, the reaction layer has a crack-free microstructure and seals the surface of the underlying LaPO4 ceramic, which consequently results in enhanced resistance to CMAS attack.

CMAS corrosion behavior of a novel high entropy (Nd0.2Gd0.2Y0.2Er0.2Yb0.2)2Zr2O7 thermal barrier coating materials

The CMAS corrosion behavior of a novel high entropy material (Nd0.2Gd0.2Y0.2Er0.2Yb0.2)2Zr2O7 with thermal conductivity of 1.215 W·m−1·K−1 was investigated. Results show that a dense layer composed of apatite type RE8Ca2(SiO4)6O2 phase and RE/Ca-ZrO2 forms in front of the reaction layer, which dramatically decreases the infiltration rate of CMAS from 20 to 5.3 µm/h at 1300 °C. Moreover, the elements with a larger ion radius are easier to form apatite type phase to slow down the further infiltration of CMAS. This study reveals that the high entropy (Nd0.2Gd0.2Y0.2Er0.2Yb0.2)2Zr2O7 ceramic is a promising candidate for TBCs with extreme resistance to CMAS corrosion.

Effect of microstructure on CMAS corrosion behavior of (Gd0.8Yb0.2)2Zr2O7/YSZ thermal barrier coatings prepared by EB-PVD

Three types of (Gd0.8Yb0.2)2Zr2O7 (GYbZ)/YSZ double ceramic layer thermal barrier coatings (TBCs) were prepared with different microstructures by electron beam physical vapor deposition. The CMAS corrosion performance of the three TBCs was studied at 1250 ℃ compared with YSZ-TBC. The results showed that the CMAS corrosion resistance strongly depends on their microstructures. TBCs consisting of smaller inter-column gaps and larger specific surface area exhibited superior CMAS corrosion performance. The reaction products of GYbZ-TBCs are mainly composed of Gd-apatite and fluorite c-ZrO2 based fluorite. The effect of microstructure on CMAS infiltration depth was discussed.

Comprehensive microstructural characterization and CMAS infiltration resistance of ytterbium disilicate coatings with lamellar and quasi-columnar structures

Degradation by CMAS deposits represents a considerable threat to the EBC service life. Current work contributes to understanding the response of Yb2Si2O7 coatings with lamellar and quasi-columnar structures to CMAS attack. It is revealed that the dual-phase structure in APS coating drastically improved the overall coating resistance to CMAS infiltration by enhancing the apatite-forming capability. The amorphous silica in PS-PVD coatings limited the formation of apatite “sealing” layer. With a continuous degradation of sand-phobic morphology, molten CMAS rapidly penetrated inside the entire coating. This study highlights that microstructure modulation of Yb2Si2O7 coating is a promising strategy for CMAS infiltration mitigation.

A new approach to protect and extend longevity of the thermal barrier coating by an impermeable layer of silicon nitride

AbstractA sample representation of a gas turbine engine blade, consisting of a nickel superalloy substrate with a deposited thermal barrier coating (TBC), was covered with silicon nitride, Si3N4, as an impermeable layer using plasma enhanced chemical vapor deposition (PECVD). The silicon nitride layer was used to seal the topcoat of yttria‐stabilized zirconia (YSZ) surface of the TBC to mitigate calcium–magnesium–aluminum–silicon oxide (CMAS) attack. CMAS testing was carried out on the covered and uncovered surfaces by melting a ratio of 25 mg/cm2 of CMAS powder onto the surface of each sample in a furnace at 1100°C for 1 h. The conformal surface reaction of the sealed layer confirmed no cracking or delamination at high temperatures. Scanning electron microscopy (SEM) micrographs confirmed that the surface of YSZ was successfully sealed. The new coating of silicon nitride was shown to be a viable solution and technique to significantly block CMAS infiltration in porous thermal barrier coatings.

Microstructure Refinement of EB-PVD Gadolinium Zirconate Thermal Barrier Coatings to Improve Their CMAS Resistance

Rare-earth zirconates are proven to be very effective in restricting the CMAS attack against thermal barrier coatings (TBCs) by forming quick crystalline reaction products that seal the porosity against infiltration. The microstructural effects on the efficacy of Electron Beam-Physical Vapor Deposition gadolinium zirconate (EB-PVD GZO) against CMAS attack are explored in this study. Four distinct GZO microstructures were manufactured and the response of two selected GZO variants to different CMAS and volcanic ash melts was studied for annealing times between 10 min and 50 h at 1250 °C. A significant variation in the microstructural characteristics was achieved by altering substrate temperature and rotation speed. A refined microstructure with smaller intercolumnar gaps and long feather arms lowered the CMAS infiltration by 56%–72%. Garnet phase, which formed as a continuous layer on top of apatite and fluorite, is identified as a beneficial reaction product that improves the CMAS resistance.

Effect of CMAS infiltration on the optical properties of thermal barrier coatings: Study of the mechanisms supported by FDTD simulations and ALD

Effect of CMAS infiltration on the optical properties of thermal barrier coatings: Study of the mechanisms supported by FDTD simulations and ALD

Effect of modulating the phase structure of YSZ ceramics by TiO2 doping on the CMAS corrosion resistance at 1250 °C

YSZ and TiO2 doped YSZ (TYSZ) sintered compacts were prepared in this study, and the modulation behaviors of Ti4+ doping on the microstructure and phase structure of YSZ ceramics were comparatively investigated, while the optimization mechanisms of Ti4+ doping on the CMAS corrosion resistance of YSZ ceramics were elucidated from these two aspects. The results indicate that although the small grain size and high densification of YSZ can mitigate CMAS infiltration at the early stage of corrosion, the high contact specific surface area of YSZ with CMAS caused by such small grains is the main reason for the rapid degradation of the samples at the later stage of corrosion. TYSZ has a higher c-phase content than YSZ, and the presence of a large amount of structurally stable c-phase was one of the main reasons for the outstanding resistance to CMAS corrosion of TYSZ. In addition, the segregation of Y and Ti elements into the surrounding CMAS was responsible for the t-m phase transition. The generated m-phase both provided a channel to segregate elements inside the grains and was easily dissolved in the CMAS, which eventually completely degraded ZrO2-based ceramics in CMAS.

Phase transformation failure in YSZ TBCs induced by component-dependent CMAS corrosion

Generally, CMAS corrosion is a key factor leading to the spalling failure in thermal barrier coatings (TBCs). Figuring out the component-dependent failure mechanism is the premise to improve the service life of TBCs. Here, CMAS corrosion behaviors with different Ca/Si components have been investigated through analyzing microstructure evolution in 8-YSZ coatings by EB-PVD. Capillary force from the gap between columnar crystals could accelerate the infiltration speed of the molten CMAS, leading to a fully populated coating with lower strain tolerance in 4 h. Subsequently, CMAS infiltration in the coatings led to preferential dissolution of yttrium ions and precipitation of yttria lean YSZ, and secondary CMAS phases. The corrosiveness of CMAS could be further tailored by optical basicity and viscosity. TBC failure could be enhanced in high-Ca case, resulting in the morphology evolution from columnar crystals to coarsening grain structures. Raman spectra indicates CMAS corrosion with high-Ca content will facilitate the phase transformation of t-YSZ to harmful monoclinic phase. In addition, DFT calculation demonstrates Ca doping into 8-YSZ will make monoclinic phase more stable and generate a 4.1 % volume expansion from the view point of energy. Consequently, the combination of the above low strain tolerance, new phases and volume expansion will accelerate the harmful morphology evolution. This work provides direct insight into the failure mechanism of CMAS corrosion and paves the way to design of new TBCs.

Novel Thermal Barrier Coatings with Phase Composite Structures for Extreme Environment Applications: Concept, Process, Evaluation and Performance

In this paper, a novel concept in the field of phase composite ceramics has been proposed and applied for creating the topcoats of durable thermal barrier coatings (TBCs), which is one of the most critical technologies for advanced high-efficiency gas turbine engines in extreme environments. The phase composite ceramic TBCs were designed to demonstrate superior and comprehensive performance-related merits, benefits, and advantages over conventional single-phase TBCs with a topcoat of 8YSZ or Gd2Zr2O7, including thermal phase stability, thermal shock durability, low thermal conductivity, and solid particle erosion resistance. In this paper, we review and summarize the development work conducted so far related to the phase composite ceramic concept, coatings processing, and experimental investigation into TBC behaviors at elevated temperatures (typically, ≥1250 °C) using different evaluation and characterization methods, including isothermal sintering, a burner rig test, a solid particle-impinging erosion test, and a CMAS corrosion test. Two-phase (t’+c) zirconia-based TBCs demonstrated improved thermal shock and erosion resistance in comparison to conventional single-phase (t’), 8YSZ TBC, and Gd2Zr2O7 TBC, when used separately. Additionally, a triple-phase (t’+c+YAG) TBC sample demonstrated superior CMAS resistance. The TBC’s damage modes and failure mechanisms for thermal phase stability, thermal cycling resistance, solid particle erosion behavior, and CMAS infiltration are also characterized and discussed in detail, in terms of microstructural characterization and performance evaluation.

Effect of CMAS Infiltration on the Optical Properties of Thermal Barrier Coatings: Study of the Mechanisms Supported by FDTD Simulations and ALD

Effect of CMAS Infiltration on the Optical Properties of Thermal Barrier Coatings: Study of the Mechanisms Supported by FDTD Simulations and ALD

Reaction mechanisms of Gd2Zr2O7 in silicate melts derived from CAS

Infiltration mechanisms at high temperatures of sand & ashes (CAS) in aircraft thermal barrier coatings (TBC) have been widely studied using the pyrochlore phase Gd2Zr2O7 (GZ) as TBC. Novelty of this study is GZ reactivity with each or a mix of oxides derived from CMAS have been scrutinized in order to better assess the diffusion, stability and role of each reactivity products in mitigation mechanisms across the CAS-GZ system. In particular, Gd+III, Si+IV, O-II diffusion mechanisms at the GZ/SiO2 interface have been discussed. These mechanisms shed light on why Gd-oxyapatites are the predominant phases at interfaces between silica-rich medium such as CAS and GZ when reactions are not complete, in contrast to thermodynamic data. Moreover, Gd2Si2O7 phases are closely linked to the growth of Gd-oxyapatites and are the seat of the diffusion barrier to Si+IV species, which further explains why GZ coatings shortly stops CMAS infiltration.