Calcium-magnesium-alumina-silicate Research Articles

CMAS + sea salt corrosion to thermal barrier coatings

In marine environment, sea salt and calcium-magnesium-alumina-silicate (CMAS) coexist largely endangering thermal barrier coatings (TBCs) service safety. In this study, the corrosion behavior of synthetic sea salt and CMAS to TBCs was investigated. Sea salt changed the CMAS structure, thereby decreasing its melting temperature and high-temperature viscosity. At 1200 ℃, CMAS + sea salt completely melted but CMAS did not; at 1250 ℃, both melted while CMAS + sea salt had stronger penetration ability in TBCs than CMAS. Thus, TBCs which have not been or are less affected by CMAS may be damaged due to CMAS + sea salt attack.

Crystallization behavior of calcium–magnesium–alumina–silicate coupled with NaCl/Na2SO4

Promoting molten calcium–magnesium–alumina–silicate (CMAS) crystallization can highly reduce the melt penetration into thermal barrier coatings (TBCs), which alleviates its corrosion to TBCs. In marine environments, many salts are coupled with CMAS. In this study, we investigated the crystallization behavior of CMAS + NaCl/NaSO4, and pointed out its influence on the corrosion behavior of TBCs. At 1200 ℃, CMAS + NaCl was crystalized into CaMgSi2O6, CaSiO3 and CaAl2Si2O8, while the crystallization products of CMAS + NaSO4 depended on NaSO4 content, i.e., 4 wt.% and 10 wt.% NaSO4 addition leading to CaAl2Si2O8 and Ca2Al(AlSiO7) phases, respectively, in addition to CaMgSi2O6 and CaSiO3. At 1100 ℃, CMAS + NaSO4 crystalized, while CMAS + NaCl had no crystallization. At lower temperatures, both NaCl and NaSO4 additions could suppress CMAS crystallization. It reveals that CMAS coupled with molten salt is hard to crystallize, which is detrimental to corrosion protection.

The mechanism in surface morphology of YSZ ceramics influencing its corrosion resistance to CMAS melt: Molecular dynamics research

In this paper, several surface morphologies such as smooth style, or mesh & stripe style containing nanostructure were constructed and simulated by molecular dynamics simulation. The results show that the diffusion area of molten calcium-magnesium-alumina-silicate (CMAS) droplets on yttria-stabilized zirconia (YSZ) surface with stripe nanostructure is smaller than that on smooth or mesh style surfaces, and its contact angle obtains the largest value. The diffusion coefficient points out that the best corrosion resistance of YSZ surface comes from the lowest diffusivity of elements in molten CMAS on stripe nanostructure. Elements radial distribution function of Ca-O verified that they mainly keep in CMAS droplets instead of permeating into YSZ ceramics. And, the influence of nanostructure on the depth of Mg, Al, Si infiltration layer in CMAS is greater than that of Ca ion. Therefore, YSZ surfaces with stripe nanostructures exhibit the best corrosion resistance.

CMAS Corrosion Resistance Behavior and Mechanism of Hf6Ta2O17 Ceramic as Potential Material for Thermal Barrier Coatings

Thermal barrier coatings (TBCs) have been seriously threatened by calcium-magnesium-alumina-silicate (CMAS) corrosion. The search for novel ceramic coatings for TBCs with excellent resistance to CMAS corrosion is ongoing. Herein, CMAS corrosion resistance behavior and the mechanism of a promising Hf6Ta2O17 ceramic coating for TBCs are investigated. The results show that temperature is the most important factor affecting the CMAS behavior and mechanism. At 1250 °C, the corrosion products are composed of dense reaction products (HfSiO4, CaXHf6−xTa2O17−x) and CMAS self-crystallization products. At 1300 and 1400 °C, the corrosion products are mainly dense CaTa2O6 and HfO2, which prevent further CMAS infiltration.

Ultra-low thermal conductivity and excellent high temperature resistance against calcium-magnesium-alumina-silicate of a novel β-type pyrosilicates

The materials which have low thermal conductivity, a good match of coefficient of thermal expansion with SiC-based ceramics matrix composites (CMCs) and excellent resistance against calcium-magnesium-alumina-silicate (CMAS) are suitable candidates for thermal/environmental barrier coating. To fulfil the above requirements we developed a new multicomponent rare earth (Y0.2Dy0.2Er0.2Yb0.2Ho0.2)2Si2O7 or (5RE0.2)2Si2O7 high entropy ceramics pyrosilicate by a single step solid solution method. The x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis revealed that the as-fabricated bulk composite (5RE0.2)2Si2O7 has a single phase β-type structure match well with single rare earth ytterbium disilicate (Yb2Si2O7) and the different rare earth oxides mixed well with each other. The excellent coefficient of thermal expansion (4 × 10−6-5 × 10−6 °C−1) and ultra-low thermal conductivity (0.7–1.6 W/m C) was observed as compared to other single and multicomponent high entropy ceramics composites. The (5RE0.2)2Si2O7 ceramics pyrosilicate show an excellent CMAS resistance at various temperatures (1300, 1400, and 1500) °C for 4 h compared with single principle RE disilicates. The above all mentioned results identify that this material is a suitable candidate for coating on the SiC-based ceramics composite for thermal/environmental barrier coating.

Comparative study on failure behavior of promising CMAS-resistant plasma-sprayed thermal barrier coatings in burner rig test with/without CMAS deposition

Along with continuous progress in inlet temperature of turbine engine, calcium-magnesium-aluminum-silicate (CMAS) deposition has become one of serious challenges for traditional yttria partially stabilized zirconia thermal barrier coatings at elevated temperature. Although lots of materials with superior CMAS resistance have been proposed, there is few comparative research on performance of corresponding coatings reported especially when subjected to thermal cycling and CMAS simultaneously. To this end, some coatings were prepared in present study, and thereafter failure behavior in condition of thermal cycling and thermal cycling-CMAS was systemically investigated and compared. Experimental results showed a varied lifetime and cracking behavior in thermal cycling test and thermal cycling-CMAS test, indicating that CMAS infiltration affected failure behavior of coatings. Besides, it was numerically found that CMAS penetration would lead to a promotion of thermal stress, which increased the tendency for cracking during thermal cycling. And the phenomenon that channel crack was the precondition of delamination crack was revealed.

The CMAS corrosion behavior of high-entropy (Y0.2Dy0.2Er0.2Tm0.2Yb0.2)4Hf3O12 hafnate material prepared by ultrafast high-temperature sintering (UHS)

High-temperature molten calcium-magnesium-alumina-silicate (CMAS) corrosion has become a fatal factor for the failure of aero-engine thermal barrier coatings. In this study, a promising entropy-stabilized (Y0.2Dy0.2Er0.2Tm0.2Yb0.2)4Hf3O12 (5YH) hafnate was prepared by the emerging ultrafast high-temperature sintering (UHS), and its corrosion and wetting behavior of molten CMAS were investigated. For the corrosion mechanism, the precipitation of the high-entropy apatite phase promotes the formation of the HfO2 phase, and it can improve the density and stability of the slow-growing reaction layer, hindering the further penetration of molten CMAS. At 1300 ℃, a reaction layer with a three-layered morphology is generated, resulting from the decreased viscosity of the molten CMAS. Moreover, computational analysis shows that molten CMAS on the 5YH surface has a larger contact angle (17°) than traditional YSZ (13°), and the spreading area is about 90 % of traditional YSZ, which benefits for its good CMAS corrosion resistance.

Unveiling of Lanthanum–Yttrium Co-Doped Zirconia and Yttrium-Stabilized Zirconia Calcium–Magnesium–Alumina–Silicate Corrosion Behavior

Unveiling of Lanthanum–Yttrium Co-Doped Zirconia and Yttrium-Stabilized Zirconia Calcium–Magnesium–Alumina–Silicate Corrosion Behavior

Effects of Yb and Sc co-doping on the thermal properties and CMAS corrosion of garnet-type (Y3-xYbx)(Al5-xScx)O12 ceramics

Thermal barrier coatings with excellent thermal performance and corrosion resistance are essential for improving the performance of aero-engines. In this paper, (Y3-xYbx)(Al5-xScx)O12 (x = 0, 0.1, 0.2, 0.3) thermal barrier coating materials were synthesized by a combination of sol-gel method and ball milling refinement method. The thermal properties of the (Y3-xYbx)(Al5-xScx)O12 ceramics were significantly improved by increasing Yb and Sc doping content. Among designed ceramics, (Y2.8Yb0.2)(Al4.8Sc0.2)O12 (YS-YAG) showed the lowest thermal conductivity (1.58 Wm−1K−1, at 800 °C) and the highest thermal expansion coefficient (10.7 × 10−6 K−1, at 1000 °C). In addition, calcium-magnesium- aluminum -silicate (CMAS) corrosion resistance of YS-YAG was further investigated. It was observed that YS-YAG ceramic effectively prevented CMAS corrosion due to its chemical inertness to CMAS as well as its unique and complex structure. Due to the excellent thermal properties and CMAS corrosion resistance, YS-YAG is considered to be prospective material for thermal barrier coatings.

Ti4+-incorporated fluorite-structured high-entropy oxide (Ce,Hf,Y,Pr,Gd)O2−δ: Optimizing preparation and CMAS corrosion behavior

Environmental barrier coatings (EBCs) with excellent chemical resistance and good high-temperature stability are of great significance for their applications in next-generation turbine engines. In this work, a new type of high-entropy fluorite-structured oxide (Ce0.2Hf0.2Y0.2Pr0.2Gd0.2)O2−δ (HEFO-1) with different Ti4+ contents were successfully synthesized. Minor addition of Ti4+ could be dissolved into a high-entropy lattice to maintain the structure stable, effectively reducing the phase formation temperature and promoting the shrinkage of bulk samples. Heat treatment experiments showed that all the samples remained a single phase after annealing at 1200–1600 °C for 6 h. In addition, high-entropy (Ce0.2Hf0.2Y0.2Pr0.2Gd0.2Ti0.2x)O2−δ demonstrated great resistance to calcium—magnesium—alumina—silicate (CMAS) thermochemical corrosion. When the content of Ti was increased to x = 0.5, the average thickness of the reaction layer was about 10.5 after being corroded at 1300 °C for 10 h. This study reveals that high-entropy (Ce0.2Hf0.2Y0.2Pr0.2Gd0.2Ti0.2x)O2−δ is expected to be a candidate for the next-generation EBC materials with graceful resistance to CMAS corrosion.

Mitigating CMAS Attack in Model YAlO3 Environmental Barrier Coatings: Effect of YAlO3 Crystal Orientation on Apatite Nucleation

Environmental barrier coatings (EBCs) are used to protect ceramic-matrix composites from undesirable reactions with steam and calcia–magnesia–alumina–silicate (CMAS) particulates found in gas-turbine engine environments. Effective EBCs contain yttria or rare earth ions that will react with molten CMAS to form a protective apatite layer that prevents further attack. Methods to improve the EBCs’ CMAS mitigation capabilities focus on improving the apatite yield but neglect optimizing the apatite formation behavior. This study investigates the effect of apatite nucleation behavior on CMAS penetration by comparing the CMAS attack at 1350 °C of four different single crystal orientations of yttria aluminate perovskite (YAP), a promising EBC candidate. The EBC/CMAS interfacial energy and, thus, reaction behavior varies with YAP orientation. In regions with low CMAS loading, rapid apatite growth is seen on YAP substrates with orientations associated with high EBC/CMAS interfacial energy. However, CMAS penetration is most significant in these samples because the apatite growth is facilitated by recession of the YAP substrate nearby. Such behavior is not observed in regions with high CMAS loading where small apatite crystals form on top of an yttrium aluminate garnet (Y3Al5O12, YAG) phase. This study shows that strategies that control the nucleation and growth of apatite will provide better protection against CMAS.

Corrosiveness of CMAS and CMAS + salt (NaVO3, Na2SO4 and NaCl) to YSZ thermal barrier coating materials

Calcium-magnesium-alumina-silicate (CMAS) is found to consist with many salts especially in marine environment, where thermal barrier coatings (TBCs) are widely used. In this study, the corrosiveness of CMAS + salt (NaVO3, Na2SO4 and NaCl) to a typical TBC material of Y2O3 partially stabilized ZrO2 (YSZ) was investigated. As compared to CMAS, CMAS + salt had lower melting temperature, viscosity and crystallization ability. This led to the CMAS + salt more aggressive to YSZ at relatively low temperature, and more permeable at 1250 ℃. Accordingly, the CMAS + salt had higher corrosiveness than CMAS to TBCs, which is worth special attention.

High temperature behavior of HfSiO4 subjected to calcium-magnesium-alumina-silicate (CMAS) and high-velocity water vapor

HfSiO4 is considered as a candidate for environmental barrier coating (EBC), but there is a lack of comprehensive evaluation of its resistance against corrosive medium. We herein study the behavior of HfSiO4 against CMAS melt and high-velocity water vapor. HfSiO4 shows poor resistance to CMAS attack. Si diffusion occurs during CMAS attack, which leads to the formation of HfO2 and CaSi2O5. HfSiO4 decomposes to form SiO2 and HfO2 under the scouring of water vapor, in which SiO2 forms volatile hydroxide and is taken away by high-velocity steam. HfSiO4 is not the preferred system for surface layer of EBC system and is expected to be used as intermediate transition layer.

Corrosion resistance of GdPO4 thermal barrier coating candidate in the presence of CMAS + NaVO3 and CMAS

GdPO4 is a promising thermal barrier coating (TBC) candidate owing to its excellent resistance to calcium-magnesium-alumina-silicate (CMAS) and molten salt corrosion. In this study, the corrosion behavior of GdPO4 in CMAS + molten salt (CN) was investigated and compared with that in CMAS. At 1200 and 1250 °C, interface reaction layers rapidly formed on both the CN and CMAS attacked GdPO4 pellets, and many crystalline products were generated in the melts. This effectively increased the melt viscosity and suppressed its penetration. As compared to CMAS, CN was more aggressive to GdPO4 at 1200 °C but less at 1250 °C.

Failure Mechanism of EB-PVD Thermal Barrier Coatings under the Synergistic Effect of Thermal Shock and CMAS Corrosion

Thermal barrier coatings (TBCs) suffer from the thermo-chemo-mechanical coupling action of thermal shock and calcium–magnesium–alumina–silicate (CMAS) corrosion. However, the failure mechanism of TBCs under the synergistic effect of thermal shock and CMAS corrosion is still unclear due to a lack of an environmental simulator. Herein, an 8YSZ ceramic coating is deposited on a PtAl bond coating/DD419 nickel-based single crystal superalloy substrate using the electron beam physical vapor deposition (EB-PVD) method. The thermo-chemo-mechanical coupling effect of TBCs is achieved in a self-developed environmental simulator. The interaction of volume expansion induced by the phase transition of ZrO2, structural degradation and thermal fatigue further increases the out-of-plane tensile stress and in-plane shear stress in the ceramic coating, which accelerates the initiation and propagation of surface vertical cracks and horizontal cracks. As multiple surface vertical cracks propagate to the interface and merge with interfacial cracks, the ceramic coating spalls from the substrate.

Evolution of the microstructure and mechanisms of performance degradation in EB-PVD YSZ thermal barrier coatings corroded by volcanic ash at 1150 °C

Volcanic ash (VA)/calcium-magnesium-alumina-silicate (CMAS) corrosion is a crucial issue causing the failure of thermal barrier coatings. This study focuses on the degradation process of electron-beam physical vapor deposition yttria-stabilized zirconia by natural VA at 1150 °C. The phase transformation and chemical reaction during VA attack were measured in real-time by in-situ high-temperature X-ray diffraction. The mechanical properties and the thermal conductivity of the as-deposited and VA corroded coatings were investigated. The correlation between these microstructural changes and the coatings’ properties are discussed. The degradation mechanisms are expected to guide the development of potential VA/CMAS mitigation methods.

Improving CMAS-corrosion resistance of YSZ-based thermal barrier coatings with Al2O3 addition

Thermal barrier coatings (TBCs), exposed to the molten calcium‑magnesium‑aluminum-silicate (CMAS) deposits, could become degraded more readily during service. In this paper, the composite Al2O3/YSZ TBCs with different Al2O3 additions were prepared through the atmospheric plasma spraying technique and their resistance to CMAS corrosion at 1250 °C was carefully compared to the 7YSZ (7 wt.% Y2O3 stabilized ZrO2) TBCs. It was found that two distinct microstructural characteristics, i.e. thermo-chemical reaction zone and CMAS physical-infiltration zone, were identified for both the 7YSZ coating and the 10 wt.%Al2O3-7YSZ coating. The thermo-chemical reaction zone was severely corroded by molten CMAS, inducing the dissolution and re-precipitation of YSZ material and generating small globular m-ZrO2 particles. Furthermore, these two coatings were completely infiltrated (approximately 300 μm) and developed significant macroscopic bending. With the Al2O3 addition increasing above 20 wt.%, the depth of the thermo-chemical reaction zone was restricted to 35 μm without obvious CMAS physical-infiltration. During CMAS attacking the Al2O3/YSZ coating, continuous dissolution of Al2O3 into CMAS melt could promote anrthite crystallization along the CMAS-coating interface, which effectively restrained CMAS penetration and improved CMAS corrosion resistance.

Stress distribution around the reaction layer of CMAS and GdPO4 thermal barrier coatings based on finite element analysis

GdPO4 thermal barrier coatings (TBCs) have high resistance to calcium‑magnesium-alumina-silicate (CMAS) attack due to the formation of a dense reaction layer mainly consisting of the apatite phase. In this study, stress evolution of CMAS covered GdPO4 TBCs during cooling, and the influence of the reaction layer roughness and thickness on the stress distribution are investigated by the finite element method. During cooling, the tensile stress concentration of σMPS occurred around the peak point of the reaction layer/GdPO4 interface. With the increase of the reaction layer roughness, the ranges of high tensile stress (σ22 and σMPS) and high shear stress (σ12) zones expanded. Increasing the reaction layer thickness also enlarged the high tensile stress (σ22 and σMPS) zone. Due to the high tensile stress and shear stress concentration, the interface between the reaction layer and the GdPO4 coating was easy to crack and separated. Decreasing the reaction layer roughness and thickness was beneficial to reduce the interface stress.

Finite element analysis on temperature field and stress distribution of thermal barrier coatings by laser modification and CMAS corrosion

Laser glazing to produce a modified layer on the surface of thermal barrier coatings (TBCs) is a promising method to alleviate calcium-magnesium-alumina-silicate (CMAS) attacks to coatings. In this study, finite element analysis is carried out to investigate the temperature field and stress distribution of TBCs after the laser glazing process and the modified TBCs after CMAS corrosion. Results revealed that along the direction of laser scanning, the principal stress was in a tensile state, which increased along the direction of the laser spot movement; while perpendicular to the laser scanning direction, the maximum principal stress appeared at the interface between the glazed layer and the unmodified coating, where could be a potential danger zone for crack initiation and coating spallation. For the modified TBCs with CMAS attack, in regions that are located in the range of 0–6 mm along the radial direction, the radial stress and maximum principal stress were high (∼1094 MPa), and the coating edge had complex shear stress state; as a result, these areas were easy to crack.

Calcium-magnesium-alumina-silicate (CMAS) corrosion behavior of A6B2O17 (A=Zr, Hf; B=Nb, Ta) as potential candidate for thermal barrier coating (TBC)

Developing new systems with excellent properties is of great significance to the progress of TBC. We herein study the CMAS corrosion behavior of four A6B2O17 ceramics to evaluate the feasibility for TBC application. Zr6Nb2O17 has the worst corrosion resistance and Hf6Ta2O17 shows the best. The effects of elements A and B are reflected in the differences of corrosion products and reverse dissolution process, respectively. The dense corrosion layer containing CaTa2O6 and HfSiO4 is an important factor for Hf6Ta2O17 to prevent further corrosion. The present results highlight the merit of Hf6Ta2O17 as a promising candidate for CMAS-resisted TBC.