Calcium-magnesium-alumina-silicate Research Articles

Effects of pellet surface roughness and pre-oxidation temperature on CMAS corrosion behavior of Ti2AlC

Calcium-magnesium-alumina-silicate (CMAS) corrosion is a serious threat to thermal barrier coatings (TBCs). Ti2AlC has been proven to be a potential protection layer material for TBCs to resist CMAS corrosion. In this study, the effects of the pellet surface roughness and temperature on the microstructure of the pre-oxidation layer and CMAS corrosion behavior of Ti2AlC were investigated. The results revealed that pre-oxidation produced inner Al2O3 layer and outer TiO2 clusters on the pellet surfaces. The content of TiO2 decreased with decreasing pellet surface roughness and increased along with the pre-oxidation temperature. The thickness of Al2O3 layer is also positively related to the pre-oxidation temperature. The Ti2AlC pellets pre-oxidized at 1050 °C could effectively resist CMAS corrosion by promoting the crystallization of anorthite (CaAl2Si2O8) from the CMAS melt rapidly, and the resistance effectiveness increased with the pellet surface roughness. Additionally, the CMAS layer mainly spalled off at the interface of CaAl2Si2O8/Al2O3 layer after thermal cycling tests coupled with CMAS corrosion. The Al2O3 layer grown on the rough interface could combine with the pellets tightly during thermal cycling tests, which was attributed to obstruction of the rough interface to crack propagation.

Peridynamic-based investigation of the cracking behavior of multilayer thermal barrier coatings

Numerical simulations of the cracking behavior of the top layers of multilayer thermal barrier coatings (TBCs) can effectively reveal the failure mechanisms of the TBCs. Current finite element method (FEM)-based simulation means have been applied to solve certain simple cracking problems in TBCs; however, they cannot effectively describe complex cracking problems in TBCs such as coalescence, intersection, and interference among multiple cracks. Peridynamic (PD), a newly developed mechanical theory, has been widely studied to provide analysis for cracking problems in TBCs. In this paper, a numerical model of TBCs is built by the bond-based PD (BB-PD) theory. Complex cracking behaviors, such as spontaneous crack propagation at both interfacial and internal regions, coalescence, and interference among multiple cracks, are simulated under isothermal cooling and gradient cooling conditions. In addition, the effects of interfacial roughness and calcium–magnesium–alumina–silicate (CMAS) inclusions on the cracking behavior are discussed. The results show that the PD model accurately captures complex cracking behaviors observed via scanning electron microscopy (SEM). Given the ability of the model for analyzing discontinuities in TBCs, it can help to further clarify the fracture mechanisms of TBCs.

Failure mechanism of YSZ coatings prepared by EB-PVD under partial penetration of CMAS attacking

The development of thermal barrier coating has been limited by calcium–magnesium–alumina–silicate (CMAS) corrosion. ~13 wt% YSZ coating under partial penetration of CMAS attacking is used as materials of thermal cycling experiment to exclude the influence of phase transition. Failure modes at boundary of CMAS permeable and impermeable zone at 1100 ℃ and 1250 ℃ are not completely consistent. The boundary of two conditions is inclined to occur vertical, transverse and oblique cracks. However, step-type cracks developed by the vertical crack tip are formed at 1100 ℃, and straight oblique penetrating cracks are formed at 1250 ℃.

Study on properties of Hf6Ta2O17/Ta2O5 system: A potential candidate for environmental barrier coating (EBC)

Ta2O5 doped Hf6Ta2O17 system (Hf6Ta2O17/Ta2O5) is considered to have potential application prospect in the field of aero-engine. We herein focus on the thermo-physical, mechanical properties and CMAS corrosion resistance of Hf6Ta2O17/Ta2O5 to systematically evaluate the possibility for the application of environmental barrier coating (EBC). By changing the content of Ta2O5, the gradient adjustment of thermal expansion coefficient can be realized while maintaining low thermal conductivity (1.5–2.2 W/(m·K)). The introduction of Ta2O5 significantly reduces the modulus and improves the fracture toughness. Single-phase Hf6Ta2O17 shows excellent corrosion resistance against molten calcium-magnesium-alumina-silicate (CMAS). The crystallization of CaTa2O6 and HfSiO4 is the important factor to prevent further corrosion. The introduction of Ta2O5 weakens the ability to prevent Si penetration and greatly increases the thickness of the corrosion layer. The results highlight the merit of Hf6Ta2O17/Ta2O5 system as potential candidate for multi-layer gradient coating on the surface of ceramic matrix composites.

Interfacial Stresses of Thermal Barrier Coating with Film Cooling Holes Induced by CMAS Infiltration

To obtain high gas turbine efficiency, a film cooling hole is introduced to prevent the destruction of thermal barrier coating systems (TBCs) due to hot gases. Furthermore, environmental calcium-magnesium-aluminum-silicate (CMAS) particulates plug the film cooling hole and infiltrate the TBCs to form a CMAS-rich layer, which results in phase transformations and significant modifications in the thermomechanical properties that impact the TBCs during cooling. This study aimed to establish a three-dimensional thermo-fluid-solid coupling TBCs model with film cooling holes and CMAS infiltration to analyze the temperature and residual stress distribution via simulations. For the interfacial stress around the cooling hole at the TC/BC interface, the film cooling holes alleviated the interfacial residual stress by 60% due to the reduction in temperature by 40%. In addition, CMAS infiltration intensified the interfacial residual stress via phase transformation. As a result of the influence of larger penetration depths and expansion rates of phase transformation, a significant increase in residual stress was observed. At the beginning of CMAS infiltration, the interfacial stress would be more dominated by the effect of infiltration depth. In addition, the failure due to interfacial normal and tangential stresses was more likely to be found at the infiltration zone near the cooling hole.

Effect of CMAS viscosity on the infiltration depth in thermal barrier coatings of different microstructures

Calcium magnesium aluminum silicate (CMAS) is one of the leading concerns for the gas turbine industry. The effects of CMAS viscosity and the coating microstructure on CMAS infiltration depth were explored by conducting a time dependent interaction study. Three CMAS compositions were used from literature, and their viscosities predicted through FactSage viscosity module were drastically different. The interaction was carried out on three different TBCs synthesized using the solution precursor plasma spray process (SPPS): two of the TBCs were made of yttrium aluminum garnet (YAG) having different microstructures that promote different modes of CMAS infiltration, and one TBC was made of gadolinium zirconate (GZO). All samples had stress relieving vertical cracks and different intensities of horizontally banded porosity, (inter pass boundaries IPBs). A concentration of 100 mg/cm2 of CMAS was applied on the TBCs which were then subjected to a 5-minute interaction at 1300 °C. Samples were analyzed using scanning electron microscopy (SEM), electron dispersive X-ray spectroscopy (EDXS), and transmission electron microscopy (TEM). Low viscosity CMAS readily penetrated the TBCs while more viscous CMAS showed less penetration. The depth of CMAS infiltration depended on the coating microstructure. In the YAG with IPBs, the CMAS spread horizontally in the IPBs before infiltrating deeper, resulting in reduced infiltration depth compared to other samples in spite of having wider vertical cracks. TEM and EDXS analysis were performed to investigate the phases present in the CMAS-TBC interaction region in YAG. Two regions were chosen, the top TBC surface in direct contact with the sea of CMAS, and the region at the CMAS penetration ended within the coating. The results showed that no secondary phases like apatite were observed in YAG, thus it can be concluded that the arrest of CMAS happened solely because of CMAS viscosity and the short infiltration time.

Sc-doped Gd2Zr2O7 coating on YSZ thermal barrier coatings to resist CMAS + molten salt attack

Coupling corrosion resulting from calcium-magnesium-alumina-silicate (CMAS) and molten salt causes severe attack to YSZ thermal barrier coatings (TBCs). In this study, a Sc-doped Gd2Zr2O7 (GZO-Sc) coating is prepared on the YSZ coating to alleviate CMAS + NaVO3 (CN) corrosion. Results revealed that the GZO-Sc coating largely prevented CN (with a melting point of 1188 °C) attack to YSZ coatings at 1200 °C and 1250 °C. At the test temperatures, CN melted and penetrated the GZO-Sc coating, during which the melt and the coating reacted to form a dense reaction layer, consisting of Gd8Ca2Si6O26-based apatite and c-ZrO2 based fluorite phase independent on the temperature and time. With the increase of the dwell time, V was separated from CN melt during infiltration, causing many CaAl2Si2O8 crystals at the bottom of the reaction layer and a V-rich layer beneath the reaction layer, which were beneficial to the CN corrosion resistance of the coatings.

Unusual temperature-dependent reactivity of ultra-high temperature ceramic (UHTC) borides with calcia-magnesia-alumina-silicate (CMAS)

The resistance of ultra-high temperature ceramic (UHTC) borides to calcia-magnesia-alumina-silicate (CMAS) attack is evaluated for the first time. ZrB2 and HfB2 are reacted with 23 mol% of CMAS 1000 °C, 1300 °C and 1600 °C, respectively, for 1 h, 10 h and 100 h, respectively, in atmosphere. Interestingly, borides first oxidize before reacting with CMAS to form ZrSiO4 and HfSiO4, respectively, which peaks at 1300 °C. At 1600 °C, the reaction with CMAS to form HfSiO4 is greatly suppressed; moreover, ZrB2 exhibits only oxidation and no reaction with CMAS nor formation of ZrSiO4 or any other CMAS induced reaction product. This unusual inertness at temperatures above 1600 °C could enable new approaches to addressing the CMAS problem in thermal and environmental barrier coatings.

Reaction products of Sm2Zr2O7 with calcium-magnesium-aluminum-silicate (CMAS) and their evolution

During flight, many silicates (sand, dust, debris, fly ash, etc.) are ingested by an engine. They melt at high operating temperatures on the surface of thermal barrier coatings (TBCs) to form calcium-magnesium-aluminum-silicate (CMAS) amorphous settling. CMAS corrodes TBCs and causes many problems, such as composition segregation, degradation, cracking, and disbanding. As a new generation of TBC candidate materials, rare-earth zirconates (such as Sm2Zr2O7) have good CMAS resistance properties. The reaction products of Sm2Zr2O7 and CMAS and their subsequent changes were studied by the reaction of Sm2Zr2O7 and excess CMAS at 1350 °C. After 1 h of reaction, Sm2Zr2O7 powders were not completely corroded. The reaction products were Sm-apatite and c-ZrO2 solid solution. After 4 h of reaction, all Sm2Zr2O7 powders were completely corroded. After 24 h of reaction, Sm-apatite disappeared, and the c-ZrO2 solid solution remained.

CMAS corrosion behavior of Sc doped Gd2Zr2O7 /YSZ thermal barrier coatings and their corrosion resistance mechanisms

YSZ thermal barrier coatings (TBCs) are vulnerable to calcium-magnesium-alumina-silicate (CMAS) attack, and a CMAS resistant TBC of Sc doped Gd2Zr2O7/YSZ are developed in this study. At 1250 ℃, corrosion products form a reaction layer at the CMAS/coating interface, which largely suppresses CMAS further infiltration. Compared with Gd2Zr2O7, Sc doped Gd2Zr2O7 has a higher ability to keep its lattice structure in the presence of CMAS, which is beneficial to maintain the coating microstructure under CMAS conditions, alleviating the coating degradation. First-principles calculation was conducted to investigate the corrosion products formation mechanisms, and the characteristics of CMAS/reaction layer and reaction layer/coating interfaces.

Microstructure design of the laser glazed layer on thermal barrier coatings and its effect on the CMAS corrosion

Calcium-magnesium-alumina-silicate (CMAS) corrosion is a great challenge for the long lifetime of thermal barrier coatings (TBCs). In this study, the surface microstructure of TBCs is tailored by laser glazing to improve the CMAS resistance, and the modified coatings are designed to have single and double laser-glazed layers. In the double laser-glazed coating, the vertical cracks in the glazed layer which penetrate directly for the case of the single laser-glazed coating become bifurcated and staggered. Under molten CMAS conditions, the coating with double laser-glazed layers highly resists to melt penetration, and the glazed layer exhibits excellent phase stability and structural integrity.

Comparison of calcium–magnesium-alumina-silicate (CMAS) resistance behavior of produced with electron beam physical vapor deposition (EB-PVD) method YSZ and Gd2Zr2O7/YSZ thermal barrier coatings systems

Thermal Barrier Coatings (TBCs) are generally used in both industrial gas turbines and aircraft engines. In this study, metal powders with CoNiCrAlY content were coated on Inconel 718 nickel-based superalloy substrate materials using the cold gas dynamic spray (CGDS) method, which is a new generation thermal spray coating method. As a top coating, YSZ and Gd2Zr2O7/YSZ (GZO/YSZ), respectively, single and double layer ceramic layers were deposited with an innovative method, physical vapor deposition by electron beam (EB-PVD). All produced TBC systems were sprinkled with vermiculite powder containing CMAS minerals and subjected to CMAS corrosion tests and the effect of CMAS damage on the phase stability and microstructure of TBC systems was determined using elemental mapping analysis, X-ray diffraction (XRD) analysis, and scanning electron microscope (SEM). According to the results obtained, it has been determined that the double layer TBC system with GZO/YSZ is more resistant to CMAS damage than the single layer YSZ system.

Crystallization behavior of CMAS and NaVO3+CMAS mixture and its potential effect to thermal barrier coatings corrosion

Calcium-magnesium-alumina-silicate (CMAS) and molten salt corrosion pose great threats to thermal barrier coatings (TBCs), and recently, a coupling effect of CMAS and molten salt has been found to cause even severer corrosion to TBCs. In this study, the crystallization behavior of CMAS and CMAS+NaVO3 is investigated for potentially clarifying their corrosion mechanisms to TBCs. Results indicated that at 1000 °C and 1100 °C, CMAS was crystallized to form CaMgSi2O6, while at 1200 °C, the crystallization products were CaMgSi2O6, CaSiO3 and CaAl2Si2O8. The introduction of NaVO3 in CMAS reduced the crystallization ability, and as the NaVO3 content increased, glass crystallization occurred at a lower temperature, with crystallization products mainly consisting of CaAl2Si2O8 and CaMgSi2O6. At 1200 °C, CMAS+10 wt% NaVO3 was in a molten state without any crystallization, which suggested that NaVO3 addition in CMAS could reduce its melting point, indicating enhanced penetration ability in TBCs and thus increased corrosiveness.

Influence of the grain size on CMAS attack of Sm2Zr2O7 ceramic

The temperature resistance of thermal barrier coatings (TBCs) has increased with the continuous development of the aviation industry. This increase in temperature resistance has resulted in a new challenge for TBCs, namely, calcium-magnesium-aluminum-silicate (CMAS) attack. As a new generation of thermal barrier coating candidate materials, Sm2Zr2O7 has good CMAS resistance properties. However, this material cannot meet the actual needs of aero-engines. Therefore, a change in the structure of Sm2Zr2O7 was used to improve the CMAS resistance properties in this paper. The relationship between the grain size of the ceramic and its resistance to CMAS penetration in the microstructure was investigated in detail.Nonpressure and SPS sintering processes were used to prepare Sm2Zr2O7 ceramics with different grain sizes that were then tested at high temperatures with CMAS. With the extension of penetration, the depth of CMAS penetration in microscale Sm2Zr2O7 ceramics increased sharply with increasing reaction time, while the penetration depth of CMAS into nanoscale Sm2Zr2O7 ceramics increased slowly. After 48 h of penetration, the penetration depth of the microscale Sm2Zr2O7 ceramics was 86 μm, and the penetration depth of the nanoscale Sm2Zr2O7 ceramics was only 47 μm. Compared with the microscale Sm2Zr2O7 ceramics, the nanoscale Sm2Zr2O7 ceramics had better CMAS resistance because the lower diffusion activation energy of the nanocrystalline grains accelerated the formation of a dense barrier layer.

In-flight particle states and coating properties of air plasma sprayed ytterbium disilicates

The effects of processing parameters on the in-flight particle behavior, phase composition, and properties of air plasma sprayed ytterbium disilicate (Yb2Si2O7) coatings were investigated in this study. With the increase in plasma enthalpy, in-flight particle temperature increases. Higher particle temperature caused an increase in silica (SiO2) evaporation from the Yb2Si2O7 particles and, as a result, additional Yb2SiO5 phase appears in the Yb2Si2O7 coating. Thermal expansion tests showed the coating containing a higher volume of Yb2SiO5 phase has a larger coefficient of thermal expansion (CTE). Thermal cycling tests in a steam environment indicated that the Yb2SiO5 phase content in the APS Yb2Si2O7 coatings has a significant effect on the EBC's durability. Thermal cycling lives decrease with the increase of Yb2SiO5 phase content in APS Yb2Si2O7 coatings. Analysis of the failed coatings indicated that the predominant EBC failure mechanisms are due to growth of the thermally growth oxides (TGO) as well as the stresses induced by CTE mismatch. Furthermore, Calcium–magnesium–alumina-silicate (CMAS) corrosion tests showed that the recession rate decreases with the increase of Yb2SiO5 phase content in APS Yb2Si2O7 coatings.

In situ observation of environmental barrier coating against calcium–magnesium–alumina–silicate attack at elevated temperature

AbstractEnvironmental barrier coating (EBC) plays a crucial role in improving the property of ceramic matrix composites at harsh environment. In this work, an in situ observation method was employed to systematically study the resistance of EBC to calcium–magnesium–alumina–silicate (CMAS) phase at elevated temperatures, which were 1200℃, 1300℃, and 1500℃, respectively. The melting and spreading process of the CMAS phase on the surface of the EBC was obtained via this method. In addition, the erosion effect of CMAS phase on EBC at different temperatures was analyzed by combining the scanning electron microscope (SEM) and energy dispersive spectrum (EDS), which provided a guidance for the modification of EBC.

PS–PVD Alumina Overlayer on Thermal Barrier Coatings Against CMAS Attack

Glassy deposits mainly comprising of calcium–magnesium–alumina–silicate (CMAS) accelerate the spallation of thermal barrier coatings (TBCs). In this work, an Al2O3 layer was produced on yttria-stabilized zirconia (YSZ) coating by plasma spray–physical vapor deposition (PS–PVD). The effects of processing parameters during PS–PVD process on the microstructures of the deposited Al2O3 coatings were investigated. A homogeneous Al2O3 coating with porosity less than 1% was deposited at the spray distance of 1400 mm, which is much denser than the coating produced by atmospheric plasma spray (APS) and other PS–PVD coatings sprayed at 1000 and 1900 mm. The CMAS corrosion test at 1250 °C showed that the APS Al2O3 layer was dissolved in CMAS after 24 h thermal exposure, whereas a dense reaction layer mainly composed of anorthite formed on the top region of the PS–PVD Al2O3 layer effectively protected the underlying Al2O3 layer from CMAS infiltration. Thermal cycling behavior of the PS–PVD Al2O3/YSZ TBC and the APS Al2O3/YSZ TBC at 1050 °C was compared. Bulk spallation of the APS TBC occurred within 300 h thermal cycling, while little spallation was observed on the PS–PVD TBC even after 500 h thermal cycling, indicating superior durability of the PS–PVD coating.

Effects of surface roughness on CMAS corrosion behavior for thermal barrier coating applications

Calcium-magnesium-alumina-silicate (CMAS) corrosion is an important cause for thermal barrier coating (TBC) failure, which has attracted increased attentions. In this study, some thermal barrier coating (TBC) materials including YSZ (yttria partially stabilized zirconia), GdPO4, and LaPO4 were prepared into bulks, and the effects of their surface roughness on wettability and spreading characteristics of molten CMAS were investigated. As-fabricated and polished bulks with different surface roughness were exposed to CMAS corrosion at 1250 °C for 1 and 4 h, following by macro and micro observations. Results revealed that compared with the as-fabricated bulks, molten CMAS on the polished samples had lower wettability and a smaller spreading area, mainly attributable to the reduced capillary force to drive the melt spreading. Meanwhile, GdPO4 and LaPO4 bulks exhibited lower CMAS wettability than YSZ bulk. It is thus considered that reducing the surface roughness is beneficial to CMAS corrosion resistance of TBCs.

Corrosion behavior and thermos-physical properties of a promising Yb2O3 and Y2O3 co-stabilized ZrO2 ceramic for thermal barrier coatings subject to calcium-magnesium-aluminum-silicate (CMAS) deposition: Experiments and first-principles calculation

In this study, corrosion behavior of Yb2O3-Y2O3 co-stabilized ZrO2 (YbYSZ) was systematically investigated under CMAS melt attack. The result was found that YbYSZ exhibited a better CMAS resistance than yttria stabilized zirconia (YSZ), which was attributed to the excellent phase stability. The degradation of grain caused by CMAS attack was greatly related to the diffusion rate of element substantially. Yb3+ exhibited a low diffusion rate contributing to good phase stability during corrosion. A corresponding first-principles calculation further confirmed this conclusion. Besides, YbYSZ showed a higher thermal expansion coefficient and lower thermal conductivity as compared with YSZ counterpart.

Calcium-Magnesium-Aluminum-Silicate (CMAS) Corrosion Resistance of Y-Yb-Gd-Stabilized Zirconia Thermal Barrier Coatings

It was published that zirconate coatings containing Gd/Yb and yttria-stabilized zirconia (YSZ) coatings with high Y content had good calcium-magnesium-aluminum-silicate (CMAS) corrosion resistance. However, there are few reports on CMAS resistance of zirconia-based thermal barrier coatings (TBCs) doped with Y, Gd and Yb. This article intends to explore the role of Y, Gd and Yb in resisting CMAS corrosion of zirconia-based TBCs. The purpose is to optimize the rare earth modification of zirconia-based TBCs. 8YSZ and ZrO2-9.5Y2O3-5.6Yb2O3-5.2Gd2O3 free-standing TBCs were produced by air plasma spraying (APS) with commercial powders. The CMAS corrosion behavior and failure mechanism of these coatings were studied by heat treatment at 1350 °C. The microstructure and composition were characterized. The results showed the CMAS deposited on the surface of 8YSZ/YSZ-Yb-Gd coatings gradually penetrated into coatings. ZrSiO4 and m-ZrO2 were observed in the shallow layer of coatings. The diffusion rate of Y3+ in coatings into CMAS was greater than that of Gd3+ and Yb3+. Compared with YSZ coatings, CMAS penetrated deeper in YSZ-Yb-Gd coatings after the same corrosion time. The spallation of YSZ-Yb-Gd coatings occurred earlier than 8YSZ. In general, adding a small amount of Yb and Gd did not improve the CMAS resistance of YSZ TBCs.