CMAS Infiltration Research Articles

Erosion behavior of CMAS/VA infiltrated EB-PVD Gd2Zr2O7 TBCs: Special emphasis on the effect of mechanical properties of the reaction products

Aero-engines operating in sand-laden (CMAS/CaO–MgO–Al2O3–SiO2) environments often encounter severe problems with thermal barrier coatings (TBCs) due to CMAS infiltration and erosion damage. This study focuses on a deeper understanding of the erosion behavior of CMAS-infiltrated EB-PVD Gd2Zr2O7 TBCs. The study includes isothermal infiltration of different CMAS and subsequent erosion tests at room temperature.In addition to the erosion behavior of the entire coating, the influence of different reaction products within the reaction layer on erosion failure was investigated by measuring the hardness and Young's modulus of the individual phases using in-situ REM-Nanoindentation. It was found that a garnet layer above the reaction layer and spinel inclusions within a thick apatite/fluorite reaction layer, can improve the erosion resistance of this reaction layer by 30–40%. Furthermore, a correlation between the erosion behavior and the hardness vs. Young's modulus relation, obtained from nanoindentation over the entire coating, was observed for a consistent microstructure.

Preparation and CMAS Wettability Investigation of CMAS Corrosion Resistant Protective Layer with Micro-Nano Double Scale Structure

Solution precursor plasma spray (SPPS) can prepare thermal barrier coatings (TBCs) with nanostructures, which can modify the adhesion and wettability of molten silicate environmental deposits (CMAS) on the surface of TBCs, thereby improving the resistance of TBCs to CMAS corrosion. In this study, SPPS layers with micro-nano double scale structures were prepared on the surface of conventional atmospheric plasma spraying (APS) coatings. The effect of process parameters on the micro-nano double scale structures and the wetting and infiltration behavior of molten CMAS on the surface of coatings were investigated. The results show that micron structure is more sensitive to process parameters. Lower precursor viscosity, closer spraying distance, and smoother APS layer are favorable to form more typical and dense micron structures. After covering the SPPS layer, the CMAS wetting diameter is reduced by about 40% and the steady-state contact angle increased up to three times. The reason is that the micro-nano double scale structures can effectively trap air and form an air layer between the coating surface and the molten CMAS. In addition, nano-particles play a more important role in the formation of the air layer, which in turn determines the steady-state wettability properties. While micron structures can influence the time needed to reach the steady state. However, the SPPS layers composed of nano-particles have a very loose structure and weak cohesion, and they degrade and fail rapidly after the infiltration of molten CMAS. Therefore, maintaining the excellent CMAS wetting resistance of the SPPS layers while taking into account their lifetime and reliability has become the focus of further research.

Determination of residual stresses in thermal barrier coating due to the amount of CMAS infiltration

In this work, we present the effect of the amount of CMAS infiltration into YSZ of Thermal barrier coatings (TBC) on the magnitude of residual stresses. The (TBC) were deposited by thermal spraying of CoNiCrAlY (Bond Coat-BC) and YSZ (Top Coat-TC) powders. The deposition of the BC was through the high velocity oxygen fuel (HVOF) system. The TC was deposited via an atmospheric plasma-spraying gun (APS). The TBCs were heat treated at 1250 °C, with a CMAS attack at a concentration of 10 mg/cm².The attack exposure was for 2 and 4 hours respectively. In this evaluation, the measured parameter was the magnitude of the residual stress state in Yttria Stabilized Zirconia (YSZ). The residual stress profiles were obtained using the Modified Layer Removal Method for Duplex Coatings (MRCMRB) and the Noda equations. An increase of 26.446 MPa was determined for 2 hours of thermal treatment and 30.743 MPa for 4 hours.

Equimolar YO1.5 and TaO2.5 co‐doped ZrO2 as a potential CMAS‐resistant material for thermal barrier coatings

AbstractCalcium‐magnesium‐alumino‐silicates (CMAS) corrosion in thermal barrier coatings (TBCs) is becoming more serious with increasing operation temperature of turbine engines. Here, we report an equimolar YO1.5 and TaO2.5 co‐doped ZrO2 (Zr0.66Y0.17Ta0.17O2, ZYTO) as a potential CMAS‐resistant material for TBCs, which shows a significantly enhanced CMAS resistance than the conventional 17 mol% YO1.5‐stabilized ZrO2 (17YSZ). After exposure at 1300°C for 100 hours, the CMAS infiltration depth in ZYTO bulk is ~80 μm (for a 20 mg/cm2 CMAS deposition), in contrast to ~700 μm in 17YSZ bulk (50 hours). Compositional and morphological analyses on the CMAS reaction zone reveal that the excellent CMAS resistance of ZYTO originates from the uniform corrosion through grain and grain boundary, along with densification of the reaction layer. The high CMAS infiltration rate of 17YSZ is attributed to the severe dissolution and infiltration through grain boundary. The reaction mechanisms of CMAS with ZYTO and 17YSZ bulks are discussed and a strategy of enhancing the CMAS resistance is proposed for ZrO2‐based TBC materials.

A novel CMAS‐resistant material based on thermodynamic equilibrium design: Apatite‐type Gd10(SiO4)6O3

AbstractCalcium‐magnesium‐alumino‐silicates (CMAS) melt attack has been a critical issue for the thermal barrier coatings (TBCs) with ever‐increasing engine operating temperature. In this study, a novel CMAS‐resistant material apatite‐type Gd10(SiO4)6O3 is developed for TBCs application based on thermodynamic equilibrium design. The chemical reaction of Gd10(SiO4)6O3 bulk and CMAS melt is investigated at 1300°C. The CMAS corrosion resistance of Gd10(SiO4)6O3 bulk is evaluated and compared with the well‐studied CMAS‐resistant material Gd2Zr2O7 (GZO). It is found that Gd10(SiO4)6O3 shows a significantly enhanced CMAS resistance, including lower intrinsic CMAS infiltration rate (~1.09 μm/h1/2) and smaller infiltration upper limit (50‐62 μm) for a 20 mg/cm2 CMAS deposition. More importantly, for Gd10(SiO4)6O3, the CMAS infiltration only alters the composition but does not change the crystal structure or destroy microstructural integrity. The reaction mechanism is elucidated as following two stages: (a) surface Gd10(SiO4)6O3 quickly transforms into Ca2Gd8(SiO4)6O2 in suit by interdiffusion with CMAS melt and then is thermodynamically stable with CMAS melt, thereby effectively inhibiting the further CMAS infiltration and (b) with the ongoing interdiffusion of Gd/Ca, the CMAS‐infiltrated layer slowly thickens and follows a parabolic law. Meanwhile, the CMAS melt gradually precipitates Ca2Gd8(SiO4)6O2 and CaAl2Si2O8 (anorthite) until the melt is exhausted.

Effects of yttria content on the CMAS infiltration resistance of yttria stabilized thermal barrier coatings system

The effects of YO1.5 doping in yttria-zirconia based thermal barrier coatings (TBCs) against CMAS interaction/infiltration are discussed. The TBCs with an YO1.5 content ranging from 43–67 mol.% (balance ZrO2) were produced by electron beam physical vapor deposition (EB-PVD) techniques. The results reveal a trend of higher apatite formation probability with the higher free YO1.5 available in the yttria-zirconia system. Additionally, the infiltration resistance and amount of consumed coating appears to be strongly dependent on the YO1.5 content in the coating. The thinnest reaction layer and lowest infiltration was found for the highest produced 67YO1.5 coating. Complementary XRD experiments with volcanic ash/YO1.5 powder mixtures with higher yttria contents than in the coatings (80YO1.5 and pure YO1.5) also showed higher apatite formation with respect to increasing yttria content. The threshold composition to promote apatite-based reaction products was found to be around 50YO1.5 in zirconia which was proved in the coatings and XRD powder experiments. An YO1.5-ZrO2-FeO-TiO2 bearing zirconolite-type phase was formed as a reaction product for all the coating compositions which implicates that TiO2 in the melt acts as a trigger for zirconolite formation. This phase could be detrimental for CMAS/volcanic ash infiltration resistance since it can be formed alongside with apatite which controls or limits the amount of Y3+ available for glass crystallization. The Fe rich garnet phase containing all the possible elements exhibited a slower nucleation compared to apatite and its growth was enhanced with slow cooling rates. The implications of phase stability and heat treatment effects on the reaction products are discussed for tests performed at 1250 °C.

Laser surface modification of porous yttria stabilized zirconia against CMAS degradation

Here, we present a new combined freeze-casting and laser processing method for the design of yttria-stabilized zirconia (YSZ) based thermal-barrier coatings. YSZ ceramics with unidirectionally-aligned pore channels were created using the freeze-casting method. After sintering, top view and cross-sectional scanning electron microscopy (SEM) revealed the structural features of the preform, which exhibits a 74 ± 2% volume fraction of porosity and an average pore channel size of 30 ± 3 μm. The measured thermal conductivity of this porous structure was 0.27 ± 0.02 W/(m K), which is eight times lower than that of reported values for dense YSZ. Though high porosity is beneficial both from a structural and thermal response perspective, the open porosity could potentially be an issue from an application stand-point when evaluating the resistance of materials to calcium–magnesium–aluminum–silicon oxide (CMAS) attack. CMAS attack, which can originate from deposits of molten sand, ash, and dust, is one of the major causes of thermal barrier coating failure. Therefore, the surface of the porous samples was modified using a laser process to create a barrier to CMAS infiltration. SEM micrographs aided in determining the optimum laser parameters required to fully seal the surface using a laser treatment. The performance of the original porous and surface-modified YSZ was compared by conducting CMAS infiltration studies. Laser modification was shown to be a viable technique to significantly reduce CMAS infiltration in porous thermal barrier coatings.

Preparation of Al2O3 nanowires on 7YSZ thermal barrier coatings against CMAS corrosion

Abstract The commonly-employed material for thermal barrier coatings (TBCs) is 7 wt.%Y2O3–ZrO2 (7YSZ), generally deposited by electron beam-physical vapor deposition (EB-PVD). Due to the increasing demand for higher operating temperature in aero-derivative gas turbines, a lot of effort has been made to prevent the premature failure of columnar 7YSZ TBCs, which is induced by the microstructure degradation, sintering and spallation after the deposition of infiltrated siliceous mineral (consisting of calcium–magnesium–aluminum–silicate (CaO–MgO–Al2O3–SiO2, i.e., CMAS)). A new method called Al-modification for columnar 7YSZ TBCs against CMAS corrosion was present. The Al film was magnetron-sputtered on the surface of the columnar 7YSZ TBCs, followed by performing vacuum heat treatment of the Al-deposited TBCs. During the heat treatment, the molten Al reacted with ZrO2 to form α-Al2O3 overlay that effectively hindered CMAS infiltration. Moreover, the Al film could evaporate and re-nucleate, leading to the generation of Al2O3 nanowires, which further restrained the moving of molten CMAS.

Estimation of CMAS infiltration depth in EB-PVD TBCs: A new constraint model supported with experimental approach

Two standard 7YSZ coatings were deposited by EB-PVD techniques and tested against CMAS infiltration at short time intervals (up to 8 min.) at 1250 °C in air. They exhibited different microstructures, i.e. porosities and microstructural features. Two species of CMAS with different compositions were used and their viscosities were determined using the concentric cylinder method and their contact angles were measured using high temperature heating microscopy. The theoretical viscosities, which were calculated using a statistical model based on the chemical composition of the melts, differed from the measured values of the viscosities by one order of magnitude. A large variation in the contact angles within a very short range of temperature (1243–1266 °C) was observed as well. The porosity and surface area measurements were performed on both EB-PVD microstructures using the nitrogen physisorption method. Additionally, the produced coatings exhibited porosities of 14.5 and 29.5 percent and the infiltration experiments have shown that the more porous coating provides higher infiltration resistance. The effect of porosity on CMAS infiltration kinetics was investigated and the results elucidate that the porosity network plays a more preeminent role than the amount of porosity. The experimental infiltration results have been compared with calculated infiltration data using a novel mathematical approach proposed in previous studies in which the permeability of the coatings is assessed with two contrasting methods termed “concentric pipe” and “open pipe” models. The infiltration was calculated by incorporating the experimentally determined properties such as contact angle, viscosity and porosity. A fitting parameter has been derived from the equations for the geometry factor for both microstructures. The calculated and experimental results are in good agreement with the concentric pipe model supporting the validity of this CMAS infiltration model.

Erosion behavior of EB-PVD 7YSZ coatings under corrosion/erosion regime: Effect of TBC microstructure and the CMAS chemistry

Aero-engines operating in dust-laden environments often encounter a lot of dust/sand that causes a severe problem to the TBCs by means of erosion. As the turbine entry temperatures are rising, molten sand is also a big concern to the life-time of TBCs.This paper deals with the TBC behavior under the combined influence of erosion and corrosion attack. Variations in TBC morphology, CMAS infiltration time and CMAS composition and their influence on the erosion resistance at room temperature were investigated. Two different EB-PVD 7YSZ morphologies consisting of a different porosity arrangement were tested in the erosion/corrosion regime. The more ‘Feathery’ structure has a better resistance to erosion compared to a more columnar ‘Normal’ structure, which leads to less degradation of the TBC. However, under the influence of CMAS infiltration the effect was found to be reversed. In general, CMAS-infiltrated EB-PVD TBCs exhibit a higher erosion resistance than the non-infiltrated ones.

Influence of microstructure on the durability of gadolinium zirconate thermal barrier coatings using APS & SPPS processes

Excellent CMAS resistance in Gadolinium Zirconate, Gd2Zr2O7 (GZO) coatings is attributed to vigorous reactions between the CMAS melt and the GZO that form secondary phases that rapidly crystallize and block further infiltration of CMAS. In this study, Gd2Zr2O7 (GZO) coatings were deposited using air plasma spraying (APS) and solution precursor plasma spraying (SPPS) processes. The relative performance of GZO in furnace thermal cycle tests with and without CMAS was determined. SPPS GZO TBCs showed 8× longer lives in thermal cycling but 10× shorter lives in CMAS testing, as compared to APS GZO TBCs. Superior performance of the SPPS coatings without CMAS can be attributed to characteristic stress relieving vertical cracks in the microstructure, which are absent in APS TBCs. These vertical cracks also act as channels to CMAS melt infiltration, which causes the loss of strain compliance and hence poorer CMAS resistance. X-ray diffraction of the failed SPPS GZO coatings revealed the formation of apatite phases, but the majority of vertical cracks were not sealed. The denser than typical APS coatings on the other hand effectively blocked the CMAS infiltration with CMAS arrest observed at a depth of ~25μm. The SPPS results suggest that the width of vertical cracks is an important parameter for sealing and hence impeding CMAS infiltration Sealing was observed in cracks with <1μm in width. Wider cracks completely defeat the beneficial CMAS blocking behavior of GZO.

Tailoring the EB-PVD columnar microstructure to mitigate the infiltration of CMAS in 7YSZ thermal barrier coatings

The CMAS associated degradation of 7YSZ TBC layers is one of the serious problems in the aero engines that operate in dusty environments. CMAS infiltrates into TBC at high temperatures and stiffens the TBC which ultimately loses its strain tolerance and gets delaminated. The EB-PVD technique is used to coat TBCs exhibiting a columnar microstructure on parts such as blades and on vanes. By varying the EB-PVD process parameters, columnar morphology and porosity of the 7YSZ coating is changed and its effect on the CMAS infiltration behaviour is studied in detail. Two different TBC pore geometries were created and infiltration experiments were carried out at 1250°C and 1225°C for different time intervals. The 7YSZ coating with more ‘feathery’ features has resulted in higher CMAS resistance by at least by a factor of 2 than its less ‘feathery’ counterpart. These results are explained on the basis of a proposed physical model.

Synthesis of yttria by aqueous sol-gel route to develop anti-CMAS coatings for the protection of EBPVD thermal barriers

Anti-CMAS yttria coatings have been prepared by sol-gel routes. Yttria powders with controlled morphology are prepared via auto-combustion of yttrium precursors in a polymerized matrix. The influence of key parameters of the water-based sols is assessed. Indeed, the pH of the initial sol and the temperature of thermal treatment play a major role in the morphology and grain size of yttria powders. To prevent infiltration of CMAS, yttria powders are proposed to be synthesized at pH=1 of the aqueous sol, with drying of the sol and heating at 900°C. After optimization of the synthesis and deposition conditions via sol-gel route, yttria-based coatings with high specific surface area are obtained. They promote the interaction with melt CMAS and consequently limit the degradation of the thermal barrier coatings situated underneath. It was proved that anti-CMAS yttria coating is effective against the infiltration of CMAS at 1250°C for 15min and even 1h.

The Accelerating Effect of CaSO4 Within CMAS (CaO–MgO–Al2O3–SiO2) and Its Effect on the Infiltration Behavior in EB‐PVD 7YSZ

Infiltration and deposition of CaSO4 in thermal barrier coatings (TBC) in addition to the CMAS deposits was found in many occasions on real aviation engines. The source and role of CaSO4 on the degradation of TBC is not well understood. CaSO4 containing CMAS was synthesized and a systematic study of its role on the CMAS infiltration behavior in EB‐PVD 7YSZ is presented in this work. Its influence on the melting and crystallization behavior of CMAS was studied with the help of differential scanning calorimetry. The decomposition of CaSO4 into CaO and SO3 was observed at 1050°C in laboratory air under the presence of CMAS using mass spectroscopy and in situ high‐temperature XRD. The same amount of CaO is brought into the CMAS system by means of adding CaCO3, which will eventually decompose into CaO and CO2 at 700°C. CMAS infiltration tests were carried out at different temperatures with and without CaSO4/CaCO3 and the results demonstrate that the sulfur has no direct effect on the aggressiveness of the anhydrite containing CMAS with regard to its infiltration behavior in EB‐PVD 7YSZ at high temperatures. The extra amount of calcia added to CMAS that is introduced by the evaporating species is responsible for enhanced infiltration of the deposits into the TBC.

Plasma-Sprayed Thermal Barrier Coatings with Enhanced Splat Bonding for CMAS and Corrosion Protection

The infiltration of molten CMAS in thermal barrier coatings (TBCs) at high temperature is significantly affected by the microstructure of the ceramic coating. Enhancing the bonding ratio between splats can reduce the interconnected pores and suppress the infiltration of the molten CMAS into the coating. In this study, a dual-layered (DL) TBC with the dense 8YSZ on the top of the conventional porous 8YSZ was proposed to enhance CMAS corrosion of atmospheric plasma-sprayed YSZ. The dense YSZ coating with improved lamellar bonding was deposited at a higher deposition temperature. The microstructure of the coatings before and after CMAS attack test was characterized by scanning electron microscopy. It was clearly revealed that by adjusting the microstructure and applying a dense ceramic layer with the improved interface bonding on the top of porous TBC, the infiltration of CMAS into porous YSZ coating can be effectively suppressed. Moreover, by designing DL TBCs, the thermal conductivity of the TBC system exhibits a limited increase. Thus with the design of DL structure, the TBCs with high CMAS corrosion resistance and low thermal conductivity can be achieved.

OS1201-428 遮熱コーティングの機械的特性および皮膜はく離に及ぼす無機質燃焼生成物の侵入の影響

OS1201-428 遮熱コーティングの機械的特性および皮膜はく離に及ぼす無機質燃焼生成物の侵入の影響

Stability and CMAS Resistance of Ytterbium‐Silicate/Hafnate EBCs/TBC for SiC Composites

Multilayer ytterbium‐hafnate/silicate coatings deposited by directed vapor deposition and designed to protect SiC‐based ceramic matrix composites were assessed to determine their thermochemical stability and resistance to attack by molten silicate deposits (CMAS). The study revealed that reactions occurring at the interface between Yb2Si2O7 and Yb4Hf3O12 layers promote coating delamination following isothermal annealing for 100 h/1500°C while coating architectures involving Yb2SiO5 in contact with Yb4Hf3O12 do not experience similar degradation. The outer Yb4Hf3O12 layers, segmented for compliance, were only moderately effective in mitigating CMAS infiltration at 1300°C and 1500°C. The results indicate that the reaction between the melt and coating forms large volumes of a silicate garnet phase at 1300°C, or a cuspidine‐type aluminosilicate at 1500°C, in addition to the apatite and reprecipitated fluorite phases observed in related systems.

Effect of CMAS Deposits on MOCVD Coatings in the System Y2O3–ZrO2: Phase Relationships

Three metal‐organic chemical vapor deposition (MOCVD) coatings (ZrO2, Y2O3·ZrO2, Y2O3) were studied in contact with model CMAS to investigate the microstructural evolution and phase formation. The MOCVD coatings were covered with CMAS powder deposits and annealed for 1 h at 1250°C, respectively. The ZrO2 coating was completely infiltrated by CMAS, whereas the yttria containing coatings show a higher resistance against CMAS infiltration. This is explained by the formation of a continuous oxyapatite layer in the reaction zone of the coating and the CMAS deposit. The Y2O3·ZrO2 coating shows the best infiltration resistance despite the fact that the Y2O3–CMAS sample is the only completely crystallized. As crystallization products, oxyapatite, melilite, anorthite as well as new garnets bearing all available cations were formed. The garnet phase was confirmed by XRD and TEM. EDS measurements were used to calculate structural formula A3B2T3O12 (A = Ca, Y, Zr; B = Mg, Al; T = Al, Si).

Electrophoretically deposited alumina as protective overlay for thermal barrier coatings against CMAS degradation

TBCs are increasingly susceptible to degradation by airborne CMAS deposits. In order to mitigate the CMAS attack, we fabricated a dense, crack-free alumina overlay for TBCs by electrophoretic deposition (EPD) technique. Overlay coatings of controlled thickness were successfully fabricated for YSZ TBCs, by EPD followed by controlled sintering at 1200 °C. YSZ with alumina overlay coatings were tested for CMAS attack at 1300 °C. Detailed examination of microstructural changes and phase evolution in CMAS tested specimens was performed by X-ray diffraction and electron microscopy. Dense alumina overlay produced by EPD was found to physically suppress the infiltration of CMAS. Furthermore, CMAS was found to crystallize into anorthite (CaAl 2Si 2O 8) and MgAl 2O 4 spinel by chemically interacting with EPD α-Al 2O 3. A shift in CMAS glass composition to a crystallizable Al-rich glass composition promoted the formation of anorthite platelets (CaAl 2Si 2O 8) and localized enrichment of Mg promoted the formation of MgAl 2O 4 spinel. EPD alumina overlay on commercial-production TBCs retained its adhesion and structural integrity after 20 cycles of 1-hour furnace thermal cycle test at 1100 °C.

Infiltration‐Inhibiting Reaction of Gadolinium Zirconate Thermal Barrier Coatings with CMAS Melts

The thermochemical interaction between a Gd2Zr2O7 thermal barrier coating synthesized by electron‐beam physical vapor deposition and a model 33CaO–9MgO–13AlO3/2–45SiO2 (CMAS) melt with a melting point of ∼1240°C was investigated. A dense, fine‐grained, ∼6‐μm thick reaction layer formed after 4 h of isothermal exposure to 1300°C. It consisted primarily of an apatite phase based on Gd8Ca2(SiO4)6O2 and fluorite ZrO2 with Gd and Ca in a solid solution. Remarkably, melt infiltration into the intercolumnar gaps was largely suppressed, with penetration rarely exceeding ∼30 μm below the original surface. The microstructural evidence suggests a mechanism in which CMAS infiltration is arrested by rapid filling of the gaps with crystalline reaction products, followed by slow attack of the column tips.