Calcium-magnesium-alumina-silicate Research Articles

Reactivity between rare‐earth oxides based thermal barrier coatings and a silicate melt

AbstractThe reactivity between rare‐earth (RE‐) oxide stabilized ZrO2 or HfO2 thermal barrier coatings (TBCs) and a calcium‐magnesium‐aluminum‐silicate (CMAS) melt was studied at 1310°C. These reactions are representative of the ingestion of siliceous materials by the intake air of gas turbines (e.g., in aircraft engines) at high temperatures (>1200°C). These materials can melt and react with coated components in the hot section, resulting in premature failure. The goal of this work was to probe the effect of various RE (RE = Y, Yb, Dy, Gd, Nd, and Sm) oxides in the melt phase equilibrium and stability of the top‐coating system. Thermodynamic calculations of the phase assemblage of the (1−x) ZrO2‐xY2O3 coating materials and CMAS melt are compared with the experimental findings. CMAS was found to penetrate the samples at the grain boundaries and dissolve the coating materials to form silicate phases containing the RE elements. Furthermore, apatite and garnet crystalline phases formed in the samples with total RE‐oxide content higher than 16 mol% in the reaction zone for the ZrO2 system. In general, samples with nominal compositions ZrO2‐9Dy2O3, HfO2‐7Dy2O3, ZrO2‐8Y2O3, HfO2‐6Er2O3, ZrO2‐9.5Y2O3‐2.25Gd2O3‐2.25Yb2O3, and ZrO2‐30Y2O3 exhibited lower reactivity, or more resistance, to CMAS than the other coating compositions.

Hot corrosion behavior of BaLa2Ti3O10 exposed to calcium-magnesium-alumina-silicate at elevated temperatures

Calcium-magnesium-alumina-silicate (CMAS) attack has been regarded as one of the significant failure mechanisms for thermal barrier coatings (TBCs). In this study, CMAS corrosion behavior of BaLa2Ti3O10, a novel TBC material, is investigated at 1300 °C and 1350 °C for 0.5 h, 4 h, 12 h and 24 h. Results reveal that BaLa2Ti3O10 has high resistance to molten CMAS infiltration, attributable to the formation of a dense reaction layer. X-ray diffraction, scanning electron microscope, energy dispersive spectroscope, transmission electron microscope confirm that the layer consists of apatite, celsian and perovskite phases. With increased corrosion duration, the layer retains good phase stability and the thickness increases. The formation of corrosion products and the reaction layer are discussed according to a dissolution-reprecipitation mechanism and the optical basicity theory.

Molten Particulate Impact on Tailored Thermal Barrier Coatings for Gas Turbine Engine

Commercial/military fixed-wing aircraft and rotorcraft engines often have to operate in significantly degraded environments consisting of sand, dust, ash, and other particulates. Marine gas turbine engines are subjected to salt spray, while the coal-burning industrial power generation turbines are subjected to fly ash. The presence of solid particles in the working fluid medium has an adverse effect on the durability of these engines as well as performance. Typical turbine blade damages include blade coating wear, sand glazing, calcia–magnesia–alumina–silicate (CMAS) attack, oxidation, and plugged cooling holes, all of which can cause rapid performance deterioration including loss of aircraft. This research represents the complex thermochemomechanical fluid structure interaction problem of semimolten particulate impingement and infiltration onto ceramic thermal barrier coatings (TBCs) into its canonical forms. The objective of this research work is to understand the underpinning interface science of interspersed graded ceramic/metal and ceramic/ceramic composites at the grain structure level for robust coatings and bulk material components for vehicle propulsion systems. This research enhances our understanding of the fundamental relationship between interface properties and the thermomechanical behavior in dissimilar materials for materials by design systems, and creates the ability to develop and fabricate materials with targeted macroscale properties as a function of their interfacial behavior. This project creates a framework to enable the engineered design of solid–solid and liquid–solid interfaces in dissimilar functionalized materials to establish a paradigm shift toward science from the traditional empiricism in engineering TBCs and high temperature highly loaded bulk materials. An integrated approach of modeling and simulation, characterization, fabrication, and validation to solve the fundamental questions of interface mechanisms which affect the properties of novel materials will be validated to guide component material solutions to visionary 2040+ military vehicle propulsion systems.

CMAS (CaO–MgO–Al2O3–SiO2) resistance of Y2O3-stabilized ZrO2 thermal barrier coatings with Pt layers

Calcium–magnesium–alumina–silicate (CMAS) corrosion significantly affects the durability of thermal barrier coatings (TBCs). In this study, Y2O3 partially stabilized ZrO2 (YSZ) TBCs are produced by electron beam-physical vapor deposition, followed by deposition of a Pt layer on the coating surfaces to improve the CMAS resistance. After exposure to 1250°C for 2h, the YSZ TBCs were severely attacked by molten CMAS, whereas the Pt-covered coatings exhibited improved CMAS resistance. However, the Pt layers seemed to be easily destroyed by the molten CMAS. With increased heat duration, the Pt layers became thinner. After CMAS attack at 1250°C for 8h, only a small amount of Pt remained on the coating surfaces, leading to accelerated degradation of the coatings. To fully exploit the protectiveness of the Pt layers against CMAS attack, it is necessary to improve the thermal compatibility between the Pt layers and molten CMAS.

Interaction and infiltration behavior of Eyjafjallajökull, Sakurajima volcanic ashes and a synthetic CMAS containing FeO with/in EB-PVD ZrO2-65 wt% Y2 O3 coating at high temperature

Yttria rich-zirconia thermal barrier coatings (TBCs) with a nominal composition of 65 wt % Y2O3 balanced ZrO2 were deposited by electron-beam physical vapor deposition (EB-PVD) and tested for calcium-magnesium-aluminum-silicate (CMAS) infiltration resistance. The infiltration studies were performed with a set of one synthetized CMAS composition and two real volcanic ashes from the Eyjafjallajökull volcano located in Iceland and the Sakurajima volcano located in southern Japan. The coatings were tested at 1250 °C for short term (5 min) and long term (intervals from 1 to 20 h). The results indicate a significantly different reaction process for the synthesized CMAS compared with the natural volcanic ashes. The yttria-rich zirconia coatings demonstrate promising results against infiltration by vigorously reacting against the molten glass inducing its crystallization by forming apatite and garnet phases. The formed reaction products effectively sealed the columnar gaps of the TBC and generated a uniform reaction layer that prevented further infiltration.

Air Plasma-Sprayed La2Zr2O7-SrZrO3 Composite Thermal Barrier Coating Subjected to CaO-MgO-Al2O3-SiO2 (CMAS)

La2Zr2O7-SrZrO3 composite thermal barrier coatings (TBCs) were prepared by air plasma spray (APS). The La2Zr2O7-SrZrO3 composite TBCs covered with calcium-magnesium-aluminum-silicate (CMAS) powder, as well as the powder mixture of CMAS and spray-dried La2Zr2O7-SrZrO3 composite powder, were heat-treated at 1250 °C in air for 1, 4, 8, and 12 h. The phase constituents and microstructures of the reaction products were characterized by x-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. Experimental results showed that the La2Zr2O7-SrZrO3 composite TBCs had higher CMAS resistance than 8YSZ coating. A dense new layer developed between CMAS and La2Zr2O7-SrZrO3 composite TBCs during interaction, and this new layer consisted mostly of apatite (Ca2La8(SiO4)6O2) and c-ZrO2. The newly developed layer effectively protected the La2Zr2O7-SrZrO3 composite TBCs from further CMAS attack.

Calcium-magnesium-alumina-silicate (CMAS) resistant Ba2REAlO5 (RE = Yb, Er, Dy) ceramics for thermal barrier coatings

Calcium-magnesium-alumina-silicate (CMAS) attack has been a great challenge for the application of thermal barrier coatings (TBCs) in modern turbine engines. In this study, a series of prospective TBC candidate materials, Ba2REAlO5 (RE=Yb, Er, Dy), are found to have high resistance to CMAS attack. The rapid formation of a continuous crystalline layer on sample surface contributes to this desirable attribute. At 1250°C, Ba2REAlO5 dissolve in the molten CMAS, accumulating Ba, RE and Al in the melt, which could trigger the crystallization of celsian, apatite and wollastonite crystals. Especially, the formation of the crystalline layer in the Ba2DyAlO5 sample is the fastest. This study also reveals that Ba is a useful element for altering CMAS composition to precipitate celsian. Thus, doping Ba2+ in yttria partially stabilized zirconia or other novel TBCs might be an attractive way of mitigating CMAS attack.

Calcium-magnesium-alumina-silicate (CMAS) resistance property of BaLn2Ti3O10 (Ln=La, Nd) for thermal barrier coating applications

Abstract Calcium-magnesium-alumina-silicate (CMAS) has posed enormous threat to thermal barrier coatings (TBCs). In this study, a series of newly developed TBC ceramics, BaLn 2 Ti 3 O 10 (Ln=La, Nd), are found to have high resistance to the penetration of molten CMAS at 1250 °C. The formation of a continuous, dense crystalline layer, mainly composed of apatite and CaTiO 3 phases, on the sample surfaces contributed to this desirable attribute. The accumulation of Ba in the molten CMAS triggered the crystallization of the melt, leading to the formation of many BaAl 2 Si 2 O 8 celsian crystals above the crystalline layer, which could reduce the mobility of the molten CMAS. The mechanisms by which the CMAS attacks BaLn 2 Ti 3 O 10 samples are discussed. The results indicate that Ba is an effective element for altering CMAS composition, and doping Ba in TBCs might be an attractive way of mitigating CMAS attack.

Plasma sprayed nanostructured GdPO4 thermal barrier coatings: Preparation microstructure and CMAS corrosion resistance

AbstractNanostructured GdPO4 thermal barrier coatings (TBCs) were prepared by air plasma spraying, and their phase structure evolution and microstructure variation due to calcium–magnesium–alumina–silicate (CMAS) attack have been investigated. The chemical composition of the coating is close to that of the agglomerated particles used for thermal spraying. Nanozones with porous structure are embedded in the coating microstructure, with a percentage of ~30%. CMAS corrosion tests indicated that nanostructured GdPO4 coating is highly resistant to penetration by molten CMAS at 1250°C. Within 1 hour heat treatment duration, a continuous dense reaction layer forms on the coating surface, which are composed of P–Si apatite based on Ca2+xGd8−x(PO4)x(SiO4)6−xO2, anorthite and spinel phases. This layer provides effective prevention against CMAS further infiltration into the coating. Prolonged heat treatment densifies the reaction layer but does not change its phase composition.

Investigation of the resistance of open-column-structured PS-PVD TBCs to erosive and high-temperature corrosive attack

In modern gas turbines, highly loaded components are internally cooled and furthermore covered with thermal barrier coatings (TBCs) to withstand the harsh operating conditions with temperatures exceeding the application limit of such coatings. Under realistic operating conditions, siliceous minerals, of a calcium-magnesium-aluminum-silicate (CMAS) composition, are ingested into the turbine and deposited on the TBCs. Besides erosion, this also leads to degradation by chemical interaction.The plasma spray-physical vapor deposition (PS-PVD) process is an advanced method for manufacturing TBCs, which fills the gap between traditional thermal spray processes and electron beam physical vapor deposition (EB-PVD). Due to the unique plasma conditions, coatings with columnar microstructures exhibiting high strain tolerance can be created. However, because of the high amount of open porosity the resistance of such structures to CMAS and erosion attack was expected to be low.In the present work, PS-PVD TBCs were investigated in a burner rig facility under thermal gradient cycling conditions and simultaneous CMAS attack. The interactions of the PS-PVD-deposited YSZ and the CMAS melt were studied by means of scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS) and compared to EB-PVD coatings. Additionally, the resistance of PS-PVD TBCs to erosion is compared to APS TBCs by means of room temperature tests according to ASTM G76-13.

Effects of CMAS penetration on the delamination cracks in EB-PVD thermal barrier coatings with curved interface

Abstract During the high-temperature operation of electron beam physical vapor deposited (EB-PVD) thermal barrier coating (TBC), the penetration of environmental calcium-magnesium-alumina-silicate (CMAS) compositions into the ceramic top-coat would affect the growth of delamination cracks. In this work, the effects of CMAS penetration on the delamination cracks in EB-PVD TBC with curved interface are investigated by finite element analysis. In the numerical model, the curved interface evolves as the cyclic displacement instability of the thermally grown oxide (TGO) layer. The penetration of CMAS into the columnar gaps of EB-PVD TBC mainly increases the in-plane modulus of TC layer. It is demonstrated that, with the increase of in-plane modulus in an intact TC, the level of tensile stress, which mainly occurs in the region above the curved interface and responsible for initiating the delamination cracks, presents a decrease; meanwhile, the level of shear stress, which mainly occurs in the region at the periphery of the curved zone to drive the delamination crack when it propagates into this region, presents a increase. Furthermore, the calculation of the strain energy release rate shows that, for the crack located above the curved interface, the increase of in-plane modulus in TC layer can prevent the accumulation of strain energy release rate, and therefore make it more difficult for delamination initiation. However, once the crack propagates into the flat periphery, CMAS penetration would begin to enhance its growth.

Preparation of nanostructured Gd2Zr2O7-LaPO4 thermal barrier coatings and their calcium-magnesium-alumina-silicate (CMAS) resistance

Nanostructured 30mol% LaPO4 doped Gd2Zr2O7 (Gd2Zr2O7-LaPO4) thermal barrier coatings (TBCs) were produced by air plasma spraying (APS). The coatings consist of Gd2Zr2O7 and LaPO4 phases, with desirable chemical composition and obvious nanozones embedded in the coating microstructure. Calcium-magnesium-alumina- silicate (CMAS) corrosion tests were carried out at 1250°C for 1–8h to study the corrosion resistance of the coatings. Results indicated that the nanostructured Gd2Zr2O7-LaPO4 TBCs reveals high resistance to penetration by the CMAS melt. During corrosion tests, an impervious crystalline reaction layer consisting of Gd-La-P apatite, anorthite, spinel and tetragonal ZrO2 phases forms on the coating surfaces. The layer is stable at high temperatures and has significant effect on preventing further infiltration of the molten CMAS into the coatings. Furthermore, the porous nanozones could gather the penetrated molten CMAS like as an absorbent, which benefits the CMAS resistance of the coatings.

Failure mechanisms of calcium magnesium aluminum silicate affected thermal barrier coatings

AbstractDepth profiles of the phase composition of two examples of calcium magnesium aluminum silicate (CMAS) affected thermal barrier coatings (TBCs) from an aero gas turbine engine were obtained using a monochromatic and collimated beam of synchrotron radiation. One TBC was deposited by plasma spray and the other by electron beam physical vapor deposition. These examples were complemented with an X‐ray diffraction (XRD) study of mixtures of TBC zirconia powder and sand heated in a furnace. The XRD results were compared with electron backscatter images and energy dispersive spectroscopy studies of the cross sections and mixtures. It was found that when liquid, the CMAS enhances mass transport leading to the densification of the zirconia, which then leads to spalling because of the increased residual stresses generated on cooling. Even without spalling densification will reduce a TBC's ability to thermally insulate. The enhanced mass transport can also lead to destabilization of the zirconia if yttrium ions preferentially transfer to the liquid or greater stabilization if calcium or magnesium ions transfer from the liquid to the zirconia. Zircon also precipitates when the zirconium from the TBC reacts with the silicon in the liquid CMAS.

Effect of Particle Size on the Micro-cracking of Plasma-Sprayed YSZ Coatings During Thermal Cycle Testing

The failure of plasma-sprayed thermal barrier coatings (TBCs) during service or thermal cycle testing usually results from internal cracking in the top coat, erosion and CMAS (calcium-magnesium-alumina-silicate)-induced damage, etc. The microstructure of ceramic coatings affects their durability and other properties of TBCs. In the present study, yttria-stabilized zirconia (YSZ) coatings were deposited by atmospheric plasma spraying (APS) using feedstocks with different particle sizes. In addition, the effect of particle size on damage evolution in the top coat was investigated. It is found that the coatings deposited using coarse particles show the higher thermal cycle life. Crack length grew with increasing numbers of thermal cycles. The faster crack growth rate can be found for the coatings deposited from fine particles. The porosity of the coating made from the coarse powder is larger than the porosity of the coating made from fine powder both in the as-sprayed condition and after thermal cycling. The changes in crack growth rate and the porosity are related to the effect of sintering and stress evolution in coatings during the thermal cyclic tests.

Calcium-magnesium-alumina-silicate (CMAS) resistance characteristics of LnPO4 (Ln = Nd, Sm, Gd) thermal barrier oxides

Calcium-magnesium-alumina-silicate (CMAS) attack has been considered as a significant failure mechanism for thermal barrier coatings (TBCs). As a promising series of TBC candidates, rare-earth phosphates have attracted increasing attention. This work evaluated the resistance characteristics of LnPO4 (Ln=Nd, Sm, Gd) compounds to CMAS attack at 1250°C. Due to the chemical reaction between molten CMAS and LnPO4, a dense, crack-free reaction layer, mainly composed of Ca3Ln7(PO4)(SiO4)5O2 apatite, CaAl2Si2O8 and MgAl2O4, was formed on the surface of compounds, which had positive effect on suppressing CMAS infiltration. The depth of CMAS penetration in LnPO4 (Ln=Nd, Sm, Gd) decreased in the sequence of NdPO4, SmPO4 and GdPO4. GdPO4 had the best resistance characteristics to CMAS attack among the three compounds. The related mechanism was discussed based on the formation ability of apatite phase caused by the reaction between molten CMAS and LnPO4.

Deposition and properties of a multilayered thermal barrier coating

Abstract The widely used air plasma sprayed thermal barrier coatings (TBCs) in industrial gas turbine engines are limited by its durability. The failure of plasma sprayed TBCs in thermal cycling is usually induced by the spallation of ceramic coating within the ceramic coating near the top coat/bond coat interface. In this research, a multilayered ceramic coating architecture is introduced comprised of the layers of segmentation cracks, porous and dense. The segmented yttria stabilized zirconia (YSZ) layer was deposited at the substrate temperature of 750 °C without any artificial cooling, which resulted in the formation of YSZ layer with obvious columnar crystal structure and segmentation cracks. And a dense top YSZ layer was also included on the top to restrain the penetration of oxygen and the molten calcium–magnesium–alumina-silicate (CMAS) glass deposits. Additionally, the conventional porous YSZ coating was preserved in order to get the low thermal conductivity. The thermal cycling tests show that the lifetime of the multilayered TBCs increases by a factor of 2 compared with that of the conventional one, which could attribute to the high strain tolerance and fracture toughness of the segmented and columnar microstructure. Oxidation resistance and CMAS corrosion resistance are also better for the multilayered TBCs compared with that of the conventional one, due to the suppression function of dense top YSZ layer. It is potential to obtain the high performance TBCs by adjusting plasma spraying.

The degradation of thermal barrier coatings by molten deposits: introducing the concept of basicity

Airborne particles such as dust, sand and volcanic ash, otherwise termed ‘CMAS’ (Calcia Magnesia Alumina-Silicate) are ingested into the engine where at the elevated temperatures this deposit can melt and induce degradation of the thermal barrier coating (TBC). The columnar, high porosity microstructure of electron beam physical vapour deposited (EB-PVD) TBCs, typically found on HP turbine blades, makes them particularly susceptible to CMAS molten deposit attack. This paper investigates the effect of modifying the chemical composition of molten CMAS deposits and, therefore, the degradation mechanisms of EB-PVD TBCs. This paper identifies a parameter termed ‘Basicity Index’, which modifies the severity and damage caused by these molten deposits to TBCs. It is believed that with the higher temperatures likely to result in next generation aero-engines and with sand chemistry varying around the world, basicity driven TBC degradation will become more prevalent.

Numerical analysis of CMAS penetration induced interfacial delamination of transversely isotropic ceramic coat in thermal barrier coating system

During the high-temperature operation of the electron beam physical vapor deposited (EB-PVD) thermal barrier coating system (TBCs), the environmental calcium–magnesium–alumina–silicate (CMAS) deposition would penetrate into the gaps between the columnar microstructure of top-coat. This penetration affects the mechanical behavior of coating and then induces the interfacial delamination of the coating from the substrate. In this paper, a physical model incorporating the CMAS penetration and the columnar microstructure of EB-PVD ceramic top-coat (Y2O3-Stabilized ZrO2, YSZ) is developed. Firstly, continuum analysis is carried out to investigate CMAS induced interfacial delamination in TBCs, in which the ceramic top-coat is simplified as a transversely isotropic material. Theoretical analysis indicates that CMAS penetration primarily affects the overall in-plane modulus of the top-coat. It is also shown that the in-plane modulus of the top-coat is the most critical parameter determining the energy release rate and mode mixity. Moreover, the effect of columnar microstructure of top-coat on the interfacial delamination is analyzed. The results demonstrate that the increase of both CMAS modulus and penetration thickness can greatly promote the energy release rate for driving the interfacial delamination. The mechanism of CMAS induced interfacial delamination is that the infiltration of CMAS into columnar gaps increases the overall effective in-plane modulus of the top-coat, thereby facilitating delamination of the coat.

Protectiveness of Pt and Gd2Zr2O7 layers on EB-PVD YSZ thermal barrier coatings against calcium–magnesium–alumina–silicate (CMAS) attack

Thermal barrier coatings (TBCs) are susceptible to degradation caused by environmental deposits mainly composed of calcium–magnesium–aluminum–silicon (CMAS). In this work, the degradation of electron beam physical vapour deposition (EB-PVD) yttria stabilized zirconia (YSZ) TBC due to CMAS attack was investigated. To protect the YSZ coating, a Pt film and a Gd2Zr2O7 (GZO) layer were deposited onto the YSZ coating surface by electroplating and EB-PVD, respectively. The coatings were heat treated at 1250°C for 4h with CMAS deposits, and their microstructure and chemical composition after CMAS attack were investigated. Electroplating a dense and defect-free Pt film on YSZ coating could provide effective protection from CMAS attack, but EB-PVD GZO coating was unable to prevent molten CMAS infiltration, mainly due to the large inter-columnar gaps. The associated mechanisms were discussed in detail.

Mechanical and cementitious characteristics of ground granulated blast furnace slag and basic oxygen furnace slag blended mortar

Reusing waste materials and reducing carbon emissions are crucial environmental concerns. Ground granulated basic oxygen furnace slag (GGBOS) and ground granulated blast furnace slag (GGBS) are the by-products of the steel industry and has positive effects on the environment because it reduces the problems associated waste disposal. This study reused these two products to completely replace cementitious materials, thus contributing to waste recycling, reducing the production demand for cement, and mitigating carbon emissions. Twelve mixture proportions were designed in this study, including an ordinary Portland mortar (OPM) as the control group and 11 steel/iron slag blended mortar (SISBM) experimental groups for the mechanical and cementitious characteristic experiments. The optimal mixing ratio for SISBM compressive strength ranged from GGBOS (steel slag): GGBS (iron slag)=3:7 to 5:5 (by weight). At the age of 91days, the compressive strength of SISBM reached 80–90% compared with that of the control group. Based on the pH values, free-CaO, and microanalysis results, the cementitious characteristics were mainly generated because the GGBOS increased the free-CaO or Ca(OH)2 concentrations in the SISBM curing water and provided alkaline environments for Ca(OH)2 to engage in the pozzolanic reaction with the SiO2 and Al2O3 in GGBS, forming crystals such as calcium aluminum silicate hydrate, (C–A–S–H), calcium silicate hydrate (C–S–H), and calcium–magnesium–alumina–silicate (C–M–A–S), which generated strength and strengthened microstructure.