Oxide Thermal Barrier Research Articles

Phase stability and initial phase high-temperature corrosion behavior of non-stoichiometric lanthanum cerium oxide thermal barrier coatings

Three different compositions of lanthanum cerium oxide thermal barrier coatings with varied cerium oxide (1.8CeO2, 1.9CeO2 and 2.0CeO2) compositions were synthesized and plasma sprayed to achieve free-standing coatings. The phase stability studies showed that the coatings are stable at 1400 °C after 100 h. The high-temperature corrosion was performed in the presence of sulfate-vanadate (50 wt%Na2SO4+50 wt%V2O5) at 1000 °C for 4 h. The result depicts LaVO4 as the major corrosive product formed in all three compositions along with CeVO4, LaCeVO4 and CeO2. The hot corrosion test result gives information that the composition of CeO2 greatly influences the hot corrosion resistance. The increase in CeO2 percentage increases the resistance against the infiltration of molten sulfate-vanadate compounds. This investigation provides a feasible way to understand the coating composition for better hot corrosion resistance in sulfate-vanadate mixtures.

Химический состав, структура и микротвердость многослойных высокотемпературных покрытий

Introduction. Plasma spraying is one of the modern and effective methods for coatings application for various purposes and compositions. With the help of thermal plasma flows, it is possible to spray almost any powder materials (ceramic, metal, metal-ceramic). Plasma spraying of multilayer protective coatings can be successfully used to increase the durability of pierced mandrels, which are the main tool in the production of hollow billets. The purpose of this work is to study the chemical composition, structure, and microhardness of multilayer high-temperature coatings of two different compositions deposited by plasma spraying, which are supposed to be used to increase the durability of pierced mandrels. Materials and research methods. The deposition of multilayer coatings of two compositions was carried out on a plasma-powder spraying unit with contact excitation of an arc discharge UPN-60KM TSP2017. Coatings were obtained by sequential deposition of three layers with different powder compositions. After deposition of all three coating layers, oxidative annealing was carried out at a temperature of 900°C to form a dense scale layer of FeO + Fe2O3 + Fe3O4 on the surface. The chemical composition of the coatings was studied by X-ray microanalysis using a TESCAN scanning electron microscope with an OXFORD attachment. The microstructure of the coatings was studied using a NEOPHOT metallographic microscope. Phase X-ray diffraction analysis was performed on a SHIMADZU diffractometer in Kα chromium radiation. Microhardness was measured on a LEICA hardness tester at a load of 50 g. Results and discussion. The nature of the distribution of chemical elements over the thickness of the coating, consisting of four layers, is established: an inner metal layer that provides protection against high-temperature corrosion; a transitional metal layer designed to equalize the thermal properties between the layers; α-Fe metal oxide layer and iron oxides and external thermal barrier oxide layer FeO + Fe2O3 + Fe3O4. Coatings are characterized by a non-uniform distribution of structural components and microhardness over its thickness. The microhardness of the inner layer reaches 1,400 HV0.05, the transition layer is 800 HV0.05, and the metal oxide layer is 300 HV0.05.

Investigation on bi-layer coating with La2(Ti0.2Zr0.2Sn0.2Ce0.2Hf0.2)2O7/YSZ prepared by laser cladding

Thermal barrier coatings are an effective technology for improving the high-temperature performance of hot section components in gas turbine engine. Due to their excellent properties, high-entropy oxides are considered to be promising materials for thermal barrier coatings. Laser cladding is a coating preparation technology and the top coat prepared by laser cladding technology has an important application value for thermal barrier coatings. In this work, to improve the thermal cycling behavior of the La2(Ti0.2Zr0.2Sn0.2Ce0.2Hf0.2)2O7 high-entropy oxide coating, a bi-layer coating with the La2(Ti0.2Zr0.2Sn0.2Ce0.2Hf0.2)2O7 high-entropy oxide layer and the YSZ layer was designed and fabricated by laser cladding on the NiCoCrAlY alloy surface. The microstructure, phase and mechanical properties of the coating were analyzed by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and micro-hardness and nanoindentation tests, respectively. The results show that a bi-layer La2(Ti0.2Zr0.2Sn0.2Ce0.2Hf0.2)2O7/YSZ coating was successfully prepared by the laser cladding method, and shows good bonding at the interface between the layers. The high-entropy oxide layer maintains a relatively stable defective fluorite structure and its microstructure exists in the stable cellular and dendrite crystalline state after laser cladding. The high-entropy oxide layer prepared by laser cladding showed an average elastic modulus of 167 GPa and an average hardness of 1022.8HV in nanoindentation tests. Thermal cycling of the coating was carried out at 1050 °C. Failure of the bi-layer coating occurred after 60 thermal cycles at 1050 °C. Thermal stresses between different layers are calculated during thermal cycling. Due to its excellent mechanical properties, the bi-layer coating with the La2(Ti0.2Zr0.2Sn0.2Ce0.2Hf0.2)2O7 high-entropy oxide and YSZ layers is expected to become an effective high-entropy oxide thermal barrier coating.

Grain boundary corrosion mechanism of YSZ thermal barrier oxides under CMAS attack

Grain boundary (GB) corrosion of yttria-stabilized zirconia (YSZ) under calcium-magnesium-alumino-silicates (CMAS) attack was investigated. Dense YSZ ceramics with YO1.5 contents varying from 8 to 57 mol.% were prepared, and exposed to CMAS at 1300 ºC. The results show preferential GB corrosion occurs for all YSZ ceramics, and the extent of corrosion significantly depends on the YO1.5 concentration. For low YO1.5 YSZ (e.g. 8YSZ and 20YSZ), GB is rapidly dissolved and infiltrated by CMAS melt. In contrast, for YSZ with high YO1.5 contents (e.g. Y2Zr2O7 and Y4Zr3O12), a uniform corrosion through the GB and grains is observed. The reason of the preferential GB corrosion is attributed to segregation of Y element in YSZ, which promotes the CMAS reactivity with the GBs. Furthermore, the mechanism of extent difference of GB corrosion in YSZ materials is discussed. It is proposed that the GB corrosion of YSZ can be mitigated by decreasing the Y segregation, increasing the relative volume of reaction products, and/or promoting the precipitation kinetic of products.

Protocol for selecting exemplary silicate deposit compositions for evaluating thermal and environmental barrier coatings

AbstractA protocol for selecting representative silicate compositions for comparative testing of gas turbine coating materials is presented. It begins with a curated dataset of compositions of engine deposits and naturally occurring siliceous debris including volcanic ashes, sands, and dusts. The compositions are first reduced to the five major oxides—those of Ca, Mg, Fe, Al, and Si—and then distilled further using principal component analysis and k‐means clustering. The process ultimately yields four classes of possible deposits with common chemical characteristics. Each class is represented by a composition centroid and a range in Ca:Si ratios. Key thermophysical properties of the possible deposits are calculated and related to the glass network connectivity, characterized by the Si:O ratio. Finally, deposits from each of these classes are compared in terms of their reactions with prototypical thermal and environmental barrier oxides, with due consideration of the effects of composition variations within each deposit class. The protocol is, in principle, adaptable to datasets compiled by OEMs and researchers in gas turbine coatings.

Experimental Study on Yttria Stabilized and Titanium Oxide Thermal Barrier Coated Piston Effect on Engine Performance and Emission Characteristics

Most of the energy was being lost through the cooling system and exhaust gas, to utilize that energy and convert it to a useful job thermal barrier coatings is widely used. Tests were conducted on a four stroke, single cylinder diesel engine for which its piston crown was coated with a thickness of 100/100 microns YSZ/TiO2 over 100 microns NiCr bond coat with plasma spray coating technology and then the results were compared with the uncoated piston. Thermal barrier coating was used for better performance, emission and combustion characteristics. The tests were performed at different load conditions using both the pistons and compared the results. At maximum load there is a rise in BTE and reduction in BSFC, CO, HC emissions compared to uncoated piston at maximum load. With the use of coated piston NOx emissions were increased and the smoke opacity is decreased compared to the uncoated piston. Finally, the results convey that thermal barrier coated piston is more efficient than uncoated piston.

Enhancing the efficiency benefit of thermal barrier coatings for homogeneous charge compression ignition engines through application of a low-k oxide

Prior experiments reported by the authors have proven the hypothesis that achieving a dynamic temperature swing on the combustion chamber surface will lead to improved thermal and combustion efficiencies of the homogeneous charge compression ignition engine. A thin layer of yttria-stabilized zirconia, roughly 150 μm, was plasma sprayed on the piston top. It led to markedly advanced ignition and heat release in the gasoline homogeneous charge compression ignition engine, accompanied with reduced unburnt hydrocarbon and carbon monoxide emissions, improved combustion efficiency, and a higher thermal efficiency. A related computational study highlighted the critical role of coating thermal conductivity in achieving a desired dynamic response; hence, the second phase of experimental investigations focused on introducing structured porosity in the yttria-stabilized zirconia coating, as a means of reducing effective conductivity. Indeed, additional incremental improvements were observed, as well as limitations related to adverse effects of the surface roughness and the fuel interactions with the surface roughness and open pores. Erosion can also be a problem in a direct injection engine. Therefore, the third round of investigations focused on a material with a natively low conductivity (low-k), sprayed on the top of an Al piston in a relatively dense form, and in a way that yields a smooth surface. The objective was to capitalize on the low conductivity, while avoiding the pitfalls accompanying high-porosity formulations. The heat-storage capacity was limited by keeping the thickness relatively low. The results verify the paramount importance of thermal conductivity in the context of high “temperature swing” behavior and indicate a potential to improve the homogeneous charge compression ignition engine’s combustion efficiency roughly 1.5%, with the overall indicated efficiency improvement on the order of 5%, on a relative basis. In addition, the low-k oxide thermal barrier applied to the piston extended significantly the low-load homogeneous charge compression ignition operability limit.

Interactions between zirconia–yttria–tantala thermal barrier oxides and silicate melts

The effects of TaO2.5 additions to Y4Zr3O12 (YZO) on its interactions with calcium-magnesium alumino-silicate (CMAS) melts are investigated. YZO is a candidate material for the protection of novel ZrO2-YO1.5-TaO2.5 based thermal barrier coatings (TBCs) against CMAS attack, but thermochemical considerations dictate that YZO contain TaO2.5 for interfacial compatibility. Oxides based on Y4Zr3-xTaxO12+0.5x (x = =0, 0.7, 1.4) were mixed with various ternary and quinary silicate compositions ranging in Ca:Si ratio from 0.35 to 1.13 and equilibrated at 1300 °C. The results show that all combinations form the desired apatite phase except for the mixture of the higher Ta5+ content with the lowest Ca:Si melt. It is found that the addition of Ta5+ increases the capture of Y3+ by the reprecipitated fluorite, decreasing the amount available to form apatite and eventually suppressing it at low Ca:Si. The effect is ascribed to the attractive interactions between Y3+ and Ta5+ in solid solution, similar to that found in tetragonal zirconia, but with concurrent incorporation of anion vacancies associated with excess Y3+ that stabilize the cubic form. As the Ca2+ content of the melt increases, so does the amount of Ca2+ incorporated in fluorite. The latter also interacts attractively with Ta5+ and leads to a reduction of captured Y3+ in fluorite as the Ca content in the reacting deposit increases. This serves to increase the amount of Y3+available in the melt to form apatite in TBC-CMAS interactions.

Structural Phase State and Thermal Cyclic Stability of the Thermal Barrier Zr–Si–O Coatings Deposited on a Copper Substrate by the Microplasma Method

In this work, preparation of thermal barrier coatings based on zirconium oxide is shown. The phased treatment of the copper substrate is proposed in order to obtain a layered thermal barrier oxide coating on it. The sample surface is nanostructured, titanium is deposited layer-by-layer (by the vacuum-arc method) and then zirconium (by the magnetron method), and then zirconium is converted into zirconium dioxide by the microplasma method. The formed oxide-ceramic coatings contain elements from a solution, according to the results of elemental analysis, and zirconium dioxide in tetragonal and monoclinic modifications, according to the results of X-ray diffraction. A study of thermal cyclic stability was carried out. It is revealed that an increase in the time of microplasma treatment to a certain value has a positive effect on the thermal cyclic properties of the obtained layer material and it is able to sustain more than 90 cycles without serious damage to the surface layer.

High-temperature oxidation and hot corrosion behavior of Er2Sn2O7 + Inconel 625 composite

Abstract In this work, a strategically designed method is employed to attain the oxidation and hot corrosion rate constants of thermal barrier oxide materials without being developed as coatings. For this, low thermal conducting oxide, Er2Sn2O7 is considered as thermal barrier oxide material and blended with metallic Inconel 625 in 1:1 wt ratio to make composite. The oxidation and hot corrosion experiments conducted on bare Inconel 625, Er2Sn2O7 + 50 wt % In 625 and YSZ +50 wt % In 625 samples. The oxidation and hot corrosion results indicate that Er2Sn2O7 pyrochlore showed superior performance than that of YSZ. This work also addresses the role of Cr on destabilization, which may be interesting for fundamental understanding.

Phase equilibria in the zirconia–yttria/gadolinia–silica systems

Phase equilibria in ZrO2-YO1.5-SiO2 (ZYS) and ZrO2-GdO1.5-SiO2 (ZGS) were experimentally assessed at 1400 °C and 1600 °C as they can offer insight on reactions between thermal barrier coatings (TBCs) based on ZrO2-YO1.5/GdO1.5 and molten silicate deposits in gas turbine engines. Features shared in both systems include the absence of ternary compounds and no ternary solubility in the binary phases. In ZYS however, a quaternary invariant reaction was observed that eliminates the zircon-disilicate equilibrium at higher temperatures. The results suggest no appreciable difference in the reactions between silica and thermal barrier oxides based on ZrO2-YO1.5 or ZrO2-GdO1.5, or environmental barrier coatings based on the corresponding Y/Gd silicates. The phase diagrams derived from these experiments are part of a broader effort to develop thermodynamic databases that can help guide the design of next-generation TBCs.

Novel thermal barrier oxides for electronics thermal management: an assessment of WO3

WO3 was evaluated as a candidate thermal barrier oxide material for protection of thermally sensitive systems (e.g., electronics packages) exposed to thermal cycling between −51 and 71 °C. A key factor for thermal barrier oxides is low thermal conductivity. Thermal conductivity serves as a quantifiable metric for assessing the thermal shielding/insulation behavior of a material. This paper assesses structural and phase effects on the thermal properties of WO3. Results suggest that the thermal conduction behavior of WO3 powders is driven by crystal/grain packing and in the temperature range of interest.

Hot corrosion evaluation of Gd2O3-Yb2O3 co-doped Y2O3 stabilized ZrO2 thermal barrier oxides exposed to Na2SO4+V2O5 molten salt

1mol% Gd2O3 and 1mol% Yb2O3 co-doped 3.5mol% Y2O3 stabilized ZrO2 (GdYb-YSZ) ceramics in metastable tetragonal (t′) structure were fabricated, and their hot corrosion behaviors were evaluated. The corrosion tests were performed at 700°C, 800°C, 900°C and 1000°C for 4h in the presence of Na2SO4+V2O5 molten salt. After corrosion, t′ phase could still be detected on the corroded surfaces and its content decreased with increasing the corrosion temperature. The corrosion products were composed of doped YVO4 and m-ZrO2, but some amounts of Na4V2O7 were found at low corrosion temperatures (700°C and 800°C). The corrosion resistance characteristic of GdYb-YSZ was compared with that of YSZ. Under an identical corrosion condition, GdYb-YSZ had less monoclinic phase formed, indicative of a better hot corrosion resistance. The hot corrosion mechanism was discussed in detail.

Equilibrium relationships between thermal barrier oxides and silicate melts

The phase equilibria between thermal barrier coating (TBC) materials and calcium–magnesium–aluminosilicate melts (CMAS, representing deposits formed when siliceous debris is ingested into modern turbine engines), were investigated at 1300 °C. The primary goal was to understand the influence of the deposit and thermal barrier oxide compositions on the melt solubility limits and reaction product constitution. Model deposit compositions with SiO2 to CaO ratios from 1.4 to 4.7 and with or without MgO were reacted with yttria- and gadolinia-zirconia thermal barrier oxides ranging from pure ZrO2 to the pure yttria and gadolinia. The reactions formed various crystalline silicate phases; rare earth–calcium–silicate apatite and zircon (ZrSiO4) were observed most frequently. Following the reactions, the residual melts were depleted in SiO2 and generally enriched in CaO, MgO, and Al2O3. The implications of the anticipated changes in the melt viscosity and the cation partitioning between the melt and various solid solution phases on the efficacy of degradation-mitigating crystallization reactions are discussed.

Experimental investigation of thermal barrier (8YSZ-TiO2-Al2O3) coated piston used in direct injection compression ignition engine

Thermal barrier coatings are becoming increasingly important in providing protection from high temperature degradation for heat engine components and allow further increase in engine temperatures for higher efficiency. The main objective of this research work is to experimentally investigate the air plasma sprayed yttria stabilized zirconia with addition of titanium oxide and aluminum oxide thermal barrier coating on Al-13% Si piston material. The mechanical properties of the coated and uncoated samples were comparatively analyzed. The test revealed that hardness values of coated samples are ten times higher than the hardness values of uncoated samples. The microstructure and surface morphology of the coating were evaluated by scanning electron microscopy. The delamination behaviour of thermal barrier coating was evaluated by thermal cycle test. Finally, the performance test of the coated and uncoated engine was evaluated with the same engine operating conditions. The brake thermal efficiency is increased by 5.99%. The brake specific fuel consumption was decreased by 0.06 kg/kWh, in TBC engine with 8YSZ + Al2O3 + TiO2. The CO and HC was greatly decreased in thermal barrier coating engine. There was the greater reduction of NOx is observed due to coating because of nitrogen has absorbed by zirconia.

Influence of Yb:Hf Ratio on Ytterbium Hafnate/Molten Silicate (CMAS) Reactivity

This study investigated the influence of YbO1.5 concentration on the reactivity between YbO1.5–HfO2 thermal barrier coating (TBC) materials and silicate melts [calcium–magnesium aluminosilicate (CMAS), specifically 33CaO–9MgO–13AlO1.5–45SiO2, all in mol%]. Three thermal barrier oxides (TBO) with YbO1.5 concentration between 30 and 80 mol% were tested at 1300°C and 1500°C. Porous pellets were used to study reaction layer growth and the behavior within infiltrated porosity. Complementary experiments examined the phase equilibria in melts containing varying quantities of the candidate TBOs. The effectiveness of reactive crystallization to limit interaction depth increased with the YbO1.5 concentration in the TBO. The results indicate that the TBO must contain sufficient YbO1.5 to satisfy the equilibrium between the melt and HfO2‐based fluorite phase before YbO1.5‐bearing silicates can precipitate. Increasing the YbO1.5 availability leads to the crystallization of apatite, garnet, silicocarnotite, and/or cuspidine (alumino)silicates. Apatite was observed at 1300°C and 1500°C, garnet and silicocarnotite only appeared at 1300°C, and cuspidine was only evident at 1500°C. The implications for the design of CMAS‐resistant coatings are discussed in the context of the experimental results.

Composition Effects on TBC/Silicate Melt (CMAS) Interaction Dynamics

Thermal barrier coatings (TBC) employed in the hot section of modern gas turbine engines are susceptible to degradation by molten deposits formed when siliceous debris is ingested into the engine [1]. These deposits, generically known as CMAS, may contain SiO2, CaO, MgO, AlO1.5, FeOx, and minor fractions of other oxides; the compositions can vary widely based on the geological source and engine service history [2]. Regardless of the specific deposit composition, the melt wicks into the open features in the TBC microstructure and solidifies under the imposed thermal gradient, leading to stiffening of the coating material and, upon cycling, elevated thermal stresses in the infiltrated region may produce cracking and/or spallation. Thermochemical attack may compound the damage, especially if the CMAS interacts with the thermally grown aluminum oxide that protects the alloy from oxidation. These degradation mechanisms can be mitigated, in part, with coatings designed to minimize the depth of melt penetration for a given exposure condition. In this context, the most effective coatings react readily with the deposit, converting the penetrating melt to a combination of stable crystalline phases that block the open porosity near the coating surface. The desired behavior is most clearly demonstrated by rare earth (RE) zirconate TBCs, notably Gd2Zr2O7, which react with CMAS to form a combination of fluorite and apatite (~Ca2Gd8Si6O26)[3]. Work with one model CMAS composition (33CaO-9MgO-13AlO1.5-45SiO2 in mol%, or Ca33Mg9Al13Si45) has shown that related RE zirconate and hafnate phases also form fluorite and apatite, among other reaction products. The efficacy of the crystallization reactions, however, varies with the RE cation radius, with the larger RE’s (i.e. La) acting as more potent apatite formers [4]. Efforts to design new TBC compositions would benefit from improved understanding of the role of the silicate deposit composition on the reaction processes. Elucidating these effects is the focus of this study. This work examined the effect of melt composition on the phase equilibria between the melts and candidate TBC materials such yttria stabilized zirconia (YSZ) and yttrium zirconate (Y4Zr3O12, YZO). The silicate melts were selected to span a relevant range of compositions while leveraging existing knowledge about the intrinsic melting and crystallization behaviors [5]. Initial experiments were performed in the simple, well-characterized CaO-AlO1.5-SiO2 (CAS) system with additional work in more complex silicate melts. Pre-reacted crystalline silicate powders were mixed with thermal barrier oxide (TBO) particles and annealed to identify the equilibrium crystalline phases and determine how the solubility limit of the TBO constituents in the melt changes with the melt composition. The results reveal that the melt composition plays a significant role in determining the crystallization product identity and volume fraction. Figure 1 highlights the effect of systematic changes in melt composition on the phase equilibria with YZO at 1300ºC. Figure 1(a,b) show melt compositions in the CAS system. Both systems form apatite while the primary ZrO2-bearing phase changes from zircon (ZrSiO4) to fluorite with a moderate shift in the SiO2 concentration. Meanwhile Ca33Mg9Al13Si45, the CMAS composition used in many past laboratory studies and shown in Figure 1(c), forms only fluorite. The latter behavior is related to the relatively high solubility of YO1.5 in both the melt and precipitated fluorite so the melt is not sufficiently saturated in YO1.5 to promote apatite crystallization. The findings shed new light on observations from ex-service TBCs that exhibit a range of deposit compositions and corresponding crystallization products and offer guidance for improved laboratory tests. The conclusions underscore the importance of considering a variety of factors related to melt chemistry when assessing the CMAS degradation of existing TBC materials and designing novel TBCs for improved CMAS resistance. Figure 1: Reaction between Y4Zr3O12 TBC material and selected silicate melt compositions (initially 25mol% YZO in the melt) produced a variety of crystallization products after 50h/1300ºC equilibration. Compositions (a) and (b) represent changes in the Ca:Si ratio but similar AlO1.5 concentration in the CAS system; (a) has sufficient SiO2 activity to form zircon (ZS) while (b) forms fluorite (F) as the only ZrO2-containing reaction product. Melt composition (c) has lower SiO2 activity and higher YO1.5 solubility; as such, no apatite (Ap) crystallization is observed even with the higher CaO concentration than (a) or (b). [1] M.P. Borom, C.A. Johnson, L.A. Peluso, Surf. Coat. Technol, 86 (1996) 116-126. [2] C.G. Levi, J.W. Hutchinson, M.-H. Vidal-Sétif, C.A. Johnson, MRS Bulletin, 37 (2012) 932-941. [3] S. Krämer, J. Yang, C.G. Levi, J.Am. Ceram. Soc., 91 (2008) 576-583. [4] D.L. Poerschke, C.G. Levi, J.Eur. Ceram. Soc., 35 (2015) 681-691. [5] E.M. Zaleski, C. Ensslen, C.G. Levi, Submitted (2014) Figure 1

Effects of cation substitution and temperature on the interaction between thermal barrier oxides and molten CMAS

Abstract The effectiveness of candidate rare earth (RE) bearing oxides to mitigate degradation of thermal barrier coatings by molten silicates is determined by the constitution and crystallization kinetics of the reaction products. The relationships between the oxide composition, test temperature, and reaction product constitution were investigated using sintered pellets of hafnates or zirconates containing YbO1.5, GdO1.5, or LaO1.5. The results suggest that the composition of the reprecipitated ZrO2- or HfO2-based phase, typically fluorite, is a useful indicator of the effectiveness of the silicate crystallization reactions. Meanwhile, the composition of the primary crystalline reaction product, typically a RE oxy-apatite, is relatively insensitive to the experimental variables considered herein. The observed trends indicate that the larger RE cations are more potent in rapidly crystallizing the silicate melt and that their relative effectiveness increases with the REO1.5 concentration in the coating material.

Finite element study of multi-modal vibration damping for thermal barrier coating applications

A physically-based computational model is developed to predict the damping behavior of oxide thermal barrier coating systems. The constitutive damping model is derived from the theory of point defect relaxation in crystalline solids and implemented within a finite element framework. While oxide coatings have been primarily employed as thermal barriers for gas turbine blades, there is a growing interest in developing multifunctional coatings combining thermal protection and damping capabilities. The direct frequency response method, as well as the modal strain energy method, have been implemented to evaluate the functional dependance of damping on temperature and frequency. Numerical results are validated through the limited experimental data available in the literature, and new results are presented to illustrate the effects of different topcoat oxides. The paper also illustrates how the developed methodology enables the damping capacity under different vibrational modes to be predicted, and to estimate the sensitivity of the design for varying geometrical parameters. Finally, the computational model is applied to investigate the damping performance of an oxide-coated turbine blade.

Segmented lanthanum cerium oxide thermal barrier coatings by atmospheric plasma spray

Recently, efforts have been focussed on seeking for new candidates as ceramic topcoat in thermal barrier coating (TBC) system, instead of yttria stabilised zirconia (YSZ) currently used for TBC topcoat. Among the candidates, lanthanum cerium oxide (La2Ce2O7, LCO) is a promising TBC topcoat material, because of its higher temperature stability and better thermal insulation performance relative to yttria stabilised zirconia. In this work, the LCO TBC with high segmentation crack density was prepared by atmospheric plasma spraying, while keeping the stoichiometric composition of the coating. High plasma power and high substrate temperature Ts contribute to the formation of the segmentation structure. The segmented LCO TBC had a lifetime of >730 cycles, exhibiting a significant improvement in thermal shock resistance as compared to the nonsegmented coating. Spallation failure of the segmented TBC was caused by both chipping and delamination cracking.