EB-PVD Thermal Barrier Coatings

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Multiscale approach to predict the lifetime of EB-PVD thermal barrier coatings

Multiscale approach to predict the lifetime of EB-PVD thermal barrier coatings

Abstract Thermal barrier coatings (TBCs) are used to protect hot components from combustion gases in gas turbines. One of the most widely used TBC systems is that applied by an electron beam-physical vapour deposition (EB-PVD) onto a Ni-base intermediate or bond coat. The resulting top zirconia based thermal insulator exhibits a characteristic columnar morphology. During service, the combination of severe thermal loads and high temperatures leads to the selective oxidation of the intermediate metallic coating, to TBC degradation and, eventually, to the development of microcracks. This may, in turn, be followed by spalling of the top coating, which constitutes the life limiting event for the component. Different approaches have been proposed to predict these phenomena, generally based on macroscopic TBC stresses as the driving force for TBC failure or on fracture mechanics approaches to predict interfacial or cohesive failure. However, no previous work integrates local interface damage and macroscopic stresses or stored strain energy in the prediction of TBC spallation. The objective of this thesis is to develop a multi-scale life predictive approach for TBC life which accounts for the evolution of local interface damage, and its effect on the fracture resistance relevant to the dominant failure mode, such as oxide interface spallation. Even though the study focuses on an EB-PVD TBC system, the proposed approach is generic and can be adapted to other types of TBCs.
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Outstanding durability of sol-gel thermal barrier coatings reinforced by YSZ-fibers

Outstanding durability of sol-gel thermal barrier coatings reinforced by YSZ-fibers

2.3. Cyclic oxidation Before cyclic oxidation, the TBCs are cleaned in an ethanol bath and dried with hot, dry and dust free air to remove any impurities that may cause premature spalling. TBCs were characterized by cyclic oxidation in air in an open furnace working at 1100 °C. A cycle is composed of 1 h dwell at 1100 °C (including a very rapid heating) and a cooling of 15 min to room temperature (using a high flow of air, free from oil and pollution). In the furnace TBCs were positioned vertically on sample holders and it is this assembly that moved according to the cycle. The resulting thermal cycling is consistent with the one performed by the motorist Safran. About every 50 cycles, TBCs were weighted with a SARTORIUS GENIUS ME 215 P precision balance and were photo graphed to determine mass variation and the percentage of surface spalling respectively. For comparison, a TBC system with a top coat made by EB PVD and provided by Safran Aircraft Engines was added to the test to serve as a reference. Compared to a sol gel TBC system, the TBC system manufacturing using EB PVD has only one face and a thickness of TGO around 200 nm. Concerning the mass variation, it takes into account both faces of the sol gel TBC samples, namely the
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Reinforced sol–gel thermal barrier coatings and their cyclic oxidation life

Reinforced sol–gel thermal barrier coatings and their cyclic oxidation life

randomly distributed porosities. Properties and induced char- acteristics are very different compared to those of EB-PVD and PS coatings. This should result in an attractive compro- mise between thermal conductivity and mechanical strength. In a previous paper, 8 the mechanisms responsible for the damage of sol–gel coatings subject to cyclic oxidation was identified and discussed as a function of the oxidation temperature and the TBC thickness. Initially, the degradation of sol–gel TBCs proceeds through the formation of a regular crack network occur- ring, either during the post-deposition thoroughly controlled by thermal treatment or during the very first cycles of oxidation, in both cases as the result of the thermally-activated sintering of the zirconia deposit. This network can further develop itself dur- ing the cumulative oxidation cycles promoting the enlargement and the coalescence of cracks that finally lets appear individ- ual TBC cells prone to spall off under the effect of the cyclic thermomechanical stresses. In order to improve the durability of sol–gel coatings upon oxidation, it is proposed to control and stabilize the process-induced crack network by filling the cracks with fresh material brought either by supplementary dip- coating passes 9 or additional spray-coating passes. By providing an appropriate adjustment and control of the process, it has been shown that the crack filling by dip-coating significantly improves the cyclic oxidation behaviour of the TBCs 9 through an overall reinforcement of the barrier resulting from an optimised mechanical pegging.
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Reinforced sol-gel thermal barrier coatings and their cyclic oxidation life

Reinforced sol-gel thermal barrier coatings and their cyclic oxidation life

randomly distributed porosities. Properties and induced char- acteristics are very different compared to those of EB-PVD and PS coatings. This should result in an attractive compro- mise between thermal conductivity and mechanical strength. In a previous paper, 8 the mechanisms responsible for the damage of sol– gel coatings subject to cyclic oxidation was identified and discussed as a function of the oxidation temperature and the TBC thickness. Initially, the degradation of sol– gel TBCs proceeds through the formation of a regular crack network occur- ring, either during the post-deposition thoroughly controlled by thermal treatment or during the very first cycles of oxidation, in both cases as the result of the thermally-activated sintering of the zirconia deposit. This network can further develop itself dur- ing the cumulative oxidation cycles promoting the enlargement and the coalescence of cracks that finally lets appear individ- ual TBC cells prone to spall off under the effect of the cyclic thermomechanical stresses. In order to improve the durability of sol– gel coatings upon oxidation, it is proposed to control and stabilize the process-induced crack network by filling the cracks with fresh material brought either by supplementary dip- coating passes 9 or additional spray-coating passes. By providing an appropriate adjustment and control of the process, it has been shown that the crack filling by dip-coating significantly improves the cyclic oxidation behaviour of the TBCs 9 through an overall reinforcement of the barrier resulting from an optimised mechanical pegging.
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Characterisation of thermal barrier sensor coatings synthesised by sol–gel route

Characterisation of thermal barrier sensor coatings synthesised by sol–gel route

Two main processes are used to apply TBCs to components, Air Plasma Spray and Electron Beam Physical Vapour Deposition (EB-PVD) [12]. The successful implementation of TBsCs has been demonstrated for both of these deposition methods [6,13]. Sol–gel processing, however, has gained increasing credibility over recent years for the production of coatings with non-directional poros- ity because it offers benefits in processing costs and flexibility, and the potential to repair damaged TBCs [14–16]. Using this method it is possible to produce multi-layered TBCs with control over the thickness, depth and concentration of dopants in each layer. Fur- thermore, the advances in the processing by some of the co-authors have demonstrated the durability of sol–gel coatings under cyclic oxidation at 1100 ◦ C [17]. Hence, the objective of the present paper is to solely investigate the use of sol–gel to produce sensor TBCs for phosphor thermometry measurements with samarium as the active dopant. Firstly, the optimum dopant concentration is deter- mined for samarium in an yttria stabilised zirconia (YSZ) host. Secondly, this material is integrated into a YSZ bulk coating at dif- ferent depths to evaluate the effectiveness of embedding the sensor layer. Finally, the decay lifetime of the phosphorescence is recorded over a range of temperatures to establish its working limits for detection.
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Processing thermal barrier coatings via sol-gel route: crack network control and durability

Processing thermal barrier coatings via sol-gel route: crack network control and durability

As compared to EB-PVD coatings, sustaining typically around 1000–1200 cycles, when cyclically oxidized in similar conditions, very good performances is achieved with sol-gel coatings. Indeed lifetime of the thermal barriers, reaching > 1000 cycles for the TBC obtained with PVP, confirms that the use of PVP dispersant can beneficially replace the Beycostat C213 as dispersing agent as well as filling agent for cracks, if necessary. However, it is shown that the reinforcement of the micro-crack network is no more required in this case, the gain in life- time being non-significant. Indeed, the process implemented in only one step is e fficient enough to promote the development of reliable and competitive sol-gel thermal barrier coatings. They remain without spallation for > 1000 oxidation cycles, in perfect agreement with re- quired lifetime expected by engine makers.
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Sol–gel thermal barrier coatings: Optimization of the manufacturing route and durability under cyclic oxidation

Sol–gel thermal barrier coatings: Optimization of the manufacturing route and durability under cyclic oxidation

3. Results of cyclic oxidation and discussion In direct relation with the processing route, the sol –gel thermal barrier coatings show very speci fic microstructures. Namely, the intimate structure of the deposit is basically non columnar and mostly equiaxial including a signi ficant density of porosity. The thickness of the barrier depends on the number of dip-coatings and other process parameters such as the rate of withdrawal from the composite sol or the viscosity of the sol. Though higher thicknesses, more than 100 μm, are achievable, the present work will focus on barriers, deposited using the controlled process parameters described in Section 1 , with thickness typically ranging from 20 μm to 100 μm. As the final heat treatment of specimen proceeds, the sintering of deposits causes the formation of a regular network of surface cracks, delimitating YSZ cells with size in the range 100 μm to 200 μm. This results from the thermally activated material shrinkage prone to induce bi-axial stresses higher than the stress to rupture of the coatings. The bi- axial nature of the stress leads to the formation of an almost isotropic pattern of individual cells separated by sintering-induced cracks. Providing it is perfectly controlled, in terms of kinetics and morphology, this initial crack network (heat checking) may be bene ficial to the further mechanical strength of the barrier while thermally cycled. Indeed, the occurrence of penetrating cracks perpendicular to the specimen surface can signi ficantly enhance the lateral compliance of the deposit as sought, to some extent, in standard EB-PVD TBC or in plasma sprayed TBC as mentioned in [10] . 3.1. Cracking, heat-checking and spalling of TBC
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Optimized sol–gel thermal barrier coatings for long-term cyclic oxidation life

Optimized sol–gel thermal barrier coatings for long-term cyclic oxidation life

3.2.3. Cyclic oxidation behaviour of the optimized reinforced sol–gel TBC: comparison with an EB-PVD TBC As previously illustrated in the literature, the failure mecha- nism of TBCs is well different for EB-PVD and sol–gel systems. Basically, EB-PVD TBCs generally exhibit long term resistance to spallation following cyclic oxidation exposure with very lit- tle degradation up to, say, one to two thousands numbers of one hour-cycles at 1100 ◦ C. Once, the mechanical strain energy stored in the system and the development of rumpling are large enough for the onset to spallation, the EB-PVD TBC generally fails upon one single cooling subsequent to an ultimate expo- sure at high temperature. Failure affects the whole TBC or at least a large surface fraction of the TBC following the initiation and propagation of cracks at the substrate/TGO interface. In the case of sol–gel TBC, the initial crack network resulting from the sintering heat treatment, concentrates stress during cyclic exposure and acts as zones of crack formation that generally tend to propagate at the TGO/TBC interface. Individual cells, delineated by this network, can subsequently spall off contin- uously and gradually as oxidation cycles cumulate. Spallation kinetics, possibly established within the very first cycles, is much more progressive than for EB-PVD TBCs characterized by sharp and sudden degradation. For no pre-oxidised and no reinforced
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Outstanding durability of sol-gel thermal barrier coatings reinforced by YSZ-fibers

Outstanding durability of sol-gel thermal barrier coatings reinforced by YSZ-fibers

Flbers Cydlc oxldatlon Thermal barrier coatings (TBC) were fabricated with commercial powders of yttria stabilired zirconia with spherical and fiber-like morphologies. The influence of fiber percentage and sintering temperature on the thennomechanical behavior was studied. TBCs with 60%-80% fibers content had the best lifetime in cyclic oxidation with Jess than 10% of coating spallation after 1000 cycles, with very good reproducibility. They reached lifetimes higher than industrial TBCs made by EB-PVD. The enhancement of durability is believed to be due to an increase in the thennomechanical constraints accommodation thanks to higher porœity and higher tenacity due to the presence of well anchored fibers, indeed deviation of the cracks were observed. Moreover, the morphology of the thermally grown oxide CTGO) layer is also favorable as it includes anchorage points of the TGO with fibers. This increased the adherence at the substrate interface and improved lifetime.
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Optimized sol–gel thermal barrier coatings for long-term cyclic oxidation life

Optimized sol–gel thermal barrier coatings for long-term cyclic oxidation life

3.2.3. Cyclic oxidation behaviour of the optimized reinforced sol–gel TBC: comparison with an EB-PVD TBC As previously illustrated in the literature, the failure mecha- nism of TBCs is well different for EB-PVD and sol–gel systems. Basically, EB-PVD TBCs generally exhibit long term resistance to spallation following cyclic oxidation exposure with very lit- tle degradation up to, say, one to two thousands numbers of one hour-cycles at 1100 ◦ C. Once, the mechanical strain energy stored in the system and the development of rumpling are large enough for the onset to spallation, the EB-PVD TBC generally fails upon one single cooling subsequent to an ultimate expo- sure at high temperature. Failure affects the whole TBC or at least a large surface fraction of the TBC following the initiation and propagation of cracks at the substrate/TGO interface. In the case of sol–gel TBC, the initial crack network resulting from the sintering heat treatment, concentrates stress during cyclic exposure and acts as zones of crack formation that generally tend to propagate at the TGO/TBC interface. Individual cells, delineated by this network, can subsequently spall off contin- uously and gradually as oxidation cycles cumulate. Spallation kinetics, possibly established within the very first cycles, is much more progressive than for EB-PVD TBCs characterized by sharp and sudden degradation. For no pre-oxidised and no reinforced
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Characterisation of thermal barrier sensor coatings synthesised by sol–gel route

Characterisation of thermal barrier sensor coatings synthesised by sol–gel route

Two main processes are used to apply TBCs to components, Air Plasma Spray and Electron Beam Physical Vapour Deposition (EB-PVD) [12] . The successful implementation of TBsCs has been demonstrated for both of these deposition methods [6,13] . Sol–gel processing, however, has gained increasing credibility over recent years for the production of coatings with non-directional poros- ity because it offers benefits in processing costs and flexibility, and the potential to repair damaged TBCs [14–16] . Using this method it is possible to produce multi-layered TBCs with control over the thickness, depth and concentration of dopants in each layer. Fur- thermore, the advances in the processing by some of the co-authors have demonstrated the durability of sol–gel coatings under cyclic oxidation at 1100 ◦ C [17] . Hence, the objective of the present paper is to solely investigate the use of sol–gel to produce sensor TBCs for phosphor thermometry measurements with samarium as the active dopant. Firstly, the optimum dopant concentration is deter- mined for samarium in an yttria stabilised zirconia (YSZ) host. Secondly, this material is integrated into a YSZ bulk coating at dif- ferent depths to evaluate the effectiveness of embedding the sensor layer. Finally, the decay lifetime of the phosphorescence is recorded over a range of temperatures to establish its working limits for detection.
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Sol–gel thermal barrier coatings: Optimization of the manufacturing route and durability under cyclic oxidation

Sol–gel thermal barrier coatings: Optimization of the manufacturing route and durability under cyclic oxidation

3. Results of cyclic oxidation and discussion In direct relation with the processing route, the sol –gel thermal barrier coatings show very speci fic microstructures. Namely, the intimate structure of the deposit is basically non columnar and mostly equiaxial including a signi ficant density of porosity. The thickness of the barrier depends on the number of dip-coatings and other process parameters such as the rate of withdrawal from the composite sol or the viscosity of the sol. Though higher thicknesses, more than 100 μm, are achievable, the present work will focus on barriers, deposited using the controlled process parameters described in Section 1 , with thickness typically ranging from 20 μm to 100 μm. As the final heat treatment of specimen proceeds, the sintering of deposits causes the formation of a regular network of surface cracks, delimitating YSZ cells with size in the range 100 μm to 200 μm. This results from the thermally activated material shrinkage prone to induce bi-axial stresses higher than the stress to rupture of the coatings. The bi- axial nature of the stress leads to the formation of an almost isotropic pattern of individual cells separated by sintering-induced cracks. Providing it is perfectly controlled, in terms of kinetics and morphology, this initial crack network (heat checking) may be bene ficial to the further mechanical strength of the barrier while thermally cycled. Indeed, the occurrence of penetrating cracks perpendicular to the specimen surface can signi ficantly enhance the lateral compliance of the deposit as sought, to some extent, in standard EB-PVD TBC or in plasma sprayed TBC as mentioned in [10] . 3.1. Cracking, heat-checking and spalling of TBC
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Advances in the field of new smart thermal barrier coatings

Advances in the field of new smart thermal barrier coatings

Our first objective has been to discriminate the suitable powder load content for the filling-up. First, cyclic oxidation tests have been performed on a sample with no filling-up, which is used as a reference for oxidation behavior. Figure 5 shows a damaged sol-gel thermal barrier. In opposition to damaged EB-PVD thermal barrier in which cyclic oxidation induces a non-progressive delamination of the major part of the coat, damaged sol-gel thermal barrier progressively proceeds through spallation. We can notice that the samples are more damaged on edges due to edge effects. For the filling-up of sample C containing a loaded sol at 40% of powder, damage mechanism is not the same. Indeed in figure 6, the topographies exhibit local buckling which can be due to two main reasons : first, the deposit is probably too loaded and we can see an important recovering of the upper surface of the sample. So, this impregnation is not suitable because the active matter is mainly located on the surface. The second reason is, of course, that 1150°C is probably a too high temperature to prevent from delamination, however, note that this choice has been done to accelerate the cracks network formation. According to Evans [8], observation of buckling suggests a poor adhesion between coating and substrate : cracks propagate at the interface and lead to delamination as a consequence of high stresses in the coating. Stresses resulting from the mismatch between thermal expansion coefficients of multilayers and alumina oxide. In our case, additional stresses may be due to the additional matter on the surface top during the filling-up.
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Modelling thermal conductivity of porous thermal barrier coatings

Modelling thermal conductivity of porous thermal barrier coatings

doping of elements, co-doping of elements, use of pyrochlore oxides, multilayer TBCs and some other techniques to lower thermal conductivity [ 26 – 29 ]. Further, the efficiency of a gas turbine engine can be increased with the selection of material combination that holds a low thermal conductivity over a specific elevated temperature range [ 30 – 32 ]. This holds true in the case of 8YSZ under prolonged heating. However, some materials/oxides may have even lower thermal conductivity compared to 8YSZ. But their use is limited due to drawbacks in material properties. Some of the issues are related to poor phase stability, lower fracture toughness and lower resistance to CMAS induced damage [ 33 , 34 ]. Low thermal conductivity and higher turbine inlet temperature are beneficial for higher engine efficiency. This, in turn, sets the requirement that the value of thermal conductivity should remain low for a longer interval under service conditions of high-temperature and high pressure [ 35 ]. Performance of the high-temperature TBC system depends on several parameters based on the microstructure of the coating. Such parameters can include the shape and orientation of the porosity, intrinsic properties of coatings, interfacial influences, doping elements, presence of rare earth metals, thickness of the coating layer, the number of defects present, and the process used to fabricate coating [ 20 , 36 – 41 ]. Some of the advanced coating techniques such as the solution precursor plasma spray (SPS) method provides the ability to develop a wide range of microstructures that may have more splats with a structure similar to EB-PVD. These kinds of coatings can be deposited at an ultra-fine level that will have a longer life than EB-PVD under set conditions [ 42 – 44 ].
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Optimizing Compliance and Thermal Conductivity of Plasma Sprayed Thermal Barrier Coatings via Controlled Powders and Processing Strategies

Optimizing Compliance and Thermal Conductivity of Plasma Sprayed Thermal Barrier Coatings via Controlled Powders and Processing Strategies

Single splat morphologies provide a frame of reference for microstructural analysis (Ref 39 , 40 ). Figure 5 com- pares these splat morphologies represented within the first-order process map. Near-perfect disk-shape splat morphologies were obtained for E3 coating. Similar results were identified for E4 with the exception of somewhat increased fragmentations along the periphery. This is attributable to increased KE of the particles due to the higher particle velocity (82 m/s compared to 144 m/s while average temperature is almost the same around 2788 C). This fragmentation concerns the material excess in the periphery of the splat). It was also noted that these splats in some cases contain unmelted nodules within the splat indicative of partial melting of the particle. This observation is also corroborated with microstructures of the sprayed coatings. The E3 morphology has a distinctive appearance consisting of successive dense zones inter- rupted by large and long cracks probably originating from the stacking of these well-formed circular lamellae while cracks could have been produced to minimize thermal strains in the coating during built up.
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Sol–gel processing and characterization of (RE-Y)-zirconia powders for thermal barrier coatings

Sol–gel processing and characterization of (RE-Y)-zirconia powders for thermal barrier coatings

process for thermal barrier applications. For each compound, structural and microstructural analyses were performed. LZ powders moves from a pure tetragonal structure for a low doping concentra- tion, to a pure pyrochlore phase for 30 LZ. Samarium and erbium doped zirconia powders crystallise mainly in the cubic form. The microscopic study suggests quite a similar behaviour between these rare earth doping elements. For each of them, the microstructure moves from compact monoliths (20 μm–50 μm) with heterogeneous size to agglomerates of thinner particles for an average doping amount of 20 mol%. This new structure can explain the higher specific surface areas of the compounds when increasing the doping content. The ceramics (heat treated at 950 °C) with a doping amount of 9.7 and 30 mol% were hot-pressed using the Spark Plasma Sintering method Table 4
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Influence of post-spraying heat treatments on the oxidation and cracking behaviour of thermal-sprayed thermal barrier coatings

Influence of post-spraying heat treatments on the oxidation and cracking behaviour of thermal-sprayed thermal barrier coatings

/ La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur. Access and use of [r]

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Optimizing Compliance and Thermal Conductivity of Plasma Sprayed Thermal Barrier Coatings via Controlled Powders and Processing Strategies

Optimizing Compliance and Thermal Conductivity of Plasma Sprayed Thermal Barrier Coatings via Controlled Powders and Processing Strategies

Single splat morphologies provide a frame of reference for microstructural analysis (Ref 39 , 40 ). Figure 5 com- pares these splat morphologies represented within the first-order process map. Near-perfect disk-shape splat morphologies were obtained for E3 coating. Similar results were identified for E4 with the exception of somewhat increased fragmentations along the periphery. This is attributable to increased KE of the particles due to the higher particle velocity (82 m/s compared to 144 m/s while average temperature is almost the same around 2788 C). This fragmentation concerns the material excess in the periphery of the splat). It was also noted that these splats in some cases contain unmelted nodules within the splat indicative of partial melting of the particle. This observation is also corroborated with microstructures of the sprayed coatings. The E3 morphology has a distinctive appearance consisting of successive dense zones inter- rupted by large and long cracks probably originating from the stacking of these well-formed circular lamellae while cracks could have been produced to minimize thermal strains in the coating during built up.
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Residual Stress Analysis of Laser-Drilled Thermal Barrier Coatings Involving Various Bond Coats

Residual Stress Analysis of Laser-Drilled Thermal Barrier Coatings Involving Various Bond Coats

e ij ¼ 2A ij r i ; with 1 ' i ' j ðEq 5Þ In Eq 5 , Aij is a calibration coefficient that corresponds to a strain generated after the jth drilling step and induced by a unit hydrostatic stress in the ith increment. In this work, the series of calibration coefficients Aij for each incre- mental step were computed through a 3D finite element analysis (ABAQUS/CAE software). Two axisymmetric 3D models were built to simulate the strain relaxation eij after each increment. Details of the FE numerical ap- proach were described in previous works involving strain gauge measurements (Ref 11 , 12 ). In this work, the pre- sence of a pre-existing through-hole has also been con- sidered in one of the FE models. This development made this residual stress analysis of laser-drilled TBC fairly original. By incrementing all i and j indices in the FE model, the series of coefficients Aij were calibrated by predicting eij (ri = 1 MPa in Eq 5 ). A schematic repre- sentation of the principle for FE calibration is given in Fig. 5 for both models. These models for multi-layered coatings involved three different materials and a blind- hole of 2 mm in diameter. The pre-drilled mesh geometry exhibited a centred through-hole with a 0.5 mm diameter. Bottom nodes of the substrate were encastred and a per- fect adhesion between the material domains was assumed in the numerical models. The location of the two nodes to achieve the strain prediction in the FE models was similar to those of the two pixels for ESPI strain analysis.
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Processing thermal barrier coatings via sol-gel route: crack network control and durability

Processing thermal barrier coatings via sol-gel route: crack network control and durability

1. Introduction Thermal barrier coatings (TBC) are used as thermal insulators in aircraft jet engines and more speci fically as a protection for turbine blades engine. Service temperatures have been increased for some years and these temperatures are sometimes higher than the degradation temperature of the used superalloys. In these conditions, the role of TBCs is to maintain or even to increase the lifetime of such complex turbine blade systems [1]. TBCs systems are made up of a single crystal nickel-based superalloy, which ensures good mechanical stability. The bondcoat, deposited onto the superalloy, plays an important role in the accommodation of lateral thermomechanical constraints. It also plays the role of a reservoir of aluminium allowing the formation of a so- called Thermal Grown Oxide (TGO) on its surface. Finally, the topcoat is a protective ceramic layer of yttria-stabilized zirconia. This ceramic has the great advantage of having a low thermal conductivity, which allows to enhance the achievable thermal gradient up to 1 °C per μm.
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