Thermal barrier coatings (TBC) fabricated by plasma spray can exhibit a wide range of microstructures due to differences in feedstock powders and spraying conditions. Since different microstructures naturally result in different thermal and mechanical properties and service life as thermal barrier coatings, it is of great importance to understand the relationship among the feedstock characteristics, spray conditions and the coating microstructures. Recent research efforts of the author's group to understand fundamental phenomena in plasma spraying of TBC are reviewed from microscopic to macroscopic viewpoints, i.e., direct observation of single droplet impact of molten zirconia by an ultra fast video camera, detection of acoustic emission (AE) signals during plasma spraying by using laser AE technique, and in-situ measurement of the curvature and temperature of a substrate during plasma spraying, from which strain-stress relationships and residual stresses of TBC can be evaluated.

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Porous and feather-like yttria-stabilized zirconia coatings have been deposited by plasma spray physical vapor deposition (PS-PVD) at the deposition rate of around 200 µm/min. The porosity of the coating reached > 50% and the overall thermal conductivity was reduced less than 0.5 W/mK, both of which were fundamentally independent of the coating thickness up to 120 µm. Such coating structures were characterized by their high scattering coefficients at light wavelength from 2 to 6 µm, and thus the transmittance to infrared was reduced significantly compared to the splat structure coating with the identical thickness. The thermal properties of these coatings were retained even after annealing at temperature ∼1500°C for 130 hours.

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In plasma spray process, the individual droplet behavior at impact is the fundamental element to understand the resulting coating microstructure and corresponding coating properties. In this study, the flattening behavior of the sprayed individual particle was systematically investigated by changing the substrate preheating temperature and ambient pressure in deposition chamber. The splat shape change transitionally from a splash shape to a disk shape by substrate preheating or ambient pressure reduction. A transition temperature, Tt, and transition pressure, Pt, were defined and introduced, respectively. Furthermore, the wetting behavior of water droplet and flattening behavior of thermal sprayed particles were studied on the substrate with different elapsed time in an air atmosphere after preheating. It is clearly found that the contact angle increase gradually with an increase of the elapsed time. More splash splats were observed on the substrate with increase of the elapsed time, which agreed with the contact angle measurement results well. Experiment results indicate that wetting of substrate by molten droplet may dominate the flattening behavior of the thermal sprayed particles. Good wetting may be generated by removing the adsorbed gas/condensation through substrate preheating or ambient pressure reduction. Based on the study above, a three-dimensional transition map was proposed as a controlling principle of the thermal spray process.

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Current use of TBCs is subjected to premature spallation failure mainly due to the formation of thermally grown oxides (TGOs). Although extensive research has been carried out to gain better understanding of the thermo - mechanical and -chemical characteristics of TBCs, laboratory-scale studies and simulation tests are often carried out in conditions significantly differed from the complex and extreme environment typically of a modern gas-turbine engine, thus, failed to truly model service conditions. In particular, the difference in oxygen partial pressure and the effects of contaminants present in the engine compartment have often been neglected. In this respect, an investigation is carried out to study the in-service degradation of an EB-PVD TBC coated nozzle-guide vane. Several modes of degradation were observed due to three factors: 1) presence of residual stresses induced by the thermal-expansion mismatches, 2) evolution of bond coat microstructure and subsequent formation of oxide spinels, 3) deposition of CMAS on the surface of TBC.

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Local area fatigue crack propagation resistance of a retired gas turbine vane made of a Ni-based superalloy which had been operating for about 20000 h. in a 1100 °C level land-based gas turbine, was experimentally evaluated. Here, special efforts were made to build up a new testing apparatus to assess the fatigue crack propagation resistance, using the miniature size specimens extracted from the vane. The miniature specimens were extracted from the leading and the trailing edges, respectively. It was shown that the fatigue crack grew associating with the rate significantly higher at the trailing than at the leading edge part. It was also indicated that the crack propagation rates in the miniature specimen were generally higher than those in the standard size specimen. Furthermore, it was also shown that an application of a simple reheat treatment was effective to recover the crack propagation resistance for the post service vane.

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Thermal barrier coatings (TBCs) increase the service lifetime of specific components in, for example, gas turbines or airplane engines and allow higher operating temperatures to increase efficiency. Lifetime prediction models are therefore of both academic and applied interest; either to test new coatings or to determine operational conditions that can ensure a certain lifetime, for example 25,000 hr for gas turbines. Driven by these demands, the equations used in lifetime prediction have become more and more sophisticated and consequently are complicated to apply. A collection of software tools for lifetime assessment was therefore developed to provide an easy to use graphical user interface whilst incorporating the recent improvements in modeling equations. The Windows based software is compatible with other Windows applications, such as, Power Point, Excel, or Origin. Laboratory lifetime data from isothermal, thermal cyclic and/or burner rig testing can be loaded into the software for analysis and the program provides confidence limits and an accuracy assessment of the analysis model. The main purpose of the software tool is to predict TBC spallation for a given bond coat temperature, temperature gradient across the coating, and thermal cycle frequency.

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The use of thermal barrier coatings (TBCs) made from yttria stabilized zirconia (YSZ) on superalloy base materials has been a significant step to a new level of operational limits in high temperature applications. By the application of a TBC in conjunction with cooling of the component material the operating temperature can be raised and higher efficiencies are achieved. As a consequence of the raised temperature failure of a TBC leads to an increased oxidative attack of the underlying bond coat material and therefore needs to be avoided. The lifetime prediction of thermal barrier coatings is therefore of interest to ensure safe operation within the inspection intervals. Several mechanisms have been identified to play a critical role in the degradation of TBC systems. Here we discuss failure of TBC systems due to bond coat aluminum depletion. This type of chemical failure may occur when the bond coat material is critically depleted of aluminum and instead of a dense slow growing α-alumina the formation of voluminous and fast growing spinels is promoted. Lifetime prediction for this failure mode requires a fundamental understanding of diffusion mechanisms and in particular the interaction of different diffusion rates in the bond coat and substrate material. Aim of this work was therefore to develop software tools that allow user friendly analysis of measured Al profiles for the assessment of diffusion rates and consequently for lifetime prediction.

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Oxidation behavior of TiAl₃ and L1₂ coated Ti-45at%Al-5at%Nb alloys at 1173K in air was investigated using mass gain measurement, field emission scanning electron microscopy, transmission electron microscopy, electron-probe microanalysis, glow discharge optical electron spectroscopy, and glancing angle X-ray diffraction analysis. The TiAl₃ formed meta-stable alumina θ-Al₂O₃, while the oxide scale on the L1₂ consisted of a duplex structure with an outer rutile TiO₂ and an inner α-Al₂O₃. During the very initial stages of oxidation both Ti and Al in the L1₂ coating could be oxidized, and then the faster diffusing Ti goes out to form an outer, continuous Ti-rich oxide which covers the slow growing Al₂O₃. In case of the TiAl₃ an outer, continuous TiO₂ layer was not observed because of the smaller amount of Ti in the TiAl₃. It could be concluded that the outer, Ti-rich oxide enhanced a phase transformation of Al₂O₃ from θ to α. To elucidate the Ti effect, Ti-vapor treated TiAl₃ and Ni-50at%Al were oxidized at 1173K in air, and showed formation of an α-Al₂O₃, in contrast to θ-Al₂O₃ on their bare alloys.

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Micro-cracking process in TBCs(Thermal Barrier Coatings) due to thermal exposure was investigated using SEM(Scanning Electron Microscope) and EBSD(Electron BackScatter Diffraction) observation. Splat particle morphologies in TBCs were clearly visualized by IPF (Inverse Pole Figure) mapping. The splat structure of top coat was consisted with large granular particle region and cluster of small columnar particle region. Cracks were observed along the interfaces of large granular particle region and the cluster of small columnar particle region, along the interfaces among large granular particles and across the cluster of small columnar particles transgranularly perpendicular to columnar crystal growth direction. The onset time of extensive macro crack formation was corresponding to the Cr oxide growth at TGO(Thermally Grown Oxide) to some extent. Thus EBSD observation can be used as a powerful tool for crystallographic observation of TBC, associated with SEM and optical microscope observation.

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Currently a new generation of relaunchable space transportation system using liquid hydrogen/ liquid oxygen rocket engines is under development. The inner combustion chamber is exposed to extreme thermal loads and environmental attack during starts. To prevent failure of the cooling channels, a thermal barrier coating to provide thermal and oxidation protection could be applied. Thermal barrier coatings are state of the art for gas turbines and this concept should be transferred to copper substrates in rocket engine applications. The thermomechanical loading conditions are quite different from the gas turbine applications as heat fluxes and temperature gradients are much higher while overall service time is much shorter. As a start for optimization of a suitable coating, a material system known for gas turbines is employed. In this work a thermal barrier coating system is applied by atmospheric plasma spraying to the copper-based high strength alloy Cu-1%Cr-0.3%Zr. The bond coat consists of a NiCrAlY alloy, while partially stabilized zirconia is used as a top coat. Spraying parameter optimization for the new substrate is described. The reached coating system is tested in thermal cycling experiments, where no failure of the coating could be detected. In oxidation experiments good environmental protection of the coating is shown.

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The effect of the bond coat on residual adhesion strength after thermal cycle fatigue was investigated in plasma-sprayed thermal barrier coatings (TBC). This study used CoNiCrAlY powder with two different particle sizes for spraying bond coat material to examine the effect of interface roughness between the bond coat and top coat. In addition, the bond coat was sprayed on either by a high velocity oxy-fuel (HVOF) or a low pressure plasma spray (LPPS). The residual adhesion strength of the TBC top coat was evaluated as a function of the number of thermal cycles by the modified 4-point bending test. In addition, SEM observations of thermal fatigue cracking morphologies and measurements of the residual stress in the ceramic top coat were carried out. The experimental results indicated that, after thermal cycle fatigue, microcracks were generated in the ceramic top coat; however, they were moderated in a rough interface TBC compared to a smooth interface TBC. In addition, the bond coat sprayed by the HVOF method showed a higher resistance to microcracking than the coat sprayed using the LPPS. Residual stress in the ceramic top coat is almost zero at 0 thermal cycles. After thermal cycle fatigue, it becomes compressional stress; however, it is independent of the bond coat. There was little difference in the adhesion strength by bond coat in as-sprayed conditions. On the other hand, the specimen with a rough interface exhibited higher residual adhesion strength after thermal cycle fatigue compared with the specimens with a relatively smooth interface. In addition, if the bond coat is sprayed by HVOF, the residual adhesion strength increases. It was revealed that the difference in residual adhesion strength by bond coat is related to the distribution morphology of thermal fatigue microcracks.

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The influence of the bond coat roughness on the life time of air plasma-sprayed (APS) thermal barrier coating systems (TBCs) was investigated under thermo-mechanical (TMF) load. The TBC system was applied on hollow cylindrical specimens made of the single crystal super alloy CMSX-4 in the orientation <001> with a MCrAlY-bond coat. Two different values of the bond coat roughness were investigated. In order to study the influence of the thicknesses of the thermally grown oxide layer (TGO), the specimens were isothermally oxidized at 1000 °C for a long term prior to the TMF experiments. The thermo-mechanical experiments show a higher number of cycles-to-failure for TBCs corresponding to an increase of the bond coat roughness. Furthermore, it could be demonstrated that a certain TGO thickness is needed to produce a total delamination of the top coat in the TMF experiments. This minimum thickness varies with the surface roughness of the bond coat and the TMF cycle's phase shift and strain range. Crack initiation and crack propagation were investigated by microscopical analyses, for example, SEM and EDX. Therefore most of the experiments were completed before a total delamination of the top coat occurred. On the basis of these investigations, crack initiation and crack propagation under thermo-mechanical load were described systematically.

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A fundamental study for an inelastic deformation of freestanding plasma-sprayed thermal barrier coatings (TBCs) has been conducted. Cantilever-type bending tests are carried out to obtain a stress-strain curve of the freestanding ceramic coating peeled from the TBC coated sample by an electrochemical treatment. In order to investigate about an inelastic deformation and its mechanism appeared in the sample, in-situ scanning electron microscope (SEM) observation is performed by means of a small tensile testing device that can be inserted into the SEM vacuum chamber. The bending test result indicated that the coating deforms with a nonlinear behavior under a monotonic loading and with a hysteresis loop under cyclic loading, in spite of that it is ceramic material. In-flight particle velocity in the spraying parameter affected the stress-strain curve significantly. In-situ SEM observation during the bending test revealed that sliding at boundary between splats plays an important role in an inelastic deformation.

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In order to investigate crack initiation sites and the crack propagation behavior in connection with the microstructure of thermal barrier coating (TBC) systems under creep and fatigue loadings, the failure behavior was observed in situ for plasma-sprayed TBC systems by optical microscopy, as a first step for elucidating the thermo-mechanical failure mechanism. Two types of TBC systems with differing top-coat (TC) microstructures were prepared by changing the processing conditions. The mechanical failure behavior of TBC system was found to depend strongly on the loading conditions. Under static creep loading, many segmentation cracks in the TC widened with increasing creep strain in the substrate. However, the propagation of these cracks into the bond-coat (BC) and alloy substrate was prevented due to the stress relief induced by plastic flow in the BC layer at elevated temperatures. As a result, the TBC system exhibited typical creep rupture behavior with nucleation and coalescence of microcracks in the alloy substrate interior regardless of the TC microstructure. Under dynamic fatigue loading, on the other hand, many fatigue cracks initiated not only from the tips of segmentation cracks in the TC layer but also from the TC/BC interface. Furthermore, it was found that the fatigue cracks propagated into the BC and alloy substrate even at elevated temperatures above the ductile-brittle transition temperature of the BC; the fatigue failure behavior under dynamic fatigue loading was dependent on the TC microstructure and the properties of the TC/BC interface.

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The local stress distributions in thermally grown oxide (TGO) layer of thermal barrier coating before and after thermal exposure were measured by photo-stimulated luminescence spectrum. The effect of isothermal oxidation on the residual stress in the TGO was investigated. It was found that the compressive stress in the TGO increased with an increase in thermal exposure time up to 100 hours and then decreased. The residual stresses in the TGO were also influenced by interfacial damage introduced by cyclic indentation.

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This paper introduces the brief summary of the recent collaboration test results on thermal barrier coatings (TBCs) in the JSMS Subcommittee, which covers the measurements of elastic modulus, tensile strength, residual stress and thermal conductivity of the TBC specimens. Here, the round-robin TBC specimens consisting of 8% yttria stabilized zirconia, CoNiCrAlY alloy bond coat and Ni-base superalloy were prepared by plasma spraying method. The isothermal exposure and the thermal cycles were applied to the TBC specimens by several conditions at high temperatures, to measure both the residual stress and the remaining adhesion strength of the ceramic top, as well as to characterize the thermally grown oxide at the top coat/bond coat interface. The round-robin test results clearly demonstrated that the adhesion strength was significantly changed by the application of thermal cycles and the isothermal exposure. Of particular important results was found in the remaining adhesion strength that were strongly dependent on the testing method to give the thermal cycles.

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Fracture strength of WC-12Co thermal sprayed coating is investigated experimentally and analytically. In the experiments, one pair of butt cylindrical specimen with coating is subjected to combined tension with torsion stresses. Fracture loci were obtained for three kinds of thickness of the coating in σ-τ stress plane. Stress distributions at crack tip singular point on fractured surfaces are analyzed by Finite-Element-Method and approximated by the expression σ=KR^-λ where R means normalized thickness coordinate. It is found that the normal stress distributions are common to all cases of testing stress conditions and so fracture condition of the brittle coating is represented as K≥Kcr in the normal stress distribution even under mixed deformation mode I and III. The delamination critical shear stress distribution could also be obtained.

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Young's modulus of thermal barrier coating (TBC) is one of the most essential mechanical properties on the designing of high performance TBC system. This paper describes one of the round-robin test results of New Energy and Industrial Technology Development Organization's (NEDO's) project titled “the survey-research of standardization on testing methods for thermo-mechanical performance of ceramic thermal barrier coatings”, for the fiscal years of 2006 and 2007. The bending method of the coating bonded to substrate is straightforward in preparing the specimen and loading to the specimen. Previous study has offered recommended testing methods and proper geometries of the specimens for the Young's modulus of thermal barrier coating and the bond coating. This paper confirmed that the recommended method and the geometry of the specimen provided the reasonable estimation as far as the other type of substrate material. It was also confirmed that the ratio of coating thickness to substrate thickness should be high and the threshold ratio was independent of the substrate material.

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Understanding of Young’s modulus of Thermal Barrier Coatings (TBCs) top coat is one of the important factors about improvement reliability of TBCs that is key technology of increasing thermal efficiency of gas turbine. Some measurement procedures have been proposed, but not established yet. Indentation test evaluated only hardness value before, but recently it has developed to be able to evaluate other mechanical properties such as yield strength, Young’s modulus, etc. From such background, application of indentation test for measurement of Young’s modulus of TBCs top coat is effective means. Although pyramidal indenter and calculation method regulated by ISO14577 is usually selected for measurement of Young’s modulus of TBCs top coat, author have proposed spherical indenter and calculation method based on elastic contact theory by Hertz. In this study, influence of different correcting specimens on measurement Young's modulus of TBCs top coat was discussed and it was concluded that the results of HMV500 hardness standard specimen show high uniformity on measured Young’s modulus of TBCs top coat (i.e. less load dependency).

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Thermal spraying is widely used in various industrial fields as effective surface modification method. In this study, mechanical properties of thermal sprayed CoNiCrAlY alloy coating were investigated. The coatings were prepared by three kinds of thermal spray methods. High-temperature exposure of the coated specimens was carried out in the atmosphere. After high-temperature exposure, apparent Young's modulus, strain tolerance, residual stress and hardness of the CoNiCrAlY coating were measured. These tests were also carried out using specimens without high-temperature exposure. As a result, the changes of mechanical properties caused by high-temperature exposure seemed to be correlated with properties of boundary between sprayed particles. From the results, effects of high-temperature exposure on mechanical properties of thermal sprayed CoNiCrAlY alloy coating were discussed.

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The hybrid effect on the orientation dependence of the composite strength of an aluminum cast alloy reinforced by Al₂O₃ whiskers and SiC particles is studied experimentally and numerically. Two types of specimens are prepared for monotonic bending tests. The longitudinal specimen orientation (maximum stress direction) is parallel to or normal to randomly oriented whiskers in plane. The monotonic strength is 18% higher when the hybrid metal matrix composite (MMC) is subjected to an external load parallel to the random whisker orientation in plane than when the load is perpendicular to the whisker orientation. The whisker orientation dependence of composite strength in hybrid composite is weaker than that in whisker-reinforced composite. On the fracture surface of the specimen loaded along the direction parallel to the random whisker orientation in plane, most whiskers are broken while many de-bonded interfaces between the whiskers and matrix are observed on the fracture surface of the specimen loaded along the direction perpendicular to the whisker orientation. To characterize the hybrid effect on the whisker orientation dependence of composite strength, a three-dimensional hybrid composite unit cell model including one whisker and a few particles under a periodic boundary condition is developed using the finite element method. The hybrid composites have higher whisker stress than whisker-reinforced composite when subjected to an external load parallel to the whisker orientation if these composites have the same total volume fraction of reinforcement and the particles are distributed randomly. Under an external load perpendicular to the whisker orientation, the interface stress of hybrid composites is lower than that of whisker-reinforced composite. As a result, the strength difference for parallel and perpendicular loading conditions of the hybrid composites is smaller than that of whisker-reinforced composite. Thus, the weak whisker orientation effect in the hybrid composites is due to a change in microscopic stress distribution induced by interaction between whiskers and particles.

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In this study, it is investigated experimentally and analytically that fracture process and fatigue behavior of Thermal Barrier Coatings (TBCs) used in a high temperature component in a land based gas turbine. The effects of the thermal load on TBCs are investigated by measuring residual stress of top coat, finite element method and torsion test. It is found that delamination of top coat under thermal load occurs at a peak of undulation in top coat and bond coat interface. Around the peak, a tearing stress is generated. Fracture and fatigue tests under torsion are carried out by using cylindrical butted specimens. It is found that the mechanical property of bond coat changes after thermal aging and the shear fatigue behavior of TBCs is affected by bond coat strength.

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Anode-supported solid oxide fuel cells (SOFCs) have attracted extensive attention due to low operating temperatures and high electrical performance. In the anodesupported SOFCs, the key is to prepare dense electrolyte films on porous anode substrates. In this study, a simple and cost-effective technique, spray coating process, is introduced to prepare dense electrolyte films. In the spray coating process, the preparation of powder suspension is very important, which affects the quality of resulting electrolyte films. A well-dispersed Ce_0.9Gd_0.1O_1.95 (GDC) suspension were prepared in isopropyl alcohol by adding 2 mass% surfactant. Using the well-dispersed GDC suspension, dense and crack-free GDC electrolyte films with thickness of 2-10 µm were successfully prepared on porous NiO-GDC anode substrates by the spray coating process. In order to evaluate the GDC electrolyte film as electrolyte for SOFC, anode-supported SOFC was fabricated and its electrical performance was measured. Results show that the GDC electrolyte film prepared by the spray coating process can be used as electrolyte of SOFC. The GDC electrolyte film is well adhered to the anode substrate, and there is no crack and delamination observed after electrical performance test.

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The utilization of anode-supported electrolytes is a very promising strategy to improve the electrical performance in solid oxide fuel cells (SOFCs) application, because it is possible to decrease considerably the electrolytes thickness. In this paper, ultra-thin ceria-based electrolyte films were successfully prepared on porous NiO/GDC anode support. The electrolyte films with thickness of 0.5-1 µm were deposited by a novel citrate sol-gel route combined with a suspension spray coating technique. The characterization and microstructure of the ultra-thin films were investigated by DTA/TGA, XRD and FE-SEM. The results showed that ceria-based films prepared were pure fluorite type nanocrystalline, homogenous and almost fully dense. Electrochemical performance of single cells based on the ultra-thin electrolyte films was also tested. The single cell with electrolyte thickness of 1 µm provided an OCV of 0.832 V at 500 °C which was close to that of the reported single cell with thicker ceria-based electrolyte film of 10 µm, and maximum power densities of 59.6, 121.9 and 133.8 mW/cm² at 500, 600, and 700 °C, respectively. These ultra-thin electrolyte films showed good combination with the porous NiO/GDC anode supports, and good insulating ability for inactive electron migration at temperatures less than 600 °C.

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A new testing method to evaluate adhesion strength of ceramic top coat has been proposed, employing a ring shape of TBC specimen specifically designed. It was shown by the experiments that a delamination behavior of the top coat was successfully reproduced in the proposed method, associating with a buckling mode; a similar mode frequently observed in actual gas turbine components. A method to quantitatively evaluate a resistance to delamination was also introduced, based on an energy release rate criterion. The experiments demonstrated that the testing method provided reasonable adhesion strength in terms of energy criterion, that almost agreed with the values measured by other researchers employing different type of testing method. It was also shown that the present method has many advantages, compared with the traditional methods.

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