Evaluation of microstructural evolution in thermal barrier coatings during thermal cycling using impedance spectroscopy

Air plasma sprayed thermal barrier coatings are thermally cycled in air up to 1030 °C and evaluated using impedance spectroscopy in conjunction with scanning electron microscopy. When the number of cycles is less than 15, impedance measurements cannot be used to detect the thermally grown oxide (TGO, usually alumina) scale because it does not fully cover the top coat-bond coat interface and the YSZ (yttria-stabilised zirconia used as the top coat) is much more electrically conductive than the alumina, leading to most of the current passing through the YSZ rather than the alumina. After the specimens are subjected to 100 until 250 cycles, impedance measurements show that a continuous alumina scale is formed. In the impedance spectra, there are four relaxation processes, which correspond to the YSZ grains, the YSZ grain boundaries, the TGO, and the electrode effect. Impedance analyses demonstrate that the resistance of the alumina scale increases and the capacitance decreases with increasing cycling. When the specimen is subjected to 400 cycles, the impedance response to the continuous TGO vanishes due to the TGO degradation.     

Studies of the sintering kinetics of thick thermal barrier coatings by thermal diffusivity measurements

Better thermal insulation of the hot path components is needed in state-of-the-art gas turbines and diesel engines, because of the increasing demands of the higher process temperatures. In these processes, thermal barrier coatings (TBCs) and various cooling techniques mainly control the component surface temperatures. For this reason low thermal conductivity of the TBCs are extensively studied. One of the main factors determining the TBC thermal conductivity is the coating microstructure, with specific attention to the porosity content, as well as to its morphology and orientation. One important feature of TBCs is the stability of their thermal properties as a function of time at service conditions. In fact the prolonged exposure to high temperature can promote shrinkage phenomena within the TBC, which make the coating less strain tolerant and more heat conductive. This leads to a drastic reduction of the functional effectiveness of this ceramic protective top layer.In order to study the evolution of thermal properties of TBC, as a function of time and temperature, thermal diffusivity evaluation by laser flash method has been performed. The measurements have been performed on freestanding yttria-stabilized zirconium oxide (YPSZ) TBCs. In particular, measurements have been carried out at five different temperatures in the range 900–1300 °C, for different ageing times (from 1 up to 150 h). The data show a significant increase of the thermal diffusivity also after exposures of few hours, especially at the highest testing temperatures. Microstructural analysis carried out by optical and electron microscopy clearly showed that the observed thermal diffusivity variations can be ascribed to sub-micrometric crack healing and sintering neck formation. Mechanical testing confirmed the microhardness increase of TBC as well. Finally the data have been summarised in order to experimentally define a “functional life” curve of the TBC, as a function of ageing time and temperature.     

Tailorable shielded compliance of thermal barrier coatings through laser texturing and microstructural modification: microfeature design and validation

Thermal barrier coatings (TBCs) are widely used for protecting components from an aggressive environment and excessive temperatures. However, the top coat can lack thermal compliance and environmental resistance, promoting premature failure. This study provides an approach to design, produce, and evaluate the novel shielded-compliant TBCs to address these challenges. Top coat laser grooving can lower thermal mismatch levels and hinder crack propagation, whereas laser glazing can enhance strength and anti-corrosion performance. Material analysis for the glazed layer revealed large columnar grains alongside a stress-free structure compared to the as-sprayed coating. The digital image correlation (DIC) in-situ heating test showed vertical cracks after glazing already to be effective stress relief features, with the controlled grooves showing the most prominent compliance, as suggested by finite element analysis (FEA). Such a concept may be applied to any coating where increased thermal compliance or environmental protection is needed, making it a highly versatile tool.     

Electrophoretic fabrication of thermal barrier coatings

An electrochemical method of fabrication of (NiCoCrAlY)/MgO/yttria-stabilized zirconia (YSZ) multilayered coating was proposed. This multilayered coat is expected to work as a thermal barrier coating for nickel superalloy substrates. The (NiCoCrAlY) layer was deposited using the electrophoretic deposition technique, the MgO layer was deposited by the electrolytic deposition technique and the YSZ layer was electrophoretically deposited. The process of depositing (NiCoCrAlY) alloy particles revealed that the electrophoretic technique can be used for particles with submicron dimensions. The MgO intermediate layer was introduced to accommodate the difference in thermal expansion coefficient between the YSZ ceramic and the NiCoCrAlY metal layers.     

Atmospheric plasma sprayed 7%-YSZ thick thermal barrier coatings with controlled segmentation crack densities and its thermal cycling behavior

Yttria-stabilized zirconia (YSZ)-coatings are deposited on Ni-based superalloy IN738 by atmospheric plasma spraying (APS). For the first time, controlled segmentation crack densities are manually developed in the coatings, even after the APS deposition. This method allows to user to control segmentation densities as well as cracks depth, which could be designed as per coating thickness and required application. Thermal cycling test shows promising strain tolerance behavior for the segmented coatings, whereas coating without segmentation could not sustain even for its first thermal cycle period. Further, microstructural studies reveal that a very thin layer of TGO was formed and obvious no coating failure or spallation was observed after thermal cycling test at 1150 °C for 500 cycles.     

Comparative study on thermal shock behavior of thick thermal barrier coatings fabricated with nano-based YSZ suspension and agglomerated particles

Two kinds of segmentation-crack structured YSZ thick thermal barrier coatings (TTBCs) were deposited by suspension plasma spraying (SPS) and atmospheric plasma spraying (APS) with nano-based suspension and agglomerated particles, respectively. The phase composition, microstructure evolution and failure behavior of both TBCs before and after thermal shock tests were systematically investigated. Microstructure of the APS coating exhibits typical segmentation-crack structure in the through-thickness direction, similar with the SPS coating. The densities of segmentation-crack in APS and SPS coatings were about 3 cracks mm⁻¹ and 4 cracks mm⁻¹_, respectively. The microstructure observation also showed that the columnar and equiaxed grains existed in the SPS coating. As for the thermal shock test, the spallation life of the APS TTBCs was 146 cycles, close to that of the SPS TTBCs (166 cycles). Failure of the APS coating is due to the spallation of fringe segments and splats.     

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

The effect of doping on the structural, morphological and thermal properties of ZrO₂-XO_1.5 (X = Y, La, Sm, Er) solid solutions for thermal barrier (TBC) applications was investigated. Oxide powders of various compositions from 9.7 to 40 mol% XO_1.5 (X = Y, La, Sm, Er) were synthesised by the sol-gel route. The structural analysis of the powders was performed using X-ray diffraction analysis coupled with Rietveld refinements and the measurement of their specific surface area with the BET method. For each rare earth dopant, the morphology of the powders varies from monoliths to agglomerates of thinner particles when the doping amount increases. In order to determine the specific heat, the thermal diffusivity at room temperature and the thermal expansion coefficient of some selected compositions, DSC, laser thermal diffusivity and high-temperature dilatometry measurements were performed on samples densified by Spark Plasma Sintering. Working thermal characterisation indicated that zirconia doped with 30 mol% SmO_1.5 and ErO_1.5 have better insulation properties and a lower thermal expansion coefficient than our reference YSZ ceramic. These various compositions are very promising for the elaboration of multilayer TBCs by the sol-gel process.     

Microstructural characterization of GZ/CYSZ thermal barrier coatings after thermal shock and CMAS+hot corrosion test

In this study, first, Gd₂Zr₂O₇/ceria–yttria stabilized zirconia (GZ/CYSZ) TBCs having multilayered and functionally graded designs were subjected to thermal shock (TS) test. The GZ/CYSZ functionally graded coatings displayed better thermal shock resistance than multilayered and single layered Gd₂Zr₂O₇ coatings. Second, single layered YSZ and functionally graded eight layered GZ/CYSZ coating (FG8) having superior TS life time were selected for CMAS + hot corrosion test. CMAS + hot corrosion tests were carried out in the same experiment at once. Furthermore, to generate a thermal gradient, specimens were cooled from the back surface of the substrate while heating from the top surface of the TBC by a CO₂ laser beam. Microstructural characterizations showed that the reaction products were penetrated locally inside of the YSZ. On the other hand, a reaction layer having ∼6 μm thickness between CMAS and Gd₂Zr₂O₇ was seen. This reaction layer inhibited to further penetration of the reaction products inside of the FG8.     

Research progress on hafnium-based thermal barrier coatings materials

Thermal barrier coatings (TBC) are important materials applied to hot part components of aero-engines in order to improve their service temperature. Increasing inlet temperature is an important factor to achieve elevated thrust-to-weight ratio and high heat engine efficiency. In recent years, traditional TBC materials have gradually reached their operating limits due to the increase in turbine operating temperature. Hafnium-based materials become promising new candidates for TBC because of the similar structure, higher temperature phase stability and lower thermal conductivity compared to traditional zirconium-based materials. In this review, recent progresses in the research and development for hafnium-based TBC materials are summarized. The phase stability, thermal and mechanical properties of rare-earth (RE)-doped HfO₂ and RE hafnate materials are introduced. RE-doped HfO₂ has good thermal properties and phase stability at high temperatures whereas relatively low fracture toughness. The RE hafnates possess the advantages of a higher phase transition temperature, lower thermal conductivity and superior fracture toughness than RE zirconates. However, the thermal expansion coefficients of most RE hafnates are quite different from the alloy matrix. Finally, further research directions for hafnium-based TBC materials are prospected in this study.     

Stress-dependent sintering behavior of porous thermal barrier coatings

Thermal barrier coatings (TBCs) are subjected to high temperature and complex stress fields during service in gas turbines. In this process, densification and hardening take place as the result of sintering, which is sensitive to boundary condition/external load. The stress-dependent sintering behaviors of porous TBCs were investigated in this work using a customized four-point bending method. Furthermore, stress-dependent sintering model was developed and implemented in finite element analysis to elucidate sintering mechanisms. It was found that stress gradient induced nonlinear differential sintering behavior, due to the accelerating and retarding effects of compressive and tensile stresses, respectively. In addition, microstructure-mechanical property relation was determined following the exponential law and high-throughput method was proposed for the characterization of stress dependence. The in-depth understanding of stress-dependent sintering behavior could provide guidance to the design and failure analysis of TBCs applied on complex shaped components in the hot section of gas turbines.     

Steam oxidation and microstructural evolution of rare earth silicate environmental barrier coatings

A primary failure mode for environmental barrier coatings (EBCs) on SiC ceramic matrix composites (CMCs) is the oxidation of the intermediate Si-bond coating, where the formation of SiO₂ at the bond coating–EBC interface results in debonding and spallation. This work compares the microstructure evolution and steam oxidation kinetics of the Si-bond coating beneath yttrium/ytterbium disilicate ((Y/Yb)DS) and ytterbium disilicate/monosilicate (YbDS/YbMS) EBCs to better understand the impact of EBC composition on oxidation kinetics. After 500 1-h cycles at 1350°C, (Y/Yb)DS displayed a decreasing concentration of the monosilicate minor phase and increasing concentration of porosity as furnace cycling time increased, whereas the YbDS/YbMS EBC displayed negligible microstructural evolution. For both EBC systems, thermally grown oxide growth rates in steam were found to increase by approximately an order magnitude compared to dry air oxidation. The (Y/Yb)DS EBC displayed a reduced steam oxidation rate compared to YbDS/YbMS.     

Lanthanum cerate thermal barrier coatings generated from thermal plasma synthesized powders

Lanthanum cerate (La₂Ce₂O₇, LC) is one of the promising advanced thermal barrier coating (TBC) materials due to its high melting point, no phase transformation between room temperature and operating temperature, low thermal conductivity, comparable coefficient of thermal expansion (CTE) with metallic substrate. The present study investigates plasma transferred arc synthesis of LC powder, its subsequent spheroidization in a thermal plasma jet and plasma spray deposition. The PTA-synthesized LC powder, spheroidized as well as the plasma sprayed coatings was found to possess excellent phase stability; the single phase cubic fluorite structure of LC was found to be retained even after prolonged arc-melting, corroborating that the material was stable from room temperature up to its melting point. It was observed that PTA melting for longer duration resulted in small deviation from stoichiometry, although the phase structure of LC was retained. Spheroidization efficiency was found to increase with the input power of the torch. Very good adherent LC coatings could be deposited on nickel super alloy with reasonably good deposition efficiency.     

Characterization of multi-scale synergistic toughened nanostructured YSZ thermal barrier coatings: From feedstocks to coatings

The nanostructured 8YSZ thermal barrier coatings were deposited by atmospheric plasma spraying onto K417 G nickel-based superalloy with high velocity oxygen fuel sprayed NiCoCrAlYCe bond-coat using as-prepared nanostructured t´-Zr_0.9Y_0.1O_1.95 feedstocks for the first time. The microstructure and mechanical properties of nanostructured and conventional 8YSZ coatings were comparatively investigated systematically. The results revealed that both coatings were composed of t´-Zr_0.9Y_0.1O_1.95 phase and the formation mechanism of t´ phase was elucidated. The nanostructured 8YSZ coatings demonstrated typical bi-modal microstructure, whereas the conventional 8YSZ coatings exhibited mono-modal microstructure. Furthermore, the bi-modal microstructure of nanostructured 8YSZ coatings was analysed by elastic modulus and nanohardness Weibull distribution plots. The high and low slopes in Weibull distribution plots corresponded to unmelted and melted regions of nanostructured 8YSZ coatings, respectively. The fracture toughness and bonding strength of nanostructured coatings were higher than that of conventional 8YSZ coatings. Finally the reasons were explained in detail.     

High-temperature interface stability of tri-layer thermal environmental barrier coatings

Thermal environmental barrier coatings (TEBCs) are capable of protecting ceramic matrix composites (CMCs) from hot gas and steam. In this paper, a tri-layer TEBC consisting of 16 mol% YO_1.5 stabilized HfO₂ (YSH16) as thermal barrier coating, ytterbium monosilicate (YbMS) as environmental barrier coating, and silicon as the bond coating was designed. Microstructure evolution, interface stability, and oxidation behavior of the tri-layer TEBC at 1300 °C were studied. The as-sprayed YSH16 coating was mainly comprised of cubic phase and ～3.4 vol% of monoclinic (M) phase. After 100 h of heat exposure, the volume fraction of the M phase increased to ～27%. The YSH16/YbMS interface was proved to be very stable because only slight diffusion of Yb to YSH16 was observed even after thermal exposure at 1300 °C for 100 h. At the YbMS/Si interface, a reaction zone including a Yb₂Si₂O₇ layer and a SiO₂ layer was generated. The SiO₂ grew at a rate of ～0.039 μm²/h in the first 10 h and a reduced rate of 0.014 μm²/h in the subsequent exposure.     

Strain-induced multiscale structural changes in lamellar thermal barrier coatings

The thermal mismatch stress, as well as residual stress, in coating/substrate systems often leads to structural changes and subsequent coating debonding in the systems. This study focused on the changes induced in the microstructure and properties of lamellar yttria-stabilized zirconia coatings upon heating, with the aim of elucidating their starting microstructure prior to sintering. The results showed that the combined effect of the residual stress and the thermal mismatch stress results in scale-sensitive changes in the properties of the coatings. The macroscale properties changed significantly, while the microscale properties changed only slightly. Structural characterization revealed that a certain degree of expansion at the tips of both the intersplat pores and the intrasplat cracks occurs, contributing to the microscale structural changes observed in most regions. Moreover, a few mesoscale cracks covering several layers were also observed. A lamellar structural model was developed to correlate the multiscale structural changes observed with those in the properties. Finally, this study revealed that the actual starting structure of plasma-sprayed thermal barrier coatings prior to sintering is different from that in the as-deposited state. This should aid in obtaining an in-depth understanding on the microstructural and properties evolution of the constrained coatings under actual service conditions.     

Repressing high-temperature radiative heat transfer in thermal barrier coatings

Photon diffusion in thermal barrier coatings (TBCs) significantly deteriorates the overall performance of gas turbines operating at high temperatures. This study presents the strategy of high-temperature photon suppression, based on a ceramic composite consisting of the second component with a smaller refractive index and controlled particle size. Using the Mie theory, it is theoretically demonstrated that controlling the second component particle size closer/equal to the infrared radiation wavelength region (1–5 μm) could reduce photon diffusion. Ceramic composites comprised of 8 wt.% yttria-stabilized zirconia (8YSZ, matrix) and corundum (second component) with different particle sizes were prepared. The total and the photon thermal conductivity of the 8YSZ/corundum composites are lower than pure 8YSZ by ∼48.9% and ∼96.4% at 1200°C, respectively. With the addition of corundum into 8YSZ, the thermal radiation transport of 8YSZ is significantly suppressed due to the photon scattering produced by the lower refractive index and proper particle size of the corundum. Besides, the fracture toughness and hardness of composites increased by ∼20% and ∼13%, respectively, compared to the 8YSZ. Composite with the corundum particles size of 1 μm displays the lowest values of total and photon thermal conductivity at high temperature.     

Crack propagation studies of thermal barrier coatings under bending

The trends recently observed in crack propagation studies under bending for thermal barrier coatings (TBCs) in power plant application are highlighted in this paper. These studies described were performed with plasma sprayed zirconia bonded by a MCrAlY layer to Ni-base superalloy. Such thermal barrier composites are currently considered as candidate materials for advanced stationary gas turbine components. The crack propagation behaviour of the ceramic thermal barrier coatings (TBCs) at room temperature, in as received and oxidized conditions reveals that cracks grow linearly in the TBC with increase in bending load until about the yield point of the superalloy is reached. Approaching the interface between the ceramic layer and the bond coat, a high threshold load is required to propagate the crack further into the bond coat. Once the threshold is surpassed, the crack grows rapidly into the brittle bond coat without an appreciable increase in the load. At a temperature of 800°C, the crack is found to propagate only in the TBC (ceramic layer), as the ductile bond coat offers an attractive sink for stress relaxation. Effects of bond coat oxidation on crack propagation in the interface regime have been examined and are discussed. ©     

Mechanical properties of lanthanum zirconate-based composite thermal barrier coatings

Lanthanum zirconate is a promising candidate material for thermal barrier coating (TBC) applications due to its low thermal conductivity and high temperature phase stability. However, its application is limited by thermal durability caused by low fracture toughness and low coefficient of thermal expansion. We recently developed LZ/8YSZ composite TBC systems using blended LZ and 8YSZ powders, which have demonstrated excellent thermal cycling performance. In this study, the mechanical properties of the composite TBCs were characterised using both nanoindentation and Vicker’s microhardness tests. The nanoindentation results show that both Young’s modulus and nanohardness increase with increasing 8YSZ content, suggesting the mechanical properties can be tailored by changing the volume ratio of 8YSZ. The ratios of Young’s modulus to nanohardness remain constant, ～18, irrespective to the coating’s composition. The microhardness results show the same dependence with 8YSZ content, which is confirmed by the analytic models based on composite theory.     

Microstructure and thermal shock resistance of AlBOw- and BNw-whisker-modified thermal barrier coatings

To improve the crack propagation resistance of YSZ thermal barrier coatings during the thermal cycle, three kinds of thermal barrier coatings were prepared by atmospheric plasma spraying: YSZ, AlBOw-modified YSZ and BNW-modified YSZ. SEM, EDS and XRD were used to analyse the morphology, composition and phase composition of the sprayed powder and coating section. The phase structures of the YSZ, YSZ+AlBOw and YSZ+BNw coatings were t' phase. The cross-section of the coating presents a layered structure with pores inside. The porosity values of the YSZ, YSZ+AlBOw and YSZ+BNw coatings are 10.33%, 14.17% and 12.52%, respectively. The thermal shock resistance of three groups of coatings after 5 min at 1000 °C was analysed. The failure behaviour of the coatings after several thermal cycles was studied. The results show that the thermal shock resistance of the coatings with AlBOw is slightly lower than that of the YSZ coatings. The thermal shock resistance of the BNw coatings is 62.2% higher than that of the YSZ coatings. The whisker inhibits the crack propagation and prolongs the life of the coatings via crack deflection, whisker pull-out and whisker bridging.     

YAG thermal barrier coatings deposited by suspension and solution precursor thermal spray

Yttrium aluminium garnet (YAG) is a promising topcoat material for thermal barrier coatings due to its high temperature stability and better CMAS (calcium-magnesium-alumino-silicate) resistance. YAG topcoats were deposited by suspension and solution precursor high-velocity oxy-fuel (HVOF) thermal spray. The relationships between processing, microstructure and final properties were studied through a range of characterization techniques and thermal cycling tests. The microstructure of the as-sprayed YAG topcoat from stoichiometric solution precursor (SP-YAG) had distributed pores and inter-splat boundaries, while the as-sprayed topcoat produced from suspension (S-YAG) had vertical and branched micro cracks, pores, and inter-splat boundaries. Both as-sprayed coatings were composed of amorphous phase, hexagonal yttrium aluminium perovskite (YAP) and cubic YAG. In thermal cycling tests, 20% of SP-YAG failure was reached after the 10th cycle; whereas, S-YAG reached the failure criteria between the 60th and 70th cycle. The failure of both the SP-YAG and the S-YAG topcoats occurred due to thermal stresses during the thermal cycling.