Interfacial stability and contact damage resistance by incorporating buffer layer in thermal barrier coatings

A buffer layer was introduced between the bond and top coats in air-plasma sprayed (APS) zirconia (ZrO₂)-based thermal barrier coating (TBC) system, to improve contact damage and interfacial stability. The microstructure is relatively continuous in the TBC system with the buffer layer, showing a step like distribution of Zr element between the top and bond coats. The TBC system with the buffer layer shows less strain than that without the buffer layer in the higher stress regions above about 1.3 GPa, while both TBC systems become soft by forming the top coat in the lower stress regions compared with the substrate. The TBC system with the buffer layer shows the lower stress–strain curves than that without the buffer layer in the thermal exposure with the relatively short dwell time of 1 h, showing the reverse trend with the relatively long dwell time of 10 h. Subsurface damage in the substrate is reduced at both indentation loads of P = 500 N and P = 2000 N by incorporating the buffer layer, independent of thermal exposure condition. The damage zone formed in the TBC system without the buffer layer increases with increasing exposure time, while the damage does not extend far in the case of the TBC system with the buffer layer. In fracture under contact environments, cracking or delamination is developed between the top coat and the buffer layer in the TBC system with the buffer layer, whereas the fracture is created at the interface of the bond coat and the substrate. The buffer layer is more efficient in protecting the substrate from contact environments and enhances the damage resistance of the TBC system.     

Effect of neck formation on the sintering of air-plasma-sprayed thermal barrier coating system

Sintering neck is a featured microstructure that may have significant effect on the sintering behaviour of air-plasma-sprayed thermal barrier coating system (APS TBCs). Based on experimental observations, a multi-necking wedge-shaped model for the sintering of APS TBCs was proposed by considering the sintering stress as surface tension and by employing the thermal-elasto-viscoplastic constitutive relation. Deformation pattern, stress distribution, sintering induced shrinkage, stiffening behaviour and temperature field were analysed by using finite element method. It is shown that the formation of sintering neck significantly affects thermal and mechanical properties related to sintering. Mechanisms of thermal and mechanical degradation induced by sintering were further elucidated.     

Thermal properties of plasma-sprayed thermal barrier coating with bimodal structure

Nanostructured zirconia coatings have been prepared by atmospherical plasma spraying (APS) on NiCrAlY-coated superalloy substrates. The isothermal oxidation test results indicate that the oxidation kinetics of nanostructured TBC follows a parabolic law and the oxidation resistance of the nanostructured TBC is comparable to that of the conventional TBC. The nanostructured thermal barrier coatings exhibit excellent thermal cyclic resistance and low thermal diffusivity. The failure of the nanostructured TBC occurs within the top coat and close to the YSZ/thermal growth oxide interface. The thermal diffusivity of the coating is 90% of that of conventional thermal barrier coatings, and it increases after heat treatment at 1050 °C for 34 h. The increase in the thermal diffusivity of the coating is ascribed to grain growth, the crack healing as well as sintering neck formation.     

Thermal failure of thermal barrier coating with thermal sprayed bond coating on titanium alloy

A thermal spray technology high-velocity oxygen fuel (HVOF) was used to deposit NiCoCrAlY as a bond coating between the titanium alloy substrate and top 8 wt% yttria-stabilized zirconia thermal-barrier coating (TBC) deposited by electron beam-physical vapor deposition (EB-PVD). The thermal cycling and isothermal exposure tests were conducted to evaluate the durability of the TBC. Investigations using OM, SEM, EPMA, and XRD revealed that the thermal-sprayed BC makes the TBC more durable in isothermal exposure tests but more short-lived in thermal cycling tests, in comparison to our previous study in which the BC was prepared by EB-PVD. This is because the thermal-sprayed imperfections, such as microcracks and voids, elevate the diffusion resistance and degrade the mechanical properties of the BC, simultaneously. To current TBC systems in which the BC is deposited by HVOF, thermal failure behaviors—such as the formation of the Ti/Al mixture oxides at some individual places in the BC, and the Ti₂Ni gaps formed around the BC/substrate interface—were also discussed.     

Enhanced properties of Al-modified EB-PVD 7YSZ thermal barrier coatings

7 wt% yttria-stabilized zirconia (7YSZ) thermal barrier coating (TBC) prepared by electron beam-physical vapor deposition (EB-PVD) has been used in gas turbine engines for many years, where the TBC must successfully withstands the damage caused by a variety of environmental and mechanical aspects. The primary failure modes for TBC are oxidation of bond coating, particle erosion and CMAS (calcium-magnesium-alumina-silicates) corrosion. The lifetime of TBC associated with above three failure factors will be reduced significantly. In order to prolong the operation time, an alternative approach depositing Al film on 7YSZ TBC surface by magnetron sputtering is proposed. An α-Al₂O₃ overlay was in-situ synthesized on each 7YSZ column through reaction of Al and ZrO₂ during vacuum heat treatment. And the results indicate that the Al-modified EB-PVD 7YSZ TBC shows better oxidation resistance, as well as lower particulate erosion and CMAS corrosion.     

Comparison of microstructure and mechanical properties of plasma-sprayed nanostructured and conventional yttria stabilized zirconia thermal barrier coatings

The main goal of this paper was to evaluate and compare the microstructure and mechanical properties of plasma-sprayed nanostructured and conventional yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). To this end, NiCrAlY bond coat, nanostructured, and conventional YSZ coatings were deposited on Inconel 738LC substrate by atmospheric plasma spraying (APS). The mechanical properties of the coating were evaluated using nanoindentation and bonding strength tests. The microstructure and phase composition of the coating were characterized by field emission scanning electron microscopy (FESEM) and X-ray diffractometry (XRD). The nanostructured YSZ coating contained both nanosized particles retained from the powder and microcolumnar grains formed through the resolidification of the molten part of the powder, whereas the microstructure of the conventional YSZ coating consisted of columnar grain splats only. The phase composition of the as-sprayed nanostructured coating consisted of the non-transformable tetragonal phase, while the conventional coating showed the presence of both the monoclinic and non-transformable tetragonal phases. The results of nanoindentation and bonding strength tests indicated that the mechanical properties of the nanostructured coating were better than those of the conventional coating.     

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.     

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.     

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.     

Phase stability,thermo-physical properties and thermal cycling behavior of plasma-sprayed CTZ,CTZ/YSZ thermal barrier coatings

ZrO₂ co-stabilized by CeO₂ and TiO₂ with stable, nontransformable tetragonal phase has attracted much attention as a potential material for thermal barrier coatings (TBCs) applied at temperatures >?1200?°C. In this study, ZrO₂ co-stabilized by 15?mol% CeO₂ and 5?mol% TiO₂ (CTZ) and CTZ/YSZ (zirconia stabilized by 7.4?wt% Y₂O₃) double-ceramic-layer TBCs were respectively deposited by atmospheric plasma spraying. The microstructures, phase stability and thermo-physical properties of the CTZ coating were examined using scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric-differential scanning calorimeter (TG-DSC), laser pulses and dilatometry. Results showed that the CTZ coating with single tetragonal phase was more stable than the YSZ coating during isothermal heat-treatment at 1300?°C. The CTZ coating had a lower thermal conductivity than that of YSZ coating, decreasing from 0.89?W?m^?1 K^?1 to 0.76?W?m^?1 K^?1 with increasing temperature from room temperature to 1000?°C. The thermal expansion coefficients were in the range of 8.98?×?10^?6 K^?1 – 9.88 ×10^?6 K^?1. Samples were also thermally cycled at 1000?°C and 1100?°C. Failure of the TBCs was mainly a result of the thermal expansion mismatch between CTZ coating and superallloy substrate, the severe coating sintering and the reduction-oxidation of cerium oxide. The thermal durability of the TBCs at 1000?°C can be effectively enhanced by using a YSZ buffer layer, while the thermal cycling life of CTZ/YSZ double-ceramic-layer TBCs at 1100?°C was still unsatisfying. The thermal shock resistance of the CTZ coating should be improved; otherwise the promising properties of CTZ could not be transferred to a well-functioning coating.     

Effect of material properties on residual stress distribution in thermal barrier coatings

Residual stress has a significant influence on the crack nucleation and propagation in thermal barrier coatings (TBC) system. In this work, the residual stress in the air plasma spraying (APS) TBC system during cooling process was numerically studied, and the influence of the material properties of each layer on the residual stress was investigated. The morphologies of the interface were described by a piecewise cosine function, and the amplitude for each segment gradually increases. The elasticity, plasticity and creep of top coat (TC), thermally grown oxide (TGO) layer and bond coat (BC) were considered and the elasticity and creep of the substrate layer were taken into account. The material properties of all layers vary with temperature. The results show that the material properties have complex influence on the residual stress during cooling. The effect of the material properties of TC and BC on the residual stress at the interface is relatively large, and that of TGO and substrate is relatively small. These results provide important insight into the failure mechanism of air plasma spraying thermal barrier coatings, and important guidance for the optimization of thermal barrier coating interfaces.     

Effect of TGO thickness on the thermal barrier coatings life under thermal shock and thermal cycle loading

Effect of thermally grown oxide (TGO) thickness on thermal shock resistance of thermal barrier coatings (TBCs) and also their behavior under a cyclic loading (including aging at maximum temperature) was evaluated experimentally. In order to form different thicknesses of TGO, coated samples experience isothermal loading at 1070?°C for various periods of times. Heat-treated samples were heated to 1000?°C and cooled down rapidly in water from the substrate side using a mechanical fixture. The life of samples was investigated as a function of TGO thickness. Furthermore, by performing an experiment the simultaneous effect of the TGO growth and thermal expansion mismatch– on the failure of thermal barrier coatings was evaluated. The results demonstrated that the presence of TGO with a thickness of 2–3?µm has a positive effect on the resistance against thermal shock.     

Failure mechanisms in model thermal and environmental barrier coating systems

The durability of environmental barrier coating (EBC) systems in gas turbine engine environments depends upon their temperature dependent rates of degradation by processes such as steam volatilization and bond coat oxidation. While addition of a thermal barrier coating (TBC) reduces the temperature within the EBC system, it introduces new failure mechanisms. Deposition of a segmented HfO₂ TBC with a reduced in-plane Young’s modulus is essential to avoid bifurcated TBC channel cracking into a Yb₂Si₂O₇ EBC, and delamination, as a result of an approximately 50% difference in coefficients of thermal expansion (CTE) of the coating layers. During prolonged high temperature steam cycling, a thin fluorite phase reaction layer is observed to develop at the HfO₂-YbDS interface consistent with recent thermochemical assessments. The CTE of the fluorite phase is shown to be substantially higher than that of either of the layers to which it is bonded, resulting in tunnel cracking of the fluorite, and eventual coating delamination of the TBC at either the fluorite-HfO₂ or YbDS-fluorite interfaces upon cooling. The study highlights the importance of matching the CTEs of the TBC and EBC layers during coating system design, and those of the reaction products that may form between them.     

Residual stress distribution along interfaces in thermal barrier coating system under thermal cycles

The residual interfacial stress plays an important role in crack initiating and propagating along the interface, which could result in delamination failure of the thermal barrier coatings (TBCs). In this study, the finite element model of air plasma spraying(APS) TBCs was established to assess the level and distribution of residual stress along top coat(TC)/thermally grown oxide (TGO) and bond coat (BC)/TGO interfaces under thermal cycles. Instead of using vertical stress S₂₂ in global coordinate system, the normal and tangential components in the local system along the interfaces, transformed from stress components S₁₁, S₂₂, and S₁₂ in the global one, were used to evaluate the way the cracks initiate and propagate along the interfaces. Firstly, the effect of the number of thermal cycles on residual stress was investigated. It was found that, for the TBCs model without TGO growth and crack, the impact of the number of thermal cycles on the stress is very insignificant and could be ignored. So the present study only chose to focus on the first thermal cycle. Then the influence of the TGO thickness and the interface amplitude on the normal and tangential residual stresses for both homogeneous and inhomogeneous temperature fields was explored. The results show that the TGO thickness, interface amplitude and temperature field affect the residual stress level and distribution, leading to different fracture mechanisms along TC/TGO and TGO/BC interfaces. Finally, the difference between the vertical stress in the global coordinate system and the normal stress in the local coordinate system was studied. Compared with vertical stress S₂₂, the stress components normal and tangential to the TC/TGO and TGO/BC interfaces are more appropriate to describing the stress distribution along the interfaces and predicting the propensity of crack initiating and propagating along the interfaces.     

Microstructure analyses and thermophysical properties of nanostructured thermal barrier coatings

Nanostructured 8 wt% yttria partially stabilized zirconia coatings were deposited by air plasma spraying. Transmission electron microscopy, scanning electron microscopy, and X-ray diffraction were carried out to analyze the as-sprayed coatings and powders. Mercury intrusion porosimetry was applied to analyze the pore size distribution. Laser flash technique and differential scanning calorimetry were used to examine the thermophysical properties of the nanostructured coatings. The results demonstrate that the as-sprayed nanostructured zirconia coatings consist of the nonequilibrium tetragonal phase. The microstructure of the nanostructured coatings includes the initial nanostructure of powder and columnar grains. Moreover, micron-sized equiaxed grains were also exhibited in the nanostructured coatings. Their evolution mechanisms are discussed. The as-sprayed nanostructured zirconia coating shows a bimodal pore size distribution, and has a lower value of thermal conductivity than the conventional coating.     

Research on strain distribution and damage behavior of thermal barrier coatings based on digital image correlation

Due to the complicated structure and serving environments, thermal barrier coatings (TBCs) usually encounter failure in the form of surface coating cracking and interface spalling without warning. At present, although many experimental techniques and equipment were developed to predict their service life, the transfer process of stress between different layers and the strain characteristics of ceramic surface are not clearly explained. In this paper, the nondestructive digital image correlation method was used to observe the character of surface strains of supersonic plasma-sprayed TBCs systems in tensile failure processes. Also, mathematical model was established basing on the principle of minimum function to calculate interface stress and coating strain expressions. The results show the characteristics of strain change in the whole tensile stage can be divided into four stages. At first, strain concentration occurs in the range of 9%-27% of the effective distance between one end of the tensile specimen, second, a certain number of strain fringes are formed and distributed at a certain distance, and then the first crack appears in the initial strain concentration area; as the load continues, more and more cracks on the coating surface reach saturation and finally fail. In the microlinear elasticity stage, the shear strain in the coating and the interface shear stress are in a linear relationship. As the thickness of the single-layer coating increasing, the strain value of the surface strain of the coating decreases, the surface strain value of the single-layer coating is about six times larger than that of the double-layer coating.     

Novel-structured plasma-sprayed thermal barrier coatings with low thermal conductivity,high sintering resistance and high durability

The microstructure of the ceramic topcoat has a great influence on the service performance of thermal barrier coatings (TBCs). In this study, conventional layered-structure TBCs, nanostructured TBCs, and novel-structured TBCs with a unique microstructure were fabricated by air plasma spraying. The relationship between the microstructure and properties of the three different TBCs was analysed. Their thermal insulation ability, sintering resistance, and durability were systematically evaluated. Additionally, their failure modes after being subjected to two kinds of thermal shock tests were analysed. The results revealed that the novel-structured TBCs had remarkably superior performances in all the examined aspects. The thermal conductivity of the novel-structured TBCs was significantly lower than those of the conventional and nanostructured TBCs both in the as-sprayed state and after thermal treatment for 500 h at 1100 °C. The macroscopic elastic modulus of the novel-structured TBCs after sintering at 1300 °C for 100 h was similar to those of the conventional and nanostructured TBCs in the as-sprayed state. During both a burner rig thermal shock test and a furnace cyclic oxidation test, the thermal shock lifetime of the novel-structured TBCs was much longer than those of the conventional and nanostructured TBCs. This study has demonstrated novel-structured plasma-sprayed TBCs with high thermal insulation ability and high durability.     

Evaluation of microstructure evolution of thermal barrier YSZ coating after thermal exposure

Understanding the microstructural transformation of plasma sprayed (APS) yttria-stabilized zirconia (YSZ) after experiencing the thermal shocking cycles is practically important for the coating optimization in terms of structure and performance. In this study, thermal shocking tests were conducted on the YSZ coated piston crown. The microscopic morphology and structure alteration across the YSZ coating interface over the piston crown was characterized by the ex-situ techniques. The results revealed that the YSZ coating primarily consisted of a stable tetragonal phase, without the monoclinic phase even after 800 cycles of thermal shocking. As the thermal shocking test continued, the pore number within the YSZ coating gradually decreased due to their spontaneous closure and the grain size correspondingly increased. Some visible cracks parallel to the interface consisting of YSZ and bonding layer happened at the localized regions of the YSZ coating. The stress state of YSZ coating was from originally tensile to compressive after thermal exposure, which helped prolonging the service lifetime of YSZ coating. In particular, the thermal shock resistance of plasma sprayed YSZ coated piston crown in association with the varying microstructure was also discussed.     

Microstructure and thermal shock resistance of Nd2O3-doped YSZ-based thermal barrier coatings

To study the impact of rare earth oxide doping on the thermal failure of thermal barrier coatings, 0.5 mol%, 1.0 mol% and 1.5 mol% Nd₂O₃-doped YSZ coatings were prepared by explosive spraying. SEM, XRD, EDS and microhardness testing were used to analyse the effect of different rare earth oxide doping contents on the morphology, composition and mechanical properties of the coatings. With an increase in the Nd₂O₃ doping content, the porosity of the coatings was reduced. The decrease in the porosity increased the compactness of the coatings and improved the microhardness and fracture toughness. The bonding strength and thermal shock resistance of the coatings were the highest among the samples herein when the rare earth doping content was 1.0 mol%, and the values were 37.6 MPa and 200 times, respectively. The thermal shock failure mode of the coating was mainly due to the exfoliation of the inner layer of the ceramic layer. The luminous intensity of the coating increased with increasing rare earth oxide doping content, and the emission spectrum of the Nd₂O₃-modified YSZ coating after the thermal shock test produced a new emission peak at 594 nm, which decreased at 708 nm.     

Microstructure and thermal properties of a promising thermal barrier coating: YTaO4

High temperature gas turbine sealing can increase the thermal efficiency of a gas turbine. In this paper, monoclinic phase YTaO₄ ceramics were fabricated via solid-state reaction. Phase composition and microstructures of the high-temperature-sintered YTaO₄ ceramics were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman Spectroscopy. Specific heat capacity rose gradually as temperature increased, due to volumetric expansion and phonon excitations. The thermal diffusivities and conductivities decreased significantly due to the effects of the porosity and phonon scattering. However, the thermal conductivities of the specimens were lower than that of 7–8 wt% yttria-stabilized zirconia (7-8YSZ), and YTaO₄ ceramics have better thermal stability than current (TBCs) material. The Vickers hardnesses of YTaO₄ ceramics as a function of sintering temperature were lower than that of 8YSZ, indicating YTaO₄ has better fracture toughness and thermal tolerance. The results demonstrate that YTaO₄ ceramics would be an excellent candidate for use as a thermal barrier coating material for high temperature gas turbines.