Overview on advanced thermal barrier coatings

During the last decade a number of ceramic materials, mostly oxides have been suggested as new thermal barrier coating (TBC) materials. These new compositions have to compete with the state-of-the-art TBC material yttria stabilized zirconia (YSZ) which turns out to be difficult due to its unique properties. On the other hand YSZ has certain shortcomings especially its limited temperature capability above 1200 °C which necessitates its substitution in advanced gas turbines.In the paper an overview is tried on different new materials covering especially doped zirconia, pyrochlores, perovskites, and aluminates. Literature results and also results from our own investigations will be presented and compared to the requirements. Finally, the double-layer concept, a method to overcome the limited toughness of new TBC materials, will be discussed.     

Effect of heat treatment on the thermal conductivity of plasma-sprayed thermal barrier coatings

The effect of heat treatment on the thermal conductivity of plasma-sprayed Y₂O₃ stabilized ZrO₂ (YSZ) and Al₂O₃ coatings was investigated. A heat treatment of 1300 °C in flowing argon for 50 h was found to significantly increase the thermal conductivity of the coatings when compared to measurements in the assprayed condition. Transmission electron microscopy (TEM) examination of the microstructures of the coatings in the as-sprayed and heat-treated conditions revealed that sintering of microcracks at the splat interfaces was the main cause for the increase in thermal conductivity. In the YSZ coatings, complete closure of microcracks was frequently observed. In contrast, microcrack closure in the Al₂O₃ coatings was characterized by the isolated necking of particles across a microcrack rather than complete closure. A model for thermal conductivity in a solid containing oriented penny-shaped cracks was used to explain the observed increase in thermal conductivity after heat treatment.     

Application of USAXS analysis and non-interacting approximation to determine the influence of process parameters and ageing on the thermal conductivity of electron-beam physical vapor deposited thermal barrier coatings

Electron-beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) display a lower thermal conductivity compared with the deposited bulk material. This effect is achieved due to the presence of pores within these films. The spatial and geometrical characteristics of the porosity influence directly the magnitude of the achieved reduction of the thermal conductivity. In this work, three EB-PVD coating containing different microstructures were manufactured by varying the manufacturing process parameters during the deposition process. Their corresponding thermal conductivities were measured via the laser flash analysis method (LFA) in both the as-coated state and after ageing (1100 °C/100 h). Analysis of the pore formation during processing was carried out by ultrasmall-angle X-ray scattering (USAXS). This technique is supported with a computer based modeling developed by researchers at Advanced Photon Source (APS) in ANL, USA, and in National Institute of Standards and Technology (NIST), USA. The model enables the characterization of the size, shape, volume and orientation of each of the pore populations in EB-PVD TBCs. The effect of these spatial and geometrical characteristics of the porosity on the thermal conductivity of the EB-PVD coatings were studied via a non-interacting approximation based on Maxwell's model. Results of LFA measurements and the applied approximation indicate an interrelation between the microstructure and the thermal properties of the analyzed EB-PVD coatings. Microstructures containing a higher volume fraction of fine anisotropic intra-columnar pores, and larger voids between feather-arms oriented at lower angles toward the substrate plane correspond to lower thermal conductivity values. Inter-columnar gaps do no significantly contribute to lowering the thermal conductivity due to their orientation parallel to the heat flux and their lower volume fraction compared with the volume occupied by the primary columns. On heat treatment, the deepest section of the gaps between feather-arms break-up into arrays of nano-sized low aspect ratio voids. The anisotropic, elongated intra-columnar pores evolve toward low aspect ratio shapes that are less effective in reducing the thermal conductivity.     

Mullite-rich plasma electrolytic oxide coatings for thermal barrier applications

A study has been undertaken of the characteristics exhibited by mullite-rich plasma electrolytic oxide coatings grown on aluminium alloys by using silicate-rich electrolytes. It is found that they can be grown at a higher rate, and to a greater thickness, than alumina PEO coatings on aluminium. The thermal conductivity of these coatings has been measured using a steady-state method. It is shown to be of the order of 0.5 W m^− 1 K^− 1, which may be compared with ∼ 1.5 W m^− 1 K^− 1 for pure alumina PEO coatings and ∼ 10-15 W m^− 1 K^− 1 for dense polycrystalline mullite. Coupled with excellent substrate adhesion and good mechanical properties, this relatively low conductivity makes these coatings attractive for thermal barrier applications. Furthermore, they are shown to exhibit a relatively low global stiffness (∼ 40 GPa), which will reduce the magnitude of thermally-induced stresses and improve the resistance to spallation during temperature changes.     

Combined pre-annealing and pre-oxidation treatment for the processing of thermal barrier coatings on NiCoCrAlY bond coatings

A combined pre-annealing and pre-oxidation treatment was developed for the processing of partially yttria stabilized (PYSZ) thermal barrier coatings (TBC) on top of NiCoCrAlY bond coatings (BC). To develop this pre-treatment, the influence of the oxygen potential during pre-annealing and pre-oxidation on the life span and failure mechanisms of the entire high temperature coating system upon thermal cycling was investigated. The results of this study showed that the service life of the coating system depended strongly on the composition and microstructure of the thermally grown oxide (TGO) after pre-oxidation. The longer life spans were obtained if the TGO thickened very slowly during thermal cycling due to a large α-Al₂O₃ grain size. Such a slow-growing TGO corresponded with a pre-treatment for which θ-Al₂O₃ was formed during pre-oxidation and for which the yttrium was located within a high density of pegs along the TGO/BC interface after pre-oxidation. If the yttrium was present on top of the TGO after pre-oxidation, a thick mixed alumina-zirconia layer formed upon thermal cycling. This mixed oxide layer contributed significantly to the total oxide layer thickness, resulting in short life spans. The formation of NiAl₂O₄ spinel in between the TBC and the α-Al₂O₃ should be avoided, since this can lead to premature failure along the spinel/α-Al₂O₃ interface.     

Ambient and high-temperature thermal conductivity of thermal sprayed coatings

Aside from its importance as a design parameter for thermal barrier coatings, measuring thermal conductivity of thermal sprayed coatings itself provides a unique method to critically characterize the nature, quantity, and anisotropy of the defect morphologies in these splat-based coatings. In this paper, the authors present a systematic assessment of thermal conductivity of wide range using the flash diffusivity technique. For the case of plasma sprayed yttria-stabilized zirconia (YSZ), coatings obtained from wide-ranging initial powder morphologies as well as those fabricated under different particle states were characterized. Both in-plane and through-thickness properties were obtained. Other material systems that were considered include: metallic alloys and semiconductors of interests. Issues such as reproducibility and reliability in measurements were also considered and assessed. Finally, work in collaboration with the Oak Ridge National Laboratory (ORNL) for alternate approaches to characterization of thermal conductivity as well as high-temperature measurements was performed. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Mechanical properties of solution-precursor plasma-sprayed thermal barrier coatings

The microstructure of thermal barrier coatings (TBCs) of 7 wt.% Y₂O₃ stabilized ZrO₂ (7YSZ) deposited using the solution-precursor plasma spray (SPPS) method has: (i) controlled porosity, (ii) vertical cracks, and (iii) lack of large-scale “splat” boundaries. An unusual feature of such SPPS TBCs is that they are well-adherent in ultra-thick forms (~ 4 mm thickness), where most other types of ultra-thick ceramic coatings fail spontaneously. Here a quantitative explanation is provided as to why as-deposited ultra-thick SPPS TBCs are so well-adherent. The mode II toughness of thin (0.2 mm) SPPS TBCs has been measured using the “barb” shear test, which is found to be 66 J m^− 2. Residual stresses in SPPS TBCs of thickness 0.2, 1.5, and 4.0 mm have been estimated using a microstructure-based object-oriented finite element (OOF) method. These stresses are found to be low, as a result of the strain-tolerant microstructure of the SPPS TBCs. The corresponding strain energy release rates that drive mode II cracks in the three different thickness SPPS TBCs have been found to be less than the mode II toughness.     

High temperature properties of thermal barrier coatings obtained by detonation spraying

NiCrAlY/YPSZ and NiCrAlY/NiAl/YPSZ thermal barrier coatings (TBCs) were successfully deposited by detonation spraying. The results indicated that the detonation sprayed TBCs included a uniform ceramic coat containing a few microcracks and a bond coat with a rough surface. The lamellar structure and the presence of cracks and impurities could reduce the thermal conductivity of the ceramic coat. Oxidation kinetics at 1000–1150 °C of detonation sprayed TBCs have been measured and discussed. The role of a Ni–Al intermediate layer in improving the oxidation resistance of duplex TBCs has also been studied.     

Thermal properties of oxides with magnetoplumbite structure for advanced thermal barrier coatings

Oxides having magnetoplumbite structure are promising candidate materials for applications as high temperature thermal barrier coatings because of their high thermal stability, high thermal expansion, and low thermal conductivity. In this study, powders of LaMgAl₁₁O₁₉, GdMgAl₁₁O₁₉, SmMgAl₁₁O₁₉, and Gd_0.7Yb_0.3MgAl₁₁O₁₉ magnetoplumbite oxides were synthesized by citric acid sol-gel method and hot-pressed into disk specimens. The thermal expansion coefficients (CTE) of these oxide materials were measured from room temperature to 1500 °C. The average CTE value was found to be ∼ 9.6 × 10^− 6/C. Thermal conductivity of these magnetoplumbite-based oxide materials was also evaluated using steady-state laser heat flux test method. The effects of doping on thermal properties were also examined. Thermal conductivity of the doped Gd_0.7Yb_0.3MgAl₁₁O₁₉ composition was found to be lower than that of the undoped GdMgAl₁₁O₁₉. In contrast, thermal expansion coefficient was found to be independent of the oxide composition and appears to be controlled by the magnetoplumbite crystal structure. Preliminary results of thermal conductivity testing at 1600 °C for LaMgAl₁₁O₁₉ and LaMnAl₁₁O₁₉ magnetoplumbite oxide coatings plasma-sprayed on NiCrAlY/Rene N5 superalloy substrates are also presented. The plasma-sprayed coatings did not sinter even at temperatures as high as 1600 °C.     

等离子喷涂YPSZ陶瓷梯度涂层的组织与抗热震性能

利用等离子喷涂工艺在汽车铝活塞表面制备了氧化钇部分稳定的氧化锆(YPSZ)陶瓷梯度涂层,用电子扫描显微镜、金相显微镜等手段对涂层从组织形貌、物相、显微硬度和热震性能进行了分析.结果表明:基体与涂层结合紧密;涂层在冷却过程中发生了t-ZrO2→m-ZrO2的相变过程;涂层的热震性能良好,可满足活塞的使用要求.     

Influence of thermal shock on insulation effect of nano-multilayer thermal barrier coatings

Thermal barrier coatings (TBCs) with nano-multilayer structure were investigated by thermal shock test. The change of insulation effect during thermal shock test was studied by in-situ temperature monitor with a thermal couple set into the substrate. Microstructure and electrical properties of TBCs were characterized by SEM and Impedance Spectroscopy, respectively. Initial increase in insulation effect was observed and related to the formation and growth of perpendicular microcracks in top coat and transversal microcracks in TGO. With thermal shock, the insulation effect decreased due to the further growth of microcracks in top coat and TGO which induced the failure of TBCs.     

The effects of heat treatment and gas atmosphere on the thermal conductivity of APS and EB-PVD PYSZ thermal barrier coatings

The effects of heat treatment and gas atmosphere on thermal conductivity of atmospheric plasma sprayed (APS) and electron beam physical vapor deposited (EB-PVD) partially Y₂O₃ stabilized ZrO₂ (PYSZ) thermal barrier coatings (TBCs) were investigated. Two-layer samples that had an EB-PVD coating deposited on bond coated nickel-base superalloy IN625 substrates, free-standing APS and EB-PVD coatings as well as a quasi-free-standing EB-PVD PYSZ coating (coating on semitransparent sapphire) were included in the study. Thermal diffusivity measurements for determining thermal conductivity were made from room temperature up to 1150 °C in vacuum and under argon gas using the laser flash technique. To investigate the effect of heat treatment on thermal conductivity, coatings were annealed at 1100 °C in air. For both the APS and EB-PVD PYSZ coatings the first 100 h heat treatment caused a significant increase in thermal conductivity that can be attributed to microstructural changes caused by sintering processes. Compared to the measurements in vacuum, the thermal conductivity of APS coatings increased by about 10% under argon gas at atmospheric pressure, whereas for the EB-PVD coatings, the influence of gas on thermal conductivity was relatively small. The effect of gas on the thermal conductivity of APS and EB-PVD PYSZ coatings can be attributed to amount, shape, and spatial arrangement of pores in the coating material.     

Image-based extended finite element modeling of thermal barrier coatings

Further advances in Thermal Barrier Coating (TBC) design are linked with the evolution of numerical models for TBCs. The present paper, therefore, enhances the idea of a currently available FEM package (OOF) that has been designed for microstructural level simulations. The approach of Extended FEM (XFEM) is incorporated in an in-house developed program to account for the existence of cracks in TBCs; both for stress-strain analysis and for heat transfer analysis. The new XFEM program is then employed to carry out the analyses of a YSZ deposit and a multilayered TBC to predict the effective Young's moduli, the overall thermal conductivities, and to assess the fracture behavior of the coating.     

Thermal modeling of various thermal barrier coatings in a high heat flux rocket engine

One- and two-dimensional thermal models were developed to predict the thermal response of tubes with and without thermal barrier coatings (TBCs) tested for short durations in a H₂/O₂ rocket engine. Temperatures were predicted using median thermophysical property data for traditional air plasma sprayed ZrO₂–Y₂O₃ TBCs, as well as air plasma sprayed and low pressure plasma sprayed ZrO₂–Y₂O₃/NiCrAlY cermet coatings. Good agreement was observed between predicted and measured metal temperatures. It was also shown that the variation in the reported values of the thermal conductivity of plasma sprayed ZrO₂–Y₂O₃ coatings can result in temperature differences of up to 180°C at the ceramic/metal interface. In contrast, accounting for the presence of the bond coat or radiation from the ceramic layer had only a small effect on substrate temperatures (<20°C). The thermal models were also used to show that for the short duration test conditions of this study, a 100 μm thick ZrO₂–Y₂O₃ coating would provide a metal temperature benefit of approximately 300°C over an uncoated tube while a 200 μm thick coating would provide a benefit greater than 500°C. The difference in the thermal response between tubes and rods was also predicted and used to explain the previously-observed increased life of TBCs on rods over that on tubes.     

Effect of heat treatment at 900 °C on microstructural and mechanical properties of thermal barrier coatings

The effect of heat treatment at 900 °C in air on the microstructural and mechanical properties of electron beam physical vapour deposited thermal barrier coatings (TBCs) was investigated in this work. To clarify this effect, the phase and the microstructure of the zirconia ceramic top coat (TC) before and after the treatment were first characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) respectively. The changes in Young's modulus and hardness of the TC before and after the treatment were then measured by indentation tests at room temperature. The delamination behavior, especially the evolution of the interfacial load capacity between the ceramic TC and the thermally grown oxide (TGO), was finally examined by the finite element analysis with surface-based cohesive behavior. Correlations between the changes in microstructure and mechanical properties were also discussed. Results showed that after the treatment neither the phase transformation from the tetragonal zirconia to the monoclinic nor the transformation to the cubic was detected, whereas the TC sintered. The combined effect of the sintering related densification and the cracks formed during the treatment led to an initial increase and a subsequent decrease in Young's modulus and hardness of the TC with increasing treatment time. The interfacial load capacity increased with increasing Young's modulus of the TC and increasing thickness of the TGO.     

Models of thermal conductivity of multilayer wear resistant coatings

The thermal conductivity of non-metallic nano- and microstructured materials is a key parameter when considering wear resistant coatings. Here, different models of phonon transport and estimation of thermal conductivity are analyzed. The hopping mechanism of phonon transport was found to be applicable for small-grain-size materials in understanding thermal conductivity, whereas large grain size materials can be studied using the correlation function approach. The Chapman–Enskog approach to the Boltzmann transport equation assuming that the transport of phonons is controlled by scattering on the grain boundaries has been analyzed. The Fourier law of thermal conductivity is obtained with the thermal conductivity inversely proportional to the specific surface of the boundaries.     

激光改性对YSZ涂层结构与性能的影响

本文采用电子束物理气相沉积技术在Ni基单晶基体表面制备双层结构的热障涂层后,采用脉冲Nd:YAG激光对其进行激光表面改性处理,获得具有复合陶瓷层结构的热障涂层。然后,对改性前后试样进行了1100℃高温氧化处理,并采用SEM和XRD分析了激光改性对涂层微观形貌和组织结构,以及抗氧化性能和耐腐蚀性能的影响。结果发现：激光改性处理后,试样表面形成致密平整的重熔层组织,该组织与原柱状晶组织共同构成了复合结构的陶瓷层,复合结构的陶瓷层减缓了有害元素向基体的扩散行为,提高了涂层的抗氧化性能和耐腐蚀性能。     

GdTaO4陶瓷的制备、力学性质和热导率研究

为寻找性能更为优异的陶瓷热障涂层材料,采用固相反应法合成了单斜结构的GdTaO_4陶瓷材料,分析了其微观组织形貌。第一性原理计算结果表明其沿[100]晶向的杨氏模量值约为[010]和[001]方向上的3倍。实验测得800℃下其热导率约为1.70 W·m~(-1)·K~(-1),明显低于7YSZ和8YSZ在800℃下的热导率(分别约为2.37和2.47 W·m~(-1)·K~(-1)),是一种潜在的低热导陶瓷热障涂层材料。     

An investigation into the correlation between nano-impact resistance and erosion performance of EB-PVD thermal barrier coatings on thermal ageing

Nanomechanical testing (nano-impact and nanoindentation mapping) has been carried out on the top surfaces of as-received and aged 8 wt.% yttria stabilised zirconia (YSZ) thermal barrier coatings (TBCs) produced by electron-beam physical vapour deposition (EB-PVD). The correlation between the nanomechanical test results and the previously reported erosion resistance of the TBCs has been investigated. The experimental results revealed that aged TBCs on zirconia for 24 h at 1500 °C or on alumina for 100 h at 1100 °C resulted in large increases in their hardness (H), modulus (E), H/E and H³/E² ratios but their erosion resistance was reduced. Nano-impact tests showed a dramatic decrease in impact resistance following the ageing of these TBCs, which is consistent with the erosion results. The strong correlation between the nano-impact and erosion resistances has confirmed the premise that rapid laboratory impact tests must produce deformation with similar contact footprint to that produced in the erosion tests.     

Non-destructive thermal property measurements of an APS TBC on an intact turbine blade

The ability to measure the properties of thermal barrier coatings (TBCs) applied to engine components is challenging due to the complex geometry of parts and the difficulty of preparing samples suitable for conventional techniques. As a result, there is a shortage of information related to the morphology and thermal properties of coatings on engine components. Phase of photothermal emission analysis (PopTea) is a relatively new non-destructive technique that is suitable for measuring the thermal properties of coatings on serviceable engine parts. To demonstrate this capability, measurements are performed on an intact turbine blade coated with air plasma sprayed (APS) 7 wt.% Y₂O₃-stabilized ZrO₂ (7YSZ). The average thermal diffusivity of the coating applied to the blade was ~ 0.5 mm²/s which is typical for thermal diffusivity previously measured on 7YSZ APS coatings made on test coupons with PopTea and laser flash. Furthermore, trends in thermal properties over the blade are studied and compared. It is discovered that variations in thermal properties are the result of differences in coating porosity.