Effect of morphology on thermal conductivity of EB-PVD PYSZ TBCs

Partially yttria stabilized zirconia (PYSZ) based thermal barrier coatings (TBC) manufactured by electron beam-physical vapour deposition (EB-PVD) protect turbine blades, working under severe service conditions in aero engines and stationary turbines. These coatings show a high strain tolerance relying on their unique morphology which is comprised of weakly bonded, preferred-oriented columns, voids between feather-like sub-columns and, finally, of intra-columnar closed pores.The results obtained in this work demonstrate that variation of the EB-PVD process parameters alters the resulting columnar morphology and porosity of the coatings. The physical properties and, most importantly, thermal conductivity, are greatly affected by these morphological alterations. This study investigates three morphologically different EB-PVD PYSZ TBC top coats in terms of the spatial and geometrical characteristics of their porosity and correlates those with the thermal conductivity values measured in as-coated state and after heat treatment at 1100 °C for 1 h and 100 h. Changes in the open and closed porosity caused by heat-treatment are characterized by small-angle neutron scattering (SANS), Brunauer-Emmett-Teller Method (BET) and scanning electron microscope (SEM). Correlation of shape and surface-area changes in all porosity types of the analysed coatings revealed that the thermal conductivity of these coatings is influenced primarily by size and shape distribution of the pores and secondarily by the pore surface-area available at the cross section perpendicular to the heat flux.     

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.     

Thermal conductivity of nanoporous ZrO2-4 mol% Y2O3 multilayer coatings fabricated by EB-PVD

The effect of multilayer configurations on the thermal conductivity of 4 mol% Y₂O₃⁻ stabilized ZrO₂ coatings fabricated by EB-PVD has been investigated. The deposited coating layers consist of columnar grains containing nano-sized pores. Multilayer specimens are found to contain many pores at the interfaces between layers. The density and thermal conductivity of the multilayer coatings decreases with increasing number of coating layers for one to six layers. The thermal conductivities of coatings deposited onto rotating substrates are lower than those of coatings deposited on stationary substrates. The decreased thermal conductivity of multilayer coatings is ascribed to the increased total porosity resulting from an increase in the number of interface pores concomitant with the formation of non-uniform interfaces between layers, which causes increased phonon scattering.     

熔盐环境下热障涂层的等温热腐蚀行为研究

采用电子束物理气相沉积（EB-PVD）在镍基高温合金表面制备了YSZ/NiCoCrAlY双层结构热障涂层,研究了其在熔盐环境下的等温热腐蚀行为。结果表明,涂层在加热过程中发生相变引起体积收缩,导致涂层内部产生微裂纹。熔盐填充涂层内微裂纹和柱状晶间隙,降低了涂层的应变容限,引起涂层内应力升高。同时,熔盐促使热生长氧化层（TGO）碱性溶解,产生疏松多孔的氧化物层,导致TGO层加速增厚。在热应力作用下,疏松层氧化物破碎,最终导致涂层剥落。     

Superior thermal barrier coatings using solution precursor plasma spray

A novel process, solution precursor plasma spray (SPPS), is presented for depositing thermal barrier coatings (TBCs), in which aqueous chemical precursors are injected into a standard direct current plasma spray system. The resulting coatings microstructure has three unique features: (1) ultra fine splats (1 μm), (2) nanometer and micron-sized interconnected porosity, and (3) closely spaced, through-thickness cracks. Coatings over 3 mm thick can be readily deposited using the SPPS process. Coating durability is excellent, with SPPS coatings showing, in furnace cycling tests, 2.5 times the spallation life of air plasma coatings (APS) and 1.5 times the life of electron beam physical vapor deposited (EB-PVD) coatings. The conductivity of SPPS coatings is lower than EB-PVD coatings and higher than the best APS coatings. Manufacturing cost is expected to be similar to APS coatings and much lower than EB-PVD coatings. The SPPS deposition process includes droplet break-up and material arriving at the deposition surface in various physical states ranging from aqueous solution, gel phase, to fully-molten ceramic. The relation between the arrival state of the material and the microstructure is described.     

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.     

Formation and behavior of thermal barrier coatings on nickel-base superalloys

Plasma-sprayed thermal barrier coatings (TBCs) have been used to extend the life of combustors. Electron beam physical vapor deposited (EB-PVD) ceramic coating has been developed for more demanding rotating as well as stationary turbine components. Here 3 kW RF magnetron sputtering equipment was used to gain zirconia ceramic coatings on hollow turbine blades and vanes, which had been deposited NiCrAIY by cathodic arc deposition. NiCrAlY coating surface was treated by shot peening; the effects of shot peening on the residual stress are presented. The results show that RF sputtered TBCs are columnar ceramics, strongly bonded to metal substrates. NiCrAlY bond coat is made of β, γ‘ and Cr phases, ZrO2 ceramic layer consists of t‘ and c phases. No degradation occurs to RF ceramic coatings after 100 h high temperature oxidation at 1150℃ and 500 thermal cycles at 1150℃ for 2 min, air-cooling.     

Effect of thermal treatment on the effective thermal conductivity of YPSZ coatings

Thermally sprayed coatings present effective properties strongly different from those of the primary bulk material. In particular, the actual thermal conductivity of Yttria Partially Stabilized Zirconia (YPSZ) coatings is typically twice lower than the thermal conductivity of dense YPSZ. The architecture of the porous network plays a major role on this decrease: thin inter-lamellar cracks act as thermal resistance and contribute to decrease the effective thermal conductivity more efficiently than globular pores.From this situation, an in-house code has been developed since a few years: this code implements a finite difference method to perform calculations directly on micrographs of coating cross-sections obtained by SEM. Each pixel of the intermediate binary picture is interpreted as a cell of integration of the heat conduction equation. A thermal gradient is applied between the top and bottom edges and a system of linear equations is formed and solved, providing the thermal flux flowing through the structure and the corresponding effective thermal conductivity.In the present study, the case of YPSZ coatings before and after thermal treatment was considered. The numerical results are in rather good agreement with experimental data: the thermal treatment tends to close a part of the thinnest pores, thus providing a decrease of the pore level and an increase of the effective thermal conductivity of the produced coatings.     

Modelling and experimental study of impedance spectra of electron beam physical vapour deposition thermal barrier coatings

A 2-dimensional finite element model has been developed to calculate the impedance spectra of electron beam physical vapour deposition (EB-PVD) thermal barrier coatings (TBCs). The model has been used to examine the effect of thermally grown oxide (TGO) growth and the TGO conductivity change on impedance spectra of TBCs. According to modelling, different spectra were generated due to the TGO growth and TGO conductivity change. Impedance measurements have been carried out on both as-deposited and thermally treated TBCs where the thermal treatments lead to the TGO growth. In addition, the thermal treatment of TBCs at different temperatures produced TGO with different compositions, probably leading to different electrical conductivities of TGO. Measured impedance spectra of TBCs with different TGO thicknesses and TGO compositions agree with modelled spectra of TBCs with different TGO thicknesses and conductivities.     

热喷涂热障涂层孔隙与涂层性能关系研究进展

孔隙在热障涂层中较为常见,孔隙对热障涂层的性能有利有弊。对热障涂层陶瓷层中孔隙的形成机理进行了综合分析,总结了热障涂层孔隙结构的调控方法,讨论了孔隙结构特征对热障涂层隔热性能和力学性能的影响。孔隙结构的引入将引起力学性能的下降,同时降低热障涂层的热导率,提高隔热效果。孔隙结构特征参数包括孔隙形状、孔隙间距、孔隙倾斜角、孔隙高宽比等,其中孔隙的倾斜角和高宽比对涂层导热性能的影响尤为重要,是孔隙结构的关键特征参数。通过原始粉末孔隙结构的保留、造孔剂(有机造孔剂、无机造孔剂)的搭配造孔、制备工艺(临界等离子喷涂参数、粒子扁平率等)的调节以及后续的孔隙处理,可实现热障涂层孔隙结构的调控。在实际应用过程中应同时兼顾力学性能和隔热性能,最重要的是保证热障涂层的有效寿命,需要综合考虑力学性能与导热性能的匹配。通过热障涂层孔隙结构特征的设计及分布控制,可实现孔隙结构高性能、高可靠性热障涂层的制备。     

Assessing Coating Reliability Through Pore Architecture Evaluation

Plasma-sprayed thermal barrier coatings (TBCs) exhibit many interlamellar pores, voids, and microcracks. These microstructural features are primarily responsible for the low global stiffness and the low thermal conductivity commonly exhibited by such coatings. The pore architecture thus has an important influence on such thermophysical properties. In the present work, the effect of heat treatment (at temperatures up to 1400 °C, for times of up to 20 h) on the pore architecture of detached YSZ top coats with different impurity levels have been characterized by mercury intrusion porosimetry and gas-sorption techniques. Stiffness and thermal conductivity were also monitored to assess the effect of change in pore architecture on properties. While the overall porosity level remained relatively unaffected (at around 10-12%) after the heat treatments concerned, there were substantial changes in the pore size distribution and the (surface-connected) specific surface area. Fine pores (<~50 nm) rapidly disappeared, while the specific surface area dropped dramatically, particularly at high-treatment temperatures (~1400 °C). These changes are thought to be associated with intrasplat microcrack healing, improved intersplat bonding and increased contact area, leading to disappearance of much of the fine porosity. These microstructural changes are reflected in sharply increased stiffness and thermal conductivity. Increase in thermal conductivity and stiffness were found to be more pronounced for coatings with higher impurity content (particularly alumina and silica). Reliability issues surrounding such increase in thermal conductivity and stiffness are discussed along with a brief note on the effect of impurities on TBC life.     

Thermal fatigue behavior of thermal barrier coatings with the MCrAlY bond coats by cold spraying and low-pressure plasma spraying

The thermal fatigue behavior of thermal barrier coatings (TBCs) with the NiCoCrAlTaY bond coats deposited by cold spraying and low-pressure plasma spraying (LPPS) was examined through thermal cyclic test. The TBCs were subjected to the pre-oxidation before the test in an Ar atmosphere. The results show that a more uniform TGO in both thickness and composition forms on the cold-sprayed bond coat than that deposited by LPPS. The TBCs with the cold-sprayed bond coat exhibit a longer thermal cyclic lifetime than that with the LPPS bond coat. The differences in oxidation behavior and thermal cyclic behavior between two TBCs were discussed based on the evident difference in the surface morphology of two MCrAlY bond coats deposited by cold spraying and LPPS.     

Infrared-optical properties and heat transfer coefficients of semitransparent thermal barrier coatings

Thermal barrier coatings (TBC) which are used in aircraft and land-based gas turbines for thermal insulation of thermally highly loaded components are usually semitransparent in the infrared spectral region at higher temperatures. Thus, at turbine surface conditions the total heat transfer coefficient of TBCs increases above the heat transfer coefficient caused by solid heat conduction alone.The solid thermal conductivity of electron beam physical vapor deposited (EB-PVD) partially yttria stabilized zirconia (YSZ) coatings derived from laser flash measurements were correlated with the microstructure of the coatings, which was adjusted by defined heat treatments. To obtain the contribution of the radiative transfer on the total heat transfer coefficient, infrared-optical characterizations were carried out at ambient and elevated temperatures.A theoretical model was developed which can be used to describe the heat transfer through semitransparent, absorbing and scattering media. Finally, the total heat transfer, caused by solid thermal conduction, radiative transfer and an interaction of both is derived for the coatings prepared in this work. Additionally the measurement method BBC (black body boundary conditions) which is suitable to determine spectral transmittance and emittance at elevated temperatures is introduced.     

Microstructural characterization of porous thermal barrier coatings by laser flash technique

An inversion procedure has been developed to obtain microstructural parameters describing the porosity morphology of porous thermal barrier coatings (TBCs) from the thermal diffusivity measured in different environments by a laser flash technique. The model base for the inversion procedure combines the Knudsen effect (i.e. the thermal conductivity variation of gases within a pore as a function of pore size and shape) with the asymmetric Bruggeman model that describes the effect of porosity on the thermal conductivity of a porous medium in terms of pore shape, orientation and content. A simplified approach in the inversion procedure has been proposed, and the reliability has been checked by simulating different microstructures within the TBC. The inversion procedure has been also applied to experimental thermal diffusivity values of a TBC which were obtained by filling pores with He, N_2, Ar and in vacuo.     

Miniaturized bend tests on partially stabilized EB-PVD ZrO2 thermal barrier coatings

The elastic properties of thermal barrier coatings (TBCs) are important for modelling the lifetime of these coatings. A new test setup has been developed to measure the system modulus of electron-beam enhanced physical vapour deposited (EB-PVD) TBC coatings by miniaturized bend tests.Due to the brittleness, low stiffness and small thickness of the top coat and its complex microstructure, it is difficult to measure its Young's modulus by standard mechanical testing. For this reason, a special sample material has been prepared which consists of a 1 mm thick layer of EB-PVD TBC. This material was isothermally heat treated for different times at 950 °C, 1100 °C and 1200 °C and then tested in a specially developed miniaturized bend test. The bend test setup permits mechanical tests with a high resolution in stress and strain, where the strain is measured by digital image correlation. So the stiffness of the free-standing TBC samples could be measured with a high accuracy and the sintering behaviour of the EB-PVD TBC and the consequent rise of Young's modulus could be determined. The results show a significant increase of the system modulus with heat treatment time and temperature caused by sintering of the coating. An activation energy of 220 kJ/mol for the process has been determined.In addition, the material was tested by nanoindentation in order to measure Young's modulus on a local scale, and the porosity of the samples was determined by quantitative image analysis.     

Oxidation resistant coatings in combination with thermal barrier coatings on γ-TiAl alloys for high temperature applications

Titanium aluminide alloys based on γ-TiAl are considered of growing interest for high temperature applications due to their attractive properties. To extend the service temperatures above 750 °C, the oxidation behaviour has to be improved predominantly by protective layers. In the present study environmental and thermal protection coatings on gamma titanium aluminides were investigated. Nitride and metallic overlay coatings based on Ti-Al-Cr-Y-N and Ti-Al-Cr, respectively, were produced by magnetron sputtering techniques. Thermal barrier coatings (TBCs) of partially yttria stabilized zirconia were deposited onto Ti-45Al-8Nb, either pre-oxidized or coated with protective layers, applying electron beam physical vapour deposition (EB-PVD).Cyclic oxidation tests were performed at 900 °C and 950 °C in air. The nitride coating exhibited poor oxidation resistance when exposed at 900 °C providing no protection for γ-TiAl. The oxidation behaviour of the Ti-Al-Cr coating was reasonable at both exposure temperatures. During prolonged exposure the coating was depleted in chromium, resulting in the breakdown of the protective alumina scale. EB-PVD zirconia coatings deposited on γ-TiAl exhibited promising lifetime, particularly when specimens were coated with Ti-Al-Cr. The adherence of the TBC on the thermally grown oxide scales was excellent; failure observed was associated with spallation of the oxide scale. At 950 °C, TBCs on specimens coated with Ti-Al-Cr spalled after less than 200 thermal cycles caused by severe oxidation of γ-TiAl and reactions between the zirconia coatings and the thermally grown oxides.     

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.     

Effects of annealing on the microstructure and the mechanical properties of EB-PVD thermal barrier coatings

The effects of thermal annealing at 1000 °C in air on the microstructure and the mechanical properties (Young's modulus and hardness) of thermal barrier coatings consisting of a 4 mol% Y₂O₃ partially stabilized ZrO₂ top coat and a NiCoCrAlY bond coat, deposited by electron beam physical vapour deposition on nickel-based superalloy IN 625, have been investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), image analysis and nanoindentation. During annealing, the ceramic top coat undergoes sintering and recrystallization. These processes lead to stress relaxation, an increase of the intra-columnar porosity and the number of large pores as measured by image analysis of SEM micrographs. An increase of the grain size of the γ-phase in the bond coat, accompanied by changes in the morphology of γ-grains with annealing time, is also observed. Correlations between these microstructural changes in the top coat and the bond coat and their mechanical properties are established and discussed.     

Ultrafast thermal plasma physical vapor deposition of yttria-stabilized zirconia for novel thermal barrier coatings

This research aims to develop advanced thermal plasma spraying technology for the next-generation thermal barrier coatings (TBCs) with a high power hybrid plasma spraying system. By using thermal plasma physical vapor deposition (TP-PVD), various functional structured yttria-stabilized zirconia (YSZ) coatings were deposited. Parameters, such as powder feeding rate, hydrogen gas concentration, and total mass flow rate of the plasma gas, were optimized, and their influences on the evaporation of YSZ powder were investigated. Ultrafast deposition of a thick coating was achieved at a rate of over 150 μm/min. The deposited porous coating has a low thermal conductivity of 0.7W/mK and the dense coating with interlaced t′ domains possesses a high nanohardness of 27.85 GPa and a high reflectance. These characteristics show that the TP-PVD technique is a very valuable process for manufacturing novel TBCs.     

Non-contact sensing of TBC/BC interface temperature in a thermal gradient

Luminescence lifetimes of rare-earth ions in yttria-stabilized zirconia have been shown to exhibit temperature sensitivity from 500-1150 °C [Gentleman, M.M. and Clarke, D.R. (2005) Surface and Coatings Technology 200, 1264; Gentleman, M.M. and Clarke, D.R. (2004) Surface and Coatings Technology 188-189, 93.]. These doped zirconias can be deposited along with standard thermal barrier coatings to create thin temperature sensing layers within the coating. Of particular interest is the temperature at the coating/bond coat interface as the oxidation life of a TBC system is exponentially dependent on this temperature. In this study, thin (∼ 10 μm) layers of europia-doped yttria-stabilized zirconia were deposited by EB-PVD onto bond-coated CMSX-4 superalloy buttons to achieve sensor layers located next to the TBC/BC interface. These coatings were then used to measure the interface temperature in a thermal gradient. Combined with pyrometric measurements of the coating-surface temperature and metal-surface temperature, the thermal conductivity of the coating (1.5 W/mK) and heat flux (∼ 1 MW/m²) in the tests were calculated.