Measurements of the thermal gradient over EB-PVD thermal barrier coatings

Conventional two-layered structure thermal barrier coatings (TBCs), graded thermal barrier coatings (GTBCs) and graded thermal barrier coatings with micropores were prepared onto superalloy DZ22 tube by electron beam physical vapor deposition (EB-PVD). Thermal gradient of the TBCs was evaluated by embedding two thermal couples in the surfaces of the tube and the top coat at different surrounding temperatures with and without cooling gas flowing through the tube. The results showed that higher thermal gradient could be achieved for the GTBCs with micropores compared to the two-layered structure TBCs and GTBCs. However, after the samples were heated at 1050°C, the thermal gradient for the GTBCs with or without micropores decreased with the increase of heating time. On the other hand, the thermal gradient for the TBCs increased with the increase of heating time. Cross-section observations by scanning electron microscopy showed that the change in microstructure was the main reason for the change of the thermal gradient.     

Thermocyclic Behavior of Differently Stabilized and structured EB-PVD thermal barrier coatings

The electron-beam physical vapor deposition (EB-PVD) process provides distinctive coatings of a unique columnar microstructure for gas turbine components. Main advantage of this structure is superior tolerance against straining, erosion and thermoshock, thus giving it a major edge in lifetime. This paper outlines the interaction between chemical composition and microstructural evolution EB-PVD zirconia-based thermal barrier coatings (TBCs) and their respective lifetimes in cyclic burner rig and furnace tests. Customizing TBC microstructure by adjusting EB-PVD processing parameters is emphasized. A structural zone diagram for PVD is modified by interconnecting the influence of substrate rotation with microstructural evolutions. Finally, some basic aspects of single source and dual source evaporation are compared.     

EB-PVD热障涂层对高温合金基体断裂特征影响的研究

采用EB－PVD方法在K3合金上（包括铸态和经标准热处理两种状态）沉积了由NiCoCrAlY金属粘结层和YSZ涂层顶层组成的双层结构的热障涂层,对未涂层和涂层试样的拉伸性能进行了评估,并分析了涂层的制备和中间处理过程中的基体的微观结构的变化。结果表明,在热障涂层的沉积以及中间处理过程中（真空前处理及后处理）,基体的铸态组织得到改善,产生了析出强化,使得在铸态K3合金基体上沉积热障兴层后基体的拉伸强度由800MPa提高到1050MPa,而在经过标准热处理的合金基体上沉积热障涂层对基体的力学性能几乎没有影响。     

Architecture of thermal barrier coatings produced by electron beam-physical vapor deposition (EB-PVD)

Extremely high temperatures and severe atmospheric conditions in the hot section of aircraft engines during operation result in degradation and structural failures of turbine components. Replacing these components is very expensive. Thermal barrier coatings (TBC) composed of ZrO₂-8wt%Y₂O₃(8YSZ) applied by Electron Beam-Physical Vapor Deposition (EB-PVD) to turbine components offer excellent properties for thermal protection and resistance against oxidation - induced erosion and corrosion. However, the life of turbine components is still limited due to premature failure of the TBC. It is hypothesized that the life of the coated components can be extended by lowering the thermal conductivity of the TBC by creating multiple non-distinct or distinct interfaces and alloy additions such as Nb-oxide which will result in a reduction in the thermal conductivity and oxygen transport through the coating. This paper presents the microstructural results of standard 8YSZ, layered 8YSZ, Nb-oxide alloyed 8YSZ and functionally graded 8YSZ with Nb-oxide deposited by EB-PVD. TBC samples were examined by various methods including scanning electron microscopy (SEM), high-resolution optical microscopy (OM), X-ray diffraction (XRD), and thermal cycling tests. The preliminary results strongly suggest that multiple interfaced TBC exhibits better oxidation resistant properties as compared to standard and alloyed TBC.     

A study on gradient thermal barrier coatings by EB-PVD in a cyclic high-temperature hot-corrosion environment

Gradient thermal barrier coatings (GTBCs) have been produced by electron beam physical vapor deposition (EB-PVD). Their performance was evaluated by isothermal oxidation and cyclic high-temperature hot-corrosion tests. It is found that the GTBCs exhibited better resistance to high-temperature oxidation and cyclic high-temperature hot-corrosion (HTHC) than traditional two-layered TBCs. A dense Al₂O₃ layer on the bond coat of GTBCs can effectively prohibit inward diffusion of oxidants such as O and S and outward diffusion of Al and Cr. On the other hand, an inlaid interface, the formation of which resulted from the oxidation of Al diffusion into the gaps between the columns of bond coat during the fabrication of the GTBCs, contributes to reinforce the adherence of the Al₂O₃ layer to the bond coat. During fluxing of the Al₂O₃ layer, S and O diffused into the bond coat. Cracks developed in the surface layer of bond coat by the combined effect of sulfidation of the bond coat and thermal cycling, and finally led to failure of the GTBC.     

CMAS环境下电子束物理气相沉积热障涂层的热循环行为及失效机制

航空发动机涡轮叶片工作时表面经常产生CaO-MgO-Al₂O₃-SiO₂(简称CMAS)等沉积物。本文中研究了电子束物理气相沉积(EB-PVD)制备ZrO₂热障涂层(TBCs)在CMAS环境下的热循环行为及失效机制。结果表明, 在1200℃热冲击条件下, 表面涂覆CMAS的热障涂层的热循环寿命低于100次, 而未涂覆CMAS的涂层寿命达到500次以上, CMAS 的存在加速了热障涂层的剥落失效。在1200℃经过210次循环后, ZrO₂陶瓷层与CMAS之间形成了约8 μm厚的互反应区, 其形成主要与CMAS中Ca²⁺内扩散有关。CMAS环境下热障涂层陶瓷层产生大量横向裂纹, 涂层的失效主要以陶瓷层片状剥落为主。     

EB-PVD process and thermal properties of hafnia-based thermal barrier coating

Thermal barrier coatings (TBCs) are being developed for the key technology of gas turbine and diesel engine applications. In general, 8 mass% Y₂O₃–ZrO₂ (8YSZ) coating materials are used as the top coating of TBCs. The development of hafnia-based TBC was started in order to realize the high reliability and durability in comparison with 8YSZ, and the 7.5 mass% Y₂O₃–HfO₂ (7.5YSH) was selected for coating material. By the investigation of electron-beam physical vapor deposition (EB-PVD) process using 7.5YSH ceramic ingot, 7.5YSH top coating with about 200 µm thickness could be formed. The microstructure of the 7.5YSH coated at coating temperature of 850 °C showed columnars of laminated thin crystals. On the other hand, the structure of the 7.5YSH coated at coating temperature of 950 °C showed solid columnars. From the result of sintering behavior obtained by heating test of 7.5YSH coating, it was recognized that the thermal durability of 7.5YSH coating was improved up to about 100 °C in comparison with 8YSZ coating. This tendency was confirmed by the experimental result of the thermal expansion characteristics of sintered 7.5YSH and 8YSZ.

©2003 Elsevier Science Ltd. All rights reserved.     

Tailored microstructure of zirconia and hafnia-based thermal barrier coatings with low thermal conductivity and high hemispherical reflectance by EB-PVD

Zirconia and hafnia based thermal barrier coating materials were produced by industrial prototype electron beam-physical vapor deposition (EB-PVD). Columnar microstructure of the thermal barrier coatings were modified with controlled microporosity and diffuse sub-interfaces resulting in lower thermal conductivity (20–30% depending up on microporosity volume fraction), higher thermal reflectance (15–20%) and more strain tolerance as compared with standard thermal barrier coatings (TBC). The novel processed coating systems were examined by various techniques including scanning electron microscopy (SEM), X-ray diffraction, thermal conductivity by laser technique, and hemispherical reflectance.     

EB-PVD在热障涂层中的研究及应用

EB-PVD是以高能电子束为热源的一种蒸发镀膜技术.在真空的环境下,高能离子束轰击靶材(金属,陶瓷等),使其融化、升华、蒸发,最后沉积在基片上.由于EB-PVD技术具有蒸发和沉积速率高,涂层致密,化学成分易于精确控制,可得到柱状晶组织,无污染,热效率高,基片与薄膜之间有较强的结合力等诸多优点,已被广泛应用于国防和民用领域.本文介绍了EB-PVD技术在制备热障涂层时优势、不足与改进措施.     

Erosion of thermal barrier coatings

Abstract

Thermal barrier coatings have been used within gas turbines for over 30 years to extend the life of hot section components. Thermally sprayed ceramics were the first to be introduced and are widely used to coat combustor cans, ductwork, platforms and more recently turbine aerofoils of large industrial engines. The alternative technology, electron beam physical vapour deposition,(EB-PVD) has a more strain-tolerant columnar microstructure and is the only process that can offer satisfactory levels of spall resistance, erosion resistance and surface finish retention for aero-derivative engines.

Whatever technology is used, the thermal barrier must remain intact throughout the turbine life. Erosion may lead to progressive loss of TBC thickness during operation, raising the metal surface temperatures and thus shortening component life. Ballistic damage can lead to total TBC removal.

This paper reviews the erosion behaviour of both thermally sprayed and EB-PVD TBCs relating the observed behaviour to the coating microstructure. A model for the erosion of EB-PVD ceramics is presented that permits the prediction of erosion rates. The model has been validated using a high velocity erosion gas gun rig, both on test coupons and samples removed from coated components. The implications of erosion on component life are discussed in the light of experimental results and the model predictions.     

Failure aspects of thermal barrier coatings

Abstract

The paper describes aspects of thermal barrier coating (TBC) microstructure and the physical and mechanical properties which they influence. The stress-strain behaviour of air plasma sprayed (APS) TBCs is discussed, including the role of residual stresses. Failure phenomena as well as the TMF behaviour of TBC coated nickel base superalloys are described. The role of bond coat oxidation on TBC life is discussed as well as some mechanical properties of vacuum plasma sprayed MCrAlY-bond coatings. Finally, life prediction methodologies are addressed and discussed in terms of a critical strain accumulation concept. From this is derived an equation which covers time dependent effects such as bond coat oxidation and sintering. The paper concludes with a brief summary of the evolution of TBCs in aero and industrial gas turbines, and the failure modes in each. In particular the increased importance of erosion, in industrial gas turbines, due to water injection is highlighted.     

Failure during thermal cycling of plasma-sprayed thermal barrier coatings

The thermal cycling behavior of plasma-sprayed ZrO₂?12wt.%Y₂O₃ coatings was studied. Coatings were produced with and without bond coats of Ni-Cr-Al-Zr and in some cases the substrates were heated to above the optimum temperature prior to spraying. The coatings (attached to the substrate) were thermal cycled to 1200 °C and their cracking behavior was followed by acoustic emission (AE) techniques. It was possible to examine the failure mechanisms by statistical analysis of the AE data and to evaluate the influence of preheating and bond coating. It is shown that the AE spectrum changes when a bond coat is used because of the presence of microcracks which, in turn, dissipate energy and improve the coating integrity. The preheating effect is reflected by a decrease in the peak count rate and an increase in the temperature at which AE activity is initiated.     

Analytical electron microscopy of the mixed zone in NiCoCrAlY-based EB-PVD thermal barrier coatings: as-coated condition versus late stages of TBC lifetime

Abstract

The microstructural evolution of the alumina-zirconia mixed zone in a NiCoCrAlY-based electron beam physical vapor deposited (EB-PVD) yttria partially stabilized zirconia (Y-PSZ) thermal barrier coating (TBC) system from the as-coated condition into the advanced stages of TBC lifetime is monitored by analytical transmission electron microscopy (TEM). In the as-coated condition yttria-rich islands at the thermally-grown oxide (TGO)/TBC interface locally impede zirconia uptake of the scale during TBC deposition and give rise to the formation of an “off-plane” alumina-zirconia mixed zone textured perpendicular to the TGO/TBC interface. During prolonged isothermal/cyclic oxidation an increased chromium diffusion through the TGO scale turns the mixed zone into a reaction zone introducing a morphological instability of the mixed zone/TBC interface due to solutioning of the bottom TBC layer.

This microstructural pattern is corroborated by a triple-stage growth model for the mixed zone during three successive stages in TBC lifetime: (i) during TBC deposition, the thickness of the mixed zone increases due to predominant outward aluminum diffusion and uptake of zirconia. No columnar alumina zone (CAZ) has formed at this stage, (ii) upon completion of the transition alumina-to-corundum phase transformation the thickness of the mixed zone remains constant while the change in diffusion mechanism for an inward oxygen diffusion process now initiates parabolic growth of the columnar alumina sublayer of the TGO scale, (iii) in the late stage of TBC lifetime an marked outward chromium diffusion from the bond coat causes the mixed zone to resume growth due to TBC destabilization and the formation of a (Al, Cr)₂O₃ mixed oxide matrix phase.

A transient YCrO₃ phase is proposed for driving the destabilization of yttria-rich sections of the bottom TBC layer.     

Microstructure of zirconia-yttria plasma-sprayed thermal barrier coatings

The objective of this paper is to report on the characterization of the highly complex microstructure of zirconia coatings, which arise as a result of the plasma-spraying process. The fine structure has been observed to change through the thickness of the coating, behaviour which has been related to the cooling rate and crystallization of the deposited material. Microstructural features such as an amorphous bond coat/ceramic interfacial film and a grain-boundary glassy phase, which are believed to have a significant effect upon coating properties such as adhesion and compliance, have been shown to be present.     

Pulsed laser processing of thermal barrier coatings

Results are reported of the effects of surface melting (sealing) produced by a 1 kW laser in pulsed mode on the structure of plasma-sprayed 8wt% yttria partially stabilized zirconia (YPSZ); pulse lengths in the range of 1 to 90msec were used. Smooth surfaces were produced with shallow cracks at values of laser energy 5 to 40 J. Comparison of the data is made with results obtained by sealing using continuous wave CO₂ laser processing.     

On debonding of graded thermal barrier coatings

Thermal barrier coatings generally consist of a metallic substrate which is the primary structural component, a metallic bond coat which serves as oxygen diffusion barrier, a very thin layer of thermally grown oxide and a ceramic top coat that provides the main thermal shielding. Homogeneous ceramic coatings as top coats appear to have certain undesirable features such as high residual and thermal stresses, generally low toughness and relatively poor bonding strength. The new concept of compositional grading of the top coat may help to overcome some of these shortcomings by eliminating the material property discontinuities. A common mode of failure in thermal barrier coatings seems to be the debonding of the top coat. In this study the related interface crack problem for a graded ceramic/metal top coat is considered. It is assumed that the thermophysical properties of the top coat continuously vary between that of the bond coat at the top coat-bond coat interface and that of the ceramic at and near the free surface. The main objective of the study is to examine the influence of the material nonhomogeneity parameters and relative dimensions on the stress intensity factors and the crack opening displacements.