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.     

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。     

An impedance spectroscopy study of high-temperature oxidation of thermal barrier coatings

Oxidation of air-plasma-sprayed (APS) thermal barrier coatings (TBCs) was carried out in air at 950 °C and was investigated using impedance spectroscopy coupled with scanning electron microscopy and X-ray diffraction. After oxidation for between 500 and 3000 h, a continuous alumina scale was formed at the bond coat/top coat interface in the TBCs, and was evaluated using impedance spectroscopy. The impedance spectra of the oxidised TBCs showed that there were four relaxation processes, which were attributed to the yttria-stabilised zirconia (YSZ) bulk of the TBCs, the thermally grown alumina scale, the YSZ grain boundary, and the metal electrode effect. Impedance analysis showed that the resistivity of the alumina scale increased with increasing oxidation time, demonstrating the thermally grown oxide (TGO) growth. The resistance of the YSZ grain boundaries also increased after oxidation for 2000 h, suggesting the composition change at grain boundaries after a long-term oxidation.     

金属/陶瓷梯度热障涂层

综述了金属/陶瓷梯度热障涂层的国内外研究成果,对两种主要的制备工艺及其特点和热应力进行了分析,重点对梯度热障涂层的设计、微结构、性能及失效机理进行了研究.展望了进一步的研究方向.     

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.     

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.     

Composition, structure and properties of gradient thermal barrier coatings (TBCs) produced by electron beam physical vapor deposition (EB-PVD).

Gradient thermal barrier coatings (TBCs) along with the bond coat were produced by one and the same technological cycle using electron beam physical vapor deposition (EB-PVD) of an MCrAlY ingot, then of an Al–Al₂O₃–ZrO₂(Y₂O₃) tablet (pressed multicomponent powder mixture) and finally of a ZrO₂–7 wt.% Y₂O₃ ceramic ingot. At the evaporation temperature used, vapor pressures of the tablet components decrease in the direction: Al→Al₂O₃→ZrO₂(Y₂O₃). The evaporation of these constituents also proceeds in the same order. As a result, a transition zone [with composition and structure gradients — transition gradient zone (TGZ)] — forms between the bond coat and outer ZrO₂–7 wt.% Y₂O₃ ceramic layer during deposition. The TGZ constitution and structure are primarily determined by the aluminum, Al₂O₃ and ZrO₂ contents of the tablet. As a consequence of liquid aluminum participation in the coating deposition process, a thin layer of β-phase (NiAl), smoothly transitions to an Al₂O₃ layer and then to ZrO₂–7 wt.% Y₂O₃.     

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.     

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.     

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。     

Microstructural evolution and degradation modes in cyclic and isothermal oxidation of an EB-PVD thermal barrier coating

Abstract

Thermal barrier coatings constituted of a partially stabilised zirconia layer (～100 μm thick) deposited by EB-PVD on top of a platinum-modified aluminide bondcoat to protect a single crystal superalloy substrate, have been studied in cyclic and isothermal oxidation at 1,100°C in air. Close examination by high resolution SEM (with field emission gun) of the oxide scale and of the metallic surfaces after various durations, up to 600 hours at 1,100°C, show several morphological features which may affect the integrity of the systems: partial decohesion regions with large grain areas at the inner surface of the scale, subsequently replaced by a cellular morphology, cavities within the oxide scale as well as at the zirconia–alumina interface. The nature and distribution of these features depend on the type of test (isothermal or cyclic oxidation) and on the systems studied (standard superalloy or modified to lower the sulphur content). Scenarios are proposed to explain the observations reported.     

TiAl合金表面激光重熔热障涂层组织及抗高温氧化性能

为了进一步提高TiAl合金表面等离子喷涂ZrO2-7%(质量分数)Y203热障陶瓷涂层的性能,采用激光重熔工艺对涂层进行处理,研究了激光重熔对涂层微观组织和抗高温氧化性能的影响.用扫描电镜(SEM)分析了涂层形貌和微观结构,同时对其抗高温(850℃)氧化性能进行了考察.结果表明,等离子喷涂热障陶瓷涂层呈典型的层状堆积特征,有一定的孔隙且分布有微裂纹;经过激光重熔处理后,陶瓷涂层片层状组织得以消失,致密性提高,获得了基本没有裂纹等缺陷的重熔层;整个重熔层包括界面没有明显特征的平面晶和上部沿热流方向生长的柱状晶组织.等离子喷涂热障涂层具有较好的抗高温氧化性能,经过激光重熔后可进一步提高其抗高温氧化能力.     

Improving thermal barrier coatings by laser remelting

Thermal barrier coatings are extensively used to protect metallic components in applications where the operating conditions include aggressive environment at high temperatures. These coatings are usually processed by thermal spraying techniques and the resulting microstructure includes thin and large splats, associated with the deposition of individual droplets, with porosity between splats. This porosity reduces the oxidation and corrosion resistance favouring the entrance of aggressive species during service. To overcome this limitation, the top coat could be modified by laser glazing reducing surface roughness and sealing open porosity. ZrO2(Y2O3) top coat and NiCrAlY bond coating were air plasma sprayed onto an Inconel 600 Ni base alloy. The top coat was laser remelted and a densified ceramic layer was induced in the top surface of the ceramic coating. This layer inhibited the ingress of aggressive species and delayed bond coat oxidation.     

Evaluation of cyclic oxidation of thermal barrier coatings exposed to NaCl vapor by finite element method

The cyclic oxidation of NiCrAlY + YSZ coating exposed to NaCl vapor has been investigated under atmospheric pressure at 1050 °C, 1100 °C and 1150 °C. The result showed that the cyclic oxidation life of NiCrAlY + YSZ coating in the presence of NaCl vapor was shortened compared with that in air. The failure of the TBC exposed to NaCl vapor occurred within the top coat and close to the YSZ/thermal growth oxide (TGO) interface. A finite element analysis was employed to analyze the stress distribution in the coatings. The computed result showed that maximum stresses occurred at the interface between the bond coat and TGO near the edge of the sample and the increased thickness of TGO caused the value of stress in TGO/YSZ interface to increase. The comparison of the maximum stresses indicated that the spinel TGO resulted in significantly higher stresses than Al₂O₃ TGO. This implies that the formation of spinel plays a dominant role in shortening the coating cycling lifetime.     

Yttria-stabilized hafnia-zirconia thermal barrier coatings: The influence of hafnia addition on TBC structure and high-temperature behaviour

The search for more reliable and durable thermal barrier systems is a key factor for future aircraft turbine engines success. Hafnia is therefore an attractive ceramic component due to its similarity to zirconia and its elevated structural transformation temperatures. We report here structural and thermomechanical investigations of various plasma-sprayed coatings composed of ZrO₂+x mol% HfO₂ (x=0, 25, 50 and 100), partially stabilized by 4.53 mol% yttria. X-ray diffraction studies show that, a metastable, non-transformable, high yttrium content, tetragonal solid solution is the only phase observed on the as-sprayed samples. This phase is crystallographically equivalent to the t phase described for classical yttrium-partially stabilized zirconia (Y-PSZ) thermal barrier coatings (TBCs). Upon high-temperature annealing in air (T=1200C), however, the return of this t phase to equilibrium differs from the classical tt+c reaction. According to literature data, reactions of the type tt+c+m should prevail at the highest hafnia contents (x50). Indeed, important quantities of monoclinic phase are accordingly being observed upon cooling. Thermal cycling of TBC samples in air has been performed at 1100C. The Young's modulus of the ceramic coating, which progressively increases when hafnia is substituted for zirconia, has a strong influence on TBC thermomechanical resistance.     

Comparative study of thermal barrier coatings for internal combustion engine

The paper presents the results of investigation into the thermal fatigue resistance of thermal barrier coatings (TBC). Two groups of double-layered thermal barrier coatings (TBC) were investigated: plasma sprayed with ZrO₂-8%Y₂O₃, Al₂O₃-40%TiO₂ or Al₂O₃-40%ZrO₂ top coats and powder flame sprayed with ZrO₂-30%CaO, Al₂O₃-40%TiO₂ and Al₂O₃-30%MgO. The extent of TBC deterioration experienced in thermal fatigue test was evaluated in the erosion test and SEM examinations. Flame sprayed coatings were found more prone to damage than plasma sprayed ones. The highest thermal fatigue resistance revealed TBC plasma sprayed with PSZ. Numerical calculation with Abaqus 6.7 finite element code was used to calculate temperature and stress variations in the coating throughout the test. Phase stability of plasma sprayed Al₂O₃-40%TiO₂ was evaluated by means of X-ray diffraction method. Thermal growth of oxides at the top coat/bond-coat interface and the decomposition of Al₂O₃-40%TiO₂ were found to be important degradation mechanisms leading to the spallation of coatings in the diesel engine and the petrol engine exploitation tests.     

Performance of laser-glazed zirconia thermal barrier coatings in cyclic oxidation and corrosion burner rig tests

The performance of laser-glazed zirconia (containing 8 wt.% Y₂O₃) thermal barrier coatings was evaluated in cyclic oxidation and cyclic corrosion tests. Plasma-sprayed zirconia coatings of two thickness (0.02 and 0.04 cm) were partially melted with a CO₂ laser. The power density of the focused lasser beam was varied from 35 to 75W mm^-2, while the scanning speed was about 80 cm min^-1. In cyclic oxidation tests, the specimens were heated in a burner rig for 6 min and cooled for 3 min. The results obtained indicated that the laser-treated samples had the same life as the untreated samples. However, in corrosion tests, in which the burner rig flame contained 100 ppm Na fuel equivalent, the laser-treated samples exhibited a nearly fourfold life improvement over that of the reference samples. In both tests, the lives of the samples varied inversely with the thickness of the laser-melted layer of zirconia.