Thermal cycling behavior and interfacial stability in thick thermal barrier coatings

The thermal cycling behavior of thermal barrier coatings (TBCs), which were prepared by two different air-plasma spray (APS) guns of 9 MB and TriplexPro™-200, was investigated to understand the effects of the microstructure on the interfacial stability and fracture behavior of TBCs. The porosities of the top coats could be controlled by changing the gun, showing porosity of about 15% using the 9 MB and 19% using the TriplexPro™-200, which decreased slightly with thermal exposure. Defects, such as interlamellar cracks, vertical cracks, and intrasplat cracks, were freshly produced in both TBCs after thermal exposure, showing delamination in the case of 2000 μm TBCs prepared using the TriplexPro™-200. The adhesive strength values of TBCs with 600 and 2000 μm thicknesses were about 8 and 6 MPa, respectively, indicating that the adhesive strength values of TBCs were affected by the coating thickness, independent of the gun. The hardness values increased after thermal exposure, and the TBCs prepared using the TriplexPro™-200 showed higher values than those prepared using the 9 MB for both thicknesses. The toughness values were not dependent on the gun, only showing an effect from coating thickness. The increase in coating thickness enhanced the densification, resulting in higher hardness and toughness values, and the microstructure could be controlled by changing the gun.     

Life time dependency on the pre-coating treatment of a thermal barrier coating under thermal cycling

A thermal barrier coating system consisting of the single crystalline Ni-based superalloy CMSX 4, a Pt aluminide bond coat and an EB-PVD processed ceramic top coat was thermally cycled in order to study the influence of three different treatments prior the deposition of the ceramic top coat. Besides the standard treatment, one type of treatment was annealing in vacuum, while the other was annealing in an O containing ArH atmosphere; in both cases for 4 h at 1080 °C.Compared to the standard treatment, annealing in vacuum almost doubled and annealing in ArH atmosphere almost tripled the cyclic life time of the ceramic coating. The improvement was related to the creation of a defined alumina scale before and during TBC deposition.     

Neodymium-cerium oxide as new thermal barrier coating material

Neodymium-cerium oxide (Nd₂Ce₂O₇) was proposed as a new thermal barrier coating material in this work. Monolithic Nd₂Ce₂O₇ powder was prepared by the solid-state reaction at 1400 °C. The phase composition, thermal stability and thermophysical properties of Nd₂Ce₂O₇ were investigated. Nd₂Ce₂O₇ with fluorite structure was thermally stable in the temperature range of interest for TBC applications. The results indicated that the thermal expansion coefficient (TEC) of Nd₂Ce₂O₇ was higher than that of YSZ (6-8 wt.% Y₂O₃ + ZrO₂) and even more interesting was the TEC change as a function of temperature paralleling that of the superalloy bond coat. Moreover, the thermal conductivity of Nd₂Ce₂O₇ is 30% lower than that of YSZ, which was discussed based on the theory of heat conduction. Thermal barrier coating of Nd₂Ce₂O₇ was produced by atmospheric plasma spraying (APS) using the spray-dried powder. The thermal cycling was performed with a gas burner test facility to examine the thermal stability of the as-prepared coating.     

真空热处理对APS-NiCoCrAlY+APS-YSZ热障涂层热循环寿命的影响

通过大气等离子喷涂在HA188合金基材上制备NiCoCrAlY+YSZ热障涂层,然后分别对试样在高真空和低真空中进行1080 ℃的热处理。通过在1100 ℃对热处理前后的热障涂层进行热循环考核,并在具备EDS的扫描电镜下分析了热循环前后的显微组织和成分。结果表明,真空热处理显著提高了APS-NiCoCrAlY+APS-YSZ热障涂层的热循环寿命,一方面是由于真空热处理后在APS-NiCoCrAlY/APS-YSZ界面上形成连续的TGO层受到了抑制,降低了由TGO产生的应力开裂,另一方面真空热处理后的涂层热循环时在APS-NiCoCrAlY的内部产生了更多的氧化物,在一定程度上降低了APS-NiCoCrAlY的热膨胀系数,减少了由APS-NiCoCrAlY/APS-YSZ热膨胀系数不匹配造成的应力开裂。并且低真空热处理的涂层内部失效裂纹没有像高真空热处理的涂层那样均匀连续扩展。     

等离子喷涂热障涂层的热循环氧化行为研究

本文采用大气等离子喷涂在HA188合金基材上制备NiCrAlY+YSZ热障涂层，并进行了1100 oC、1120 oC和1150 oC三个温度点的高温循环氧化行为对比研究。结果表明，随着考核温度的升高，热障涂层热循环失效寿命显著下降，失效主要是由YSZ/NiCrAlY界面附近YSZ 层中裂纹形成和扩展导致。循环失效后的YSZ与制备态的相结构一样，均为非平衡四方相t"-ZrO2，未发生t"→c+m相变。在热循环过程中，YSZ/NiCrAlY界面形成的热生长氧化物层（Thermally Grown Oxide, TGO）增厚基本符合“抛物线”规律，并且YSZ中裂纹的产生和扩展与TGO的增厚直接相关。     

Laser cycling and thermal cycling exposure of thermal barrier coatings on copper substrates

Thermal barrier coatings (TBCs) are successfully applied in turbines and could also protect combustion chambers in rocket engines. Apart from different loading conditions, the main difference between these applications is the substrate material, which is nickel-based for turbines and copper-based for rocket engines. To optimize the coating system, more knowledge of possible failure modes is necessary.In this work a standard coating system was applied by atmospheric plasma spraying to copper specimens. These specimens were exposed to thermal cycling with different cooling rates and to laser shock testing. A laser-cycling set-up was developed to qualify different coating systems. This set-up consists of a high-power diode laser (3 kW) which provides high heating rates to up to 1500 °C. Laser shock testing has proven to be a suitable alternative to burner rig testing.The results were different to the common failure modes for TBCs on nickel substrates as the coatings system does not fail at the interface between top coat and bond coat, but at the interface between substrate and bond coat. Two failure modes were observed: copper oxide was undermining the coatings at the substrate/bond coat-interface in the case of thermal cycling experiments, and complete delamination occurred at the same interface in the case of laser shock testing. Consequently, this interface is critical in the investigated material system.     

Finite element simulation for mechanical response of surface mounted solder joints under different temperature cycling

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型材挤压过程金属变形流动的有限元仿真

采用大变形弹塑性有限元理论,对典型型材角铝零件的挤压成形过程进行了全面有限元仿真和分析。深入研究了了型材挤压变形过程中金属变形流动规律和力学特征,对挤压工艺参数合理选择和优化奠定了基础。     

Damage mechanisms and lifetime behavior of plasma-sprayed thermal barrier coating systems for gas turbines — Part II: Modeling

A phenomenological lifetime prediction tool for plasma-sprayed ZrO₂ based thermal barrier coating systems with MCrAlY bondcoat is presented. The analytical model uses a two step approach for calculating the development of delamination cracks: The initial crack growth is considered to be proportional to the thickness of the thermally grown oxide (TGO) scale on the bondcoat. After exceeding a critical TGO thickness, crack propagation is governed by linear elastic fracture mechanics taking into account stresses induced by thermal mismatch and by TGO growth. Validation using experimental data from thermal cycling tests with high temperature dwell times from few seconds up to 96 h gave evidence of the good predictive quality of the model.     

Finite element analysis of a barb test for thermal barrier coating delamination toughness measurement

The present study analyzes interface delamination behavior in the thermal barrier coating (TBC)/substrate system during a barb pullout test when the delamination crack is short. With the use of finite element analysis, strain energy release rates and phase angles of the delamination crack are obtained for various specimen dimensions, elastic properties and delamination lengths. These results are useful for designing the geometry of the barb test to obtain various phase angles.     

Thermal cycling failure of new LaMgAl11O19/YSZ double ceramic top coat thermal barrier coating systems

New LaMgAl₁₁O₁₉ (LaMA)/YSZ double ceramic top coat thermal barrier coatings (TBCs) with the potential application in advanced gas-turbines and diesel engines to realize improved efficiency and durability were prepared by plasma spraying, and their thermal cycling failure were investigated. The microstructure evolutions as well as the crystal chemistry characteristics of LaMA coating which seemed to have strong influences on the thermal cycling failure of LaMA and the new double ceramic top coat TBCs based on LaMA/YSZ system were studied. For double ceramic top coat TBC system, interface modification of LaMA/YSZ by preparing thin composite coatings seemed to be more preferred due to the formations of multiple cracks during thermal cycling making the TBC to be more strain tolerant and as well as resulting in an improved thermal cycling property. The effects of the TGO stresses on the failure behavior of the TBCs were discussed through fluorescence piezo-spectroscopy analysis.     

Fracture toughness measurements of plasma-sprayed thermal barrier coatings using a modified four-point bending method

In this study, the adhesion strength of thermal barrier coatings 8YSZ (ZrO₂ + 8 wt.% Y₂O₃) deposited on NiCrAlY bond coats by atmospheric plasma spraying is investigated experimentally. A modified four-point bending specimen that can generate a single interface crack to facilitate the control of crack growth was adopted for testing. The fracture surfaces were examined using a scanning electron microscope. Images show that cracks are initiated along YSZ/NiCrAlY interfaces, then kink and grow uniformly within the YSZ layer. The load-displacement curves obtained indicate three distinct stages in crack initiation and stable crack growth. Based on a microstructural model, finite element analyses were performed to extract the bonding strength of the thermal barrier coatings. The fracture toughness of the plasma-sprayed 8YSZ coatings, in terms of critical strain energy release rate G_c, can be reliably obtained from an analytical solution or from a numerical simulation of the cracking process using compliance methods.     

Finite element analysis of thermal stress in magnetron sputtered Ti coating

The thermal, shear and radial stresses generated in the Ti coating deposited on glass and Si substrates were investigated by finite element analysis (ANSYS). The four-node structural and quadratic element PLANE 42 with axisymmetric option were used to model the Ti coating on glass and Si substrates. The influence of deposition temperature, substrate and coating properties on the generation of thermal stress in Ti is analyzed. It is found that the thermal stress of Ti coating exhibits a linear relationship with deposition temperature and Young's modulus of the coating, but it exhibit an inverse relationship with the coating thickness. The results of simulated thermal stress are in accordance with the analytical method. The radial stress and shear stress distribution of the coating–substrate combination are calculated. It is observed that high tensile shear stress of Ti coating on glass substrate reduces its adhesive strength but high-compressive shear stress of Ti on Si substrate improves its adhesive strength.     

Artificial disbonds for calibration of transient thermography inspection of thermal barrier coating systems

Transient thermography is commonly used for the detection of disbonds in thermal barrier coatings (TBC). As for other NDT techniques, reference test specimens are required for calibration, but unfortunately, real disbonds are very difficult to use because it is difficult to control their size, and large ones tend to spall. Flat bottomed holes are commonly used, but these over-estimate the thermal contrast obtained for a defect of a given diameter. This paper quantifies the differences in thermal response using finite element analysis validated by experiments, and proposes a form of artificial disbond that gives a better representation of the thermal responses seen with real defects. Real disbonds tend to have a non-uniform gap between the disbonded surfaces across the defect, and the effect of this on the thermal response is evaluated using finite element simulations. It is shown that the effect can be compensated for by adjusting the diameter of the calibration defect compared to the real defect.     

真空热处理双粘结层热障涂层抗氧化性能

传统单层结构粘结层热障涂层抗氧化性能不足寿命短,采用超音速火焰喷涂(high velocity oXy-fuel,HVOF)和大气等离子喷涂(atmosphere plasma spray,APS)制备双层结构粘结层,对粘结层进行真空热处理,研究热障涂层的抗氧化性能.结果 显示,经过1050℃×3 h真空热处理,粘结层...     

基于内聚力单元与XFEM的热障涂层失效分析

为了更好的理解热障涂层的失效机理,文中运用ABAQUS有限元软件来分析热障涂层的失效情况,使用内聚力单元和扩展有限元（XFEM）两种方法研究热障涂层TGO界面开裂与陶瓷涂层（TC）和氧化层（TGO）内随机裂纹的萌生与扩展,研究竖直裂纹与水平裂纹的关系.结果表明,热障涂层TGO界面的开裂首先出现在TGO/TBC波谷处.陶瓷涂层和氧化层内随机裂纹的萌生同样发生在TGO/TBC波谷处.竖直裂纹的存在可以抑制水平裂纹的萌生与扩展,且其在TGO/TBC波谷处的扩展长度比在TGO/TBC波峰处的扩展长度更长,说明TGO/TBC波谷区域是个危险区域,在此区域容易引发裂纹的萌生与扩展.     

Effects of material creep on displacement instability in a surface groove under thermo-mechanical cycling

Displacement instability along the interface between a zirconia top coat and thermally-grown oxide (TGO) forming metal substrate is a fundamental source of failure in some thermal barrier coatings (TBCs). In this work, the effects of material creep on displacement instability near the surface groove on Fecralloy substrate subjected to thermo-mechanical cycling are investigated using finite element analysis (FEA). Assumptions and references about material properties are minimized for a more realistic analysis. Most material properties, such as TGO growth and creep parameters, are based on in-house experimental measurements. Our results show that under a small magnitude of tensile stress, cyclic history plays a major role in displacement instability, and that this role significantly fades as stress increases. We also show that creep in the substrate material predominates at the holding time of a cycle. Creep significantly enhances the deformation near the groove without mechanical load applied on the specimen, while it contributes to length change rather than deformation of the groove as the load level increases.     

DD90单晶高温合金热障涂层抗氧化及热循环性能

在第3代单晶高温合金DD90上制备热障涂层,采用超音速火焰喷涂(HVOF)制备NiCrAlY+NiCoCrAlTaY双层结构的粘结层,大气等离子喷涂(APS)制备YSZ和MSZ/YSZ结构的陶瓷层。在1150℃抗氧化试验中,YSZ涂层增重量明显高于MSZ涂层。在1200℃热循环250次后,2种涂层没有发生明显相变,MSZ涂层抗烧结性更优异。2种涂层TGO主要由Al₂O₃及少量尖晶石结构的混合氧化物组成。双层粘结层减少了Al元素向基材的扩散,而Cr元素由于浓度梯度扩散导致拓扑密堆(tcp)相的析出及二次反应区(SRZ)深度的增加。     

TiC金属陶瓷/铸铁钎焊接头热应力的有限元模拟

研究了采用Ag-Cu-Zn钎料在1173K温度下钎焊TiC金属陶瓷与铸铁时，接头在冷却过程中的热应力最大值和应力集中区。模拟结果表明，在冷却过程中，铸铁／TiC金属陶瓷接头的剪应力主要集中在界面端点处，且剪应力的最大值出现在Ag-Cu-Zn／TiC金属陶瓷界面处。TiC金属陶瓷下表面的拉应力最大值出现在TiC金属陶瓷的端点处，且随着连接温度的降低拉应力的最大值逐渐降低。TiC金属陶瓷下表面的压应力最大值出现在TiC金属陶瓷中部，且随着连接温度的降低压应力值逐渐增加。     

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.