Simulation of thermal barrier coating spallation induced by the initiation/growth/coalescence of internal crack and interfacial crack based on a real image model

In the furnace cycle test, the growth of oxide film leads to the propagation and coalescence of multiple cracks near the interface, which should be responsible for the spallation of thermal barrier coatings (TBCs). A TBC model with real interface morphology is created, and the near-interface large pore is retained. The purpose of this work is to clarify the mechanism of TBC spallation caused by successive initiation, propagation, and linkage of cracks near the interface during thermal cycle. The dynamic growth of thermally grown oxide (TGO) is carried out by applying a stress-free strain. The crack nucleation and arbitrary path propagation in YSZ and TGO are simulated by the extended finite element method (XFEM). The debonding along the YSZ/TGO/BC interface is evaluated using a surface-based cohesive behavior. The large-scale pore in YSZ near the interface can initiate a new crack. The ceramic crack can propagate to the YSZ/TGO interface, which will accelerate the interfacial damage and debonding. For the TGO/BC interface, the normal compressive stress and small shear stress at the valley hinder the further crack propagation. The growth of YSZ crack and the formation of through-TGO crack are the main causes of TBC delamination. The accelerated BC oxidation increases the lateral growth strain of TGO, which will promote crack propagation and coalescence. The optimization design proposed in this work can provide another option for developing TBC with high durability.     

纳米陶瓷的制备

纳米陶瓷材料可显著降低材料的烧结致密化程度,节约能源;并可改善陶瓷材料的性能,提高其使用可靠性。主要介绍了纳米陶瓷的制备方法,包括纳米陶瓷粉体的制备、成型和烧结。     

PVD TBC experience on GE aircraft engines

The higher performance levels of modern gas turbine engines present significant challenges in the reli-ability of materials in the turbine. The increased engine temperatures required to achieve the higher per-formance levels reduce the strength of the materials used in the turbine sections of the engine. Various forms of thermal barrier coatings have been used for many years to increase the reliability of gas turbine engine components. Recent experience with the physical vapor deposition process using ceramic material has demonstrated success in extending the service life of turbine blades and nozzles. Engine test results of turbine components with a 125 μm (0.005 in.) PVD TBC have demonstrated component operating tem-peratures of 56 to 83 °C (100 to 150 °F) lower than non-PVD TBC components. Engine testing has also revealed that TBCs are susceptible to high angle particle impact damage. Sand particles and other engine debris impact the TBC surface at the leading edge of airfoils and fracture the PVD columns. As the impacting continues, the TBC erodes in local areas. Analysis of the eroded areas has shown a slight increase in temperature over a fully coated area ; however, a significant temperature reduc-tion was realized over an airfoil without TBC.     

脱粘裂纹扩展诱发热障涂层陶瓷层温度场演变与烧结行为研究

目的 研究等离子喷涂热障涂层(APS-TBCs)内部脱粘裂纹尺寸对陶瓷层温度场与烧结行为的影响.方法 采用有限元模型研究不同尺寸脱粘裂纹对其上方陶瓷层温度场变化规律的影响,并通过梯度热循环试验对裂纹上方陶瓷层烧结行为进行研究,采用扫描电子显微镜(SEM)分析陶瓷层表面和断面的组织形貌,并使用图片法对裂纹上方陶瓷层不同区域的孔隙率进行分析.结果 裂纹上方陶瓷层温度变化区域的面积取决于裂纹尺寸,且最高温度始终位于裂纹中心的上方陶瓷层表面,随着裂纹宽度向两端依次递减,其对应的上方温度依次递减.在本试验条件下,随着脱粘裂纹尺寸的增加,涂层厚度方向的最高温度以线性形式增加,裂纹尺寸每增加0.5 mm,其上方陶瓷层同一位置处的温度增加约30℃,且最高温度区域随之增大.裂纹长度超过3 mm时,在脱粘裂纹热阻效应下,裂纹上方陶瓷层区域的温度升高.裂纹越长,上方陶瓷层整体温度提升越高,不仅导致陶瓷层发生相变和烧结的区域增大,还使得相变和烧结速率升高.不同梯度热循环次数下,陶瓷层表面和内部均出现了数量和长度不等的脱粘、垂直裂纹,烧结面积逐渐增大.脱粘裂纹长度为4 mm时,其最大宽度约40μm.结论 脱粘裂纹上方陶瓷层温度变化以表面最高温度处为中心向四周呈放射性递减.越靠近陶瓷层表面,层状组织信息越少,相应区域的孔隙率越小,烧结和相变程度逐渐增加,使涂层发生脱落失效的可能性进一步增加.     

热障涂层无损检测技术研究进展

为提高发动机的涡轮前温度和热端部件服役寿命,热障涂层（TBCs）被广泛应用于燃气涡轮发动机。热障涂层具有多相、多界面和非均质特性,且其服役工况恶劣复杂。寻找一种可以表征涂层显微组织、缺陷、热物性、应力等反映涂层质量和剩余寿命的无损检测方法,对发动机的热端部件安全性和可靠性至关重要。文中综述了超声检测技术（UT）、声发射技术（AE）、红外热成像技术（IRT）、阻抗谱技术（IS）和光激发荧光压电光谱技术（PLPS）的原理以及其在热障涂层无损检测中的研究应用,并详细介绍了太赫兹时域光谱（THz-TDS）技术及其在热障涂层中的应用。最后总结了上述无损检测方法的检测能力,并对热障涂层无损检测方法进行展望。     

Synthesis and thermophysical properties of ATa2O6 (A = Co,Ni, Mg,Ca) tantalates with robust CMAS resistance

A new family of ceramic environmental/thermal barrier coating (E/TBC) materials, that is, ATa₂O₆ (A = Co, Ni, Mg, Ca), for high-temperature applications, are investigated and reported in this study. We focus on the synthesis and features of crystal structures, and on the mechanical and high-temperature properties. ATa₂O₆ oxides have an extraordinary phase stability (up to 1300°C), and their thermal expansion coefficients (6.2–7.3 × 10⁻⁶ K⁻¹) match those of SiC fiber-enhanced SiC ceramic matrix composites (3–7 × 10⁻⁶ K⁻¹). Their low thermal conductivities (min: 1.15 W·m⁻¹·K⁻¹) root in the slow phonon spreading speed and fierce phonon-phonon scattering process, and they will provide exceptional thermal insulation. Moreover, their hardness (5.6–8.8 GPa), toughness (1.4–1.9 MPa·m^1/2), and moduli (100–210 GPa) have good comparability with current E/TBCs. We propose the 33CaO-9MgO-13AlO_1.5-45SiO₂ (CMAS) corrosion mechanisms of ATa₂O₆ ceramics, and their robust CMAS resistance relies on the phase stability of CaTa₂O₆ oxides. The excellent high-temperature properties ensure that ATa₂O₆ can be used as E/TBCs to provide thermal insulation and CMAS corrosion protection.     

The phase stability and thermophysical properties of InFeO3(ZnO)m (m = 2, 3, 4, 5)

The phase stability and thermophysical properties of InFeO₃(ZnO)_m (m = 2, 3, 4, 5) compounds were investigated, which are a general family of homologous layered compounds with general formula InFeO₃(ZnO)_m (m = 1–19). InFeO₃(ZnO)_m (m = 2, 3, 4, 5) ceramics were synthesized using cold pressing followed by solid-state sintering. They revealed an excellent thermal stability after annealing at 1450 °C for 48 h. No phase transformation occurred during heating to 1400 °C. InFeO₃(ZnO)₃ exhibited a thermal conductivity of 1.38 W m⁻¹ K⁻¹ at 1000 °C, which is about 30% lower than that of 8 wt.% yttria stabilized zirconia (8YSZ) thermal barrier coatings. The thermal expansion coefficients (TECs) of InFeO₃(ZnO)m bulk ceramics were in a range of (10.97 ± 0.33) × 10⁻⁶ K⁻¹ to (11.46 ± 0.35) × 10⁻⁶ K⁻¹ at 900 °C, which are comparable to those of 8YSZ ceramics.     

Infrared radiative properties of plasma-sprayed BaZrO3 coatings

The room temperature directional-hemispherical reflectance and transmittance spectra of free-standing atmospheric plasma-sprayed BaZrO₃ coatings with different thicknesses were measured in the wavelength range of 0.8–15.0 μm, and the absorption coefficient and scattering coefficient as a function of wavelength were extracted using the modified four-flux model. Results showed that BaZrO₃ is a high scattering, low absorption material at the wavelength <6 μm, where turbine engine thermal radiation is most concentrated. The absorption coefficient of BaZrO₃ starts to increase rapidly at wavelength of 6 μm, indicating that BaZrO₃ is high absorbing and opaque in the long wavelength range. A pronounced absorption peak occurs at a wavelength of 7 μm, and is associated with a BaCO₃ coating impurity. The scattering coefficient of BaZrO₃ decreases with the increase of wavelength in the whole measured wavelength range, caused by the decrease of the relative size of the scattering center compared with the wavelength.     

等离子喷涂-物理气相沉积防护涂层及其失效机理研究进展

防护涂层在航空发动机和燃气涡轮发动机免受水氧腐蚀和盐雾侵蚀、耐高温、抗冲刷和服役寿命方面发挥着重要的作用。等离子喷涂-物理气相沉积(PS-PVD)技术以其大功率、喷射范围广、能够使粉末原料以气相甚至离子化和非视线沉积特性等特点被广泛应用在热/环境障涂层制备领域,形成的涂层具有沉积效率高、孔隙率较高、热导率低、应变容限高等特点。首先,对PS-PVD热障涂层的表面形貌、柱状晶特点和等离子射流特性进行了探讨。然后,讨论了PS-PVD YSZ热障涂层表面镀铝改性对涂层抗CMAS侵蚀、减少孔隙和裂纹、TGO的生成和提高热循环寿命方面的重要作用,结果表明表面镀铝能显著减少涂层表面孔隙、凹坑、微裂纹和熔融盐对于热障涂层的不利影响。其次,简要概述了近年来SiC_f/SiC CMCs代替镍基高温合金在航空发动机基体上抗高温高压和快速失效方面发挥的重要作用。SiC_f/SiC CMCs与T/EBCs的结合,有效延长了航空发动机的使用寿命,是我国航空事业的又一次重大突破。此外,重点介绍了PS-PVD非视线沉积特性和涂层阴影效应在涂层沉积方面这一特色,总结了TGO、残余...     

Impact of ZrO2 alloying on thermo-mechanical properties of Gd3NbO7

The ZrO₂ alloying effect is widely used to optimize the thermo-mechanical properties of potential thermal barrier coatings. In this study, dense x mol% ZrO₂-Gd₃NbO₇ with C222₁ space group were manufactured via a solid-state reaction. The crystalline structure was determined through X-ray diffraction and Raman spectroscopy, when the surface morphology was observed by scanning electron microscopy. ZrO₂-Gd₃NbO₇ had identical orthorhombic crystal structures, and there was no second phase. The crystalline structure of ZrO₂-Gd₃NbO₇ shrunk with the increasing ZrO₂ content as indicated by XRD and Raman results. The heat capacity and thermal diffusivity of ZrO₂-Gd₃NbO₇ were 0.31–0.43 J g⁻¹ K⁻¹ (25–900 °C) and 0.25–0.70 mm²/s (25–900 °C), respectively. It was found that ZrO₂-Gd₃NbO₇ had much lower thermal conductivity (1.21–1.82 W m⁻¹ K⁻¹, 25–900 °C) than YSZ (2.50–3.00 W m⁻¹ K⁻¹) and La₂Zr₂O₇ (1.50–2.00 W m⁻¹ K⁻¹). The thermal expansion coefficients (TECs) were higher than 10.60 × 10⁻⁶ K⁻¹ (1200 °C), which were better than that of YSZ (10.00 × 10⁻⁶ K⁻¹) and La₂Zr₂O₇ (9.00 × 10⁻⁶ K⁻¹). The mechanical properties of Gd₃NbO₇ change little with the increasing ZrO₂ content, Vickers hardness was about 10 GPa, and Young's modulus was about 190 GPa, which was lower than YSZ (240 GPa). Compared with previous work about alloying effects, much lower thermal conductivity was obtained. Due to the high melting point, high hardness, low Young's modulus, ultralow thermal conductivity and high TECs, it is believed that ZrO₂-Gd₃NbO₇ is promising TBCs candidate.