Residual Stresses and Their Effects on the Durability of Environmental Barrier Coatings for SiC Ceramics

Qualitative residual stresses in current environmental barrier coatings (EBCs) were inferred from the curvature of EBC-coated SiC wafers, and the effects of EBC stresses on the durability of EBC-coated SiC were evaluated. The magnitude of substrate curvature correlated fairly well with the EBC–SiC coefficient of thermal expansion (CTE) mismatch, EBC modulus, and thermally induced physical changes in EBC. BSAS (1− x BaO· x SrO·Al₂O₃·2SiO₂, 0≤ x ≤1) components in the current EBCs, i.e., Si/mullite or mullite+BSAS/BSAS or yttria-stabilized zirconia (YSZ: ZrO₂–8 wt% Y₂O₃), were the most beneficial for reducing the EBC stress in as-sprayed as well as in post-exposure EBCs. The reduced stress was attributed to the low modulus of BSAS. The addition of a YSZ top coat significantly increased the substrate curvature because of its high CTE and sintering in thermal exposures. There were clear correlations between the wafer curvature and the EBC durability. The Si/mullite+20 wt% BSAS/BSAS EBC maintained excellent adherence, protecting the SiC substrate from oxidation, while the Si/mullite+20 wt% BSAS/YSZ EBC suffered delamination, leading to severe oxidation of the SiC substrate, after a 100 h −1300°C exposure in a high-pressure burner rig.     

Engineered Multilayer Thermal Barrier Coatings for Enhanced Durability and Functional Performance

The durability of plasma sprayed thermal barrier coatings (TBCs) has been of significant interest ever since their introduction in gas turbine engine components. Of particular importance is the role of coating processing, microstructure and ensuing properties on their thermal cycle life. Among the coating properties of the ceramic top coat that have shown strong correlations with durability include the elastic modulus (i.e., compliance) and the fracture toughness, both of which are influenced by processing as well as thermal aging during service. In this article, we have systematically investigated furnace cycle durability of plasma sprayed TBCs produced from controlled processing conditions, yielding differences in both modulus and toughness. Following performance assessment and mechanistic insights obtained from single layer ceramic coatings, novel bilayer architectures have been proposed and fabricated, in an effort to improve furnace cycle durability. The bilayer approach targets coating properties based on location, by providing dense, high toughness coating at regions prone to delamination failure (near‐interface), while allowing for the majority of the coating to contain high porosity, resulting in reduced overall modulus. Such improved bilayers simultaneously display both high durability and low thermal conductivity enabling a promising approach for functionally optimized coatings. The plasma spray process together with its ability to dynamically change process parameters enables the fabrication of these novel architectures.     

Effects of Interface Roughness on Residual Stresses in Thermal Barrier Coatings

Using a newly developed object-oriented finite-element analysis method, both an actual microstructure and model microstructures of a plasma-sprayed thermal barrier coating system were numerically simulated to analyze the full-field residual stresses of this coating system. Residual stresses in the actual microstructure were influenced by both the irregular top-coat/bond-coat interface and cracks in the top coat. By treating the microcracked top coat as a more-compliant solid microstructure, the effects of the irregular interface on residual stresses were examined. These results then could be compared to results that have been obtained by analyzing a model microstructure with a sinusoidal interface, which has been considered by some earlier investigators.     

The Measurement of Residual Strains Within Thermal Barrier Coatings Using High-Energy X-Ray Diffraction

The residual strains through the entire thickness of the zirconia layer of pristine and heat-treated thermal barrier coatings (TBCs) were mapped to help elucidate the failure mechanisms of TBCs. The strains were measured using 80.72 keV synchrotron radiation and a transmission geometry. The heat-treated TBC showed that a compressive strain formed in the zirconia layer of the TBC on cooling but this strain was diluted and reversed by the oxidation-driven expansion of the underlying metals. It also showed large (0.0024) out-of-plane tensile strains in the zirconia layer just above its interface with a thick underlying oxide layer.     

热障涂层氧化层厚度对残余应力的影响

对热障涂层氧化过程进行了详细的阐述,推算了氧化增厚动力曲线,采用ANSYS分别建立了不同氧化层厚度的热障涂层微观二维模型,氧化层界面形状简化为正弦形式。模拟了氧化层不同特征厚度时的应力变化,分析了不同氧化增长方向对各层残余应力的影响。结果表明:随着氧化层的增厚,粘结层波峰处σy增幅较大,易在氧化层和粘结层间发生分层。氧化层分别向TBC层、BC层以及同时向两层方向增长时,发现不同的氧化层增长方向对各层内的残余应力影响不大,可以忽略其影响。     

Damage Evolution and Stress Analysis in Zirconia Thermal Barrier Coatings during Cyclic and Isothermal Oxidation

The failure mechanisms of air-plasma-sprayed ZrO₂ thermal barrier coatings with various microstructures were studied by microscopic techniques after thermal cycling. The elastic modulus ( E ) and hardness ( H ) of the coatings were measured as functions of the number of thermal cycles. Initially, both E and H increased by ∼60% with thermal cycling because of sintering effects. However, after ∼80 cycles (0.5 h at 980°C), the accumulated damage in the coatings led to a significant decrease of ∼20% of the maximum value in both E and H . These results were correlated with stresses measured by a spectroscopic technique to understand specific damage mechanisms. Stress measurement and analysis revealed that the stress distribution in the scale was a complex function of local interface geometry and damage in the top coat. Localized variations in geometry could lead to variations in measured hydrostatic stresses from −0.25 to −2.0 GPa in the oxide scale. Protrusions of the top ZrO₂ coat into the bond coat were localized areas of high stress concentration and acted as damage-nucleation sites during thermal and mechanical cycling. The net compressive hydrostatic stress in the oxide scale increased significantly as the scale spalled during thermal cycling.     

Determination of Residual Stress in Coatings by a Membrane Deflection Technique

A membrane deflection technique has been developed to measure the isotropic residual stress in biaxially-constrained coatings. The technique has been demonstrated on various materials, including polyimide, latex rubber and photoresist coatings. Stress values obtained from membrane deflection compared well with results obtained from time-averaged vibrational holographic interferometry except for values obtained from samples where rigidity effects were found to be important. A criterion based on the thickness, rigidity, stress and sample radius is also discussed, establishing the applicability of the technique to samples of low rigidity.     

Determination of Residual Stress in Coatings by a Membrane Deflection Technique

A membrane deflection technique has been developed to measure the isotropic residual stress in biaxially-constrained coatings. The technique has been demonstrated on various materials, including polyimide, latex rubber and photoresist coatings. Stress values obtained from membrane deflection compared well with results obtained from time-averaged vibrational holographic interferometry except for values obtained from samples where rigidity effects were found to be important. A criterion based on the thickness, rigidity, stress and sample radius is also discussed, establishing the applicability of the technique to samples of low rigidity.     

激光强化电刷镀Ni镀层残余应力研究

利用Nd3+:YAG激光器和电刷镀设备制备了激光强化电刷镀Ni层,采用X-射线衍射法测定了镀层的轴向残余应力,分析了残余应力随镀层厚度变化的情况.结果表明,当镀层δ从10μm增加到200μm时,激光电刷镀Ni层轴向残余应力由压应力逐渐过渡为拉应力,最大值为197 MPa(激光功率300W、镀层δ为200μm),比普通电...     

Upper Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS

Current state-of-the-art environmental barrier coatings (EBCs) for Si-based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al₂O₃·2SiO₂) or mullite + BSAS ((1− x )BaO· x SrO·Al₂O₃·2SiO₂, 0 ≤ x ≤ 1) layer, and a BSAS top coat. Areas of concern for long-term durability are environmental durability, chemical compatibility, volatility, phase stability, and thermal conductivity. Variants of this family of EBC were applied onto monolithic SiC and melt-infiltrated SiC/SiC composites. Reaction between BSAS and silica results in a low-melting (∼1300°C) glass, which can cause the spallation of the EBC. At temperatures greater than ∼1400°C BSAS suffers significant recession via volatilization in water-vapor-containing atmospheres. Both reactions can be EBC life-limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. Initial rapid increase in thermal conductivity at temperatures as low as 1300°C indicates the sintering of EBC.     

Multilayer Design and Evaluation of a High Temperature Environmental Barrier Coating for Si-Based Ceramics

Two types of environmental barrier coatings for silicon nitride are investigated. In Type A coatings, a bilayer of polymer-derived SiCN and zirconia was deposited on silicon nitride. This coating nearly completely suppressed oxidation of Si₃N₄ at 1350°C in a 900 h-long test. But the coating volatilized in a streaming water vapor environment. In Type B coatings, a topcoat of hafnia was built on to the SiCN coating with an intermediate compliant layer to accommodate the thermal expansion mismatch between hafnia and Si₃N₄. The three-layer design was successful in preventing both oxidation as well as weight loss in the silicon nitride at temperatures up to 1300°C. The compliant interlayer was made from a porous microstructure, which approximately followed the guidelines subscribed by a model based on the columnar design. The results lead to both expected and unexpected findings. The three-layer design used to accommodate thermal expansion followed the prediction from the model. But the suppression of oxidation by the SiCN and zirconia overlayer, in Type A coatings, was unexpected. Inhibition of oxygen diffusion by zircon, which appears to have formed by a reaction between SiCN and zirconia, is one possible explanation. Another explanation is that ionic diffusion of oxygen in zircon overlayer and the molecular diffusion of oxygen through the silica interlayer just below creates an electrical field, which opposes ionic diffusion of oxygen through zircon.     

氟树脂乳液复层涂料的研制

采用合成氟树脂乳液为粘结剂，通过不同种类、粒径的体质颜料合理搭配以及控制体质颜料与基料配比，并选择合适的助剂，制成了性能优异的氟树脂乳液复层建筑涂料。其耐沾污率为14．5％，耐人工老化性通过1000h。     

Piezospectroscopic Analysis of Interface Debonding in Thermal Barrier Coatings

One of the principal modes by which electron-beam-evaporated thermal barrier coatings fail is via the nucleation of local regions of debonding, which grow and link together until reaching a critically sized flaw for spontaneous buckling and spalling. This progressive-failure mode is used as a basis for analyzing the changes that can occur in photostimulated luminescence spectra that have been recorded from the thermally grown oxide. This process also provides a basis for the quantitative determination of the extent of local damage prior to spalling from an analysis of the shape of the luminescence spectra.     

Residual Stress and Microstructural Evolution in Tantalum Oxide Coatings on Silicon Nitride

Due to its coefficient of thermal expansion (CTE) and phase stability up to 1360°C, tantalum oxide (Ta₂O₅) was identified and investigated as a candidate environmental barrier coating for silicon nitride-based ceramics. Ta₂O₅ coatings were plasma sprayed onto AS800, a silicon nitride ceramic from Honeywell International, and subjected to static and cyclic heat treatments up to 1200°C in air. Cross-sections from coated and uncoated substrates were polished and etched to reveal the effect of heat treatments on microstructure and grain size. As-sprayed coatings contained vertical cracks that healed after thermal exposure. Significant grain growth that was observed in the coatings led to microcracking due to the anisotropic CTE of Ta₂O₅. High-energy X-ray diffraction was used to determine the effect of heat treatment on residual stress and phases. The uncoated substrates were found to have a surface compressive layer before and after thermal cycling. Coating stresses in the as-sprayed state were found to be tensile, but became compressive after heat treatment. The microcracking and buckling that occurred in the heat-treated coatings led to stress relaxation after long heat treatments, but ultimately would be detrimental to the function of the coating as an environmental barrier by affording open pathways for volatile species to reach the underlying ceramic.     

Microtexture Analysis of the Alumina Scale in Thermal Barrier Coatings

Transmission electron microscopy‐based grain orientation mapping method was employed to investigate the microtexture of the alumina scale formed in commercial thermal barrier coatings (TBCs) with two standard types of Pt‐enriched bond coats. Reliable orientation/phase maps with a spatial resolution down to 2 nm were acquired on the alumina grains. It was observed that the alumina scale on the Pt‐aluminide β‐phase bond coat has a stronger c‐axis texture normal to the bond coat surface, in comparison with that on the Pt‐diffused γ/γ′‐phase bond coat. The microtexture of the alumina scale could affect its effective coefficient of thermal expansion, which is a contributor to the severity of the bond coat rumpling mechanism of TBCs failure.     

Ceramic Composites with Multilayer Interface Coatings

Silicon carbide matrix composites have been fabricated from either ceramic-grade Nicalon^TM or Hi-Nicalon^TM fibers coated with an interface material consisting of six alternating carbon and silicon carbide layers. Initial efforts involved the use of chemical vapor infiltration to produce minicomposites (single tows of fibers). In subsequent work, forced-flow thermal-gradient chemical vapor infiltration was used to produce a single composite plate with a multilayer interface from ceramic-grade Nicalon fabric and two plates from Hi-Nicalon fabric, one with a single carbon layer and one with a multilayer interface. Tensile testing of the minicomposites and of specimens cut from the plates revealed typical composite behavior and strengths for the as-processed samples. Exposure of tensile specimens to 950°C air for 100 h resulted in large losses in strength and strain tolerance regardless of the interface coating. The results demonstrate that forced-flow thermal-gradient chemical vapor infiltration can be used to prepare multilayer interface material. The results also verified that relatively thick (>100 nm) single or multiple carbon layers are susceptible to oxidation that causes the loss of composite properties.     

Fatigue of Thick Thermal Barrier Coatings

Thick thermal barrier coatings (TTBCs) of plasma-sprayed 8% Y₂O₃–ZrO₂ were fatigued in compression as part of a reliability and durability study to evaluate their potential use in high-performance diesel engines. Test specimens were designed to test the bulk ceramic uniaxialiy, independent of the substrate. A test machine was designed to alleviate the mechanical gripping and alignment difficulties associated with cyclically stressing brittle ceramics in compression. Higher fatigue limits, 375 vs 200 MPa, were observed at 800°C than at room temperature. Specimens tested at room temperature after high-temperature com-pressive cycling also had higher fatigue limits, indicating that the strengthening was permanent. At temperatures of 800°C, the coatings showed evidence of low-temperature, pressure-induced sintering. The extent to which sintering occurred was determined by studying the change in the elastic modulus as a result of the application of varying temperatures and static stresses.     

液化石油气钢瓶残余应力测试分析

采用钻孔法对液化石油气钢瓶的残余应力进行测试,并分析了环焊缝、角焊缝和封头位置的残余应力分布特性与钻孔法测试残余应力的适用性,为开展液化石油气钢瓶出厂残余应力抽样检测提供借鉴。