Oxidation-Based Model for Thermal Barrier Coating Life

A procedure is described for modeling the lives of thermal barrier coatings subjected to high-temperature environments. The model is used to calculate cycles-to-failure as a function of heating cycle duration. It is based on the presumption that oxidation is the single important time-dependent factor which limits the life of these coatings, and that oxidation-induced strains combine with cyclic strains to promote slow crack growth in the ceramic layer. Good agreement is obtained between calculated and experimental lives for specimens tested in a furnace. This shows that an oxidation-based approach is promising. The importance of reproducible specimen preparation is also discussed.     

Multilayer,Multimaterial Thermal Barrier Coating Systems: Design,Synthesis, and Performance Assessment

Thermal barrier coatings (TBCs) are increasingly playing a vital role in enhancing efficiency and performance of gas turbine engines. As engine operating temperatures rise, yttria‐stabilized zirconia (YSZ), the currently principal TBC material, reaches its operational limits. Gadolinium Zirconate (GDZ)‐based pyrochlore oxides are now emerging contenders, not only due to their lower thermal conductivity, but also their ability to resist attack by silicate deposits. However, GDZ cannot be directly substituted for YSZ due to its incompatibility with the thermally grown alumina layer, therefore requiring to be a component of multilayer system. Although industry has already adopted these materials in various applications, a number of fundamental issues arise with respect to layered‐coating design, their properties and processing dependence. In this study several multilayer architectures, based on the YSZ–GDZ system, have been developed and tested for durability under furnace thermal cycling conditions. Coating designs considered optimization of microstructure and properties of individual layers based on their location within the top‐coat thickness to address competing interests of thermal conductivity, compliance, and resistance to silicates. A large variance in durability was observed in coatings made with different multilayer designs. The results and the associated failure mechanisms are rationalized through preliminary evaluation of elastic energies at the failure locations and corresponding energy release rates. The results point to new strategies in the design and manufacturing of optimal multilayer coatings.     

Noncontact Methods for Measuring Thermal Barrier Coating Temperatures

Three noncontact, optical methods for measuring temperature are reviewed with an emphasis on their application to the measurement of temperatures of thermal barrier coatings (TBCs). The methods are: infrared pyrometry, Raman spectroscopy, and photo-stimulated luminescence from lanthanide-doped coatings. Although each has the capability of measuring temperatures pertinent to monitoring TBCs, the finite thickness of typical coatings together with the optical properties of zirconia place severe restrictions on the depth from which the temperature sensing can be obtained. Some of these limitations can be circumvented using photo-stimulated luminescence with coatings containing dopants at specific locations. To illustrate this, it is demonstrated that by depositing coatings with a lanthanide dopant, such as Eu³⁺, at specific locations, for instance in contact with the metallic alloy, temperature sensing can be performed with much higher spatial resolution.     

Thermal Analysis of Reactions in Soda–Lime Silicate Glass Batches Containing Melting Accelerants: II, Multicomponent Systems

The glass melting reactions in a multicomponent system (sand–soda ash–calcite–dolomite–feldspar) were studied using data from DTA, TGA, and XRD interactively. The first-formed liquid phase occurred at 700°C from eutectic melting among CaCO₃, Na₂CO₃, and MgO. Further liquid phase formed at the CaCO₃–Na₂CO₃, eutectic at 785°C and a fusion reaction among SiO₂, CaO, and the molten phase at 812°C. Reactions between molten soda ash and silica grains to form a sodium disilicate coating also occurred in this temperature range. The effects of reaction accelerant additions (Na₂SO₄, NaNO₃, NaCI) on batch fusion were analyzed. Sodium chloride was found to be the most effective melting accelerant due to the formation of a NaCI–Na₂CO₃ eutectic liquid phase at ∼636°C, which effectively attacked the silica relic. CO₂ gas release terminated ∼80°C earlier with 1 wt% NaCI additions to the base glass.     

水性隔热保温节能涂料的制备研究

以丙烯酸弹性乳液为基料,空心玻璃微珠、金红石型钛白粉、云母及空心硼硅酸盐等为颜填料制备隔热保温涂料,该涂料性能稳定、表面光滑、涂层薄,耐沾污、热反射效率高,隔热性能良好,适应范围广。     

Thermal Analysis of Reactions in Soda–Lime Silicate Glass Batches Containing Melting Accelerants: I, One- and Two-Component Systems

To identify each glass melting reaction in a multicomponent system, one-component and two-component reaction processes were studied using DTA, TGA, and XRD. Two-component mixtures were prepared by choosing pairs in the same ratio as in a commercial container glass batch composition (sand-soda ash-calcite-dolomite-feldspar). The presence of silica in the silica-calcite system decreased the termination temperature of the decomposition of calcite, but did not alter the onset of decomposition. Similar behavior was found in the dolomite-silica system. A double carbonate (Na₂Ca(CO₃)₂) formed via solid-state reaction in the calcite-soda ash system, and metasilicate/disilicate phases were detected during the fusion process in the soda ash-silica system. The effects of reaction accelerant additions in the soda ash-silica system were investigated using 1 wt% additions of sodium sulfate, sodium nitrate, and sodium chloride. Sodium chloride was the most effective melting accelerant, lowering the termination temperature of CO₂ release by ∼80°C compared with the soda ash-silica system with no additives. NaCl additions caused complete reaction and/or fusion of Na₂CO₃ prior to its melting temperature. Sodium sulfate additions acted to suppress metasilicate/ disilicate formation by coating quartz grains and shifted consequent CO₂ release to higher temperature.     

Observation of Subcritical Spall Propagation of a Thermal Barrier Coating

Observations are reported of the room-temperature propagation of a spalling failure mode of a thermal barrier coating (TBC) from its bond coat after oxidation. The coating is a Y₂O₃-stabilized ZrO₂ coating formed by electron-beam deposition on a Ni-Co-Cr-Al-Y bond coat. The spall shape evolution and stress redistribution as the spall propagates are reported. The failure propagates primarily as an interface crack between the bond coat and the thermally grown aluminum oxide (TGO) formed on the underside of the TBC during oxidation. The observations are consistent with subcritical propagation of an interface crack between the TGO and bond coat assisted by the presence of moisture. An estimate of 9 J/m² is made of the fracture resistance in air of the interface.     

低熔点聚酰胺的结晶和熔融行为

采用差示扫描量热法和X射线衍射法对低熔点聚酰胺的结晶和熔融行为进行研究,考察了共聚单体、等温结晶温度和时间对样品结晶和熔融行为的影响。结果表明:经等温结晶后的样品再次升温时,均出现了双熔融峰,且随等温结晶温度的升高或时间的延长,低温熔融峰向高温方向移动,而高温熔融峰则基本不变。等温结晶温度80℃时,两种样品的熔融焓值均较大,并且随结晶时间的延长会变大。X射线衍射结果表明:随着结晶温度的升高或时间的延长,衍射峰变得尖锐,样品的结晶度增大。添加了十二烷基二胺的切片B比添加癸二胺的切片A具有更好的结晶能力,比较适合纺丝加工。     

非晶态化学镀Ni-Sn-P合金镀层变温晶化的研究

研究了非晶态Ni-Sn-P合金镀层的变温化过程,由实验得出Ni-Sn-P合金约在350℃开始晶化,450℃左右晶化结束,并测得Ni-Sn-P合金变温晶化激活能为233KJ·moL~(-1)。     

Concept of Functionally Graded Materials for Advanced Thermal Barrier Coating Applications

This feature article explores the concept of creating functionally graded metal-ceramic composite microstructures for thermal barrier coatings used in gas-turbine applications. From a thermomechanical perspective, this concept offers the possibility of significantly improving the life and reliability of thermal barrier coatings. However, prior research reveals that progress has been somewhat limited because of the oxidative instability exhibited by some metal-ceramic composite microstructures. The present study addresses some of the materials criteria and research issues associated with preparing chemically stable, yet mechanically durable, graded metal-ceramic microstructures for realistic application environments.     

热障涂层高温失效的研究进展

热障涂层的高温失效问题是热障涂层材料研究的重要命题,本文对国内外学者在热障涂层高温失效试验、失效机制、失效的动态监测和寿命预测方面的研究进展进行了综述。热障涂层的失效主要源自涂层中存在的热生长氧化物(TGO)的失效,文中总结出了国内外学者对TGO研究的几个重点方面,以期为热障涂层的高温失效问题提供研究思路。     

Crystallization and Melting Kinetics of Cristobalite

By measuring internal crystallization in vitreous silica, the kinetics of crystallization and melting of cristobalite were determined near the melting point. Both processes followed linear growth kinetics and could be completely represented by the equation μ=AΔT/η. Analyses of the crystallization data and observations of the crystal morphology indicate that crystallization occurs by a continuous growth mechanism. Melting was heterogeneous, occurring only at the glass-crystal interface and along grain boundaries.     

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。     

Failure Mechanisms of a Zirconia-8 wt% Yttria Thermal Barrier Coating

Isothermal and cyclic heat treatments of a plasma-sprayed zirconia-8 wt% yttria thermal barrier coating on a nickel superalloy substrate highlighted coating failure mechanisms. A reaction layer formed at the bond coat/ceramic interface, and failure of the coating initiated in the reaction layer due to opening thermal expansion mismatch stresses associated with a nonplanar interface. Crack propagation occurred in the ceramic in all cases, due to the high fracture energy of the interface. The relatively low fracture energy of the coating, in planes parallel to the interface, is the ultimate factor limiting coating lifetime.     

电沉积Ni-Fe-P非晶态合金镀层变温晶化的研究

用差热分析的方法研究了电沉积Ｎｉ－Ｆｅ－Ｐ非晶态合金镀层的变温晶化过程。通过实验得出不同铁含量的Ｎｉ－Ｆｅ－Ｐ合金在不同加热速度下的开始晶化温度、结束晶化温度,并计算出其晶化激活能。对不同Ｆｅ含量镀层的晶化激活能进行比较发现,Ｆｅ元素在Ｎｉ－Ｆｅ－Ｐ合金镀层中具有稳定非晶态组织的作用。     

The Porous Phase Barrier and Crystallization

Abstract

A porous body can be used as a phase barrier, or to safely store or transmit a metastable fluid phase (B) under certain definable conditions even when a stable phase (A) outside the porous body is in intimate contact with the metastable phase inside the body. The condition to be met can be expressed by

R < 2σV _A [(S _B -S _A ) cos θ _A δT]

where R is effective (“cylindrical”) radius of the pores; δT is degrees of supercooling (or superheating); σ is specific surface free energy of the phase boundary; θ _A is the contact anglen of Phase A with the material of the porous body; V and S are partial molal volume and entropy of the indicated phases, respectively. Phase B, the “metastable” phase by conventional test, is found to be the stable phase so long as it remains confined within sufficiently small pores. If the “metastable” phase (B) is a supercooled liquid, strongly adsorbed by the porous material (θ _A > 90°), Phase A can be crystalline, as demonstrated by the natural process of frost heaving of soil. This implies new methods of managing crystallization processes, including one whereby saline water is purified by an “ice sandwich” that sustains reverse osmosis and another whereby components of a binary eutectic mixture may be completely separated.     

Crystallization Behavior of Potassium Niobium Silicate Glasses

Crystallization behavior of potassium niobium silicate (KNS) glasses having compositions expressed by the general formula x K₂O· x Nb₂O₅·(1 - 2 x )SiO₂, with x = 0.167, 0.182, 0.200, 0.220, and 0.250, has been studied by DTA, X-ray diffraction, second harmonic optical generation (SHG), and electron microscopy. Bulk crystallization of potassium niobates in glasses with compositions near K₂O·Nb₂O₅·2SiO₂, as well as surface crystallization of KNbSi₂O₇ phase, has been established. Transparent glass-ceramics, based on potassium niobates with remarkable SHG signal values, can be obtained from glasses with the lowest silica content, by heat treatment at temperatures just above T _g, while at higher temperatures from all of the glasses under investigation the main crystallizing phase is KNbSi₂O₇ ferroelectric. Applying a dc electric field, grain-oriented crystallization is produced in KNS glasses with development of significantly anisotropic arrangements of KNbSi₂O₇ crystallites.     

Effect of CMAS Infiltration on Radiative Transport Through an EB-PVD Thermal Barrier Coating

One of the adverse effects of sand ingestion in gas turbines is that the thermal barrier coatings on the blades and vanes can be infiltrated at high temperatures by molten calcium–magnesium–aluminum–silicate (CMAS) and cause premature failure of the coating. To investigate the effect of CMAS penetration, the optical properties of a synthetic glass representative of CMAS are reported from 500 nm to 2.5 μm. Results are then presented to show that silicate infiltration of an electron beam-deposited TBC can increase radiative transport through the coating. The results are qualitatively consistent with a simple optical scattering model for radiative transport through a porous coating.     

成核PP的结晶过程与熔融过程研究

研究了PP在α成核剂作用下的结晶与熔融过程,通过差示扫描量热法(DSC)对成核PP(nPP)的降温与升温过程进行了扫描。结果发现,α成核剂使PP的结晶温度显著提高,结晶速率加快;α成核剂使PP的熔融温度和熔融焓提高,不改变PP的平衡熔点。     

New Thermal Barrier Coatings Based on Pyrochlore/YSZ Double-Layer Systems

Pyrochlore materials La₂Zr₂O₇ and Gd₂Zr₂O₇ have been used to produce thermal barrier coating systems by atmospheric plasma spraying. The materials have been applied as single-layer coatings with only a topcoat made of pyrochlore material. In addition, double-layer systems with a first layer of yttria-stabilized zirconia (YSZ) and a top layer made of pyrochlore material were produced. These systems have been tested in thermal cycling test rigs at surface temperatures between 1200-1450°C and the results were compared to the behavior of YSZ coatings. Single-layer coatings had a rather poor thermal cycling performance. On the other hand, double-layer systems showed similar results to YSZ coatings at temperatures below about 1300°C. At higher temperatures the double-layer coatings produced from our own powders revealed excellent thermal cycling behavior. At the highest test conditions, lifetime was thereby orders of magnitude better than that of YSZ coatings. Results indicate that an increase of the maximum surface temperature in gas turbines by at least 100 K becomes possible with the new coatings. Coatings produced from commercial powders showed a somewhat reduced performance.