Variation of Thermal Barrier Coating Lifetime Characteristics with Thermal Durability Evaluation Methods

The thermal durability of thermal barrier coating systems (TBCs) obtained using feedstock powders with different purity and phase content was investigated by thermal shock testing with different cycle times, including the effects on the sintering and phase transformation behaviors. Four 8 wt.% yttria-stabilized zirconia powders, with regular purity (TC1), high purity (TC2 and TC3), and without monoclinic phase (TC4), were employed to prepare the topcoat of TBC by atmospheric plasma spray on a NiCoCrAlY bondcoat deposited by high velocity oxy-fuel. The microstructure and phase stability of the topcoats affected the TBCs’ lifetime in the short-term (1 h) and long-term (24 h) furnace cyclic test (FCT) at 1100 °C and jet engine thermal shock (JETS) test. In the short-term FCT and JETS tests, in which coatings are severely subjected to thermal stress, the TBCs’ lifetime is most affected by the microstructure of the topcoat. The coating layer with the lowest monoclinic phase in the as-sprayed state showed the lowest phase-transformation characteristics in the isothermal oxidation test (1400 °C). These properties resulted in the best lifetime in the long-term FCT. Therefore, the coating material and evaluating methods of TBCs’ life should be selected depending on the usage environment.     

Oxidation behavior of thermal barrier coatings with HVOF and detonation-sprayed NiCrAlY bondcoats

The high-velocity oxygen-fuel (HVOF) technology was employed to deposit the bondcoat of a thermal barrier coating (TBC) system. The isothermal oxidation rate at 1100 °C of the TBC system with the HVOF bondcoat is two times lower than that of the TBC system with the detonation-sprayed bondcoat. The better isothermal oxidation resistance of the TBCs with HVOF sprayed bondcoats demonstrates that unlike alumina dispersoids in the HVOF sprayed bondcoat, rough surface of the detonation-sprayed bondcoat is undesirable for the detonation-sprayed TBC system concerning oxidation due to a large specific surface area and unfavorable oxides on the bondcoat.     

Thermal barrier coating life modeling in aircraft gas turbine engines

Analytical models for predicting ceramic thermal barrier coating (TBC) spalling life in aircraft gas tur-bine engines are presented. Electron beam/physical vapor-deposited and plasma-sprayed TBC systems are discussed. An overview of the following TBC spalling mechanisms is presented: (1) metal oxidation at the ceramic/metal interface, (2) ceramic/metal interface stresses caused by radius of curvature and inter-face roughness, (3) material properties and mechanical behavior, (4) component design features, (5) tem-perature gradients, (6) ceramic/metal interface stress singularities at edges and corners, and (7) object impact damage. Analytical models for TBC spalling life are proposed based on observations of TBC spall-ing and plausible failure theories. Spalling was assumed to occur when the imposed stresses exceed the material strength (at or near the ceramic/metal interface). Knowledge gaps caused by lack of experimen-tal evidence and analytical understanding of TBC failure are noted. The analytical models are considered initial engineering approaches that capture observed TBC spalling failure trends.     

Thermal conductivity and elastic modulus evolution of thermal barrier coatings under high heat flux conditions

Laser high heat flux test approaches have been established to obtain critical properties of ceramic thermal barrier coatings (TBCs) under near-realistic temperature and thermal gradients that may be encountered in advanced engine systems. Thermal conductivity change kinetics of a thin ceramic coating were continuously monitored in real time at various test temperatures. A significant thermal conductivity increase was observed during the laser-simulated engine heat flux tests. For a 0.25 mm thick ZrO₂-8% Y₂O₃ coating system, the overall thermal conductivity increased from the initial value of 1.0 W/m K to 1.15, 1.19, and 1.5 W/m K after 30 h of testing at surface temperatures of 990, 1100, and 1320 °C, respectively, Hardness and elastic modulus gradients across a 1.5 mm thick TBC system were also determined as a function of laser testing time using the laser sintering/creep and microindentation techniques. The coating Knoop hardness values increased from the initial hardness value of 4 GPa to 5 GPa near the ceramic/bond coat interface and to 7.5 GPa at the ceramic coating surface after 120 h of testing. The ceramic surface modulus increased from an initial value of about 70 GPa to a final value of 125 GPa. The increase in thermal conductivity and the evolution of significant hardness and modulus gradients in the TBC systems are attributed to sintering-induced microporosity gradients under the laser-imposed high thermal gradient conditions. The test techniques provide a viable means for obtaining coating data for use in design, development, stress modeling, and life prediction for various TBC applications.     

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.     

Experimental and numerical life prediction of thermally cycled thermal barrier coatings

This article addresses the predominant degradation modes and life prediction of a plasma-sprayed thermal barrier coating (TBC). The studied TBC system consists of an air-plasma-sprayed bond coat and an air-plasma-sprayed, yttria partially stabilized zirconia top layer on a conventional Hastelloy X substrate. Thermal shock tests of as-sprayed TBC and pre-oxidized TBC specimens were conducted under different burner flame conditions at Volvo Aero Corporation (Trollhättan, Sweden). Finite element models were used to simulate the thermal shock tests. Transient temperature distributions and thermal mismatch stresses in different layers of the coatings during thermal cycling were calculated. The roughness of the interface between the ceramic top coat and the bond coat was modeled through an ideally sinusoidal wavy surface. Bond coat oxidation was simulated through adding an aluminum oxide layer between the ceramic top coat and the bond coat. The calculated stresses indicated that interfacial delamination cracks, initiated in the ceramic top coat at the peak of the asperity of the interface, together with surface cracking, are the main reasons for coating failure. A phenomenological life prediction model for the coating was proposed. This model is accurate within a factor of 3.     

Multilayered thermal barrier coating for land-based gas turbines

Multilayered thermal barrier coatings (TBC) with different functions were proposed for the hot section components of land-based gas turbines. This article describes a multilayered TBC with an oxidation resistant layer. A conventional duplex TBC and a triplex TBC, in which an aluminized layer was added to the conventional duplex TBC to increase oxidation resistance, were prepared. It was confirmed by a burner rig test that the triplex TBC with the aluminized layer is resistant to oxidation and shows high durability in a thermal cycle test, compared with the conventional duplex TBC. The spalling in the thermal cycle test of each TBC specimen occurred at the same position, when the thickness of the oxidation layer was 11 to 13 μm. The mechanism of spalling of the coating in the thermal cycle test was discussed in terms of stress in the coating. Stress in the direction of spalling occurred by an uneven interface between the bond and top coat and increased with growth of the oxidation layer. It is thought that the high durability of the triplex TBC in the thermal cycle test is derived from suppressing the growth of the oxidation layer and decreasing the stress due to the addition of the aluminized layer.     

Recent Developments in the Field of Thermal Barrier Coatings

Conventional thermal barrier coating (TBC) systems consist of a duplex structure with a metallic bondcoat and a ceramic, heat-isolative topcoat. Several recent research activities are concentrating on developing improved bondcoat or topcoat materials; for the topcoat especially, those with reduced thermal conductivity are investigated. Using advanced topcoat materials, the ceramic coating can be further divided into layers with different functions. One example is the double-layer system in which conventional yttria-stabilized zirconia (YSZ) is used as bottom and new materials such as pyrochlores or perovskites are used as topcoat layers. These systems demonstrated an improved temperature capability compared to standard YSZ. In addition, new functions are introduced within the TBCs. These can be sensorial properties that can be used for an improved temperature control or even for monitoring remaining lifetime. Further increased application temperatures will also lead to efforts for a further improvement of the reflectivity of the coatings to reduce the radiative heat transfer through the TBC.     

温度梯度热循环下纳米7YSZ陶瓷层结构演变

目的采用低温超音速等离子喷涂(LT-HVOF)在镍基高温合金基体(K417)上制备了NiCoCrAlYTa粘结层,使用大气等离子喷涂(APS)在粘结层上制备了纳米7%Y_2O_3-ZrO_2(7YSZ)陶瓷涂层,以获得温度梯度热循环下纳米陶瓷层的结构演变机制。方法通过燃气热冲击实验仪对热障涂层模拟真实服役条件下温度梯度热循环的工作环境,采用一维稳态热传导模型计算了热障涂层中各涂层界面的温度,探讨了在热驱动作用下等径晶粒和非等径晶粒的扩散长大机制。结果热循环次数为40次时,涂层近表面出现了烧结致密化现象,而陶瓷层底部涂层保持原来的结构。热循环次数增加到460次时,整个陶瓷层断面都发生了烧结致密化现象。结论温度是涂层烧结致密化的主导因素。涂层中当等大晶粒接触形成弯曲颈时,由于弯曲颈只受水平方向静压力作用,晶粒中原子扩散速率慢,导致晶粒长大速率较慢;而当非等大晶粒接触形成弯曲颈时,在晶粒接触弯曲颈处存在一偏大晶粒方向的剪切力,其导致晶粒向弯曲颈扩散速率增加,晶粒长大速率较快。     

飞行状态下热障涂层的受力行为

本文通过在计算机中建立航空发动机叶片的工作环境,进而模拟在飞行状态下叶片表面涂敷的陶瓷热障涂层的受力行为,获得了陶瓷涂层在热气动载荷作用下应力场分布,并得出应力场分布与涂层结构和叶片几何形状及与承受的载荷之间的关系,提出了进一步提高陶瓷热障涂层的使用寿命的途径。     

Microstructural features and properties of plasma sprayed YPSZ/NiCrAlY thermal barrier coating (TBC)

0　IntroductionThermalbarriercoatings(TBCs)arewidelyusedontheturbinebladesforaircraftpropulsionorpowergenerationtoreducethemetallicsubstratetemperature,whichleadstoincreasingengineefficiencyandloweringpollutantemissionsresultingfromallowableincreaseofoperationtemperature[1,2].Today,TBCsareattractingmoreattentionandhavewiderpotentialapplicationstoprotecthightemperaturecomponents.However,thermalbarriercoatingshaveatendencytocrackandspallinserviceduetothermalshockandthermalcyclingbetweenambient…     

Technical and Economical Aspects of Current Thermal Barrier Coating Systems for Gas Turbine Engines by Thermal Spray and EBPVD: A Review

The most advanced thermal barrier coating (TBC) systems for aircraft engine and power generation hot section components consist of electron beam physical vapor deposition (EBPVD) applied yttria-stabilized zirconia and platinum modified diffusion aluminide bond coating. Thermally sprayed ceramic and MCrAlY bond coatings, however, are still used extensively for combustors and power generation blades and vanes. This article highlights the key features of plasma spray and HVOF, diffusion aluminizing, and EBPVD coating processes. The coating characteristics of thermally sprayed MCrAlY bond coat as well as low density and dense vertically cracked (DVC) Zircoat TBC are described. Essential features of a typical EBPVD TBC coating system, consisting of a diffusion aluminide and a columnar TBC, are also presented. The major coating cost elements such as material, equipment and processing are explained for the different technologies, with a performance and cost comparison given for selected examples.     

等离子喷涂ZrO2—NiCoCrAlY梯度热障涂层的抗热震性及失效机理

研究了ZrO2-NiCoCrAlY热障涂层的抗热震性和热震失效机理。实验结果表明,梯度热隙涂层能明显延缓热震裂纹的形成和扩展,具有较高的抗热震性。热震裂纹形成与扩展主要在粘结层与基体的界面处。随热循环次数的增加,热震裂纹可在表面陶瓷层内和陶瓷层与过渡层的界面处形成。实验表明热障涂层热震失效的过程主要是裂纹形成、扩展及涂层剥落,粘结层的氧化是导致涂层剥落失效的重要原因。     

CoCrAlY表面改性后热障涂层高温氧化及热震性能

采用大气等离子喷涂技术（APS）在镍基高温合金表面制备了CoCrAlY粘结层,利用电子束蒸发镀膜在CoCrAlY表面蒸镀纳米铝膜,并使用强流脉冲电子束熔敷纳米铝膜进行表面改性,最后使用APS在表面改性后的CoCrAlY表面沉积陶瓷层制备了热障涂层.在空气环境中对热障涂层进行高温氧化试验和热震试验.结果表明,CoCrAlY表面改性后热障涂层经1 050℃静态空气氧化后,界面处生成的热生长氧化物（TGO）具有较高的连续性和致密性,有效阻碍了氧化的进一步发展且避免尖角型氧化物的形成,提高了热障涂层的抗氧化能力;在1 050℃高温加热后10℃水淬热震条件下,脱落率仅为2%左右.     

Al-rich precipitation in CoNiCrAlY bondcoat at high temperature

A thermal barrier coating (TBC) is applied on a surface of a gas turbine blade to provide a thermal barrier and oxidation resistant properties for the components. The ability to resist oxidation of the coating arises from the self-healing, protective Al₂O₃ scale on top of the bondcoat, which is formed during service. However, if Al depletion occurs within the bondcoat, the protective scale will lose its self-healing ability, and hence, its oxidation-resistant property. This paper investigated the depletion of Al within the bondcoat by studying the microstructure of the bondcoat on a gas turbine blade after it has been in 4000 h service at 1200 °C. The results showed that Al depletion had occurred at different levels throughout the turbine blade. In the area where Al depletion had not yet occurred, precipitation of an Al-rich phase was detected. Most of the Al was contained within this phase, leaving only small amount of Al in the surrounding matrix. A well-defined boundary was observed between the depleted and non-depleted regions.     

航空发动机热障涂层存在的问题及其发展方向

热障涂层主要作用在发动机的热端部件,可以减少油耗,提高效率,延长热端部件使用寿命。热障涂层技术的高速发展使得航空发动机的性能得到了极大的提高,但是它在使用过程中存在着失效问题。主要对陶瓷面层存在的高温烧结、热生长氧化物生长应力、高温热腐蚀和界面应力失配四种问题进行了论述,并针对具体问题提出了解决方案。同时,对新型的低热导率热障涂层和应用在CMC基体上的热障涂层的研发情况进行了综述和展望。     

重型燃机喷嘴壳体及遮热板热障涂层剥落机制

目的 探究重型燃机喷嘴壳体及遮热板热障涂层剥落机制,为该部件的全寿命管理提供参考。方法 采用等离子喷涂方法,分别制备以06Cr25Ni20不锈钢和Hastelloy X合金为基材的热障涂层试验件,并结合水淬热冲击表征方法与瞬态热力耦合仿真方法,表征热障涂层水淬后的剥落状态,获得热障涂层残余剪应力的分布状态随基材和服役工况的变化行为,揭示热障涂层在多层热失配工况下的剥落机制。结果 在水淬热冲击条件下,2种不同基材的热障涂层试验件表现出类似的剥落行为,但由于基材热膨胀系数的差异,以06Cr25Ni20不锈钢为基材的热障涂层的残余剪应力(70.1 MPa)比Hastelloy X合金基材的热障涂层(52.7 MPa)更大,热冲击寿命更短。在梯度温度载荷下,2种不同基材热障涂层试验件的失效模式不同,前者的最大残余剪应力为39.2 MPa,后者为25.7 MPa。结论 在2种温度载荷下,以Hastelloy X合金为基材的热障涂层具有较低的残余应力和较长的服役寿命。此外,水淬热冲击可以快速表征热障涂层的寿命行为,但其失效模式与实际梯度温度载荷下的失效模式仍有一定区别。     

Long crack behavior in a thermal barrier coating upon thermal shock loading

The behavior of macroscopic long cracks in the ceramic top coat of a thermal barrier coating (TBC) system subjected to thermal shock loading and the influence of the cracks on the coating durability were investigated experimentally and numerically. Thermal shock testing was conducted until coating failure. Comparisons were made with coating samples without macroscopic cracks. The experimental results revealed that the presence of macroscopic cracks reduces the life of the TBC. The finite-element method, with a fracture mechanics approach, was applied to analyze preexisting long cracks, and the calculations correlate well with the experimental findings. It was found that the life of the coating is reduced with crack length as well as with maximum cycle temperature. It was also found that the stress-intensity factors for long cracks are initially high and decrease with the number of temperature cycles, which indicates that rapid crack growth occurs during the first number of cycles.     

The influence of bondcoat and topcoat mechanical properties on stress development in thermal barrier coating systems

A finite-element study has been undertaken to investigate the stress development within a TBC system consisting of an EB-PVD YSZ topcoat and a Pt-aluminized diffusion bondcoat. Particular attention has been paid to the role of variables such as the elastic anisotropy within the topcoat, interface roughness, variation in creep strength of the bondcoat and the volumetric strains associated with the formation of the thermally grown oxide (TGO). Bond coat oxidation and thermal loading during cooling give rise to significant tensile stresses within the topcoat and tensile tractions at the TGO interfaces. Bondcoat creep, as distinct from yield and plastic behaviour, was the dominant stress relaxation process, and strong bondcoats (in creep) tended to show higher tensile stress levels. Another important factor determining thermal barrier coating stress levels was the level of elastic anisotropy of the topcoat: an elastic isotropic yttria-stabilized zirconia gave rise to considerably higher stresses than a transversely isotropic topcoat.     

高温拉压环境下DZ125高温合金的热障涂层失效

采用电子束物理气相沉积法（EB-PVD）在定向凝固Ni基高温合金DZ125基体上制备了NiCoCrAlY粘结层和YSZ陶瓷层，研究了高温拉压环境下热障涂层的失效模式，并对其进行了有限元分析。实验结果表明，热障涂层的失效与仅受热载荷作用下的有很大不同，仅有热载荷作用下的热障涂层裂纹多萌生于热氧化层（TGO）内部，进而扩展引起热障涂层的失效。而高温拉压试验后热障涂层体系存在两种裂纹，分别萌生于TGO／粘结层界面和粘结层／扩散层界面附近。有限元模拟结果显示TGO／陶瓷层和TGO／粘结层处存在应力状态的转变和应力值的突变，径向应力的突变导致了界面分离现象的产生，而轴向应力的突变加速了垂直于界面裂纹的扩展，并导致了试样的最终断裂。