Characterization of modified thick thermal barrier coatings

In gas turbines and diesel engines, there is a demand for thick thermal barrier coatings (TTBCs) due to the increased process combustion temperatures. Unfortunately, the increased thickness of plasma-sprayed thermal barrier coatings (TBCs) normally leads to a reduced coating lifetime. For that reason, the coating structures have to be modified. When modifying the structure of TTBCs, the focus is normally on elastic modulus reduction of the thick coating to improve the coating strain tolerance. On the other hand, coating structural modification procedures, such as sealing treatments, can be performed when increased hot-corrosion resistance or better mechanical properties are needed. In this article, several modified zirconia-based TTBC structures with specific microstructural properties are discussed. Coating surface sealing procedures such as phosphate sealing, laser glazing, and sol-gel impregnation were studied as potential methods for increasing the hot-corrosion and erosion resistance of TTBCs. Some microstructural modifications also were made by introducing segmentation cracks into the coating structures by laser glazing and by using special spraying parameters. These last two methods were studied to increase the strain tolerance of TTBCs. The coating microstructures were characterized by optical microscopy, a scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The effect of sealing procedures on the basic thermal and mechanical properties of the coatings was studied. In addition, some correlations between the coating properties and microstructures are also presented, and the advantages and drawbacks of each modification procedure are discussed.     

Thermal cycling resistance of modified thick thermal barrier coatings

The thermal cycling properties of several modified thick thermal barrier coatings (TTBC) were studied in three test series in which the maximum coating temperature was fixed to 1000, 1150 and 1300 °C. The modified coating structures were all segmentation-cracked coatings and some of these coatings were surface-sealed. The segmentation-cracked coatings were produced by laser glazing or by using appropriate plasma spray parameters. The sealing treatments were made by using aluminium phosphate or sol–gel-based sealant. In this paper, it was demonstrated that regardless of whether the segmentation-cracked TTBCs were made by using specific plasma spray parameters or by laser glazing, the strain tolerance of the coating improved significantly. Instead, both sealing treatments reduced the thermal cycling resistance of the TTBCs to some degree, especially in the case of aluminium phosphate sealing. Coating microstructures, their mechanical and elastic properties and residual stresses were taken into consideration when estimating the thermal cycling properties and failure modes of the coatings.     

Sealing procedures for thick thermal barrier coatings

Zirconia-based 8Y₂O₃-ZrO₂ and 22MgO-ZrO₂ thick thermal barrier coatings (TTBC, 1000 μm), were studied with different sealing methods for diesel engine applications. The aim of the sealing procedure was to improve hot corrosion resistance and mechanical properties of porous TBC coatings. The surface of TTBCs was sealed with three different methods: (1) impregnation with phosphate-based sealant, (2) surface melting by laser glazing, and (3) spraying of dense top coating with a detonation gun. The thicknesses of the densified top layers were 50–400 μm, depending on the sealing procedure. X-ray diffraction (XRD) analysis showed some minor phase changes and reaction products caused by phosphate-based sealing treatment and some crystal orientation changes and phase changes in laser-glazed coatings. The porosity of the outer layer of the sealed coating decreased in all cases, which led to increased microhardness values. The hot corrosion resistance of TTBCs against 60Na₂SO₄-40V₂O₅ deposit was determined in isothermal exposure at 650 °C for 200 h. Corrosion products and phase changes were studied with XRD after the test. A short-term engine test was performed for the reference coatings (8Y₂O₃-ZrO₂ and 22MgO-ZrO₂) and for the phosphate-sealed coatings. Engine tests, duration of 3 h, were performed at the maximum load of the engine and were intended to evaluate the thermal cycling resistance of the sealed coatings. All of the coatings passed the engine test, but some vertical cracks were detected in the phosphate-sealed coatings.     

Indentation method for fracture resistance determination of metal/ceramic interfaces in thick TBCs

The indentation technique has been used to measure the adhesion of plasma- sprayed ceramic coatings on metals intended for thick thermal barrier coating ( TTBC) applications. This approach provides the adhesion value as the critical strain energy release rate,Gc, of the interface, which also takes into account any residual stresses. The theoretical background of the method is outlined, and specific examples are reported with respect to the effect of substrate temperature on the metal/ceramic adhesion of thick TBCs.     

Thermal shock testing of thermal barrier coating/bondcoat systems

Various methods of thermal shock testing are used by aircraft and industrial gas turbine engine (IGT) manufacturers to characterize new thermal barrier coating systems in the development stage as well as for quality control. The cyclic furnace oxidation test (FCT), widely used in aircraft applications, stresses the ceramic/bondcoat interface, predominantly through thermally grown oxide (TGO) growth stress. The jet engine thermal shock (JETS) test, derived from a burner rig test, creates a large thermal gradient across the thermal barrier coating (TBC), as well as thermomechanical stress at the interface. For IGT applications with long high-temperature exposure times, a combination of isothermal preoxidation and thermal shock testing in a fluidized bed reactor may better represent the actual engine conditions while both types of stress are present. A comparative evaluation of FCT, JETS, and a combined isothermal oxidation and fluidized bed thermal shock test has been conducted for selected ceramic/bondcoat systems. The results and the failure mechanisms as they relate to the TBC system are discussed. A recommendation on the test method of choice providing best discrimination between the thermal shock resistance of the ceramic layer, the ceramic/bondcoat interface, and even substrate related effects, is given. This paper was presented at the 2nd International Surface Engineering Congress sponsored by ASM International, on September 15–17, 2003, in Indianapolis, Indiana, and appeared on pp. 520–29.     

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。     

Insulated Piston Heads for Diesel Engines

Widely studied in the 1980s, the insulation of pistons in engines aimed at reducing the heat losses and thus increasing the indicated efficiency. However, those studies stopped in the beginning of the 1990s because of NO _x emission legislation and also because of lower oil prices. Currently, with the improvement of exhaust after treatment systems (diesel particulate filter, selective catalytic reduction, and diesel oxidation catalyst) and engine technologies (exhaust gas recirculation), there are more trade-offs for NO _x reduction. In addition, the fast rise of the oil prices tends to lead back to insulation technologies in order to save fuel. A 1 mm thick plasma sprayed thermal barrier coating with a graded transition between the topcoat and the bondcoat was deposited on top of a serial piston for heavy-duty truck engines. The effects of the insulated pistons on the engine performance are also discussed, and the coating microstructure is analyzed after engine test.     

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.     

水下机器人柴电动力系统可靠性研究

大型化是未来水下机器人发展的重要趋势,大排水量水下机器人多采用柴油机作为动力系统的动力源。本研究面向水下机器人柴电动力系统在设计过程中选用单套或双套柴油机组技术方案的问题,进行系统可靠性分析,对比单套柴油机组与双套柴油机组在不同工况下的系统可靠度。结果表明:巡航状态下双套柴油机组可靠度较单套柴油机组提高25%以上;高速航行状态下双套柴油机组可靠度较单套柴油机组降低最大可达31.2%;巡航状态与高速航行状态各占全部工作流程一半时,双套机组的系统可靠度与单套机组的系统可靠度相同。     

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。     

Ruthenium-containing bond coats for thermal barrier coating systems

Bond coats for zirconia-based thermal barrier coating systems applied to nickel-based superalloys are typically composed of the B2 NiAl phase. Since RuAl has the same B2 crystal structure but a melting point 400°C higher than NiAl, ruthenium-modified aluminide bond coats could provide improved system temperature capability. Creep experiments on ternary Al−Ni−Ru alloys demonstrate greatly improved creep properties with increasing ruthenium content. Processing paths for ruthenium-modified NiAl-based bond coatings have been established within the bounds of commercially available coating systems. The oxidation resistance of ruthenium-modified bond coats during thermal cycling has been examined, and potential thermal barrier coating system implications are discussed.     

Overview of thermal barrier coatings in diesel engines

An understanding of delamination mechanisms in thermal barrier coatings (TBCs) has been developed for diesel engine applications through rig tests, structural analysis modeling, nondestructive evaluation, and engine evaluation of various TBCs. This knowledge has resulted in improved TBCs that survive se-vere cyclic fatigue tests in high-output diesel engines. Although much conflicting literature now exists regarding the impact of TBCs on engine performance and fuel consumption, changes in fuel consumption appear to be less than a few percent and can be nega-tive for state-of-the-art diesel engines. The ability of the TBC to improve fuel economy depends on a num-ber of factors, including the fuel injection system, combustion chamber design, and initial engine fuel economy. Limited investigations on state-of-the-art diesel engines have indicated that surface- connected porosity and coating surface roughness may influence engine fuel economy. Current research efforts on TBCs are primarily directed at reduction of in-cylinder heat rejection, ther-mal fatigue protection of underlying metal surfaces, and possible reduction of diesel engine emissions. Significant efforts are still required to improve the plasma spray processing capability and the economics for complex-geometry diesel engine components.     

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。     

Methods for thermomechanical analysis of thermal barrier coatings in diesel engines

Thermal barrier coatings applied to in-cylinder surfaces of diesel engines are subject to large thermomechanical loads as a result of the transfer of heat from the combustion gases. The thermal loads produce high temperatures and high temperature gradients in the coating. Owing to the intermittent cyclical nature of the engine heat transfer and the low conductivity of thermal barrier coatings, large cyclic surface temperature transients are superimposed on the already high steady state temperatures. In addition, there are significant thermal transients produced by rapid variations in the engine operating conditions (engine speed and load).Recent methods developed by researchers rigorously couple the calculation of spatially and time-resolved in-cylinder gas phase thermal processes with the calculation of structural heat transfer and thermal stresses using finite element techniques. This makes possible a detailed analysis of the effects of steady state and transient engine heat loads on the performance of heat barrier coatings. The techniques are described in a brief and general manner but attention is otherwise directed to specific results obtained by us. Results showing the temperatures and thermal stresses produced by steady state and transient thermal loads are presented for plasma-sprayed zirconia coatings on the valves of a heavy duty highway diesel engine.     

A design perspective on thermal barrier coatings

This article addresses the challenges for maximizing the benefit of thermal barrier coatings for turbine engine applications. The perspective is from the viewpoint of a customer, a turbine airfoil designer who is continuously challenged to increase the turbine inlet temperature capability for new products while maintaining cooling flow levels or even reducing them. This is a fundamental requirement for achieving increased engine thrust levels. Developing advanced material systems for the turbine flowpath airfoils, such as high-temperature nickel-base superalloys or thermal barrier coatings to insulate the metal airfoils from the hot flowpath environment, is one approach to solve this challenge. The second approach is to increase the cooling performance of the turbine airfoil, which enables increased flowpath temperatures and reduced cooling flow levels. Thermal barrier coatings have been employed in jet engine applications for almost 30 years. The initial application was on augmentor liners to provide thermal protection during afterburner operation. However, the production use of thermal barrier coatings in the turbine section has only occurred in the past 15 years. The application was limited to stationary parts and only recently incorporated on the rotating turbine blades. This lack of endorsement of thermal barrier coatings resulted from the poor initial duratbility of these coatings in high heat flux environments. Significant improvements have been made to enhance spallation resistance and erosion resistance, which has resulted in increased reliability of these coatings in turbine applications.     

Thermal barrier coatings for industrial gas turbine applications: An industrial note

Thermal barrier coatings (TBCs) have been used in high-thrust aircraft engines for many years to pro-vide thermal protection and increase engine efficiencies. TBC life requirements for aircraft engines are typically less than those required for industrial gas turbines. This paper describes current and future ap-plications of TBCs in industrial gas turbine engines. Early testing and applications of TBCs are reviewed. Areas of concern from the engine designer’s and materials engineer’s perspective are identified and evaluated. This paper focuses on the key factors that are expected to influence utilization of TBCs in ad-vanced industrial gas turbine engines. It is anticipated that reliable, durable, and highly effective coating systems will be produced that will ultimately improve engine efficiency and performance.     

Optimization of Air Plasma Sprayed Thermal Barrier Coating Parameters in Diesel Engine Applications

In the present paper, an optimization of thermal barrier coating parameters is performed for diesel engine applications. The substrate is A356.0-T7, a cast aluminum alloy which has a vast application in diesel engines, and the alloy is coated by plasma sprayed ZrO₂-8 wt.% Y₂O₃. Parameters including the feed rate of coating powders, the nozzle distance to specimen surfaces, and the coating thickness are optimized by thermal shock fatigue tests and bending tests. Optimum values of the feed rate and the nozzle distance are 30 g/min and 80 mm, respectively, when the objective is considered as maximizing the bending strength. Thermal shock tests demonstrate that lower thickness of coating layers has a better lifetime. By increasing the coating thickness, the thermal fatigue lifetime decreases. The reason is due to higher order of stresses near the interface of the substrate and the bond coat layer, calculated by a finite element simulation. One suggestion to improve the lifetime is to use multiple layers of coatings.     

等离子喷涂热障陶瓷涂层冷却累计残余应力的有限元模拟与验证

目的 为有效预测等离子喷涂热障涂层冷却过程中累积的残余应力,降低残余应力对涂层稳定性的影响,需寻求可靠的热障涂层应力检测方法。方法 利用有限元分析软件,采用生死单元法建立了等离子喷涂ZrO₂涂层的有限元模型,高斯热源模拟等离子喷涂热源工况,研究涂层冷却至室温的残余应力及其分布。使用X射线衍射法、拉曼光谱法对等离子喷涂制备的ZrO₂涂层进行残余应力检测。结果 通过有限元模拟结果可以看出,喷涂涂层冷却到室温后其中心区域的残余应力与边缘位置相比较大,主要集中在热流中心区域;每层涂层结合界面处会产生较大应力,致使应力沿涂层厚度方向变化明显。涂层的等效应力为160～220 MPa。采用X射线衍射法检测涂层存在180～185MPa残余应力。标定ZrO₂涂层的拉曼-应力因子为8.33 (cm·GPa)^-1,计算得到涂层存在残余应力为174～180 MPa。对喷涂试样进行拉伸试验后,其残余应力有一定程度的释放。结论 使用有限元能有效模拟等离子喷涂至室温时涂层内部残余应力,与XRD、拉曼光谱检测结果具有良好的匹配性,...     

The behavior of ceramic thermal barrier coatings on diesel engine combustion zone components

Partially stabilized zirconia ceramic thermal barrier coatings were plasma sprayed over an MCrAlY (M ≡ metal) bond coat on the valve faces and tulips, piston crowns and cylinder heads of two medium speed diesel engines to a specified total thickness of 0.015 in (0.4 mm). One engine was operated for 500 h on a test stand using a cyclic range of throttle settings that was representative of a locomotive engine's operation using number 2 diesel oil fuel. The other engine was operated in a towboat for 9000 h of service on the Inland Rivers, also using number 2 diesel oil fuel.Coating performance analyses were carried out on several parts after service. Coatings applied to piston crowns and cylinder heads generally performed in an acceptable manner. One type of coating on a valve face performed acceptably without coating loss or degradation. Two other types of coatings on valve faces failed for identifiable reasons. Overall, the durability of the coatings in a diesel engine combustion zone operating environment was promising.     

Temperature measurements and adhesion properties of plasma sprayed thermal barrier coatings

Metal-ceramic coatings have been widely used for industrial applications, mainly in the gas turbine and diesel engine industries as thermal barrier coatings (TBCs). Conventional thermal barrier coatings consist of a metallic bond coat and an insulating ceramic topcoat. Temperatures and temperature gradients in the coating during plasma spraying play an important role on the final coating quality, especially the temperature of the particles just hitting the substrate surface. In this work, metal-ceramic coatings were applied on nickel-superalloy substrates. The temperatures of both the coating surface and substrate were measured during spraying. The adhesion of the coatings was determined using ASTM C 633 and correlated with the measured temperatures. Optical pyrometry and thermocouples were used to measure the interfacial and substrate temperatures, respectively. Temperature was shown to have a significant influence where lower interfacial temperatures were found to result in lower adhesion values.