Comparative study of thermal barrier coatings for internal combustion engine

The paper presents the results of investigation into the thermal fatigue resistance of thermal barrier coatings (TBC). Two groups of double-layered thermal barrier coatings (TBC) were investigated: plasma sprayed with ZrO₂-8%Y₂O₃, Al₂O₃-40%TiO₂ or Al₂O₃-40%ZrO₂ top coats and powder flame sprayed with ZrO₂-30%CaO, Al₂O₃-40%TiO₂ and Al₂O₃-30%MgO. The extent of TBC deterioration experienced in thermal fatigue test was evaluated in the erosion test and SEM examinations. Flame sprayed coatings were found more prone to damage than plasma sprayed ones. The highest thermal fatigue resistance revealed TBC plasma sprayed with PSZ. Numerical calculation with Abaqus 6.7 finite element code was used to calculate temperature and stress variations in the coating throughout the test. Phase stability of plasma sprayed Al₂O₃-40%TiO₂ was evaluated by means of X-ray diffraction method. Thermal growth of oxides at the top coat/bond-coat interface and the decomposition of Al₂O₃-40%TiO₂ were found to be important degradation mechanisms leading to the spallation of coatings in the diesel engine and the petrol engine exploitation tests.     

Thermal barrier coatings produced by laser cladding

A 2kW C0₂ laser has been used to clad a mild steel substrate with two different ceramic coatings, namely yttria partially stabilized zirconia (8 wt% YPSZ) or a mixture of YPSZ and pure alumina powder. A range of laser processing parameters has been investigated. Results have been obtained showing the possibility of using the laser beam for producing a clad layer of thermal barrier coating with different topography depending on the processing conditions.On leave from Scientific Research Council, Baghdad, Iraq.     

Thermal barrier coatings produced by chemical vapor deposition

Yttria stabilized zirconia (YSZ) can be employed as thermal barrier coatings (TBCs) on Ni-based super alloys in gas turbines and aircraft engines. The YSZ coatings have been fabricated by atmospheric plasma spraying or electron-beam physical vapor deposition. The increase in operation temperature of gas turbines demands another fabrication process to obtain high quality TBCs. Chemical vapor deposition (CVD) can be an alternative route to prepare TBCs due to excellent conformal coverage and columnar microstructure. This paper reviews the fabrication of YSZ films by conventional thermal CVD and plasma CVD intended for TBCs. A new laser CVD developed by our group with a high deposition rate of 660 µμh^-1 was also briefly introduced.     

Thermal barrier coatings produced by chemical vapor deposition

Yttria stabilized zirconia (YSZ) can be employed as thermal barrier coatings (TBCs) on Ni-based super alloys in gas turbines and aircraft engines. The YSZ coatings have been fabricated by atmospheric plasma spraying or electron-beam physical vapor deposition. The increase in operation temperature of gas turbines demands another fabrication process to obtain high quality TBCs. Chemical vapor deposition (CVD) can be an alternative route to prepare TBCs due to excellent conformal coverage and columnar microstructure. This paper reviews the fabrication of YSZ films by conventional thermal CVD and plasma CVD intended for TBCs. A new laser CVD developed by our group with a high deposition rate of 660 μm h⁻¹ was also briefly introduced.     

Thermal barrier coatings for gas turbines, automotive engines and diesel equipment

Thermal barrier coatings have been developed extensively in the aerospace industry, but it is only recently that they have been modified and tested for safe use in diesel and internal combustion engines. Although diesel operate at lower temperatures than aircraft engines, they are subjected to much greater compressive loads and more frequent thermal shock than their aircraft counterparts. In addition, many of these must cope with the contaminates found in lower-grade fuels. The different between aircraft and diesel are often ignored by coating applicators, resulting in premature failure of the coating. This article examines some of the problems and opportunities, with particular reference to the GPX range of coatings.     

陶瓷热障涂层的热导率和热扩散率测量

提出了应用3ω法进行等离子喷涂热障涂层材料的热导率和热扩散率测量的方法。测试了室温下2种典型的热障涂层材料Y2SiO5和La2Zr2O7的热导率和热扩散率,测试结果与文献中的结果吻合良好。实验中对不同孔隙率的样品的热导率在室温附近的温度区间内进行测试,结果表明,孔隙率的变化对热导率有明显的影响。另外,孔隙率对热扩散率有双向的影响,即存在某一孔隙率值使得涂层样品的热扩散率最大。     

平面电容传感器热障涂层缺陷检测系统

为有效检测热障涂层的缺陷,研究基于平面电容传感器的缺陷检测系统。该文对用于热障涂层缺陷检测的平面电容传感器的工作原理进行分析,结合热障涂层的结构,利用COMSOL有限元仿真软件对传感器结构进行优化设计,设计微小电容检测电路,并通过复导纳的方法提取检测信息,搭建热障涂层缺陷检测系统,并对系统的性能进行测试。实验测试中对3种不同厚度的氧化铝陶瓷片进行复导纳的检测,通过对检测结果进行数据分析更易区分不同厚度的陶瓷片,即基于平面电容传感器的缺陷检测系统可以有效检测出热障涂层的厚度变化缺陷。     

Thermal conductivity issues of EB‐PVD thermal barrier coatings

The thermal conductivity of electron‐beam physical vapor deposited (EB‐PVD) thermal barrier coatings (TBCs) was investigated by the Laser Flash technique. Sample type and methodology of data analyses as well as atmosphere during the measurement have some influence on the data. A large variation of the thermal conductivity was found by changes in TBC microstructure. Exposure at high temperature caused sintering of the porous microstructure that finally increased thermal conductivity up to 30 %. EB‐PVD TBCs show a distinct thickness dependence of the thermal conductivity due to the anisotropic microstructure in thickness direction. Thin TBCs had a 20 % lower thermal conductivity than thick coatings. New compositions of the ceramic top layer offer the largest potential to lower thermal conductivity. Values down to 0.8W/(mK) have been already demonstrated with virgin coatings of pyrochlore compositions.     

Thermal barrier coatings for thermal insulation and corrosion resistance in industrial gas turbine engines

Four thermal barrier coatings were subjected to a 500 h gas turbine engine test. The coatings were two Y₂O₃-stabilized ZrO₂ composites, Ca₂SiO₄ and CaTiO₃. The Ca₂SiO₄ coating exhibited significant spalling. Y₂O₃-stabilized ZrO₂ and CaTiO₃ coatings showed little degradation except in blade leading edge areas. Post- test examination showed variations in the coating due to manual application techniques. Improved process control is required if engineering quality coatings are to be developed. The results indicate that some leading edge loss of the coating can be expected near the tip.     

Thermal property evolution of metal based thermal barrier coatings with heat treatments

Predicting “in-service” lifetime of ceramic thermal barrier coatings (TBCs) is difficult due to the inherent brittle nature of ceramics used. Therefore, the study of metal-based thermal barrier coatings (MBTBCs) has been initiated to challenge the current problems of ceramic-based TBCs (CBTBCs) and create a new generation of thermal barrier coatings (TBCs). In this work, nano/amorphous structured MBTBCs, for use in internal combustion engines, have been produced using high frequency induction plasma spraying (IPS) of iron-based nanostructured alloy powders. Coatings were deposited by IPS using various spray parameters and heat treated up to 850 °C to study the thermal stability of the coating. The thermal diffusivity (α) properties of MBTBCs were measured using a laser flash method. Density (ρ) and specific heat (C _p ) of the MBTBCs were also measured for calculating thermal conductivity (k = αρC _p ).

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Thermal barrier coatings from sol-gel-derived spray-grade Y2O3-ZrO2 microspheres

For the development of ceramic thermal barrier coatings, spray-grade yttria-stabilized zirconia microspheres were prepared by the sol-gel technique. Oxide microspheres were obtained by calcination of the corresponding gel spheres at 1000 °C. Scanning electron microscopic and optical microscopic observations revealed the material thus obtained to have a predominantly spherical morphology and the requisite size distribution (5–50 m). The dense, calcined microspheres showed good flowability. X-ray diffraction studies indicated the presence of the tetragonal polymorph of ZrO₂ as the major phase, in addition to about 14% monoclinic ZrO₂. The plasma-sprayed YSZ coatings made from the sol-gel-derived microspheres showed a further decrease in the monoclinic ZrO₂ content (6%). The coatings survived 40–50 thermal cycles (30 min at 1200 °C followed by a water quench), indicating good thermal shock resistance.     

Thermal Landau jet

The author considers the steady-state temperature distribution in a jet of incompressible fluid emitted in pulsed bursts from a point source, with and without account taken of viscous dissipation. When viscous dissipation is not taken into account, an exact solution is obtained for the energy equation, bounded in the entire closed interval 0 . When viscous dissipation is considered, the energy equation is obtained using the introduced separation of variables.     

Erosion of thermal barrier coatings

Abstract

Thermal barrier coatings have been used within gas turbines for over 30 years to extend the life of hot section components. Thermally sprayed ceramics were the first to be introduced and are widely used to coat combustor cans, ductwork, platforms and more recently turbine aerofoils of large industrial engines. The alternative technology, electron beam physical vapour deposition,(EB-PVD) has a more strain-tolerant columnar microstructure and is the only process that can offer satisfactory levels of spall resistance, erosion resistance and surface finish retention for aero-derivative engines.

Whatever technology is used, the thermal barrier must remain intact throughout the turbine life. Erosion may lead to progressive loss of TBC thickness during operation, raising the metal surface temperatures and thus shortening component life. Ballistic damage can lead to total TBC removal.

This paper reviews the erosion behaviour of both thermally sprayed and EB-PVD TBCs relating the observed behaviour to the coating microstructure. A model for the erosion of EB-PVD ceramics is presented that permits the prediction of erosion rates. The model has been validated using a high velocity erosion gas gun rig, both on test coupons and samples removed from coated components. The implications of erosion on component life are discussed in the light of experimental results and the model predictions.     

Thermal cycling and isothermal oxidation behavior of quadruple EB‐PVD thermal barrier coatings

Increase of energy efficiency by increasing the turbine inlet temperature is the main driving force for further investigations regarding new thermal barrier coating materials. Today, thermal barrier coatings consisting of yttria stabilized zirconia are state of the art. In this study, thermal barrier coatings consisting of 7 weight percent yttria stabilized zirconia (7YSZ) and pyrochlore lanthanum zirconate (La₂Zr₂O₇) were deposited by electron beam physical vapor deposition. Regarding thermal cycling and isothermal oxidation behavior different layer architectures such as mono‐, double‐ and quadruple ceramic layers were investigated. The thermal shock behavior was examined by thermocycle tests at temperatures in the range between T = 50 °C ‐1,150 °C. Additionally, the isothermal oxidation behavior at a temperature of T = 1,150 °C with dwell times of t= 50 h and t = 100 h was studied in the present work. The conducted research concerning the behavior of various thermal barrier coating systems under thermal cycle and isothermal load highlights the potential of multilayer thermal barrier coatings for operating in high temperature areas.     

CMAS环境下电子束物理气相沉积热障涂层的热循环行为及失效机制

航空发动机涡轮叶片工作时表面经常产生CaO-MgO-Al₂O₃-SiO₂(简称CMAS)等沉积物。本文中研究了电子束物理气相沉积(EB-PVD)制备ZrO₂热障涂层(TBCs)在CMAS环境下的热循环行为及失效机制。结果表明, 在1200℃热冲击条件下, 表面涂覆CMAS的热障涂层的热循环寿命低于100次, 而未涂覆CMAS的涂层寿命达到500次以上, CMAS 的存在加速了热障涂层的剥落失效。在1200℃经过210次循环后, ZrO₂陶瓷层与CMAS之间形成了约8 μm厚的互反应区, 其形成主要与CMAS中Ca²⁺内扩散有关。CMAS环境下热障涂层陶瓷层产生大量横向裂纹, 涂层的失效主要以陶瓷层片状剥落为主。     

Plasma-sprayed ceramic thermal barrier coatings for turbine vane platforms

Increased resistance of plasma-sprayed ceramic coatings to cyclic thermal exposure can be achieved by increasing the strain tolerance of the ceramic and by controlling harmful residual stresses. Strain tolerance can be increased by controlling internal structure to minimize harmful flaws and to add favorable discontinuities. Structures with controlled porosity, internal microcracking and segmentation were shown to give good resitance to thermal cycling. Plasma spraying on low temperature substrates was shown to increase spalling life in a cyclic thermal test by reducing damaging compressive stresses.