Thermal Failure of Nanostructured Thermal Barrier Coatings with Cold-Sprayed Nanostructured NiCrAlY Bond Coat

Nanostructured thermal barrier coatings (TBCs) were deposited by plasma spraying using agglomerated nanostructured YSZ powder on Inconel 738 substrate with cold-sprayed nanostructured NiCrAlY powder as bond coat. The isothermal oxidation and thermal cycling tests were applied to examine failure modes of plasma-sprayed nanostructured TBCs. For comparison, the TBC consisting of conventional microstructure YSZ and conventional NiCrAlY bond coat was also deposited and subjected to the thermal shock test. The results showed that nanostructured YSZ coating contained two kinds of microstructures; nanosized zirconia particles embedded in the matrix and microcolumnar grain structures of zirconia similar to those of conventional YSZ. Although, after thermal cyclic test, a continuous, uniform thermally grown oxide (TGO) was formed, cracks were observed at the interface between TGO/BC or TGO/YSZ after thermal cyclic test. However, the failure of nanostructured and conventional TBCs mainly occurred through spalling of YSZ. Compared with conventional TBCs, nanostructured TBCs exhibited better thermal shock resistance.     

Thermal Fatigue Behavior of Thick and Porous Thermal Barrier Coatings Systems

High-temperature thermal fatigue causes the failure of thermal barrier coating (TBC) systems. This paper addresses the development of thick TBCs, focusing on the microstructure and the porosity of the yttria partially stabilized zirconia (YPSZ) coating, regarding its resistance to thermal fatigue. Thick TBCs, with different porosity levels, were produced by means of a CoNiCrAlY bond coat and YPSZ top coat, both had been sprayed by air plasma spray. The thermal fatigue resistance of new TBC systems and the evolution of the coatings before and after thermal cycling was then evaluated. The limit of thermal fatigue resistance increases depending on the amount of porosity in the top coat. Raman analysis shows that the compressive in-plane stress increases in the TBC systems after thermal cycling, nevertheless the increasing rate has a trend which is contrary to the porosity level of top coat. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Low conductivity plasma sprayed thermal barrier coating using hollow psz spheres: Correlation between thermophysical properties and microstructure

Life and thermal properties of plasma sprayed TBCs - widely used in gas turbine engines - are closely related to the microstructure of the ceramic top coating. Especially, the thermal behaviour of this coating is induced by the void shapes and networks which are in turn determined by both the spraying conditions and the feedstock material.A specific hollow yttria partially stabilised zirconia powder was produced in a one-step process by spray drying and an experimental statistical design study was conducted to investigate the influence of spraying variables (primary and secondary gas flow rates, arc current, spraying distance, spraying angle and traverse speed) on structure and properties of resulting plasma sprayed coatings. The coatings were characterized with respect to deposition efficiency, roughness, porosity and thermal conductivity. A reduction of 25% of the thermal conductivity was achieved by improving the spray and powder parameters. A quantitative characterization of the porous structure using image analysis of polished cross-sections was implemented. The parameters that have relevant influence on the coating porous structure were identified, and their relative importance was determined. An attempt was made to identify morphological criteria of the porous network (coarse/fine porosity ratio, cracks total length, cracks orientation) correlating with the thermal conductivity values.     

Thermal Shock Resistance of APS 8YSZ Thermal Barrier Coatings on Titanium Alloy

Thermal barrier coatings (TBCs) with a typical 8YSZ ceramic top coat and CoNiCrAlY bond coat were deposited on titanium alloy substrate (Ti-6Al-4V in wt.%) by air plasma spraying. Thermal insulation and thermal shock resistance of the TBCs at different temperatures as well as their failure behavior were investigated. The results showed that the test temperature had a significant effect on thermal shock life of the TBCs. Failure of the TBCs systems was caused by the formation of crack, bond coat oxidation and elemental diffusion. The vertical cracks induced by thermal shock cycles were probably responsible for the enhancement in thermal shock resistance of the TBCs. Furthermore, elemental diffusion had a great effect on the acceleration of the TBCs failure. The TBCs could provide a good thermal protection for the titanium alloy substrate.     

Microstructural evolution of CoCrAlY bond coat on Ni-based superalloy DZ 125 at 1050 °C

CoCrAlY alloy has been widely used as metallic protective coatings or the bond coats in thermal barrier coatings (TBCs) to protect the underlying superalloy from oxidation and hot-corrosion. In this paper, the TBC consisting of yttria stabilized zirconia (7YSZ) ceramic top coat and CoCrAlY bond coat was deposited onto directionally solidified nickel based superalloy DZ 125 by electron beam physical vapor deposition (EB-PVD). The microstructural evolution of the bond coat on this superalloy was investigated after thermal exposure for 100 h at 1050 °C. Due to a significant inward diffusion of Al, Co and Cr from the coating and outward diffusion of Ni, Hf, W and Ti from the substrate, the phase transformation from the Co-based Al-rich β-CoAl phase to the Al-deficient γ-CoNi solid solution phase occurred in the bond coat. Simultaneously, a large amount of Ni-based β-NiCoAl phase was present in the bond coat. In addition, the particles containing substrate strengthening elements Hf and/or W are abundant in the thermally grown oxides (TGO) and within the bond coat. The mechanism for the microstructural evolution is discussed.     

Study of microstructure and thermal shock behavior of two types of thermal barrier coatings

Gas turbines provide one of the most severe environments challenging material systems nowadays. Only an appropriate coating system can supply protection particularly for turbine blades. This study was made by comparison of properties of two different types of thermal barrier coatings (TBCs) in order to improve the surface characteristics of high temperature components. These TBCs consisted of a duplex TBC and a five layered functionally graded TBC. In duplex TBCs, 0.35 mm thick yittria partially stabilized zirconia top coat (YSZ) was deposited by air plasma spraying and ～0.15 mm thick NiCrAlY bond coat was deposited by high velocity oxyfuel spraying. ～0.5 mm thick functionally graded TBC was sprayed by varying the feeding ratio of YSZ/NiCrAlY powders. Both coatings were deposited on IN 738LC alloy as a substrate. Microstructural characterization was performed by SEM and optical microscopy whereas phase analysis and chemical composition changes of the coatings and oxides formed during the tests were studied by XRD and EDX. The performance of the coatings fabricated with the optimum processing conditions was evaluated as a function of intense thermal cycling test at 1100 °C. During thermal shock test, FGM coating failed after 150 and duplex coating failed after 85 cycles. The adhesion strength of the coatings to the substrate was also measured. Finally, it is found that FGM coating has a larger lifetime than the duplex TBC, especially with regard to the adhesion strength of the coatings.     

Influence of heat treatment on nanocrystalline zirconia powder and plasma-sprayed thermal barrier coatings

Nanostructured zirconia top coat was deposited by air plasma spray and NiCoCrAlTaY bond coat was deposited on Ni substrate by low pressure plasma spray.Nanostructured and conventional thermal barrier coatings were heat-treated at temperature varying from 1050 to 1 250oC for 2-20 h.The results show that obvious grain growth was found in both nanostructured and conventional thermal barrier coatings(TBCs)after high temperature heat treatment.Monoclinic/tetragonal phases were transformed into cubic phase in the agglomerated nano-powder after calcination.The cubic phase content increased with increasing calcination temperature.Calcination of the powder made the yttria distributed on the surface of the nanocrystalline particles dissolve in zirconia when grains grew.Different from the phase constituent of the as-sprayed conventional TBC which consisted of diffusionlesstransformed tetragonal,the as-sprayed nanostructured TBC consisted of cubic phase.     

Failure Mechanism for Thermal Fatigue of Thermal Barrier Coating Systems

Thick thermal barrier coatings (TBCs), consisting of a CoNiCrAlY bond coat and yttria-partially stabilized zirconia top coat with different porosity values, were produced by air plasma spray (APS). The thermal fatigue resistance limit of the TBCs was tested by furnace cycling tests (FCT) according to the specifications of an original equipment manufacturer (OEM). The morphology, residual stresses, and micromechanical properties (microhardness, indentation fracture toughness) of the TBC systems before and after FCT were analyzed. The thermal fatigue resistance increases with the amount of porosity in the top coat. The compressive in-plane stresses increase in the TBC systems after thermal cycling; nevertheless the increasing rate has a trend contrary to the porosity level of top coat. The data suggest that the spallation happens at the TGO/top coat interface. The failure mechanism of thick TBCs was found to be similar to that of conventional thin TBC systems made by APS.     

热障涂层系统的热梯度机械疲劳应力分析

基于IN738高温合金基体上涂覆的热障涂层系统(Thermal barrier coating system,TBCs),分析热循环和热梯度机械疲劳加载条件下涂层的应力分布及演变。通过有限元分析研究了热生长氧化层(Thermally growth oxidation,TGO)的应力分布,以预测不同载荷作用下TBCs的失效行为。结果可知,在热循环的基础上施加应变载荷会造成TGO应力性质及大小的改变。只施加温度载荷,在加热过程中TGO/粘结层(Bond coat,BC)界面波峰位置会承受轴向较大的拉伸应力,裂纹多会在此处萌生,且以层间开裂的方式失效。而在温度与机械载荷的共同作用下,冷却过程中会承受较大的拉伸应力,显著增大的轴向应力与径向应力共同作用,使垂直于TGO/BC界面的裂纹沿着界面方向扩展,从而造成陶瓷层(Top coat,TC)剥落。进一步对比分析了同相和反相加载时的应力分布,结果表明反相加载时一次循环周期内会产生拉伸平均应力,更易发生TBCs的失效。     

粉末粒径对低压等离子喷涂厚W涂层的影响

采用低压等离子喷涂(LPPS)技术,用2种不同粒径的粉末,在铜基体上制备了厚度为0.7 mm以上的W涂层.通过扫描电镜(SEM)研究了涂层的微观形貌,并对粉末对W涂层的显微结构、结合强度、氧含量及热导率的影响进行了探讨.结果表明:细小粒径W粉所制备的W涂层孔隙率较低,但涂层存在明显的分层;粗粉所制备的W涂层结构均匀,涂层的内聚力与热导率分别达到了38 MPa和98.06 W/m·k,远高于细W粉涂层的23 MPa和75.36W/m·k.细W粉涂层较低的性能和其严重的分层有关.     

Effect of Feedstock Size and its Distribution on the Properties of Detonation Sprayed Coatings

The detonation spraying is one of the most promising thermal spray variants for depositing wear and corrosion resistant coatings. The ceramic (Al₂O₃), metallic (Ni-20 wt%Cr) , and cermets (WC-12 wt%Co) powders that are commercially available were separated into coarser and finer size ranges with relatively narrow size distribution by employing centrifugal air classifier. The coatings were deposited using detonation spray technique. The effect of particle size and its distribution on the coating properties were examined. The surface roughness and porosity increased with increasing powder particle size for all the coatings consistently. The feedstock size was also found to influence the phase composition of Al₂O₃ and WC-Co coatings; however does not influence the phase composition of Ni-Cr coatings. The associated phase change and %porosity of the coatings imparted considerable variation in the coating hardness, fracture toughness, and wear properties. The fine and narrow size range WC-Co coating exhibited superior wear resistance. The coarse and narrow size distribution Al₂O₃ coating exhibited better performance under abrasion and sliding wear modes however under erosion wear mode the as-received Al₂O₃ coating exhibited better performance. In the case of metallic (Ni-Cr) coatings, the coatings deposited using coarser powder exhibited marginally lower-wear rate under abrasion and sliding wear modes. However, under erosion wear mode, the coating deposited using finer particle size exhibited considerably lower-wear rate.     

热障涂层脱粘裂纹缺陷的高温表征方法与演变规律EI北大核心CSCD

热障涂层在服役过程中相邻区域脱粘裂纹的扩展合并是造成陶瓷层最终剥落的重要原因,然而缺乏简单有效的无损测试方法。提出利用空腔高热阻在陶瓷层表面局部热积累,形成表面亮斑的特点,通过亮斑反向跟踪脱粘缺陷的新方法。结果表明,在界面处制备水溶性盐斑,继续喷涂陶瓷层后用水浴溶解的方式可在YSZ与金属粘结层界面有效预制特定外形与尺寸的人造脱粘裂纹缺陷;预制脱粘裂纹与表面高温亮斑尺寸呈正相关,且近似呈现为比例系数为1.031的线性关系,当预制裂纹直径大于0.4 mm时,可在涂层表面观测到亮斑,当预制裂纹直径大于0.7 mm时,用亮斑尺寸预测裂纹尺寸的相对误差低于15%;在梯度热冲击循环下,热障涂层随热冲击次数的增加,表面首先出现亮斑,随后亮斑长大、合并,在2500次左右热循环时合并速度加快,最终陶瓷层在亮斑处局部剥落。基于脱粘裂纹对于纵向热流的阻碍作用,提出一种人造脱粘裂纹缺陷的预制方法,并确立一种通过测量表面亮斑尺寸估计内部裂纹尺寸的热障涂层无损测量方法。解决了热障涂层高温缺陷难以实时观测的问题,并进一步研究了其高温演变规律,可为热障涂层的寿命预测提供数据支持。     

等离子喷涂热障涂层高温风洞热震行为

采用等离子喷涂工艺制备ZrO2-8%Y2O3(质量分数,下同)陶瓷层,冷喷涂制备CoNiCrAlY粘结层,在高温燃气风洞条件下测试热障涂层的热震性能,并研究了高温氧化处理对试样热震性能的影响.结果表明,等离子喷涂热障涂层具有较好的抗热震性能,经过100次热震循环后,涂层与基体结合良好,涂层较为完整,未出现大面积的剥落;经过氧化处理后的试样抗热震性较差.     

Formation of molybdenum boride cermet coating by the detonation spray process

The effects of the powder particle size and the acetylene/oxygen gas flow ratio during the detonation spray process on the amount of molybdenum phase, porosity, and hardness of the coatings using MoB powder were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), etc. The results show that the presence of metallic molybdenum in the coating results from decomposition of MoB powder during thermal spray. The compositions of the coatings are metallic Mo, MoB, and Mo₂B, which are different from the phases of the original powder. The amount of molybdenum phase increases monotonously with the oxygen/acetylene ratio, but the increasing rate for the fine powder is faster than that for the coarse powder. The porosity and hardness of the coating are related to the amount of molybdenum phase. The phase constitution of the coating is discussed.     

Thermal cycling failure of new LaMgAl11O19/YSZ double ceramic top coat thermal barrier coating systems

New LaMgAl₁₁O₁₉ (LaMA)/YSZ double ceramic top coat thermal barrier coatings (TBCs) with the potential application in advanced gas-turbines and diesel engines to realize improved efficiency and durability were prepared by plasma spraying, and their thermal cycling failure were investigated. The microstructure evolutions as well as the crystal chemistry characteristics of LaMA coating which seemed to have strong influences on the thermal cycling failure of LaMA and the new double ceramic top coat TBCs based on LaMA/YSZ system were studied. For double ceramic top coat TBC system, interface modification of LaMA/YSZ by preparing thin composite coatings seemed to be more preferred due to the formations of multiple cracks during thermal cycling making the TBC to be more strain tolerant and as well as resulting in an improved thermal cycling property. The effects of the TGO stresses on the failure behavior of the TBCs were discussed through fluorescence piezo-spectroscopy analysis.     

Plasma spray forming of tungsten coatings on copper electrodes

Both direct current dc plasma and radio frequency induction plasma were used to deposit tungsten coatings on copper electrodes. Fine tungsten powder with mean particle size of 5 um and coarse tungsten powder with particle size in the range from 45 um to 75 um were used as plasma spray feedstock. It is found that dc plasma is only applicable to spray the fine tungsten powder and induction plasma can be used to spray both the coarse powder and the fine powder. The tungsten coating deposited by the induction plasma spraying of the coarse powder is extremely dense. Such a coating with an interlocking structure and an integral interface with the copper substrate demonstrates high cohesion strength and adhesion strength.     

The Influence of Heat Treatments on the Porosity of Suspension Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

Suspension plasma-sprayed coatings are produced using fine-grained feedstock. This allows to control the porosity and to achieve low thermal conductivity which makes the coatings attractive as topcoats in thermal barrier coatings (TBCs). Used in gas turbine applications, TBCs are exposed to high temperature exhaust gases which lead to microstructure alterations. In order to obtain coatings with optimized thermomechanical properties, microstructure alterations like closing of pores and opening of cracks have to be taken into account. Hence, in this study, TBC topcoats consisting of 4 mol.% yttria-stabilized zirconia were heat-treated in air at 1150 °C and thereafter the coating porosity was investigated using image analysis (IA) and nuclear magnetic resonance (NMR) cryoporometry. Both IA and NMR cryoporometry showed that the porosity changed as a result of the heat treatment for all investigated coatings. In fact, both techniques showed that the fine porosity decreased as a result of the heat treatment, while IA also showed an increase in the coarse porosity. When studying the coatings using scanning electron microscopy, it was noticed that finer pores and cracks disappeared and larger pores grew slightly and achieved a more distinct shape as the material seemed to become more compact.     

Fracture mechanics analysis of microcracks in thermally cycled thermal barrier coatings

The effects from thermal shock loading on pre-existing microcracks within thermal barrier coatings (TBCs) have been investigated through a finite element based fracture mechanical analysis. The TBC system consists of a metallic bond coat and a ceramic top coat. The rough interface between the top and bond coats holds an alumina oxide layer. Stress concentrations at the interface due to the interface roughness, as well as the effect of residual stresses, were accounted for. At the eventual closure between the crack surfaces, Coulomb friction was assumed. To judge the risk of fracture from edge cracks and centrally placed cracks, the stress intensity factors were continuously monitored during the simulation of thermal shock loading of the TBC. It was found that fracture from edge cracks is more likely than from centrally placed cracks. It was also concluded that the propagation of an edge crack is already initiated during the first load cycle, whereas the crack tip position of a central crack determines whether propagation will occur.     

Behavior of plasma-sprayed thermal barrier coatings during thermal cycling and the effect of a preoxidized NiCrAlY bond coat

The failure mechanisms of thermal barrier coatings (TBCs) subjected to a thermal load are still not entirely understood. Thermal stresses and/or oxidation cause the coating to fail and hence must be minimized. During the present investigation, TBCs up to 1.0 mm were sprayed and withstood high thermal stresses during thermal testing. Owing to the substantial thickness, the temperature at the top coat/bond coat interface was relatively low, resulting in a low oxidation rate. Furthermore, bond coats were preoxidized before applying a top coat. The bond strength and the behavior during three different thermal loads of the preoxidized TBCs were compared with a standard duplex TBC. Finite-element model (FEM) calculations that took account of bond coat preoxidation and interface roughness were made to calculate the stresses occurring during thermal shock. It is concluded that the thick TBCs applied during this research exhibit excellent thermal shock resistance and that a preoxidizing treatment of the bond coat increases the lifetime during thermal loading, where oxidation is the main cause of failure. The FEM analysis gives a first impression of the stress conditions on the interface undulations during thermal loading, but further development is required.     

Microstructure design and mechanical properties of thermal barrier coatings with layered top and bond coats

The microstructure of layered thermal barrier coatings (TBCs) with three coating layers in the bond and top coats, respectively, prepared using a specialized coating system (TriplexPro™-200), was controlled and its mechanical properties were investigated, which were then compared with the common TBCs with a single layer in each coat. The bond and top coats were coated with 100 and 200 μm for each feedstock, resulting in 300 and 600 μm thicknesses in the bond and top coats, respectively. The microstructure of the top coat could be controlled by changing the feedstock and using a multiple hopper system—dense/intermediate/porous layers from surface to interface or reverse microstructure. In the case of the bond coat, a compositional gradient was achieved. The adhesive strength values of the top coats were strongly dependent on the microstructure, whereas the values for the bond coat were similar. The hardness and toughness values gradually changed from surface to interface, indicating that the mechanical properties corresponded well with the microstructure of the TBCs. The indentation stress-strain curves of both TBCs with the layered structure were located between the curves for TBCs with the single structure of relatively dense and porous microstructures. Damage on the surface and subsurface was strongly affected by the microstructure of the top coat, showing a similar trend with the stress-strain behavior. This evidence allowed us to propose an efficient coating in protecting the substrate from mechanical environments.