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 Cyclic Fatigue Behavior of Nanostructured YSZ/NiCrAlY Compositionally Graded Thermal Barrier Coatings

Oxidation of Metals - In this research, thermal cyclic fatigue (TCF) behavior of a compositionally graded layer (CGL) nanostructured coating was investigated and compared with the TCF behavior of a...     

Influence of TGO Composition on the Thermal Shock Lifetime of Thermal Barrier Coatings with Cold-sprayed MCrAlY Bond Coat

NiCoCrAlTaY bond coat was deposited by cold spraying to assemble thermal barrier coatings (TBCs). The microstructure of the cold-sprayed bond coat was examined using scanning electron microscopy. TBCs consisting of cold-sprayed bond coat and plasma-sprayed YSZ were pretreated at different conditions to form different thermally grown oxides (TGOs) before thermal cycling test. The influence of the TGO composition on the thermal cyclic lifetime was quantitatively examined through the measurement of the coverage ratio of the mixed oxides on the bond coat surface. The results showed that the bond coat exhibited a dense oxidation-free microstructure, and TGOs in different morphologies and constituents were present after thermal cyclic test. The formation of TGOs was significantly influenced by pretreatment conditions. Two kinds of TGO were detected on the surface of bond coat after the spallation of YSZ coatings. One is the α-Al₂O₃-based TGO and the other is the mixed oxide. It was found that the thermal cyclic lifetime is inversely proportional to the coverage ratio of the mixed oxides formed at the bond coat/YSZ interface. The high coverage ratio of the mixed oxide on the interface leads to the early spalling of YSZ coating.     

Adhesion improvements of Thermal Barrier Coatings with HVOF thermally sprayed bond coats

Thermal barrier coatings (TBC) are an effective engineering solution for the improvement of in service performance of gas turbines and diesel engine components. The quality and further performance of TBC, likewise all thermally sprayed coatings or any other kind of coating, is strongly dependent on the adhesion between the coating and the substrate as well as the adhesion (or cohesion) between the metallic bond coat and the ceramic top coat layer. The debonding of the ceramic layer or of the bond coat layer will lead to the collapse of the overall thermal barrier system. Though several possible problems can occur in coating application as residual stresses, local or net defects (like pores and cracks), one could say that a satisfactory adhesion is the first and intrinsic need for a good coating. The coating adhesion is also dependent on the pair substrate-coating materials, substrate cleaning and blasting, coating application process, coating application parameters and environmental conditions. In this work, the general characteristics and adhesion properties of thermal barrier coatings (TBCs) having bond coats applied using High Velocity Oxygen Fuel (HVOF) thermal spraying and plasma sprayed ceramic top coats are studied. By using HVOF technique to apply the bond coats, high adherence and high corrosion resistance are expected. Furthermore, due to the characteristics of the spraying process, compressive stresses should be induced to the substrate. The compressive stresses are opposed to the tensile stresses that are typical of coatings applied by plasma spraying and eventually cause delamination of the coating in operational conditions. The evaluation of properties includes the studies of morphology, microstructure, microhardness and adhesive/cohesive resistance. From the obtained results it can be said that the main failure location is in the bond coat/ceramic interface corresponding to the lowest adhesion values.     

EB-PVD热障涂层热循环性能评价方法研究

制备8批次EB-PVD双层结构热障涂层试样,采用循环加热快速冷却实验装置模拟热障涂层服役环境,开展了热障涂层试样在不同热循环保温时间条件下的热循环性能评价实验,采用指数下降的数学模型对热循环实验数据进行拟合分析,获得了表征热障涂层试样静态氧化性能和热疲劳性能的物理量.结果表明,在本实验工艺条件下制备的不同批次热障涂层试样的静态氧化性能和热疲劳性能具有不同的匹配关系,热障涂层试样静态氧化性能总体估计值为(677±194)h,热疲劳性能总体估计值为(6789±1818)次.     

Microstructure and Thermal Behavior of Thermal Barrier Coatings

Yttria stabilized zirconia thick coatings were thermally sprayed from two different feedstock powders. Coating characteristics such as density, crystalline phase composition, and microstructure were evaluated. The thermal expansion coefficient and thermal diffusivity were measured as a function of temperature up to 800 °C and analyzed in terms of the microstructural features. The ability of available models to relate the measured thermal properties to the microstructural features as characterized by readily available methods was assessed. The importance of pore shape and orientation on the thermal conductivity was evidenced. The thermal contact resistance between the substrate and the coating in these samples was estimated from the thermal diffusivity data, and found to change during cooling from 800 °C.     

Improved Thermal Cycling Durability of Thermal Barrier Coatings Manufactured by PS-PVD

The plasma spray-physical vapor deposition (PS-PVD) process is a promising method to manufacture thermal barrier coatings (TBCs). It fills the gap between traditional thermal spray processes and electron beam physical vapor deposition (EB-PVD). The durability of PS-PVD manufactured columnar TBCs is strongly influenced by the compatibility of the metallic bondcoat (BC) and the ceramic TBC. Earlier investigations have shown that a smooth BC surface is beneficial for the durability during thermal cycling. Further improvements of the bonding between BC and TBC could be achieved by optimizing the formation of the thermally grown oxide (TGO) layer. In the present study, the parameters of pre-heating and deposition of the first coating layer were investigated in order to adjust the growth of the TGO. Finally, the durability of the PS-PVD coatings was improved while the main advantage of PS-PVD, i.e., much higher deposition rate in comparison to EB-PVD, could be maintained. For such coatings, improved thermal cycling lifetimes more than two times higher than conventionally sprayed TBCs, were measured in burner rigs at ~1250 °C/1050 °C surface/substrate exposure temperatures.     

粘结层和陶瓷层厚度对纳米结构热障涂层性能的影响

采用超音速火焰喷涂+大气等离子喷涂工艺,在K403高温合金表面制备不同层厚比的NiCrA-lY/纳米7YSZ热障涂层,研究了涂层厚度变化对热障涂层表面粗糙度、结合强度、热震性能和热循环寿命的影响规律。结果表明:当粘结层厚度一定时,随着陶瓷层厚度的增加,其表面粗糙度增加,涂层结合强度下降;当粘结层厚度为50μm时,热障涂层的抗热震性能随陶瓷层厚度增加而降低,粘结层厚度提高至100μm时,热障涂层的抗热震性能随陶瓷层厚度增加先提高,后降低,热障涂层在1100℃的热循环寿命测试结果也基本对应这一规律;当粘结层厚50μm且陶瓷层/粘结层的层厚比在(1～2)∶1的范围内,或者粘结层厚100μm且陶瓷层/粘结层的层厚比在(2～2.5)∶1范围内时,热障涂层具有较优异的性能。     

Evolution of Residual Stresses in PS-PVD Thermal Barrier Coatings on Thermal Cycling

Plasma spray-physical vapor deposition (PS-PVD) is an advanced technique to fabricate quasi-columnar structured thermal barrier coatings (TBCs) with excellent thermal cyclic lifetime. In this study, PS-PVD TBCs were investigated via burner rig test. The residual stresses in both of the topcoat layer and the thermally grown oxide (TGO) scale were measured non-destructively using Raman spectroscopy and Cr³⁺ photoluminescence piezo-spectroscopy, respectively. Evolution of the microstructures and distribution of residual stresses in such kind structured TBCs before and after thermal cycling test were investigated. The accumulated tensile stress in the as-sprayed ceramic topcoat changed to compressive state after 100 cycles and then gradually increased. In addition, the mapping compressive stresses in the TGO measured through the ceramic topcoat surface decreased rapidly and then essentially maintained at a relatively stable state with further testing. Moreover, the pre-heating of the bondcoat could significantly affect the stress distribution in the TGO, in contrast, no obviously influence on the stresses in the YSZ topcoat.     

多次热循环下热障涂层热稳定过程的数值模拟

采用热弹塑性有限元方法,通过对热障涂层多次热循环降温过程中产生的残余应力的数值模拟,研究了在热梯度、热膨胀失调情况下涂层的热稳定过程.结果表明,在相同材料参数情况下,陶瓷层与粘接层的界面形貌对残余应力及稳定状态有显著的影响,凹凸不平的界面将会使界面残余应力发生突变,不利于增强界面结合强度和涂层结构的稳定.该结果对分析涂层寿命及失效机制有指导意义.     

Mechanical Properties and Thermal Shock Resistance of HVOF Sprayed NiCrAlY Coatings Without and With Nano Ceria

NiCrAlY coatings without and with 0.2?wt.% nano ceria were prepared by high velocity oxygen fuel spraying. The microstructure, mechanical properties, and thermal shock resistance of as-sprayed coatings were investigated. The results showed that in the as-sprayed coatings, the number of un-melted particles was reduced drastically, the microstructure was refined and compact due to the refinement of sprayable powders. Both the hardness and adhesive strength of the NiCrAlY increased due to the refinement of microstructure and the decrease of the defects, such as pores and oxides, after adding nano ceria. The thermal cycle life of NiCrAlY coatings was improved by 15% after adding 0.2?wt.% nano ceria, which is attributed to the low content of spinel NiCr₂O₄ and high content of Cr₂O₃ in the thermal cycling, the refined and compact microstructure, and increased interfacial boundary.     

超音速微粒轰击处理对NiCoCrAlY粘结层高温氧化行为的影响

采用超音速微粒轰击(SFPB)技术细化处理高速氧燃料喷涂法(HVOF)喷涂的粘结层,结果表明粘结层主要由γ'-Ni3Al相和γ-Ni相组成.高温氧化2 h,粘结层表面首先生成亚稳态的γ-Al2O3和稳态的α-Al2O3,且在Al2O3之间有少量NiO、Co3O4和尖晶石.亚稳γ-Al2O3和尖晶石倾向于借助β-(Ni,...     

Structural and Thermal Behavior of Fe-Cr-Mo-P-B-C-Si Amorphous and Nanocrystalline HVOF Coatings

The microstructure, thermal behavior, and mechanical properties of amorphous/nanocrystalline 70Fe-15Cr-4Mo-5P-4B-1C-1Si (wt.%) coatings produced by high velocity oxy fuel (HVOF) spraying of mechanically alloyed powders were investigated by x-ray diffractometry (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Thermal stability of samples was investigated using differential scanning calorimetry (DSC). The results show that by adjusting the HVOF parameters especially fuel/oxygen ratio and proper selection of powder composition, the desired microstructure with different amount of amorphous and nanocrystalline phases and therefore with different mechanical properties could be obtained.     

Thermal Cycling Behavior of Quasi-Columnar YSZ Coatings Deposited by PS-PVD

Columnar-structured thermal barrier coatings, owing to their high strain tolerance, are expected for their potential possibilities to substantially extend turbine lives and improve engine efficiencies. In this paper, plasma spray-physical vapor deposition (PS-PVD) process was used to deposit yttria partially stabilized zirconia (YSZ) coatings with quasi-columnar structures. Thermal cyclic tests on burner rigs and thermal shock tests by heating and water-quenching method were involved to evaluate the thermal cycling and thermal shock behaviors of such kind of structured thermal barrier coatings (TBCs). Evolution of the microstructures, phase composition, residual stresses and failure behaviors of quasi-columnar YSZ coatings before and after the thermal tests was investigated. The quasi-columnar coating obtained had an average life of around 623 cycles when the spallation area reached about 10% of the total coating surface during burner rig tests with the coating surface temperature of ~1250 °C. Failure of the coating is mainly due to the break and pull-out of center columnar segments.     

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.     

Bond Strength of Multicomponent White Cast Iron Coatings Applied by HVOF Thermal Spray Process

Multicomponent white cast iron is a new alloy that belongs to system Fe-C-Cr-W-Mo-V, and because of its excellent wear resistance it is used in the manufacture of hot rolling mills rolls. To date, this alloy has been processed by casting, powder metallurgy, and spray forming. The high-velocity oxyfuel process is now also considered for the manufacture of components with this alloy. The effects of substrate, preheating temperature, and coating thickness on bond strength of coatings have been determined. Substrates of AISI 1020 steel and of cast iron with preheating of 150 °C and at room temperature were used to apply coatings with 200 and 400 μm nominal thickness. The bond strength of coatings was measured with the pull-off test method and the failure mode by scanning electron microscopic analysis. Coatings with thickness of 200 μm and applied on substrates of AISI 1020 steel with preheating presented bond strength of 87 ± 4 MPa.     

Thermal Cycling and High-Temperature Corrosion Tests of Rare Earth Silicate Environmental Barrier Coatings

Lutetium and yttrium silicates, enriched with an additional secondary zirconia phase, environmental barrier coatings were synthesized by the solution precursor plasma spraying process on silicon carbide substrates. A custom-made oven was designed for thermal cycling and water vapor corrosion testing. The oven can test four specimens simultaneously and allows to evaluate environmental barrier performances under similar corrosion kinetics compared to turbine engines. Coatings structural evolution has been observed by SEM on the polished cross sections, and phase composition has been analyzed by XRD. All coatings have been thermally cycled between 1300 °C and the ambient temperature, without spallation, due to their porosity and the presence of additional secondary phase which increases the thermal cycling resistance. During water vapor exposure at 1200 °C, rare earth disilicates showed a good stability, which is contradictory with the literature, due to impurities—such as Si- and Al-hydroxides—in the water vapor jets. The presence of vertical cracks allowed the water vapor to reach the substrate and then to corrode it. It has been observed that thin vertical cracks induced some spallation after 24 h of corrosion.