Relationship between microstructure of plasma-sprayed 8YZ coatings and thermal fatigue life of thermal barrier coatings

Microstructure of plasma-sprayed yttria-stabilized zirconia coatings (8YZ) was characterized by the measurement of surface roughness, hardness, and pore size distribution and was correlated with thermal fatigue life. It was confirmed that the coatings which had greater roughness tended to show both lower hardness and higher porosity. Furthermore, such coatings were found to have a longer thermal fatigue life. We propose that measurement of the roughness of 8YZ coatings is useful as a non-destructive evaluation method for predicting thermal fatigue life.     

Measurement of thermal diffusivity and anisotropy of plasma-sprayed coatings

The thermal diffusivity of free-standing tungsten and zirconia plasma-sprayed coatings was measured in the directions parallel and perpendicular to their surface. The parallel thermal diffusivity was evaluated by a double-sensing Laplace-transform technique and compared to the perpendicular values obtained by the (lash technique. Ratios between the parallel and the perpendicular thermal diffusivity values were in the range of 1.1 to 1.5 for zirconia and 4 to 6 for tungsten. The results are discussed in terms of the coating thickness and microstructure.On leave from Laboratoire d'Énergetique et de Méchanique Théorique et Appliquée, B.P. 160, 54504 Vandoeuvre les Nancy, France.     

等离子体喷涂纳米结构氧化锆热障涂层高温煅烧后的组织结构和相组成变化

采用国产和进口两种大气等离子体喷涂设备,在不同功率下喷涂制备了ZrO2-8%(质量分数)Y2O3纳米结构热障涂层.在大气环境箱式电阻炉中1100～1250℃下对涂层进行了高温煅烧处理,采用扫描电子显微镜观察分析了涂层煅烧前后的微观组织变化,通过X射线衍射分析了涂层煅烧前后的相组成变化.结果表明:涂层经高温煅烧后,熔化团聚体大颗粒中新形成的纳米晶发生了明显长大,但临界熔化状态的团聚大颗粒中的小颗粒无明显变化;采用国产设备较低功率制备的纳米结构氧化锆涂层相组成为四方相和立方相,以四方相为主,经高温煅烧后四方相向立方相转变,且随煅烧温度和时间的增加而增加;采用进口设备较高功率制备的涂层全部为立方相,煅烧前后无相变发生.     

Thermal conductivity and phase evolution of plasma-sprayed multilayer coatings

Multilayer coatings were prepared using small-particle plasma spray to investigate the effect of interfaces on thermal conductivity and phase stability. Monolithic and multilayer alumina and yttria partially-stabilized zirconia coatings, with 0, 3, 20, and 40 interfaces in 200–380 m thick coatings were studied. Thermal conductivity was determined for the temperature range 25 °C to 1200 °C using the laser flash method and differential scanning calorimetry. Thermal conductivity of the multilayer coatings was accurately modeled by a series heat transfer equation, indicating that interfacial resistance plays a negligible role in heat transfer in the direction perpendicular to the coating plane. Powder X-ray diffraction results indicate that identical phase transitions occur in all the coatings. Independent of coating microstructure (i.e. layer thickness), as-sprayed -Al₂O₃ transforms to -Al₂O₃ after 100 hours at 1200°C; as-sprayed metastable t–ZrO₂ converts to a mixture of t–ZrO₂ and c–ZrO₂ after 100 hours at 1300 °C. Thus, the results indicate that the interfaces do not aid in stabilizing the as-sprayed phases after prolonged severe heat treatments.     

Effective medium theory and the thermal conductivity of plasma-sprayed ceramic coatings

The thermal diffusivity of plasma-sprayed zirconium oxide-7% yttrium oxide and aluminium oxide-3% titanium oxide ceramic coatings was measured by using a periodic heat-flow method. Samples were prepared with different porosities thus allowing the thermal diffusivity as function of porosity to be determined. The samples were also impregnated with silicone oil so that the effect on the thermal diffusivity by replacing air with silicone oil in the pores could be determined. The experimental results were compared with effective medium theories representing three different microstructures: (1) a continuous ceramic matrix with dispersed pores, (2) a continuous ceramic matrix with continuously interconnected pores, (3) dispersed ceramic particles loosely bonded together. The latter two microstructures gave the best agreement between the experimental data and the theory.     

A model for the thermal conductivity of plasma-sprayed ceramic coatings

The very low thermal conductivity of plasma-sprayed ceramic coatings can be explained in terms of a model for the coating microstructure involving limited regions of good contact between lamellae. The non-contact regions may be regarded as very thin planar pores which have limited thermal conductance at low temperatures because their width is comparable with the mean free path of the gas molecules within them. At higher temperatures there is a radiative contribution across the pores which results in a smaller temperature coefficient of thermal conductivity than for a sintered ceramic. Heating at high temperatures results in changes in the shape of the planar pores and a large increase in the thermal conductivity.     

Phase composition and properties of plasma-sprayed zirconia thermal barrier coatings

The use of X-ray diffraction combined with TEM analysis has been used to study the crystalline structure and change in phase composition of zirconia coatings containing 6–12 wt% Y₂O₃. The optimum composition for maximum durability, observed for coatings within this composition range, is believed to be related to the microstructure developed on rapid cooling and to the volume fractions of t′, c and m phases formed during the evolution of the coating. The amount of these phases present in commercial thermal barrier coatings has been determined using X-ray diffraction and the mechanisms of toughening deduced from TEM examination of the sections of the coatings. The results obtained are discussed in relation to the degree of toughness and hence the thermal shock resistance which is a major factor in determining service life.     

Thermal conductivity and thermal diffusivity of plasma-sprayed stainless steel

Results are shown of an experimental study concerning the thermal conductivity (over the temperature range 50–400°C) and the thermal diffusivity (over the temperature range 500–1100°C) of plasma-sprayed stainless steel.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 24, No. 1, pp. 112–114, January, 1973.     

Effect of the thickness of plasma-sprayed coating on bond strength and thermal fatigue characteristics

NiCrAlY bond coat and ZrO2–8 wt% Y2O3 top coat with various thicknesses were deposited on Hastelloy X by plasma spraying. Residual stress was calculated by the finite element method (FEM) to explain the variations in the bond strength and thermal fatigue characteristics with the thickness of the bond coat and top coat. The bond strength of thermal barrier coatings (TBCs) increased with decreasing maximum residual stress in the y-direction of the top coat. The thermal fatigue characteristics increased with decrease of the maximum principal residual stress of the top coat and the thickness of oxidation layer of the bond coat.     

The relationship between the mechanism of formation,microstructure and properties of plasma-sprayed coatings

The mechanism of formation of plasma-sprayed coatings was examined and related to the microstructure produced. The evidence suggests that the real area of contact between individual lamellae within the coating and between lamellae and substrate is much less than the apparent area because of adsorbed and entrapped gas, oxide films or other contamination. The measured fracture toughness parameters for cohesive failure of coatings are generally much lower than would be expected for complete wetting of previously solidified material by impinging droplets, reflecting the imperfect contact between lamellae. Similar considerations apply to the lamellae-substrate interface at which the contact angle would generally be greater than for lamellae-lamellae interfaces. The difference between the fracture toughness values for ceramic and metallic coatings and the role of a metallic subcoat under ceramic coatings can be explained in terms of plastic deformation of metallic lamellae. The very high adhesive fracture toughness of NiAl coatings on steel implies more effective contact rather than inherently stronger bonding between contact points. This may be due to aluminothermic reduction of the oxide film on steel. Improvement of the mechanical properties of plasma-sprayed coatings requires methods for increasing the real area of contact between lamellae and between lamellae and substrate.     

Modeling of ultrasonic wave propagation in teeth using PSpice: a comparison with finite element models

Ultrasound is used extensively in the medical field for the detection and characterization of a variety of features in the human body. Finite element models used to understand ultrasonic wave propagation in teeth have been developed so that ultrasound techniques could be realized in dentistry. This paper presents a hypothesis that underlies one possible design of an ultrasonic tool that can be used in a clinical environment, as well as several models that describe acoustic field simulation, propagation, and interaction with the layers of several tooth structures. A complete PSpice model of a single-element transducer based on Redwood's version of Mason's equivalent circuit, a focusing lens, and a multi-layer tooth structure is used to illustrate the validity of this hypothesis. Transmission line theory is employed as a basis for the models of the piezoceramic, the lens, and the different tooth layers. Results clearly depict the transmission and reflection of the ultrasonic waves as they travel through the layers within the tooth structure and point out the noticeable similarity to longitudinal L-wave signatures produced by axisymmetric finite element models presented in earlier studies.     

超声波对纳米Ni-TiN复合镀层的影响

采用超声-电沉积的方法制备纳米Ni-TiN复合镀层.利用原子吸收分光光度计(AAS)、高分辨率电子显微镜(HRTEM)和X射线衍射仪(XRD)研究超声波对复合镀层含量、显微组织及微观结构的影响.结果表明,超声波的引入,不仅能提高复合镀层中纳米TiN粒子的含量,还能明显改善显微组织结构,细化晶粒.在超声波功率为200W时,镀层中粒子含量达到最大值9.9%.     

Determination and analysis of crack growth resistance in plasma-sprayed thermal barrier coatings

Ceramic thermal barrier coatings (TBCs) are increasingly applied to enhance the performance of advanced gas turbine engines. However, the delamination cracks initiated in these coatings limit their applications. In this research, a sandwiched four-point bend specimen is used to evaluate the crack growth resistance in plasma-sprayed TBCs. Well controlled, stable and measurable crack extension is obtained. A rising crack growth resistance curve is found. The steady state strain energy release rate is obtained to be about 170 J/m². The delamination crack evolution behavior is in situ observed and simulated by the finite element analysis based on a crack bridging model.     

Mechanical properties and fracture behavior of interfacial alumina scales on plasma-sprayed thermal barrier coatings

Abstract

The mechanical properties and fracture behavior of Y-doped Al₂O₃ scales were investigated by furnace thermal cycling (to 1,150°C) of plasma-sprayed thermal barrier coatings (TBCs) with vacuum plasma-sprayed (VPS) or air plasma-sprayed (APS) Ni–22Cr–10Al–1Y bond coatings. No significant alterations in Al₂O₃ hardness or Young’s modulus (as measured by mechanical properties microprobe) were detected as a function of bond coat type, exposure time, or number of thermal cycles. The interfacial Al₂O₃ scales on VPS NiCrAlY exhibited progressive increases in localized fracture, buckling, and delamination during thermal cycling. The concentration of arrayed lenticular voids in the columnar Al₂O₃ grain boundaries significantly increased during cyclic oxidation (as compared to isothermal oxidation), but only in scales which formed on convex surfaces, suggesting internal void growth was stress-related. The amount and frequency of scale damage was higher on convex surfaces with a relatively large radius of curvature as compared to convex surfaces with a very small radius of curvature. Although the thermo-mechanical fracture resistance of Al₂O₃ scales on APS NiCrAlY was superior to scales on VPS NiCrAlY, TBC lifetimes on VPS NiCrAlY were greater by a factor of 2. Apparently, severe interfacial scale damage did not rapidly degrade the adherence of the ceramic top coatings.     

Effect of powder structure on microstructure and electrical properties of plasma-sprayed 4.5 mol% YSZ coating

Electrolyte coatings by atmospheric plasma spraying were prepared by 4.5 mol% ytrria-stabilized zirconia (YSZ) powders manufactured by agglomerate-sintered (A-S) and fusing-crashed (F-C) processes. Microstructure of the powders and the coatings were characterized using scanning electron microscopy. The electrical conductivity of the coatings was investigated using both impedance spectroscopy and DC methods. The results showed that the electrical conductivity of coating prepared with A-S powder was lower than that with F-C powder. It was found from the impedance analysis that both the grain and grain boundary resistances were large in the coating formed by A-S powder. This fact resulted from deposition of partially melting of spray particles. A model was proposed to explain the effect of powder structure and melting state on the coating microstructure and properties.     

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。     

CFRP复杂几何结构区相控阵超声检测建模与声传播规律

为厘清碳纤维增强树脂基复合材料(Carbon fiber reinforced plastics,CFRP)复杂几何构件相控阵超声检测中声传播规律,围绕CFRP材料R区开展了弹性特性表征、有限元建模、声场计算及实验验证工作。基于超声液浸背反射法和模拟退火算法求解了CFRP单向板刚度矩阵反问题,并借助Bond变换实现了R区弹性特性的定量描述。结合微观组织分析等获取材料、几何特征,建立了虑及曲面形状、叠层、弹性各向异性的R区相控阵超声检测有限元模型,计算了R区相控阵超声检测A、B扫描信号,发现存在结构噪声和缺陷伪像。在此基础上研究CFRP材料R区瞬态声场并与CFRP平板、弹性各向同性R区和0°单向板R区情况对比,阐明了结构噪声和缺陷伪像的形成机制:弹性各向异性叠层结构导致倾斜入射的超声波发生反射和折射,与沿肋板传播的快速波混叠在R区形成结构噪声,同时多向板R区两侧肋板反射导致缺陷伪像,即材料弹性各向异性与构件曲面叠层结构耦合共同影响缺陷的精准辨识。      

Effect of heat treatment on phase stability, microstructure, and thermal conductivity of plasma-sprayed YSZ

The effects of 50-hour heat treatments at 1000°C, 1200°C, and 1400°C on air plasma-sprayed coatings of 7 wt% Y₂O₃-ZrO₂ (YSZ) have been investigated. Changes in the phase stability and microstructure were investigated using x-ray diffraction and transmission electron microscopy, respectively. Changes in the thermal conductivity of the coating that occurred during heat treatment were interpreted with respect to microstructural evolution. A metastable tetragonal zirconia phase, with a non-equilibrium amount of Y₂O₃ stabilizer, was the predominant phase in the as-sprayed coating. Upon heating to 1000°C for 50 hours, the concentration of the Y₂O₃ in the t-zirconia began to decrease as predicted by the Y₂O₃-ZrO₂ phase diagram. The c-ZrO₂ phase was first observed after the 50-hour heat treatment at 1200°C; monoclinic zirconia was observed after the 50-hour heat treatment at 1400°C. TEM analysis revealed closure of intralamellar microcracks after the 50-hour/1000°C heat treatment; however, the lamellar morphology was retained. After the 50-hour/1200°C heat treatment, a distinct change was observed in the interlamellar pores; equiaxed grains replaced the long, columnar grains, with some remnant lamellae still observed. No lamellae were observed after the 50-hour/1400°C heat treatment. Rather, the microstructure was equivalent when viewed in either plan view or cross-section, revealing large grains with regions of monoclinic zirconia. Thermal conductivity increased after every heat treatment. It is believed that changes in the intralamellar microcracks and/or interlamellar pores are responsible for the increase in thermal conductivity after the 1000°C and 1200°C heat treatments. The increase in thermal conductivity that occurs after the 50-hour/1400°C heat treatment is proposed to be due to the formation of m-ZrO₂, which has a higher thermal conductivity than tetragonal or cubic zirconia.