Effect of functionally graded structure design on durability and thermal insulation capacity of plasma-sprayed thick thermal barrier coating

This paper aimed to design and optimize the structure of a thick thermal barrier coating by adding graded layers to achieve a balance between high thermal insulation capacity and durability. To this end, conventional TBC, conventional TTBC, and functionally graded TTBCs were deposited on the superalloy substrate by air plasma spraying. To determine the quality of the bond strength of the coatings, the bonding strength was measured. The durability of coatings was evaluated by isothermal oxidation and thermal shock tests. Then, at a temperature of 1000 °C, the thermal insulation capacity of the coatings was carried out. The microstructure of the coatings was characterized by a scanning electron microscope. The results showed that the thickness of the TGO layer formed on the bond coat in the conventional TBC and TTBC under the oxidation test at 1000 °C after 150 h was 2.79 and 2.11 μm, respectively, whereas, in the functionally graded TTBC samples, no continuous TGO layer was observed as a result of internal oxidation. The functionally graded TTBC presented higher durability than conventional TTBC due to improved bonding strength, thermal shock resistance, and the lack of a TGO layer at the bond/top coat interface. Also, the thermal insulation capacity of the functionally graded TTBC (with 1000 μm thickness of YSZ coating) was better than TTBC.     

Stress wave propagation in adhesively bonded functionally graded cylinders: an improved model

This study presents an improved mathematical model to analyse the stress wave propagation in adhesively bonded functionally graded (FG) circular cylinders (butt joint) under an axial impulsive load. The volume fractions of the material constituents in the upper and lower cylinders were functionally tailored through the thickness of each cylinder using a power-law. The effective material properties of both cylinders, which are made of aluminum (Al) and silicon carbide (SiC), at any point were predicted by using the Mori–Tanaka homogenization scheme. In this improved model, the governing equations of the wave propagation include the spatial derivatives of local mechanical properties and were discretized by means of the finite difference method. The influence of these spatial derivatives and the compositional gradient exponent on the displacement and stress distributions of the joint was investigated. The material composition variations of both cylinders affected the displacement and stress fields whereas the compositional gradient exponent had a minor effect. The stress concentrations were alleviated in time, the displacement and stress distributions/variations around/along the upper and lower cylinder-adhesive interfaces were significantly affected by the adhesive layer. The spatial derivatives also affected the temporal histories of the displacement and stress components evaluated at the selected critical points of the upper cylinder, adhesive layer and lower cylinder. The consideration of the spatial local material derivatives provided a more accurate mathematical model of wave propagations through the graded layered structures.     

Direct additive manufacturing of melt growth Al2O3-ZrO2 functionally graded ceramics by laser directed energy deposition

Functionally graded ceramics (FGC), which combine properties of different ceramics in one part, usually have better comprehensive function and structural efficiency. In this study, four different gradient transition Al₂O₃-ZrO₂ FGC samples were prepared by laser directed energy deposition (LDED) method. The results show that there is an obvious interface in direct transition sample. The transition section bears tensile stress caused by difference of thermophysical properties of materials, resulting in significant longitudinal cracks. Element transition in interface region shows a step sharp transition. The direct transition sample shows intergranular fracture and the bonding strength is very low. Gradient transition mode can effectively suppress cracks, and avoid the step transition of microstructure and elements. Elements, microhardness of 25, 20 wt% FGC samples realized a nearly linear smooth transition. The interface fracture of FGC samples changed to transgranular fracture, bonding strength was significantly improved, and the maximum flexural strength reached 160.19 MPa.     

Torsional statics of two-dimensionally functionally graded microtubes

A size-dependent governing equation is derived to investigate the torsional static behaviors of two-dimensionally functionally graded microtubes based on the modified couple stress theory. The shear modulus is assumed to vary along the tube’s length direction according to an exponential distribute function, and varies along the tube’s radius direction according to a power-law function. A generalized differential quadrature method is developed to determine the rotational angle and shear stresses. Some illustrative examples are given to investigate the effects of applied torques, the length scale parameter and various material compositions on the torsional angle and shear stresses.     

分级加载电压下考虑有效电势变化的软黏土电渗固结理论

分级加载电压技术能够有效延长电渗处理的时间,改善地基的处理效果并能有效降低能耗。在利用分级加载电压技术开展电渗试验时,随着土体的排水固结,土体各点的电势会发生明显的变化,这与现有的电渗固结理论假设土体各点电势保持不变有所差异。利用自主设计的电渗试验仪器开展了分级加载电压下的电渗室内试验,分析了电渗过程中土体有效电势变化的规律。基于该规律并结合Esrig固结理论建立了在阳极处不排水、阴极处排水条件下考虑土体有效电势变化的电渗固结理论,得到了分级加载电压条件下考虑土体有效电势变化的超静孔隙水压力和平均固结度的解析表达式。研究表明,在电压分级加载过程中,土体有效电势变化规律表现为随时间先基本保持不变,后呈抛物线状先增后减。通过试验案例验证了考虑土体有效电势变化所求得解析解的合理性,且该数值计算结果相较于Esrig数值计算结果,前者更加接近实测值,该固结方程的建立为后续分级加载电压技术的推广应用提供了理论依据,也为实际工程提供了参考。     

On processing of Ni-WC based functionally graded composite clads through microwave heating

Present study focuses on the development of four layered functionally graded clads (FGC) of Ni-WC based composite material on AISI 304 substrate through microwave heating route. Experimental trials were conducted inside a microwave applicator of domestic type at frequency range of 2.45?GHz. The optimal exposure time of 900?W microwave power was varied with compositional gradient and it was from 300 to 360?s. The mechanism of FGC formation through microwave heating was explained and developed FGC was subjected to mechanical and microstructure characterizations. The results of micro-structural analysis revealed that the FGC of ～1.8?mm thickness was produced and was free from any type of interfacial cracks and visible porosity. It was observed that WC particles were randomly dispersed in the nickel matrix. XRD study revealed the formation of inter-metallics, such as NiW₄, NiSi, and Cr₂₃C₆. Maximum value of microhardness was observed in the top FGC layer and was 880?±?30?HV.     

采用泡沫铜电极的热再生氨电池性能数值模拟

以采用泡沫铜电极的热再生氨电池（thermally regenerative ammonia-based battery，TRAB）为研究对象，建立了多孔介质内物质传输与电化学反应耦合的稳态模型，计算获得了电池性能及多孔电极内物质传输特性，并研究了电解质浓度和电极孔隙率对电池性能的影响。研究结果表明，从主流区界面到多孔电极内部，阳极氨和阴极铜离子浓度逐渐降低，存在一定的浓度梯度，而且随着反应电流的增大，浓度梯度明显增大。在一定的范围内分别增大阳极氨浓度和阴极铜离子浓度，从主流区向多孔电极内物质传输增强，电池性能均能不断提升；随着硫酸铵浓度的增大，电解质电导率增大，电池性能逐渐提升，但增幅逐渐减小。此外，多孔电极孔隙率也会影响电池性能，本研究中TRAB在电极孔隙率为0.6时获得最高的最大功率(15.3 mW)。     

A comprehensive study on the size-dependent hygrothermal analysis of exponentially graded microplates on elastic foundations

Abstract

In this paper, the effects of hygrothermal conditions on various behaviors, such as bending, free vibration, mechanical and thermal buckling, of exponentially graded microplates lying on two-parameter elastic foundations are investigated. The trigonometric four-variable plate theory incorporated to the modified couple stress theory (MCST) is employed to derive the equations of motion. The present MCST contains an internal material length scale parameter, thus it can capture the size effect. The microplate is assumed to be subjected to a temperature rise and moisture concentration which are varied linearly through the thickness of the plate. Based on an exponential law, the material properties of the microplate are graded only in z direction. The equations of motion are solved analytically to obtain the displacements, stresses, eigenfrequencies and critical buckling load and temperature of the microplates. The present results are validated by comparing them with those previously published. The numerical examples reveal that considering the size effect and/or the elastic foundations leads to an increment in plate stiffness and thereby leads to a decrement in the deflection and an increment in eigenfrequency and buckling loads. It is also shown that the size effect is negligible for the thicker plate.     

Present status and future prospects of plasma sprayed multilayered thermal barrier coating systems

Thermal barrier coatings (TBCs) play a pivotal role in protecting the hot structures of modern turbine engines in aerospace as well as utility applications. To meet the increasing efficiency of gas turbine technology, worldwide research is focused on designing new architecture of TBCs. These TBCs are mainly fabricated by atmospheric plasma spraying (APS) as it is more economical over the electron beam physical vapor deposition (EB-PVD) technology. Notably, bi-layered, multi-layered and functionally graded TBC structures are recognized as favorable designs to obtain adequate coating performance and durability. In this regard, an attempt has been made in this article to highlight the structure, characteristics, limitations and future prospects of bi-layered, multi-layered and functionally graded TBC systems fabricated using plasma spraying and its allied techniques like suspension plasma spray (SPS), solution precursor plasma spray (SPPS) and plasma spray –physical vapor deposition (PS-PVD).     

Interfacial failure analysis of functionally graded adhesively bonded double supported tee joint of laminated FRP composite plates

This paper deals with three-dimensional non-linear finite element analyses to assess the structural behavior of adhesively-bonded double supported tee joint of laminated FRP composites having embedded interfacial failures. The onset of interfacial failures is predicted by using Tsai–Wu coupled stress failure criterion with pre-determined stress values. The concept of fracture mechanics principle is utilized to study the sustainability of the tee joint having interfacial failures pre-existed at the critical locations. Individual modes of the strain energy release rates (SERR) G_I, G_II and G_III, are considered as the damage growth parameters and, are evaluated using the Modified crack closure integral (MCCI) technique based on the concept of linear elastic fracture mechanics (LEFM). Based on the stress analyses, it has been observed that the interfacial failures in tee joint structure trigger at the interface of base plate and adhesive layer from both ends of base plate. Depending on the SERR magnitudes, it has been noticed that the interfacial failure propagates under mixed mode condition. Therefore total SERR (G_T) is considered as the governing parameter for damage propagation. Furthermore, efforts have been made to retard damage propagation rate by employing functionally graded adhesive (FGA) instead of monolithic adhesive material. Series of numerical simulations have been performed for varied interfacial failure length in functionally graded adhesively bonded double supported tee joint structure in order to achieve the significant effect of FGA with various modulus ratios on SERR. Material gradation of adhesive indicates significant SERR reduction at the incipient stage of failure which necessitates the use of functionally graded adhesive for the tee joint and prolong the service life of the structure.