Thermal Expansion Mismatch Stresses Research Articles

Structure and tribological behavior of a polyetherimide/polytetrafluoroethylene matrix filled with negative thermal expansion zirconium tungstate particles

Thermal expansion mismatch stresses may be a reason for premature failure of sealing and bearing components made of polymer composites that are rubbed against metallic surfaces at elevated temperatures. It is of particular importance for polymer matrix composites loaded with anti-friction inclusions of polytetrafluoroethylene (PTFE) that possesses high coefficient of thermal expansion. In this regard, a study was undertaken to elucidate the effect of the thermal mismatch on wear and friction of amorphous polyetherimide (PEI) matrix loaded with either polytetrafluoroethylene (PTFE) particles (i) or PTFE particles covered with negative thermal expansion zirconium tungstate (ZT) α-ZrW2O8 (ii) at temperatures 23 °C, 120 °C and 180 °C. The ball-on-disk testing scheme was used according to standard with AISI 52100 steel balls rubbed against a polymer composite disks at normal force P = 5 N and sliding velocity V = 0.3 m/s. The PEI/PTFE/ZT composite demonstrated a tendency for wear rate (WR) reduction in sliding at 120 °C and 180 °C as compared to corresponding WR enhancement on the PEI/PTFE. The rationale is provided that the ZT additive effectively reduced thermal expansion of the PTFE in the PEI matrix and thus improved wear resistance of the composite. New thermal expansion-controlled wear and friction instability mechanism has been proposed and discussed.

Prediction of internal stress in alumina laminates induced by shrinkage mismatch based on elastic model modification

AbstractCracks induced by internal stresses compromise the structural integrity of ceramic laminates. Such stresses can be classified into thermal stress derived from thermal expansion mismatch and shrinkage stress derived from shrinkage mismatch. Analytical models have been proposed for the former, whereas only numerical predictions have been explored for the latter. In this study, an equation is constructed to predict the shrinkage stress by modifying an elastic solution proposed in previous studies. We fabricated laminates with three‐layer sandwiched structures using only alumina specimens with high and low densities to control the shrinkage and avoid thermal mismatch. Surface cracks were initiated when the shrinkage stress exceeded the flexural strength of the surface layer. Although the original strength of the surface layer was 308.8 MPa, the laminates’ flexural strength was lower than that because of shrinkage constraint. Our results demonstrate that the shrinkage stress was successfully estimated analytically using a few material properties measured in an ordinary environment. This study will ensure the structural integrity of ceramic laminates in laminate designs.

Perpendicular magnetic anisotropy at room-temperature in sputtered a-Si/Ni/a-Si layered structure with thick Ni (nickel) layers

In the literature, a magnetic “easy” axis perpendicular to the film plane at room temperature (i.e., perpendicular magnetic anisotropy - PMA) has been reported in Ni (nickel) layers with thicknesses below ≈15 nm. In this work, we observed room-temperature PMA in a-Si/Ni/a-Si (where a-Si denotes amorphous silicon) thin film structures with nickel layer thicker than 15 nm. Two layered structures were prepared by DC/RF triode sputtering: [a-Si/Ni/a-Si] sandwich structure and [a-Si/Ni/a-Si]5 multilayer structure. The cross sectional STEM revealed uniform Ni layers with thicknesses of ≈17 nm in [a-Si/Ni/a-Si]5 – multilayer and ≈28 nm in [a-Si/Ni/a-Si] – single-layer whereas amorphous Si layers were ≈15 nm and 170 nm thick, respectively. An amorphous Ni–Si interphase was also observed in the layered structures. The XRD showed patterns for fcc-Ni with dominant (111) orientation. No other crystalline phases were observed in the XRD patterns. To our knowledge, there are no literature reports of easy magnetization direction perpendicular to the film plane at room temperature for Ni layers with thickness of ≈28 nm as presented in this work. The origin of PMA in a-Si/Ni/a-Si films may be mainly attributed to the magnetoelastic anisotropy whereas the secondary source of PMA is believed to be the surface anisotropy and magnetocrystalline anisotropy of [111] columnar grains. Amorphous silicon layers (substrate) do not have a well-defined lattice structure like crystalline substrates. Therefore, they do not induce strains in the nickel layers through lattice mismatch as in the case of epitaxy. The strains can be caused by other factors such as diffusion-induced strain, thermal expansion mismatch or intrinsic stresses during the growth process. These results could be important for applications in memory devices, sensors, logic chips, magneto-optic, magneto-electronic and spintronic devices and in fundamental research, as well as first step toward preparation and understanding of the PMA in thick nickel layers.

Compatibility of low thermal conductivity and high infrared emissivity of plasma-sprayed Sm2Hf2O7 and Pr2Hf2O7 coatings

In order to develop thermal-protection coatings, rare earth hafnates Sm2Hf2O7 (SHO) and Pr2Hf2O7 (PHO) coatings with pyrochlore structure were prepared on different substrates by atmospheric plasma spraying (APS). The infrared radiation, flame-ablation resistance, and thermal cycle performance were investigated in relationship with their morphologies, phase stabilities, thermophysical and mechanical properties. The results showed that both SHO and PHO coatings had compatible thermal-protection performance with lower thermal conductivity (0.711 and 0.779 W·m−1·K−1 at 1000 °C) and higher infrared emissivity (0.814 and 0.894 at 1000 °C) in the 3–5 μm band. Moreover, the supersonic flame ablation resistance of PHO coatings at high temperatures was greater than that of SHO coatings. Both SHO and PHO coatings have long thermal cycling life at low and medium temperatures and short thermal cycling life at high temperatures. The failure mechanisms of both type coatings were mainly associated with thermal expansion mismatch stress and fatigue stress generated during multiple thermal cycling.

Simulation of 1500 °C Thermal Shock for Novel Structural Thermal/Environmental Barrier Coatings System

In recent years, with the development of SiC composites in aero-engine hot-end components, environmental barrier coatings (EBCs) have received extensive attention. Moreover, in order to elevate the service temperature, it is a developing trend to apply thermal barrier coatings (TBCs) with low thermal conductivity on EBCs coating system to form thermal/environmental barrier coatings (T/EBCs). However, the combination of high coefficient of thermal expansion (CTE) of TBCs with low CTE of EBCs often leads to premature failure due to excessive thermal expansion mismatch stress. However, a novel structural thermal barrier coating with embedded micro-agglomerated particles (EMAP TBC) by using atmospheric plasma spraying (APS) process has brought hope to solve this problem due to its low elastic modulus. Therefore, in this study, an innovative EMAP Gd2Zr2O7 T/EBCs coating system (EMAP Gd2Zr2O7/Yb2Si2O7/Si) under 1500 °C flame thermal shock was simulated and systematically studied on the SiC substrate. The results showed that the EMAP Gd2Zr2O7 T/EBCs coating system has much lower thermal stress than the conventional Gd2Zr2O7/Yb2Si2O7/Si T/EBCs coating system. Furthermore, when the thickness of each layer of the EMAP Gd2Zr2O7 T/EBCs coating system varies, to meet the thermal insulation requirements of Yb2Si2O7 layer and reduce the thermal shock stress, the thickness of the EMAP Gd2Zr2O7 layer is recommend being about 100 μm. Meanwhile, the thicknesses of Yb2Si2O7 and Si layers can be set as large as needed. In addition, with the increase in Yb2SiO5 doping content in the Yb2Si2O7 intermediate layer, the EMAP Gd2Zr2O7 T/EBCs coating system suffers a greater risk of spalling failure due to the increase in thermal stress.

Control of deviatoric stress in the diamond anvil cell through thermal expansion mismatch stress in thin films

Control of deviatoric stress in the diamond anvil cell through thermal expansion mismatch stress in thin films

Bonding and Thermal-Mechanical Property of Gradient NiCoCrAlY/YSZ Thermal Barrier Coatings with Millimeter Level Thickness

The thermal insulation properties of thermal barrier coatings (TBCs) can be significantly improved with increasing the coating thickness. However, due to the weak bonding of high-thickness TBCs, the low reliability and short lifetime greatly limits their application under some severe operating conditions. In this study, a novel and high-efficiency synchronous dual powder feeding method is used to deposit a series of gradient NiCoCrAlY/YSZ coatings with millimeter level thickness. The tensile bonding strengths and residual stress state of coatings are evaluated in order to explore the effect of thickness on the bonding strength of coatings. The results suggested that, due to some micro-convex structure at the “GC/TC” interface and inside “GC” layer, the bonding strength of 1000-μm-thickness gradient NiCoCrAlY/YSZ TBCs with the 4:6 and 2:8 hybrid ratios is over 44 MPa compared to the common TBCs. The fracture position gradually shifts from NiCoCrAlY bond coat to NiCoCrAlY/YSZ transition zone and finally to the YSZ top coat owing to the different position of residual stress concentrations. After thermal cycling tests, the 1000-μm-thickness gradient coating exhibits a higher thermal cycling life. Some coarse cracks initiate and propagate at the bottom region of TBCs, which is mainly due to thermal expansion mismatch stress that finally results in the failure of the gradient coating between the “BC” layer and the substrate.

Thermally adaptive axisymmetric trusses for satellite platforms

Lightweight trusses with anisotropic effective thermal expansion connecting the main satellite body and a remote platform are designed to passively accommodate large temperature variation at orbiting around the Earth. Every truss consists of identical units with members that can be made of different materials. Unit members are pin-connected to the satellite, the platform, and between each other. Effective thermal expansion of the truss coincides with the thermal expansion of the platform material on the upper level of the truss, the thermal expansion of the satellite material on the lower level of the truss, and equals to zero along the truss height. Because of this, the trusses provide a constant, independent of temperature, distance between the satellite and the platform preventing misalignment of optical systems located on the platform and serve as adapters eliminating thermal expansion mismatch stresses arising at the points of member junction to the satellite and the platform. Six different unit configurations are designed, and various material combinations for unit members are considered. The geometry of the truss unit depends on the coefficient of thermal expansion of the satellite, the platform and the truss member materials. Stiffness and structural efficiency at uniaxial loading are calculated for every configuration and compared with each other. Material combinations providing maximum stiffness or structural efficiency depend on the platform size. The trusses do not need thermal deformation control or isolation. Examples of unit design are presented; it is noticeable that the adaptive trusses can be made of a single material.

Plasma sprayed alumina-yttria composite ceramic coating for electrical insulation applications

A composite ceramic coating consisting of 50:50 weight fraction of alumina (Al2O3)-yttria (Y2O3) blend was deposited as an electrically insulating topcoat on Oxygen-Free Electronic Copper (OFEC) substrates with a cermet interlayer of 30 wt% NiAl-70 wt% Al2O3 blend, both deposited by Atmospheric Plasma Spray (APS) process, and with a NiCrAlY bond coat deposited by High-Velocity Oxy-Fuel (HVOF) process for application in Direct Current (DC) conduction pump of the sodium-cooled fast reactor. The cermet interlayer in between topcoat and bond coat is beneficial for a reduction in the thermal expansion mismatch stresses and further contribute to the improvement in the thermal fatigue life of the multilayer coating system. The coated samples were subjected to a thermal fatigue cycle between 600 °C and 200 °C, simulating sodium-cooled fast reactor temperature patterns in regular operation and transient conditions. The multilayer composite ceramic coating with cermet interlayer successfully endured up to 1000 thermal cycles with no cracking, spallation, or delamination of the topcoat. Surface cross-sectional micrographs before and after the thermal cycle revealed neither significant changes in the splat and lamellae morphology nor distribution of phases, except for the densification effects. X-ray Diffraction (XRD) analysis revealed neither phase change nor the formation of any new phase upon thermal cycling. The adhesion test as per ASTM-C633 revealed that the bonding strength of the coatings lower by ~20% after thermal cycling. The insulation resistance of thermal cycled samples showed a two order reduction in the magnitude, compared to that of the as-coated samples.

The hot corrosion behavior in vanadate environment and thermal cycling behaviors of YTZ and 2GYTZ for thermal barrier coating applications

A preliminary examination was carried out for hot corrosion resistance to V2O5 and thermal cycling behaviors of two alternative materials: zirconia co-doped with 16 mol% YO1.5 + 16 mol% TaO2.5 (YTZ) and zirconia co-doped with 14 mol% YO1.5+ 2 mol% GdO1.5 + 16 mol% TaO2.5 (2GYTZ). The compositions and microstructures of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). Results showed ZrV2O7 and YVO4/GdVO4 were the main reaction products at 750 °C and 850 °C for 2 h, while YVO4 and m-ZrO2 were detected as the main corrosion products at 850 °C for 10 h or rising temperature to 950 °C. With the depletion of V2O5, ZrV2O7 disappeared and decomposed into m-ZrO2 and V2O5. The hot corrosion mechanisms are similar for YTZ and 2GYTZ ceramics. However, the amount of m-ZrO2 in the 2GYTZ was much more than that in the YTZ at the same corrosion conditions. The weaker bonds due to partial substitution of Y3+ with larger size of Gd3+ and the size misfit between Gd3+ and Zr4+ resulted in decreasing the anti-corrosion capability of 2GYTZ ceramic. The tetragonal phase was very stable for YTZ and 2GYTZ coatings during high temperature heat treatment. The thermal durability of the YTZ and 2GYTZ TBCs can be significantly improved by using the YSZ buffer layer. While the cycling lifetimes for 2GYTZ coating was much shorter than that of YTZ coating whether designed for single-ceramic-layer structure or double-ceramic-layer structure. Failure of the 2GYTZ coating mainly resulted from the thermal expansion mismatch stress, concentrated thermal stress introduced by the severe sintering and low fracture toughness.

Influence of Gd2O3 substitution on thermal and mechanical properties of ZrO2-Ta2O5-Y2O3

The effects of the partial Y2O3 substituted by Gd2O3 on thermal and mechanical properties of ZrO2-Ta2O5-Y2O3 ceramics were studied. Results indicated the single stable tetragonal phase of zirconia co-doped with 16 mol% YO1.5 +16 mol% TaO2.5 (YTZ) and zirconia co-doped with (16-x) mol% YO1.5+ x mol% GdO1.5+16 mol% TaO2.5 (xGYTZ, x = 2, 4, 8) just existed at and above 1500 °C. The Gd2O3 substitution caused a decrease in phase stability when annealed at 1300 °C and 1400 °C. YTZ and 2GYTZ ceramics exhibited high fracture toughness, large thermal expansion coefficients (TECs), and low thermal conductivity. With the Gd2O3 substitution, the mechanical properties and thermal conductivity showed a decreasing trend, while TECs increased due to the decrease of crystal energy. The thermal durability of YTZ and 2GYTZ TBCs was evaluated. The cycling life of 2GYTZ TBCs was much shorter. The combination of sintering effect and thermal expansion mismatch stresses was significant factor for the failure.

Spray power-governed microstructure and composition, and their effects on properties of lanthanum-cerium-tantalum-oxide thermal barrier coating

Lanthanum-cerium-tantalum-oxide (LCT) thermal barrier coatings (TBCs) were prepared by air plasma spraying at three different spray powers. Then, the effects of spray power on the microstructure, composition, thermophysical properties, mechanical performances, and thermal cycling behaviors of the coatings were investigated in detail. The results indicated that when the spray power was increased from 30 kW to 42 kW in 6 kW steps, the coating became denser, the La/(Ce + Ta) ratio of the as-sprayed coating first increased slowly and then sharply, and the thermal conductivity and thermal expansion coefficient of the coating gradually increased and decreased, respectively. In addition, the Vickers hardness, Young's modulus, and fracture toughness of the coating first increased and then decreased. The spray power had a significant impact on the thermal cycling performance of LCT coating. The LCT coating prepared at low power showed better thermal shock resistance. The thermal expansion mismatch stress and the low fracture toughness of a ceramic top coat are the main reasons for the premature failure of the TBCs.

Robust lightweight multifunctional thermally tailored lattices

This paper presents conceptual designs for lightweight lattices that are able to connect two materials with differing coefficients of thermal expansion without generating thermal expansion mismatch stresses during temperature excursions. The lattices operate passively at ambient conditions to accommodate large variations of temperature and to provide constant separation, independent of temperature, between the substrate materials. When pin-connected, the lattices are free from thermal mismatch stresses both internally and at their connections with the substrates. However, previous configurations of these lattices are highly sensitive to small perturbations in geometry, as would be associated with thermal expansion or manufacturing imperfections, and hence must be designed giving consideration to such phenomena. Hence, sensitivity analysis must be a factor in the design process. To show that such sensitivity analyses can be carried out using theoretical approaches, experimental results are presented that are in accord with the theoretical calculations. Several examples are given to demonstrate strategies for reducing lattice sensitivity to geometric imperfections for different combinations of material and geometry. More importantly, novel alternative designs for lattices using simpler configurations, which are less sensitive to small perturbations, are explained. These alternative lattices employ less complicated geometry and, in some cases, require only one material, but offer less adaptibility in the situations to which they are applicable.

Microstructural and phase characterisation of pyrolytic graphite coating by CVD using propane and methane as precursor

ABSTRACTPyrolytic graphite (PyG) coatings on high-density graphite (HDG) substrates were deposited by chemical vapour deposition using propane and methane as precursor gas at aprocess temperature of 1800°C. Upon cooling to room temperature, cracking and delamination were observed in the PyG layer at certain edges of flat substrates deposited in propane cycle, attributed to large thermal expansion mismatch stresses. The PyG coatings deposited using different precursor hydrocarbon sources were characterised using polarised light microscopy, scanning electron microscope, X-Ray diffraction analysis, Laser Raman Spectroscopy and X-ray Photoelectron Spectroscopy. Microstructure analysis revealed coarse structure cones grown with larger cone angles. The large deposition rate, associated growth and thermal stress in the case of propane cycle have led to the cohesive delamination of the PyG coating. Nickel melting studies to simulate the uranium melting was carried out in PyG coated HDG crucibles whichshowed reactive dissolution of PyG at the Ni/crucible interface.

The role of surface roughness on dislocation bending and stress evolution in low mobility AlGaN films during growth

The bending and interaction of threading dislocations are essential to reduce their density for applications involving III-nitrides. Bending of dislocation lines also relaxes the compressive growth stress that is essential to prevent cracking on cooling down due to tensile thermal expansion mismatch stress while growing on Si substrates. It is shown in this work that surface roughness plays a key role in dislocation bending. Dislocations only bend and relax compressive stresses when the lines intersect a smooth surface. These films then crack. In rough films, dislocation lines which terminate at the bottom of the valleys remain straight. Compressive stresses are not relaxed and the films are relatively crack-free. The reasons for this difference are discussed in this work along with the implications on simultaneously meeting the requirements of films being smooth, crack free and having low defect density for device applications.

Zirconia-ceramic versus metal-ceramic: Thermal expansion mismatch and residual stresses

Zirconia-ceramic versus metal-ceramic: Thermal expansion mismatch and residual stresses

Performance and Durability of Thin Film Thermocouple Array on a Porous Electrode.

Management of solid oxide fuel cell (SOFC) thermal gradients is vital to limit thermal expansion mismatch and thermal stress. However, owing to harsh operation conditions of SOFCs and limited available space in stack configuration, the number of techniques available to obtain temperature distribution from the cell surface is limited. The authors previously developed and studied a thermocouple array pattern to detect surface temperature distribution on an SOFC in open circuit conditions. In this study, the performance in terms of mechanical durability and oxidation state of the thin film thermoelements of the thermocouple array on the porous SOFC cathode is investigated. A thin-film multi-junction thermocouple array was sputter deposited using a magnetron sputter coater. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) characterisation techniques were carried out to understand characteristics of the thin film before and after temperature (20 °C–800 °C) measurement. Temperature readings from the sensor agreed well with the closely placed commercial thermocouple during heating segments. However, a sensor failure occurred at around 350 °C during the cooling segment. The SEM and XPS tests revealed cracks on the thin film thermoelements and oxidation to the film thickness direction.

Microstructural analysis and thermal shock behavior of plasma sprayed ceria-stabilized zirconia thermal barrier coatings with micro and nano Al2O3 as a third layer

In this study, thermal shock behavior of three types of atmospheric plasma sprayed ceria-stabilized zirconia (CSZ) thermal barrier coatings was evaluated: usual CSZ, layer composite of CSZ/Micro Al2O3 and layer composite of CSZ/Nano Al2O3 in which Al2O3 was used as a top coat on CSZ layer. The thermal shock test was administrated by quenching the samples in the cold water with the temperature of 20–30°C from 1100°C. Microstructure evaluation, elemental and phase analysis of coatings before and after tests were performed using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometry, respectively. The experimental results revealed that the CSZ/Nano Al2O3 TBC has a superior thermal cycling behavior, in comparison to CSZ/Micro Al2O3 and usual CSZ TBCs. Also, failures of both usual CSZ and layer composite of CSZ/Micro Al2O3 TBCs mainly were associated with thermally grown oxide formation and growth stresses at NiCrAlY/CSZ interface, whereas, in CSZ/Nano Al2O3 TBC, the failure mechanism was coefficient of thermal expansion mismatch stress in NiCrAlY/CSZ interface.

Phase structure evolution and thermal expansion variation of Sc2O3 doped Nd2Zr2O7 ceramics

Abstract (Nd1−xScx)2Zr2O7 (x = 0, 0.1, 0.3, 0.5, 0.7) compounds were synthesized by solid state reaction at 1700 °C for 10 h, and characterized by XRD, Raman spectroscopy, SEM and high-temperature dilatometer. Nd2Zr2O7 exhibited pyrochlore phase, and its lattice parameter increased after Sc2O3 doping, which could be attributed to the presence of Sc3+ interstitial ions in pyrochlore lattice. Fluorite phase formed in the doped Nd2Zr2O7, and (Nd0.3Sc0.7)2Zr2O7 exhibited pure fluorite phase. The thermal expansion coefficient (TEC) of Nd2Zr2O7 was significantly enhanced by 10 mol% Sc2O3 doping, but higher Sc2O3 doping decreased the TEC. The reduced crystal energy due to the presence of Sc3+ interstitial ions could cause the initial increase in the TEC, and the formation of fluorite phase might contribute to the reduced TEC. Considering the alleviation of the thermal expansion mismatch stress for the high-temperature applications of Nd2Zr2O7, Sc2O3 was an excellent dopant and there existed an optimal Sc2O3 content for the optimization design of compound compositions.

Adaptive bimaterial lattices to mitigate thermal expansion mismatch stresses in satellite structures

Earth-orbiting satellites regularly pass from sunlight to shade and back; these transitions are typically accompanied by significant temperature changes. When adjoining parts of a satellite that are made of different materials are subjected to large temperature changes, thermal mismatch stresses arise that are a function of the temperature change and the difference in coefficients of thermal expansion (CTEs) between the two materials. These thermal stresses are linked to undesirable deformation and, through long-term cycling, fatigue and failure of the structure. This paper describes a type of anisotropic lattice that can serve as a stress-free adaptor between two materials, eliminating thermal mismatch stresses and their concomitant consequences. The lattices consist of planar nonidentical anisotropic bimaterial cells, each designed based on a virtual triangle. Physically the cells consist of a triangle made of material with higher CTE surrounded by a hexagon made of material with lower CTE. Different skew angles of the hexagon make a particular cell and the whole lattice anisotropic. The cells can be designed and combined in a lattice in such a way that one edge of the lattice has CTE that coincides with the CTE of the first part of the structure (substrate 1), while the other edge of the lattice has CTE equal to the CTE of the second part of the structure (substrate 2). If all joints between the parts of each cell, neighbouring cells, and the lattice and the substrates are pinned, the whole structure will be free of thermal stresses. This paper will discuss the fundamental principles governing such lattices, their refinement for special circumstances, and opportunities for improving the structural performance of the lattices. This will be presented coupled to a rational strategy for lattice design.