Investigation of the coefficient of thermal expansion in nanocrystalline diamond films

Nanocrystalline diamond films are a promising class of nanomaterials with tunable properties. An especially appealing field of application for NCD are nitrogen doped semiconducting films.The residual stress in the films is an important film property directly influencing the adhesion of the film on the substrate and thus the film performance. The residual stress consists of two components: a thermal part due to the different coefficients of thermal expansion of film and substrate and an intrinsic part. The residual stress in most films that are deposited at high temperatures is dominated by an effect arising from the difference in coefficients of thermal expansion in film and substrate. By measuring the residual stress in the film at different temperatures it is possible to calculate the coefficient of thermal expansion of the films. For this purpose an ex-situ optical device was used to measure the residual stress of the film.Nanocrystalline diamond films were deposited by microwave-plasma CVD at a pressure of 200 mbar from an Ar/H₂/CH₄ plasma while the hydrogen fraction in the process gas and the substrate temperature were varied between 3 to 6% to 600 to 800 °C respectively.To investigate the influence of the nitrogen admixture in the plasma on the thermal expansion coefficient more films were deposited at a pressure of 200 mbar with admixtures of nitrogen of 2.5% and 7.5%.It is shown that by controlling the process parameters the coefficient of thermal expansion in the NCD films can be matched with the silicon substrate for insulating as well as for conductive films and therefore the thermal stress component be minimized. The results are important for the development of MEMS where silicon as a substrate is widely used.     

Effect of thermal residual stress on the tensile properties and damage process of C/SiC composites at high temperatures

Due to the mismatch of the thermal expansion coefficients between the matrix and yarns, thermal residual stress will appear in C/SiC composites. In this paper, a progressive damage model was used to predict the thermal-mechanical behavior of C/SiC composites and reveal the failure mechanism. Firstly, the properties of the composites under tensile load were tested at three different temperatures in vacuum. Then, the elastic-plastic progressive damage constitutive laws were used and implemented by a user-defined subroutine UMAT in ABAQUS. The thermal residual stress evolution in the cooling and heating processes was characterized. Finally, the stress-strain curves of the composites under tensile load at different temperatures were studied. The effects of thermal residual stress on the tensile properties and progressive damage process of C/SiC composites were revealed sequentially. This work can give design guidance for strengthening of C/SiC composites.     

Hot Pressing of Annular Alumina–Zirconia Composites

An analysis is performed to predict the densification during and the state of residual stress after hot pressing of annular alumina/zirconia (3Y-TZP) composites. The objective of the analysis was to study the residual stresses resulting from stress gradients during pressing and those from thermal expansion mismatch during the cooling of the compact from the pressing temperature to room temperature. It is predicted that the residual stresses are affected by the respective densification rates of the core and the annulus, their elastic modulus, and thermal expansion coefficient. For the system analyzed in this study, it is predicted that hot pressing reduces the residual stresses that result from the mismatch in thermal expansion coefficients. This is due in part to the high densification rate and in part to the high elastic modulus of the alumina annulus compared to the zirconia core. For surface compression strengthening, a system where the annulus would have similar elastic modulus but lower densification rate and lower thermal expansion coefficient than the core would be more beneficial.     

Enhanced mechanical properties of SiC reticulated porous ceramics via adjustment of residual stress within the strut

Silicon carbide reticulated porous ceramics (SiC RPCs) with multi‐layered struts were fabricated by polymer replica technique with SiC slurry, followed by infiltrating alumina slurries containing andalusite under vacuum condition. The effects of andalusite addition on the microstructure and mechanical properties of SiC RPCs were investigated, also the residual stress within the multi‐layered strut was predicted. Theoretical calculations showed that the residual tensile stress generated in the outer layer of SiC RPCs because of its larger thermal expansion coefficient of infiltration slurry than that of SiC slurry at elevated temperature. Furthermore, the addition of andalusite reduced the thermal expansion coefficient and Young's modulus of infiltration slurries, thereby significantly reducing the residual stress of the outer layer in multi‐layered struts. The reduced residual tensile stress within the outer layer was beneficial to eliminate surface cracks on the struts, thus improving the mechanical properties and thermal shock resistance of SiC RPCs.     

基体条件对Sm2Zr2O7/YSZ双陶瓷层热障涂层界面残余热应力的影响

采用有限元分析软件ANSYS对等离子喷涂Sm2Zr2O7/YSZ双陶瓷层热障涂层界面残余热应力分布进行了数值仿真。结果表明:基体厚度不同时,涂层界面Sm2Zr2O7/YSZ及界面YSZ/NiCoCrAlY对应应力及应力梯度基本不变,表明应力及应力梯度与基体厚度无关;但基体材质热膨胀系数对涂层系统界面的径向、轴向及剪切应力梯度有决定性的影响,且各应力梯度随金属基体的热膨胀系数差异增加而增大,表明基体材质是影响涂层界面径向残余热应力及应力梯度的根本原因。采用多层陶瓷结构并合理选择各层材质的热膨胀系数将更加有利于降低涂层应力梯度,进而改善涂层性能,延长涂层寿命。     

Model for SiC fiber strength after oxidation in dry and wet air

The strengths of oxidized SiC fibers were modeled from the effects of SiO₂ scale residual stress on fracture. Surface tractions from scale residual stress were determined for SiC surface flaws. The residual stress was the sum of the growth stress from oxidation volume expansion, thermal stress from SiO₂-SiC thermal expansion mismatch, and stress from phase transformations in crystallized scale. The partial relaxation of tensile residual stress from scale cracking was also calculated. Scale thicknesses were determined using Deal-Grove oxidation kinetics for glass and crystalline scales. Kolmogorov-Johnson-Mehl-Avrami (KJMA) kinetics was used to determine scale crystallization rates. Strengths of fibers with glass and with crystalline scales formed by oxidation in dry and wet air between 600° and 1400°C were modeled. The effects of partially crystallized scales were calculated using Weibull statistical methods. Modeled strengths were compared with measurements. Slight strength increases after glass scale formation, large decreases that accompany scale crystallization, and some differences between dry and wet air oxidation were accurately modeled. This suggests that under some conditions the scale residual stress dominates the changes in strength after SiC fiber oxidation. However, modeled strengths were significantly higher than those measured for some fibers oxidized in wet air, which suggests another degradation mechanism is active for these conditions. Modeling assumptions and implications for SiC fiber strength after oxidation for long times are discussed.     

Heat treatment of thin carbon films and the effect on residual stress, modulus, thermal expansion and microstructure

Thin carbon films are used to hermetically seal and improve the performance of devices exposed to extreme conditions. Such films, which are deposited by chemical vapor deposition, develop residual thermal stresses due to a mismatch in the coefficient of thermal expansion between the film and substrate. Residual stresses reduce the adhesion of the film, and are a common cause of coating failure. This work investigates heat treatment as a potential technique to reduce residual stresses in thin carbon films. The magnitude of the residual stress has been challenging to measure due to the associated size scales and mechanical properties. In this study, experimental measurements of mechanical properties and residual stresses in thin carbon films are performed using nanoindentation and Raman spectroscopy. The results relate surface residual stresses to film thickness and heat treatment temperature. The approach presented in this study is a nondestructive and non-intrusive method for measuring residual surface stress and properties in thin films, and is ideal for small or curved-surface specimens such as optical fibers and other photonic devices.     

Effect of the Grain Boundary Thermal Expansion Coefficient on the Fracture Toughness in Silicon Nitride

The effect of thermal expansion mismatch stress between silicon nitride and different grain boundary phases on the fracture toughness of silicon nitride was investigated. Different sintering aids in the Y-Mg-Si-Al-O-N system produced silicon nitride specimens with very similar micro- structures but different grain boundary phase compositions and different values of fracture toughness. The fracture toughness of the silicon nitride increased as the thermal expansion coefficient of the grain boundary phase increased. The presence of tensile residual stress at the grain boundary caused by thermal expansion mismatch between the silicon nitride and the grain boundary phase enhanced crack deflection and grain bridging.     

Residual stress and thermal expansion of spun-on polyimide films

The residual stress in spin coated films and the effect of heat treatments on this stress were determined by the Fizeau interference method. Three types of spin coated polyimide (PI) films have been studied. Two of them were prepared by thermal conversion of their poly(amic acid)s (PAAs) and the third one by solvent evaporation of the soluble preimidized PI. For the imidized PI films the residual stress vs. bake temperature shows an inclined steplike behavior while this function for soluble PI is approximately linear. The room temperature stress relative to silicon substrates in fully baked films (400°C) is between 40 and 70 MPa and nearly independent of the film thickness. From the stress measurement, the thermal expansion coefficient and Young's moduli have also been obtained. The thermal expansion coefficient and the Young's modulus are in the order of 9.0 × 10^?6°C^?1 and 10 GPa, respectively. These values deviate from those published for bulk material which is explained by the in-plane orientation of the molecular chains in spun-on PI films.     

等离子喷涂Sm_2Zr_2O_7热障涂层残余热应力

采用有限元分析软件ANSYS对等离子喷涂Sm2Zr2O7热障涂层在金属基体材质类型、厚度、半径变化时涂层的残余热应力进行了分析。结果表明,金属基体的热膨胀系数对Sm2Zr2O7热障涂层的残余热应力有着显著影响,金属粘结层与表面陶瓷层界面处的残余应力及其应力梯度随着金属基体热膨胀系数增加而增大。金属基体厚度和半径不是影响Sm2Zr2O7热障涂层残余应力的主要因素。     

Viscoelastic Effects in a Phosphate Glass-Metal Seal

A family of phosphate glasses has been developed with thermal expansion behavior in the elastic range that nearly matches the response of 304 stainless steel. Attempts to make concentric pin-shell seals consistently have yielded fractures between 400° and 300°C during cooling. Elastic stress analyses which neglect glass transitional behavior and utilize constant glassy ("elastic") thermal expansion coefficients predict a residual stress state that is compressive. However, viscoelastic computations which include the effects of structural relaxation during glass transtion show that tensile stresses sufficient to cause failure arise during cooldown of the seal.     

Specific Features of Changes in the Properties of One- and Two-Alkali Borate Glasses Containing Water: III. Thermal Expansion and the Structural and Mechanical Relaxation Parameters of Two-Alkali Borate Glasses

The thermal expansion and stress relaxation in mixed alkali borate glasses containing lithium, sodium, and potassium oxides with a total alkali oxide content of 15 mol % are measured on an inclined quartz dilatometer and a relaxometer. The experimental data obtained are used to determine the thermal expansion coefficients and the structural and mechanical relaxation parameters. No deviations from the additivity are found in the concentration dependences of the thermal expansion coefficient and the calculated parameters determining the width of the spectra of the structural and stress relaxation times. The IR absorption spectra of the studied glasses are recorded in the range of stretching vibrations of hydroxyl groups. Analysis of the IR spectra makes it possible to assume that the content of residual water in the structure of borate glasses affects the manifestation of the mixed alkali effect in the properties of these glasses.     

Model construction and effect of thermally grown oxide dynamic growth on distribution of thermal barrier coatings

A physical geometric model of the dynamic growth of thermally grown oxide (TGO) was established based on an analysis of the TGO growth of 8YSZ thermal barrier coatings during thermal cycling. Finite-element simulation was used to simulate the evolution law between the coating residual stress and thermal cycling, and the linear elasticity, creep effect, and stress accumulation in each thermal cycle were studied. The interface between the top coat (TC) and the bond coat (BC) was covered with a TGO layer that grew vertically and slowly in a layer-like manner. The stress in the TGO was distributed with a “layer” zonal gradient, and the TGO/BC boundaries were distributed uniformly with a large compressive stress, which decreased the TGO layer thickening. With the longitudinal rapid random TGO growth, the boundaries were subjected to a tensile stress, and a high tensile stress concentration area developed at the boundaries. The internal stress consisted of an alternating and mixed distribution of concentrated compressive and tensile stresses. The concentration area of the maximum equivalent stress was distributed in the one-layer TGO near the TC/TGO interface. When a microcrack formed at the TGO/BC boundaries, the crack was subjected to a tensile stress of different size, with a higher tensile stress at both ends, which facilitated crack expansion. Thus, the 8YSZ thermal barrier coating was prone to crack formation and expansion at the TGO/BC boundaries and in the TGO layer near the TC/TGO boundaries.     

金属基烤瓷牙冠残余应力产生的原因与控制方法

详细分析了金属基烤瓷牙冠残余应力产生的原因，认为金属与烤瓷热膨胀系数不匹配，制备工艺不合理以及金属与烤瓷结合欠佳等是产生残余应力和导致烤瓷破坏的主要原因，并对如何控制残余应力，关少烤瓷开裂和肃落进行了论述。     

Fracture Induced in Al2O3 Bicrystals by Anisotropic Thermal Expansion

Bicrystals of Al₂O₃ were fabricated to study the effects of thermal expansion anisotropy on fracture in a model system containing one grain boundary. Fractures occurred perpendicular to the directions of maximum tensile stress in bicrystals with thermal expansion coefficient differences as low as 0.31×10⁻⁶⁰C⁻¹ and originated at the boundary, probably in areas of high residual stress associated with pores. Numerical stress analysis of two-dimensional model bicrystal configurations showed that stresses induced by thermal expansion anisotropy are maximal in a region localized along the bicrystal boundary.     

Residual Stresses in PZT Investigated by Polarized Raman Piezospectroscopy

Micro-probe Raman piezospectroscopy has been applied to analyse the behaviour of bulk samples of Lead Zirconate Titanate doped with Nb, Sr and Y (PZT-NSY) under mechanical load by performing 4-point bending experiments. The samples experienced different degree of silver migration from Ag electrodes. A residual stress gradient was observed in the proximity of electrodes, due mostly to thermal expansion coefficients mismatch, while in areas unaffected by Ag contamination the stress gradient could be only attributed to the externally applied load.     

Delayed curvature and residual stresses in porcelain tiles

Most industrial porcelain tiles suffer changes in their curvature after firing: such process is known as delayed curvature. One of the hypotheses used to explain this phenomenon is based on the relaxation of residual stresses by creep. In this study two types of industrial glazed porcelain tiles have been studied. One of them displayed delayed curvature after firing, whereas the other one presented a stable curvature. The main objective was to determine if the delayed curvatures were caused by the residual stresses generated during rapid industrial cooling. Both types of existing residual stresses (thermal stresses, caused by thermal gradients inside the tile during cooling, and body–glaze fit stresses, due to the thermal expansion mismatch between body and glaze) were measured, as well as related samples properties (elastic modulus, creep behaviour, thermal expansion). The results demonstrated that the residual stresses are not the main cause of the delayed curvature phenomenon.     

Residual stress in particulate epoxy resin by X-ray diffraction

Residual stress in particulate epoxy resin was investigated by X-ray diffraction. Microdeformation of incorporated Al and α-SiO₂ crystal, which was induced by the residual stress, could be detected as a shift of X-ray diffraction peak. The residual stress at the interface between the adherend and the particulate epoxy resin was found to decrease with the increase of volume fraction of filler. It was shown that the difference in the thermal expansion coefficients between the adherend and the particulate epoxy resin is much more effective on residual stress than the increment of Young's modulus owing to the incorporation of filler. When epoxy resin was cured on the Al plate, incorporated particles were subjected to a tensile stress; while cured on polytetrafluoroethylene sheet, particles were subjected to a compressive stress. The incorporation of some inorganic particles is considered effective to reduce the residual stress.     

残余热应力对复合材料修理金属裂纹板影响分析

本文建立了复合材料胶接修理金属裂纹板的三板有限元模型;采用分析金属裂纹板裂纹尖端应力强度因子的方法,分析了残余热应力对复合材料修理金属裂纹板修理效果的影响。结果表明,残余热应力对单面修理结构的影响小于双面修理结构。残余热应力对单面修理结构的影响由于裂纹板宽度比值不同而不同。补片与母板的热膨胀系数相差越大,残余热应力对修补效果影响越大。     

Functionally Graded Zircon–Molybdenum Materials without Residual Stresses

A layered, zircon–molybdenum functionally graded material was obtained by starting from commercial powders of molybdenum (median particle size ( d ₅₀) of ∼3 μm) and zircon ( d ₅₀∼ 0.8 μm). Conventional processing led to a material that was free of internal residual stress. The sintering behavior of green compacts with compositions that corresponded to the different layers was studied via dynamic sintering. The thermal expansion coefficients of each layer were measured using conventional dilatometry. The presence of residual stresses was determined using Vickers indentations. Both thermal expansion mismatch and differential shrinkage between the layers were negligible.