The effect of residual stresses on adhesion measurements

The adhesion of films and coatings is often measured by determining the load required to separate them from their substrate. If there are residual stresses that are relaxed upon delamination, then an additional contribution to the energy-release rate will affect the measurements. These residual stresses may also cause a shift in the mode-mixedness of the interface crack which, in turn, can affect the interfacial toughness. To ensure an accurate interpretation of adhesion measurements, therefore, the effects of these stresses must be considered. These effects are discussed with particular reference to two commonly used test geometries: the blister test and the peel test.     

The effect of local residual stress on the fractal dimension of silicate glasses

Local residual stress caused by impacts, machining and indentation results in a decrease in strength in most materials that fail in a brittle manner. The ratio of the critical crack size, c, and the fracture mirror size, r, also is affected by the existence of local residual stress. The global fracture toughness of non-R curve materials is not affected by the local residual stress. The fractal dimension of the fracture surface as characterized by the fractal dimensional increment, D*, is directly related to the square of the fracture toughness. This paper addresses the question of the effect of the local residual stress on the fractal dimension of the fracture surface. We derive a relationship between the fractal dimensional increment and the c/r ratio for materials fractured with and without local residual stress. We then compare the prediction with two cases of experimental results. We show the fractal dimension remains constant with the change in the c/r ratio for local residual stress conditions.     

The effect of Al2O3 on sintering and crystallization of MgSiO3-based glass-powder compacts

The influence of Al₂O₃ (8 wt.%) on sintering and crystallization features of glass powders based on magnesium silicate (MgSiO₃) was experimentally determined. The investigated compositions were Y_0.125Mg_0.875Si_0.875B_0.125O₃ and Y_0.125Mg_0.725Ba_0.15Si_0.875B_0.125O₃. For the experiments, glasses in bulk and frit forms were produced by melting in Pt-crucible at 1600 °C for 1.5 h. Glass-powder compacts were sintered at different temperatures between 900 °C and 1100 °C. The evolution of crystalline regime was determined by in situ recording of X-ray diffractograms of fine glass powders at elevated temperatures. The results and their discussion showed that addition of 8 wt.% Al₂O₃ in glass batches affected the thermal properties of the glasses and resulted in MgSiO₃-based glass ceramics well sintered between 900 °C and 1100 °C. In the BaO-free MgSiO₃ glass ceramics, clino- and orthoenstatite crystallize while the presence of BaO favours the formation of hexacelsian.     

The effect of residual stresses and dislocations on microwave dielectric loss in rutile-related (Ti0.6Zr0.4)0.8(Zn1/3Nb2/3)0.2O2

Strain and stress play an important role in functional materials, but they are often neglected in electroceramics. Here we report the stress effect on microwave dielectric loss in rutile-related (Ti_0.6Zr_0.4)_0.8(Zn_1/3Nb_2/3)_0.2O₂ solid solutions. The macroscopic, microscopic and nano-scale residual stresses have been systematically investigated, including their relationships. The average Raman spectra have demonstrated considerable macro-residual stresses, especially for the heavily cracked sample. The high-resolution Raman images reveal the inhomogeneity of residual stress distribution at the grain-level, which is reflected by the shifts of Raman-active modes. The high-resolution transmission electron microscope (HRTEM) images and geometric phase analysis (GPA) analysis show the aggregation of dislocations and resulting distortion of crystal. It can be found that the dielectric loss depends strongly on residual stresses, that samples with greater residual stress exhibit much higher dielectric loss because the residual stress increases the vibration anharmonicity of the lattice.     

Enthalpic and entropic origins of nucleation barriers during polymer crystallization: the Hoffman-Lauritzen theory and beyond

In the search for a greater understanding of polymer crystallization, numerous experimental observations with regards to microscopic structures and macroscopic properties have been reported in the past half-century. There are generally two types of experimental results to provide information about the mechanisms of polymer crystal growth, i.e. molecular dynamic/scattering and structural/morphological. Since we cannot follow the trajectory of individual chain molecules when they undergo the transition from liquid to solid state during the crystallization process, structural/morphological analysis of polymer crystals reveal information recorded during this process. Namely, the final structure and morphology of polymer crystals have atomic, stem and global chain conformation information embedded in them during crystallization which provides evidence which can be used to deduce molecular aspects of the polymer crystallization process. It is commonly understood that polymer crystallization, from the thermodynamic perspective, is a first-order transition involving the relaxation of a metastable undercooled melt towards the equilibrium state which is rarely reached in polymer crystals. This process is controlled by a free energy barrier. A molecular model is required to describe the landscape of the free energy barriers and to provide an analytical explanation concerning and predictions about polymer crystallization. The Hoffman-Lauritzen (HL) theory, which was put forward more than 40 years ago, was one of the first analytical theories to illustrate how polymers crystallize. Since then, modifications to the HL theory and suggestions for new approaches have been reported, but the core physical picture of the HL theory has largely remained intact. This article consists of four major parts: (1) we will compare the crystallization of small molecules and long chain molecules, and the relationship between crystallization and crystal habits. The diversity of crystalline structures and morphologies of semi-crystalline polymers must be taken into account when studying the crystallization mechanism of polymers (2) this article also serves as a brief review of the HL theory and its importance in our understanding of polymer crystallization (3) we have tried to answer the question: what is the nucleation barrier? Specifically, we will illustrate that the nucleation barrier in polymer crystallization consists of both enthalpic and entropic components as deduced from experimental results. This barrier, from our perspective, consists of selection processes taking place in different length- and time-scales (4) finally, there is a brief discussion on what issues remain, in particular, in the areas of undercooled liquid structures and the initial stages of crystallization.     

The effect of barium content on the crystallization and microhardness of barium fluormica glass-ceramics

Mica glass-ceramics are easily machined due to their “House-of-Cards” microstructure. Barium fluormica glass-ceramics were developed and indicated good mechanical properties. This work studies the effect of varying barium content on the crystallization and microhardness of mica glass-ceramics. Four glasses were produced with different baria contents, then converted to mica glass-ceramics using a two-step heat treatment. They were characterized using Differential Scanning Calorimetry [DSC], X-ray Diffraction [XRD], Scanning electron microscopy [SEM] and Vickers microhardness. DSC showed some formulations indicated bulk crystallisation as the dominant mechanism. XRD showed the crystallization of Barium fluorphlogopite in all the compositions with minor secondary phases. SEM showed the formation of “House-of-Cards” microstructures and with an increase in BaO content, a decrease in contrast was observed in back scattered mode. Exceptionally low hardness values (<2 GPa) were obtained for longer heat treatments/holding times and are related to the well-developed house-of-cards microstructures formed.     

Effect of different nucleating agent ratios on the crystallization and properties of MAS glass ceramics

The effects of different kinds of nucleating agents on crystallization, microstructure and performances of Magnesium Aluminosilicate (MgO-Al₂O₃-SiO₂, MAS) glass-ceramics which were fabricated by melting method in this study. Also, this paper systematically investigated the mechanism of glass stability, crystallization kinetics and element distribution of MAS glass-ceramics. Herein, we used three kinds of nucleating agents, which was TiO₂, ZrO₂ and composite nucleating agent (TiO₂/ZrO₂). The results showed after the doping of nucleating agent, the content of α-cordierite was increased, the stability and crystallization kinetics of glass was changed, the precipitated crystal phase was finer and more compact. Wherein, the sample with composite nucleating agents (TiO₂, ZrO₂) has the best performance due to the highest contents of α - cordierite, uniform distribution of elements without agglomeration in the crystal phase and the most compact structure, whose Vickers hardness and bending strength can reach 9.70 GPa and 312 MPa, respectively.     

Evaluating nanocrystallite size distributions in doped and undoped nanocrystalline ceramics by X-ray diffractometry

Analytical expressions are formulated that enable the lognormal distribution of nanocrystallite sizes and the mean-squared crystal lattice microstrain in both doped and undoped nanocrystalline ceramics to be routinely determined by X-ray diffractometry. Importantly, these expressions, which are also applicable to most well-annealed nanocrystalline metals/alloys, are developed under the consideration that both the experimental and instrumental X-ray diffraction peaks are modeled by split Voigt functions to account for the (hitherto widely ignored) intrinsic peak asymmetry. The general analytical expressions of this new single-peak line-broadening method are then particularized to the case of small nanocrystallites with uniform composition for which a reasonable simplification (i.e., symmetrization of the experimental peak) can be made without compromising the accuracy of the results. Next, as an example of application the method developed is applied to the characterization of a nanocrystalline ceramic powder, and the nanocrystallite sizes so-determined are validated by transmission electron microscopy. The more complete and reliable microstructural information offered by this single-peak line-broadening method could contribute towards the accurate characterization of both doped and undoped nanocrystalline ceramics, and of most well-annealed nanocrystalline metals/alloys as well, a topic of particular research interest today.     

Effects on the crystallite growth behavior of LaPO4 nanopowders and its mechanical properties

The effects of pH of the reaction solution and the concentration of phosphoric acid on the crystal growth behavior of LaPO₄ crystallites were investigated and the mechanical properties of rare-earth phosphates were compared. As a result, the concentration of phosphoric acid of 10% was beneficial to the crystal growth of LaPO₄ nanocrystalline. When the pH value of the reaction solution was 2, the size of LaPO₄ crystallites increased gradually with the increasing reaction temperature, and the smallest crystallite size of 43.27 nm was obtained after heat-treatment at 1000 °C. Simultaneously, the activation energy for crystal growth of LaPO₄ nanocrystalline was relatively lower (26.82 kJ mol⁻¹). With the decreasing radii of rare-earth ions, the hardness, Young's modulus and fracture toughness of the bulk rare-earth phosphates exhibited a reduced tendency, resulted from the increase of porosity under the same preparation process.     

The effect of zinc substitution on the magnetism of magnesium ferrite nanostructures crystallized from borate glasses

Glasses in the system 51.7 B₂O₃/9.3 K₂O/1 P₂O₅/10.4 Fe₂O₃/(27.6–x) MgO/ x ZnO (with x=0, 5, 10, 13.8 and 20 mol%) were prepared by the conventional melt quenching method. The as prepared glass samples were thermally treated at 560 °C for 3 or 6 h. The effect of substituting MgO by ZnO in the glass network on the crystallized phase was studied. The resulting magnetic glass ceramics were characterized using X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and transmission electron microscopy (TEM) including energy dispersive X-ray analysis (EDX). The substitution of Mg by Zn resulted in a larger lattice parameter of the precipitated crystals, while the crystallite size does not change significantly. TEM micrographs, recorded from extracted particles, showed the formation of small aggregates with about 30 nm in diameter. These agglomerates contain crystals with sizes in the range from 7 to 9 nm. EDX measurements proved the incorporation of Zn²⁺ ions into the crystal phase. Room temperature magnetic measurements of the samples with up to 10 mol% ZnO showed hysteresis loops which are characteristic for super paramagnetic (SPM) behavior. A magnetic contribution was not detected for samples with higher ZnO concentrations. The maximum magnetization varied with the composition of the glass ceramics to a great extent.     

Extended-chain and folded-chain crystallite formation during the crystallization of stretched crosslinked rubbers

The strain-induced crystallization phenomenon of a crosslinked polymer network was studied using statistical, thermodynamics. The basic approach was essentially that used by Flory (1), but in this work Flory's assumption of only a single crystalline phase (the extended-chain crystalline structure with chain parallel to stretch direction) was abandoned. The dimension of the crystalline vector-which is assumed to be parallel to the stretch direction and the percent crystallinity is taken as two independent variables instead of being treated as one single variable. A single variable treatment is inherent in the assumption of a single extended-chain crystallite. By use of the present approach, either the folded-chain crystallite or the extended-chain one is found to be thermodynamically stable depending on temperature, stretch ratio and molecular weight. The retractive force of a crystallized polymer network has been calculated. It is shown theoretically that the formation of an extended-chain crystallite will cause the retractive force to decrease, and that the formation of a folded-chain crystallite will cause the retractive force to increase in a reversible crystallization process.     

Sintering, crystallization and mechanical properties of a gel-cast cordierite glass-ceramic body

Gel-cast bodies based on cordierite glass-ceramics were prepared by sintering route. Effect of monomer and cross-linker values as well as sintering temperatures on bending strength of dried and sintered bodies were investigated. While the bending strength of dried gel-cast bodies was increased with the percentage of the polymers, bending strength of sintered bodies was changed conversely with them. Therefore, it was concluded that the least amount of monomer acrylamid (3 wt.%) and moderate amount of cross-linker (∼0.75 wt.%) guarantees the required dried and fired bending strengths. The optimum sintering temperature was about 1270 °C and specimens that was fired at this temperature showed a maximum bending strength of about 200 MPa.     

Effect of Cr2O3 on the crystallization behavior of synthetic diopside and characterization of Cr-doped diopside glass ceramics

Diopside is the main crystalline phase in silicate materials such as ceramics and glass-ceramics. Herein, the effect of Cr₂O₃ on the microstructure and crystallization behavior of synthetic diopside, as well as the solubility of Cr₂O₃ in diopside is discussed. Samples were prepared by the melting method and characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry, and confocal laser scanning microscopy. Results show that the maximum achievable solubility of Cr₂O₃ in diopside is between 1% and 3% by weight, and that the magnesiachrome spinel formed by Cr₂O₃ can act as a nucleating agent for the diopside phase. Glass ceramics was prepared by synthesis slag which simulates the chromium-containing waste. The activation energy of crystallization is 274 KJ/mol and Avrami parameter is 3.23. The leaching behavior of glass ceramics was studied. Additionally, the effect of Cr₂O₃ on the mechanisms of phase change were discussed. The study provides a theoretical basis for the preparation of chromium containing waste-based silicate materials with diopside as the main crystalline phase.     

Non-isothermal crystallisation behaviour of fluorophlogopite/nepheline glass ceramics

Abstract

Fluorophlogopite/nepheline glass ceramics were formed from the system of SiO₂–Al₂O₃–MgO–K₂O–Na₂O–F, and the thermodynamic, crystallisation behaviour and microstructure were investigated using differential thermal analysis, X-ray diffraction, and scanning electron microscopy. It was found that fluorophlogopite crystals and nepheline crystals crystallised simultaneously, and bulk nucleation was the main crystallisation mechanism. Crystal growth was prone to follow the two-dimensional direction and controlled by diffusion. Activation energy for glass transition was 797·43 kJ mol^?1, and crystallisation activation energy was 433·16 kJ mol^?1.     

Suppressing the effect of cullet composition on the formation and properties of foamed glass

The process of foaming glass is very dependent on the chemical composition of the glass. In this study we used a foaming-agent/oxidizing-agent couple and a crystallization inhibitor to foam cullets of flat, container and CRT-panel glass. Foamed glass with a density of 110–120?kg?m^–3, a thermal conductivity of 50–52?mW?m^–1 K^–1 and a homogeneous pore structure was obtained from a mixture of panel glass, 0.33?wt% carbon and 4.45?wt% Fe₂O₃. We also showed that it is possible to fabricate foamed glass with the same density or pore structure as mentioned above by adding up to 50?wt% container cullet or 70?wt% flat glass to the mixture. In the foamed samples with a low content of panel glass, crystals form, resulting in an increased open porosity, density and inhomogeneous pore structure. The crystallization can, however, be inhibited by adding calcium phosphate, so enabling the preparation of high-quality foamed glass from flat glass or flat/container-glass mixture. The pore gas is predominantly CO₂ and the pressure inside the pores is 0.36–0.47?bar. The reduced effect of the composition on the foaming process suggests that there is a great potential for stabilizing the production of foamed glass and ensuring the product's quality.     

Effect of crystallization on the macro and micro residual stress of enamel coating

In this paper, we investigate microstructural and mechanical properties of enamel coating after different heat treatment periods. The results show that the refractory BaAl₂Si₂O₈, CaAl₂Si₂O₈ and MgCr₂O₄ crystals precipitate from parent enamel, and crystallization volume fraction reaches saturation at 40 % after heat treatment 32 h. Moreover, from the experimental and theoretical analysis of residual stress, the macro and micro compressive residual stresses increase with increasing the crystallized volume fraction. And, the continuous macro compressive residual stress and small-scale microstructure inhomogeneities could significantly lead to the shear stress concentration near the enamel coating surface, which results in the typical fish-scaling failure of enamel. Meanwhile, we also find that a slit method with geometric phase analysis (GPA) under focused ion beam scanning electron microscopy (FIB/SEM) system could more reliably characterize residual stress of enamel coating because of phase transformation than traditional X-ray diffraction (XRD) method.     

Effects of imperfect adhesion on thermal micro-residual stresses in polymer matrix composites

This paper investigates the influences of imperfect bonding between the fiber and matrix on thermal micro-residual stress fields in polymer matrix composites. For this purpose, a representative volume element consisting of a three-phase composite material subjected to a uniform temperature change is considered. Based on the energy method, a three-dimensional closed-form solution for micro-residual stresses is obtained. Besides, a finite element model is developed and the results are compared with the analytical solution. Both the energy method and finite element analysis show similar trend for thermal stress distribution along the fiber length, while due to the stress singularity, the interfacial shear stress from the finite element solution cannot satisfy the stress-free condition at the fiber end. The analysis shows that the magnitude of thermal stresses and their distribution mainly depend on the bonding efficiency parameter. An increase in thermal and elastic properties bonding efficiencies leads to a considerable decrease in composite axial and shear residual stresses, while the Poisson's ratio bonding efficiency does not affect the thermal stress fields. The interfacial radial residual stress distribution is approximately independent of the bonding conditions. Inefficient bonding may result in higher residual stresses in comparison with the perfect bonding condition. It means that in cases of low bonding efficiency conditions, the ability of composites to sustain and transmit load decreases drastically. Thermal stress concentration occurs at the vicinity of the fiber ends, although peak values depend on the bonding efficiency value.