Modified thick thermal barrier coatings: microstructural characterization

Thick thermal barrier coatings were modified with laser glazing and phosphate based sealing treatments. Surface porosity of the sealed coatings decreased significantly in all cases. Structural analysis showed a strong preferred crystal orientation of the t′ZrO₂ phase in direction [002] in laser-glazed 25CeO₂–2.5Y₂O₃–ZrO₂ coating. In laser-glazed 22MgO–ZrO₂ coating the major phase was rhombohedral Mg₂Zr₅O₁₂. In phosphate sealed 8Y₂O₃–ZrO₂ coating the strengthening mechanism was identified as adhesive binding without chemical bonding. Coating microstructures were determined by scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy and optical microscopy. Coatings were also characterized by X-ray diffraction, microhardness and porosity.     

Direct observation and measurement of fiber architecture in short fiber-polymer composite foam through micro-CT imaging

A non-destructive X-ray imaging technique was used to determine internal structure in a polymer foam reinforced with short fibers. The technique, known as micro-CT (for computerized tomography), was used to measure the fiber length distribution (FLD) and fiber orientation distribution (FOD), two parameters that are critical to the behavior of short-fiber-reinforced composites. Phenolic foam reinforced with short glass fibers was used as an exemplar to demonstrate the potential of this technique, exploiting the large difference in density between the two components. Direct 2D and 3D images were generated in which individual fibers were clearly resolved, along with portions of the foam structure. The images were analyzed using computer software to obtain quantitative FLD and FOD data. A distinct preferred orientation of fibers was revealed that was attributed to shear flow during foam expansion. For quantitative analysis of microstructure in short fiber composites, the micro-CT technique affords numerous advantages over the conventional approach of parallel dissection followed by image analysis of polished surfaces, and may be useful for determining FLD and FOD in polymer composites with dense matrices.     

Numerical simulation of steel-plate strengthened concrete beam by a non-linear finite element method model

Strengthening with epoxy bonded steel plate is one of the most widely used techniques for flexural upgrading of reinforced concrete (RC) beams. However, debonding failure at the plate cut-off zone and or in the vicinity of flexure and shear cracks leads to catastrophic failure of the upgraded beams. This particular failure depends on several factors such as the distance of plate curtailment from the support, plate thickness and the provision of end anchors. Since the conventional beam theory cannot predict the debonding failure of such beams, a finite element model capable of predicting the overall behavior of strengthened beams including different failure modes accurately is developed. This paper presents the formulation of finite elements and material models and simulation results of some RC beams tested for flexural strengthening with epoxy bonded steel plates.     

Phase field microelasticity modeling of dislocation dynamics near free surface and in heteroepitaxial thin films

Dislocation dynamics near a free surface and in heteroepitaxial thin films are simulated using an extended version of the nanoscale Phase Field Microelasticity model of dislocations [Acta Mater. 49 (2001) 1847]. The model automatically takes into account the effect of image forces on dislocation motions. In particular, the operations of Frank–Read sources in epitaxial films grown on infinitely thick and relatively thin substrates are investigated. The simulation reveals different misfit dislocation behaviors at the interface. Its implication on the interface susceptibility to crack nucleation is discussed.     

Twinning and detwinning of 〈0 1 1〉 type II twin in shape memory alloy

〈0 1 1〉 type II twin is a major twinning mode frequently observed in NiTi shape memory alloy. Its response to applied stresses is responsible for various inelastic deformation processes. In order to predict the shape recovery characteristic under various conditions, it is of primary importance to predict the martensite detwinning characteristic prior to the reverse transformation. However, due to its irrational nature, the type II twin plane is not as easily comprehensible as that of other types of twins and a strong disagreement exists over the nature of the twin boundary. In the present research, the 〈0 1 1〉 type II twin boundary before and after deformation was investigated. A complete model of 〈0 1 1〉 type II twin plane and its role in the detwinning process are proposed which can be used to account for the various experimental observations.     

Lattice dynamics in wide band gap materials based superlattices

Lattice dynamics calculations in zinc blende group III nitrides superlattices were carried out assuming the existence of interface regions with variable thickness. The acoustic frequencies were observed to remain practically unchanged for all values of interface thickness from one to three monolayers. The dispersion relation, Raman spectrum and the atomic displacements of optical modes were found to be greatly affected by interfacing. Several Raman peaks shift toward the center position of the spectrum with increasing interface thickness. As a consequence, some of the Raman frequencies become quasi-coincident giving rise to highly prominent structures in the middle range spectrum. Effects of localization of atomic displacements at the interface regions are shown.     

Surface modification of 2Cr13 oil pump steel by plasma immersion ion implantation–ion beam enhanced deposition (PIII–IBED)

2Cr13 martensite steel is often used as a piston material in oil pumps. In the harsh environment of an oil field, the materials and components undergo extensive and accelerated wear and tear. In this study, we employ Ti and N plasma immersion ion implantation and ion beam enhanced deposition (PIII-IBED) to enhance the surface wear resistance of 2Cr13 steel in an effort to prolong its working lifetime. To assess the technique efficacy and surface properties of the 2Cr13 steel samples treated by PIII-IBED using different voltages, the coefficient of friction, wear tracks, microhardness, anode polarization curves, chemical composition and elemental depth profiles were determined. The experimental data show that the wear resistance of the treated 2Cr13 steel samples is improved significantly by the method, and the nitride phases formed in the modified layer play an important role in the enhancement mechanism.     

Creep behaviour of two sintered silicon nitride ceramics

The high temperature mechanical properties of two sintered silicon nitride—based ceramic materials with different microstructural features, labelled N7202 and N3208, are presented. The mechanical behaviour was examined by creep tests in compression, at temperatures between 1450 and 1700 °C in argon atmosphere. The results, analysed in terms of the creep equation, yield n=0.6±0.1 and Q=470±20 kJ/mol for N7202, and n=0.6±0.1 and Q=530±10 kJ/mol for N3208. The microstructural observations allow identify a possible microscopic deformation mechanism compatible with the set of creep parameters obtained experimentally.     

多轴振动试验控制的整型权函数法

研究了应用于多轴向多激励随机振动控制的H∞整型控制方法,并且针对在频响函数矩阵病态的频率点处,响应自谱和互谱超出工程预定参考谱目标的情况,分解设计了H∞整型加权矩阵,从而改进频响函数矩阵病态情况,并对系统进行算法解耦。然后用整型矩阵对驱动信号的傅氏谱进行修正,进入控制回路迭代运算。实验验证表明,用文中算法设计整型权函数矩阵并修正驱动信号,可以有效的改善响应自谱和互谱的控制效果,抑制某些频率点处频响函数的不良影响。     

The comprehensive in vitro evaluation of eight different calcium phosphates: Significant parameters for cell behavior

Eight different calcium phosphate nanoparticles, namely bovine bone bioapatite calcined at 500, 600, and 700°C, Mg-doped brushite, fluorinated calcium phosphate, Ca-deficient hydroxyapatite, hydroxyapatite, and tricalcium phosphate, were characterized employing physico-chemical methods. Their cytocompatibility was evaluated under human osteoblast-like cell line MG-63 culture conditions in elution media and via the direct interaction of cells with calcium phosphate nanoparticles. The main objective was to determine the correlation of the cell indices with the differently determined physical and chemical parameters of the calcium phosphates. Chemical composition, which contributes toward pH changes, and the calcium ion concentration in the medium appear to make up particularly significant factors; moreover, it was proved that the number of material types represents a further important aspect. In the case of a large number of material types, almost no correlation was determined between the analyzed parameters; however, in the case of a small number of apatite types, several positive correlations were found. It can be concluded that it is not possible to identify any monitored parameters that had a major impact on cell behavior or, at least, such an effect which can be generalized. It appears more likely that cell behavior is affected by the interplay of various parameters.