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Elastic constants of fibre-textured thin films determined by X-ray diffraction

Supposing the Hill grain-interaction model, it is demonstrated that X-ray elastic constants can be used to determine mechanical elastic constants of cubic fibre-textured thin films. The new approach is demonstrated by the experimental characterization of out-of-plane moduli of fibre-textured Cu and CrN thin films

High-precision determination of residual stress of polycrystalline coatings using optimised XRD-sin2ψ technique

The aim of the research is to optimise the XRD-sin2ψ technique in order to perform high precision measurement of surface residual stress. Residual stresses existing in most hard coatings have significant influence on the adhesion, mechanical properties and tribological performance. In the XRD-sin2ψ stress measurement, the residual stress value is determined through a linear regression between two parameters derived from experimentally measured diffraction angle (2θ). Thus, the precision coefficient (R2) of the linear regression reflects the accuracy of the stress measurement, which depends strongly on how precise the 2θ values are measured out of a group of very broad diffraction peaks. In this research, XRD experiments were conducted on a number of samples, including an electron beam evaporated ZrO2 based thermal barrier coating, several magnetron sputtered transitionmetal nitride coatings, and shot-peened superalloy components. In each case, the diffraction peak position was determined using different methods, namely, the maximum intensity (Imax) method, the middle point of half maximum (MPHM) intensity method, the gravity centre method, and the parabolic approaching method. The results reveal that the R2 values varied between 0.25 and 0.99, depending on both the tested materials and the method of the 2θ value determination. The parabolic approaching method showed the best linear regression with R2=0.93±0.07, leading to high precision of the determined residual stress value in all cases; both the MPHM (R2=0.86±0.16) and gravity centre (R2=0.91±0.11) methods also gave good results in most cases; and the Imax method (R2=0.71±0.27) exhibited substantial uncertainty depending on the nature of individual XRD scans

Changes in microfibril angle in cyclically deformed dry coir fibers studied by in-situ synchrotron x-ray diffraction.

Dry coir fibers are characterized by wideangle X-ray scattering coupled with tensile tests. The fibers exhibit elastic and plastic behavior with the yield point at a strain of about 2%. In-situ experiments document that the cyclic loading and unloading beyond the yield point does not reduce the stiffness of the fibres, since they recover their initial stiffness by every increase of the strain. The diffraction data show that the microfibril angle (MFA) of cellulose fibrils in the coir fibre cells is inversely proportional to the magnitude of the applied strain. In average, the relatively high MFA of about 45 degrees in the unstrained state decreases linearly upon straining until the fibers break at about 35% strain. When the strain is released during the tensile experiment the MFA tends to recover its original magnitude. No significant differences in the dependence of MFA on strain are detected in elastic and plastic regions, respectively. The results demonstrate that the tissue with helical architecture does not have to be saturated with water in order to exhibit the effect of the recovery of the mechanical function when cyclically loaded. This indicates differences in the architecture of the coir cell wall in comparison with that of compression wood with high MFA whereby similar phenomena were observed in the wet state. © 2008, Springer. The original publication is available at www.springerlink.co

Residual stresses in thermally cycled CrN coatings on steel

CrN coatings were deposited on polycrystalline ferritic steel substrates at 350°C by magnetron sputtering using Cr targets in Ar + N2 atmosphere. In order to simulate the thermal fatigue, the samples were repeatedly irradiated using a laser beam of 6mm in diameter. The thermal cycling was performed in the range of 50–650°C with up to 100 000cycles. Subsequently, the structures were characterized using high-energy synchrotron and high-temperature laboratory X-ray diffraction. The structures exhibit complex changes in the morphology and in residual stress state in the heated spot. The annealing results in the relaxation of compressive stresses in the coating and in the formation of high tensile stresses in the steel substrate. This effect decisively depends on the number of applied cycles. The reduction of compressive stress in the coating is caused by the annealing of point defects and by dimensional changes of the substrate due to its plastic deformation in the center of the irradiated spot. The plastic deformation of the substrate is also the probable reason for the ripples observed for samples cycled more than 3000 times. The presented approach allows a complex characterization of thermo-mechanical processes in coating-substrate composites and opens the possibility to understand phenomena related to the thermal fatigue of coated tools