Maher S. Amer

The topotactic transformation of Ti{sub 3}SiC{sub 2} into a partially ordered cubic Ti(C{sub 0.67}Si{sub 0.06}) phase by the diffusion of Si into molten cryolite

Immersion of Ti{sub 3}SiC{sub 2} samples in molten cryolite at 960 C resulted in the preferential diffusion of Si atoms out of the basal planes to form a partially ordered, cubic phase with approximate chemistry Ti(C{sub 0.67}, Si{sub 0.06}). The latter forms in domains, wherein the (111) planes are related by mirror planes; i.e., the loss of Si results in the de-twinning of the Ti{sub 3}C{sub 2} layers. Raman spectroscopy, X-ray diffraction, optical, scanning and transmission electron microscopy all indicate that the Si exists the structure topotactically, in such a way that the C atoms remain partially in their ordered position in the cubic phase.

THE RAMAN SPECTRUM OF TI3SIC2

In this paper, the Raman spectrum of Ti3SiC2 is reported and compared with that of TiC0.67. All the TiC0.67 first order Raman disorder-induced modes are active, but shifted, in Ti3SiC2. Two additional peaks at 150 and 372 cm−1 are observed in Ti3SiC2. The former is ascribed to a shear mode between the Si and Ti planes; the origin of the latter is unknown. No second order Raman bands are detected. Micro-Raman spectroscopy also reveal the presence of ≈50 A graphite crystallites in samples hot pressed in graphite dies—these crystallites are not detected in samples processed by hot isostatic pressing in molten glass containers.

Fibre interactions in two-dimensional composites by micro-Raman spectroscopy

In the study of fracture processes in composite materials, the interactions between broken and intact fibres are of critical importance. Indeed, the redistribution of stress from a failed fibre to its unfailed adjacent neighbours, and the stress concentration induced in these, determine the extent to which a break in one fibre will cause more breaks in neighbouring fibres. The overall failure pattern is a direct function of the stress concentration factors. In this paper we use laser micro-Raman spectroscopy to study the extent of stress transfer and redistribution caused by fibre fracture in two-dimensional Kevlar 149 based microcomposites. The strain along the fibres was mapped at different levels of load, and specimens with different inter-fibre distances were used to study the fibre content effect. The experimental stress concentration factors were compared with values predicted from various theoretical models. The stress concentration factors generally agreed with those literature models that include interfibre distance and matrix effects. The overall failure pattern was found not to be a direct function of the stress concentration factors in this system, as fracture propagates from fibre to fibre even at large interfibre distances, and is apparently accompanied by relatively low values of the stress concentration factors. The ‘critical cluster size’, beyond which final fracture of the composite occurs in a catastrophic manner, was found to be larger than five adjacent fibre breaks in the present system, for all interfibre distances studied.

Study of the hydrostatic pressure dependence of the Raman spectrum of single-walled carbon nanotubes and nanospheres.

We have investigated the behavior of single-walled carbon nanotubes and nanospheres (C(60)) under high hydrostatic pressure using Raman spectroscopy over the pressure range 0.2-10 GPa using a diamond anvil cell. Different liquid mixtures were used as pressure transmission fluids (PTF). Comparing the pressure dependence of the Raman peak positions for the nanotubes and the nanospheres in different PTF leads to the observation of a number of new phenomena. The observed shift in Raman peak position of both radial and tangential modes as a function of applied pressure and their dependence on the PTF chemical composition can be rationalized in terms of adsorption of molecular species from the of PTF on to the surface of the carbon nanotubes and/or nanospheres. The peak shifts are fully reversible and take place at a comparatively modest pressure (2-3 GPa) that is far below pressures that might be required to collapse the nanoparticles. Surface adsorption of molecular species on the nanotube or nanospheres provides a far more plausible rational for the observed phenomena than ideas based on the notion of tube collapse that have been put forward in the recent literature.

Stress Characterization of MEMS Microbridges by Micro-Raman Spectroscopy

Abstract In this research, micro-Raman spectroscopy is employed to examine, and characterize the residual stress in MUMPs polysilicon, micro-electro-mechanical systems (MEMS) microbridge structures. Currently, few techniques are available to measure the residual stress in MEMS devices. The residual stresses from the deposition processes can have a profound effect on the functionality of the fabricated MEMS structures. Typically, material properties of thin films used in surface micromachining are not controlled during deposition. The residual stress, for example, tends to vary significantly for different deposition methods. Several post-fabrication processes are available to reduce the inherent residual stress from these deposition methods. In an attempt to reduce the residual stress in MEMS microbridges, a phosphorous diffusion and accompanying anneals were performed. Residual stress profiles obtained through micro-Raman spectroscopy are presented, illustrating stress reduction is possible through these post-processing techniques. The stress profiles presented demonstrate the variations between the MUMPs structural layers (Poly1 and Poly2) for different microbridge widths. The improved stress levels could significantly increase device performance, reliability, and yield.

Experimental Measurement of Fiber/Fiber Interaction using Micro Raman Spectroscopy

Abstract Understanding the stress concentrations that develop due to fiber failure in multi-fiber composites is crucial because it dictates the failure mode and toughness of a composite. Composites with strong fiber/fiber interaction (high stress concentrations) are often brittle while those with low fiber/fiber interaction are more ductile. In this study, the effect of fiber failure on neighboring fibers in graphite/epoxy composites was studied using Micro Raman Spectroscopy (MRS). The effect of interphase toughness and strength on the stress concentration factor (SCF) and the radius of the zone of influence, r∞ was studied. It was found that the toughness of the interphase strongly affects SCF and r∞ A brittle interphase was found to reduce r∞ from 30 fiber diameters to 20 and caused SCF to drop more rapidly with inter-fiber distance. The maximum value of SCF was not affected by the toughness of the interphase and was found to be 1.46 ± 0.05. On the other hand, a 50% decrease in the interfacial shear stress (from 40 MPa to 20 MPa) resulted in a further reduction in r∞ down to 10 fiber diameters and a reduction in the maximum SCF to 1.16 ± 0.05. The results are explained and discussed from both micromechanics and energy viewpoints.

Stress Concentration Phenomenon in Graphite/Epoxy Composites: Tension/Compression Effects

In composite materials, fiber/fiber interaction and the resulting stress concentration phenomenon due to a fiber failure are crucial in determining the composite fracture behavior. A number of mathematical models have been developed for calculating the stress concentration factor (SCF) and predicting its dependence on the properties of the composite constituents. None of the existing models separately considers the case of tension and compression. This could be due to the force-balance approach used, which assumes the phenomenon to be the same in both cases. In this study, micro-Raman spectroscopy (MRS) was used to investigate the stress concentration phenomenon and measure experimentally the stress concentration factor in graphite/epoxy composites subjected to tensile and compressive stress. The results showed that the stress concentration in tension is different from that in compression. The radius of the zone of influence is much higher when the composite is under compressive stresses. The maximum SCF in both cases, however, was 1.5 ± 0.05, limited by the ability of the interface to transfer shear stresses. The difference in stress concentration is attributed primarily to the difference in the interfacial behavior of the composite under tension and compression. The results are discussed and explained from both fracture mechanics and energy viewpoints.

NEW METHODOLOGY FOR DETERMINING IN SITU FIBER, MATRIX AND INTERFACE STRESSES IN DAMAGED MULTIFIBER COMPOSITES

Two recent developments, experimental Micro-Raman spectroscopy (MRS) and theoretical quadratic influence superposition (QIS) analysis, have greatly improved micromechanical measurement and realistic numerical modeling of fiber, matrix and interface stresses around fiber breaks in multifiber, polymer matrix composites. In this study, these two techniques are combined to develop a methodology for determining the in situ interfacial strength parameters, such as the yield stress and frictional sliding resistance, which are major determinants of local deformation and damage propagation around fiber breaks. With a spatial resolution of 2 μπ\, MRS is used to measure fiber axial strain profiles produced by naturally occurring fiber breaks in multifiber, high modulus graphite/epoxy model composites under uniaxial tension. For the same matrix material and surfacetreated fibers, several fiber spacings and different interfacial conditions are tested including: sized fibers, unsized fibers and fibers with a PMMA coating applied using RF-Plasma grafting. The QIS micromechanical technique is then used to interpret the MRS data to quantify the complex, local in situ matrix and interface stresses and deformations, which can be accurately described by an elastic-plastic-debond (multiparameter) micromechanical model. These quantities are found to depend highly on interface condition and local fiber spacing, thus motivating questions about the usefulness of traditional single-fiber-composite tests in forecasting localized failure in large composites. Comparing the stress concentration profiles on intact neighboring fibers measured experimentally and predicted by QIS reveals the complex dependence on fiber spacing and the extent and type of interfacial damage. In a few examples, it is shown how the multi-parameter description of the interface, calibrated at the single fiber break level, can be used as input for analysis and prediction of activity around more complex fiber break arrangements in a much larger composite. Based on the present results, recommendations for further investigations, using MRS-QIS, are given.

Introduction of compressive residual stresses in Ti-6Al-4V simulated airfoils via laser shock processing

Ti-6Al-4V (Ti-64) simulated airfoils were laser shock processed with two laser power densities (4 and 9 GW/2) for each of three pulse repetition treatments (1, 3, and 5 shocks/spot). The microstructural effects of laser shock processing (LSP) on the Ti-64 were studied via scanning electron microscopy (SEM). Ultrasonic nondestructive inspection (NDI) was conducted to ensure that the LSP treatments resulted in no internal damage to the simulated airfoils. In-depth residual stress and cold work measurements were made using x-ray diffraction.No substantial changes due to LSP were found in the microstructure, and no internal damage was detected during NDI or metallographic sectioning. It was found that the in-depth residual stress and cold work states induced by LSP were a function of laser power density and pulse repetition. It was possible to induce compressive residual stresses in the direction most critical for the prevention of fatigue-crack growth throughout the thickness of the simulated airfoil leading edge.

Relating hydrothermal degradation in single fibre composites to degradation behaviour in bulk composites

Abstract Previous studies on single fibre composites using micro-Raman spectroscopy revealed that the primary interfacial degradation mechanism in graphite/epoxy composites after hydrothermal exposure is mechanical in nature. Mechanical degradation of the interface is the result of a complex state of stress created at the interface due to matrix swelling upon water absorption. The major component of such stresses is a radial tensile stress which causes a tensile failure of the interfacial bonds during exposure. To date, the effect of fibre volume fraction on the state of stress created at the interface due to matrix swelling has not been considered. In this paper, finite element analysis (FEA) is used to model bulk composites with different fibre volume fractions to determine the applicability of single fibre composite behaviour to that of bulk composites. The analysis showed that the mechanical degradation mechanism can operate at all possible volume fractions and that in the case of a non-homogeneous fibre distribution, regions with both very high or very low fibre content are more susceptible to environmental degradation. Experimental results obtained from bulk composites (V f ≈63–71%) confirmed the FEA results and showed that the interfacial degradation in that case was more severe than in the case of single fibre composites.