K. K. Chawla

Nanoindentation Behavior of Nanolayered Metal-Ceramic Composites

Small-length scale multilayered structures are attractive materials due to their extremely high strength and flexibility, relative to conventional laminated composites. In this study, nanolayered laminated composites of Al and SiC were synthesized by DC/RF magnetron sputtering. The microstructure of the multilayered structures was characterized, and the mechanical properties measured by nanoindentation testing. The influence of layer thickness on Young’s modulus and hardness of individual and multilayers was quantified. An analytical model was used to subtract the contribution of the Si substrate, to extract the true modulus of the films.

Electrophoretic Deposition of Carbon Nanotubes on Metallic Surfaces

A method based on electrophoretic deposition has been developed to produce uniform deposits of multi-walled carbon nanotubes on stainless steel substrates. Aqueous suspensions were used under constant voltage conditions in the range of 5 to 50 V, with deposition times ranging from 0.5 to 10 minutes. The thickness of the coatings was controlled by variation of voltage and deposition time during EPD. Coatings of up to 100μm thickness were achieved, which exhibit homogeneous microstructure. The EPD technique is fast, cost-effective, and it can be applied to complex shapes. Possible applications of CNT coatings are in heat extraction devices or porous nanostructured coatings for tissue engineering scaffolds.

Phase identification in reactively sintered molybdenum disilicide composites

Abstract The objective of this research was to identify and quantify the phases formed in reactively sintered MoSi 2 and MoSi 2 composites made from carbon coated MoSi 2 powders. The purpose of adding carbon was to form silicon carbide particles (SiC p ) in situ and reduce the inherent presence of SiO 2 in MoSi 2 . The carbon additions were made via polymeric coatings on MoSi 2 particles by two processes, Phenolic Resin Based Carbon by Solvent Evaporation (PRBCSE) and Aqueous Dispersion Flocculation (ADF). The sintering temperatures and times ranged from 1600 to 1800°C, and from 1 to 100 h, respectively. The addition of carbon did reduce the presence of SiO 2 and there was formation of SiC p . However, the SiC p formation was less than expected, a maximum of 1.4 vol.% SiC p was formed. The Nowotny phase (Mo ≤4 8 Si 3 C ≤0.6 ) was observed in the sintered samples. The in situ formation of SiC p would increase the toughness of MoSi 2 by serving as a reinforcement. The reduction in the amount of SiO 2 would reduce the incidence of grain boundary sliding caused by viscous flow of SiO 2 at elevated temperatures.

Processing and Microstructure of an All-Oxide Ceramic Composite

An all-oxide composite consisting of alumina fiber, alumina matrix, and barium zirconate interphase has been investigated. The barium zirconate interphase was applied on alumina fibers through coating via a sol-gel route. The incorporation of the coatings did not significantly influence the densification behavior of the composite under hot-pressing conditions. During the processing of the composite, the barium zirconate reacted in situ with alumina fiber and alumina matrix to form Ba-β-alumina platelets with an elongated morphology, which is propitious for crack deflection and thus toughness enhancement. The results reveal that it is possible to reduce fiber strength degradation by controlling the coating and densification processes.

Fabrication, mechanical properties and thermal stability of a novel glass matrix composite material reinforced by short oxycarbide fibres

An aluminosilicate glass matrix composite material reinforced by randomly oriented SiC-based (Tyranno™) chopped fibres was fabricated. Slurry dipping and hot-pressing techniques were used to prepare dense composites containing 45 vol% fibres uniformly dispersed in the glass matrix. The mechanical properties and fracture mechanisms of the composite under flexion and compression loading were studied. In flexure, the composite showed higher modulus and strength than the unreinforced glass. However, in compression, the strength of the composite was lower than that of the monolithic glass. Considering the potential application of the material at high temperatures, the thermal aging behaviour of the composite in air at temperatures between 500 and 700°C was investigated. The composite retained its room-temperature compressive strength after exposure for 26 h at 500°C. The variation of compressive strength measured after exposures at higher temperatures was ascribed to mechanisms of fibre/matrix interface oxidation and to the softening of the glass matrix.

Preliminary Studies in the Processing and Characterization of Al2O3/SnO2 Laminated Composites

All oxide composites (reinforcement and matrix both being oxides) exhibit high temperature oxidation resistance in addition to high strength and hardness. A major drawback of these materials is that the oxide fiber and oxide matrix tend to react, which strengthens the interface and therefore drastically reduces the damage tolerance. To overcome this problem, a mechanically weak interphase material, which also serves as a diffusion barrier, is generally used. One such materials system is tin dioxide (SnO2) in alumina-based composites. Previous attempts to fabricate such alumina matrix composites have been unsuccessful due to the higher temperatures needed to densify Al2O3 coupled with the fact that SnO2 decomposes to SnO in reducing environments. SnO has a relatively low melting point (1125 °C). In this paper we report the successful fabrication of Al2O3/SnO2, laminated composites and some observations on microstructural and mechanical characterization of the laminates. As expected from the phase diagram, no chemical compound formation was observed between Al2O3 and SnO2 which means that no primary chemical bonding developed between individual laminae. TEM observations showed, however, a strong mechanical interlocking at the SnO2/Al2O3 interfaces. In spite of the relatively strong interfacial bond, cracks did deflect. Our microstructural studies showed that SnO2 served as a weak interphase material.

Two-dimensional microstructure based modelling of Young's modulus of long fibre thermoplastic composite

Abstract The dependence of physical and mechanical properties on microstructure is well known. Various numerical and analytical methods are routinely employed to predict the properties of multiphase materials but these models make certain simplifying assumptions about the microstructure of the material, such as homogeneity, that are not accurate. In the present work, a microstructure based finite element code called object oriented finite element method (OOF) has been used to investigate the longitudinal elastic modulus of a long fibre reinforced thermoplastic (LFT) composite (glass fibre/polypropylene). The modulus value predicted by OOF was then compared with the experimental value and values predicted by various models. It is shown that by taking into account the important microstructural parameters in the composite, i.e. the actual fibre orientation and distribution, accurate prediction of modulus can be obtained.

Characterization of the binder phase in a three-phase carbon microballoon syntactic foam

In three-phase syntactic foams, the (often polymeric) binder phase plays the vital role of holding the compressive load bearing microballoon components in place. When characterizing three-phase syntactic foams, it is important to characterize this binder phase. To obtain thermal and mechanical properties, we have performed differential scanning calorimetry and nanoindentation on samples of the binder in a three-phase carbon microballoon syntactic foam system. The binder is a specific variety of bismaleamide, APO-BMI, differing from the prototypical bismaleamide by an extra methyl group and two sulfur atoms between the central benzene rings. Through differential scanning calorimetry, we determined that the melting temperature of neat APO-BMI is 120 °C and that curing begins at 230 °C. The presence of carbon microballoons was seen to affect the curing reaction of APO-BMI, reducing the onset of curing temperature. The Youngs modulus of cured APO-BMI was determined by nanoindentation to be 6.6 GPa.

Dynamic elastic modulus and vibrational damping in nicalon SiCxOy fiber/borosilicate glass composites : Effects of thermal cycling

Measurements of dynamic elastic modulus and vibrational damping were made at room temperature for DURAN (a borosilicate glass) and SiCxOy Nicalon™/DURAN (a glass matrix composite). Both sets of materials had been thermal cycled to 500 and 700°C which are below and above the glass transition temperature (Tg = 530°C), respectively. The piezoelectric ultrasonic composite oscillator technique (PUCOT) was used to determine the values of the Young’s modulus and damping. Archimedes’ method was used to find the density of the specimens, and the impulse excitation technique was used to find the flexural modulus. Microstructural examinations were made on selected specimens. The experimental results showed that thermal cycling of the composites below Tg had no distinguishable effect on the density, dynamic Young’s modulus or flexural modulus values; however, an increase in damping of 56% was observed. For thermal cycling above Tg, the density decreased by about 0.5%, the Young’s modulus decreased by 8%, the flexural modulus decreased by 15% and the damping increased by 608%. The simultaneous decrease of elastic modulus and density, and increase of damping in the composites with increasing thermal cycling temperature were analyzed in terms of microstructural degradation due to thermal effects on the matrix, fibers and interfaces.