Raj N. Singh

Determination of fibre-matrix interfacial properties in ceramic-matrix composites by a fibre push-out technique

A simple concentric cylinder model is developed for the fibre push-out test in order to interpret the experimentally observed indenter load-displacement curves in ceramic-matrix composites. The fibre-matrix interface is assumed to be partially bonded and partially frictionally coupled. It is shown that a slope change in the loading curve corresponds to bonding at the fibre-matrix interface. In contrast an insignificant change in the slope is predicted for composites in which the fibre-matrix interface is frictionally coupled. This model also provides a framework for determining the interfacial debond energy and the interfacial shear strength in ceramic composites using the fibre push-out tests. The predictions of this model are compared with the push-out test results performed on zircon-SiC composites uniaxially reinforced with either uncoated or BN-coated SiC monofilaments, which suggested that the fibre-matrix interfaces in both of these composites are frictionally coupled.

Influence of testing method on mechanical properties of ceramic matrix composites

Mechanical properties of a monolithic zircon and zircon-matrix composites reinforced with silicon carbide monofilaments and/or whiskers were measured in three-point flexure and uniaxial tension modes to study the influence of testing methods on mechanical behaviour. A number of composite characteristics, such as the first-matrix cracking stress and strain, the ultimate composite strength and strain, and the modulus were obtained from the load-deflection behaviour in flexure and tension tests. The results indicated that the modulus values and the qualitative dependence of mechanical properties on composite parameters were similar in flexure and tension tests. In contrast, all of the other mechanical properties of the monolithic and composites were different in tests performed in flexure and tension modes. Typically, the first-matrix cracking stress and strain were higher in flexure tests than in tension tests, and these stress and strain values were independent of the filament-matrix interfacial properties. Similarly, the ultimate strengths of the monolithic and composites were higher in flexure than in tension, and these strengths were independent of interfacial properties. Therefore, the mechanical properties of composites obtained in flexure should not be used for a quantitative comparison with the predictions of micromechanical models, which are derived under the assumption of a uniform tensile stress. However, the flexure data are perfectly valid in demonstrating the qualitative dependence of mechanical properties on composite parameters.

A uniaxially reinforced zircon-silicon carbide composite

Zircon matrix composites uniaxially reinforced with as-supplied and BN-coated silicon carbide filaments were fabricated, and their mechanical properties were measured in flexure mode. A toughened-composite behaviour was displayed by the reinforced samples with strengths between 681 and 700 MPa and toughness between 24 and 41 kJ m−2. In comparison, the monolithic zircon failed in a brittle manner, had an average strength of 280 MPa and toughness of 0.95 kJ m−2. Influence of fibre-matrix interfacial shear stress on the first matrix cracking stress, ultimate failure strength and strain, toughness, and mode of failure were studied. The composite toughness was found to be dependent on the interfacial shear stress whereas the first matrix cracking stress was independent of the interfacial shear stress. These results on mechanical properties are compared with predictions from composite models.

INFLUENCE OF HIGH-TEMPERATURE EXPOSURE ON MECHANICAL PROPERTIES OF ZIRCON-SILICON CARBIDE COMPOSITES

Thermal stability of zircon matrix composites uniaxially reinforced with either uncoated or BN-coated silicon carbide monofilaments was determined by measuring mechanical properties and fibre-matrix interfacial characteristics in the as-fabricated state and after annealing treatments between 25 and 1430 °C for times up to 100 h. Composites reinforced with uncoated silicon carbide filaments retained their mechanical properties and fibre-matrix interfacial characteristics up to 1350 °C for 100 h. In contrast, composites reinforced with BN-coated silicon carbide filaments displayed changes in mechanical properties and fibre-matrix interfacial characteristics when annealed beyond 1300 °C for 100 h. Both types of composite displayed a significant reduction in strength and toughness after annealing at 1430 °C for 20 h. These results are consistent with changes in fibre-matrix interfacial properties, and with changes in mechanical characteristics of zircon matrix and silicon carbide filament as a result of the high-temperature annealing treatments.

Chapter 2 - Metallurgical Properties of Thin Conducting Films

Publisher Summary This chapter discusses the metallurgical properties of thin conducting films. It describes the important aspects of microstructure, adhesion, oxidation, diffusion, and interaction in thin films of polysilicon, pure metal films, alloy metal films, suicides, and amorphous metal films with the help of theoretical background and experimental results relevant to very-large-scale-integration metallization. It also discusses the importance of grain boundaries in oxidation, diffusion, and interaction. The trends toward lower temperature processing will enable the applications of thin films that are unacceptable for high processing temperatures. Alloy metal films and multilayered structures will be used to enhance electromigration, adhesion, or provide diffusion barrier characteristics. Oxidation of polysilicon and suicides at lower processing temperatures will be studied to grow good quality thin oxide for isolation.