Ceramic matrix composites characteristics

As discussed previously, ceramic matrix composites were originally developed to overcome the brittleness of monolithic ceramics. Thermal shock, impact and creep resistance can also be improved, making CMCs premium replacement choices for some technical ceramics. Industrial applications such as in automotive gas turbines or advanced cutting tools are already taking advantage of such characteristics. 

The applications of glass/glass-ceramic matrix composites (CMC) can be divided into two specific categories aerospace applications and non-aerospace applications. In aerospace applications, performance is the prime consideration, while in non-aerospace applications cost-effectiveness is paramount. The characteristic properties of materials for aerospace applications should be... 

This chapter describes the preparation and examination of ceramic matrix composites realized by the addition of different carbon polymorphs (carbon black nanograins, graphite micrograins, carbon fibers and carbon nanotubes) to silicon nitride matrices. In the following sections, structural, morphological and mechanical characteristics of carbon-containing silicon nitride ceramics are presented. 

SiC whiskers are known to vary in their physical and chemical characteristics from manufacturer to manufacturer and in some cases from batch to batch. While the differences can be quite minor, they can make major impacts in the performance of ceramic matrix composites both at room and elevated temperature. 

Note that for the asymptotic equations of Eqns. (2) and (3) to be valid, r

characteristic length, and is normally the crack length or the remaining ligament, whichever is the smaller, of a fracture specimen. Also, the above asymptotic equations are not valid for an orthotropic elastic continuum, such as a ceramic fiber/ceramic matrix composite. While the static crack tip state for an orthotropic elastic continuum has been derived, to the author s knowledge, no dynamic counterpart is available to date. Nevertheless, the above crack tip state should be applicable to particulate/whisker-filled ceramic matrix composites which macroscopically behave like an isotropic homogeneous continuum. 

Characteristics of Fibre-reinforced Ceramic-matrix Composites... 

For each phenomenon, there are also many elements involved which determine the behaviour of each phenomenon. These phenomena are described by a wide range of characteristic time and length values. For the case of CVI fabrication of fibre-reinforced ceramic-matrix composites, the diameter of a molecule and the thickness of the interfacial phase are about 10 1 run and 102nm respectively, whilst the sizes of the substrate/component and the reaction are around 1 m. In addition, elementary chemical reactions occur in a time range of 10 " to 10 4 s, the time for heat transfer and mass transfer is around 1 s to 10 min. By contrast, the total densification time for one CVI run is as long as approximately 102 h. In such cases, it is necessary to establish multiscale models to understand and optimise a CVD process. 

An understanding of the above characteristics and requirements for materials science and engineering forms the basis of the structure of this book, as it summarises precisely the essential knowledge requirements for CVD technology. Whilst the authors tackle a wide range of theoretical topics, the focus of the book is on the fibre-reinforced ceramic matrix composites used by the CVD or chemical vapour infiltration (CVI) processes. Based on the requirement of a systematic understanding of CVD processes, the related materials by some special CVD techniques and their potential applications, the book is structured as follows. 

As fiber is a primary component in continuous fiber reinforced ceramic matrix composites, its characteristic is an important factor that confines the thermal conductivity of the composites. The ideal SiC fiber should be highly crystalline, oxygen-free, and stoichiometric. As shows in Table I,... 

Originally, requirements in aerospace applications played a decisive role in developing ceramic matrix composites. Selection criteria for materials in power plants, heat shield systems for space shuttles and rockets, were a desperate temperature resistance and good characteristics considering its mass. In practice, one of the first... 

Depending upon what type of host matrix material is used in creating the composite material, the composites may be classified into three classes (1) polymer-matrbe composites, (2) metal-matrix composites, and (3) ceramic-matrix composites. We discussed the characteristics of matrix materials earlier when we covered metals and plastics. 

The mechanical behavior of CVI SiC/SiC composites exhibits features which differentiate these composites from monolithic ceramics, from other ceramic matrix composites and from other composites. These features depend on composite microstructure, interphases, fiber and matrix properties. The main characteristics of CVI SiC/SiC composites are examined i.e. the applications, the Chemical Vapor Infiltration process and properties. Main properties are discussed with respect to features of the mechanical behavior. 

Xu Y, Cheng L, Zhang L, Yan D, Mechanical properties and microstructural characteristics of carbon fibre reinforced silicon carbide matrix composites by chemical vapour infiltration, Niihara K, Nakano K, Sekino T, Yasuda E eds.. Ceramic Society of Japan, High Temperature Ceramic Matrix Composites III, Proc 3rd Int Conf, Osaka, Sep 6-9 1998, 73-16, Key Eng Mater, Vol 164-165. 

All the SiC fibres are circular in cross-section with diameters of 15 p,m or less. The earliest SiC fibres, the Nicalon 100 series, showed all the fracture characteristics of a glassy structure, although TEM studies showed the presence of very small SiC grains of less than 2 nm as well as even smaller free carbon particles. The Nicalon 200 series became the standard fibre used for most ceramic matrix composites. The fibre has a diameter of 15 p.m and also shows a glassy fracture morphology, as Fig. 2 reveals. 

The classification of C/C composites, and ceramic matrix composites, is generally based on the number of directions of the reinforcement architecture. Sometimes it is also completed by the nature of the fiber and/or of the matrix. Occasionally it can also take into account a physical characteristic (high or low density high or low thermal conductivity). A more sophisticated classification is under development, based on the type of fiber, reinforcement architecture, matrix type, fiber fraction volume, density, porosity, tensile strength, and modulus at room temperature (ASTM WK49676). 

Metals and ceramics (claylike materials) are also used as matrices in advanced composites. In most cases, metal matrix composites consist of aluminum, magnesium, copper, or titanium alloys of these metals or intermetallic compounds, such as TiAl and NiAl. The reinforcement is usually a ceramic material such as boron carbide (B4C), silicon carbide (SiC), aluminum oxide (A1203), aluminum nitride (AlN), or boron nitride (BN). Metals have also been used as reinforcements in metal matrices. For example, the physical characteristics of some types of steel have been improved by the addition of aluminum fibers. The reinforcement is usually added in the form of particles, whiskers, plates, or fibers. 

Composites may use ceramics as the matrix phase and/or the reinforcing phase. The purpose of a composite is to display a combination of the preferred characteristics of each of the components. In CMCs one of the principal goals has been to increase fracture toughness through reinforcement with whiskers or fibers. When ceramics are the reinforcement phase in, for example, metal matrix composites the result is usually an increase in strength, enhanced creep resistance, and greater wear resistance. Three issues must be solved ... 

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