Fibre-matrix composite processing

Naslain, R. (1993). Fibre-matrix interphases and interfaces in ceramic matrix composites processed by CVI. Composite Interfaces, 1, 253-286. 

Lanxide A process for making composites of metals with oxides. A molten metal reacts with an adjacent oxidant and is progressively drawn through its own oxidation product so as to yield a ceramic/metal composite. Fibres or other reinforcing materials can be placed in the path of the oxidation reaction and so incorporated in the final product. The Lanxide Corporation was founded in 1983 in Newark, DE, to exploit this invention. In 1990 it formed a joint venture with Du Pont to make electronic components by this process. Variations are Dimox (directed metal oxidation), for making ceramic metal composites, and Primex (pressureless infiltration by metal), for making metal matrix composites. 

Whisker-reinforced glass-ceramic matrices are expected to find several applications in automotive components, metal forming, cutting tools, etc., due to their low thermal expansion, high thermal shock resistance, high reliability and low material and processing costs. Some industrial applications for continuous fibre-reinforced ceramic matrix composites (CMCs) are listed below. 

Short fibre polymer composites are being increasingly used as engineering materials because they provide mechanical properties superior to neat polymers and can be processed easily by the same fabrication methods, e.g. injection moulding. The mechanical properties of these materials are dependent on a complex combination of several internal variables, such as type of matrix, fibre-matrix interface, fibre content, fibre dimensions, fibre orientation, and external... 

Thermal residual stresses are inherent to fibre reinforced composites due to the heterogeneity of the thermo-mechanical properties of their two constituents. Such stresses build up when composite structures are cooled down from the processing temperature to the test temperature. Residual stresses will be present on both a fibre-matrix scale (micro-scale), and on a ply-to-ply scale (macro-scale) in laminates built up from layers with different orientations. It is recognised that these stresses should be taken into account in any stress analysis. 

The market needs for more advanced materials provide the driving force of the CVD processes. For example, the development of high performance aero-engines requires the strong and tough fibre reinforced ceramic matrix composites with low density which can potentially be used in ultra-high temperatures. 

Chemical vapour infiltration (CVI) is an extension of CVD processes only when a CVD process occurs on an internal surface of a porous substrate (especially for the fibre preform). As compared with CVD, the CVI process for ceramics is much more effective and important because it is the optimal technique to fabricate fibre reinforced ceramics and particularly carbon fibre reinforced carbon and advanced ceramic matrix composites. Both CVI and CVD techniques share some common features in overall chemistry, however, the CVI is much more complex than the CVD process in mass transport and chemical reactions. 

In such conditions the brittle ceramic fibres, such as Nicalon SiC and A1203 fibres, remain undamaged during the CVI process. However, conventional techniques for the fabrication of ceramic-matrix composites such as hot pressing take place at extremely high temperatures (2000°C) and under high mechanical stresses (30 MPa), which usually severely damage the fibres. 

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. 

The fibres are not resistant to the temperatures encountered in metal and ceramic matrix composite melt processing, and protective coatings or low-temperature manufacturing routes are needed. 

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