Developments in Ceramic Materials Research

176
M. A. Sheik
composites the reinforcements usually increase the strength indirectly by increasing the
toughness of the matrix.
The selection of matrix materials for ceramic composites is strongly influenced by
thermal stability and processing considerations. Common matrix materials include oxides,
carbides, nitrides, borides and silicides.
Engineered ceramics are used in thermal and structural applications requiring high
temperature resistance, high hardness and chemical inertness. Applications that exploit the
thermal structural properties of ceramics commonly include cutting tool inserts, wear resistant
components, ballistic armour, heat exchangers, burner tubes, prosthetics, dental implants, heat
engine components and thermal barrier coatings [11].
2.1. Manufacturing of CMCs
CMCs can be processed either by conventional powder processing techniques used for
making polycrystalline ceramics or through novel techniques specifically developed for such
purposes [11]. Since this chapter includes the modelling of a composite manufacturing
porosity, it is instructive to outline some of the important processing techniques [2], [12],
utilised in the fabrication of CMCs. Technical details of the manufacturing processes as well
as the relative merits and disadvantages of the different fabrication methods are also
discussed.
2.1.1. Infiltration
Infiltration of a preform constructed of reinforcement can be executed with a matrix
material in solid, liquid or gaseous form. Liquid infiltration demands proper control of the
fluidity of liquid matrix. The process results in a high density matrix without pores in the
matrix. Besides, only a single processing step is required to attain a homogeneous matrix.
However, high processing temperatures can cause unfavourable chemical reactions between
the reinforcement and the matrix. Ceramics have relatively high melt viscosities, which
render the infiltration of the preform a difficult operation. In addition, thermal expansion
mismatch between the reinforcement and the matrix, a large temperature interval between the
processing and room temperature as well as the low strain to failure of ceramics are factors
that impede the production of a CMC devoid of cracks. Similarly, the matrix is prone to
cracks due to the differential shrinkage between the matrix and the reinforcement on
solidification. Figure 1 schematically represents the liquid infiltration process.
2.1.2. Polymer Infiltration and Pyrolysis
Ceramic matrix in a composite can be accomplished from the usage of polymeric
precursors. Polymer infiltration and pyrolysis (PIP) constitutes an attractive processing route
because of the relatively low cost as compared to the generally high costs of CMC
fabrication. Apart from the cost factor, the process is able to maintain small amounts of
residual porosity and minimal degradation of fibres. Furthermore, this approach allows near
net-shape moulding and fabrication of composites near their full densities. In PIP, fibres are
infiltrated with an organic polymer, which is heated with raised temperatures and pyrolysed to
form a ceramic matrix. Due to the relatively low yield during the conversion from polymer to