Developments in Ceramic Materials Research

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M. A. Sheik
Ten sheets of the woven carbon fibre fabric are assembled between forming platens;
which are then housed within a chemical vapour infiltration (CVI) chamber. The assemblage
is then CVI processed, and the effect of temperature and pressure results in the fibres within
each tow being embedded within a carbon matrix, created by diffusion of carbon from the
fibres. The tows are, in turn, embedded within the infiltrated silicon carbide matrix. The
process involves time-dependent temperature and pressure distributions within the structure
of the laminate. Figure 5 schematically represents a plain weave arrangement and Figures 4
and 6 illustrate the geometric configuration and coordinate global system of this material.
3.2. Classes of Porosities
Internal damage in the form of cracks and voids can be introduced either during
manufacturing or in service. During manufacturing, damage is caused by a mismatch of
thermo-mechanical properties of the constituent materials, thus inducing thermal gradients,
thermal stresses, localised failure and hence damage when cooled. This manifests itself after
cooling as micro-porosity. Additionally, damage is created in service by mechanical
overloads, fatigue, time-dependent and environmental effects. As damage evolves, a limiting
condition is reached when an engineering component becomes mechanically unserviceable. It
then requires either repair or replacement. The dominant effect on material serviceability is
that the thermal transport properties are dramatically reduced due to the evolution of damage,
which can be highly directional, ultimately rendering the component thermally unserviceable
due to impaired thermal efficiency. Despite the existence of a very strong coupling between
mechanical behaviour and thermal properties, these relations are currently ill comprehended
and are not capable of being accurately predicted at the present time.
Paolo et al [16] have achieved the quantification as well as classification of initial
porosity through the usage of optical electron microscopy and SEM. These techniques have
been selected for their ease of use and availability; and, for their potential to cross check
observations and measurements. 4 classes of porosities have been identified and
schematically shown in Figure 7.
Figure 7. Schematic drawing of a general plane orthogonal to either the X or the Y-direction illustrating
the four classes of porosity. [Circled letters denote the porosity type].