Developments in Ceramic Materials Research

Modeling of Thermal Transport in Ceramics Matrix Composites 175
But first, for each porosity classification, finite element analysis techniques are used to
quantify the effect of each class of porosity on the spatial heat transport properties assessed at
the level of a micro Unit Cell. In the analysis care is taken to accurately model porosity
volume fractions and characteristic defect lengths. However, thermal properties determined
using one model is included in subsequent models. It is in this way that the synergy between
different classes of porosity is assessed.
The two CMC materials used for modeling here are: (1) DLR-XT, a [0/90] plain weave
laminate, and (2) HITCO, a complex 8-Satin Weave CMC. Geometric models for both are
built from SEM Micrographs. For DLR-XT a Unit Cell model is built using 4 quarter parts
which assemble together to form the Unit Cell with fibre tows and matrix together. In
contrast, the Hitco Unit Cell is created as a single part since this Representative Volume
Element (RVE) is the smallest and unique geometric entity that cannot be further simplified.
For the thermal characterization of the two chosen materials through extensive finite
element modelling, two thermal properties are evaluated using steady-state and transient
analysis respectively. First is Thermal conductivity which is one of the driving forces in
designing materials for thermal applications. The other is thermal diffusivity, which is an
important parameter for controlling thermal transport, defined as the ratio of a material’s
capacity to conduct heat versus its capacity to store it. Experimental work is conducted for
validation purposes by making thermal diffusivity measurement by the laser flash method [8].
The materials are thermally analysed with and without the external mechanical load in order
to establish a correlation between the mechanical damage on a CMC causing degradation of
its thermal diffusivity.
With the increase in the geometric complexity of the CMCs, the requirement of the
computing resources rises considerably. This is reiterated by the current modeling effort with
HITCO 8-harness satin weave geometry. It is shown that with the multiplication of the Unit
Cell (each containing around a million elements) across the lamina and then the laminate,
demand for computational power increases drastically. Special arrangement is therefore
required for conducting analysis of such large FE models. The solution is shown here to be
the use parallel processors in a High Performance Computing (HPC) environment.
2. CERAMIC MATRIX COMPOSITES (CMCS)
Generally, ceramics cover a wide variety of non-metallic, inorganic compounds that are
frequently processed at high temperatures. Starting compounds of ceramics can be borides,
carbides, nitrides, oxides, silicides, phosphides, chalcogenides and their complex compounds,
both natural and synthetic [9]. Compared to metals these compounds have higher melting
temperatures, elastic moduli and hardness, and lower densities, and electrical and thermal
conductivities.
Ceramics can be broadly grouped into two classes, namely, conventional ceramics and
high performance ceramics. Technical ceramics, which are employed for the fabrication of
CMCs, exhibit extraordinary resistance to heat, chemicals and wear [10].
Fibres used in ceramic matrix composites fall into three general categories based on their
diameter: monofilaments, textile fibres and whiskers. In addition, reinforcements in the form
of particulates and platelets are also being utilised in ceramic composite designs. In ceramic