Developments in Ceramic Materials Research

174
M. A. Sheik
1. INTRODUCTION
Ceramic Matrix Composites (CMCs) are considered to be possible replacement for
metallic super-alloys in high-temperature parts of aero-engines and heavy-duty gas turbines,
and for applications such as nozzles in rocket engines. Increased operating temperatures from
900-1200°C for coated super-alloys to above 1300°C for CMCs have the potential to achieve
higher thermal efficiencies and lower emissions [1]. Such applications require good thermal
properties for heat transfer in CMC engine components. Compared to metallic alloys, a
deficiency of CMCs is the degradation of thermal transport properties due to internal damage.
An ability to predict thermal transport in CMCs is a primary requirement at the design stage.
The presence of damage and cracks can be introduced in either manufacturing or in service.
Firstly, damage during manufacturing is introduced as a result of the different thermomechanical
properties of the constituent materials, which during cooling introduce thermal
gradients, thermal stresses, localized failure and hence damage. This manifests itself after
cooling as micro-porosity. Secondly, damage is created in service [2], [3] 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. The overall impact is to render the component thermally
unserviceable due to impaired thermal efficiency. There is a very strong coupling between
mechanical behavior and thermal properties which is not currently well understood; and is not
capable of being accurately predicted at the present time. Hence one of the drivers for current
research on CMCs is the need to describe and predict these effects at the design stage. This is
to be achieved through the establishment of effective modeling procedures.
The modelling of CMCs requires a number of factors to be taken into account, which
influences their thermal transport, such as the architecture of the composite, the properties of
constituent material, and the influence of defects [4]. Early approaches to the thermal finite
element modelling were two dimensional; for example unidirectional long fibre composites
have been studied by Lu and Hutchinson [5] for longitudinal thermal transport, and by Klett
et al.[6] for longitudinal and transverse thermal transport. The main limitation of such models
is their simplicity, which does not reflect the complexities of real composites. A complex
weave model of a plain weave CMC was presented by Sheikh et al. [7]. This model was 3dimensional,
and represented a relevant development towards the modelling of complex
composites architectures. They introduced the effect of directionality in thermal transport by
introducing the individual properties of fibre and matrix. However, the model was deficient
since the influence of initial porosity was not taken into account.
A developmental modelling strategy is set, which took account of the following factors:
(i) categorise manufacturing or initial porosity;
(ii) create an ability to numerically model it; and
(iii) subsequently, further develop this to a point where growth and coalescence of initial
porosity due to applied loadings and thermal strains can be accurately predicted; and,
in addition, couple this with the degradation of mechanical and thermal properties.