Slurry infiltration, liquid infiltration,

1. Lecture 6.5: Ceramic Matrix Composites: Processing
This lecture mainly focuses on the two important techniques of processing ceramic matrix
composites. These techniques are slurry infiltration and liquid infiltration process. In these
techniques, reinforcement is in solid state and matrix is in liquid state.
Slurry Infiltration/Impregnation
Slurry infiltration process is used to fabricate ceramic matrix composites. In this process, the
matrix in the form of a liquid or slurry which infiltrates the fibre preform resulting in a
composite. In other words, in this process, impregnation of the reinforcing phase takes place in a
tank containing the matrix phase (liquid slurry).
The slurry typically consists of the following:
 Matrix powder
 Liquid carrier (water or alcohol)
 An organic binder
Various parameters such as particle size distribution, binder type and amount, powder content
and carrier medium have a significant effect on composite part quality. The matrix powder is the
most important aspect of this process as the liquid carrier and the organic binder is removed
during the process.
The particle size of the matrix powder should be less than the fibre diameter as it results in
reducing porosity as well as leads to thorough impregnation. Infiltration into the fiber preform
can be improved by adding certain wetting agents in the slurry. After infiltration, the liquid
carrier is allowed to evaporate.
The resulting prepreg (a combination of fibre reinforcement and the slurry) can then be layed-up
on a tool for consolidation. The organic binder must be burnt out before starting the
consolidation process.
Processing stages
The slurry infiltration process involves two main stages (Figure 1):
Stage-1: Incorporation of the reinforcing phase into slurry of the consolidated matrix.

2. The fibres are impregnated by passing them through the slurry tank. The impregnated fibres are
then taken over by the take up drum. The prepreg thus formed is in the form of tapes which is cut
into the desired size. Then, the different layers of tapes are stacked together and the binder is
burnt out before the consolidation takes place.
Stage-2: Matrix consolidation by hot pressing.
After the burning of the binder from the stacked tapes, it is subjected to consolidation by hot
pressing which results in the desired ceramic matrix composite.
Figure 1 Slurry Infiltration Process
Advantages
 Low porosity (due to hot pressing and use of continuous fiber reinforcement).
 Good mechanical properties. (because of continuous fibre reinforcement).
 Composites with uniform fiber distribution can be processed.
Disadvantages
 High melting point matrix materials cannot be processed.
 Damage of reinforcing phase may occur during the hot pressing.

3.  Relatively small and simple parts can be be fabricated.
Liquid infiltration
It is similar to the resin transfer molding process which is used for the processing of polymer
matrix composites. The schematic of liquid infiltration process is shown in figure 2.
The three major issues to be considered in the liquid infiltration process are:
a) Chemical reactivity: The process is done at an elevated temperature and at high
temperatures; the reinforcement and the matrix may react resulting in unnecessary
reaction which deteriorates the bonding between the reinforcement phase and the matrix
phase.
b) Melt viscosity: The melt viscosity of the ceramics is high compared to that of
metals, which can result in the inability of the ceramic infiltrant to infiltrate into the
fibrous ceramic preform.
c) Wettability of the reinforcement: The infiltrant may not be able to wet the
reinforcement resulting in improper bonding which further leads to the failure of the
product at the interface of reinforcement and the matrix.
Figure 2 Liquid Infiltration Process

4. The preform is kept in a closed chamber which is surrounded by the heating coils to attain the
desired temperature for the infiltration to take place as shown in the figure 2. The infiltrant
infiltrates into the preform under pressure with the help of a piston as shown in figure. 2.
Advantages
 Matrix of homogeneous structure can be processed.
 The matrix can be formed in a single processing step.
 Fiber preform in any form (fiber, whisker or particle) can be infiltrated by this
process.
Disadvantages
 Infiltration of performs is a complicated task as the ceramics have higher melt
viscosities than metals.
 The differential shrinkage between the matrix and reinforcing phase causes crack
formation during solidification.
 High melting points of ceramics results in greater possibility of reaction between
the liquid matrix and the reinforcement.