Ceramic matrix cimposites

1. Lecture 6.3: Powder Processing: Ceramic Matrix Composites
Challenges in Processing of Ceramic Matrix Composites
There are certain issues and challenges which limit the processing of ceramic matrix composites
and therefore, there application spectrum is also limited. The following points should be taken
care of during processing of ceramic matric composites:
 Processing routes for CMCs involve high temperatures – can only be employed
with high temperature reinforcements.
 The high temperature properties of the reinforcement are also important during
service.
 Difference in the coefficients of thermal expansion between the matrix and the
reinforcement lead to thermal stresses after cooling from the processing
temperatures.
Processing approaches for CMCs
Solid, liquid, or gas phase processing of ceramic matrix composites typically involve the
infiltration of the matrix onto the reinforcement while processing. During processing of ceramic
matrix composites, main objectives are attaining least porosity, uniform distribution of
reinforcement and excellent bonding strength between fiber and matrix.
Number of processing techniques has been explored for ceramic matrix composites, the most
predominant processing methods for ceramic matrix composites include:
(1) Powder Processing/Hot /Cold Pressing
(2) Slurry Infiltration/Impregnation
(3) Polymer Infiltration and Pyrolysis
(4) Chemical Vapor Infiltration/Impregnation
(5) Reaction Bonding Processes
(6) Directed Oxidation
Powder Processing
This technique is generally used to fabricate discontinuously reinforced ceramic matrix
composites. The process is only efficient for very small reinforcement such as whiskers and

2. particulates. Processing of long discontinuous fibers is troublesome as it breaks into short fibers
during mixing and consolidation phase. The basic processing steps involved in powder
processing are mixing (short fibers/ whiskers with slurry of ceramic powder), drying and hot
pressing. Typical fabrication sequence for powder processing is shown in Figure 1.
Figure 1 Fabrication steps in powder processing
The common challenge in processing of composite materials is voids content in the final product
that can be minimized through uniform dispersion of the reinforcement and matrix powder.
Another way to improve consolidation and diminish porosity is to use super fine ceramic
particles and hot pressing or hot isostatic pressing (HIP). Hot pressing is typically used to
produce various cutting tools that can be used to machine hard-to-machine materials. Optimum
packing can be achieved when the particle size distribution contains about 30% by volume of
small particles and 70% by volume of large particles. Addition of whiskers to slurry can result in
undesirable increase in viscosity. Furthermore, whiskers with a large aspect ratio, greater than
50, tend to tangle and form bundles or loose clumps.
Mixing of raw materials
(matrix and reinforcement)
Green body fabrication
(cold pressing, injection molding)
Green body machining
Binder burn-out
Consolidation and densification
(sintering, hot pressing & HIP)
Inspection

3. Near net shape can be typically produced by introducing binder to the reinforcement and matrix
through cold forming processes such as uniaxial pressing, cold isostatic pressing, tape casting,
extrusion and injection molding. Secondary operations can be performed on green body without
facilitating any damage after cold pressing. Binders should be burnt out during the consolidation
process.
Challenges in powder processing
The steps involved in powder processing are simple but there are many challenges involved in
processing of ceramic matrix composites through powder processing. Some of these challenges
are:
1. Hydrostatic tensile stress may be developed due to difference in coefficient of
thermal expansion of the reinforcement and matrix.
2. Homogeneous mixture of the constituents cannot be achieved readily.
3. High proportions of the toughening phase cannot easily be achieved.
4. Mixing and pressing operations results in damage to whiskers.
5. Fibers and whiskers can form a network that may inhibit the sintering process.
6. Reduced packing efficiency.