13
1
1.4.3 Metal Matrix Composites (MMCs)
MMCs are composed of a metallic matrix (such as aluminium,
magnesium, and beryllium) using a continuous phase with highmelting temperature metals (such as tungsten and molybdenum) or
of non-oxide ceramics (such as silicon carbides, carbon, boron, and
alumina) using a dispersed phase. The reinforcing fibres with ionic
bonds have lower surface energies than the metals to be used as
matrix. The incompatibility in surface energy does not allow the
reinforcing fibres to make bonds with the metal matrix. To overcome this, the ceramic fibres are coated with a thin film of highmelting temperature metal that has a higher melting point that that
of the metal matrix. The MMCs are fabricated by many methods,
and the following are the most common:
5 Alternate stacking of reinforcing fibres (SiCs) and metal matrix
films (Al foil) are placed in the mould, and with the application
of heat and pressure, the MMC is formed.
5 Powder metallurgy technology uses a high-temperature
isostatic press to apply heat and pressure to form the MMC. It
uses metal matrix in the form of fine powder and reinforcement in the form of short fibre or whiskers.
5 Plasma spray or chemical vapour deposition (CVD) of metal
matrix over the reinforcing fibres yields a prepreg followed by
pressing this prepreg in an isostatic press to yield MMC.
5 Co extrusion of reinforcing fibres with a metal matrix yields
towpregs. Fabrication of MMC is carried out by placing a
towpreg in the mould and pressing it in isostatic press.
1.4.4 Carbon–Carbon Composites (CCCs)
CCC is a class of CMC. Although the initial processing of CCCs is
similar to PMC, there is no polymer present in the final composite.
It has carbon fibre as reinforcement and carbon as matrix. The common process of making CCC is by making a PMC having a high
char yield containing a polymeric matrix such phenol formaldehyde. This prepared PMC is pyrolyzed at a high temperature followed by carbonization and graphitization. Graphitization takes
place above 2700 °C. After every heat cycle, the polymeric resin is
infiltrated to fill the voids caused due to charring in order to have
good densification. After the last heating cycle, no polymeric resin
infiltration is carried out.
? Example 1.4 What makes the processability of CCC possible
starting with raw materials, such as pitch-based carbon fibres
and phenolic resin?
v Answer
In CCC, the matrix and reinforcement are both carbon.
Therefore, it is essential to have a matrix that yields into high
carbonous content (i.e. charring yield). Special grades of
Point to Ponder…
Ablation is a process in which an
ablative material provides thermal
protection from high temperatures
by sacrificing its mass under
thermal load. The temperature
in a rocket engine is in excess of
2500 °C whereas, ablative liners
keep the temperature < 50 °C
at the outside surface across
the thickness of approximately
50 mm.
1.4 · Classification
1
1.4.3 Metal Matrix Composites (MMCs)
MMCs are composed of a metallic matrix (such as aluminium,
magnesium, and beryllium) using a continuous phase with highmelting temperature metals (such as tungsten and molybdenum) or
of non-oxide ceramics (such as silicon carbides, carbon, boron, and
alumina) using a dispersed phase. The reinforcing fibres with ionic
bonds have lower surface energies than the metals to be used as
matrix. The incompatibility in surface energy does not allow the
reinforcing fibres to make bonds with the metal matrix. To overcome this, the ceramic fibres are coated with a thin film of highmelting temperature metal that has a higher melting point that that
of the metal matrix. The MMCs are fabricated by many methods,
and the following are the most common:
5 Alternate stacking of reinforcing fibres (SiCs) and metal matrix
films (Al foil) are placed in the mould, and with the application
of heat and pressure, the MMC is formed.
5 Powder metallurgy technology uses a high-temperature
isostatic press to apply heat and pressure to form the MMC. It
uses metal matrix in the form of fine powder and reinforcement in the form of short fibre or whiskers.
5 Plasma spray or chemical vapour deposition (CVD) of metal
matrix over the reinforcing fibres yields a prepreg followed by
pressing this prepreg in an isostatic press to yield MMC.
5 Co extrusion of reinforcing fibres with a metal matrix yields
towpregs. Fabrication of MMC is carried out by placing a
towpreg in the mould and pressing it in isostatic press.
1.4.4 Carbon–Carbon Composites (CCCs)
CCC is a class of CMC. Although the initial processing of CCCs is
similar to PMC, there is no polymer present in the final composite.
It has carbon fibre as reinforcement and carbon as matrix. The common process of making CCC is by making a PMC having a high
char yield containing a polymeric matrix such phenol formaldehyde. This prepared PMC is pyrolyzed at a high temperature followed by carbonization and graphitization. Graphitization takes
place above 2700 °C. After every heat cycle, the polymeric resin is
infiltrated to fill the voids caused due to charring in order to have
good densification. After the last heating cycle, no polymeric resin
infiltration is carried out.
? Example 1.4 What makes the processability of CCC possible
starting with raw materials, such as pitch-based carbon fibres
and phenolic resin?
v Answer
In CCC, the matrix and reinforcement are both carbon.
Therefore, it is essential to have a matrix that yields into high
carbonous content (i.e. charring yield). Special grades of
Point to Ponder…
Ablation is a process in which an
ablative material provides thermal
protection from high temperatures
by sacrificing its mass under
thermal load. The temperature
in a rocket engine is in excess of
2500 °C whereas, ablative liners
keep the temperature < 50 °C
at the outside surface across
the thickness of approximately
50 mm.
1.4 · Classification