7. Ceramic Matrix Composites

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7. Ceramic Matrix Composites
strenth , stiffness
temperature
Ceramic matrials
chemical inertness
low density
sensitive flaw
thermal shock

Ceramic matrix composites
? CMC is low on the ratio of fiber and matrix
1. Fabrication of CMC
Porosity !!
The matrix powder particle smaller than the fiber
diameter !
Powder size , temp, pressure are parameter that
Control the matrix prosity
2
Melt infiltration , in situ chemical reaction,
and sol-gel and polymer pyrolysis are the other
techniques That have been used to produced
Chemical vapor infiltration (CVI)
? low stress
process combines thermal gradient and
pressure gradient approaches
Chemical vapor deposition (CVD)
? low temp
? lower temp, greater homogeneity in
single-phase matrix, potential of unique
multiphase matrix materials
Sol-gel polymer pyrosis technique
2. properties of CMC
The ratio Ef/Em determines the extent of matrix
microcracking
? ceramic matrix fracture strain tend to be
rather low
strongly bonded CMC- fiber matrix would fail
at matrix failure train
weakly bonded CMC- the fibers will be bridging
the matrix blocks But a weaker interface, would
lead to fiber-bridging of matrix microcracks.
High matrix mouldus composites, matrix cracking
would occur at much lower stress.
3
fiber length ( the fiber ratio
length/diameter), fiber orientation, strength
moduli of fiber matrix, Thermal mismatch,
matrix porosity, fiber flaw factor of CMC
process !
aligned continuous fibers do lead to a real
fiber reinforcement effect !
Stress at fiber ends is minimized and higher
fiber volume fractions can be obtained but it
becomes Difficult to remove matrix porosity
3. Interface in CMC
?- linear expansion coefficients
?? ?f-?m

?? () the matrix is compressed on cooling
good !
?? (-) the matrix is tensioned on cooling
matrix cracking
-In the radial direction
?? () the fiber tend to shrink away from the
matrix on cooling
?? () the fiber and matrix bond strength can
even be improved.
Matrix cracking resulting from thermal mismatch
is a more serious problem in short-fiber
composites
? short-fiber composite owing to increase stress
at the fiber ends, matrix cracking occurs in all
direction
? thus, the composite is very weak !
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Mechanical bonding
chemical bonding
chemical compatibility between matrix and the
fiber zirconia reinforced magnesia composite
system
? heat treatment is improved bonding !
4. Toughness of CMC
Fibers can play the role of toughening agents in
ceramic matrices
? tensile stress increases from a minium ar both
fiber Ends and attains a maxium along the central
portion Of the fiber
5
Use of a volume fraction of fiber ?stiffer than
matrix ?needed to produce matrix microcracking ?
higher composite ultimate tensile stress as well
as a high creep resistance !
?A high volume fraction and a small fiber
diameter also provide a sufficient number of
fibers for crack bridging and postpone Crack
propagation to higher strain levels
5. applications of CMC
heat engines, special electronic/electrical
applications, energy conversion , military
system..
Commercial application of CMC is in cutting
tools