Biomedical Engineering Reference
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matrix chemical interactions. This chemical composition flexibility also
enables the development of glasses with a wide range of thermal expansion
coefficients (TECs) in order to match the TEC of the reinforcement. In
addition, by controlling the viscosity of glasses, high composite densities can
be achieved by promoting viscous flow during sintering. Finally, the low
elastic modulus of most glasses means that high-modulus fibers can provide
a true reinforcement, unlike the case of polycrystalline ceramic matrix
composites where the ratio of moduli is close to one (Chawla, 2003).
However, the main drawback of glasses is the low-temperature capability
compared with polycrystalline ceramics, which limits applications to modest
temperatures.
Carbon, silicon carbide, alumina, boron and metallic reinforcements have
been used in glass and glass-ceramic matrices in a variety of shapes,
including continuous and chopped fibers, particles, platelets and whiskers.
Continuous carbon fibers were the first fibers applied to silica reinforcement
(Crivelli-Visconti and Cooper, 1969), producing excellent toughness and
crack growth resistance; the composites exhibited non-catastrophic failure.
Borosilicate glasses and calcium-silicate glass-ceramic matrices have also
been reinforced with aligned carbon fibers for improved composite
toughness (Sambell et al., 1972), which was attributed to low fiber/matrix
interface strength promoting crack deflection. Prewo used continuous
carbon fibers to obtain exceptionally high elastic modulus glass matrix
composites (Prewo, 1982). The combination of carbon fibers with a glass
matrix offers other benefits since carbon fibers impart lubricity to the
composite surface and the glass matrix provides high hardness and wear
resistance, as well as dimensional stability (Prewo, 1981).
A wealth of literature on silicon carbide (SiC) fiber reinforced glass and
glass-ceramic composites is available, showing improved fracture strength
and toughness together with the potential for high-temperature oxidation
resistance. Monofilaments and large-diameter SiC fibers have been used as
reinforcements in different glass matrices (Aveston, 1971, Bansal, 1997, Cho
et al., 1995, Prewo and Brennan, 1980). Moreover, SiC yarns have been used
to develop high-strength composites (Prewo and Brennan, 1980, 1982); the
highest strength was achieved by using a lithium aluminosilicate matrix
(Brennan and Prewo, 1982). The addition of SiC whiskers also substantially
increased the strength of lithium aluminosilicate (LAS) matrices (Gadkaree
and Chyung, 1986).
Glass and glass-ceramic matrix composites containing nano-reinforce-
ments such as CNTs represent an emerging family of composites that
potentially exploits the small size of fillers of high-strength values.
Furthermore, the enormous interfacial area generated due to the incorpora-
tion of nano-fillers in composites may give rise to new kinds of composites,
possessing improved properties.
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