Biomedical Engineering Reference
In-Depth Information
speculation is supported by direct observation of the interfaces: the
alumina-alumina interface contained a markedly wider non-crystalline
structure than the silicon carbide-alumina interface. A strong particle-
matrix interface in a ceramic nanocomposite is supposedly attributable to a
great internal compression stress acting on the interface during the cooling
process due to thermal expansion mismatch (Ohji et al., unpublished).
Transgranular fracture of nanoparticles in the grain boundary crack
propagation in the nanocomposite also suggests rigid bonding of the
particle-matrix interface. In tensile creep and creep rupture tests, the creep
life of the ceramic nanocomposite was 10 times longer and the creep strain at
fracture was 8 times smaller than those of the monolith at 1200
Cand
50MPa. The nanocomposite demonstrated transient creep until failure,
while accelerated creep was observed in the monolith. Microscopic
characterization suggested the following microstructural evolution during
creep: as grain boundary sliding proceeded, intergranular silicon carbide
nanoparticle rotated and plunged into the alumina matrix, significantly
increasing creep resistance.
Fibre-reinforced ceramic matrix composites are being developed for
potential use in gas turbine engines and other industrial applications
involving high service temperatures and both static and dynamic loads.
Ceramic composites have shown low density, enhanced fracture toughness,
higher damage tolerance and excellent thermal stability compared to
monolithic ceramics. Critical to the successful implementation of fibre-
reinforced ceramic matrix composites in such applications is the presence of
a suitably weak fibre/matrix interface that is also resistant to oxidation at
high temperatures. High strength and toughness at room temperature have
been demonstrated in graphite fibre reinforced composites and SiC fibre
reinforced composites with a carbon interfacial layer (Levitt 1973, Philips
et al, 1972, Prewo and Brennan 1980, 1982). However, both of these
materials are susceptible to oxidation during long-term high-temperature
exposure. Research has been devoted to the design of suitable interfaces to
improve the oxidation resistance of composites, and several approaches
have been investigated, including the use of fibre coatings and matrix doping
(Brennan 1987, Naslain et al. 1991, Rice 1987). Sun et al. (1996) selectively
designed a composite consisting of a barium magnesium aluminosilicate
(BMAS) glass-ceramic matrix reinforced with SiC fibres with a SiC/BN
coating. The material exhibited retention of most tensile properties up to
1200
8
C. Monotonic tensile fracture tests produced ultimate strengths of
230-300MPa with failure strains of 1%, and no degradation in ultimate
strength was observed at 1100 and 1200
8
8
C. Tensile fatigue experiments were
conducted in which the composite survived 10
5
cycles without fracture at
temperatures up to 1200
C (Sun et al. 1996). The short- and long-term
properties observed in this study were derived largely from the degree to
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