Biomedical Engineering Reference
In-Depth Information
response of individual phases, but they also depend on the method of
measurement.
1.5
Interface characteristics of ceramic
nanocomposites
The phenomenal progress in materials science and technology has led to
very high and critical performance demands on ceramic materials having
high-mechanical strength and toughness in high-temperature applications. It
is becoming increasingly necessary and sometimes absolutely essential to
control parameters such as thermal expansion mismatch between reinfor-
cing agents (fibres, whiskers or nano-phase particles) and matrix materials,
creep deformation, grain boundary sliding, crack growth, high-temperature
oxidation and densification of materials during the processing of ceramic
nanocomposites. To achieve high performance, an excellent balance of
properties of each constituent of the nanocomposite and the interaction
between component items at the interface are of utmost and equal
importance. For the development of high-strength ceramic nanocomposites,
it is necessary to build strong interfaces between the reinforcements (either
micro- or nano-size) and the nano-size matrix materials. However, for the
development of high-toughness ceramic nanocomposites, the interfaces
between the reinforcements (either micro- or nano-size) and the nano-size
matrix should have a low fracture toughness to propagate a crack through
the matrix and deflect along the fibre reinforcement/matrix interface rather
than continue through the fibre reinforcements.
In many early ceramic matrix composite (CMC) systems, the weak
interface was observed in between the Nicalon-grade woven silicon fibre
reinforcement (Si-C-O) and the matrix (Si-N-C) contained carbon.
However, such a carbon layer will be oxidized in oxygen-rich environments
at temperatures as low as 400
C. After the carbon is gone, the oxygen reacts
with the fibre to form a silica (SiO
2
) layer on the surface of the fibre (Bonney
and Cooper 1990, Brennan 1986, Cooper and Chung 1987). As an
alternative to carbon, boron nitride (BN) (Brennan et al. 1992,
Hanigofsky et al. 1991, Kmetz et al. 1991, Naslain et al., 1991, Shen et al.
1992, 1993, Sigh and Burn 1987, Sun et al. 1994, Veltri and Galasso 1990)
has been considered as a new fibre coating, and several CMC manufacturers
have switched to a BN-containing interface between the fibre and the
matrix. Microanalysis of fibre/matrix interfaces has been conducted on as-
received material using a fibre push-in technique (Hartman and Ashbaugh
1990, Jero et al. 1992, Kerans and Parthasarathy 1991). All the interface
data were collected at room temperature using a Micro Measure Machine
(Process Equipment Co., Tipp City, OH), which is designed specifically for
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