Biomedical Engineering Reference
In-Depth Information
grains may improve high-temperature creep behaviour by dislocation
pinning with SiC particles.
1.4.2 Si
3
N
4
/SiC based nanocomposites
Among the available high-temperature material systems currently being
developed, fibre-reinforced ceramic matrix nanocomposites (FRCMNCs)
have attracted great attention for aerospace applications. Compared with
monoilithic ceramics, FRCMNCs exhibit remarkable damage tolerance,
with a fracture toughness several times higher than most monolithic
ceramics. Defect sensitivity, weak thermal shock resistance and lack of
toughness continue to be the two main technical hurdles that exclude
monolithic ceramics from aerospace turbine engines. However, high-
toughness FRCMNCs require an oxidation-resistant interface between the
fibre and the matrix. Due to a weak interface, cracks propagate through the
matrix rather than continuing along the fibre during high-temperature
applications. In many early ceramic matrix composite systems, the weak
interface between the fibre and the matrix contained carbon. However, the
carbon layer is 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). The SiO
2
layer significantly
weakens the fibre and also allows strong bonding to the matrix, which
results in a significant decrease in fracture toughness of the FRCMNC. It is
thus necessary to establish an oxidation-resistant interface coating over the
surface of the fibre before fabrication of a FRCMNC.
Advanced high-temperature material systems are considered key techno-
logical tools in the evolution of current aerospace propulsion systems. In the
past decade, the aerospace design community has greatly improved
understanding of the thermal environment within an aerospace turbine
engine, thus allowing for new concepts in aerofoil design, cooling and
combustion. However, major advances can only be realized with greatly
improved materials and a thorough understanding of their high-temperature
mechanical behaviour and performance. One such composite was manu-
factured by Kaiser Ceramic Composites, which is a joint venture of Dow
Corning (Midland, MI) and Kaiser Aerotech (San Leandro, CA), and is
sold under the trade name Sylramic
TM
202
. The silicon-carbon-oxygen (Si-
C-O) fibres used in the composite were ceramic-grade Nicalon
TM
fibres
(Nippon Carbon Co., Tokyo, Japan). The fibres were supplied in the form
of an eight-harness satin weave (8HSW) cloth. Individual sections of cloth
were placed in a CVD furnace and a boron-containing coating was applied
to the fibres. Once coated, a total of eight sections of cloth were prepregged
with a mixture of polymer, fillers and solvent. The individual sections of
8
