Biomedical Engineering Reference
In-Depth Information
prepregged cloth were then placed in a vacuum bag using a hand lay-up
procedure and a warp aligned stacking sequence. Once in the bag, the
assembly was warm moulded and cured in an autoclave to produce a flat
tile. Further
curing was
followed by pyrolysis under nitrogen at
temperatures
C. During pyrolysis, the polymer was pyrolyzed to
an amorphous SiNC ceramic matrix. After pyrolysis, the assembly was
reinfiltrated with polymer solution and then pyrolyzed again; this procedure
was repeated several times to increase the matrix density. Test coupons were
then cut from the tiles using a laser, reinfiltrated with polymer solution and
pyrolized several times to densify the cured matrix further. Because the
porosity in this composite was predominantly open, this may suggest that
either additional infiltrations with the polymeric precursor would have been
advisable or the viscosity or molecular weight of the final precursor was too
high to penetrate and fill the open pores effectively. Typical distributions of
fibre, porosity and filler of the transverse cross-section of the nanocompo-
sites were shown in the optical micrographs by the investigators. Large
pores in the 8HSW Nicalon/SiNC nanocomposites were also shown by the
investigators in optical micrographs (Lee et al. 1998).
Tensile tests have demonstrated that this FRCMNC exhibits excellent
strength retention up to 1100
>
1000
8
C. The room-temperature fatigue limit was
160MPa, 80% of the room-temperature tensile strength. In the high-
temperature tension tests, each specimen was heated to the test temperature
for 15min and then held there for about 20min to allow the specimen to
equilibrate. When the 20min soak was complete, the load was applied. A
high-temperature extensometer with alumina rods was used to measure
strain. All high-temperature tension, creep rupture and fatigue tests
followed this heating procedure (Butkus et al. 1992).
For the fabrication of thermal shock resistant Si 3 N 4 based nanocompo-
sites, an amorphous Si-C-N precursor powder was prepared by the reaction
of [Si(CH) 3 ] 2 NH + NH 3 +N 2 in a vapour phase gas system at 1000
8
8
Cand
￿ ￿ ￿ ￿ ￿ ￿
by heat treatment in N 2 atmosphere at 1300
C for 4 h to stabilize the
powder. The amorphous precursor powder thus obtained was almost
spherical and homogeneous, and the average particle size was 0.2
8
m. This
Si-C-N amorphous precursor powder actually converts to a mixture of
Si 3 N 4 and SiC particles when heated at high temperatures. The amorphous
Si-C-N powder with various carbon contents was mixed with 8 wt% Y 2 O 3
as the sintering aid in a plastic bottle using Si 3 N 4 balls and ethanol for over
10 h. The dried mixtures were hot-pressed at 1700-1800
μ
CinN 2 atmo-
sphere. In the Si 3 N 4 /SiC nanocomposites, the grain morphology of Si 3 N 4
was strongly influenced by the SiC dispersions, depending on the volume
fraction of SiC. Up to approximately 25 vol% SiC, the growth of elongated
Si 3 N 4 grains was accelerated by the SiC dispersion. As compared with
monolithic Si 3 N 4 prepared under the same conditions, the nanocomposites
8
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