Page 238 - Engineered Interfaces in Fiber Reinforced Composites

220 Engineered interfaces in $her reinforced composites

~
""V
700 - -.-.- Monolithic mullite
-
Uncoated SIC fibers
600 - .-............ BN-coated SIC fibers
500 -

6
Displacement (mm)
Fig. 5.33. Load-displacement records of monolithic mullite, and Sic fiber reinforccd mullitc composites
with and without BN coating. After Singh (1988).

composites. The coatings are deposited isothermally onto the fiber preform of plain
weave fabric from an argon/polypropylene mixture at 1100°C. By varying the
deposition time, coating thickness in the range 0.1-1.0 pm are obtained. Fig. 5.35
shows the load-displacement curves obtained in flexure tests of the composites with
and without coating layers of varying thickness. The uncoated fiber composite
exhibits poor strength response and brittle fracture without any fiber pull-out, as do
the specimens with prior oxidation treatments. Incorporation of a graphite coating
promotes fiber pull-out, whose length increases with coating thickness. The linear
inverse relationship between interfacial shear strength and coating thickness has also
been identified, Fig. 5.36. The uncoated fiber composite with a strong chemical
bonding possesses the highest interfacial shear stress, while the specimen with an
oxidized interlayer shows no interfacial bonding because the oxidation of carbon
produces a gap between the fiber and matrix.
Apart from functioning as a reaction barrier, another important role of the low
modulus interlayer by the pyrolytic graphite, is the reduction of the radial
compressive stress arising from differential thermal contraction between fiber and
matrix. Thicker carbon coatings give more reduction in thermal stresses than thinner
coatings, which is partly responsible for the inverse relationship between interfacial
shear strength and coating thickness (Fig. 5.36). A compliant coating is most useful
when the CTE for the fiber is smaller than the matrix material (Arnold et al., 1990),
as in most practical CMCs.
BN is also proven to be an efficient reaction barrier coating for Nicalon fiber-Sic
matrix systems (Naslain et al., 1991a, b). The coating promotes a non-linear and
non-catastrophic fracture behavior under tensile and impact loading. A coating of
0.5 pm in thickness exhibits the best mechanical properties (Prouhet et al., 1994).
The use of C, BN, BN/SiC coatings on Nicalon Sic fibers is also suggested for Zr-
based matrix materials. Bender et al. (1986) have shown that the BN-coated Nicalon
fibers in a zirconia-based matrix (Zr02 + 50 mol% SO2 and ZrOz + 50 mol%