Page 240 - Engineered Interfaces in Fiber Reinforced Composites

222 Engineered interfaces in fiber reinforced composites

500 I

I 400
m
a
E
300
c
L
u)
kj
5 200
-
IQ ._
u
tu
-
re
kJ 100
+
c
0
0 2 4 6 a 10 12
Inverse of coating thickness Qm-ll
Fig. 5.36. Interface shear strength as a function of coating thickness for Nicalon Sic fiber-Sic matrix
composites. After Lowden (1991).
fibers by a CVD process is also shown to be quite promising for improved high-
temperature mechanical properties of barium magnesium aluminosilicate (BMAS)
matrix composites (Sun et al., 1994). The duplex coating produces a stable interface
structure which is resistant to gross reaction at 1100°C for extended periods of time.
The turbostratic BN layer offers a relatively weak interfacial zone, allowing crack
deflection with associated high fracture toughness.
Some interesting results have been reported based on the studies using Nicalon
Sic fibers-glass matrix composites containing lithium aluminosilicate (LAS). X-ray
diffraction study shows two distinct carbon-rich reaction layers (Bender et al., 1986).
The first layer is essentially amorphous carbon of thickness about 100 nm, which is
adjacent to the Sic fiber. The carbon layer originates from excess carbon in the Sic
fiber which migrates to the fiber surface (Chaim and Heuer, 1987). Another source
of carbon is the residue left after heat cleansing of the organic size from the Sic and
of the organic binder from the prepreg. The second reaction layer is located between
the first layer and the LAS matrix, consisting predominantly of microcrystals of
NbC. The niobium originates from the additive to the LAS glass as a crystallization
aid. The thickness of the second layer varies from 20 to 100 nm. Trace amounts of
impurities like Zr, Mg, Ti, K, Ba, Fe and As are also detected in the second layer,
that migrate to the interface region during processing. In this SIC-LAS system, the
interface is relatively weak compared to the bulk fiber and matrix. It follows then
that debonding occurs inevitably between the two reaction layers where the NbC
microcrystals are well developed (see Fig. 5.37).