Page 247 - Engineered Interfaces in Fiber Reinforced Composites

228 Engineered interfaces in jber reinjorced composites

The improvement in fracture toughness of a Nextel 480 mullite (3Al2O3-2SiO~)
fiber in a glass matrix has also been achieved by incorporating a BN coating on the
fiber surface (Vaidya et al. 1992). The uncoated fiber composite shows a brittle and
planar fracture, while those containing BN coated fibers exhibit extensive fiber pull-
out, in a similar manner shown for SnOz coated PRD-166 fibers (Fig. 5.39(b)).
However, when a very thin, say about 0.3 pm, coating is applied, no BN layer is
observed after the process, because the thin coating becomes easily oxidized,
followed by vaporization of the oxidation product. Otherwise, the BN coating tends
to decompose during the hot pressing of the matrix material. This indicates that the
choice of coating thickness is an important factor which controls the effectiveness of
the coating material. Ha and Chawla (1993) and Ha et al. (1993) used a similar BN
coating successfully to obtain tough mullite fiber-mullite matrix composites. A
duplex SiC/BN coating is also recommended for use to reduce the interface bond
strength.
A diffusion barrier coating has also been successfully applied to aluminide-based
intermetallic matrix composites (Misra, 1994). For example, Ti coating on A1203
fiber for reinforcements of NiAl or FeAl matrices produces a rather strong bonding
at the interface which is desirable to eliminate the longitudinal matrix cracks arising
from thermally induced residual stresses. However, a weak interface is needed for
easy debonding and fiber pull-out which are required for improvement of fracture
toughness.
Alloying elements can also have a significant effect on reaction processes at the
interface region. For example, the addition of a small amount of magnesium, say
less than 0.4 wt% (Chapman et al., 1991), or about 3 wt% lithium (Birchall et al.,
1985; Birchall, 1986) in A1203 fiber-aluminum matrix composite is found to be
beneficial for metal infiltration and fracture resistance without causing a harmful
reaction at the interface. Increasing the magnesium content, however, deteriorates
the flexural strength due to a corresponding increase in thickness of the reaction
product, MgAl2O4, at the interface region (Johnston and Greenfield, 1991).

References

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