Page 62 - Fiber Fracture

MODELS OF FIBRE FRACTURE 47
COATING -

MANTLE
INTERFACE
CORE

Fig. 9. Cross-section of B and SIC composite monofilaments produced by chemical vapour deposition,
showing the concentric core, interface, mantle and coating regions.

Sic or B to accommodate the growth strains that appear during B deposition and the
thermal strains generated by the mismatch in the coefficients of thermal expansion. The
pyrolytic graphite interface does not react with either B or Sic, and it is weakly bonded
to the fibre mantle. On the contrary, Sic and B react with the W core, leading to the
formation of an interfacial reaction zone of different thickness and composition. The
reaction zone (of about 100 nm in thickness) is a mixture of W2C, WSi2, and WSSi3
in the SiC/W monofilament, and is strongly bonded to both the W core and the Sic
mantle. In the case of B monofilaments, the interfacial reaction zone made up of WB4
and W2B5 completely replaces the original W substrate, and is also well bonded to the B
mantle. Various surface coatings (C, C/SiC, TiBz for Sic, and Sic, BN for B) are finally
applied on the mantle surface. They protect the monofilaments against abrasion during
handling and act as diffusion barriers during composite processing at high temperature.
In addition, it was observed that some of these coatings significantly increased the
strength, and this was attributed to the rearrangement of the residual stresses as well
as by the healing effect on the surface flaws. It has been suggested that the amorphous
carbon coating seals the Sic grain boundaries at the surface, hence minimising the
stress concentration. Moreover, the amorphous C prevents any grain boundaries from
extending to the free surface (Wawner, 1988; Chawla, 1998; Cheng et al., 1999).
Fracture characterisation of these composite monofilaments has demonstrated that
the strength-limiting flaws were always found at or near the interfacial region between
the core and the mantle (Lara-Curzio and Sternstein, 1993; Gonzalez and Llorca, 2000).
In the case of B/W monofilaments, the fracture origins have been associated with voids
nucleated along axial die-marks on the W surface (Vega-Boggio and Vingsbo, 1976;
Vega-Boggio et al., 1977). This led to the simplest fracture model for these composite
fibres, which assumed that the void behaved as an internal elliptical crack, and obtained
the fibre strength using the LEFM criteria for homogeneous fibres described above
(Vega-Boggio and Vingsbo, 1976). Evidently, this was a first-order approximation
which included neither the effect of the residual stresses at the interfacial region nor that
of the crack propagation through materials with different properties.