Page 235 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 5. Surface treutments of jibers and effects on composite properties 217

thickness. SCS-2 and SCS-8 fibers are designed for and functional in A1 matrices,
whereas SCS-6 fibers with a thicker stoichiometric p-Sic coating are designed
specifically for Ti matrices which require more protection from the high temperature
fabrication process.
The original coatings present on SCS-2 fibers are found to be effective for
retaining the fiber strength after exposure in molten aluminum for up to 1.5 h.
However, the strength decreases systematically with continuing exposure due to
fragmentation of the coating and formation of reaction products such as aluminum
carbides. The fracture behavior of Ti alloy matrix composites containing SCS-6
fibers and their reaction barrier coatings have received much attention in recent
years. SCS-6 fibers show little sign of degradation after 5 h exposure at 900°C when
incorporated in a Ti-6A1-4V matrix (Strife and Prewo, 1982; Martineau et al.,
1984). On the contrary, when the same SCS-6 fibers are incorporated in a Ti-24A1-
11Nb intermetallic alloy at the solidification temperature of 1O4O0C, a reaction layer
is created at the interface region. Its thickness depends on the time exposed: 1.1 and
1.7 pm for 0.5 and 4 h exposure, respectively (Cantonwine and Wadley, 1994). Fiber
push-out tests identify an increase in the interface bond strength with increasing
reaction layer thickness, and associated characteristic debond behavior. In the
composite with a thin reaction layer, debonding occurs predominantly at the outer
SCS coating-reaction layer interface, whereas the composite with a thick reaction
product displays multiple debond paths branching between the fiber-SCS coating-
reaction product interfaces. The interfacial reaction between the SCS-6 fiber and Ti
matrix is identified as a result of interdiffusion between Ti, Si and C atoms, and the
major reaction products are TIC, Ti5Si3 and Ti3Si2 (Lancin et al., 1988).
Additional surface modifications on vapor deposited Sic fibers, including WC,
TaC, TIN, B4C, Al, Ni and Fe, have been applied with varying degree of success
(Wawner and Nutt, 1980; DeBolt, 1982; Wawner, 1988). After exhaustive trial and
error, TiB is selected as an additional coating material to further prevent the
diffusion-induced reactions between the SCS-6 fibers and matrix materials, including
Ti alloys and Ti-A1 intermetallic alloys (e.g. Ti3A1, TiAl and TiA13) (Donnellan and
Frazier, 1991; James et al., 1991). When the coated fiber is subjected to tensile

carbon-nch 6 - Sic

\- Carbon-rich surface. / '
coating(O-4ym) 1
Fig. 5.30. Schematic illustrations of two SCS-type SIC fibers. After Chawla (1993), Fig. 3.35, p. 95.
Reproduced by permission of Chapman & Hall.