Page 237 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 5. Surface treatments of.fiber.7 and efects on coniposite properties 219

Fig. 5.32. Scanning electron microphotograph of a duplex Y/YzO3 coating on a Sic monofilament. After
Clyne and Withers (1993). Reproduced by permission of Cambridge University Press.

coating of oxide: reduced danger of crack propagation from the oxide layer to the
fiber, and ability to self-cure the damage to the oxide layer as a result of the gettering
action of dissolved oxygen from the matrix by the exposed yttria (Kiescheke et al.,
1991a). The thin coating on Sic fibers is also found to provide considerable
protection for Mg-Li matrix composites at temperatures up to 400°C, while the
uncoated fibers suffer catastrophic embrittlement by penetration of Li into the grain
boundaries (Kiescheke et al., 1991b).
A study of mechanical properties of uniaxial SCS-6 fiber reinforced zircon matrix
composites (Singh, 1988; Singh and Gaddipati, 1991; Singh, 1993) shows significant
streqgthening and improvement of fracture toughness compared to those without
fiber reinforcement. A BN coating on the fiber further improves the stability of the
interface and the toughness values in particular, at both ambient and elevated
temperatures. This is evidenced by the forcedisplacement curves given in Fig. 5.33,
where the differences in strength and ductility between the unreinforced zircon and
the reinforced composites with and without the BN coating are quite remarkable.

5.5.5.3. Reuction hrrrrier corrtings on Niculon Sic ,fibers
The properties of Nicalon Sic fibers start to degrade above about 600°C because
of the thermodynamic instability of composition and microstructure, leading to the
evolution of Si0 and CO (Johnson et al., 1988). Nicalon fibers are coated with
pyrolytic graphite, normally in an argon atmosphere, to protect the fibers from
chemical damage during composite processing and thus to improve the strength and
modulus of the CMCs made therefrom (Hwang and Jang, 1991). The coating,
shown in Fig. 5.34, reduces the interface bond strength due to the reduced chemical
reaction taking place at the interface region.
Lowden and More (1989) and Lowden (1991) studied extensively the effects of
graphite coating thickness on the mechanical properties of Nicalon fiber-Sic matrix