Page 236 - Engineered Interfaces in Fiber Reinforced Composites

loading, the brittle TiB2 layer cracks easily, promoting interfacial failure, which in
turn allows the inherent fiber strength to be preserved (Jeng et al., 1991).
Later, aiming to improve the stability of SCS-6 monofilaments at high
temperatures, a TiB/C duplicate coating has been developed. However, its
application in Ti-6A1-4V matrix composites is marred by the formation of TiB
needles at the interface region (Jones et al., 1989; Guo and Derby, 1994). Fig. 5.31
compares the interfacial morphologies between the uncoated and TiBjC coated
SCS-6 fibers after exposure at 970°C for 2 h. The needles are thought to increase the
stress concentration and the tendency of cracking at the interface region, even in the
absence of external loading, which are highly undesirable. In contrast, the TiB2/C
duplicate coating performs very effectively in protecting the SCS-6 fibers from
chemical reactions in a Ti3Al matrix (Guo et al., 1993; Guo and Derby, 1994). Only
limited reaction is observed without the harmful TiB needles at the interface region,
and the reaction mechanism arises due to the diffusion of boron through the
reaction layer toward the matrix. Thermo-mechanical and high temperature
isothermal fatigue loading of SCS-6 fibers embedded in a Ti-6Al4V matrix show
that the matrix cracking with unbroken fiber bridging is a major failure mechanism
(Jeng et al., 1992). Damage initiation appears to be controlled by the rate of
oxidation layer formation on the specimen surface. Oxygen diffuses through the
matrix internal cracks into the interface layer, resulting in severe fiber-matrix
separation and the formation of oxidation pits on the SCS-6 fiber surface.
Further studies on duplex barrier coatings consisting of a metallic layer with an
overlayer of metal oxide have been performed on SCS-6 monofilaments for
reinforcement of Ti matrices (Kiescheke et al., 1991a, b; Warwick et al., 1991). After
depositing an initial layer of yttria by a sputtering process, a covering oxide layer of
Y203 is applied in the second sputtering process. Fig. 5.32 shows a duplex coated
SCS-6 monofilament. The duplex layer offers some advantages over a single layer

Fig 5.31 Scanning electron microphotographs of the interface morphology for (a) uncoated SIC
monofilament and (h) Ti 6AI4V coated SIC monofilament after exposure at 1070°C for 2 h After Guo
et al (1993). Reproduced by permission of Blackwell Science Ltd.