Page 234 - Engineered Interfaces in Fiber Reinforced Composites

216 Engineered interfaces in fiber reinforced composites

5.5.5. Silicon carbide jibers

5.5.5.1. Introduction
One of the most important fibers for high temperature applications is the Sic
fiber. There are two major processes developed to produce continuous Sic fibers.
The first is achieved by coating Sic on either a tungsten or a carbon filament by a
CVD process. The Sic fiber obtained from a CVD process is very thick (say, 100-
150 pm in diameter) and is rigid. The surface of Sic fibers, Le. SiCjW fiber,
produced on a tungsten fiber is bumpy and nodular. Although the nodules are
smaller than those seen in boron fibers, they are very sensitive to self-abrasion which
may reduce significantly the fiber tensile strength. More importantly, prolonged
exposure to elevated temperature causes significant degradation of the fiber
strength, due probably to the deleterious tungsten core reaction. Later, the tungsten
core is replaced with a carbon monofilament substrate to produce SiC/C fibers. The
carbon core is routinely coated with a thin (about 1 pm) layer of pyrolytic graphite
before deposition of Sic to minimize the reaction between the carbon core and Sic.
To reduce the extreme surface sensitivity of SiC/C fibers, a thin layer of carbon is
deposited onto the surface. The carbon coating may also lower the stress
concentrating ability of grain boundaries at the fiber surface. However, the
amorphous carbon-rich surface also has a serious disadvantage: a low wettability
with metal matrices particularly cast aluminum, which causes poor adhesion and
forms reaction products, such as aluminum carbide with an aluminum matrix. To
reduce this detrimental problem, the fiber surface is further modified by covering the
carbon layer with an additional Sic coating. This is designated the SCS fiber
(Textron Specialty Materials Co).
The other process is the transformation of an organic precursor into a continuous
thin ceramic fiber. In the spinning process, polycarbosilane, a high molecular weight
polymer containing Si and C, is obtained by thermal decomposition and
polymerization of polydimethylsilane. The fiber thus produced consists of a mixture
of p-SiC, carbon crystallite and SO2. The presence of carbon crystallite suppresses
the growth of Sic crystals. Yajima and coworkers (Yajima et al., 1976, 1978, 1979)
were the first to produce fine (10-30 pm in diameter), continuous and flexible fibers,
which are commercialized with the trade name of Nicalon (Nippon Carbon Co.).
Sic monofilaments produced by the CVD process is generally superior to Nicalon
Sic fibers in mechanical properties because of its almost 100% 6-Sic purity while
Nicalon is a mixture of Sic, Si02 and free carbon. Representative properties of Sic
monofilaments and Nicalon fibers are given in Table 5.15.

5.5.5.2. Reaction barrier coatings on Sic monofilaments
There are four types of SCS fibers depending on the thickness of the final Sic
coating designed for different metal matrices. They are the standard SCS, SCS-2,
SCS-6 and SCS-8. Fig. 5.30 illustrates schematically the cross sections of two
commercially produced Sic fibers, the standard SCS and SCS-6 fibers, according to
DiCarlo (1988). Both types of fibers consist of a carbon core of 37 pm in diameter, a
Sic sheath of varying thickness and a carbon-rich surface coating of (PI pn in