Page 34 - Fiber Fracture

FIBER FRACTURE: AN OVERVIEW 19

derived ceramic fibers. The following mirror constants were obtained from the slope of
their respective lines in Fig. 12 1.6 MPa m-'j2 for E-glass fiber and 2.2 MPa m-1/2
for optical glass and CA fibers. The mirror constant for the fused-silica optical fiber is
in good agreement with the Ai value of 2.22 MPa m-'/2 and 2.1 MPa m-1/2 that was
obtained for fused-silica fiber from Chandan et al. (1994) and Mecholsky et al. (1977),
respectively. The mirror constants for E-glass and CaO/A1203 fibers are also in good
agreement with previously determined Ai values (Gupta, 1994; Sung and Sung, 1996).
Fractographic work has been done by a number of researchers on polymer-derived
Sic fibers (Sawyer et al., 1987; Soraru et al., 1993; Taylor et al., 1998), eutectic
oxide-oxide fiber (Yang et al., 1996), and glass fibers (Mecholsky et al., 1977; Chandan
et al., 1994; Gupta, 1994; Mecholsky, 1994; Sung and Sung, 1996). Polymer-derived
Sic fibers have been tested in several studies and data containing the mirror radius
measurements and flaw size measurements were obtained. Fracture surface examination
showed two types of flaws to be responsible for the fracture of most of the ceramic
fibers: internal and surface flaws or defects. Internal flaws can consist of pores (Taylor
et al., 1998), granular defects (Sawyer et al., 1987), and voids. Pores can agglomerate
at a certain region and form clusters, and these pore clusters behave similar to a single
flaw. Granular defects (e.g. a region of high carbon concentration in a Nicalon fiber) are
aggregates of small (0.1 to 2 pm) granular particles.
Surface flaws are common in optical fiber because of the processing technique used
for the fabrication of fused-silica fibers. They are also very common in other ceramic
fibers such as alumina-based or silicon carbide-based fibers. Airborne particles as well
as other elements tend to attach to the surface of the fiber during process or handling.
Alumina-based continuous fibers are available commercially from a number of
sources. For example, Nextel series of alumina-based fibers from 3M Co. Nextel 3 12,
440, 480 (discontinued) have a mullite composition with varying amounts of boria to
restrict grain growth. Nextel 720 consists of 85 wt% alumina + 15 wt% silica. Almax
fiber from Mitsui Mining Co. fiber while Saphikon is a continuous monocrystalline
alumina fiber (diameter between 75 and 225 pm) grown from melt by a process known
as edge-defined film-fed growth process. Sometimes Saphikon fibers show undulations
on the fiber surface, see Fig. 13. Typically fracture surface of such single crystal fibers
show cleavage planes, see Fig. 14. These fibers typically have microvoids formed during
fiber growth from the melt. One important feature of ceramic fibers is the surface
texture. Their surface roughness scales with grain size. Fig. 15a shows the rough surface
of an alumina fiber while Fig. 15b shows the grain structure of an alumina fiber. The
rough surface of such brittle fibers makes them break at very low strains and it makes
very difficult to handle them in practice. The grain boundaries on the surface can act as
notches and weaken the fiber. A surface layer of silica or boron nitride on alumina fiber
can heal surface flaws and increase fiber strength. A polycrystalline mullite fiber (Nextel
480), 10 pm in diameter, coated with boron nitride, showed an increase in the Weibull
mean strength with increasing coating thickness up to 0.2 pm vis h vis the uncoated
fiber, see Table 1 (Chawla et al., 1997). This was due to the surface flaw heating effect
of the smooth BN coating. However, for a thicker BN coating (1 pm), there was a
decrease in strength. This was because the soft BN coating at 1 pm thickness on a 10
pm diameter fiber had a volume fraction of 0.31.