Page 128 - Fiber Fracture

FRACTURE CHARACTERISTICS OF SINGLE CRYSTAL AND EUTECTIC FIBERS 113

Fig. 2. Representative examples of fracture surfaces of single-crystal (1 11) Y203 (A), (1 1 I) Y3AI5Ol2 (B),
and (0001) A1203 (C). Fracture-originating flaws were consistently easy to find for Y3A15012 and A1203
but less obvious for Y203 (due to disintegration). The (1 11) plane is the highly preferred cleavage plane for
Y203 and is a less common cleavage plane for Y3A15012. (0001) A1203 fails without preferred cleavage
tendency, but shows some degree of preference for fracture near certain planes.

from internal voids. The tensile strengths reported here are among the highest tensile
strength values reported in the literature for continuous fibers and a considerable fraction
(-20%) of the theoretical strength of sapphire (-45 GPa). This high tensile strength
is related to the avoidance of internal voids, and large-scale facets thus leaving only
surface-related defects to control fracture of the fibers. The surface flaws (Fig. 2C) are
apparently related to three surface features that arise in the molten zone during laser
processing and are then frozen into the solid fiber. These are diameter deviations along
the fiber length, localized ripples that predominate during non-symmetric heating and
periodic striations. The tensile strength of the fibers may be closely related to the shape
of the meniscus at the crystal-liquid-vapor interface and the departure of the contact
angle from its steady-state value. This difference may also promote small-scale facets.
SEM characterization of the fibers grown in the (0001) direction showed no visible
facets. The (0001) basal plane in sapphire is one of the low free-energy planes, the
other being the (0712) r-plane (Kitayama, 1999). It was expected that the fiber may
have some growth facets relating to these planes at the solid-liquid interface providing
an array of re-entrant corners. SEM and optical microscopic characterization of fracture