Page 127 - Fiber Fracture

112 A. Sayir and S.C. Fanner

Table 1. Summary of room temperature mechanical properties (mean of 40 tests)
Single crystal Young’ modulus SD Tensile strength SD
(GPa) (GPa) Wa) @Pa)
(0001) AI203 453 f36 6.7 2.2
(111) Y3Als012 290 *2 1 3.4 0.8
(111) y203 164 f17 0.7 0.1
SD, standard deviation.

did not exceed 700 MPa, with an average strength of 570 MPa (fl10 MPa). The tensile
strength of single-crystal (OOO1) A1203 and (111) Y3A15012 were 6.7 and 3.4 GPa,
respectively. The standard deviations were 2.2 GPa for single-crystal (OOO1) A1203 and
0.7 GPa for single-crystal (1 11) Y3A15012 (40 fibers tested for each composition). The
underlying reason for the difference in strength can be traced back to the anisotropy
of each crystal, but is not necessarily attributable to the intrinsic difference in Young’s
moduli.
The strength controlling flaws for each fiber were different in nature and originated
from the crystal growth conditions. The single-crystal fibers of Y2O3 contained growth
facets, forming periodic defects along the (1 1 1) direction of the fiber. All fibers fractured
by octahedral cleavage (Fig. 2), and cleavage was often perpendicular to the fiber axis
confirming that the fiber axis coincides with the (111) crystallographic direction as
determined from X-ray characterization. X-ray analysis indicated that (1 1 1) Y2O3 had
cubic symmetry and was optically isotropic. The selection of the (1 11) growth direction
and carefully controlled solidification conditions made it possible to eliminate the
undesirable phase transformations that may occur in the solid state as expected from
the phase diagram (Roth and Schneider, 1960). The tested fibers were all single crystals
of the same cubic phase, and the periodic defects were most likely a result of changes
in the interface energetic and molecular attachment kinetics during solidification rather
than being associated with phase transformations. The layers needed for the growth of
the interface are mostly generated in the small curved rim of the liquid-solid interface
at the outer edge of the crystal near the meniscus contact line and are located at the
different crystallographic positions of the planes. The issue is the ‘anisotropy factor’ of
the interface for the different planes at the liquid-solid interface and the stability of a
particular crystal face with respect to facet formation. Although the surface energies of
Y2O3 are not known, the expected deviation for different planes from the (1 11) direction
would be small and prone to periodic instability during crystal growth. Yet, the degree
of anisotropy is large enough to produce step like defects on the surface of the crystal
leading to low fiber strength.
For A1203 fibers the (OOO1) growth direction was selected to achieve two objectives.
First, the mechanical properties of sapphire are closely related to its crystallography,
and selection of the (0001) growth direction eliminates the three known slip systems
(basal, prismatic, and rhombohedral slip). Second, the selection of the (OOO1) growth
direction provided large enough anisotropy so that the meniscus shape during solidifi-
cation was constant. The tensile strength of single-crystal (Oool) A1203 fibers was 6.7
GPa (32.2 GPa), Table 1. A few sapphire fibers with low strength consistently failed