Page 129 - Fiber Fracture

114 A. Sayir and S.C. Farmer

surfaces failed to show twins. The tensile strength values reported here are very high.
Therefore, the expected twin size is beyond the resolution of SEM and may require
transmission electron microscopy (TEM) characterization to resolve the question of their
occurrence.
The tensile strength of (111) Y3A15012 was moderately high, 3.4f0.8 GPa, but
considerably less than A1203. The fractographic analysis of the fibers revealed that
low-strength fibers had a stress concentration associated with [lo01 planes along the
fiber axis, Fig. 2B. The occurrence of these stress concentrations was sensitive to the
pull-rate during crystal growth. Y3A15012 has 160 atoms per unit cell (A1203 has 12)
and thus a sluggish crystallization process. Due to the high viscosity and low thermal
conductivity of the Y3A15012 melt the changes in tangential temperature gradient along
the free surface of the melt did not readily initiate instabilities. The significance of these
physical characteristics of the melt was twofold. First, the meniscus angle does not
change readily; therefore, smooth exterior surfaces are produced. Fibers have strengths
around 3 GPa even though they contain macroscopic facets. This is in strong contrast
to single-crystal Y2O3, although they both have cubic symmetry. Second, as reported in
the literature (Caslavsky and Viechnicki, 1980), the Y3Al5O12 system can move toward
metastable solidification resulting in solidification of perovskite phase with YA103 com-
position. Fibers grown at faster rates failed mostly from internal flaws of precipitated
YA103 phase as determined by X-ray and Raman spectroscopy analysis. Hence, any
attempts to increase pull speed for high volume production are technically not viable.
From the foregoing discussion of the fracture characteristics of single-crystal fibers,
it is apparent that single-crystal A1203 and Y3Al~012 are possible candidates for load
bearing applications. Yet, Y3A15012 cannot be produced in an economically practical
manner due to sluggish crystallization kinetics and hence is not a viable reinforcement
for structural composites that require a considerable amount of reinforcement, Hence,
Y203 and Y3A15012 are most attractive for sensors, waveguides, and laser host types
of functional applications where the amounts of fiber material required are lower.
Accordingly, with the emergence of single-crystal A1203 fibers as the more promising
candidate for reinforcing fibers in structural applications, the strength of A1203 fibers at
elevated temperatures needs to be studied for high-temperature use.

Fracture Strength of (0001) Alto3 Fibers at Elevated Temperatures

Single-crystal (0001) A1203 fibers with room temperature strengths of 6.7 GPa
exhibit strengths at 1400°C of (1 GPa. This reduction in strength may be due to the
decrease of the material resistance to crack propagation, that is, the decrease in fracture
toughness KI, with increasing temperature. However, this would require a dramatic
decrease of Young’s modulus and/or a substantial decrease in the surface energy of
sapphire. Young’s modulus of single-crystal A1203 decreased monotonically with a very
small slope as a function of temperature. The modulus at 1450°C was only 10 to 14%
less than the room temperature value. Since the relative decrease in the elastic modulus,
-(T/E)(dE/dT), for A1203 is not profound, the decrease of other thermodynamic
properties with increasing temperature is not expected to be large enough to account for
the dramatic decrease of tensile strength at elevated temperatures.