Page 122 - Fiber Fracture

Fiber Fracture
M. Elices and J. Llorca (Editors)
0 2002 Elsevier Science Ltd. All rights reserved

FRACTURE CHARACTERISTICS OF

SINGLE CRYSTAL AND EUTECTIC FIBERS

Ali Sayir and Serene C. Farmer

NASA Glenn Research Cewez Cleveland, OH 44135, USA

Introduction ..................................... 109
Experimental .................................... 109
Results and Discussion ............................... 110
Fracture Characteristics of Single-Crystal Y2O3, Y3A15012 and A1203 .... 110
Fracture Strength of (0001) A1203 Fibers at Elevated Temperatures ..... 114
Directionally SolidifiedAl203/Y3A1sO12 EutecticFibers ........... 117
Conclusions ..................................... 121
References ...................................... 122

Abstract

Single-crystal fibers are attractive for functional ceramic applications as active de-
vices and are equally important for structural ceramic components as load bearing
applications. The fracture characteristics of single-crystal fibers from a variety of crystal
systems including the A1203/Y3Al5Ol2 eutectic were examined. The Young moduli of
(0001) Al2O3, (111) Y3A15012 and (111) Y203 fibers were 453, 290, and 164 GPa,
respectively, and agreed well with the literature. Single crystals of (1 11) Y2O3 were the
weakest fibers and their strength did not exceed 700 MPa. The moderate tensile strength
of single-crystal (11 1) Y3A15012 was controlled by the facet forming tendency of the
cubic garnet structure and in some cases by the precipitation of cubic perovskite phase
YA103. High-strength single-crystal (0001) A1203 fibers did not retain their strength
at elevated temperatures. The data suggest that single-crystal (0001) A1203 failure is
dependent on slow crack growth at elevated temperatures. The high-temperature tensile
strength of A1203/Y3A15012 eutectic fibers is superior to sapphire (1.3 GPa at llOO°C)