Abstract
Recrystallised alumina is used as a high performance crucible material. Its
thermal shock resistance is known to be affected by component shape, and
also by processing variables, since the defects and internal stress at both the
microscale (i.e. between grains due to anisotropic crystal properties) and
macroscale (i.e. due to differential shrinkage during sintering) influence the
fracture strength. The aim of this thesis is to study the nucleation and growth
of defects in pure alumina and Cr-doped alumina, and to investigate how
their behavior is affected by residual stresses, such those introduce by
thermal expansion of the crystal grains.
In this thesis, digital image correlation is applied to polycrystalline aluminas
(i.e. Cr-doped alumina and pure alumina with average grain 3.6 μm and 1.5
μm respectively) that are stressed in an optical microscope. The defect size
and the surface crack opening displacement were measured using digital
image correlation. The distribution and population of crack nucleating
defects were obtained by in-situ observation of the stressed surface and by
analysis with digital image correlation. These data are then compared with
independent measurements of the defect population using Hertzian
indentation, from which defect populations are derived for the pure and Crdoped alumina samples.
Grain boundary plane and grain orientations in the vicinity of crack nuclei
were characterised by electron microscopy. Crack nuclei were shown to
develop at boundaries predicted to have high tensile thermal strains,
caused by the orientation of the grain boundary plane relative to the
adjacent grains, such as basal plane grain facets. The techniques of focused
ion beam (FIB) milling and electron backscatter diffraction (EBSD)
characterization of the crystallographic orientations and structure of
cracked grain boundaries were used to provide data for a model to explain
the cracking of these boundaries as a result of the thermal strains and the
anisotropic thermal expansion behaviour of alumina.