Crack Initiation Behavior of Optical Glasses from Vickers Indentation
Date
2005-11
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Abstract
The crack initiation behavior of nine optical glass compositions was
investigated in the current work. A recording microindenter equipped with
optical observation and an acoustic emission detection system were used to
monitor the crack initiation behavior in-situ from contact with a Vickers
diamond.
Seven of the optical glasses had a deformation response that was shear-
‘flow’ controlled, i.e., ‘normal.’ However, median-radial cracks formed readily
on indenter loading. Thus, the popular notion that ‘normal’ glasses crack
only on unloading, for relatively small loads, does not apply to these glasses.
The two remaining optical glasses behaved ‘anomalous’ in the sense that
compaction was the main indentation response, with accompanying ring-cone
cracks, particularly at higher loads.
The quantities G/K (shear modulus/bulk modulus), E/HV (Young’s
modulus/Vickers hardness), and RO2 (RO2 = moles RO2/moles RO+R2O) were
found to govern the overall cracking behavior of the glasses examined. The
ratio G/K was found to characterize a glass’s relative tendency to undergo
shear-‘flow’ vs. volume compaction (densification), and hence ‘normal’ or
‘anomalous’ behavior, respectively. The ratio E/HV correlated with the
relative plastic zone sizes and crack driving force around the indentations. In
general, higher amounts of RO2 were correlated with increased resistance to
median-radial cracking. Glasses with G/K less than ≈0.6, E/HV greater than
≈14, and RO2 less than ≈2.0, tended to form median-radial cracks readily
on indenter loading, while glasses with the opposite characteristics tended to
form median-radial cracks primarily on unloading. Glasses with either
tightly packed structures, characterized by relatively high density, little
interstitial free volume, low G/K, and high E/HV, or weak, loosely held
structures, sheared readily beneath the indenter, generating large strains
and stresses, and consequently median-radial cracked at low loads on
indenter loading. Both glass types had low RO2 and lacked a true threedimensional
network structure. For tightly packed structures the lack of
open space, even in glasses with strong intrinsic bonds, resulted in shear-
‘flow’ as the only means to accommodate the indenter. Weak, loosely held
structures, even those with relatively low density, preferred to shear, since
this offered the least resistance to deformation.
Description
Advisory committee members: William Lacourse, William Carty, Arun Varshneya. Dissertation completed in partial fulfillment of the requirements for the degree of Doctorate of Philosophy in Ceramics at the Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University
Type
Thesis