Crack Initiation Behavior of Optical Glasses from Vickers Indentation

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.