Quantitative evaluation of indentation-induced densification in glass
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stimate the ratio of densification to Vickers indentation volume, three-dimensional images of Vickers indentations on several glasses, including silicate glasses and bulk metallic glass (BMG), were obtained before and after annealing using an atomic force microscope. Large volume recovery of Vickers indentation by annealing was observed for all glasses but BMG. Following previous studies, this recovered volume almost corresponded to the densified volume under a Vickers indenter, and the compositional dependence of densification was discussed. The ratios of densification to the total indentation volume for silica and soda-lime glasses were 92% and 61%, respectively. It was concluded that densification was a general property for silicate glasses and that the ratios of densification to the total indentation volume for all the glasses correlated well with Poisson’s ratios of the glasses.
I. INTRODUCTION
Vickers indentation testing allows for a simple comparison of the apparent hardness of material including glass. Vickers hardness is defined as the ratio of the indentation load to the indentation surface area. The surface area is estimated from the length of the projected diagonal, assuming that the indentation shape is identical to the one of the pyramidal Vickers diamond indenter. Although hardness correlates to the resistance of material to permanent deformation, it does not shed light on the mechanism of irreversible deformation of glass under sharp contact loading, which can be divided into mainly two modes, namely plastic (or shear) flow and densification.1–3 The former is a volume-conservative deformation process, and the latter includes volume contraction. Plastic flow is responsible for residual stress due to elastic/plastic mismatch,4–6 but densification is not. Therefore, two glasses having the same hardness numbers but for which shear flow and densification contribute to different extents to the indentation deformation may exhibit significantly different overall indentation behaviors regarding indentation-induced cracking, scratchability, damageability, machinability, and so on. Therefore, a method allowing for the discrimination of both deformation contributions is of great interest.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0418 3404
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 12, Dec 2005 Downloaded: 27 Mar 2015
According to Peter,1 densification of glass by indentation is a general property, and shear (plastic) flow, related to piling-up, occurs in glasses with significant amounts of network modifiers. By using an interferometric technique, Ernsberger also concluded that Vickers hardness could be interpreted as a measure of the critical stress for yield by densification.7 Later, Hagan3 investigated the flow lines, which occur in deformed zone under a Vickers indenter, of soda-lime glass and suggested that the flow lines were produced by shear displacements but that a hardening effect accompanied the shear flow. According to Aror
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