Insight into the Deformation Mechanism under a Sharp Contact-Loading in Glass by Atomic Force Microscopy
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Insight into the Deformation Mechanism under a Sharp Contact-Loading in Glass by Atomic Force Microscopy Tanguy ROUXEL, Satoshi YOSHIDA@, Haixia SHANG and Jean-Christophe SANGLEBOEUF LARMAUR, FRE-CNRS 2717, Bât. 10 B, Campus de Beaulieu, Université de Rennes 1, 35042 Rennes Cedex, France. ABSTRACT The response of a material to a sharp contact loading, as in the case of Vickers indentation for instance, provides a unique insight into the material constitutive law, including elastic and irreversible deformation parameters as well. However, under such peculiar thermodynamical and mechanical conditions (the mean contact pressure on the contact area reaches values typically higher than 1 GPa, corresponding to the hardness of the material) the deformation processes are complex and the matter located just beneath -and around- the contact area may experience some structural changes and behave in a way different to the expected - or known - macroscopic behaviour. It is showed in this study by means of detailed topological investigations of the residual indentations by Atomic Force Microscopy (AFM) that the elastic recovery typically represents 50 to 70 % of the indentation volume at maximum load and that the densification contribution may reach 90 % of the residual deformation volume. Besides, most glasses exhibit indentation-creep phenomena, which become significant over time scale of only few minutes because of a pronounced shear-thinning behavior. INTRODUCTION The way matter deforms beneath a sharp-contact loading is of primary interest both for glassmakers and users who want to understand the mechanism of surface damage and for materials scientists who intend to estimate material properties from instrumented indentation experiments. However, while the reversible strain stems from elasticity, the permanent contribution still remains a matter of controversy. In a first approximation the permanent deformation may be viewed as the result of a combination of volume-conservative processes such as shear-plasticity and shear-viscosity, where viscosity may refer to a non-linear stress/strain-rate relationship, and non-volume-conservative processes such as flow densification, which is well known to occur in a-SiO2 [1] and damage. In order to get insight into the deformation mechanism beneath the indenter, we have developed a topological analysis of the indentation geometry using Atomic Force Microscopy. For this rheological study, microcracking must be avoided and therefore an indentation load as low as 100 mN was selected for Vickers indentation experiments. The essential steps of the experimental investigations lie in the discrimination between the reversible and irreversible contributions and between the different irreversible deformation processes as well. In particular, the indentation depth was found clearly insufficient to provide a realistic picture of the elasticity process and doesn’t allow for the estimation of the densification contribution. Quantitative results were obtained by means of accurate volume
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