Indentation size effect of alumina ceramic shocked at 12GPa

The motivation behind this study was the urge to understand how the high strain rate flyer plate impact affects the nanohardness of alumina ceramics. Therefore, the load controlled nanoindentation experiments were conducted with a Berkovich indenter on an as received coarse grain (~10μm), high density (~3.98g.cm3) alumina and the shock recovered tiny fragments of the same alumina. The shocked alumina fragments were obtained from an earlier flyer plate shock impact study in a two stage gas gun. The nanohardness of the as received alumina was much higher than that of the shocked alumina. The shocked alumina showed a relatively much stronger indentation size effect (ISE) while the as received alumina exhibited a mild ISE. A new explanation was given for the presence of the relatively strong ISE in the shock recovered alumina. Additional characterizations such as scanning electron microscopy, field emission scanning electron microscopy, transmission electron microscopy and analysis of the experimental load depth data were utilized for this purpose. Finally, a new, qualitative model was proposed to provide a rational picture of the nanoindentation responses of the as received and shocked alumina ceramics. ► Strong ISE in Nanohardness of Shocked Alumina. ► Weak ISE in Nanohardness of As Received Alumina. ► Explanation of ISE by concept of Interaction between Nanoindent and Microstructural Damage. ► Extent of ISE controlled by relative ease of Indenter penetration governed by relative frequency of Multiple Micro pop in Events. ► Important role of Shear stress generated just underneath the nanoindenter.