Nanohardness of Sintered and Shock Deformed Alumina

Abstract

To understand how high-strain rate, flyer-plate impact affects the nanohardness of a coarse (~10 lm) grain, high-density (~3.978 gm cc -1 ) alumina, load controlled nanoindentation experiments were conducted with a Berkovich indenter on as-sintered disks and shock-recovered alumina fragments obtained from an earlier flyer-plate shock impact study. The nanohardness of the shock-recovered alumina was much lower than that of the as-sintered alumina. The indentation size effect was severe in the shock-recovered alumina but only mild in the as-sintered alumina. Extensive additional characterization by field emission scanning electron microscopy, transmission electron microscopy, and analysis of the experimental load depth data were used to provide a new explanation for the presence of strong indentation size effect in the shockrecovered alumina. Finally, a qualitative model was proposed to provide a rationale for the whole scenario of nanoindentation responses in the as-sintered and shock-recovered alumina ceramics.

Grain Boundary Nanohardness of Coarse Grain Alumina

“…However, it was reduced to about 21 GPa when measured at an applied load of 10,000 μN. In other words, the nanohardness data evaluated at both CG and ACG regions reflected the presence of an indentation size effect (ISE), as expected for structural ceramics and ceramic matrix composites …”

“…Assuming thereby that these defects may reduce the nanohardness of glass by even as much as 50%, the effective nanohardness of the grain boundary glass may be assumed to be about 2.1 GPa. The typical, load‐independent nanohardness of alumina can be taken as approximately 20 GPa …”

“…This is exactly from where the motivation and the scope of the present work start to blossom out. It is important to recognize in this context that in recent times the nanoindentation technique has emerged as a very effective technique to determine the surface mechanical properties of a truly wide variety of materials at the microstructural length scale . With the advent of truly revolutionary development of nanoscience and technology, application of loads well below 10 mN and sensing the ultrasmall depth of penetrations both accurately and reproducibly are possible today.…”

Comparative Study of Indentation Size Effects in As-Sintered Alumina and Alumina Shock Deformed at 6.5 and 12 GPa

“…Recently, an explanation for the observation of strong ISE in nanohardness of alumina shocked at 6.5 GPa was reported by us [28]. The objective of the present work is to examine the presence of ISE in naonohardness of alumina shocked at 12 GPa and compare the results with those of alumina shocked at 6.5 GPa [28] so that a generic mechanism of ISE in nanohardness of shock-loaded alumina can be provided.…”

“…The experimental details have been already reported in [27][28][29][30][31] and will be only briefly mentioned here. The sintered alumina discs (diameter ∼50 mm, thickness ∼2.48 mm) of 10 μm grain size and 99.95% theoretical density had a Hugoniot elastic limit (HEL) of 1.91 GPa [27].…”