Nanomechanical properties of coarse grain alumina ceramic

The hardness of alumina ceramics determines their performance in high end structural applications like wear resistant inserts, coatings, cutting tools, biomedical load bearing implants and armor face plate etc. Therefore, the nano-hardness (H) and Young’s modulus (E) of a pressureless sintered, 99.99% dense, 10 μm grain size alumina were measured by nanoindentation at 10 mN to 1000 mN load with a Berkovich indenter. The decrease in nano-hardness with the increase in indentation depth revealed the presence of an indentation size effect (ISE) which was explained by the application of the well established Nix and Gao model. The measured value of Young’s modulus (400 GPa) was insensitive to variation in loads applied during the nanoindentation experiments

Shock deformation of coarse grain alumina above Hugoniot elastic limit

Symmetric shock experiments were conducted on a 10 mu m grain size coarse alumina ceramic with a gas gun to identify its Hugoniot elastic limit (HEL). To understand the damage initiation and their subsequent growth mechanisms in coarse grain alumina subjected to shock impact at levels much above the HEL, additional asymmetric shock recovery experiments with the same gas gun were then deliberately conducted on the same alumina at shock pressure levels more than three times as high as the HEL and the fragments collected by a dedicated catcher system. Detailed characterization of the shock recovered alumina fragments by X-ray diffraction, nanoindentation, scanning electron microscopy, field emission scanning electron microscopy and transmission electron microscopy were utilized to understand the nature and process of failure initiation, incubational growth, coalescence and propagation leading to fragmentation. Based on these data a new qualitative damage model was developed to explain the deformation mechanism

Nanohardness of Sintered and Shock Deformed Alumina

To understand how high-strain rate, flyer-plate impact affects the nanohardness of a coarse (similar to 10 mu m) grain, high-density (similar to 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 shock-recovered 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

Indentation size effect of alumina ceramic shocked at 12 GPa

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 (similar to 10 mu m), high density (similar to 3.98 g.cm(3)) 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. (C) 2012 Elsevier Ltd. All rights reserved

Nanoindentation of shock deformed alumina

In the current study, the experimental results on the nanoindentation response of both as prepared and shock recovered alumina of 10 mu m grain size and identical processing history are presented and analyzed. The shock recovery experiments were deliberately conducted with gas gun arrangements at shock pressures much above the Hugoniot Elastic Limit (HEL) of alumina. The nanoindentation experiments were conducted at 10-1000 mN load with a Berkovich indenter. The nanohardness and Young's modulus value of shock recovered alumina were always lower than those of the as prepared alumina samples. Subsequently, the detailed characterizations of the shock recovered alumina samples by X-ray diffraction, scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) were utilized to understand the reasons behind the drop in nanohardness and Young's modulus of shock recovered alumina samples. (C) 2010 Elsevier B.V. All rights reserved

Electron microscopy of shock deformation in alumina

Shock recovered samples of a coarse grain (10 mu m), high density (>99.9% theoretical) alumina from asymmetric impact tests conducted at 6.5 GPa (e.g. 3.2 times its Hugoniot Elastic Limit) in a single stage gas gun and characterized by X-ray diffractometry, scanning and field emission scanning electron microscopy, and transmission electron microscopy showed prolific presence of reduced crystallite size, higher average microstrain, grain localized micro/nano-scale deformations, micro-cleavages, grain-boundary microcracks, micro-wing crack formation, extensive shear induced deformations and fractures localized at grains, grain boundaries and triple grain junctions, grain localized entanglement of dislocations and their pile up impeded at grain boundaries. A new qualitative model based on micro-shear and micro-twist induced deformation and fracture in single and/or multiple planes in suitably oriented grain and/or grain assembly was developed to explain the experimentally observed damage evolution process. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved