Search

Abstract: 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. [Copyright &y& Elsevier]

Many advanced applications of glass demand fabrication of engineering parts of utmost dimensional precision which require very accurate grinding and polishing that involves controlled removal of glass. Despite the wealth of literature, however; the mechanism of material removal in glass grinding and polishing is still far from well understood. For instance, it is not known at all to what extent the mechanical properties are compromised inside a scratch groove so as to optimize the machining parameters. Therefore, to develop better understanding about the mechanism of material removal, a series of combined nanoindentation and single pass scratch experiments were conducted on a commercially available soda-lime-silica glass as a function of various normal loads (2-20 N) and scratch speeds (0.1-1 mm/s). It was found that the nanohardness and Young's modulus at the local microstructural length scale inside the scratch groove could decrease quite dramatically (~30% to 70%) depending on the combination of load and scratching speed. Further, the tribological properties, the severity and the spatial density of damage evolution were sensitive to the normal loads, scratching speeds, and tensile stresses. Extensive scanning electron microscopy leads to interesting observations on material removal mechanisms. These observations were explained by the theoretical predictions of a model for a brittle, microcracked solid. [ABSTRACT FROM AUTHOR]

To understand how high-strain rate, flyer-plate impact affects the nanohardness of a coarse (~10 μm) grain, high-density (~3.978 gm cc) 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. [ABSTRACT FROM AUTHOR]

We report the fracture toughness ( K) measured by nanoindentation technique at five different loads in the range of 100-1000 mN on the cross section of a highly porous microplasma-sprayed hydroxyapatite (MIPS-HAP) coating. The unique advantage of this technique was that it could evaluate K at a scale comparable with that of the local microstructure of the coating. In spite of high porosity (∼11 vol%), the coating showed a toughness (∼0.6 MPa m) comparable to typical literature data (∼0.5 MPa m) on dense HAP coating. Scanning electron microscopy of nanoindents suggested some plausible reasons for achieving a relatively higher toughness in the coating. [ABSTRACT FROM AUTHOR]