Hardness of alumina/silicon carbide nanocomposites at various silicon carbide volume percentages

Vickers indentation was employed to measure the microhardness of monolithic alumina and six alumina-based nanocomposites consisting of variable silicon carbide nanoparticle volume percentages of 0.3% to 20%. Indentation tests were performed over a broad range of loads from 0.5N to 40N. The resultant hardness-load curves exhibit cumulative increases in the apparent hardness based on the silicon carbide content and reveal each sample suffers from a prominent indentation size effect (ISE). Herein, we present a comprehensive analysis of this data using Meyer’s Law, the proportional specimen resistance model (PSR) and the modified proportional specimen resistance model (MPSR) and employ TEM imagery to detail potential mechanisms by which silicon carbide nano-reinforcements influence the “true hardness” and the ISE

Plastic deformation and cracking resistance of SiC ceramics measured by indentation

Hardness is deemed as a measurement of resistance to plastic deformation of materials, but cracking, accompanying the deformation of ceramics, has recently been reasonably interpreted as one of the essential contributors to the widely known size effect of hardness, also called the indentation size effect. On the basis of the physical activities behind the indentation size effect, we intend to have the indentation size effect measurement adapted as a means to quantitatively measure the plastic deformation and the cracking damage of ceramics. We studied three types silicon carbide ceramics that were manufactured from same chemical formulation and processing route. Two of the three were given a post-sintering annealing at different dwell times, giving a distinctive microstructure from homogeneous equiaxial grains to highly heterogeneous elongated grains. By fitting the indentation size effect data with proportional specimen resistance model, the a1 and a2 parameters are extracted with good confidence. The a2 value is interpreted as the “true hardness”, the determined values of which were found to be similar for all samples. However, the a1, relatable to the indentation size effect, increased dramatically together with the indentation-induced cracking around Vickers indents observed under SEM. Considering that the cracking resistances of all three samples showed limited variance, we discuss, using a simple illustrative model, the possible microstructural factors which may contribute to the cracking damage exhibited under a defined loading condition

Chemically bonded phosphate ceramics reinforced with carbon nanotubes

We report herein, a scalable method for the preparation of alumina (Al2O3)-phosphate ceramics reinforced with carbon nanotubes (CNTs). All composites were manufactured by direct on-site growth of CNTs on ceramic particles via catalytic chemical vapour deposition. Introduction of catalyst metals to the substrate was achieved through two simple approaches, drip-coating and vacuum filtration, both of which have been reviewed. Transmission electron microscopy was utilised to investigate the interface between the Al2O3 surface and the in-situ CNTs. Resultant ceramics were produced by impregnating phosphoric acid into the Al2O3+CNT nanocomposite powder followed by die-pressing. In order to maintain the integrity of the CNTs, dehydration/curing was performed at 130-150○C. Scanning electron microscopy was elected to comparatively characterise the microstructure of this type of ceramic nanocomposite against its monolithic equivalent. Possible mechanisms by which specific features have formed are discussed

Plastic deformation of polycrystalline aluina introduced by scaled-down drop-weight impacts

We present our findings after scaled-down drop-weight tests, performed under relatively low loading conditions and employing a small-scale spherical indenter as a projectile, to boost the strain rate and energy density of the impact, resulted in the generation of a cavity of measurable depth on the surface of a pure, fully dense, alumina ceramic. We demonstrate that activated dislocations are a main contributor in the formation of the residual impression with an estimated maximum density of ~4.02×1014 m−2

Contact damage of silicon carbide ceramics with different grain structures measured by Hertzian and Vickers indentation

We have used Hertzian and Vickers indentation to investigate contact damage in sintered SiC ceramics, one consisting of uniform, fine-grains and the other a coarse, elongated grain structure. Cracking-resistance measured by Hertzian indentation, showed no discernible difference, nor did the Vickers hardness. However, numerical analysis of the Vickers indentation size effect, performed using the proportional specimen resistance model, indicates 77.3% greater surface energy, mostly realised through cracking, is experienced by heterogeneous SiC per unit area of indentation impression. This is typified by an observable increase in the number of radial cracks generated around Vickers impressions, which has been found to artificially increase the K IC determined by Vickers indentation fracture. Quantitative measurements of pre-existing flaws by Hertzian indentation show that heterogeneous SiC retains a higher density of larger flaws. Relationships between the differences in cracking around Vickers indents and the pre-existing flaw populations of these two SiC ceramics are discussed