Development of Dense Hydroxyapatite Nanoceramic by Pressureless Sintering

In order to improve fracture toughness, nanocrystalline hydroxyapatite (HA) powder was used for fabricate dense HA nanoceramic. The nanocrystalline HA powder was obtained with ball milling for 12 h from calcined natural bovine bone. The green compacts of HA nanopowder were sintered by pressureless sintering with non-linear heating rate sintering at 1200°C for 3 h. Fracture toughness and hardness properties of HA nanoceramic have been assessed using indentation technique. The fracture toughness values of sintered nanosized-HA powder is 3.50.1 MPa.m1/2, which is within those of human cortical bone. The nanostructure with liquid phase sintering of sintered nanosized-HA powder contributed to the increase of fracture toughness. The toughening mechanisms of pull-out of nanocrystalline HA and grain boundary sliding have been observed.

Microstructural Evolution and Densification Kinetics During Sintering of Oxide-Dispersed Tungsten Alloys

The present paper discusses the role of ceria and hafnia dispersions in tungsten alloys on the microstructural evolution and densification kinetics during sintering. Densification kinetics were measured using dilatometry, and microstructural changes were examined using scanning electron microscopy and Auger electron spectroscopy. Activation energies for sintering were obtained by analyzing the shift of the iso-density points as a function of linear heating rate. Sintering of both tungsten and ceria-dispersed tungsten were found to be controlled by grain boundary diffusion, with apparent activation energies of 318±21 and 385±15 kJ/mole, respectively. However, densification of hafnia-dispersed tungsten is not controlled by a single mechanism. Under different conditions hafnia can enhance or retard densification; the mechanisms associated with this behavior are discussed. In particular, the relationships between sintering behavior and the tungsten-ceria and tungsten-hafnia interfaces are examined. Comparison with conventional oxide dispersoids, such as thoria, will also be made.