Initial Stage Of Densification Research Articles

Vacuum Hot Pressing of Tungsten Powders

In order to study the densification behaviors, several tungsten powders were hot-pressed in vacuum at 1100-1300°C for 10 min under the pressure of 750-875kg/cm2. The density of the sintered specimens was 60-82% of the theoretical value, indicating that they are still in the initial stage of densification. Without additives, higher density is achieved with fine powders, but the density is not a simple function of the average particle size. In mixed powders, different densification behaviors were observed at lower temperatures and the densification is not affected significantly by the powder parameters. Summarizing, important factors in densification are powder, temperature and pressure in the order stated.

Densification of the Plastic Ball Compacts during the Hot-Pressing

For the purpose of getting a better insight into the initial stages of densification during the hot-pressing, the hot-pressing experiments using the compacts of amorphous polystyrene sphere were performed. The size of the amorphous polystyrene plastic sphere was 0.695mm in diameter.The obtained experimental data showed that the densification curves were divided into three parts. These parts were characterized by an initial rapid densification followed by an intermediate stage such as Murray's relation and finally a much slower process. On the other hand, it has been reported that the rapid densification in the initial stage was attributed to; (a) the particle rearrangement, (b) the influence of the porosity on the applied load, (c) the plastic flow within the solid particle.To ensure these effects, the creep experiments using the sintered polystyrene sphere compacts which had various porosities were performed. From these experiments, it was found that the visco-elastic deformation within polystyrene itself and the influence of the porosity on the applied load could be effective, the former in paticular. In spite of these facts, the rearrangment of the spheres within the sintered body could not be observed in these experimental conditions. These were the contrast to the reports which had been presented.In the intermediate stage of densification, Murry's relation was valid for the experimental data. In this stage, it was reasonable that the densification proceeded by the viscous flow characteristics of polystyrene sphere.The final stage of the densification which was a much slower process was attributed to the pore closure as some investigaters reported.Besides, the effects of porosity to the creep rate in the sintered polystyrene plastic sphere compacts were determined. As the results, the following experimental equations were obtained.J/J0=1+5PJ=J0 exp 3.7PJ0: creep compliance for the zero porosity specimenJ: creep compliance for the P % porosity specimenP: prorosity %

Hot‐Pressing of Alumina Powders at Low Temperatures

A better insight into the initial stages of densification during the hot‐pressing of ceramics can be obtained by performing the experiments at low temperatures. The parameters which have been examined are pressure, temperature, and the particle size of the alumina used. The results obtained preclude plastic flow as the predominant mechanism responsible for the large increases in density observed initially. Particle rearrangement by boundary sliding is suggested as the process responsible for the initial increase in compact density.