Pressure-assisted sintering

The possibility to obtain a uniformly dispersed composite powder was shown for the a-Fe-Al203 system where metal particles with an average size of 55 nm were formed in an amorphous/nano alumina matrix.18 Other studies attempting to obtain dense bulk composites based on the sol-gel route using conventional pressure-assisted sintering ( 1400°C and an applied force of 10 MPa) resulted in a coarse microstructure.16 However, if reaching theoretical density is not a necessary requirement, a porous ceramic microstructure containing nanometer-sized metal particles can be used as a catalytic material.19 Certain combinations of composite materials demand... 

Application of pressure during sintering results in a lower final porosity as compared with pressureless processes. External pressure increases the pressure difference created by curved surfaces, Eq. (3.6), and facilitates pore elimination. For the usual pressures in hot press sintering (or pressure-assisted sintering), this increase in pressure difference is 5-10 times (McColm Clark, 1988). As a result, sintering temperatures can be reduced by 300°C and higher final densities are obtained. 

Because RBSN employs gaseous reactants, the SiC/RBSN material tends to have higher levels of frequently interconnected, residual porosity than the SiC/Si3N4 composite fabricated by pressure assisted sintering methods. Interconnected residual porosity remains an important issue for two reasons oxidation and thermal conductivity. Internal oxidation can lead to internal stresses which may cause premature matrix cracking and fiber delamination. Thus, to avoid internal oxidation protective coatings may be necessary for these materials. 

Power-law creep can also be a major densification mechanism in pressure-assisted sintering. At the early stage of densification achieved by power-law... 

Comparison of Eqs (5.27) and (5.28) with Eqs (5.9) and (5.10) shows that the densification rate by the capillary pressure is higher than that by the externally applied pressure under certain conditions. This result is due to the reduction of pore size and the increase in capillary pressure above the external pressure with pore size reduction. The very final densification is thus expected always to be governed by pressureless sintering kinetics. The densification rate in pressure-assisted sintering is, of course, the sum of those due to the capillary pressure (Eqs (5.9) and (5.10)) and to the external pressure (Eqs (5.27) and (5.28)). The contribution of capillary pressure increases as the external pressure decreases and also as the pore size decreases with densification. 

Discuss the effects of particle shape on densification during pressure-assisted sintering and pressureless sintering. 

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