Sintering surface shrinkage

During pressure sintering, interiDarticle compressive stress, approximated by the externally applied stress and nonnalized by the relative density of the compact p, supplements the surface tension driving force for pore shrinkage ... 

In the same year as Kuezynski s research was published, Shaler (1949), who had done excellent work on measuring surface energies and surface tensions on solid metals, argued that surface tension must play a major part in fostering shrinkage of powder compacts during sintering his paper (Shaler 1949) led to a lively discussion, a feature of published papers in those more spacious days. 

Stainless steel powder with a mean particle diameter of 50 mm has been compacted to a green density of 58% and sintered in pure H2. The resulting shrinkage measurements are given below. Published diffusion data for this stainless steel show that the activation energies are 225 kJ/mol for surface diffusion, 200 kJ/mol for grain boundary diffusion, and 290 kJ/mol for volume diffusion. Use the data below to determine the mechanism. 

Remove the organic polymer in an ammonia gas atmosphere and sinter the remaining in a furnace. Upon heating, the volatile components escape and the nickel oxide fiber surface becomes uniformly porous to form a nickel oxide membrane. Significant shrinkage can be expected. 

Processes (c) and (d) are of less significance for ceramics where densification is usually required. Surface diffusion, however, is probably involved in the early stage of sintering when partial strengthening of the powder compacts occurs with no shrinkage involved. 

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