Materials for engineering, 3rd Edition - (Malestrom)

Glasses and ceramics 141
Thus, the smaller the void, the greater the closure pressure. Efficient
sintering is, therefore, promoted by the use of precursor powders of fine
particle size. The diffusional process requires the presence of lattice vacancies
and ceramics of covalent bonding have a very high formation energy for
vacancies and, therefore, exhibit low solid state diffusion rates, giving poor
densification properties. Atom transport is predominantly by grain boundary
diffusion so, again, a fine grain-size is essential for efficient densification. In
most cases, a ‘densification aid’ or grain growth inhibitor is added to the
ceramic to achieve maximum density and minimum grain size. In the case of
carbide and nitride ceramics these are metal oxides; LiF is added to alumina
and magnesia for this purpose. The additives segregate to the newly-formed
grain boundaries during sintering and increase the diffusion coefficient by
forming low melting-point or low viscosity glass phase. The best sintering
aids also suppress grain growth in the component, which would otherwise
lead to a reduction in the number of diffusion paths, thus slowing the
densification rate. Voids would become ‘stranded’ in large grains, with no
fast pathway for mass transfer, and remain as a likely source cracking when
the component is under stress in service.
The microstructure of a pure, polycrystalline engineering ceramic can be
seen by polishing, etching and magnifying. The important features are the
grain size and the degree of porosity, and a dense ceramic is thus similar in
microstructure to a polycrystalline metal (Fig. 4.3). Table 4.3 summarizes
the properties of some of these materials. The data give a general indication
1 µm
4.3 Pure polycrystalline alumina (Courtesy of Dr R. L. Riley and
Dr M. Miranda-Martinez.)