Solid-state sintering is the preferred method used to produce fine-grained ceramics with high relative density because of process simplicity. A large variety of high purity precursor powders are commercially available with common refractory oxides such as Al2O3, ZrO2, MgO, and others produced in industrially significant quantities. The process of sintering will only be briefly reviewed here since several excellent texts [36, 37] and overviews are available [38, 39]. In addition, numerous papers have been published on the sintering of specific ceramic compounds.
During solid-state sintering, porosity in powder compacts is reduced from 40 to 60 vol% in green bodies to values that can approach zero in finished parts . As the porosity is removed, the volume of the part decreases while modulus and mechanical strength increase . Solid-state sintering is driven by the reduction of surface free energy that occurs when high energy solid-vapor interfaces (e. g., particle surfaces) are replaced by lower energy solid-solid interfaces (e. g., grain boundaries) . For academic study, the sintering process is divided into stages: (1) initial sintering, (2) intermediate sintering, and (3) final sintering . These stages can be defined in terms of physical changes in the compacts such as grain size or total volume, variations in physical properties such as relative density, or differences in mechanical properties such as moduli . The sintering rate, ultimate relative density, and final grain size are affected by the particular oxide chemistry that makes up the compact, its initial particle size, and the efficiency of particle packing after consolidation. In general, effective solid-state sintering is limited to powders with relatively fine (~10 pm or less) particle size.
Densification of powder compacts requires mass transport. In solid-state sintering, material is transported from the bulk or the surface of particles into pores. To overcome kinetic limitations and promote mobility of atoms, a powder compact is heated to a significant fraction of its melting temperature. Sintering temperatures for single phase oxides typically fall in the range of 0.75-0.90 of the melting temperature (Tm). For example, mullite (incongruent melting point 1890°C or 2,163 K ) with an initial particle size of approximately 0.2 pm can be sintered to ~98% relative density by heating to 1600°C (1,723 K or 0.80 Tm) for 2 h [41, 42]. The resulting ceramic had a final grain size of approximately 1 pm (Fig. 3) and a microstructure typical of solid state sintered, fine grained ceramics.
Sintering temperature and rate are also affected by particle size. Precursor powders with a “fine” grain size reach the same density at lower temperatures compared with “coarse” grained powders . Smaller particles have a greater surface area to volume ratio and, therefore, a higher driving force for densification, which can lower the temperature required for densification . In addition to precursor powder particle size, the packing of particles prior to sintering affects densification. Nonuniform particle packing can result in the formation of stable pores in fired microstructures . As the pore size approaches the grain size, the driving force for pore removal approaches zero; pores that are larger than the grains are, therefore, stabilized due to a lack of driving force for removal . Stable pore formation is especially problematic when
Fig. 3 Microstructure of a solid-state sintered mullite ceramic with a relative density > 99%. The ceramic had equiaxed grains with a grain size of ~1 |im and no apparent glassy phase
nano-sized particles are employed because of their propensity toward formation of hard agglomerates.
The grain size of a sintered ceramic affects its performance. During solid-state sintering, grains grow during the final stage of sintering as full density is approached . As with the densification process, grain growth is also driven by a reduction in surface energy; however, elimination of grain boundaries (solid-solid interfaces) in dense solids is less energetically favorable than the elimination of free surfaces (solid – vapor interfaces) in porous compacts . The grain size required to achieve the performance requirements for a particular application may be smaller or larger than the grain size that would result from the optimal heat treatment. Grain growth can be altered by changing the time and temperature of the heat treatment . In many cases, trace additives or dopants can be used to further modify grain growth . Some dopants dissolve into the matrix altering its defect chemistry and thereby affecting material transport rates. Other dopants remain as discrete particles that affect grain growth simply by their presence in grain boundaries. The classic example of a particulate dopant that inhibits grain growth while promoting sintering is the addition of MgO to Al2O3. When a-Al2O3 is sintered at 1600°C in air, the average grain size is ~5.0 |jm and a density of ~97% is achieved . For the same a-Al2O3 doped with 250 ppm MgO sintered under the same conditions to the same density, the grain size is only ~3.5 |dm .