**2.2 Addition of particles of a second phase**

A strategy used to achieve nanometric grain sizes is through addition of solutes or particles of a second phase in single-phase ceramics, which reduce the grain boundary mobility or fix the grain boundary, respectively (Novkov, 2006). This strategy has been used successfully by many researchers. Chaim et al. (Chaim et al., 1998) added 4 wt% trivalent oxides (Y, La, Bi) and tetravalent oxides (Ce, Th) in nanocrystalline zirconia powder and found that Y2O3, CeO2 and ThO2 inhibit grain growth during sintering. According to Mayo (Mayo, 1996), Hahn et al. added to a powder Y2O3 nanocrystalline TiO2 to limit grain growth. Part of Y2O3 dissolved in the regions of grain boundaries and partly reacted with TiO2 to form a second phase in grain boundaries. These two effects have limited the growth of grains so that the Y2O3 sintered without applying pressure reached 90% density with 50nm grain size and Y2O3; when adding TiO2, sintered under the same conditions, it showed 30nm grain size with 99% density.

Recent studies have shown that grain growth inhibition during sintering, which favors increase in mechanical properties of the nanocomposite, occurs by adding small amounts of nanosized zirconia inclusions in a ceramic body of alumina matrix. Grain growth inhibition has also been observed with nanometric inclusions of silicon carbide. However, densification during sintering is difficult by the presence of zirconia in alumina. Other problems were reported in the literature: tendency to particles agglomeration and difficulty to dispersion of nanosized particles of zirconia in alumina matrix, particularly for mechanical mixing methods (Sakka & Hiraga, 1999; Susuki, 2001).

Trombini et al. (Trombini et al., 2007) dispersed powder of alumina and zirconia separately, which allowed them to obtain a complete and homogeneous dispersion of nanosized particles of zirconia in alumina matrix. The Spark Plasma Sintering (SPS) could be used to obtain samples with densities close to theoretical density with very homogeneous microstructure and grain size similar to the initial particle size of powder with at 1300 ºC sintering temperature. Pierri et al. (Pierri et al., 2005) observed that the presence of small amounts of zirconia (1 vol%) was sufficient to cause an grain growth inhibition of alumina, allowing the sintering process without application of pressure that results in higher final densities and increased mechanical strength and wear resistance.
