**3. Experimental procedure**

406 Sintering of Ceramics – New Emerging Techniques

Zhou et al. (Zhou et al., 2003) showed that triple junction at large grain sizes is not significant since its volume fraction is negligible compared with the total interface fraction. It is believed that occurs when the passage to the second stage of sintering, the energy in the triple junction during the whole period of time, remains constant. If there is increase in temperature it may be due to increased energy of the system, so there may be a greater mobility of the triple junction in comparison with the grain boundary, so the contour can move freely without any difficulty, indicating a common growth grain. At low temperature, the triple junctions make difficult the movement of grain boundaries not allowing grain

Nanometric and sub-micrometric alumina powders (Hesabi et al., 2009; Li & Ye, 2006) were also sintered in two steps. Ye and Li (Li & Ye, 2006) found that it is necessary that nanosized alumina powders reach 85% theoretical density in the first stage of sintering, so it can be fully densified at the second level, while Bodisova (Bodisova et al., 2007) showed that density should not be less than 92% theoretical density to achieve full densification without

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

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

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,

growth occurrence (Czubayko et al., 1998).

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

with 99% density.

grain growth in the second level for post sub-micron alumina.

mechanical mixing methods (Sakka & Hiraga, 1999; Susuki, 2001).

Initially, the alumina powder was processed to remove the hard agglomerates. The following procedure was used: powder was dispersed in isopropyl alcohol with 0.2 w% of PABA (4-aminobenzoic acid) and 0.5 w% of oleic acid. The suspension was submitted to a ball mill during 10 h, using zirconia balls (ball/powder in mass ratio of 2:1) in a polypropylene vial. Suspension was dried at 75ºCand then pulverized and sieved.

For dispersion of zirconia nanometric powder in the alumina powder a ZrO2 suspension was prepared through traditional balls milling (ZrO2 balls with 5mm diameter) using 0.5 wt% of deflocculant PABA (4-aminobenzoic acid) in alcoholic medium with a balls/powder mass ratio of 4:1. After 12 hours milling, suspension was separated through the milling and reserved. Simultaneously, Al2O3 suspension in alcoholic medium was prepared with 0.2 wt% PABA with a balls/powder ratio of 5:1 for 1 hour in balls mill. 5 vol% ZrO2 previously prepared were added to this suspension under agitation. Then, final suspension was mixed in conventional balls mill for 22 hours. Finally, 0.5w% oleic acid was added to the suspension and mixed for 2 more hours. The obtained mixtures were dried at room temperature under flowing air.

Prior to sintering experiments, samples of pure alumina were uniaxially pressed under 80 MPa into cylindrical compacts (ø=10 mm, and height of about 5 mm) and isostatically cold-pressed under 200 MPa. Samples were heat-treated at 600°C in air for 1 h to eliminate organic materials. Green density of samples was about 59% of the theoretical density (%TD).
