Y. Li et al. /Materials Research Bulletin 37(2002)1401-1409 1405 react with urea on the surface of a-Al2O3grains. It shows the disappearance of peaks of boric acid and urea, and the appearance of peaks arising from t-BN (20= 26.20 after reduction, which agrees with the XRd data of the mixtures comprising only boric acid and urea under the same condition. t-BN grain size was about 30 calculated using the Scherrer equation [10] according to the typical t-BN peak (20= 26.2%)in Fig. 2b. Additionally, Al-O-B compounds as semifinished product mainly containing Al18B4033, appeared during the reduction procedure h-Bn (from the peak of 1002) reflection, 20= 26.8%)appeared, taking the place of t-Bn during the hot-pressing process. Most of the Al18B4O33 disappeared in this step, as evidenced by the much lower intensity(I/lo) of the XRD peaks, compared to that before hot-pressing. Notably, Sialon phases(containing varied a-Sialon with different components) appeared in the hot-pressing step, and no Si3N4 peaks remained; while no Sialon phases appeared in Al2O3/10 wt %o Si3 N4 mic ites.However, Si3N4 peaks disappeared and glass phase peaks appeared arising from Sio2. It is indicated that the semifinished products, Alg B4O33, turned into Al2O3 and BN, and si3 N4 reacted with the matrix Al2O3 in the hot-pressing step. The estimated equations are expressed as following Al18B4O3+N2+6C→9A2O3+4BN+6CO↑ (1) xAlO3+ySi3 N4 - zSialon In order to obtain fully dense bodies at lower temperatures, a small quantity of Si3n4 was added into the composites as sintering aids. The dispersion of nano-sized BN made sintering temperatures rise due to its inhibition to Al2O3 boundary migration. Furthermore, if Si3 N4 is not added into the sintering bodies, macro-cracks ill emerge when BN content is more than 20 vol %, which leads to poor strength This was mainly caused by a difference in the coefficient of thermal expansion between a-Al2O3 and h-BN, in our point of view. The coefficient of thermal expansion of a-Al2O3 is 8 x 10-/C, but that of h-BN is-2.9 x 10/C in a-axis direction and 40.5 x 10-b/oC in c-axis direction, respectively. The tensile stress that originated at the interfaces between the matrix and the dispersoid accumulated with the increase of bn volume fraction and, finally, resulted in destructive cracking. Microcracks were also observed in Si3//BN nanocomposites [11]. The formation of Sialon phases in the composite ceramics contributed to reduce thermal expansion due to its lower coefficient of thermal expansion. < Fig 3a shows a representative TEM picture for the reduced powders. It is observed hat the al-Al2O3 particles are surrounded with a disordered layer of t-BN, which was identified using XRD and EDS Fig 3b shows the corresponding SEM micrograph of polished and etched surface of hot-pressed body. The surface is parallel to the hot- pressing direction Matrix grains are regular, about 1 um in size. Coarse grains larger than 2 um were not found. BN grains 100-200 nm in diameter can be seen dispersed within the matrix grains as well as club-shaped bn grains 400-600 nm in length, 100-200 nm in width at the grain boundaries. No evident directional alignment for bN grains was observed in the materials for the surface morphology perpendicular to the hot-pressing direction being similar to that of the parallel one.      % %
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