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Y. Li et al. /Materials Research Bulletin 37(2002)1401-1409 H3BO3+ CO(NH2)2 +a-SiaN(10wt Riddling(#300) Fig. 1. Flow chart of fabrication processes of the Al2O/BN composites The crystalline phases of the powder mixtures and hot-pressed bodies were identified by X-ray diffraction(XRD) analysis. The microstructure was observed by transmission electron microscope (TEM, JEM-200CX, Japan Electron Co. ) and scanning electron microscope (SEM, AMRAY-1000B, USA), respectively. The microchemical analysis was done using the energy dispersion spectroscopy(EDs) analyzer attached to the sem Bulk density was measured by the Archimedes immersion technique in distilled water. The Vickers hardness was measured by the vickers indentations method fracture strength was determined by a three-point bending test using a rectangular bar (mm x 4 mm x 25 mm) with the bend span of 20 mm and a crosshead speed of 0.5 mm/min. The tensile surface of specimens was parallel to the hot-pressing direction. The fracture toughness was measured using single edge notched beam (SENB) with three-point bending. The beam dimensions were 2 mm x 4 mm x 20 mm and the notch width and depth were 200 um and 1.5 mm, respectively. Each test for both or and Kic was done eight times in order to eliminate large occasional errors. Machinability was evaluated using hard metal alloy drill bits of two diameters under the varied conditions 3. Results and discussions 3.1. Phase compositions and microstructures Fig. 2 illustrates the transformation of crystalline phases in the processes of preparing the Al2O,BN composite ceramics through the chemical route using boric acid and urea as BN source. It can be observed in this figure that the starting powders before reduction in H, consisted of a-Al2O3, boric acid and urea boric acid does not6 , % & 3 & - & & & H & , N-, &%% #N$7' ,/ 6 ( & , #62 "2- ))8N " 2 8/' & #12 $ -*))) K1 ' ( ,/ 6 , & , & , #271' ,. & 12/ 5 & , & , & G & & / 6 45 & & , 45 & &/ L & & , - & # < ? ' & % ) & & & % )/? / 6 % % - & / 6 % & & & #12' - &/ 6 & < ) & & & & )) & */? ( ,/ 2 % % & C8 & & / , ( & & , & % & & ( & &/ -* * (2 $ ( , , L/ % % , % & & / C ( & H & % & : & % - & & / & & L/ */ L % % % / )* * + ! , - ./ /0 1 *<)