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Y. Li et al. /Materials Research bulletin 37(2002)1401-1409 (b) t-BN a-Al2O3 Fig. 3. Micrographs from: (a) TEM of Al2O3/BN nanocomposite powders; and(b) SEM of the polished surface from Al,O/30 vol. BN nanocomposite ceramics. 3.2. Mechanical properties Variations in fracture strength and fracture toughness with BN volume fraction for AL2O3/BN nanocomposite ceramics are plotted in Fig. 4. The fracture strength of composite ceramics in this work decreased with an increase of BN content. Even so, the materials exhibited a high strength of 478 MPa with BN content up to 20 vol %, which was approximately 85%0 strength of Al2O3 monolithic ceramics. While the hardness was only 60% of that(8 10 GPa) for the composite and 13.52 GPa for Al2O3 monolith Considerably low strength and toughness for Al2O,Si3N4 microcomposite ceramics were caused by a large amount of Sio2 glass in the material after hot-pressing Based on linear fracture mechanics concept of brittle ceramics, fracture strength of ceramics is controlled by fracture toughness and the size of critical flaws introduced by processing defects, such as large pores, agglomerates of addition phases and abnormally grown grains. Soft h-Bn particles with layer-structures can relax stress and absorb energy at the crack tip through microcracking or crack-particle interac tion,and then prevent the main crack from extending which is propitious to improve fracture toughness. BN-toughened oxide composites have been studied systemically [12, 13]. Thus, as shown in Fig 4, the fracture toughness of Al2O3/BN nanocomposite ceramics increases with BN content and a maximum toughness of 5.5 MPa m-is obtained. Although, the addition of bn no doubt introduces faws and influences strength of the materials to some degree, a remarkable decrease in fracture strength has not been observed in this work because the fracture toughness was improved due to fine h-Bn dispersions 3. 3. Mach The Al2O3/BN nanocomposite ceramics containing BN, no less than 20 vol % in this work, exhibited excellent machinability, which is very important and has practical applications for engineering ceramics. Machinability improved with the increase of bn volume fractions or the decrease of vickers hardness-*/* 2 2 4 % & % ( % % & L/ </ 6 % % 5 & & % / 2( 3 & % <;@ B ) (/0 3 , @?0 % / Q & , E)0 % #@/*) B' % & * /? B % / 8& , & % 1 < & , % 1 % - / & % % % % & , % & . % M & & , & % % && & , / 1% - , - 3 & , 5 5 5- - & ( 5 % 3 & ( % / - & 3& ( & & , , I* * J/ 6 L/ < % % & 3 % ?/? B * &/ && % & & M & M % & 5 & % ( & 5 % ( & & H - & / -*-* 6 ) (/0 5 3 & 3 , ( , & % / , ( & % ( % & % 45 & / L/ / %D #' 62 % & + & #' 12 % & % % ) (/0 / *<)E )* * + ! , - ./ /0 1