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1402 Y. Li et al. /Materials Research bulletin 37(2002)1401-1409 engineering ceramic components have complex shapes and hence, require machining by diamond tools. They cannot be machined using conventional metal tools such as cemented carbide, high-speed steel, etc. One of the significant disadvantages of ceramics is the high machining costs, which limits their application fields. It has been shown that rendering ceramics machinable usually caused significant decrease in fracture strength [1, 2] In recent years, attempts have been made to develop machinable ceramics with high fracture strength and high fracture toughness. Uno et al. [3 showed mica-based nanocomposite glass-ceramics, consisting of fine tetragonal zirconia particles, 20- 50 nm in size, embedded in plate-like mica crystals, which exhibited excellent mechanical properties with bending strength of 500 MPa, fracture toughness of 3.2 MPa mand good machinability. Baroum and El-Raghy [4]reported that Ti3SiC not only could be drilled and taped using high-speed steel tools, but also had a fracture strength up to 600 MPa. Its crystal structure was comprised of planar Si layers linked gether by TiC octahedra, and the microstructure was an analogous texture with plate shaped graphite, leading grains to be easily cleaved. Suganuma et al. [5]had fabricated machinable SiC/C ceramic composites, which still retained high strength over 200 MPa at 1500"C Niihara and co-workers [6, 7] reported Si3N4/BN nanocomposites with dispersed nano-sized h-BN. They claimed excellent machinability and high fracture strength(1100 MPa in max), which were attributed to the weak grain boundaries and nano-sized h-BN dispersions. These results imply that the super fine dispersoids with laminar structure and a low modulus could be expected to play a crucial role in improving the machinability of ceramics and its mechanical properties Since there has been no information about Al2O3-based machinable ceramics with both high strength and high fracture toughness, to our knowledge, we report in this paper the fabrication processes and mechanical properties of Al2O3/BN nanocom posite ceramics, and also discuss the machinable mechanism. 2. Experimental procedure In order to have homogeneous dispersion of h-BN into Al2O3 ceramics, a chemical process to precipitate the bn precursor on a-Al2O3 powders were adopted [8, 9]. In nis study, boric acid and an excess of urea were selected as the bn source to coat the a-AL2O3 particle surface. Fig. I shows schematically the experimental procedure. The Bn content was adjusted to be 10, 20 and 30 vol %o a-Al2O3 powders were ball milled with boric acid and urea in a plastic bottle with ethanol as medium using alumina balls. The slurry was dried at 60-65'C. Before drying, the electric-blower was used to evaporate most of the ethanol with mechanical stirring. The dried mixtures were kept at 1000C for 6 h in flowing hydrogen gas. The products were ball milled with the sintering aids (10 wt. %0 Si3N4, 0.5 um in grain size), and then after drying, the composite powders were hot-pressed at 1600-1800C in nitrogen gas. For compar ison, Al2O3 with 10 wt. Si3N4 was also hot-pressed at 1600-1800"C and pure Al2O3 powders at 1500"C in nitrogen gas. ( 3 & G , && / 6 , & ( & & - & / % H &&( % H &/ C & , & H & % I* J/ C , ( & & ( % & % / K / I J & - & - % H . )= ?) . && & -5 , 3 & 3 & % ?)) B % % / B * & & ,/ & 2-$, I<J & 6 18 , & & & & & - & & % E)) B/ C , & % 1 , 5 & , 68 & & 3 - & & , ( &/ 1 / I?J & % & 188 & ( )) B *?))8/ & -5 IE ;J & 1 < & & -. & -/ 6 , & 3 , & % #**)) B 3' & 5 & & -. & - & / 6 , H & & & & & 3 & , ( , % & / 1 % - & & % 5 & % & % - & & / C & ( & % - - & & & I@ >J/ C &, & & 3 % & - % / L/ * , 3 & / 6 & & *) ) & ) (/0/ - & & & & & / 6 , & & E)=E?8/ % &, - & ( % / 6 & & 3 5 *)))8 % E M ,& / 6 & & & #*) /0 1 < )/? . ' & % &, & - & *E))=*@))8 / L - *) /0 1 < - & *E))=*@))8 & & *?))8 / *<) )* * + ! , - ./ /0 1