PROCESSING AND PROPERTIES OF ALN MATRIX COMPOSITE CERAMICS CONTAININGDISPERSED HARD MATERIALS

The fabrication of AlN-(Y2O3) matrix ceramics reinforced with disperse d TiN or TiB2 particles is described and the resulting specimens are c haracterized by their mechanical properties and thermal conductivity. The microstructures of the TiN-reinforced specimens sintered at 1700 d egrees C without isothermal hold show homogeneously distributed TiN pa rticles (approximate to 1 mu m) in the fine-grained (1-2 mu m) AlN mat rix. In contrast, the coarse TiB2 grains (2-3 mu m) of the specimens s intered at 1750 degrees C for 60 min are distributed inhomogeneously i n the AlN matrix. Furthermore, the surface of the TiB2 grains reacted to TiN and BN during densification in 0.1 MPa N-2. The mechanical prop erties of the AlN-(0.5 wt% Y2O3)/5 wt% TiN composites sintered at 1700 degrees C without holding time were determined to be 12.5 GPa, 390 MP a and 3.6 MPa m(1/2) for the Vickers hardness HV, the four-point bendi ng strength sigma(B) and the fracture toughness K-Ic, respectively. Th e AlN-(4 wt% Y2O3)/25 wt% TiN specimen sintered at 1900 degrees C for 60 min gave 10.7 GPa for the Vickers hardness and 270 MPa for the four -point bending strength. The fracture toughness was determined to be 8 .3 MPa m(1/2). In addition, the thermal conductivity of the AlN-TiN co mposites was of the order of 65 and 105 W m(-1) K-1 for composites sin tered at 1700 degrees C without isothermal hold and at 1900 degrees C for 60 min, respectively. The AlN-(2 wt% Y2O3)10 wt% TiB2-containing s pecimen sintered at 1750 degrees C for 60 min has HV of 9.5 GPa, sigma (B) of 220 MPa and K-Ic of 3.7 MPa m(1/2).The influence of fracture mo de and intrinsic stress on the mechanical properties is illustrated an d discussed. Finally, the processing of AlN-(4 wt% Y2O3) ceramics with out reinforcing particles coated with the TiN-particle reinforced AlN is reported. These layered materials exhibit a high thermal conductivi ty of 150 W m(-1) K-1 inside the body and enhanced hardness, fracture strength and fracture toughness at the surface. (C) 1996 Elsevier Scie nce Limited.