The phase composition, microstructure, and electrical resistivity of hot-pressed AlN ceramics with 0-4.8 wt% Sm 2 O 3 additive were investigated. The phase composition was approximately consistent with that estimated from the Sm 2 O 3 -Al 2 O 3 phase diagram using the amount of added Sm 2 O 3 and oxygen content of the AlN raw material. When sintered at more than 18001C, the AlN ceramics with 1.0-2.9 wt% Sm 2 O 3 additive contained an Sm-b-alumina phase wetting the grain boundaries, and their electrical resistivity considerably decreased to 10 10 -10 12 X . cm. This resistivity decrease was caused by the continuity of the Smb-alumina phase with a resistivity lower than that of bulk AlN.

To clarify the foreign object damage (FOD) resistance of ceramics, chipping fracture mode and flexural fracture mode were investigated using several types of Si3N4 and Sic. The critical velocity which is the threshold impact velocity of the projectile for chipping fracture and flexural fracture was determined. The critical velocity of the chipping fracture mode is explained as a function of Kga-", and depends on the hardness and the shape of the projectile. The critical velocity of the flexural fractu.-e mode is explained as a function of ~y t ' '~. The mechanisms of impact damage are discussed. [Key words: mechanical properties, fracture toughness, silicon nitride, silicon carbide, impact.]

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Electrical resistivity of AlN ceramics was examined with various amounts of Y2O3 within 0 to 4.8 mass%. The electrical resistivity at room temperature varied from 10 16 to 10 10 Ω·cm with different Y2O3 amounts and at sintering temperatures. In the typical samples sintered at 1900°C, a smaller amount of Y2O3 addition with 0.1 to 0.5 mass% gives the lowest electrical resistivity of 10 10 Ω·cm, whereas the higher amount of Y2O3 maintains high resistivity of more than 10 13 Ω·cm. The results derived from different analytical techniques such as impedance analysis, cathodoluminescence spectrum and microstructural analysis explain the importance of the oxygen concentration in the AlN grain for the electrical resistivity of AlN ceramics.