Silicon nitride materials containing various compositions of Yb2O3 were investigated. The effects of sintering additives on the microstructure, fracture and R-curve behaviors were studied. The average grain size and aspect ratio increased with the amount of Yb2O3 additives. The growth of beta-grains started growing from pre-existing beta-grains (seeding) until impeded by other beta-grains. Growth of crystalline phase with planar font was observed inside amorphous pockets. Heterogeneous growth of secondary crystalline phases from two different ends of amorphous pockets was also noticed. The difference in Yb:Si ratio between the amorphous pocket and secondary crystalline phase could inhibit the completion of crystallization. R-curve behaviors were observed in Yb2O3-doped silicon nitride. (C) 2002 Elsevier Science B.V. All rights reserved.