EFFECTS OF STARTING POWDERS ON PROPERTIES OF NORMALLY SINTERED AND HIP`ED Si3N4.

Six kinds of Si//3N//4 starting powders which were mixed with 6 wt% Y//2O//3, 2 wt% Al//2O//3 and 3 wt% MgO as densification aids, were pressed and normally sintered at 1550 degree -1650 degree C. The sintered compacts were HIP'ed without capsule in a nitrogen atmosphere of 100 MPa at 1700 degree C. Density, flexural strength and alpha - to beta -phase transformation were studied. The results obtained are summarized as follows: (1) Density and strength of HIP'ed Si//3N//4 compacts were strongly affected by characteristics of starting powders such as specific surface area, green density and amount of alpha -phase. (2) The mean strength of sintered compacts of which starting powder had a specific surface area of 14. 2 m**2/g and contained 97% alpha -phase, increased from 370 to 870 MPa by HIP treatment. (3) Improvement of strength depends on alpha - to beta -phase transformation and the formation of fibrous grain in Si//3N//4 compacts during HIP treatment.