DUPLEX ALPHA,BETA-SIALON CERAMICS STABILIZED BY DYSPROSIUM AND SAMARIUM

Duplex alpha,beta-sialon ceramics with a minimum volume fraction of re sidual intergranular glass have been prepared using Dy or Sm as the al pha-sialon stabilizing element. These microstructures contained high a spect ratio beta-sialon grains homogeneously distributed in an alpha-s ialon matrix, A number of the larger alpha-sialon grains contained dis locations and showed a core/shell structure, Dy gave an alpha-sialon w hich was stable over a wide temperature range (1350-1800 degrees C) fo r long holding times, while the use of Sm resulted in less stable alph a-sialon structures at medium temperatures (1450 degrees C) and the fo rmation of melilite, R(2)Si(3-x)Al(x)O(3+x)N(4-x), beta-sialon, and th e 21R sialon polytype during prolonged heating, High alpha-phase conte nts gave a very high hardness (H-V10 approximate to 22 GPa) but a comp aratively low indentation fracture toughness (around 4.4 MPa . m(1/2)) . Duplex sialons fabricated from powder mixtures corresponding to an a lpha-to-beta sialon ratio of around 50:50 resulted in a sialon materia l with a favorable combination of high hardness (around 22 GPa) and in creased toughness (to around 5.5 MPa . m(1/2)).