CREEP OF SILICON-NITRIDE TITANIUM NITRIDE COMPOSITES

The effect of particulate TiN additions (0-50 wt%) on creep behaviour of hot-pressed (5 wt% Y2O3 + 2wt%Al2O3)-doped silicon nitride (HPSN)-b ased ceramics was studied. Creep was measured using a four-point bendi ng fixture in air at 1100-1340-degrees-C. At 1100-degrees-C, very low creep rates of HPSN with 0 30 wt% TiN are observed at nominal stresses up to 160 MPa. At 1200-degrees-C the creep rate is slightly higher, a nd at 1300-degrees-C the creep rate is increased by three orders of ma gnitude compared to 1100-degrees-C and rupture occurs after a few hour s under creep conditions. It was established that the formation of a T iN skeleton could detrimentally affect the creep behaviour of HPSN. An increase in TiN content leads to higher creep rates and to shorter ru pture times of the samples. Activation energies of 500-1000 kJ mol-1 i n the temperature range of 1100-1340-degrees-C at 100 MPa, and stress exponents n less-than-or-equal-to 4 in the stress range 100-160 MPa at 1130-1200-degrees-C were calculated. Possible creep mechanisms and th e effect of oxidation on creep are discussed.