Microstructural Modeling and Toughening of Si3N4 Ceramics for High TemperatureEngine Applications

摘要

R-curve behavior was achieved in hot isostatically pressed (HIP) silicon nitridewith low additive levels. Two HIP densification processes which produced R-curve in HIP silicon nitride were demonstrated. The first densification process was extended soaks at 1840 deg C; the second included a pre-HIP sinter step. The strongest R-curve behavior was observed with the rare earth mixture of erbia-samaria, HIPed at 1840 deg C for 240 minutes. The fracture toughness approached 11 MPa/sq m at crack lengths of only 600 to 800 microns. Other rare earth combinations were also evaluated. The final microstructure was influenced by the type of rare earths and silica content. The mixed rare earths resulted in a finer and higher aspect ratio microstructure than materials densified with yttria. The development work strongly suggests that HIP silicon nitride which combines the creep resistance of NT154 type materials with R-curve behavior is possible. Silicon nitride, Rare earths, Microstructure, Toughness, Heat treatment, Hot isostatic pressing.