Influence of powder characteristics and powder processing routes on microstructure and properties of hot pressed silicon nitride materials

摘要

Four different commercial Si_(3)N_(4) powders were hot pressed with the addition of La_(2)O_(3) and Y_(2)O_(3) as sintering aids. Two powder processing routes were set up: addition of sintering aid powders by ultrasonic dispersion, addition of nanodispersed amorphous additive species by chemical coprecipitation. The following aspects were analyzed: characteristics of starting powders and powder mixtures, with reference to surface modification (electrokinetic behaviour and surface properties) induced by the powder treatment; sintering behaviour of the powder mixtures; influence of raw powders characteristics and processing route on microstructure and properties of dense materials. The microstructural characteristics of hot pressed materials (grain size, aspect ratio, grain boundary phases) were found to be dependent on powder characteristics and its process history. Significant variation of the mechanical properties (Young modulus, hardness, toughness and strength) were related to microstructural features. Strength, for example, ranges from 600 to 1200 MPa at room temperature and from 400 to 1000 MPa at 1200℃; toughness ranges from about 4 to about 6 MPam~(1/2).