The Development of the Mechanical Strength of Reaction-Bonded Silicon Nitride

The report describes research aimed at furthering understanding of the nature and mechanism of development of the bond in RBSN. The project formed a part of an overall programme aimed at securing control of the microstructure of RBSN and, in consequence, over the mechanical properties of the ceramic. The study embraced three grades of starting silicon powder: a specially prepared high purity powder (99.99% Si), a similar powder contaminated with Fe (approx. 10 ppm Fe), and a commercial powder (approx. 98% Si; 0.5% Fe; 1.5% AL). Sonic Young's modulus was determined at stages throughout the nitriding reaction for all three powders and the data correlated with microstructural observations. In the case of the high-purity silicon powder it was concluded that the bond forms as a result of the deposition of massive, vapour-formed, Si3N4, and that Si3N4 'whisker' formation played no part in the bonding process. It was also observed that RBSN made from the two higher purity powders had Young's modulus values significantly higher than commonly encountered. A 3-phase composite model developed by Cohen and Ishai satisfactorily described the rate of development of Young's modulus with extent of reaction. (Author)