Silicon nitride joining for high temperature applications

Techniques for joining silicon nitride (Si3N4) to itself and to structural metal alloys for high–temperature applications were developed. Two materials joining systems were used; high–temperature brazes and oxynitride glasses. Three commercially available precious metal braze alloys and modifications of these brazes were evaluated for applications in which Si3N4 is to be joined to structural metals. Active metal coatings were used to promote wetting and optimise the bonding to the Si3N4. Average bend strengths of 330 MPa at room temperature and 160 MPa at test temperatures as high as 800°C were measured for the brazed Si3N4 specimens. Oxynitride glass joints were formed after reaction of an oxide glass from the yttria–alumina–silica ternary system with the Si3N4. Extensive interdiffusion between the glass joint and the Si3N4 plus Si3N4 dissolution into the glass joint promoted the formation of a strong bond. Average four–point bend strengths were 370 MPa at room temperature and 490 MPa at 1000°C.