SELF-STRENGTHENING AND HIGH-TEMPERATURE SINTERING IN BINDER-FREE SILICON-NITRIDE - A THERMOANALYTICAL STUDY

A thermal strengthening process, which occurs during low-temperature h eating of binder-free silicon nitride, has been investigated using sim ultaneous thermal analysis, dilatometry and FTIR and shown to occur in separate stages over clearly identifiable temperatures. Reactions whi ch give the strengthening are the loss of physically and chemically co mbined water and the decomposition of ammonium carbonate and various h ydrosilicates. Compacts have bend strengths of 8-10 MPa after strength ening at 500-degrees-C and 30-34 MPa after strengthening at 900-degree s-C. High-temperature dilatometry shows several stages of sintering. T he maximum rate occurs at 1800-degrees-C with shrinkage commencing at 1450-degrees-C. Densities of 98.3% theoretical are obtained on heating to 1900-degrees-C.