Joining of Reaction Bonded Si<sub>3</sub>N<sub>4</sub> Using Al

Abstract

In order to develop a joining technique for reaction bonded Si3N4 which ensures good high-temperature strength of joined body, Al foil was used as an insert material. The joining was conducted in 1atm N2 atmosphere at temperatures from 1470 to 1970K, under pressures up to 20MPa for 1800s. Direct joining without any insert material was also carried out for comparison. The microstructure of joined regions was observed with a high resolution electron microscope, and the strength of the interface was measured by 3-point bending test as a function of joining temperature and joining pressure. The results obtained are summarized as follows.(1) Direct joining is almost impossible under the above conditions.(2) When joined by using Al foil at 1970K under 20MPa, the bending strength at 1670K attains to 154MPa.(3) The interface layer is composed of unreacted Al and AlN-polytype Sialon, and Al joins to Si3N4 matrix with the Sialon layer in-between. As the joining temperature rises, the amount of Sialon increases at the expense of Al and Al is completely converted to Sialon at 1970K.(4) Along grain boundaries of Sialon, there exists a non-crystalline phase of 1-3nm in thickness. The non-crystalline phase can be an extended grain boundary.(5) When joined at 20MPa, the bending strength is insensitive to the joining temperature at room temperature but very sensitive at 1670K. This is explained by, (a) the amount of unreacted Al and its degree of adhesion at the interface, and (b) the residual stress supposedly induced by the difference in thermal expansion coefficient between the Sialon and the Si3N4 matrix.(6) The strength increases with increasing joining pressure, but saturates above about 10MPa. This is explained by the joining-pressure dependence of the amount of the Sialon formed.