Joining of austenitic stainless steel and ferritic stainless steel to sialon

Joining of ceramics to metal is not easy due to the difference in the thermal expansion of the two materials. In this research work, the joining of sialon to austenitic stainless steel and ferritic stainless steels were compared. At above a joining temperature of 1200°C, sialon will react with steel to form iron silicide and nitrogen is given off. Thus to counteract this effect, a high nitrogen content in the steel is one way of overcoming this problem by nitriding both austenitic and ferritic stainless steels at 1200°C. These materials were then joined to sialon using a hot press at 1250°C. The results show that inter-reaction and interdiffusion on both sides of the materials occur in the layer. Good interfaces were found between sialon and steels. Analyses on the joinings were carried out using a scanning electron microscope, an optical microscope, a hardness tester, and XRD. The analysis also indicated different morphology of the interfaces that were formed. A ductile interlayer was formed between the ferritic stainless steel and sialon, which enabled good joining. However, this layer is absent in the joining of austenitic stainless steel to sialon. In both cases of joining, the chemical reactions between the steels and sialons produce an intermediate hardness in the reaction layer. Thus the interfaces have the important role in reducing the effect of the thermal mismatch between the sialon and steel, where the formation of a composite interlayer provides properties that are intermediate between those of the steel and sialon. However, the joining of sialon to ferritic stainless steel is more successful compared to joining to austenitic stainless steel.