Use of Si3N4 as a reactant in preparation of TiN–Ti5Si3 composites by solid-state SHS reactions

An experimental study on in situ formation of TiN–Ti5Si3 composites with a wide range of compositions by self-propagating high-temperature synthesis (SHS) was conducted in two reaction systems. One adopted silicon nitride (Si3N4) as the solid source of nitrogen in a solid-state combustion system. The other employed both solid and gaseous reagents and performed the reactions of the Ti–Si powder compact under nitrogen pressures. Self-sustained reactions featuring a distinct combustion front were observed in both systems. TiN–Ti5Si3 composites containing Ti5Si3 from 20 to 80 mol% were effectively produced by solid-state combustion synthesis of Ti–Si3N4 and Ti–Si3N4–Si powder compacts. This result verifies the contribution of Si3N4 to the formation of TiN and Ti5Si3. However, the solid/gas reaction of Ti–Si compacts with gaseous nitrogen was proved to be an inappropriate route in terms of product composition. For the production of a TiN-rich composite such as 80 mol% TiN, it was found that the nitrogen uptake by the Ti–Si sample compact was inadequate, thus resulting in formation of a great amount of Ti2N in the final product. On the contrary, when preparing the TiN–Ti5Si3 composite with a low TiN content like 20 mol% or even 50 mol%, Ti–Si powder compacts in gaseous nitrogen were subjected to excessive Ti nitridation and yielded a Si-rich silicide phase TiSi2 in addition to TiN and Ti5Si3.