Nitridation enhancement of TiSi2 powders by addition of nickel

The nitridation enhancement of a TiSi2 powder by nickel addition was examined. The quantities of the added metal were comprised between 2.5 and 25mol.%, the balance being TiSi2. The isothermal heat treatments were made at 1100 °C for durations up to 40 h under normal pressure and continuous flow of pure nitrogen. Comparing to pure TiSi2 powder, an improvement of the conversion in TiN and Si3N4 is obtained for compositions containing 10.0; 12.5 and 15.0 mol.% of nickel. According to the quaternary Ti-Si-Ni-N phase diagram, nickel was found to form Ni4Ti4Si7 compound. Thermogravimetric analysis of these three compositions showed the existence of three successive stages during the conversion. First, an initiate reaction occurred with a very weak weight gain. Then, the second stage exhibited an acceleration of the weight gain. Those two stages are both controlled by nucleation and growth, as represented by the kinetic equation [-ln(1-α)]1/2 = k.t. Finally, the reaction is limited by three-dimension diffusion, as represented by the kinetic equation: [1-(1-α)1/3]2 = k.t. From this behavior, an activation mechanism is proposed.