Isothermal nitridation kinetics of TiSi2 powders

Abstract

The aim the present work is to determine the reaction kinetics between a TiSi 2 powder with gaseous nitrogen. Isothermal nitridation of TiSi 2 powders with fine (1.4µm) and medium (4.5µm) particle size has been studied in pure nitrogen atmosphere ranging from 1000 to 1200°C for duration up to 50 hours. The isothermal nitridation kinetics of TiSi 2 powders were investigated by thermogravimetry. The nitridation rate strongly depends on the particle size and temperature. The smaller particle size exhibits the higher nitridation rate due to its larger surface area. The conversion process is complex with nucleation and growth of TiN at the surface of the grain and Si 3 N 4 inside the grain promoted by Kirkendall effect with an influence of the volume increase.

Simultaneous synthesis of TiN–Si₃N₄ composite powders from TiSi₂ in NH₃ atmosphere

“…In previous studies using the same starting material under the flow of nitrogen gas, 13), 14) a TiNSi 3 N 4 particulate composite was obtained, and 48.8 % of the reaction ratio without additives was achieved at 1,200°C. In this study, we obtained TiNSi 3 N 4 mixed powders with a fibrous Si 3 N 4 structure under the flow of NH 3 gas.…”

“…Moreover, Maillé et al reported the influence of temperature, grain size, and holding time on the nitriding rate of TiSi 2 powder and identified chemical mechanisms at temperatures between 1,000 and 1,200°C for durations up to 50 h using a direct nitriding process under nitrogen gas flow. 13), 14) They did not, however, investigate the optimum conditions for achieving the highest percentage of reaction ratio without additives (a ratio of 48.8 % was achieved). The researchers also reported nitridation enhancement by the addition of nickel, 5) for which they achieved a 96.3 % reaction ratio.…”

Nitridation enhancement of TiSi2 powders by addition of nickel

“…During the first step, only TiN is formed and generates a layer that behaves as a diffusion barrier limiting silicon nitridation. The second step occurs after significantly longer exposures and involves a sluggish nitridation of silicon at the center of the TiSi2 grain thanks to the formation of opened porosities generated by the Kirkendall effect [29]. This mechanism slows down the progress of the reaction so that the nitridation kinetics are very low and the conversion is inadequate.…”

“…The exothermicity of this reaction and the formation of liquid silicon could be detrimental in the case of fibrous composites. In a previous work, we focused our attention on the nitridation of pure TiSi2 compacts at lower temperatures (1000-1200°C) to produce the matrix of a fibers-reinforced composite [29]. Accordingly to previous papers [30][31][32][33], it was found that the nitridation is sluggish in these conditions because of diffusional limitations.…”

“…Mass conversion rate versus time at 1100°C of pure TiSi2 powder[29] in comparison to (1-x) TiSi2+ x Ni mixtures with x = 0.100; 0.125 and 0.150. (fits of the three successive stages with the A2 and D3 models plotted in dot lines) Backscattered electrons images of a TiSi2 powder heat treated at 1100°C in nitrogen with addition of: a) Fe and b) Co a) b) Nitridation of a TiSi2 grain in presence of Ni: a) proposed conversion mechanism with four successive steps and b) backscattered electrons image of a large grain in the sample 0.900 TiSi2+ 0.100 Ni treated at 1100°C…”