Nanocrystalline HfN produced by mechanical milling:: Kinetic aspects

Nanocrystalline HfN powders were obtained by mechanically assisted gas-solid reaction. An analysis of the reaction kinetics under different milling frequencies and gas pressure is presented. The results shown the formation of an interstitial solid solution alpha -Hf, prior to its transformation to cubic HfN. Rietveld refinements of the X-ray diffraction data were done in order to analyze the progress of the structural change, The short range order was characterized by perturbed angular correlations, which indicated a random distribution of N around Hf. The reaction kinetics were obtained from the measured value of absorbed nitrogen per Hf (n). Two main features were evident; the reaction advance dependence on the dose of mechanical energy and a slowing down in the reaction kinetics for n similar to0.5 associated with the saturation of surface sites, These findings are discussed taking into account the structural change and the role of surface adsorption and volume diffusion. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.