Evidence for self-sustained MoSi 2 formation during room-temperature high-energy ball milling of elemental powders
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We present evidence indicating that rapid, self-sustained, high-temperature reactions play an important role in the formation of tetragonal MoSi2 during room-temperature high-energy ball milling of elemental powders. Such reactions appear to be ignited by mechanical impact in an intimate, fine-grained, Mo-Si physical mixture formed after an initial milling period. Under certain conditions, limited propagation of self-sustained reactions in these uncompacted powder mixtures renders the compound formation seemingly gradual in bulk-averaged analysis. It is suggested that this type of reaction is an important mechanism in the mechanical alloying of highly exothermic systems. Results are discussed in comparison with similar reactions we observed in ball-milled Al-Ni powders, with self-sustained combustion synthesis previously reported for Mo-Si powders, and with interfacial diffusional reactions in Mo-Si powders or thin-film diffusion couples.
I. INTRODUCTION The refractory molybdenum disilicide is currently of considerable interest because of its importance and potential in industrial applications.1 The combination of high melting point (2050 °C), low density (6.24 g/cm 3 ), and very high stability in oxidizing and corrosive environments makes MoSi2 an excellent high-temperature structural material. This is demonstrated, for example, by the increasing use of M0S12 for heating elements in furnaces that operate in air at temperatures up to 1700 °C. M0S12 has been synthesized using various methods such as arc-melting and casting, or powder reactions involving sintering and hot-pressing.2 4 There are at least two drawbacks to these traditional techniques. First, most processes require very high temperatures and industrial furnaces. Second, contaminants are usually incorporated into the material during processing and affect its properties. For example, oxygen, when reacting to form SiO2 particles inside the MoSi2 matrix, degrades its high-temperature mechanical properties. Combustion synthesis, often termed self-propagating high-temperature synthesis (SHS), has also been used to produce MoSi2.5'6 This type of reaction makes use of the high heat of formation of MoSi2 from the constituent elements (A/// = 31 kcal/mole MoSi 2 5 ). The reaction to form MoSi2 is induced in the surface layer of a compacted Mo-Si powder mixture pellet by radiative heat from an ignition coil placed close to the pellet surface. The strong exothermic reaction at the surface liberates sufficient heat to raise the temperature of adjacent regions of reactants and the reaction becomes a
'Present address: Department of Mechanical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803.
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http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 8, Aug 1993
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self-sustained. The reaction front propagates at a speed on the order of 10 cm/min, without additional external heating.5-6 It has also been reported that SHS volatilizes impurities, and that its products can be sintered at lower temperatures compared with
