Milling and mechanical alloying of inorganic nonmetallics
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The versatility of mechanochemical processing was investigated in a number of nonmetallic inorganic systems. It is shown that high energy grinding can be used to produce amorphous carbon from synthetic graphite and some forms of natural graphite. Elemental sulfur can likewise be amorphized by prolonged high energy grinding. Phase transformations of aFe 2 O 3 (hematite) and mechanochemical reactions of this phase with ZnO and NiO are strongly influenced by the presence of iron resulting from wear of the grinding media. Thus spinel type ferrites were obtained by grinding of such mixtures for short times ( 1 - 3 h in a Spex mill); however, longer grinding times resulted in the formation of FeO or (Fe, Zn)O (when grinding ZnO) or FeO or (Fe, Ni)O (when grinding NiO), presumably as a result of the reaction of mill wear debris with the mill charge. The suspected (Fe, Zn)O phase is most likely a nonequilibrium solid solution. Negligible wear accompanied the mechanochemical synthesis of NiS and ZnS from elemental powders. These sulfides were formed for short milling times. In contrast, sulfides of tungsten were not formed even when rather long milling times were employed. The survey of mechanochemical reactions presented here further reinforces the concept that this low temperature synthesis method is a robust process route for production of a wide range of materials.
I. INTRODUCTION In 1970 Benjamin1 reported on the successful fabrication of nickel based superalloys by mechanical alloying (MA). When applied to metallic materials, MA is associated with repeated cold welding and fracturing of metal powder particles during the high energy ball milling characteristic of the process.2 While MA was originally used for production of oxide dispersion strengthened superalloys, it later proved to be a versatile technique for synthesizing unique microstructures. Examples include nonequilibrium solid solutions,3 stable and metastable intermetallic compounds,4^8 and amorphous metallic phases.9"17 Rather extensive research during the last two decades has resulted not only in a number of "new" materials fabricated via MA, but also in improved understanding of the process.18"20 In addition, higher energy, more efficient mills have been utilized for MA, at least in a laboratory setting. In contrast to the potential commercial advantages of mechanically alloyed metallic materials, physical and/or chemical changes introduced by intensive grinding of nonmetallic materials (so-called mechanochemical reactions) have found little application. This is so even though such reactions in nonmetallics were observed well before MA was conceived. For example, Clark and Rowan reported in 1941 on the polymorphic transformation of PbO induced by grinding,21 and referenced even
a 'Permanent
address: Institute Jozef Stefan, Ljubljana, Slovenia. J. Mater. Res., Vol. 7, No. 6, Jun 1992
http://journals.cambridge.org
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earlier work on this phenomenon. The kinetics of phase transformations induced by grinding of various nonmetallic in
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