Nanophase Materials by Mechanical Attrition: Synthesis and Characterization

Abstract

Mechanical attrition characterized by a cyclic shear deformation of powder particles has been found to be a versatile method in producing nanophase materials with a broad range of chemical composition and atomic structure including nanocrystalline as well as amorphous states. In this process, lattice defects are produced by “pumping” energy into initially single-crystalline powder particles of typically 50 μm particle diameter. This internal refining process with a reduction of the average grain size by a factor of 10³ - 10⁴ results from the creation and self-organization of high-angle grain boundaries within the powder particles during the milling process. By TEM analysis the mechanism of grain size reduction has been revealed to occur in three distinctively different steps involving the formation of grain boundaries. Together with the microstructural evolution of attrited nanophase materials, a change of the thermodynamic, mechanical and chemical properties of these materials has been observed with the properties of nanophase materials becoming controlled by the free volume and cohesive energy of the grain boundaries. As such, it is expected that the study of mechanical attrition processes in the future not only opens new processing routes for a variety of advanced nanophase materials but also improves the understanding of technologically relevant deformation processes, e.g. surface wear, on a nanoscopic level.