Mechanically induced gas–solid reaction for the synthesis of nanocrystalline ZrN powders and their subsequent consolidations

Abstract

Equiatomic nanocrystalline ZrN powders with an average grain size of less than 8 nm in diameter have been fabricated by high energy ball-milling elemental powders of Zr under nitrogen gas-flow at room temperature. The ductile powders of Zr tend to agglomerate during the first stage of the reactive ball milling (RBM; <11 ks) to form powder particles with larger diameters. The powders are then intensively disintegrated into smaller particles during the second stage of milling (11–43 ks). These disintegrated particles that have fresh or new surfaces begin to react with the milling atmosphere (nitrogen) during this stage of milling to form a cubic phase of ZrN powder coexisting with unreacted hcp-Zr powder. Towards the end of milling (86–173 ks), a single phase of nanocrystalline ZrN (NaCl-structure) is obtained. The powders of this end-product have spherical like morphology with average particle size of about 0.4 μm in diameter. Cold and hot pressing techniques were employed to consolidate the powders at the several stages of the RBM. The as-milled and as-consolidated powders were characterized as a function of the RBM time by means of X-ray diffraction, transmission electron microscopy, scanning electron microscopy, optical metallography and chemical analyses. The results have shown that the consolidated ZrN compact of the end product (173 ks of RBM) still maintains its unique nanocrystalline characteristics with an average grain size of less than 80 nm. Density measurements of these consolidated samples of the end-products (86–173 ks of the RBM) show that they are essentially fully dense (above 99% of the theoretical density for ZrN). The dependences of the hardness on the grain size and the consolidation temperatures were investigated.