SYNTHESES OF FULL-DENSITY NANOCRYSTALLINE TITANIUM NITRIDE COMPACTS BY PLASMA-ACTIVATED SINTERING OF MECHANICALLY REACTED POWDER

Nearly equiatomic nanocrystalline titanium nitride (Ti56N44) powder wi th an average grain size of 5 nm has been synthesized by ball milling elemental Ti powder under nitrogen gas flow at room temperature. Durin g the first stage of reactive ball milling (RBM) (time < 3.6 ks), the metallic Ti powder tends to agglomerate to form powder particles with a larger diameter. At the second stage (3.6 to 22.0 ks), the agglomera ted particles of Ti fragment to form smaller particles, These smaller particles that have new or fresh surfaces begin to react with the mill ing atmosphere (nitrogen) during the third stage of milling (22 to 86 ks) to form TiN powder coexisting with unreacted Ti powder. Toward the end of milling (86 to 173 ks), a single phase of nanocrystalline TiN (NaCl structure) is obtained. The powder of this end-product has a sph erical-like morphology with an average particle size of about 0.4 mu m diameter. A sintering procedure using plasma activation has been empl oyed to consolidate the powder particles at several stages of the RBM. The as-milled and as-consolidated powders have been characterized as a function of the RBM time by means of X-ray diffraction (XRD), transm ission electron microscopy (TEM), scanning electron microscopy (SEM), optical metallography, and chemical analyses. Density measurements of the consolidated samples show that after 86 to 173 ks of the RBM time, the compacted samples are essentially fully dense (above 96 pct of th e theoretical density for TiN). The results also show that the consoli dated TiN compacts still maintain their unique nanocrystalline propert ies with an average grain size of about 65 nm. The hardness and some m echanical properties of the consolidated TiN compacts have been determ ined as a function of the RBM time.