Mechanical enhancement of the carbothermic formation of TiB2

Abstract

A mixture of rutile, boron oxide, and graphite has been mechanically milled for 1, 10, and 100 hours with the intention of forming titanium diboride during subsequent thermal treatment. The resultant powders were examined after isothermal annealing by X-ray diffraction (XRD) to determine the effect of milling on the formation of TiB2. No reaction was found to occur during milling, even for milling times of 100 hours. Annealing of the milled samples resulted in the formation of TiB2 at temperatures as low as 1000°C for the 100 c-hour milled powder. The reaction mechanism during thermal processing changed with milling time, with TiBO3 predominant in samples milled for >1 hour, whereas TinO2n−1 phases predominated for 1 hour of milling. Titanium carbide became an increasingly abundant intermediate phase with longer milling times, but was converted to TiB2 at high temperatures. The annealed products showed the presence of acicular crystals of TiB2, with those from 1 hour of milling being somewhat larger than those for longer milling times, where submicron particles were formed. This was attributed to the presence of a larger fraction of molten B2O3 compared to the longer-milled powders where TiBO3 predominated.