Influence of stearic acid on mechanochemical reaction between Ti and BN powders

Abstract

The influence of stearic acid as a process control agent on the mechanochemical reaction between Ti and BN to form TiN/TiB 2 nanocomposite powder by high energy ball milling has been investigated. A powder mixture of pure Ti and hexagonal BN powders with a molar ratio of 3:2 was milled for up to 40 h with up to 1.75 wt.% of stearic acid. The XRD analysis shows that an intimate mixture of TiN and TiB 2 crystalline powders was formed during milling by a displacement reaction. It has been observed by monitoring the temperature of the vial surface during the milling process that the addition of over 1.5 wt.% of stearic acid changed the reaction mode from a mechanically induced self-propagating reaction (MSR) to a gradual reaction. It has also been found by XRD and TEM analyses that the MSR produces TiN and TiB 2 particles initially of sub-micron size, which are reduced gradually by further milling. However, there still remain some coarse TiB 2 particles larger than a few hundred nanometers even after 16-h milling due to their extremely high hardness. On the other hand, the gradual reaction induced by stearic acid successfully inhibits the formation of coarse TiB 2 particles and eventually decreases the crystallite size of the products below 40 nm after 16 h of milling.