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/TiB2 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 TiB2 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 TiB2 particles initially of sub-micron size, which are reduced gradually by further milling. However, there still remain some coarse TiB2 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 TiB2 particles and eventually decreases the crystallite size of the products below 40 nm after 16 h of milling. (C) 2003 Elsevier B.V. All rights reserved.