Microwave-assisted combustion synthesis in a mechanically activated Al–TiO2–H3BO3 system

The Al2O3–TiB2 in-situ composite has been fabricated by different techniques. In this work, the mechanical activation process has been used to aid microwave-assisted combustion synthesis (MACS) to produce the Al2O3–TiB2 in-situ composite. For this purpose, the thermite mixture of Al, TiO2 and boric acid (H3BO3) powders was used as the raw materials, and was mechanically activated at different milling speeds. The results of X-ray phase analysis of the mechanically activated samples after combustion synthesis showed that the Al2O3–TiB2 in-situ composite has been successfully fabricated by thermal explosion mode of combustion synthesis in microwave, while no combustion synthesis occurred for the unmilled sample. Also, it was found that by increasing the milling speed from 250 to 400 rpm, the purity of the final products has been increased; while further milling speed up to 550 rpm reduced the purity of the final products. The effects of milling speed were also studied by means of differential scanning calorimetry (DSC) measurements. It was shown that by increasing the energy level of the reactants via milling speed, the ignition temperature and the intensity of exothermic peaks in the DSC curves have been changed. Finally, in order to have a good understanding about the in-situ formation of such ceramic composites, a reaction mechanism was proposed based on the experimental results. The synthesized composite exhibited high microhardness value of about 1950 Hv in dense parts.

Research Highlights
► Using boric acid instead of boron oxide as a boron source.
► Investigation of the effects of milling speed on the combustion synthesis process.
► Using microwave assisted combustion synthesis.
► Suggesting a precise reaction mechanism.