Influence of Mechanical and Electric Current Activation on the Mechanism of Formation and the Properties of Bulk B₄C-TiB₂ Composites Obtained by Reactive Sintering

Abstract

The preparation of dense nanostructured B4C-TiB2 is investigated through the combination of mechanical (Ball Milling) and electric current (Spark Plasma Sintering or SPS) activation of Ti, B and graphite powders. The full conversion of 8h milled reactants is obtained at much lower temperature (about 1200 °C), as compared to simply blended mixtures (>1600 °C). The formation of TiB2 generally precedes that of B4C. In addition, regardless of the milling treatment, an increase of the heating rate during SPS is found to produce a transition of the mechanism governing the formation of TiB2 and B4C, i.e. from gradual solid- solid to rapid combustion-type behavior. However, when unmilled powders are used, unreacted and intermediate species are still present in the product resulting after the combustion synthesis reaction takes place during sintering. In contrast, ball milled powders reacted completely under combustion regime, even when relatively milder heating rate conditions are adopted. SPS product density increases from about 82 % to 94 % of the theoretical value (I = 1100 A), as a consequence of the mechanical treatment. Correspondingly, a material with homogeneous phase distribution and grain size down to 100-200 nm is obtained. A further improvement of product density (> 96.5 %) is produced, at the expenses of a certain grain growth, when the applied current intensity is augmented to 1200 A.