Thermodynamic of liquid metal infiltration in TiC–SiC or SiC porous compacts

Abstract

This work is the study of the reactivity during the capillary infiltration of liquid silicon or Si–Ti molten alloys in SiC or SiC + TiC compacts to form TiSi2/SiC composites. The main aim was to identify the thermodynamic and kinetic limitations of this process. Preliminary thermodynamic analyses of the equilibria in the Ti–Si–C system were performed to select the compositions of the liquid and the operating temperatures. Three cases were chosen: 1) infiltration of molten TiSi2 in pure SiC compacts at 1550 °C, 2) reactive infiltration of pure liquid silicon in SiC + TiC compacts at 1450 °C, and 3) reactive infiltration of the molten eutectic Ti0.16Si0.84 alloy in SiC + TiC compacts at 1380 °C. The compacts were prepared from mixtures of micronic SiC (α or β polytypes) and TiC powders. The compositions of the powder mixtures were calculated to fill totally the porosity of the compacts of about 50% or with an excess of TiC. The heat treatments of the pellets at 1380, 1450 and 1550 °C were performed with a holding duration of 1 h in high vacuum. Experimental results evidenced that the interactions between the liquid and the powders are complex. The obtained materials differ more or less from the expected composites that are generally not dense and contain variable quantities of free silicon. It is found that these experimental results can be explained by advanced thermodynamic calculations. This work proves that the activity gradients play a determining role during the infiltration process by initiating the dissolution and the diffusion of atoms in the liquids.