Decomposition of process control agent during mechanical milling and its influence on displacement reactions in the Al-TiO2 system

The process control agent (PCA) stearic acid has been used to prevent excessive cold welding during mechanical milling of an Al-TiO2 powder mixture. Gradual decomposition of the stearic acid during high-energy ball milling caused contamination of the powder with carbon. The decomposition rate was found to be 3-4 times faster when 2.5 wt.% instead of 5 wt.% of PCA have been used. This resulted in a higher degree of powder contamination-in the early stages of milling-for a lower initial PCA addition. The degree of contamination with carbon can readily be estimated by thermogravimetry and by the size of an endothermic peak at 875 °C, due to a reaction between aluminium carbide and titanium aluminide, in the trace of differential scanning calorimetry experiments.