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.