Reaction-bonded aluminum oxide process: I, the effect of attrition milling on the solid-state oxidation of aluminum powder

The effect of attrition milling on the solid-state oxidation of aluminum powder is important for the reaction-bonded aluminum oxide process. Attrition milling increased the surface area to 14.4 and 20.2 m²/g versus 1.2 m²/g for unmilled powder and smeared the Al particles, and the surface was hydrolyzed to form bayerite and boehmite. Upon heating the hydroxides decompose to form an 11-13 nm thick amorphous plus γ-Al₂O₃ layer which subsequently retards oxidation kinetics. The oxidation per unit area decreases for the higher surface area powders at temperatures below the critical temperature but the total oxidation of the milled powder is approx. 70% versus approx. 9% for the as-received powder because of the higher surface area. The critical temperature depends on Al particle surface characteristics and is defined as the transition temperature above which the initial rate of oxidation is linear, not parabolic. Above the critical temperature the oxidation per unit area decreases significantly. In addition, linear oxidation occurs faster than parabolic oxidation and thus the initial fast oxidation kinetics (i.e., linear) can cause thermal runaway during oxidation. Therefore, oxidation below the critical temperature is essential to maximize solid-state oxidation and to prevent thermal runaway. The critical temperatures for the as-received (1.24 m²/g), the 6 h (14.4 m²/g), and 8 h (20.2 m²/g) attrition-milled Al powders were 500°, 475°, and 500°C, respectively. A model for oxidation during the parabolic and linear oxidation stages is presented.