The 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 po wder is important for the reaction-bonded aluminum oxide process. Attrition milling increased the surface area to 14.4 and 20.2 m(2)/g versus 1.2 m(2) /g for unmilled powder and smeared the Al particles, and the surface was hy drolyzed to form bayerite and boehmite, Upon heating the hydroxides decompo se to form an 11-13 nm thick amorphous plus gamma-Al2O3 layer which subsequ ently retards oxidation kinetics. The oxidation per unit area decreases for the higher surface area powders at temperatures below the critical tempera ture but the total oxidation of the milled powder is similar to 70% versus similar to 9% for the as-received powder because of the higher surface: are a. The critical temperature depends on Al particle surface characteristics and is defined as the transition temperature above which the initial rate o f oxidation is linear, not parabolic. Above the critical temperature the ox idation per unit area decreases significantly. In addition, linear oxidatio n occurs faster than parabolic oxidation and thus the initial fast oxidatio n kinetics (i.e., linear) can cause thermal runaway during oxidation, There fore, oxidation below the critical temperature is essential to maximize sol id-state oxidation and to prevent thermal runaway. The critical temperature s for the as-received (1.24 m(2)/g), the 6 h (14.4 m(2)/g), and 8 h (20.2 m (2)/g) attrition-milled Al powders were 500 degrees, 475 degrees, and 500 d egrees C, respectively. A model for oxidation during the parabolic and line ar oxidation stages is presented.