Oxidation kinetics, microstructure evolution and texture development in the reaction-bonded aluminum oxide (RBAO) process

The reaction-bonded aluminum oxide (RBAO) process involves the solid state (T < 660°C) and liquid state (1000° < T < 660°C) oxidation of compacts of attrition milled Al and alpha alumina. When heated in air, the oxidation per unit area decreases for the higher surface attrition milled Al powders (i.e., 14.4 and 20.2 m2/g) but the total oxidation of the milled powder is ∼70% versus ∼9% for the as-received powder (surface area = 1.2 m2/g) because of the higher surface area. The solid state oxidation of Al powder sequentially follows parabolic, linear and non-linear rate laws. The solid state oxidation of RBAO compacts, liquid Al formation, and alpha alumina phase transformation during the RBAO process and their effect on final microstructure were investigated. Compact oxidation is controlled by heat transfer and the rate-controlling heat transfer mechanism is convection. Low heat transfer from the surface results in too rapid oxidation and a core-shell oxidation of the compact. The α-Al₂O₃ particles in the RBAO precursor behave like seed particles for the gamma to alpha alumina transformation in the RBAO process. Highly textured, dense alumina ceramics were fabricated by a new processing route which utilizes a mixture of Al metal powder, alpha alumina powder, alpha alumina platelet (template) particles and a liquid phase former. Texture development in liquid-phase sintered RBAO ceramics was studied during templated grain growth (TGG); a technique for developing crystallographic texture in ceramic bodies via the grain growth of aligned anisometric particles in a dense and fine grain size matrix. The process of TGG occurs in 3 stages: densification, initial growth of individual template particles, and template impingement and thickening. Texture development is directly related to the initial number of template particles and the inter-template spacing. The growth of alpha alumina template particles is anisotropic. Template growth in radial direction is controlled by diffusion through the liquid phase during a solution-precipitation growth process. Template growth in the thickness direction occurs by 2-D nucleation and growth.