High‐Temperature Behavior of the Aluminum Oxycarbide Al2OC in the System Al2O3–Al4C3 and with Additions of Aluminum Nitride

Abstract

A large number of high‐temperature experiments involving binary mixtures of alumina and aluminum carbide (Al4C3) were performed to clarify the behavior of the aluminum oxycarbide Al2OC. It is first shown from heat treatments on the molar composition 55Al2O3·45 Al4 C3 that, in conditions of stable equilibrium, Al2OC decomposes to Al4O4 C and Al4 C3 at 1715°C according to the reaction 4Al2 OC → Al4 O4 C+Al4 C3. However, Al2 OC can be obtained at room temperature from compositions of lower carbide contents (<20 mol%) and by rapid cooling. In that case Al2 OC is solidified in a metastable state, and, when subsequently annealed to temperatures above 1200°C, its lattice reorganizes and progressively transforms into the lattice of Al4 O4 C according to the reaction Al2 O3+xAl2 OC → (1—x)Al2 O3+xAl4 O4 C. This transformation is described in terms of TTT curves and is accompanied by a decrease in hardness and wear resistance. Additions of AlN to Al2 O3 and Al4 C3 have been found to create a solid‐solution Al2 OC—AlN in an unexpected region of the ternary system Al2 O3—Al4 C3—AlN. As a result the quasi‐binary section Al2 OC—AIN of the ternary diagram Al2 O3—Al4 C3—AlN was extensively investigated. We report the solubility limit of AlN in Al2OC, the improved high‐temperature stability of the Al2OC—AlN solid solution compared with pure Al2OC, and eventually the existence of a possible intermediate ternary compound of formula Al10O3C3N4.