Ceramic composites by melt oxidation

Abstract

Melt oxidation offers a route in ceramic processing to fabrication of ceramic matrix composites. It has its advantages and drawbacks as may be seen from the following example. A suitable 0 alloy, when heated to a temperature above its melting point in an oxidizing atmosphere, will form, on its surface, an interconnected network of Al₂O₃ and residual alloy. This composite microstructure can be made to grow into the interstices of a preform. Growth inhibitors applied to the external boundaries of the preform enable net shape fabrication, while melt infiltration and in-situ oxide formation ensure near full density. The fabrication temperature is governed by the melting point of the parent metal, thereby reducing processing temperatures and residual stresses. In its simplest form the alumina matrix composite may be grown into free space on the surface of an alloy. The mechanisms associated with this process will be described first, paying attention to the role of solute elements and the relevant phase equilibria. Secondly, the influence of preforms, both reactive and inert, will be examined in modifying the growth of the oxidation product. Finally, we consider modifications of the basic process by which the volume fraction of melt in the final composite may be increased, including the fabrication of a metal-matrix composite.