Graded ceramic preforms: various processing approaches

The paper reviews several approaches aimed at generating ceramic preforms with an in-built porosity gradient. Some of these approaches rely on variations of the sinterability in stacked powder layers, e.g., as a function of the carbon content (x) in monophased stacked TiCx layers, of the TiC-to-TiB2 ratio in biphased TiC–TiB2 ceramics, and of particle size in B4C preforms. Graded porosity may be achieved by the controlled mass loss through evaporation of graded amounts of volatile sintering additives (ionic salts) or as a result of the reaction between a base ceramic (B4C) and varying amounts of TiO2 additions. The essential experimental details and the hardness profiles obtained in graded ceramic preforms that had been infiltrated with molten Al are presented.