Sintering
Since the plasticity needed to shape the new ceramics is not normally based on a water mixture, the drying step so commonly required to remove water from the traditional green ceramics can be omitted in the processing of most new ceramic products. The sintering step, however, is still very much requred to obtain maximum possible strength and hardness. The functions of sintering are the same as before: (1) to bond individual grains into a solid mass. (2) to increase density, and(3) to reduce or eliminate porosity. Temperatures around 80% to 90% of the melting temperature of the material are commonly used in sintering ceramics. Sintering mechanisms differ somewhat between the new ceramics, which are based predominantly on a single chemical compound(eg, Al:03), and the clay-based ceramics, which are usually made of several compounds having different melting points. In the case of the new ceramics, the sintering mechanism is mass diffusion across the contacting particle surfaces, probably accompanied by some plastic flow. This mechanism causes the centers of the particles to move closer together. resulting in densification of the final material. In the sintering of traditional ceramics, this mechanism is complicated by the melting of some constituents and the formation of glassy phase that acts as a binder between the grains.