7.1 Hot Pressing

Hot pressing is a suitable method for densifying materials with poor sintering behaviour. This technique, which combines powder pressing and sintering into one single operation, offers many advantages over conventional powder consolidation. By simultaneous application of temperature and pressure, it is feasible to achieve near theoretical density in a wide range of hard to work materials. As the resistance of metal particles to plastic deformation decreases rapidly with increase in temperature, much lower pressures are required for consolidation by hot pressing. Further, densification by hot pressing is relatively less sensitive to powder characteristics - shape, size and size distribution which are important in cold pressing and sintering. Hot pressing parameters pressure, temperature, time and the working atmosphere largely control the properties of compacts. High speed tool steels, super alloys, beryllium and the refractory metals are particularly amenable to hot pressing. Hot pressing is perhaps the only method of producing dense fine grained shapes of materials such as pure carbides, nitrides and borides which are otherwise difficult to sinter due to the lack of adequate atomic mobility at the sintering temperature.

The various steps involved in the hot pressing procedure are as follows:

1. Powder or a cold compacted preform is placed into the die mould;

2. The mould...