Gas Pressure Bonding

GAS PRESSURE BONDING

Hot isostatic pressure

The Hot Isostatic Press (HIP) uses the simultaneous application of heat and pressure. We refer to this process as HIPing and the product as being HIPed. A furnace is constructed within a high-pressure vessel and the objects to be pressed are placed inside. Figure 4.23 shows a typical HIP arrangement.

Temperatures can be upto 2000°C, and pressures are typically in the range of 30 - 100 MPa. A gas is used as the pressure medium unlike in the CIP where a liquid is often used.

Argon is the most common gas used for HIPing, but oxidizing and reactive gases can be used. Note that the high-pressure vessel is not inside the furnace.

There are two variants of HIPing.

• Encapsulated: using a deformable container

• Not encapsulated: shape and sinter first, then HIPed

• Now HIPing is used for a wide variety of ceramic (and metallic) components, such as alumina-based tool bits Sis bez and the silicon nitride nozzles used in flue-gas desulfurization plants by the utility industry.

The advantages of the HIPing process are becoming more important as interest in structural ceramics (e.g., Si,N) grows.

• Nonoxide ceramics can be HIPed to full density while keeping the grain size small and not using additives. Very high densities combined with grain sizes (because of the relatively low temperatures) leads to products with special mechanical properties.