Ceramics at TWI

Hot isostatic pressing

Hot isostatic pressing (HIPing) is the simultaneous application of temperature and isostatic pressure. It differs from hot pressing in that the pressure is applied uniformly in all three directions rather than uniaxially. HIPing is used in both metallurgical and ceramics industries, for example in the production of defect-free castings, complex shape components, or high density ceramics and composites and in the solid phase (diffusion) bonding of similar or dissimilar materials.

The component is surrounded by a gaseous (typically argon) medium and undergoes a thermal treatment at high pressure to consolidate the materials.

Because of the isostatic nature of the pressurising medium, greater flexibility with regard to sample shape is achieved through this process, and because of plastic deformation at the interface, surface preparation need not be so stringent.

It may be possible to alleviate some of the coefficient of thermal expansion mismatch problems associated with bonding dissimilar materials, since component design is more flexible. Therefore the design can be arranged such that a compressive stress is obtained at the interface on cooling.

This process also allows simultaneous densification and bonding of powders or porous bodies.

The critical aspect of this technique is that the interface must be isolated from the gaseous pressure medium. This stops the pressurising gas from interacting with surface or interconnected porosity within the component. This is often achieved by encapsulating the component. There are three main types of encapsulation:

1. direct sealing of the circumference of the contact area between the two parts;
2. placement of a sleeve of material around the contact area between the two parts;
3. full, or partial encapsulation of the entire component.

After HIPing, the encapsulant material is normally machined off.

TWI offers a full range of encapsulation techniques, depending on temperature requirements:

For temperatures up to 1000°C

• TIG welding of metals
• Diffusion bonding

For temperatures in excess of 1000°C

• Glass encapsulation
• Electron beam welding of metals

More information:

• Joining ceramics - a guide to best practice. Section 5. Solid state bonding of ceramics
• How are oxide dispersion strengthened (ODS) alloys produced?