Perhaps the most useful starting point is to begin with the definition of the technical term “HIPing”, an acronym for the process known as Hot Isostatic Pressing.
In rudimentary terms, the HIPing process uses the combination of high temperatures and high pressures to densify engineering ceramics and hard metals. The densification and removal of porosity thereby leading to improved mechanical properties such as strength and reliability.
A schematic of a typical pressure vessel set-up is shown in Figure 1 below.
Figure 1. Schematic of the Hot Isostatic Pressing Process
The HIPing Process
Typically, an inert gas (Argon or Nitrogen) is used within the pressure vessel to ensure that pressure is applied uniformly from all sides (Isostatic Pressure). The inert nature of the gas also assists in reducing any oxidation effects.
Pressures of up to 207 MPa (30,000 psi) may be used with the temperature of operation being as high as 2000°C (3,632°F). Some HIP’s can go as high as 414 MPa (60,000 psi).
Cycle times and temperatures for HIPing vary due to the optimum sintering and densification temperatures of materials and their ability to be heated and cooled quickly without suffering thermal shock effects. However, it is not unusual for the heat up and cool down cycles to be 8 hours in duration.
With many engineering ceramics the HIPing process uses what is known as “cladless”. As the name suggests, this technique, first invented in 1938 by Johan Romp of the Philips organization in the Netherlands, begins with a sintered ceramic, hence it is often known as “Sinter-HIP”.
In 1955 Chuck Boyer furthered the development of HIPing in the USA through work conducted at Battelle Columbus Labs, Ohio.
The sintered ceramic is introduced into the HIPing chamber typically with a density of >97% of its theoretical density, the HIPing process then effectively closes the remaining pores to produce, in an ideal world, a “defect free” ceramic.
The other form of HIPing which can be applied to engineering ceramics is to encase the raw ceramic powder within a metal can or glass capsule, subject the assembly to the HIPing process and then remove the cladding to reveal the densified product.
Although the use of Argon as the inert gas in the process is the most common approach, it can lead to discolouration or a darkening of oxide ceramics such as zirconia. In these cases it is possible to introduce oxygen partial pressure HIPing to avoid the discolouration effect.
Similarly with the HIPing of Silicon Nitride based ceramics it is often advantageous to use a pure Nitrogen gas to prevent the dissociation of the microstructure.
How Does HIPing Benefit Engineering Ceramics
The HIPing process is used in many engineering ceramic applications, for example;
- Silicon Nitride Ball Bearings
- Body Armour
- Zirconia and Alumina Dental implants
- Multi-Layer Capacitors
- Downhole Oilfield Components
- Sputtering Targets
In all of these applications the primary use of HIPing is to increase the density of the material and increase the strength and reliability of the components.
Engineering ceramics, fail under stress due to pre-existing defects. By removing defects the ultimate strength of the material can be increased significantly. In the case of zirconia ceramics for example, this can be > 40%, from a 3 point bend strength of 1,000 MPa to over 1,400 MPa.
In parallel with the reduction in defect failure sites, it’s also the case that HIPing generally reduces the range of defect sizes, in turn this leads to more reliable products as their failure is limited to a narrow band of failure stress levels. This is also reflected by an increase in the reliability parameter the Weibull modulus which defines the reliability of the product.
Machining Improvements Due to HIPing
With high quality engineering ceramic components that are designed for use in demanding wear or corrosion environments such as pump parts or oilfield valves, it is often the case that after sintering and HIPing the component must be machined to demanding tolerances.
The improved density of the HIP’ed component surface aids in micro-machining processes as it is easier for the operator to achieve high levels of surface finish. The diamond grinding process removes surface grains of the machined material and a tighter grain structure will yield a better finish.
However, it should be noted that the machining of such components requires highly skilled operators and high precision equipment to achieve the highest levels of precision, surface finish and low levels of surface and sub-surface damage. For further details and advice on machining HIP’ed components contact Insaco at www.insaco.com.
If you are considering an engineering application that you feel may be suited to a HIPing then Insaco with 65 years of experience is second to none.
They have many decades of experience with HIPing materials for a wide range of applications.
Presently they have an 85,000 ft2 facility with more than 300 machine tools capable of grinding and polishing glass ceramics and other ceramic materials to useful geometries and tolerances.
This information has been sourced, reviewed and adapted from materials provided by INSACO Inc.
For more information on this source, please visit INSACO Inc.