Reaching Maximum Density
The promise of high-performance ceramics lies primarily in two areas. The first is the ability to create from ceramic material(s) a product that has specific levels of wear resistance, high temperature functionality, weight-to-strength ratios and/or other unique properties. The second is as a replacement material for current metal components to either reduce material costs or final component weight, or to expand the useful specification range beyond the useful range of a metal component.
The potential forming methods for these advanced ceramic materials are equally as varied, and each has its advantages and disadvantages. Hot isostatic pressing (HIPing), sintering, hot pressing, cold isostatic pressing (CIPing), injection molding, extrusion, slip casting, and a number of pseudo-isostatic processes are often used individually or in combination to form ceramic products for various high-performance applications.
When maximum uniform densities and/or a variety of sizes and shapes are needed, HIPing is often the forming method of choice. With HIPing, companies can either form components directly from powder, or they can use the technique in combination with a pre-forming step, such as CIPing, sintering or injection molding. The attainable densities and basically limitless size capability of the HIP operation are not found in any other forming process.
HIPing AdvantagesA hot isostatic press applies both pressure and temperature to form and uniformly densify a particular material. Pressure is applied inside a vessel by compressing a gas medium, such as argon, nitrogen or oxygen, while the heat is supplied through a resistance-heated furnace, also located inside the pressure vessel. Temperature and pressure are monitored and controlled within a specified tolerance throughout the process, and a computer provides man/machine interfacing.
The main advantage of the HIP process is its ability to densify a powder or preformed material to 100% of theoretical density. Additional advantages include the creation of a more homogeneous microstructure, the ability to bond dissimilar materials, or the ability to process to net or near net shapes.
Saint-Gobain Advanced Ceramics in East Granby, Conn., is one of a number of companies that have experienced the advantages of HIPing first hand. According to Tom Leo, plant manager, “The real benefit of HIPing is that it achieves uniform pressure and repeatable consolidation of the product. The process enables you to densify materials that under most conditions cannot be densified.”
Leo’s plant has been using HIPing for the past decade to manufacture high-precision Cerbec® ceramic balls, which are used in high-speed bearings. “Bearing balls are highly loaded structural ceramics, and any porosity in the end product would be detrimental to performance. We have a special composition that we developed for our bearings, and with the use of HIPing we virtually eliminate any porosity of the material,” says Leo.
“Uniformity and repeatability are also important,” he adds. “We make millions of balls every week, and we’re trying to achieve no variation, so that our customers consistently get a good quality product. HIPing makes our product very uniform and homogenous, and it’s extremely repeatable—it gives us the quality we need.”
Technological AdvancesA number of advances have been made in the HIP process in recent years, as ceramic manufacturers such as Saint-Gobain and others have requested new processing parameters. These advances include increases in the maximum operating temperature up to 3000 degrees C, the ability for the HIP furnace to operate in partial pressures of oxygen and more reliably in nitrogen, the replacement of typical temperature measurement via thermocouples with more accurate and durable temperature measurement systems, controlled and uniform furnace cooling rates to decrease thermal cracking and improve throughput, and improvements in the reliability of graphite furnace designs for longer component life and less maintenance. While most of these improvements were developed in the 1990s, their use has been sporadic as the overall commercial demand and use for HIPed ceramics has been confined to a few systems and companies.
For many ceramic manufacturers, it can be hard to balance the capital cost of the equipment and supporting technology with the overall demand for the properties of HIPed ceramics. However, according to Leo, there is no substitute for HIPing when extremely high-quality parts with the maximum possible densities are required. “HIPing is a bit more expensive than other processes if you look at just the sintering step. However, if you look at the total process and include the cost of rejects and the quality of the product being made, it’s a pretty competitive technology. If you’re trying to make a high-quality product for a demanding application, HIPing is definitely a process to consider,” Leo says.
For More InformationFor more information about HIPing, contact Flow Autoclave Systems Inc., 3721 Corporate Dr., Columbus, OH 43231; (614) 891-2732; fax (614) 891-4568; or visit http://www.flowae.com.
SIDEBAR: HIPing ApplicationsExamples of various current and potential applications for HIPed advanced ceramic components include:
Wear and Tool Parts
- Textile thread guides
- Mechanical seals
- Chemical process liners, filters and bearings
- Cutting tools
- Dies, guides, valves and nozzles
- Substrates and capacitors
- Oscillators, thermistors and varistors
- Turbocharger rotors
- Piston and cylinder liners
- Valve lifters
- Piston valves and seats
- Dental implants and replacements
- Prosthetic joint devices