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.
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.”
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.
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