Process developments create opportunities for ceramics

Paul Boughton

Design engineers are usually comfortable working with metals and plastics but most have little or no knowledge or experience of ceramics. This is a pity, as these materials can offer major advantages when the other classes of materials cannot provide the performance characteristics required.

For example, ceramics are commonly used in the medical and aerospace industries, as their various attributes include high heat resistance and biocompatibility.

Designers working in other sectors may be able to redesign products in ceramics to gain a performance improvement.

One of the traditional difficulties with ceramics has been in obtaining prototypes. Today prototype components are expected to represent the final part as closely as possible - which implies that prototypes of ceramic components must be made from ceramics with properties very similar to those specified for the actual product, especially for functional testing.

Cerampilot is a French company that has been established to supply this growing need for ceramic prototypes for medical implants and other products. The company's proprietary Fast Ceramic Production (FCP) technique was developed from research carried out at the Centre for Technology Transfer in Ceramics (CTTC).

In FCP, a CAD file of the part - perhaps a prosthetic bone fragment - is created from a 3D scan, and used to make a model using stereolithography (SLA). This builds up the part, layer by layer, from a material comprising UV-sensitive resin and a ceramic powder. Once the 'green' part has been built, it is sintered to produce the implant. This technique can be used to manufacture parts in biocompatible materials such as hydroxyapatite (hydroxylapatite) or hydroxyapatite/tri calcium phosphate. It has already been used to make long-term implants to repair or replace bones in the skull, spine and leg.

According to Cerampilot, "FCP produces better bone substitute than traditional methods. For the same value of porosity - 60 per cent - the mechanical strength is three to five times higher."

Tailored material properties

One of the advantages of FCP is that the process ensures that all pores are the same size. Furthermore, porous and solid elements can be combined in the same part. Because the original CAD file is created from scanned data, the part is customised to each patient, with the first prototype delivered in around 15 days.

FCP can also be used with a range of other ceramics, including: alumina, which is used for high-temperature electrical insulators; zirconia, for applications such as jewellery and heating elements; aluminium nitride, which is found in radar components; mullite, which has high resistance to thermal shock; and cordierite, a ceramic that exhibits good thermal conductivity. In addition to these 'standard' products, Cerampilot also produces tailor-made 'technical' ceramics for specific applications.

Phenix Systems, another French company, supplies direct laser sintering (DLS) machines that can handle either ceramic or metal powders. Its PM series of laser sintering machines can build parts with a repeatability of 20 microns. Parts built on the PM series machines are subsequently sintered in a furnace. The company's PM100T is suitable for manufacturing small components, as well as for training, research and development purposes, being capable of building alumina parts up to 100 mm in diameter and 100 mm tall. The PM250 is a larger, updated version of the PM100T that can build parts up to 250 mm diameter and 300 mm tall.

Another rapid prototyping system for creating ceramic parts is Javelin 3D's Steamroller, which uses the technique known as laminated object manufacturing (LOM). Whereas stereolithography, selective laser sintering and similar additive rapid prototyping techniques lay down thin layers of material that are selectively cured with a laser, LOM is a process in which thin sheets of material are cut to size then bonded together to build a 3D model.

USA-based Javelin's technique - which it calls CerLAM - is based on the LOM concept; however, instead of the paper used in conventional LOM, it uses tape that is impregnated with a ceramic material. The Steamroller automatically feeds the sheets one at a time, whereupon the machine's laser cuts the sheets according to the 3D digital data supplied. Finally the profiled sheet is added to the model.

When it is complete, the model goes through two heating processes to produce a true ceramic prototype. The first is de-binding, which heats the part to 90°C, then slowly to 300°C and more quickly to 600°C, before cooling to room temperature. Next, it is sintered, with the exact heating process depending on the material composition. Some ceramics, such as carbides and nitrides, can be sintered using microwaves - which saves both cost and time. However, Javelin's process can also output parts in a wide variety of other ceramics, including alumina, silicon nitride and hydroxyapatite.

Satellites and microsatellites

Javelin has been involved in a project with the Materials & Manufacturing Directorate - part of the US Air Force Research Laboratory - to develop ceramics that will increase the life of Hall thruster insulators for spacecraft - including satellites and microsatellites. Hall thrusters are propulsion devices that keep satellites in the correct orbit. Small amounts of xenon gas are fired out of the thrusters, but this can erode the chamber walls. The research aims to identify ceramics that have better erosion resistance than the existing boron nitride materials used in Hall thrusters and manufacture them more cheaply and efficiently by using rapid prototyping techniques. Javelin used its CerLAM method to prepare prototype ceramic insulators for testing. If successful, the technique could be used to manufacture the parts in order to avoid large tooling costs. So far two ceramics with improved erosion resistance.

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