Process developments create opportunities for ceramics
Ceramics offer properties that are not generally available from metals, plastics or composites, but ceramic components can be difficult to source, especially if time is of the essence. Alistair Rae reports.
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.
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.
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 have been identified, although erosion patterns were seen - possibly due to impurities and imperfections caused by traces of the organic material used to bind the ceramic particles together.
Many short-run metal and plastic production parts are now being made using prototyping techniques and it is inevitable that the same will happen in ceramics. Phil Reeves, managing director of prototyping consultancy Econolyst, comments: "The production of ceramic layer-manufactured parts for end-use applications is in its infancy. However, processes and materials are being developed that will see rapid manufacturing used in the consumer goods and toy industries, biomedical industry and the aerospace and automotive sectors."
3D printing
One USA company, Therics, which is part of medical device manufacturer Integra Lifesciences, has developed a 3D printing process called Theriform to make synthetic bone products using its own line of beta tri calcium phosphate (beta-TCP) materials. Originally developed at the Massachusetts Institute of Technology (MIT), Theriform is a process in which a print heads travel across a fine layer of beta-TCP, adding tiny droplets of binder solution to build a 3D component. The company states: "Using this method, we can control both essential elements of true scaffolding materials: the micro-architecture - or internal inter-connectivity of the pores - and the macro-architecture, or profile shape." The porous nature of the scaffold helps to promote bone growth, and the implant can be combined with bio-additives, including bone marrow, blood and drugs.
At the same time, German research - published in the Journal of Dental Research - offers an alternative to CAD/CAM milling for making dental prostheses. Researchers at the Dental Materials and Biomaterials Research unit at the University Hospital Aachen in Germany - together with four partners - prepared a zirconia-based ceramic suspension with 27 percent solid content by volume. A modified drop-on-demand inkjet printer was used in a 3D printing process to produce a crown, which was then sintered.
UK materials consultancy CERAM uses a similar process, which it calls Direct Ceramic Jet Printing (DCJP), to build ceramic prototypes and short-run production parts. Giuliano Tari, the director of operations, explains: "We use an ink with a fine ceramic powder inside." The company routinely uses this process to print patterns directly onto ceramic tiles (Ferro's Kerajet is an example of a commercial product). But Tari says the technique could also be used to make more sophisticated products such as sensors.
A good example of the consultants' work is a project undertaken with German ceramic furnace manufacturer DEKEMA Dental-Keramiköfen GmbH, relating to a heating element. The company needed to ensure that its new design would meet the requirements for a low-wear, highly dynamic furnace operation while improving on long-term temperature stability. Consultants worked with the client to develop a recipe formulation that matched the product performance requirements. At the same time, computer modelling was used to simulate the heating element, enabling a new element support to be developed that can operate at high temperature gradients thanks to the enhanced material properties and optimised geometry.
The new part was manufactured using a visco-plastic processing (VPP) technique in which a material impregnated with ceramics can be extruded thanks to the material's clay-like consistency. In this case, no physical prototype was needed; instead, the consultants produced a virtual prototype, which was then approved prior to production commencing. Tari says: "There was no aesthetic aspect to the design, so we did not need to build a physical prototype."
Better design of ceramic components can also improve the performance of metal parts. For example, Morgan Technical Ceramics has developed a flow modelling technique for the design of ceramic cores that are used in the manufacture of metal castings. The software optimises the injection moulding of the cores, enabling designers to create cast parts with more complex geometries. Flow simulation software is commonly used in plastics manufacturing, but has not previously been used by the ceramics industry before, according to Morgan, whose specialists use the software to predict how the ceramic core cavity will fill. Areas of stress and potential breakage are highlighted early on, so the design of the core can be refined to ensure the mould is filled in a controlled, uniform way.
Morgan claims the process is more efficient than traditional methods because simulation is carried out before any metal is cast.
