Dr. Uibel, how would you describe additive manufacturing in the simples possible terms? How does it differ from conventional manufacturing processes?
Uibel: Part of the answer is already in the question. “Additive” means nothing more than that something is built up – in most cases, layer by layer. Additive processes are mainly aimed at the production of plastic objects, but they can also be used for metals and ceramics. Conventional processes, on the other hand, are subtractive, which means that material is removed, for example by milling and drilling, to produce an object.
What are the basic processes in additive manufacturing?
First of all, 3D printing, because this is also an additive process. In 3D printing, a binder is deposited onto a powder bed, where it solidifies the material. This process is repeated layer by layer, with the binder also connecting the layers to one another to produce the printed product. Then there’s also “direct” 3D printing. In this process, the material is applied directly from the print heads onto a substrate, where the layers solidify through the evaporation of solvents. What’s special about this is that I can use several print heads and therefore also several materials.
In stereolithography, a liquid plastic is used for the production of 3D objects, which is laid down in layers and solidified using a laser.
Fused filament fabrication is probably the process with the simplest technical structure and currently the most widely used. In this process, a polymer filament is melted and the liquid melt is then deposited in layers, solidifying at the same time. A variant of this is liquid deposition modeling, in which a ceramic mass is deposited. The 3D object is then dried and solidified through means of a sintering process.
Selective laser sintering allows for the use of different materials, including metals. It involves a powder bed process in which powder is used in layers and sintered with a laser. The material melts under the effect of the laser and thereby also connects the layers with one another.
Finally, we have laminated object manufacturing. In this process, a film is applied to a substrate and cut out with the laser. The film is adhesive on one side and is then glued to the next one until the object is completed.
Additive manufacturing not only allows users a high degree of design freedom, it also makes it possible for them to map very complex structures.
Dr. Krishna Uibel
Looking at the overall process, what are the advantages of additive manufacturing?
Additive manufacturing not only allows users a high degree of design freedom, it also makes it possible for them to map very complex structures. Another important point is that compared to conventional processes, the number of process steps and the use of materials can be reduced. It’s also entirely possible that series production may become cheaper in the future. When used for rapid prototyping, additive manufacturing also involves the comparatively fast production of prototypes or small series. This makes it possible for designers to check whether new ideas – for example for manufacturing new components – are feasible.
Of the processes listed, which are of any interest for ceramics?
I conducted an assessment in collaboration with my colleagues and our result was that direct inkjet printing, stereolithography, and liquid deposition modeling were the methods that worked best. No other methods are currently suitable for the use of ceramic materials.
And conversely, which ceramic materials are particularly suitable for use in additive manufacturing?
For oxide-ceramic components, that would be aluminium oxide and zirconium oxide; for non-oxide ceramics, it’s silicon nitride and silicon carbide, as well as boron carbide. Other technical ceramics require processes that cannot currently be achieved with 3D processes.
In this type of application, there are no differences from the conventional process chain. There’s always a fine-grained ceramic powder that’s mixed with polymer additives. This creates a green body in which the particles are connected through a polymer binder, which is then sintered at high temperatures so that a dense ceramic component is created at the end.
One thing is particularly important with a view to the future: even with perfect 3D printing, the use of ceramics never results in a finished component. It still requires thermal processes such as those that can take place in vacuum or pressure sintering furnaces – especially in the case of non-oxides. This means that in the future additive manufacturing will probably remain within the domain of ceramic manufacturers, especially since the requirements regarding strength, freedom from defects and surface quality also affect other devices that are used.
One thing is particularly important with a view to the future: even with perfect 3D printing, the use of ceramics never results in a finished component. It still requires thermal processes such as those that can take place in vacuum or pressure sintering furnaces – especially in the case of non-oxides.
Dr. Krishna Uibel
For which areas are ceramic components manufactured through additive processes particularly suitable?
Mainly for areas where they’re also manufactured conventionally, because ceramic is a material that has a whole range of special properties in comparison with other materials. This includes a high degree of hardness, such as that which is required for grinding or cutting tools or in ballistic protection in armour. A high degree of resistance to chemical corrosion is a characteristic in the use of silicon carbide, which is used for bearings and seals in pumps, for heat exchangers and for microreactors – in other words, in areas where aggressive substances are used. The high temperature stability of ceramics is always required in areas where metals and plastics are stretched to their limits. In environments where temperatures exceed 500 or even 1000 degrees Celsius, there are no real alternatives to ceramic materials. Another field relates to functional use, for example with electrical insulation or cables in high temperature ranges. It is actually already being used for making implants, crowns and bridges in dental ceramics – i.e. when it concerns producing individual “components” in small numbers. In the future, however, there will definitely be other applications that we cannot yet know of.
Can you foresee how the market will develop in the coming years?
That’s difficult. At the moment, the printers that have the very high degree of precision that’s required are still extremely expensive. But we need this accuracy when it comes to combining materials in a microstructure, something that’s not possible with conventional methods. One could imagine that in the future, we will actually be able to build individual sensors “additively” in a single printing process. In looking to the future, we should always bear in mind that the additive manufacturing processes were all designed for use with plastic. Apart from stereolithography, for which there are already commercial suppliers, we are now at the point of checking whether and where the existing processes can be adapted to the requirements of ceramics. There are currently a great number of such machines in the research laboratories of large corporations, where attempts are being made to adapt them to the special requirements of metal or ceramics, so that they may later be put into commercial use.
Examples where printing with ceramics has already stepped outside laboratory development are dental products, such as crowns and bridges made of zirconium oxide, which are about to be commercialised via stereolithography, and direct inkjet printing, or products for use in high-temperature applications, such as recuperative burners made of Si-SiC, which are already part of the daily business of the ceramic manufacturer Schunk Engineering Ceramics.
Right now we can only outline the enormous benefits of the current state of development. The starting points I have already mentioned, but we must not forget that the design freedom that we get with additive manufacturing can enable us to develop completely new products. This applies, for example, to the construction of complex internal structures – or for integrating new materials in a microstructure in the scale of a few 10 or 100 micrometres and, depending on the requirement profile, combining them with the properties of other materials.
Dr. Krishna Uibel has been working in the field of ceramic raw materials, technical ceramics and ceramic coating systems since the end of 2000. While working on his doctorate at RWTH Aachen University, he developed materials and coatings for high-temperature applications and laid the cornerstone for direct inkjet printing. In a patented process, zirconium oxide ceramics with a strength of >1000 MPa could be manufactured additively for the first time.
Today Dr. Krishna Uibel is head of Product Management at Friatec AG.
As an inventor and co-inventor, he has registered industrial property rights for innovative products in the areas of 3D printing, high-temperature materials and processes, inorganic ceramic coatings, thermally conductive fillers and molding processes for BN polymer compounds and ceramic components in a total of 22 patent applications.