24/04/2018

Testing a ceramic sample

Testing a ceramic sample by X-ray computed tomography to detect potential defects in the material

Source: BAM

Many products can now be manufactured by 3D printing – whether bone substitutes, tools or car parts. Layered objects from different materials can be created by using three-dimensional computer models. The base material is powdered, most often ceramics, plastic or metal. The powder is applied layer by layer and the individual layers are then glued together with resin or fused by a laser beam.

Prof. Dr. Jens Günster is researching and developing 3D powder printing processes for so-called additive manufacturing. His team has already contributed to the improvement of additive manufacturing. For example, they have optimised the flowability and the grain size of the powders used in 3D printing, which improved the printed product. The idea of using suspensions as a starting material is also promising.

3D powder printing shows promise

Many companies use 3D printing for industrial production, at the moment, however, it is more suited for the production of small numbers of individual products, for small batches or for rapid prototyping. “3D powder printing can also be suitable for very large products, such as telescope mirrors for space research,” says Günster.

The industry is also interested in the use of ceramics for 3D printing. Due to their high heat resistance and strength, ceramics are particularly popular for industrial applications. But ceramic powders do not cross-link through chemicals as easily as plastic resins and metals, nor can they be plastically deformed by heat. The desired structures are difficult to form and difficult to consolidate. “It all depends on the material,” explains Günster. Even if ceramic products from the printer are then heat treated (sintered) in a furnace, the result is often still not satisfactory. “We have conducted many tests on how the ceramic materials can be better adapted to the additive manufacturing process and changed them accordingly,” Günster describes his approach. “The processes that we developed have also become very popular in the industry,” he says.

Polymers provide more stability

The use of pre-ceramic polymers as starting material for the industrial printing process is imminent. Pre-ceramic polymers are special polymers that can be converted into ceramics. The compact ceramic is formed only when heated above 1200 °C and the precursor substance turns into silicon oxycarbide, a glassy substance. “We are using an inexpensive commercial powder also used in the cosmetics industry,” Günster explains.

But the idea nearly failed because heating the preceramic polymer is not possible since it melts at 60 °C. However, the researchers used this weakness for their advantage: Günster’s team added a substance to the material, which makes the structure cross-link better and uses two printheads. One printhead applies the binder and cross-linking agent, the other printhead only the binder. The skeleton of the structure and the shell are printed with these two liquids in one step. The cross-linking agent ensures that the desired structure is maintained during heating. As soon as it melts, the added polymer flows into the gaps and fills them completely. As a result, no edges or pores remain, creating a ceramic component with greater stability. “This self-assembled form has optimised the structure and thereby withstands greater pressure,” explains Günster.

A ceramic structure from a 3D printer

A structure from a 3D printer, which can be converted into a ceramic material at a temperature of 1400 °C

Source: BAM

Customised ceramics are particularly interesting for medical purposes. “Porous lattice structures in the upper layers of an implant or in the coating of a prosthesis are currently in high demand,” explains Prof. Dr. Giovanni Bruno. “The existing human bone grows into these cavities thereby improving grafting.” But this is only viable if the lattices are correctly distributed and the pores are not too large, otherwise the material will break.

Computed tomography helps to quickly control the dimensions of 3D products, estimate defects and displacements in the structure and establish their size and number. BAM is investigating another method that enables this precise overview, the so-called X-ray refraction. In this method X-rays are refracted at microscopically small interfaces on the surface or inside the material, which makes it possible to detect microfine cracks or pores. “The finer the resolution, the better we can characterise the material,” explains Bruno.

Non-destructive testing improves manufacturing

Researchers are no longer just looking for production errors, they are also interested in the material’s structure. They analyse the composition and the spatial arrangement of individual components. “Computed tomography or X-ray refraction helps us investigate larger volumes non-destructively even in 3D,” Bruno explains. This is important because many effects that characterise new materials are based on threedimensional structures. This analysis improves the manufacturing process. When Bruno’s team finds unwanted pores or other defects, it investigates the cause of the abnormality. “We want to understand why the results of 3D printing deviate from the digital image of the computer,” he says.

Laboratory computed tomography and X-ray analysis are not suitable for daily use. They are used, for example, when a particularly high resolution is required or for high-quality components such as ceramic implants. They are simply too expensive for typical industrial components. BAM is developing further non-destructive, mobile and at the same time reliable examination methods accessible to all – namely ultrasound and thermography.