Once a bone defect has exceeded a critical size, the human bone is no longer able to heal the defect on its own. In such cases, the implantation of an artificial structure becomes necessary to bridge the defect and to support the natural healing process. Bioceramics like hydroxyapatite (HA) and tricalcium phosphate (TCP) are commonly used as artificial implants due to their remarkable ability to significantly promote bone ingrowth and activate natural bone cells. However, their relatively low mechanical properties limit their application to small bone defects in areas of minimal mechanical stress. In contrast to HA and TCP, zirconia is also a bioceramic which possesses exceptional mechanical properties characterised by high hardness and strength, making it suitable for areas of high mechanical stress. However, zirconia lacks the ability to enhance bone ingrowth and activate natural bone cells.

Our aim in this study is to combine the benefits of both materials, using HA or TCP in areas intended for osseointegration, and zirconia in areas of increased mechanical stress.

Our manufacturing process uses Digital Light Processing (DLP) based vat polymerisation, where a photocurable ceramic slurry containing ceramic particles and organic binder is selectively cured by blue light to build up the part layer by layer. Subsequent thermal post-processing produces bioceramic parts characterised by high density with high resolution (<40 µm).

For our sinter-joining experiments, we chose a ring-in-ring design where the outer and inner ring each consisting of a different ceramic material. First, both ceramic materials are thermally treated to remove the organic binder. Second, the inner ring optionally undergoes a pre-sintering step to reduce its size to fit inside the outer ring. Third, the sinter-joining process takes place during the final co-sintering step, in which the materials that are then combined are subjected to the maximum sintering temperature. We use the different shrinkage behaviour of the ceramic materials to achieve a press fit between the rings. This involves dimensioning the diameters to ensure that the inner diameter of the outer ring is smaller than the outer diameter of the inner ring after co-sintering. Precise dimensional adjustments, facilitated by the high resolution of the printing process, reduce the risk of inducing excessive stresses in the outer ring and thereby avoid part failure.

To understand the effects and establish process control, our investigations include parameters such as varying pre-sintering temperatures and different co-sintering temperatures and profiles. We also investigate parts with a fine surface texture at the interface. A final quantitative analysis provides insight into the bond strength between the two different ceramic materials.

Acknowledgment: Christian Doppler Research Association, Austrian Federal Ministry for Digital & Economic Affairs, National foundation for Research, Technology & Development