Natural bone consists of a hard outer shell, called cortical bone, and an inner mesh-like structure, called cancellous bone. When developing bone implants, this hybrid structure should be considered. Our study focused on combining two different bioceramic materials via sinter-joining, using zirconia as mechanically strong material and hydroxyapatite or tricalcium phosphate as osteoconductive material. We produced the green parts individually with a lithography-based ceramic manufacturing process. After debinding and pre-sintering the two individual parts were joined and co-sintered to a final hybrid part. The connection is created by inducing a press-fit between the outer and inner material due to their different shrinking behaviour. We tested the sinter-joining process first on a simplified ring-in-ring design and biaxial bending plates to quantify the mechanical resistance of the hybrid part. We found a significant increase in the maximal measured force in the piston-on-three-balls test set-up from (72 ± 53) N to (366 ± 88) N for a 5% and 10% press-fit, respectively. Furthermore, we introduced sintering aids to ensure exact alignment of the two parts during co-sintering and measured the deformation after co-sintering. We continued to increase the complexity of the hybrid sinter-joined parts and resulted in successfully manufacturing a complex implant-design consisting of an outer load-bearing shell and an inner porous structure. Moreover, we investigated the interface with a scanning electron microscope and additional electron backscatter diffraction (EBSD) analysis to characterize chemical and crystallographic gradients.

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