*Article* **New Perspectives on Zirconia Composites as Biomaterials**

**Giuseppe Magnani, Paride Fabbri, Enrico Leoni, Elena Salernitano and Francesca Mazzanti \***

ENEA, Laboratory of Materials Technologies Faenza, Via Ravegnana 186, 48018 Faenza, Italy; giuseppe.magnani@enea.it (G.M.); paride.fabbri@enea.it (P.F.); enrico.leoni@enea.it (E.L.); elena.salernitano@enea.it (E.S.)

**\*** Correspondence: francesca.mazzanti@enea.it

**Abstract:** Zirconia–alumina composites couple the high toughness of zirconia with the peculiar properties of alumina, i.e., hardness, wear, and chemical resistance, so they are considered promising materials for orthopedic and dental implants. The design of high performance zirconia composites needs to consider different aspects, such as the type and amount of stabilizer and the sintering process, that affect the mechanics of toughening and, hence, the mechanical properties. In this study, several stabilizers (Y2O3, CuO, Ta2O5, and CeO2) were tested together with different sintering processes to analyze the in situ toughening mechanism induced by the tetragonal–monoclinic (t–m) transformation of zirconia. One of the most important outcomes is the comprehension of the opposite effect played by the grain size and the tetragonality of the zirconia lattice on mechanical properties, such as fracture toughness and bending strength. These results allow for the design of materials with customized properties and open new perspectives for the development of high-performance zirconia composites for orthopedic implants with high hydrothermal resistance. Moreover, a near-net shape forming process based on the additive manufacturing technology of digital light processing (DLP) was also studied to produce ceramic dental implants with a new type of resin–ceramic powder mixture. This represents a new frontier in the development of zirconia composites thanks to the possibility to obtain a customized component with limited consumption of material and reduced machining costs.

**Keywords:** zirconia–alumina composite; stabilizing oxides; critical grain size; tetragonality; mechanical properties; fracture toughness; flexural strength; ceramic additive manufacturing; DLP
