Fabrication and multiscale mechanical characterization of Ti alloy/Al2O3 functionally graded materials for orthopaedic applications

In this study, the mechanical and tribological properties of Ti6Al4V-alloy/Al2O3 Functionally Graded Materials (FGM) were evaluated through multiscale indentation tests and metallographic analysis. The FGM, consisting of 11 uniform layers graded in 10% composition steps of 1 mm thickness each, was generated by Spark Plasma Sintering (SPS) technique with a non-uniform sintering temperature. Metallographic analyses evidenced the presence of a Ti3Al intermetallic phase at the metal/ceramic interface of the intermediate FGM layers. Nano and micro-scale indentation tests were performed with a NanoTest Indenter (Micro Materials Ltd., Wrexham, U.K.), while an ad hoc set-up was designed and realized to characterize the FGM at the macroscopic scale. Elastic and inelastic material properties have been obtained on both the cross-section and the surfaces of the external pure metal and pure ceramic layers. The mechanical characterization exhibited the capability to quantify the problems related to the sintering techniques already evidenced in the metallographic analyses. The lack of an optimal sintering temperature led to sintering defects that most likely resulted in imperfect interfaces. This work proved that the combination of advanced micro and nano mechanical characterization techniques in a multiscale investigation framework represents a reliable feedback for the development and optimization of the SPS as a process for the fabrication of FGM.