STUDY ON CHARACTERISTICS OF DIRECT ENERGY DEPOSITED NITINOL AND A NOVEL COATING METHOD FOR ORTHOPEDIC IMPLANT APPLICATIONS
Creators: name: Lee, Jeongwoo
Publisher: name: Purdue University Graduate School
Year: 2022
Subjects: Numerical modelling and mechanical characterisation
Orthopaedics

description: This study is focused on synthesizing Nitinol by additive manufacturing that can provide desirable mechanical properties for orthopedic implants and adding functionally gradient coating that can enhance both safety and biocompatibility for orthopedic implant applications. <br> The characteristics of additively manufactured Nitinol, by using the direct energy deposition (DED) technique, were experimentally studied. Because of a unique layer-by-layer manufacturing scheme, the microstructure and associated properties (mechanical and thermo-mechanical properties) of the DED Nitinol is different compared to conventionally produced Nitinol. Both the feasibility of manufacturing defect-free microstructure and the precise control of chemical composition were demonstrated. Effects of chemical compositions and post heat-treatment conditions on the phase transformation temperatures of the DED Nitinol were systematically analyzed and compared with those of conventional Nitinol. More precise control of phase transformation temperature from DED Nitinol was possible due to incoherent precipitate formation during aging heat treatment. In a similar way, the mechanical properties of the DED Nitinol were less sensitive to its chemical compositions and post heat-treatment conditions. The feasibility of the precise control of both mechanical and thermo-mechanical properties of the DED Nitinol was demonstrated which can broaden its applications. <br> The bulk polycrystalline properties of the NiTi phase were studied via molecular dynamics (MD) simulations. Thermo-mechanical properties that are highly sensitive to chemical composition were not precisely predicted from previous reports and studies. In this study, realistic boundary conditions were applied to calculate bulk polycrystalline properties. Thermally driven phase transitions of NiTi between martensite and austenite are simulated with external stresses in both normal and shear directions. It is shown that phase transformation temperatures are affected by applied external stresses, and realistic values compared to experimental data are correctly predicted only when external stresses in both normal and shear directions are similar to the experimentally observed values of 0.05 – 0.1 GPa. The experimentally observed grain orientation and grain boundary thickness were applied to simulation domains for the prediction of the elastic moduli. The elastic moduli of polycrystalline NiTi structure was calculated as 52 GPa which is close to the experimentally reported value of 20-40 GPa while other studies predicted over 85 GPa. <br> Lastly, pure titanium gradient layers were coated on the Nitinol surface for orthopedic implant applications to eliminate potentially toxic Ni ion release. Using the DED technique, both the core Nitinol and titanium gradient layers were manufactured with high purity and without microstructural defects. An additional biomedical coating of Hydroxyapatite (HA) was deposited on the outer surface using the cold spray technique. The resultant bonding strength was determined to be 26 MPa which exceeded the requirement of the ISO-13779 standard (15 MPa). The <em>in vitro</em> test of the Ni release rate from the entire gradient Nitinol structure was very low, which was comparable to drinking water.