Surface passivation of porous NiTi shape memory alloy via thermal oxidation

Abstract

Porous NiTi alloy has been developed especially in the biomedical sector due to its shape memory ability and mechanical properties which is similar to human bones. However, NiTi alloy carries the risk of Ni ions leaching when implanted in the human body making it less favorable for biomedical use for long term. Therefore, this project aimed to passivate porous NiTi by oxidizing its surface through annealing treatment. The porous NiTi alloy was initially fabricated using the powder metallurgy method, incorporating CaH₂ as a pore-forming agent in varying amounts from 0.25 to 15.00 wt%. Meanwhile, the annealing process was conducted at temperatures ranging from 300°C to 700°C for 15 minutes to achieve surface oxidation. Here, the morphology, porosity level, phase identification, transformation behavior, and mechanical properties were evaluated using various characterization techniques such as densitometer, scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and universal testing machine (UTM) in compression mode, respectively. The results reveal that when the percentage of CaH2 is increased to 15wt.%, the porosity of sintered sample can reach up to 42%. The shape of the pores become irregular and partially interconnected. Higher percentage of CaH2 tend to create more interconnected pore structure that beneficial for the transport of bodily fluids and bone healing process. In terms of phase identification, during phase formation, the undesired phases like NiTi2, Ni3Ti and Ni-rich phases co-exist with NiTi. For the transformation behavior, martensitic transformation peaks were observed for samples with ≤3wt% CaH2 with an enthalpy value of approximately 4 J/g. However, when the concentration of the CaH2 increased to ≥6 wt. %, the enthalpy change (ΔH) values decrease significantly, and the samples no longer exhibit martensitic transformation. This is attributed to the pores structure and the presence of undesirable phases alongside NiTi formation, both of which hinder the transformation enthalpy for porous NiTi. For mechanical properties, as the CaH2 content increases to 15 wt. %, both the strength and stiffness decrease substantially, dropping from 78 MPa to 4 MPa and from 3 GPa to 0.31 GPa, respectively. Notably, a stiffness of 2 GPa was recorded with 3 wt. % of CaH2, which is comparable to the stiffness of cancellous human bones (typically in the range of 2-4 GPa). This similarity minimizes the stress shielding effect. Consequently, porous NiTi alloy, with its lower stiffness and unique shape memory properties, emerges as an excellent choice for surgical implants. Based on these findings, the sample with 3 wt. % of CaH2 was identified as the optimal sample. The subsequent phase of the study focused on surface passivation via thermal oxidation using annealing treatment. The sample was annealed at various temperatures ranging from 300°C to 700°C. It is evident that higher annealing temperatures promote the formation of thicker oxide layers due to thermodynamic principles. The evaluation of biocompatibility revealed that raising the annealing temperature to 500°C significantly lowered the measurement of Ni ion release. However, further temperature increments led to the development of an unstable and brittle oxide layer, resulting in an increased release of Ni ions. Overall, the development of a porous NiTi alloy with a passivated surface through thermal oxidation treatment has exhibited promising outcomes for applications in the field of biomedical.