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Porous Nickel-Titanium (NiTi) alloy has been developed especially in the biomedical sector due to its shape memory ability and its mechanical properties which is comparable to human bones. However, NiTi alloy carries the risk of Nickel (Ni) ions leaching when implanted in the human body making it less favourable for biomedical use forlong term. Therefore, the goal of this research is to passivate the porous NiTi by oxidizing the surface via annealing treatment. Initially, the porous NiTi alloy was fabricated via powder metallurgy technique with addition of Calcium Hydride (CaH2) as a pore forming agent. Here, the degree of porosity was measured and the pore morphology, phase identification and transformation behavior were characterized by Scanning Electron Microscope (SEM), X-Ray Diffractometer (XRD), and Differential Scanning Calorimeter (DSC), respectively. Then, the surface treatment was performed by varying annealing temperatures where the oxide layer formation was characterized by using SEM and Energy Dispersive Spectrometer (EDS). The result shows that the sample's porosity increased by up to 42% from its theoretical density, demonstrating that the weight percentage of the pore-forming agent has a significant effect on theporosity of the NiTi alloy. For surface treatment, at higher annealing temperature i.e. 500°C, it produced a thicker oxide layer of TiO2 as compared to samples annealed at 300°C and 400°C. This indicates that the annealing temperature highly affects the formation of oxide layers on the surface. This oxide layer adheres well to the NiTi alloysurface and is expected to prevent the Ni ion from releasing, thus making this porous NiTi biocompatible metal. [ABSTRACT FROM AUTHOR]

Ni-based Fe-Ni-Cr alloy is an important engineering material for a variety of applications especially at high temperature conditions. However, during service, this alloy experienced an oxide exfoliation that may reduce the material lifetime and its potential applications. To enhance the protective properties of this alloy at high temperature application, the modification on the alloy surface was done to introduce the subsurface region of refined grain that will induce diffusion. This project is to study the effect of different surface preparation on the isothermal oxidation behavior of Fe-Ni-Cr alloy. Different grit of SiC papers were used during grinding processes, namely P180, P600 and P1000 in order to achieve different surface roughness of the samples. The ground samples were exposed to oxidizing temperature at 700°C in air for 150 hours. The isothermally oxidized alloys were analyzed using kinetics of oxidation and scanning electron microscope equipped with energy dispersive x-ray spectrometer. Oxides layers were formed on the alloy surface which act as a protective layer at the isothermal temperature condition. As a result, all samples exhibited a weight gain pattern, following a parabolic rate law showing diffusion-controlled oxide growth mechanism. P180 sample recorded the lowest parabolic rate constant, showing the lowest oxidation rate. In addition, P180 sample also displayed a uniform and incessant oxide layer without evidence of oxide exfoliation. [ABSTRACT FROM AUTHOR]