Properties and optimization of TiN coating in Ti-51 at. %Ni, Ti-27at. %Nb and Ti-25at. %Ta alloys using magnetron sputtering for biomedical applications

Abstract

Ti-based alloys belong to the categories of metals that are being used extensively in biomedical implants because they possess unique properties, such as high strength, corrosion resistance and biocompatibility. Among many Ti-based alloys, Ti-Ni alloys have been widely applied for biomedical applications. However, it has been discovered that Nickel is a toxic element that can cause hypersensitivity on human body. Hence, the need to develop Ni-free Ti-based alloys for biomedical applications is of paramount importance. P type Ti alloys, such as Ti-27at.%Nb and Ti-25at.%Ta consisting of non-toxic elements are some of the strong candidates for the replacement of Ti-Ni alloys. The aim of this research was to modify the Ti- 51at.%Ni, Ti-27at.%Nb and Ti-25at.%Ta alloys by applying TiN coating on the surface of the alloys to improve their corrosion resistance, wear resistance, surface hardness and biocompatibility. Titanium Nitride (TiN) was selected as the coating material deposited on the substrates through physical vapour deposition magnetron sputtering method with varying deposition parameters namely, temperature, power, bias voltage and Nitrogen flow rate. The Taguchi method of parameter optimization through design of experiment was adopted and Taguchi orthogonal array standard 9- run matrix L9(34) was applied to reduce the number of experimental runs to only 9 experiments. Microstructural and phase variation of the coated and uncoated Ti- 51at.%Ni, Ti-27at.%Nb, and Ti-25at.%Ta alloys was determined using scanning electron microscope (SEM), energy dispersive spectrometer and x-ray diffractometer. The surface hardness (344 ± 12.5 HV, 325 ± 26.5 HV and 359 ± 7.9 HV) adhesion strength, (2999 ± 149.5mN, 2110 ± 100mN, 2145 ± 12.3mN) and coating thickness (1.171p,m, 1.92p,m, 1.78p,m) were measured using micro hardness and scratch adhesion test equipment, respectively. Corrosion properties were evaluated using both electrochemical and immersion tests in simulated body fluids (SBF). Antibacterial test was performed on the coated samples using agar disc diffusion technique with Escherichia coli bacteria. Based on microstructural characterisation, all the alloys showed typical features and morphologies of Ti- 51at.%Ni, Ti-27at.%Nb, and Ti-25at.%Ta. The coating materials deposited on the alloys were found to be composed of TiN coatings. The coatings showed improvement of surface hardness: 88.8%, 30%, 35.5%, and adhesion strength: 10.7%, 30.6% and 15.9% for all coated alloys of Ti-51at.%Ni, Ti-27at.%Nb, and Ti- 25at.%Ta, respectively. The results of the bio-corrosion test showed that both the coated and uncoated alloys had excellent corrosion resistance after 28 days of immersion in SBF solution at a constant temperature of 37oC. Similarly, the electrochemical test conducted at 37oC in SBF solution, showed that the uncoated and coated samples had high resistance towards corrosion. The antibacterial test results indicated that the uncoated alloys exhibited sign of the presence of antibacterial activities with small inhibition zones formed around them. However, no inhibition zones were observed in the coated alloys due to the presence of the deposited TiN coatings that acted as a physical barrier between the alloys and their surroundings. Nevertheless, the TiN coated Ti-based alloys have tremendous potential as materials for biomedical applications.