Fabrication of (Ti-O-N-Si)/Ti Composite Coating on NiTi Shape Memory Alloy Using PIIID and Coating Evaluation
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Fabrication of (Ti, Si, O, N)/Ti Composite Coating on NiTi Shape Memory Alloy Using PIIID and Coating Evaluation Tao Sun 1, Lang-Ping Wang 2, Min Wang 1, * 1 Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong 2 State Key Lab of Advanced Welding Production Technology, Harbin Institute of Technology, Harbin, China * Corresponding author: [email protected] ABSTRACT A compact and uniform (Ti, Si, O, N)/Ti composite coating was fabricated on the surface of a NiTi shape memory alloy (SMA) (containing 50.8 at.% Ni) using plasma immersion ion implantation and deposition (PIIID) with radio-frequency (RF) magnetron sputtering. The coating and coated NiTi SMA were studied using various techniques. Analysis showed that the Ni content was drastically reduced on the surface of coated samples due to coating formation. This could greatly improve the biocompatibility of NiTi SMA. There was no TiO2 or TiN formation in the coating. The shape memory ability of NiTi SMA samples was no deteriorated by the coating process. The coating significantly improved the corrosion resistance and wear resistance of NiTi SMA and also rendered the material bioactive. INTRODUCTION NiTi shape memory alloys (SMAs) are attracting increasing attention in orthopedics and dentistry due to their unique shape memory ability and superelastic property. Even though they possess excellent mechanical properties that are desirable for medical implants, their long-term clinical applications have so far been limited because of Ni ions release which can cause cytotoxic, allergic and potentially carcinogenic effects [1]. In addition, metallic biomaterials such as stainless steel and titanium alloys are generally considered as bioinert, i.e., implants made of these metals are usually encapsulated by fibrous tissues after being implanted in human bodies, resulting in loosening and subsequent failure of the implants [2]. Furthermore, long-term wear and wear debris generation are also critical issues affecting the longevity of NiTi SMAs implants in human bodies. Wear particles can affect osteoblast viability, proliferation and function [3], ultimately leading to bone resorption and loosening of the implant. Forming a dense, uniform bioactive and wear-resistant coating on NiTi SMA implant surface is an effective way to improve biocompatibility, bioactivity and wear resistance of the implants, making them suitable for long-term implantation in human bodies. Many surface modification techniques, including sol-gel process [4], biomimetic deposition [5], electrochemical deposition [6] and magnetron sputtering [7] have been investigated for forming suitable coatings on NiTi SMA for medical applications. Compared to other techniques such as plasma spraying, plasma immersion ion implantation and deposition (PIIID) has many advantages, such as suitability for complex object geometry, processing of large numbers of objects in batches, high bonding strength of coatings, and varying and control of coating composition, in creating high
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