Introduction: Plasma immersion ion implantation (PIII) is a useful material modification technique that can effectively tune the surface properties of metallic material for biomedical applications. Ti alloys, Ti-based shape memory alloys, Co-based alloys and stainless steel[1], [2], [3], [4] are substrates currently used for this kind of surface treatment. This technique is known to affect not only the chemical composition, but also its crystalline structure and hence its biological response and corrosion properties, increasing biocompatibility and decreasing the release of toxic ions by metallic alloys [5], [6]. Structural, mechanical and chemical features of an oxygen plasma implanted Co alloy surface are studied in this work. Specifically, the correlation between these properties were investigated for two varying implantation parameters (antenna power, P, and implantation time, d).
Materials and Methods: Specimens cut from an L605 Co-Cr alloy was used as substrate after a specially designed and tested electropolishing protocol. Plasma implantation was performed at an accelerating pulsed voltage of 10 kV. Transmission electron microscopy was used to study the microstructure of the implanted layer, while X-ray photoelectron spectroscopy and glowing discharge optical emission spectroscopy provided information on the chemical composition and thickness of the modified layer. Scanning electron microscopy provided information on the surface state before and after the implantation process. Mechanical properties were assessed by nanoindentation and scratch test.
Results and Discussion: Oxygen implantation led to the formation of a mostly hexagonal Co-oxygen rich phase on the surface of the material, with some presence of Si due to sputtering of the quartz window. The surface layer created by the implantation process modified the chemical composition, the structure and the thickness of the oxidized layer. In fact, the natural oxide layer formed after electropolishing was thinner and richer in Ni and Cr than the implanted one, composed only by cobalt oxide. The implantation process appeared to remove the grain boundaries through amorphization of the topmost layer, as evidenced by SEM micrographs
Scratch tests, performed in the range 0.03 – 10 N, evidenced for all the implantation conditions a relevant plastic deformation of the substrate (presence of slip lines), but no severe delamination of the implanted layer was found. A critical load of ~5 N was found for the thicker layer. The hardness, as measured by nanoindentation experiments, was found to be in the range ~ 4 to 5 GPa.
Conclusions: PIII oxidation allows the formation of a Co-O rich layer on the surface of L605. The modified layer composition, structure and thickness can be modulated according to the implantation conditions. Oxygen implantation, even for higher doses, is responsible for a change in the mechanical properties of the modified layer, increasing both elastic modulus and hardness; the modified layer adhesion did not change in a significant way for all the considered conditions.
This work was partially funded by NSERC-Canada, FRQ-NT-Quebec and CFI-Canada. LMA was awarded of an undergraduate scholarship from CNPq - CAPES Foundation and Ministry of Education of Brazil. CP, DM and MV were recipiendaries of the Linkage Grant from Quebec/Italy sub-commission of the Quebec Ministry of Intl Relations. The authors would also thank Daniele Ciccarelli (DIISM technician) and Adriano Di Cristoforo (SIMAU technician) from Marche Polytechnic University.
References:
[1] Liu Hongxi, Xu Qian, Zhang Xiaowei, Wang Chuanqi, Tang Baoyin, Wear and corrosion behaviors of Ti6Al4V alloy biomedical materials by silver plasma immersion ion implantation process, Thin Solid Films, 521 (2012) 89-93
[2] Kelvin W.K. Yeung, S.L. Wu, Y. Zhao, X.M. Liu, c, R.Y.T. Kao, K.D.K. Luk, K.M.C. Cheung, Paul K. Chu, Antimicrobial effects of oxygen plasma modified medical grade Ti-6Al-4V alloy, Vacuum, 89 (2013) 271-279
[3] A. Lacoste, S. Béchu, Y. Arnal, J. Pelletier, R. Gouttebaron, Plasma-based ion implantation of oxygen in stainless steel: influence of ion energy and dose, Surface and Coatings Technology 156 (2002) 225–228
[4] C. Díaz, J. W. Gerlach, S. Mändl, J. A. García, Reduction of corrosion current of CoCr alloys by post-PIII oxidation, Surface & Coatings Technology 256 (2014) 59-63
[5] Guosong Wu, Kai Feng, Ali Shanaghi, Ying Zhao, Ruizhen Xu, Guangyin Yuan, Paul K. Chu, Effects of surface alloying on electrochemical corrosion behavior of oxygen-plasma-modified biomedical magnesium alloy, Surface and Coatings Technology, 206 (2012) 3186-3195
[6] Chih-Hsiung Yang, Yu-Tsai Wang, Wen-Fa Tsai, Chi-Fong Ai, Mau-Chin Lin, Her-Hsiung Huang, Effect of oxygen plasma immersion ion implantation treatment on corrosion resistance and cell adhesion of titanium surface, Clin. Oral Impl. Res. 22 (2011) 1426-1432.