Corrosion behavior of titanium boride composite coating fabricated on commercially pure titanium in Ringer's solution for bioimplant applications Bose Sivakumar, Raghuvir Singh ⁎, Lokesh Chandra Pathak CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India Academy of Scientific and Innovative Research, New Delhi 110025, India abstract article info Article history: Received 12 August 2014 Received in revised form 20 October 2014 Accepted 2 December 2014 Available online 4 December 2014 Keywords: Titanium Bioimplant Boride coating Corrosion resistance Ringer's solution The boriding of commercially pure titanium was performed at 850 °C, 910 °C, and 1050 °C for varied soaking pe- riods (1, 3 and 5 h) to enhance the surface properties desirable for bioimplant applications. The coating devel- oped was characterized for the evolution of phases, microstructure and morphology, microhardness, and consequent corrosion behavior in the Ringer's solution. Formation of the TiB 2 layer at the outermost surface followed by the TiB whiskers across the borided CpTi is unveiled. Total thickness of the composite layer on the substrates borided at 850, 910, and 1050 °C for 5 h was found to be 19.1, 26.4, and 18.2 μm respectively which includes b 3 μm thick TiB 2 layer. The presence of TiB 2 phase was attributed to the high hardness ~2968 Hv 15 gf of the composite coating. The anodic polarization studies in the simulated body fluid unveiled a reduction in the pitting corrosion resistance after boriding the CpTi specimens. However, this value is N 0.55 V SCE (electro- chemical potential in in-vivo physiological environment) and hence remains within the safe region. Both the un- treated and borided CpTi specimens show two passive zones associated with different passivation current densities. Among the CpTi borided at various times and temperatures, a 3 h treated shows better corrosion resis- tance. The corrosion of borided CpTi occurred through the dissolution of TiB 2 . © 2014 Published by Elsevier B.V. 1. Introduction Surface modification is a proven alternative to minimize the surface damage of metallic materials including bioimplants. Titanium and its al- loys are extensively preferred bioimplants due to their low density, ex- cellent corrosion resistance, and acceptable biocompatibility. The Ti based bioimplants, however, suffer from the poor fretting corrosion and sliding wear resistance when subjected to the two body contact such as in the hip and knee joints [1–3]. The naturally formed passive oxide layer on the titanium is easily removed from the contact zone, due to poor mechanical properties of the oxide, and as a result fresh me- tallic surface is exposed to the physiological fluid [4]. This increases cor- rosion of metals/alloys and subsequent release of metallic ions (such as Ti, Al and V from Ti–6Al–4V alloy) along with the wear debris to the physiological fluid. A number of surface modification techniques and processes such as plasma based, laser based and assisted, thermal oxida- tion, organic and inorganic coatings, diffusion coatings have been devel- oped to overcome the inferior tribological resistance of the titanium and its alloys [4–9]. Over the past several years, boriding, a diffusion based coating method has been increasingly used to improve the tribological and corrosion behavior of metallic materials. Titanium borides (TiB 2 and TiB) have shown the remarkable properties such as low electrical resistivity (similar to metals), high melting points, high hardness and excellent wear, and corrosion resistance [10,11]. Boriding of the titani- um, therefore, can be an extremely viable method to improve its tribo- logical behavior. Interestingly, TiB 2 and TiB coatings can be developed in-situ on the surface of titanium by heating the Ti specimen encapsu- lated by the mixture containing boron source, an activator, and filler material. The coating by reinforcement of the borides (a non-diffusive ex-situ method) performed on the surfaces of several metals/alloys in which coating adherence may be lost can, thus, be avoided. The other important reason for choosing the boride coating is the comparable thermal expansion coefficient of TiB 2 /TiB and Ti, which may ensure much less thermal stresses and so the distortion at the interface. The coefficients of thermal expansion of TiB 2 , TiB, and Ti are 8.1 × 10 -6 , 8.5 × 10 -6 and 8.6 × 10 -6 /°C respectively. The undesirable thermal mismatch between Ti matrix and several other ceramics such as Ti 5 Si 3 , CrB, B 4 C, SiC, and TiC is reported to cause high residual stresses that de- grade the coating–matrix interface [12–14]. The boriding by thermochemical diffusion process (namely pack boriding) has been extensively investigated on the ferrous substrates for various industrial applications. The pack boriding is a very simple Materials Science and Engineering C 48 (2015) 243–255 ⁎ Corresponding author at: CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India. E-mail address: rsr@nmlindia.org (R. Singh). http://dx.doi.org/10.1016/j.msec.2014.12.002 0928-4931/© 2014 Published by Elsevier B.V. Contents lists available at ScienceDirect Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec