Nanodiamonds for improving lubrication of titanium surfaces in simulated body fluid Asghar Shirani a , Nicholas Nunn b , Olga Shenderova b , Eiji Osawa c , Diana Berman a, * a Materials Science and Engineering, University of North Texas, Denton, TX, 76203, USA b Ad amas Nanotechnologies, Inc., Raleigh, NC, 27617, USA c NanoCarbon Research Institute, Asama Research Extension Centre, Shinshu University, Ueda, Nagano, 386-8567, Japan article info Article history: Received 11 October 2018 Received in revised form 18 November 2018 Accepted 1 December 2018 Available online 3 December 2018 abstract Hip implants are often made from titanium or titanium-based alloys. However, wear during the oper- ation inside the human body is a key source of implant failure and adverse health effects. We propose new insight on the lubrication of titanium components. Addition of small amounts (less than 0.2 wt%) of nanodiamonds (NDs) to simulated body fluid promotes a substantial improvement in friction (3 times reduction) and wear (up to 2 orders of magnitude wear reduction) behavior of the titanium surfaces. Interestingly, the amount of NDs needed for improvement of friction and wear characteristics is critically dependent on the applied loads. With higher contact loads, larger concentrations of NDs are needed for better friction and wear reduction. Analysis of the wear track formed during sliding indicates the for- mation of a carbon-rich tribolayer which improves tribological properties of the contacting surfaces. Our results suggest that the carbon layer is formed from the nanodiamonds embedding in the top layer of titanium. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction Premature natural joint degeneration is a common problem in the population over the age of 40 as a result of excessive loading conditions as well as failure of normal repair processes [1]. Artificial joints made from metal, ceramic, or plastic materials have become the only long-term solution for relief from pain, mobility, or other adverse health effects related to joint degradation and failure [2]. In recent years, the number of orthopaedic surgeries substantially increased, though reliability and lifetime of the artificial joints remain a major issue. During operation, artificial joints are exposed to a complex environment and subjected to mechanical degradation [3,4]. Additionally, biocompatibility of the materials, or their ability not to cause an inflammatory or toxic response, is an important aspect to consider [5]. The search for biocompatible, tribologically efficient materials led to the exploration of different ceramic and metal alloy com- ponents [6]. Ideally, the joint replacement material should exhibit an identical performance to the bone when in operation [7]. Ultra- high-molecular-weight polyethylene (UHMWPE) was used in earlier years [8], but raised concerns with regard to adverse tissue reactions [9]. UHMWPE was replaced with stainless steel and then with Co-Cr-Mo and alumina [10], which demonstrated good wear resistance but lead to inflammation and pain in long-term. So far, titanium remains the most favorable materials for artificial joints. This has led to extensive research on titanium-based alloys for biomedical applications, such as Ti-Nb-Ta-Zr or TNZT [11], Ti-6Al- 7Nb [12], Ti-6Al-4V [13], and Ti-5Al-2.5Fe [14], among others. Ti- tanium and titanium-based alloys are the preferred materials used for hip cup shells due to their high corrosion resistance and biocompatibility over other materials, such as conventional stain- less steels and cobalt-based alloys [15]. However, high wear of the titanium components during exposure to normal and shear stresses is a major cause for their failure [16]. As a result, degradation of the metal implants during movement of the joints limits their lifetime [17]. An alternative to modification of the alloy composition is to modify the lubrication media with biocompatible additives [18e21]. Nanodiamonds have proven to be excellent friction and wear modifiers in various sliding systems [22e25]. Specifically, adding small amounts of nanodiamonds to oils [26] or water [27,28] resulted in substantial decrease in friction and wear of sliding metal * Corresponding author. E-mail address: Diana.Berman@unt.edu (D. Berman). Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon https://doi.org/10.1016/j.carbon.2018.12.005 0008-6223/© 2018 Elsevier Ltd. All rights reserved. Carbon 143 (2019) 890e896