Mechanics of ZnO morphological dependence on wear resistance of ultra high molecular weigh polyethylene Rajeev Kumar Sharma, Ambreen Nisar, Kantesh Balani * Biomaterials Processing and Characterization Laboratory, Indian Institute of Technology Kanpur, Kanpur 208016, India article info Article history: Received 9 October 2016 Received in revised form 5 January 2017 Accepted 27 March 2017 Available online 1 April 2017 Keywords: Ultra high molecular weight polyethylene ZnO Compression molding Wear Fretting abstract Ultra high molecular weight polyethylene (UHMWPE) is widely used as an acetabular cup liner for cartilage replacement in hip arthroplasty. The implant failure mainly occurs due to the wear of polymeric liner. Thus, in order to increase the wear resistance of polymer liner, 20 wt % ZnO is reinforced in compression molded UHMWPE. Herein, the role of various ZnO morphologies, viz. microrods (R), nanoparticles (NP), and microdisks (D) is assessed on the tribological properties (such as wear rate and coefficient of friction) of UHMWPE matrix. Sliding wear (bulk length scale) and fretting wear (micro- meter length scale) have shown highest wear resistance of ZnO(D) reinforced UHMWPE. On one hand, ZnO(R) experience pronounced buckling and weak interfacing in matrix and show an increased sliding wear rate of 8.08  10 14 m 3 /Nm (when compared to that of 5.88  10 14 m 3 /Nm for UHMWPE). Con- trastingly, ZnO(D) exhibited strong interfacing and demonstrated a reduced wear rate of 1.80  10 14 m 3 / Nm. A similar trend of wear rate was observed at micrometer length scale i.e. least (1.17  10 21 m 3 /Nm) for ZnO(D) and highest (3.19  10 21 m 3 /Nm) for ZnO(R) when compared to that of UHMWPE (2.40  10 21 m 3 /Nm). Damage mechanism in ZnO(NP) is attributed to the formation of tribofilm on counter surafce. Further, wear-resistance rendered by ZnO morphologies is found to be higher (up to 19%) in sliding wear when compared to that of fretting. In summary, enhanced tribological resistance of UHMWPE-ZnO(D) can make it a potential polymeric liner in total joint arthoplasty. © 2017 Elsevier Masson SAS. All rights reserved. 1. Introduction Cartilage, a soft tissue available in body joints, can be replaced by ultra high molecular weight polyethylene (UHMWPE) via total joint arthroplasty. Till date, UHMWPE is taken as the gold standard of articulating surface in total joint replacement (TJR) (Patel and Balani, 2015; Rishabh et al., 2010; Gupta et al., 2012, 2013; Kurtz, 2009; Liu and Sinha, 2013). Notwithstanding high fatigue resis- tance, toughness, creep resistance, excellent biocompatibility, low coefficient of friction, nonstick self lubricating surface, and biocompatibility, some issues associated with UHMWPE include infection and wear due to articulation against hard ceramic/ metallic femoral head (Afzal et al., 2013; Pandey et al., 2015; Sharma and Balani, 2014). Unfortunately, the probability of joint replacement is: for hips and knees (>90%) and for shoulders, el- bows, ankles etc. (<10%) (Kurtz, 2009). TJR is one of the most threatening topics in medical science because of trauma associated with the implant replacement. The importance of such TJRs can be adjudged by more than 800,000 total hip arthroplasties (THAs) that are annually performed over the world (Cabrer et al., 2004) and around 1,50,000 THRs are conducted per year alone in USA (Kurtz, 2009). As far as wear is concerned, wear resistance of polymeric ma- terials can be improved by various techniques, for example, cross linking and fillers/fibers reinforcement etc. (Alam et al., 2015; Xie et al., 2003; Anderson et al., 2002; Gencur et al., 2003; Ikada et al., 1999; Paital et al., 2009; Chang et al., 2013, 2015). Cross linking in UHMWPE (Muratoglu et al., 2001a,b, 2003, 2004, 2005; Estok et al., 2007) causes modification in its internal structure such as crystallinity, strain to failure, etc. (Gupta et al., 2013; Wang et al., 2015). In an investigation, UHMWPE samples with different crystallinity were tested by micro-scratch tester and it was found that UHMWPE with a high degree of crystallinity (55.1%) has a higher scratch resistance (scratch depth and scratch width of 0.46 mm and 85.2 mm, respectively) whereas low degree of crys- tallinity (45.6%) possessed lower scratch resistance (scratch depth and scratch width of 0.52 mm and 102.3 mm, respectively) * Corresponding author. E-mail address: kbalani@iitk.ac.in (K. Balani). Contents lists available at ScienceDirect European Journal of Mechanics A/Solids journal homepage: www.elsevier.com/locate/ejmsol http://dx.doi.org/10.1016/j.euromechsol.2017.03.010 0997-7538/© 2017 Elsevier Masson SAS. All rights reserved. European Journal of Mechanics A/Solids 65 (2017) 149e158