Role of transfer layer on tribological properties of nanocrystalline diamond nanowire film sliding against alumina allotropes R. Radhika a , N. Kumar b, ⁎, A.T. Kozakov c , K.J. Sankaran d , S. Dash b , A.K. Tyagi b , N.-H. Tai d , I.N. Lin e a Crystal Growth Centre, Anna University, Chennai 600025, TN, India b Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, TN, India c Research Institute of Physics, Southern Federal University, Rostov-on-Don, Russian Federation d Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC e Department of Physics, Tamkang University, Tamsui, Taiwan, ROC abstract article info Article history: Received 15 March 2014 Received in revised form 28 April 2014 Accepted 12 June 2014 Available online 20 June 2014 Keywords: Nanocrystalline DNW film Tribological properties Sliding counterbodies Al 2 O 3 Sapphire Ruby Interface chemical behavior The tribological properties of nanocrystalline diamond nanowire (DNW) film treated in CH 4 atmosphere at 400 °C were studied in ambient atmosphere at room temperature using various allotropes of alumina ball as sliding counterbodies. Super low value of friction coefficient (~0.003) and extremely high wear resistance (~ 2.8 × 10 -21 mm 3 /Nm) were observed when the Al 2 O 3 ball slides against the film. In contrast, high friction coefficients with the values ~0.06 and ~0.07 were observed while using sapphire and ruby balls, respectively. Wear loss was also high ~4 × 10 -19 mm 3 /Nm and 2.8 × 10 -15 mm 3 /Nm in DNW/sapphire and DNW/ruby sliding pairs, respectively. Such a behavior is fundamentally explained in terms of the chemical nature of the sliding in- terfaces and surface energy of ball counterbodies. As a consequence, the chemical affinity of Al 2 O 3 ball towards the carbon atoms is less, which resulted in the absence of carbonaceous transfer layer formation on the Al 2 O 3 ball scar. However, in the case of sapphire and ruby balls, the wear track was found to be highly deformed and significant development of carbonaceous transfer layer was observed on respective ball scars. This phenomenon involving transfer layer formation is related to high surface energy and strong chemical affinities of sapphire and ruby balls towards carbon atoms. In such a condition, sliding occurs between film and the carbonaceous transfer layer formed on the ball exhibiting high energy due to covalent carbon bonds that chemically interact and enhance sliding resistance. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Tribological efficiency needs advanced materials for specific engi- neering applications where sliding occurs between alternate materials [1,2]. In this regard, it is technically important to investigate ceramic counterbodies sliding against carbon based films like nanocrystalline diamond (NCD) and DNW [3,4]. In NCD films, the typical size of the dia- mond crystallites falls in the range of 15–30 nm. However, in DNW films, ultrananocrystalline diamond grains with sizes 3–5 nm are embodied in the wires. These wires have typical dimensions of 100–130 nm in length and 10–15 nm in diameter. Such films are an advanced class of materials which have normally low/ultra-low friction coefficients with high wear resistance in a wide range of operating conditions [5–8]. It is revealed from several investigations that mainly, formation of carbonaceous transfer layer on sliding ceramic counterbodies often influences tribolog- ical properties of diamond-like carbon and NCD films [2,3]. Interestingly, high friction coefficient and high wear rate were measured when NCD slides against SiC and Si 3 N 4 balls [3]. Such a characteristic was explained by the formation of carbonaceous transfer layer on these balls that causes the development of covalent chemical bonding between the sliding interfaces, resulting in high friction. In contrast, ultra-low friction coeffi- cient and high wear resistance were obtained while sliding against Al 2 O 3 , ruby and Zr 2 O 3 balls [2,3]. Furthermore, super-low friction coefficient and high wear resistance were measured on H 2 /O 2 plasma treated nano- crystalline DNW film while using Al 2 O 3 balls [9,10]. This happens due to weak chemical interaction between sliding interfaces which restricts formation of covalently bonded transfer layer. In the early work, it is shown that the value of friction coefficient consistently decreases with increasing the humidity levels during the tribo-test performed on ultra-nanocrystalline diamond (UNCD) films [7]. This value is ~0.13 in low humidity which is significantly decreased to ~ 0.004 at highly humid conditions. Such a reduction in friction coefficient was ascribed to passivation of dangling covalent bonds of carbon atoms occurring due to the formation of chemical species such as C\COO, CH 3 COH and CH 2 \O. In another work, friction coefficient of as-deposited DNW film showed a high value of ~ 0.2 while sliding against Al 2 O 3 ball [9]. However, this value was decreased to a super low value ~ 0.0001 in H 2 plasma treated DNW film. Such super low valued friction coefficient is described by the passivation of uncompensated carbon dangling bonds by Diamond & Related Materials 48 (2014) 6–18 ⁎ Corresponding author. Tel./fax: +91 4427480081. E-mail address: niranjan@igcar.gov.in (N. Kumar). http://dx.doi.org/10.1016/j.diamond.2014.06.005 0925-9635/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond