Tribology International 150 (2020) 106341 Available online 10 April 2020 0301-679X/© 2020 Elsevier Ltd. All rights reserved. Fabrication of bio-inspired deterministic surfaces by photochemical machining for tribological applications J.C. S� anchez a , A. Toro a , H.A. Estupi ~ n� an a , G.J.T. Leighton b , J.L. Endrino b, c, d, * a Tribology and Surfaces Group, Universidad Nacional de Colombia, Medellín, Colombia b School of Aerospace, Transport and Manufacturing, Cranfield University, Cranfield, MK43 0AL, UK c Basque Center for Materials, Applications & Nanostructures, UPV/EHU Science Park, Barrio Sarriena s/n, 48940, Leioa, Spain d IKERBASQUE, Basque Foundation for Science, Maria Diaz de Haro 3, 48013, Bilbao, Spain A R T I C L E INFO Keywords: Deterministic surface Photochemical machining Pin-on-disc tests Coefficient of friction ABSTRACT Surface texturing of AISI 52100 steel was carried out by photochemical machining. The geometrical parameters of the textured patterns were inspired by the characteristics of the skin of Vipera Ammodytes snake. The influence of the etching time and the size of the features on the quality of the textured surfaces was studied, as well as the effect of chemical modification on the surfaces etched by photochemical machining. The textured surfaces were tested in a pin-on-disc tribometer to evaluate the frictional response of the deterministic surfaces in comparison to stochastic surfaces. The results showed a marked influence of the etching time on the size and shape of the texture features, as well as a correlation between the coefficient of friction and the apparent contact area of the texture patterns. Also, a reduction of coefficient of friction was observed due to the deposition of WS 2 and Graphite coatings onto the textured surfaces. 1. Introduction Many issues related to friction and wear under dry conditions or with inadequate lubrication have been widely discussed in the literature [1–7]. In the process of formulating solutions for specific tribological pairs (bearing-shaft, piston-plunger, hip implants-bone), a number of surface texturing methods have been proposed to improve the tribo- logical performance of contacting surfaces. In the case of metallic sur- faces, the photochemical machining (PCM) process, also known as photoetching, photo-fabrication or photochemical milling [8,9], is a widely accepted manufacturing process, together with electro discharge machining (EDM), electrochemical machining (ECM) and laser beam machining (LBM). PCM is a material removal technique that does not require conventional machine tools, in which light, chemicals, micro- lithography and photosensitive resist materials are used to define areas to be cut from a volume [9]. Allen et al. established four main advan- tages of PCM that make it technically one of the best methods for surface modification [10]. First, the finished surface has excellent resolution; second, no burrs are produced during the process; third, PCM has consistent rates of metal removal because the surface finishing is easily controlled, and finally, PCM is a low temperature process so that the chemical and physical characteristics of the finished product are identical to those of the starting material. Accordingly, PCM has been used on different materials including copper, magnesium, Ni-based su- peralloys, steel and Co-Cr alloys with positive results [11–16], and several authors have worked on the optimization of the process for specific materials and applications by playing with the main variables involved [17–19]. PCM plays a major role in the industry in the fabrication of precision parts in different fields, with an average of US $6.1 billion in sales by the end of the 20th century [20]. It is used in the fabrication of diverse products, from decorative articles and jewellery to functional products used in the medical, electronic, telecommunication, automotive, and aerospace industries, among others [21]. The technique is widely used in the fabrication of microelectronic mechanical systems (MEMS) due to its high resolution and its facility for texturing small parts [20]. Several companies have concentrated their PCM operations in metals, especially steels. Greiner estimated that in the year 2000, out of 31 companies, 94% worked with stainless steel and 52% worked with other steel alloys [22]. However, PCM has not been used previously on AISI 52100 steel for tribological applications, so the results of this study may contribute to the understanding of the influence of surface texturing on the friction and wear behavior of bearings, especially under inadequate lubrication conditions or even in absence of lubricant at the contact surface. * Corresponding author. Basque Center for Materials, Applications & Nanostructures, UPV/EHU Science Park, Barrio Sarriena s/n, 48940, Leioa, Spain. E-mail addresses: haestupinand@unal.edu.co (H.A. Estupi~ n�an), jose.endrino@bcmaterials.net (J.L. Endrino). Contents lists available at ScienceDirect Tribology International journal homepage: http://www.elsevier.com/locate/triboint https://doi.org/10.1016/j.triboint.2020.106341 Received 28 October 2019; Received in revised form 24 March 2020; Accepted 26 March 2020