Friction and wear durability studies on the 3D negative fingerprint and honeycomb textured SU-8 surfaces Sandar Myo Myint a , Myo Minn a , Ren Yaping a , N. Satyanarayana a , Sujeet K. Sinha a,c,n , Charanjit S. Bhatia b a Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive, Singapore 117576, Singapore b Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore c Department of Mechanical Engineering, Indian Institute of Technology, Kanpur 208016, India article info Article history: Received 6 August 2012 Received in revised form 24 October 2012 Accepted 28 October 2012 Available online 3 November 2012 Keywords: Coefficient of friction Perfluoropolyether (PFPE) Negative fingerprint texture Honeycomb texture and SU-8 abstract The effects of two different textures (a 3D negative fingerprint texture and a honeycomb texture) on the tribological performance of SU-8 polymer surface have been investigated with a ball-on-disc tribometer. Friction and wear behaviors of the textured surfaces are conducted against a 4 mm diameter silicon nitride (Si 3 N 4 ) ball counterface. The coefficient of friction for the negative fingerprint textured surface (m ¼0.08) is much lower than that of the untextured surface ( 0.2) and the honeycomb textured surface ( 0.41) under a normal load of 100 mN and a rotational speed of 2 rpm. The coefficients of friction of the textured surfaces decrease with increasing normal loads between 100 mN and 300 mN. Above the normal load of 300 mN, the coefficient of friction of the negative fingerprint textured surface increases due to the occurrence of plastic deformation. The honeycomb textured surface has shown the highest coefficient of friction. The wear durability tests are also conducted at a normal load of 100 mN and a rotational speed of 500 rpm on the untextured/textured surfaces on SU-8 in the presence of an overcoat of a nano-lubricant, perfluoropolyether(PFPE). Six samples i.e. the untextured surface (Si/SU-8 and Si/SU-8/PFPE), the 3D negative fingerprint textured surface (Si/SU-8/FP and Si/SU-8/FP/PFPE) and the honeycomb textured surface (Si/SU-8/HC and Si/SU-8/ HC/PFPE), each with and without PFPE nano-lubricant, have been investigated for their tribological behaviours. The negative fingerprint pattern on SU-8 with PFPE coating has shown the highest wear life of 60,000 cycles under a normal load of 100 mN. The reasons for excellent tribological performance of 3D fingerprinted SU-8 surface are analyzed using the Hertzian contact area calculation. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Soft polymer coatings have emerged as durable wear protec- tive layers in MEMS/NEMS (Micro/Nano Electro-Mechanical Sys- tems). Polymers are capable of providing low friction and low shear strength [1]. SU-8 polymer is utilized in the fabrication of MEMS/NEMS devices. SU-8, an epoxy-based negative photoresist, exhibits good structural strength, thermal resistance and chemi- cal resistance [2,3]. SU-8 is compatible with the micro-fabrication technique (photolithography) and involves much less number of processing steps than those for silicon (Si). Tribology plays an important role in the performance of MEMS materials. However, SU-8 shows high friction and high wear. Hence, for SU-8 to be used as bulk or coatings for MEMS/NEMS applications, it is necessary that the tribological problems are solved a priori. Surface texturing is one of the many ways of controlling tribolo- gical performance. In recent times, many researchers have investi- gated the tribological behaviors of SU-8 polymer with various surface patterns. Further, improvements in the tribological proper- ties have been shown by SU-8 micro-dot textures on Si [4]. Experiments on the micro-patterned surfaces of SU-8 have shown a reduction in the coefficient of friction and an increase in the wear life of SU-8 when slid against Si 3 N 4 counterface [5]. Different textures on surfaces have been tested on magnetic hard disks [6–8] and mechanical components [9] as well. Many studies have shown that friction can be decreased by surface texturing due to a reduction in the real contact area [1,10,11]. He et al. [12] investigated friction of poly (dimethylsiloxane) (PDMS) elastomeric material with surface textures such as pillars and grooves. They observed that the coefficient of friction on the pillar-textured surface reduced by approximately 59% at macro- scale tests and 38% at the micro-scale tests, when compared to the untextured surface. Wang et al. [13] studied friction reduction due to the presence of micro-dimples on various surfaces Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/triboint Tribology International 0301-679X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.triboint.2012.10.020 n Corresponding author. Tel.: þ65 65164825; fax: þ65 67791459. E-mail addresses: sujeet10@hotmail.com, sujeet@iitk.ac.in (S.K. Sinha). Tribology International 60 (2013) 187–197