ORIGINAL PAPER Friction Study of a Ni Nanodot-patterned Surface Hengyu Wang Rahul Premachandran Nair Min Zou Preston R. Larson Andrew L. Pollack K. L. Hobbs Mathew B. Johnson O. K. Awitor Received: 15 June 2007 / Accepted: 6 August 2007 / Published online: 21 August 2007 Ó Springer Science+Business Media, LLC 2007 Abstract Nanoscale frictional behavior of a Ni nanodot- patterned surface (NDPS) was studied using a TriboInd- enter by employing a diamond tip with a 1 lm nominal radius of curvature. The Ni NDPS was fabricated by thermal evaporation of Ni through a porous anodized alu- minum oxide (AAO) template onto a Si substrate. Surface morphology and the deformation of the NDPS were char- acterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM), before and after friction/ scratch testing. SEM images after scratching clearly showed that, similar to what was assumed at the macro- scale, the frictional force is proportional to the real area of contact at the nanoscale. It was found that adhesion played a major role in the frictional performance, when the normal load was less than 20 lN and plastic deformation was the dominant contributor to the frictional force, when the normal load was between 60 lN and 125 lN. Surprisingly, a continuum contact mechanics model was found to be applicable to the nanoscale contact between the tip and the inhomogeneous Ni NDPS at low loads. The coefficient of friction (COF) was also found to depend on the size of the tip and was four times the COF between a 100 lm tip and the Ni NDPS. Finally, the critical shear strength of the Ni nanodots/Si substrate interface was estimated to be about 1.24 GPa. Keywords Friction Á Nanoscale Á Nickel Á Nanodot-patterned surface Á Nano-patterning Á Anodized aluminum oxide (AAO) 1 Introduction Friction is a phenomenon that has been studied for centuries since the early days of Leonardo da Vinci (1452– 1519). Amontons’ law, which states that friction is proportional to the normal load, has described many macroscopic and microscopic non-adhesive sliding con- tacts, surprisingly well [1, 2]. For miniaturized systems, such as micro-electro-mechanical systems/nano-electro- mechanical systems (MEMS/NEMS), however, Amontons’ law no longer applies, because the adhesion contribution to friction can no longer be neglected due to the large surface- area-to-volume ratio of MEMS/NEMS structures and the increased surface smoothness [3]. In fact, adhesion and friction are two of the main issues affecting the reliability of MEMS/NEMS devices involving contact interfaces [4]. Therefore, a fundamental understanding of friction at nanoscale dimensions and controlling friction through nano-surface-engineering are of great scientific and tech- nological significance. Recently, surface nano-patterning with Ni nanodot arrays on a Si substrate was investigated for adhesion and friction reduction of contacting interfaces for potential tribological applications in miniaturized systems [5]. The results showed that the adhesion forces and the coefficients of friction (COF) between a 100 lm diamond tip and the Ni nanodot-patterned surface (NDPS) were reduced up to H. Wang Á R. Premachandran Nair Á M. Zou (&) Department of Mechanical Engineering, University of Arkansas, Fayetteville, AR 72701, USA e-mail: mzou@uark.edu P. R. Larson Á A. L. Pollack Á K. L. Hobbs Á M. B. Johnson Department of Physics & Astronomy, University of Oklahoma, Norman, OK 73019, USA O. K. Awitor De´partement Mesures, Physiques, Universite´ d’Auvergne, Clermont, France 123 Tribol Lett (2007) 28:183–189 DOI 10.1007/s11249-007-9263-4