Size-dependence transition from nano-friction to micro-friction

The interpretation of tribological processes is presently very different for nano-scale and micro-scale phenomena. This is mainly related to the presence of adhesive forces, which are not negligible at the nano-scale and, in some cases, dominate the tribological behaviour of the system. The morphological characteristics of sliding surfaces have been demonstrated to strongly influence both the nano-scale and the micro-scale tribology [1-4]. The phenomena which rule these properties are different depending on the dimensions of surface micro/nano-structures, on the contact area and on the applied load. At the micro-scale the presence of a regular array of micro-cavities could lead to the reduction of friction and wear, due to the enhancement of the hydrodynamic lift-off effect and to the role played by the micro-cavities as lubricant reservoir and debris pocket [1, 2]. At the nano-scale parallel nano-grooves induce a hydrophobic character to the surface, resulting in a reduction of the coefficient of friction and adhesion [3, 4]. The aim of the present study is to determine the critical size at which the transition from nano- to micro-scale regimes occurs, investigating the effect of surface patterning on the tribological properties of Si(100). With this purpose the Si flat surface have been patterned by means of Focused Ion Beam with regular arrays of parallel grooves 50nm wide and 4nm deep, varying the pitch from 125nm to 1m. Adhesion and friction have been studied as a function of the pitch by means of AFM in air and in vacuum, using a dedicated flat tip sensitive to the integrated effect of the nano-structures on the tribology of the system. The hydrophobicity of 125nm pitch array, already described in ref. 4, decreases between 400 and 500nm pitch, resulting almost absent for larger values. This finding identifies 400-500nm pitch interval as the threshold between nano- and micro-scale regimes. This research has been supported by PRRIITT (Regione Emilia Romagna) and Centro Interdipartimentale per la Ricerca Applicata e i Servizi nella Meccanica Avanzata e nella Motoristica (Università di Modena e Reggio Emilia). [1] A. Kovalchenko et al., Tribol. Int. 38 (2005) 219. [2] A. Borghi et al., Wear 265 7-8 (2008) 1046. [3] Y. Ando, Tribo. Lett. 19 (2005) 29. [4] D. Marchetto et al., Wear 268 (2010) 488.