Philippe Stempflé

Multi-axis MEMS force sensor for measuring friction components involved in dexterous micromanipulation: design and optimisation

At the nanoscale and for particular applications such as dexterous micro-manipulation, two degrees of freedom nanotribometers are no longer adequate for studying and characterising the contacts. This paper deals with the specifications and working principle of a new multi-axis friction sensor designed for nanotribological testing applied to this purpose in order to extract each contribution independently (i.e., sliding, rolling and spin motion). It is composed of a central platform with a fixed ball and surrounded by a compliant table. Its sensing ability is based on piezoresistivity: four sets of piezoresistors are symmetrically distributed at the root of four central beams. Finite elements method simulations are performed to find the optimal dimensions of the sensor. As results, this sensor could measure independently normal and friction forces in the range of 1 mN and 100 µN, respectively and the three rotation components. Estimated crosstalk is lower than 1% with a good sensitivity.

Multi-asperity Nanotribology of Self-Assembled Monolayers Grafted on Silicon Wafers Displaying Various Crystallographic Orientations and Nanostructures

Micro/nanotribological study of self-assembled monolayers (SAMs) derived from n-alkyltrichlorosilanes deposited on silicon wafers displaying various crystallographic orientations—Si (100), Si (111) and Si (110)—has been conducted using a ball-on-disc nanotribometer. The parameters that have been varied are (i) the alkyl chain length, (ii) the tribological parameters (i.e. the normal load, the sliding velocity, the sliding distance and the relative humidity level) and (iii) some surface characteristics of the silicon substrates (i.e. crystallographic orientation, roughness, and thickness of the amorphous native oxide). Experimental results show that the key parameter controlling the tribological behaviour is not the alkyl chain length as generally reported for adsorbed and grafted monolayers, but rather the film’s homogeneity—i.e. the degree of packing and the surface coverage of the monolayer—in connection with the crystallographic orientation of the substrate.