Download Friction Coefficient Mapping (FCM) and Triboadhesion

Friction Coefficient Mapping (FCM) and Triboadhesion Mapping (TM):
Surface Microstructure and Function
Rubén Álvarez Asencio1, Eleonora Bettini1, Jinshan Pan1, Esben Thormann1, and
Mark W. Rutland1,2*
1. Department of Surface and Corrosion Science, School of Chemical Science and Engineering, The
Royal Institute of Technology, SE-100 44 Stockholm, Sweden
2. Institute for Surface Chemistry, Stockholm, Sweden
* To whom correspondence should be addressed. [email protected]. Tel: +46 8 790 9914.
Keywords: Friction; LFM; adhesion.
A lot of effort has been devoted recently to study micro-nanonanotribology that evolves with
the suitable techniques, such as surface force apparatus (SFA) and atomic force microscope (AFM),
especially with lateral AFM (LFM) where the sample is scanned perpendicular to the fast scan
direction of the cantilever. The friction forces originated by the interaction between the tip and the
surface, lead to the twisting of the cantilever. This torsion of the cantilever provides the friction in the
system.
Despite the interest in this area, the majority of the research has been qualitative in nature,1-3
and when quantitative friction information was targeted, either they did not provide enough
tribological information of the system,4 the methods were based on theoretical approximation to
calculate friction forces and friction coefficients,5 or the techniques were just difficult to perform
and/or interpret.6 Additionally, when the systems studied present multiple domains with different
friction properties; the study became even more complicated.
Nowadays, a very common practice is to study friction based on a single applied load but it
has disadvantages. Friction is a force that is proportional to the applied load, and this proportionality
is given by the friction coefficient, μ, and varies with the system studied.
Consequently, a single friction value obtained with a specific normal force can not provide a
full description of the tribological properties in the system. It is necessary to understand how friction
changes with load in order to have a full understanding of the tribological properties of the system.
Besides, the prediction of how friction evolves with load becomes crucial within the range of interest.
Another parameter that contributes to the friction on the system is the adhesion. This force
acts as an additional load and has to be also considered. This parameter has been often extracted
from the “jump-out” measurement of a force curve.7 However, in this work, is presented an
alternative way of calculating the adhesion or triboadhesion where the surfaces do not in fact need
to be separated during adhesion measurements, and the result is therefore independent of several
factors,7 such as, surface sliding and thermal fluctuations, which otherwise may lead to
underestimation of the adhesion in normal force measurements.
Friction coefficient and triboadhesion maps (Figure 1) have been created for the first time from
lateral atomic force microscope (LAFM) images. By imaging the surface systematically as a function
of load, a series of images can be tiled, and pixelwise fitted to a modified Amonton´s Law to obtain
friction coefficient and triboadhesion maps. This removes the ambiguity of friction contrast in LAFM
imaging which can be a function of the load used for imaging. Two commercial alloys with
heterogeneous microstructure, a nitride powder tool steel sample and a biomedical CoCrMo alloy,
have been scanned using a standard silicon cantilever in order to obtain the friction data. The
friction coefficient and the triboadhesion maps provide unique information regarding to the
heterogeneous alloy microstructures as well as shedding light on the tribological behaviour of the
alloys.
[1] Baselt, D. R.; Baldeschwieler, J. D. Journal of Vacuum Science & Technology B:
Microelectronics and Nanometer Structures 1992, 10, (5), 2316-2322.
[2] Sasaki, K.; Koike, Y.; Azehara, H.; Hokari, H.; Fujihira, M. Applied Physics A: Materials
Science & Processing 1998, 66, (0), 1275-1277.
[3] McMullen, R. L.; Kelty, S. P. Scanning 2001, 23, (5), 337-345.
[4] Morant, C.; López, M. F.; Gutiérrez, A.; Jiménez, J. A. Applied Surface Science 2003,
220, (1-4), 79-87.
[5] Breakspear, S.; Smith, J. R.; Nevell, T. G.; Tsibouklis, J. Surface and Interface Analysis
2004, 36, (9), 1330-1334.
[6] Labardi, M.; Allegrini, M.; Salerno, M.; Frediani, C.; Ascoli, C. Applied Physics A:
Materials Science & Processing 1994, 59, (1), 3-10.
[7] Cappella, B.; Dietler, G. Surface Science Reports 1999, 34, (1-3).
Figure 1. Silicon-CoCrMo alloy images studied in PBS, (a) height, (b) friction coefficient
and (c) triboadhesion.