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Development of measures and processing data protocol for the characterization of tribological properties of coatings at the sub-micrometer scale usin

Here we report on the protocol that INFM has developed for the characterization of tribological properties of coatings at the sub-micrometer scale using an atomic force microscope. This protocol is designed to provide quantitative results on corrugated samples, which is recognized as the most problematic issue in such measurements. The protocol developed by INFM is thus particularly interesting and adequate for the study of coatings provided by NANOCOAT partners. Results of first characterizations are shown. The major outlook of this work is the possibility of carrying on nano-tribological measurements on tribo-coatings in a liquid cell. This will allow to test the effect of different lubricants on the coatings.

An atomic force microscope (AFM) operated in the friction force mode (Friction Force Microscopy - FFM) is one of the most powerful techniques for the investigation of friction at the sub-micrometer scale. The slider in this case is represented by a micrometer-sized tip, with a radius of curvature from 10 to 100 nm. Friction and adhesion between such a small probe and a surface at this scale depend sensitively on the physico-chemical environment (lubricants, humidity, &) and local composition. It can be expected that any modification in the composition of a tribological coating, which affects the chemistry and structure at the submicrometer, down to the nanometer scale, as well as any change in the environmental conditions, will influence the nano-tribological behavior. In the optimization of performances of industry- and application-oriented tribological coatings, such as those developed in the framework of the NANOCOAT project, results of AFM/FFM characterization can therefore provide an additional and complementary feedback for the production of the coatings, when considered together with those of macroscopic testings.

In FFM topographic and friction maps of surfaces are acquired together. This is made possible by the simultaneous acquisition of the vertical and lateral deflections of the cantilever supporting the AFM tip. The formers are related to changes in the topographic relief, while the latter are proportional to the friction force between the tip and the sample surface. In order to perform quantitative nanofriction measurements, it is necessary to control and accurately measure both the magnitude and the direction of the forces acting on the AFM tip.

However, because of the local tilt of the surface, forces acting on the tip can be different from those inferred using the standard reference frame of the laboratory. In particular, the measured forces in the directions parallel and perpendicular to the AFM reference plane do not necessarily coincide with the forces acting parallel and perpendicular to the sample surface, which actually define the friction coefficient and the friction versus load characteristics of the interface under investigation. A topographic correction is thus required in order to obtain the value of the true friction coefficient.

We have developed a complete quantitative protocol for the characterization of the friction properties of corrugated surfaces using the AFM. The first part of the protocol consists in the simultaneous acquisition of topographic, applied loads, and lateral force maps. The local tilt of the surface is calculated from the topographic map. The external applied load is remotely controlled and synchronized with the end-line and end-frame triggers from the microscope. We can change the applied load after each scanned line during the acquisition of a single frame. A complete load ramp can be applied and recorded in a single AFM scan. The load range is typically 0 100 nN. The second part consists in the application of the topographic correction to experimental data, in order to extract the corrected values of the friction coefficient µ and the adhesive offset c.

We have applied the protocol to measure several coating produced by coater partners of Nanocoat. The results are published in the Nanocoat technical report.
We observed the smaller friction coefficient on the DLC samples from IWS. The biased DLC samples have smaller friction coefficient than the reference DLC.
Ns-carbon CrN from Microcoat have larger friction coefficient. Moreover, we noticed that on a 1 µm scale, the friction behavior of ns-carbon/CrN coatings is not uniform. Our analysis revealed the co-presence of several linear trends in the friction-load characteristics, corresponding to different friction coefficients (and adhesion). This can be due to non-homogeneity of the chemical composition of the coatings on the sub-micrometer scale, as well as to the presence of contaminants at the surface. The second stronger trend on ns-carbon/CrN coatings was observed mainly at higher loads (in parenthesis in the table; only the two strongest trends are reported).

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