Using AFM to Gain Nanoscale Insights on Macroscale Tribofilms

Areas of sliding contact between load-bearing surfaces like gears, pistons, and bearings, are common sites of mechanical system failure [i]. These interfaces are usually under high mechanical stress and experience material (wear) and energy (friction) dissipation. In turn, this can lead to waste, and sometimes worse, to disastrous failure.

“Tribo-”, also known as “rubbing”, interfaces are especially important to understand to aid with the development of new materials, like lubricants and additives in order to ameliorate losses and reduce failure. A few serious economic problems are friction and wear, which contribute more than one fifth to global energy consumption [ii]. However, examining their mechanisms, especially at the microscopic level, is particularly difficult: these phenomena are literally concealed between the contacting bodies.

AFM Optical Detection System

From the 1990s until today, AFMs have become everyday tools in tribological research, from visualizing the texture of wear tracks after sliding experiments, to measuring friction forces at the nanoscale [iii]. The AFM tip signifies a model single asperity contact, and sliding is usually where friction is highest which is within boundary conditions.

The AFM optical detection system is useful in many ways and can monitor the normal motion of the cantilever spring, which denotes the normal load, as well as the torsional motion, which represents the frictional force. Consequently, one can measure friction response at a precise load, or, even better, the dependence of friction on load, which is basically the coefficient in Amonton’s law.

In the example below, tribofilms are investigated. They are formed on a hard, higher-friction surface after 1h in comparison to 6h of sliding experiments with a reciprocating tribometer. Tribofilms are identified as the thin coatings formed during the sliding of contacting surfaces that can greatly affect their friction, in addition to their wear behavior [iv].

Gaining Insights on Nanomechanics of Tribofilms

The formation of tribofilms is classically complex, thus often poorly understood. Materials are transformed by contacting surfaces, or their surrounding environment (e.g., lubricants and additives) and is developed through mechanical, chemical, and thermal reactions. Once formed and secured to the surface, these tribofilms can influence and lead the tribological performance of the sliding contact giving way to the important targets of investigation and engineering.

On top of mapping out friction response via lateral force microscopy, other techniques can be used to gain insight on the nanomechanics of tribofilms, as described above. For instance, a technique unique to Asylum Research is known as AM-FM Viscoelastic Mapping Mode. This technique yields both modulus and dissipation maps, and simultaneously images the sample in tapping mode [v]. These represent elastic response and viscous (and adhesive) damping: imperative mechanical properties to know about polymer films.

Evolution of tribofilm texture over time. Topographical images: the variation in texture of the tribofilm when the duration of reciprocal sliding increases from 1 h to 6 h. (A & C) After 1 h of sliding, a patchy film develops with very uneven patch sizes. (B & D) After 6 h of sliding, patches are still seen, but they are smaller, more monodisperse in size, and more evenly distributed on the surface.

Figure 1. Evolution of tribofilm texture over time. Topographical images: the variation in texture of the tribofilm when the duration of reciprocal sliding increases from 1 h to 6 h. (A & C) After 1 h of sliding, a patchy film develops with very uneven patch sizes. (B & D) After 6 h of sliding, patches are still seen, but they are smaller, more monodisperse in size, and more evenly distributed on the surface.

Evolution of tribofilm dissipation and modulus over time. Dissipation and modulus maps were attained using AM-FM Viscoelastic Mapping Mode. (Left) Dissipation histograms help to explain that the film exposed to 6 h of reciprocal sliding exhibits lower damping behavior (about half as much as the film at 1 hour), on top of a more homogenous distribution. (Right) Additionally, the modulus shifts somewhat to lower values for the film subjected to 6 h of reciprocal sliding compared to the film at 1 h.

Figure 2. Evolution of tribofilm dissipation and modulus over time. Dissipation and modulus maps were attained using AM-FM Viscoelastic Mapping Mode. (Left) Dissipation histograms help to explain that the film exposed to 6 h of reciprocal sliding exhibits lower damping behavior (about half as much as the film at 1 hour), on top of a more homogenous distribution. (Right) Additionally, the modulus shifts somewhat to lower values for the film subjected to 6 h of reciprocal sliding compared to the film at 1 h.

Topographic and friction images and friction loops for the tribofilm: after 1 h of reciprocal sliding. Comparing the topography and friction images reveals lower friction response on the raised tribofilm patches (i.e., plateaus) versus the valleys. This proves that the valleys have different material properties (and possibly composition) compared to the top of the film. Friction loops were acquired on 500 nm segments of the different areas, as indicated on the images. Valleys, like area A, show a wider friction loop, representative of a higher friction, while plateaus, like area B, show a tighter friction loop (lower friction), but with significant stick-slip sliding, as shown by the saw tooth pattern.

Figure 3. Topographic and friction images and friction loops for the tribofilm: after 1 h of reciprocal sliding. Comparing the topography and friction images reveals lower friction response on the raised tribofilm patches (i.e., plateaus) versus the valleys. This proves that the valleys have different material properties (and possibly composition) compared to the top of the film. Friction loops were acquired on 500 nm segments of the different areas, as indicated on the images. Valleys, like area A, show a wider friction loop, representative of a higher friction, while plateaus, like area B, show a tighter friction loop (lower friction), but with significant stick-slip sliding, as shown by the saw tooth pattern.

Friction loops for the tribofilm: after 6 h of reciprocal sliding. Similar to the film subjected to wear for only 1 hour, the friction loop data for the 6 hour film shows higher friction in the valleys, though the friction is more irregular. The plateau areas similarly indicate lower friction and more jagged sliding behavior, yet again consistent with the 1 h film. Note: the same tip mounting and normal loads were used for both the 1 h and 6 h tribofilm measurements. This means that the magnitudes of the lateral signal can more or less be compared. From this information, it can be seen that the width of the friction loop increased eightfold on the tribofilm patches or plateaus, and eight- to twelvefold in the valleys.

Figure 4. Friction loops for the tribofilm: after 6 h of reciprocal sliding. Similar to the film subjected to wear for only 1 hour, the friction loop data for the 6 hour film shows higher friction in the valleys, though the friction is more irregular. The plateau areas similarly indicate lower friction and more jagged sliding behavior, yet again consistent with the 1 h film. Note: the same tip mounting and normal loads were used for both the 1 h and 6 h tribofilm measurements. This means that the magnitudes of the lateral signal can more or less be compared. From this information, it can be seen that the width of the friction loop increased eightfold on the tribofilm patches or plateaus, and eight- to twelvefold in the valleys.

References

[i] Williams, John. Engineering tribology. Oxford University Press, 1994.
[ii] Holmberg, Kenneth, and Ali Erdemir. "Influence of tribology on global energy consumption, costs and emissions." Friction 5, no. 3 (2017): 263-284.
[iii] Carpick, Robert W., and Miquel Salmeron. "Scratching the surface: fundamental investigations of tribology with atomic force microscopy." Chemical reviews 97, no. 4 (1997): 1163-1194.
[iv] Luo, Quanshun. "Tribofilms in solid lubricants." Encyclopedia of Tribology (2013): 3760-3767.
[v] Kocun, Marta, Aleksander Labuda, Waiman Meinhold, Irene Revenko, and Roger Proksch. "Fast, high resolution, and wide modulus range nanomechanical mapping with bimodal tapping mode." ACS nano 11, no. 10 (2017): 10097-10105.

This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.

For more information on this source, please visit Asylum Research - An Oxford Instruments Company.

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