FFM Imaging of Low-Friction Materials
AFM Instrumentation from KLA Tencor
It has been estimated that reducing friction and wear in the engine and its associated components could save the U.S. economy as much as $120 billion a year. The escalation of regulations geared towards energy conservation has stimulated the development of more effective lubricants to further increase fuel efficiency.
In terms of fuel lubricity additives, the dispersion of solid lubricants in lube media is always a big challenge. Solid lubricants with shrinking size down to the nanometer scale have dramatically enhanced dispersion and stability in fuel. In addition, nanoscaled lubricants exhibit extremely high surface areas, thus positively impacting their tribological performance.
As a result, much effort has been invested in exploring novel low-friction nanomaterials as high-performance dry lubricants for targeted applications. For example, it has recently been proven that chemically functionalized graphene oxide (GO) can be used as a novel additive to 10W-40 engine oils to significantly reduce the friction and wear of steel balls.
In this article, data from AFM-based tribological investigations of lubricating thin films using the KLA Tencor7500 atomic force microscope will be presented.
The KLA Tencor7500 AFM/SPM microscope (see Figure 1) is a high-performance scientific instrument that delivers high-resolution imaging with integrated environmental control functions. Keysight’s patented magnetic AC mode (MAC Mode) is offered as a system option. Switching imaging modes with the KLA Tencor7500 AFM/SPM microscope is quick and convenient, owing to the scanner’s interchangeable, easy-to-load nose cones.
Figure 1. The KLA Tencor7500 AFM/SPM microscope.
Every aspect of the KLA Tencor7500’s design and construction is optimized to reduce mechanical noise and deliver industry-leading performance. The compact and completely encapsulated scanner provides easy cantilever exchange, a slot for (optional) preamps for STM and CSAFM operation, as well as an integrated, high-reliability connector to interface with the control electronics. All 7500 AFM/SPM microscopes come with the lowest-noise closed-loop position detectors so as to provide the ultimate convenience and performance in imaging — without sacrificing resolution or image quality.
FFM Imaging of Low-Friction Materials
Friction force microscopy (FFM) is a variation of atomic force microscopy (AFM) in contact mode. FFM can detect lateral force variations on the atomic scale when sliding a sharp tip over a flat surface. The sliding often takes the form of a stick-slip movement with the same periodicity as the atomic lattice.
Tungsten disulfide (WS2) is one of the most lubricous materials known to science. With its 0.03 coefficient of friction, WS2 offers excellent dry lubricity unmatched by any other substrate. We expect to verify this using FFM. Figure 2 is an optical photograph of exfoliated WS2 flakes with various thicknesses on silica. Some ultrathin WS2 films exhibiting triangle shapes can be identified easily in the photograph.
Figure 2. An optical photograph of exfoliated WS2 flakes with various thicknesses on silica.
Figure 3 shows the FFM data. Friction images with both trace (left, top) and retrace (left, bottom) scan directions are included. On the right, the cross-sectional cursor profiles of the same line drawn on both images are displayed to construct a friction loop. Quantitative measurements of the friction forces between the tip and sample can be derived.
As can be seen, the gap between the trace and the retrace loop at the WS2 location is only 40 (arb. units) while that of the substrate is 210 (arb. units), indicating much less friction force between the WS2 films and the AFM tip.
Furthermore, nanoscale frictional characteristics of tungsten disulfide ultrathin films are captured. Cross-sectional cursor profiles at the WS2 region are much smoother, and less stick-slip movement of the probe at smooth and lubricating areas is observed.
Figure 3. Friction force microscopy studies of WS2 nanosheets on silica.
Figure 4 presents Kelvin force microscopy (KFM) images of WS2 nanosheets on silica. The measured surface potential of the WS2 films is lower than that of the silica substrate, showing a different behavior in comparison with graphene-based lubricating coatings.
Figure 4. Kelvin force microscopy studies of WS2 nanosheets on silica.
Previously, we have performed KFM studies of exfoliated graphene flakes on silica and proven that their surface potentials are usually higher than that of the silica substrate. This implies that the local electric properties of tungsten disulfide thin films are distinct from those of graphene layers.
Utilizing FFM imaging of ultrathin tungsten disulfide films on silica as an example, we have demonstrated that AFM is a useful tool in tribological studies of nanomaterials. Quantified measurements of the friction force at the tip/sample interface and spatially resolved tribology properties at the nanometer scale can be achieved via friction force microscopy.