Scanning probe microscope observations of monolayers of a classic boundary lubricant, stearic acid (STA), reveal long-range dynamics of wear and reconstruction of monomolecular films under the shear forces caused by the sliding tip. The STA monolayer in a fluid state displays a flow of material from the worn area and its redistribution resulting in multilayer formation within the range of 80 µm. Surface diffusion of mobile organic material is responsible for the observed long-range effects of the local shear stresses produced within the contact area. Solid and fluid monolayers have very different velocity dependencies of the friction forces. For solid monolayers, we observe a monotonic increase of the friction forces with velocity rising from 0.02 to 1000 µm/s. In contrast, for the fluid STA monolayers the friction forces behave nonmonotonically with a maximum value around 0.2 µm/s. We observe significant compression of the STA monolayers under the tip reaching 35% of initial thickness before the fatal damaging. The observed compression can be related to the collective tilting of the molecules under normal loads due to a formation of gauche conformers in alkyl chains. Estimated Young's modulus is in the range of 0.2-0.7 GPa for very small deformations (<3% of thickness compression). However, the elastic modulus decreases sharply to tens of MPa at higher deformations.
A combination of finite element analysis (FEA) calculations and resonant frequency measurements are applied for determining normal and lateral spring constants of microfabricated ceramic/gold cantilevers for friction force microscopes. The cantilever Si3N4 and Au layers are combined analytically into an equivalent single composite layer. Bending and torsion behavior of the cantilever under typical operating forces are determined through FEA. Effective Young’s modulus for the composite Si3N4-Au beam from 172 to 185 GPa is determined through assimilation of FEA and fundamental resonant frequency measurements. Several current analytical solutions are compared to the full FEA evaluation. A new analytical expression is derived for obtaining the ratio of lateral to normal spring constants and thereby evaluation of absolute values of friction coefficients. Calibration plots are presented for assessment of both vertical and torsion spring constants of bicomponent cantilevers by measuring their resonant frequencies and thickness of gold overlay.
Scanning thermal microscopy (SThM) was used for probing surface microthermal properties of a wide range of materials from polymers to metals. We demonstrated that SThM measurements in contact mode can provide unique capabilities of unambiguous measurements of localized thermal conductivity of a wide variety of surfaces with sensitivity better than 0.05 W m -1 K -1 and lateral resolution in the range from 0.03 µm for hard materials to 1 µm for compliant materials. Variation of surface microthermal conductivity correlates fairly well with known bulk values for hard materials. For compliant materials, significant contribution of local deformation to measured values of thermal response is noted.
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