Dependence of the friction process on the molecular structure and architecture of thin polymer films

The frictional properties of a homologous series of poly(n-alkyl methacrylates) (PnAMA) and a series of poly(methyl methacrylate) (PMMA) films, cast from a variety of solvents, are characterized. The choice of polymer film was driven by the consideration of the possible mechanisms for the accommodation of a macroscopically applied shear stress by molecular entities. Two possible mechanisms are proposed: (i) the relative flexibility of the polymer backbone chain. For this purpose the PnAMAs have been chosen. By varying the length of the substituent chain, the relative molecular freedom around the backbone chain is altered. These molecular differences are sensed in the frictional properties at the macroscopic level, and (ii) the molecular organization is also proposed to be a factor in determining the friction response of a particular polymer film. For this purpose, the frictional properties of PMMA films cast from different solvents are investigated. There is observed to be a strong influence of the molecular organization on the frictional properties of the solvent cast PMMA films. The molecular probe employed to characterize the molecular environment is vibrational spectroscopy. Conformationally sensitive vibrational modes are used to determine the relative flexibility of the backbone chain and the organization of the chain network.     

Measurements of tribological properties of poly(pyrrole) thin film bearings

We report the results of a recent study on the tribological properties of electropolymerised thin films at light loads and low speeds. Poly(pyrrole) films incorporating different counter-ions have been electrochemically deposited onto gold electrodes on the plano-convex glass substrates and studied extensively. The measuring apparatus has been greatly improved from that reported earlier and now provides simultaneous monitoring of frictional force and wear. High precision capacitive gauging is employed to provide high resolutions of frictional force of better than 100 μN and height variation (wear) of 2 nm. A large number of specimens of poly(pyrrole) grown from five different counter-ions were prepared and their performances evaluated. The film morphology of each type of film was examined by atomic force microscopy (AFM) for control of the variability of film formation. Results are presented for the friction coefficients and wear rates observed for the films typically at a load of 2 N and a sliding speed of 5 mm s⁻¹. The effects of normal loading force and sliding speed on the friction coefficient are also discussed with a load range of 0.2–5 N and a sliding speed up to 30 mm s⁻¹.     

Load dependence of sticking-domain distribution in two-dimensional atomic scale friction of NaF(100) surface

Remarkable transitions were found in atomic-scale friction-images of the NaF(100) surface and the corresponding sticking-domain distribution by detailed investigation of the load dependence using the two-dimensional frictional force microscope. The tip-position map reveals a new type of sticking-domain distribution pattern, which is different from the simple lattice periodicity.     

Electrostatic force microscopy study on the domain switching properties of the Pb(Zr0.2Ti0.8)O3 thin films with different crystallographic orientations for the probe-based data storage

The domain switching properties of the ferroelectric Pb(Zr_0.2Ti_0.8)O₃ (PZT) thin films with two types of crystallographic orientations were investigated by electrostatic force microscopy (EFM). The crystallographic orientations of the PZT thin films were random on the (1 1 1)Pt/MgO(1 0 0) and c-axis preferred on the (1 0 0)Pt/MgO(1 0 0), respectively. When dc bias was applied to the films for writing in micro-scale area, electrostatic force images showed that the domain switching was hard in the PZT thin films with random orientation, while the pattern could clearly be written in the PZT films with c-axis orientation. The differences in the domain switching properties of each PZT thin film were investigated in the crystallographic orientations point of view, and the domain switching dynamics were also measured by investigating the nano-sized dot switching behavior with respect to the width of the applied voltage pulse.