Zinc dialkyldithiophosphate (ZDDP)-based antiwear additives are crucial in automotive lubricants, where its effectiveness in reducing wear of ferrous alloys is well established. However, prior studies of light-weight aluminum-based alloys reveal that ZDDP is not as effective an anti-wear agent on Al-based surfaces for reasons that remain under debate. Here we use in situ atomic force microscopy (AFM) to study nanoscale ZDDP-derived tribofilms at the sliding interface between an alumina microcolloid probe and substrates comprised of either aluminum (with native oxide) or aluminum oxide (single crystal sapphire). The experiments reveal that ZDDP tribofilms form on both substrates, supporting the idea that tribofilm formation crucially involves thermally-activated, stress-assisted chemical reactions, and does not require cation exchange from wear debris originating from the substrate.

Tribological investigations of macroscopic lubricated sliding contacts are critical for a wide range of industrial applications including automotive engines, gears, bearings, and any other contacting surfaces in relative motion. However, the inability of existing techniques to access buried sliding interfaces with high spatial resolution inhibits the development of fundamental insights into the tribological processes at play. Here we demonstrate a novel and general in situ method, based on atomic force microscopy (AFM), in which micrometer-scale spherical probes are attached to a standard microfabricated AFM cantilever which is then slid over a substrate while immersed in a liquid lubricant. In this case, steel colloidal probes and steel substrates were used, and the contact was immersed in a commercial polyalphaolefin oil with zinc dialkyl dithiophosphate (ZDDP) additive at both room temperature and 100 °C, but the method can be used for a broad range of material combinations, lubricants, and temperatures. We demonstrate that the in situ measurements of friction force and the morphological evolution of the tribochemical films on the substrate can be simultaneously achieved with nanometer-level spatial resolution. In addition, we demonstrate that the sliding zone is readily accessible for further characterization with higher spatial resolution using standard AFM probes with nanometer-scale tip radii. Ex situ characterization of the micrometer-scale probe and the sample is also feasible, which is demonstrated by acquiring high-resolution AFM topographic imaging of the final state of the probe.

Adsorption of 4He and para-H2 is predicted to exhibit exotic properties on weak-binding surfaces. For 4He, alternative scenarios include nonwetting, prewetting and a Bose gas monolayer. For H2, a monolayer superfluid phase can be created by a suitable choice of the substrate. Intriguing phenomena are predicted for 3He/4He mixtures in multilayer films.

Conjugated polymers doped with alkali metals typically exhibit ordered two-dimensional (2D) superlattices of linear chains and alkali-metal channels, analogous to guest-host monolayer sequences in layer intercalates. A common feature is a superlattice periodicity that varies with dopant concentration. Unique to doped polymers, the local 2D symmetry that defines the superlattice building block can vary with concentration and with the relative sizes of the dopant and chain projection normal to its axis. We present x-ray-diffraction measurements of stage-2 and stage-1 K-doped trans-polyacetylene, and of stage-1 Cs-doped polyparaphenylene vinylene (PPV). In all three cases, detailed profile fits are inconsistent with the highly symmetric intercalation channels previously proposed. We give evidence for new structural models in which the symmetries are broken by rotations and translations of the polymer chains. In the case of stage-2 K-(CH)x, this takes place by a rotation and translation of (CH)x chains, lowering the 2D lattice symmetry from P4mm to P4. In stage-1 K-(CH)x, translational distortions reduce the symmetry from P4mm to P4gm, while in stage-1 Cs-PPV a similar local symmetry reduction arises from chain rotations that do not affect the space-group symmetry. We offer several candidates as the driving force for the broken symmetries and discuss the implications for phase-diagram and band-structure calculations.

X-ray data for trans-polyacetylene (CH${)}_{\mathit{x}}$ in the range 129 KT513 K show that the a parameter expands twice as fast as b, accompanied by a 6\ifmmode^\circ\else\textdegree\fi{} increase in the setting angle. Polyethylene (${\mathrm{CH}}_{2}$${)}_{\mathit{x}}$ exhibits a similar behavior (but in the opposite sense), such that for both materials the interchain packing becomes closer to that of a two-dimensional triangular lattice at high temperature, as would be expected for close-packed disks. In contrast, poly(paraphenylene vinylene) and the emeraldine-base form of polyaniline both show small, nearly isotropic interchain thermal expansions, and the packing does not approach close packing of disks at high T. We suggest that for polymers consisting of relatively flat and rigid chains, the average interchain structure reflects the T-dependent interchain dynamics, specifically rigid-chain rotational modes about the chain axes. This effect is absent in materials for which ring torsion modes are the lowest-energy deformations. The coupling between rotational and translational degrees of freedom, and between intrachain dynamics and the average interchain structures, is discussed in light of the planar-rotor model of Choi et al. and the rotator phases observed in finite-length alkanes.

The x-ray powder profile of heavily doped [(CH)${\mathrm{Cs}}_{\mathrm{y}}$${]}_{\mathrm{x}}$ is analyzed on the basis of three dimensionally ordered intercalate channels. Fifteen peaks representing 27 unique reflections are well described (positions and intensities) by a tetragonal pseudocell with \ifmmode \bar{a}\else \={a}\fi{}=b\ifmmode\bar\else\textasciimacron\fi{}=9.093 A\r{}, c\ifmmode\bar\else\textasciimacron\fi{}=7.950 A\r{} containing 24 (CH) and 4 Cs units (y=0.167). The Cs and (CH) sublattices are actually incommensurate along c\ifmmode\bar\else\textasciimacron\fi{}, with 6.46 undistorted (CH) units per Cs implying y=0.15. Reflections with nonzero L are significantly less intense than calculated neglecting the Debye-Waller factor, indicating large-amplitude c-axis thermal motion of Cs ions. The fractional linear in-plane dilation upon replacing K with Cs is within 20% of the corresponding value in graphite intercalates.

We use neutron and X-ray diffraction to characterize the morphology of (CD)x and (CH)x films prepared by the Naarmann-Theophilou route. The mosaic distribution of chain axes normal to the plane of the film, and the longitudinal and transverse coherence lengths, are reported for cis- and trans-isomers. The only significant difference with respect to other new preparation methods is a slightly greater coherence length along the chain axes. The mosaic is intermediate between those of stretched Shirakawa and Graz-Durham materials. Bulk thermal isomerization from cis to trans is complete at 140°C.

A review is given of crystal structures and order/disorder phenomena in the fullerene solids C60, C60O and C70 and in the alkali metal intercalation compounds of C60. Recent results on orientational melting and quenched orientational disorder in the pristine solids are summarized, and the effect of small perturbations on the quasi-spherical molecular shape are described. The intercalation properties of C60 are discussed in the context of other host lattices such as graphite. The variety of orientational correlations in fullerenes and fullerides is rationalized in terms of competing contributions to the orientational potentials. Structural properties of alkali fullerides are correlated with electronic properties and superconductivity, and an updated binary phase diagram is described.