Use of very lightweight sliding parts that require almost zero wear rates has begun recently in magnetic recording devices and micromechanical systems. The ultimate goal of microtribology is to create practical zero-wear devices with very small mass and very light load. Friction involves wear, but do not if only the surface forces interact. There are two types of microwear processes: one involves only depression, and the other process forms upheavals on a surface as the pre-stage of wear. Microtribology is an important technology for the development of new microdevices, and is also an important science for understanding the origin of friction and wear. Close cooperation between scientists and engineers is needed.
Recently Frictional-Force Microscopy (FFM) has enabled us to measure the friction on an atomic level, and has opened a new research area in Micro-Tribology. However, from the theoretical standpoint of view, the question of what physical quantity is observed by FFM has not been fully clarified yet. Therefore we introduce a theoretical method of interpretation of two-dimensional frictional-force images based on numerical simulation and experimental analysis. As a case study, cleaved graphite surface is adopted as a sample surface. Simulated images reproduce experimental images quite well. We clarify that the part of the stable domain boundary of the cantilever basal position appears as a fringe between the bright and the dark area along the scan direction in the FFM image. Furthermore it is also clarified how the FFM images are influenced by the macroscopic conditions such as the load, scan direction, and anisotropy of the cantilever. By this analysis, physical meaning of the FFM image patterns can be clearly understood together with that of the stick-slip motion of the tip atom.
Superlubricity, that is a phenomena of vanishing friction has been studied theoretically and experimentally. It is theoretically shown that certain unique cases exist where friction force exactly vanishes when the atomic arrangements of contacting surfaces satisfies the conditions for the appearance of superlubricity. The experiments agree with the theoretical predictions. The measurements with mica and molybdenum disulfide show that friction forces decrease as the contacting conditions approach those for the appearance of superlubricity. Friction is not observed in the superlubricity regime in measurements capable of resolving a friction force of 3 ×10-9 N by scanning tunneling microscope.
A two-dimensional frictional force microscope study has been made on an atomic scale tribology between a sharp tip of a single asperity of Si3N4 and an atomically flat surface of NaF(100), where the frictional force becomes two-dimensional vector. By increasing the normal load from 1.4 nN to 2.2 nN, the periodicity of the two-dimensional stick-slip motion of the tip is remarkably changed from lattice periodicity to atomistic periodicity. This load dependence of the periodicity qualitatively agrees with that of the tip-surface interaction due to surface relaxation by increasing the load, which is theoretically predicted using computer simulation. Within the load range where the atomistic periodicity appears, the load dependence of the amplitude of the friction shows anomalous behavior. Further, by increasing the load more than ∼3.6 nN it is suggested that the tip and/or NaF surface break down, because the two-dimensional stick-slip motion disappears.
The important role of surface chemistry was reviewed in order to understand microtribology. Tribological properties are closely dependent on surface chemical states at the friction interface. Organic films provide a good lubricity at the thickness more than monolayer. Conformation of molecules in the surface films affects tribological properties. The thin layer lubricants on magnetic recording disks are decomposed tribochemically even under a low load and the decomposition was strongly affected by the surface activity of the slider materials. The chemical nature of nascent surfaces and exoemission were discussed as an active source for the tribochemical reactions.
As the dimensions of micromachine components decrease, the surface area decreases in proportion to the square of the linear decrease while the volume decreases in proportion to the cube power of the linear decrease. Therefore, the gravity and inertia force proportional to the volume are negligible but the influences of the attraction and friction forces proportional to the surface area are predominant in micromachines. This paper describes some examples of applications of micromachine technologies such as a microfactory and a microsatellite and introduces products of MEMS (microelectromechanical systems). The tribological problems in micromachines are then discussed. First, we show that the attraction force increases the friction force and tribological phenomena in a micro scale are different from the macro tribology. Second, we show a reduction of friction force by creating regular asperity arrays. Finally, the relation between the electrostatic charge and friction is discussed.
Recording density of magnetic disks has markedly increased as much as 60% a year by decreasing spacing between a magnetic disk and a magnetic head. Micro-tribology is the most significant science and technology to improve mechanical reliability of ultrahigh recording disks. This report will introduce a tribological approach to analyze the phenomena at the interface between the head and the disk.
We applied neural networks to identification of chemical species. The input data corresponding to the spectra of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were prepared using Gaussian patterns with noise component. The neural networks with a Kohonen's self-organized feature map and a back-propagation algorithm were used in this work. From the results, we found that the input patterns were able to be classified using the neural networks without any pre-treatments such as smoothing and so on. Therefore, neural networks are thought to be useful in chemical analysis.
A brief perspective on the present diamond surface studies has been attemped. First, unique properties of diamond surface have been described with a brief summary of those of hydrogenated and oxygenated surfaces. Some topics have been presented with an emphasis on the studies motivated by the new method of diamond synthesis, i.e., chemical vapor deposition (CVD) techniques. Finally, possible directions of future studies have been pointed out.
Solar energy conversion by organic photoelectrochemical cells has been intensively investigated. Dye-sensitized cells such as Grätzel cell which is made from titanium dioxide coated with ruthenium complex seem to be promising as solar cell in a level of practical application. Organic-based photovoltaic cells, which mimic photoinduced multistep electron transfer processes in photosynthesis, have been developed in recent years. Sequential electron relay in primary processes of photosynthesis has been realized in multi-arrays of pigments connected with covalent bonds as photosynthetic model. A variety of techniques, such as Langmuir-Blodgett (LB) films, lipid bilayer membranes, and self-assembled monolayers, have been applied for construction of photovoltaic device where pigments including donor, sensitizer, and acceptor, are arranged unidirectionally at the molecular level on electrode surfaces. In the light of the rapid progress in these fields organic-based solar cells will be much improved in the near future.