Layered crystals, friction

Most of the solid lubricants mentioned above owe their low-Mction characteristic primarily to a lamellar or layered crystal structure (see two of them in Figure 6.1 as typical examples). When present at a sliding contact interface, these solids shear easily along their atomic shear planes and thus provide low friction. Some of the solid lubricants do not have such layered crystal structures, but in applications, they too provide very low friction and wear. For example, certain soft metals (In, Pb, Ag, Sn, etc.), PTFE, a number of solid oxides and rare earth fluorides, diamond and diamondlike carbons, etc., can also provide fairly good lubrication despite the lack of a layered crystal structure like the ones shown in Figure 6.1 [1]. In fact, diamondlike carbon films are structurally amorphous but provide some of the lowest friction and wear coefficients among all other solid materials available today [8]. 

While the interlayer shear mechanism is believed to be generally responsible for the low friction of most lamellar solid lubricants, extensive research by many scientists in previous years has also confirmed that a favorable layered crystal structure in itself is not sufficient... 

Diamond behaves somewhat differently in that n is low in air, about 0.1. It is dependent, however, on which crystal face is involved, and rises severalfold in vacuum (after heating) [1,2,25]. The behavior of sapphire is similar [24]. Diamond surfaces, incidentally, can have an oxide layer. Naturally occurring ones may be hydrophilic or hydrophobic, depending on whether they are found in formations exposed to air and water. The relation between surface wettability and friction seems not to have been studied. 

A number of substances such as graphite, talc, and molybdenum disulfide have sheetlike crystal structures, and it might be supposed that the shear strength along such layers would be small and hence the coefficient of friction. It is true... 

Solid ozone is very expl, and at its freezing pt is very sensitive. If liq ozone in a tube is suddenly immersed to the full length of the ozone layer into solid nitrogen, detonation usually occurs. This probably is due to the fact that ozone crystals appear over the entire height of the tube, and by friction of one set of crystals against another, enough heat is developed to initiate ozone detonation. On the other hand, if only the bottom of the ozone tube is inserted into solid nitrogen, the crystallization of solid ozone proceeds slowly from the bottom toward the top and no detonation takes place (Ref 3, p 225)... 

T ike metals minerals also exhibit typical crystalline structures. As an example, the structure of molybdenite is shown in Figure 1.17. It is hexagonal with six-pole symmetry and contains two molecules per unit cell. Each sulfur atom is equidistant from three molybdenum atoms and each molybdenum atom is surrounded by six sulfur atoms located at the comers of a trigonal prism. There are two types of bonds that can be established between the atoms which constitute the molybdenite crystal stmcture. They are the covalent bonds between sulfur and molybdenum atoms and the Van der Waals bonds between sulfur-sulfur atoms. The Van der Waals bond is considerably weaker than the covalent sulfur-molybdenum bond. This causes the bonds of sulfur-sulfur to cleave easily, imparting to molybdenite the property of being a dry lubricant. Molybdenite adheres to metallic surfaces with the development of a molecular bond and the friction between metallic surfaces is replaced by easy friction between two layers of sulfur atoms. 

Friction between the impacting surfaces, explosive crystals, and/or grit particles in the explosive layer. 

Another tool used to study friction on the molecular scale is the quartz crystal microbalance (QCM) introduced in Section 9.4.1. The QCM has been used to monitor the adsorption of thin films on surfaces via the induced frequency shift [385], In the years since 1986, Krim and coworkers could show that the slippage of adsorbed layers on the QCM leads to a damping of the oscillator [472], This damping is reflected as a decrease in the quality factor Q of the oscillator. From the change in Q, a characteristic time constant rs, the so-called slip-time, can be derived. This corresponds to the time for the moving object s speed to fall to 1 /e, i.e. 

However, Jamison has intensively studied the relationship between the crystal and electronic structures of layer-lattice solid lubricants and their frictional properties, and has shown that other aspects of its electron distribution give a particularly favourable structure to molybdenum disulphide. In its structure the molybdenum atoms in one layer do not lie directly above or below the molybdenum atoms in an adjacent layer, but are opposite holes in that layer. The sulphur atoms are directly opposite other sulphur atoms, but do not have any unpaired electrons to provide strong bonding. It is this lack of electronic interactions which leads to the high interlamellar spacing, and low interlamellar attraction. 

As will be explained later, it is considered that the surface of such a film normally consists of a thin layer of fully-ordered crystalline material with the basal planes oriented parallel to the plane of the substrate surface. Conformal contact between two such films will then be similar to the contact between two adjacent lamellae within a crystal. As a first approximation it might therefore be assumed that interfacial slip will resemble intracrystalline slip. However each surface may be degraded by the presence of contaminants, surface defects, and deviations from planarity, and it cannot be assumed that interfacial friction will be completely governed by the same considerations as intracrystalline friction. 

Gamulya and co-workers also concluded on the basis of electron microscopy and micro-electron diffraction that the production of a highly reflective surface occurs when the coefficient of friction reaches a minimum level. The full orientation appears to be limited to a very thin surface layer, which they found to be about 0.1 tjm thick in their tests, while Brudnyi and Karmadonov described it as being only one crystal thick, regardless of the force used for burnishing. On the other hand, a fully densified, fully-oriented layer without discontinuities may not be distinguishable in practice from an extended single crystal. 

Their crystal structures have been mentioned briefly in connection with intercalation in Section 14.2. All five compounds can be obtained in the layered hexagonal crystal form, and most are also found in rhombohedral or trigonal form. The compounds of the Group 6 metals, molybdenum and tungsten, as well as niobium diselenide, have a hexagonal form similar to that of molybdenum disulphide, in which the metal atoms in one layer are displaced sideways from those in the layers immediately above and below. This structure results in the widest interlamellar spacing, the easiest interlamellar shear, and the lowest friction. 

When the temperature is decreased the viscosity decreases for a liquid crystal consisting of prolate ellipsoids. The reason for this is that a smectic A phase is formed at low temperatures. In this phase the layers can fairly easily slip past each other thus decreasing the friction. 

Alternative methods, including friction transfer [71] and directional epitaxial crystallization [76], have also been used successfully in the alignment of PFs. In the friction transfer method one provides a crystalline templating substrate, (e.g., PTFE) and then applies a CP over layer. As in the case of a rubbed substrate, the CP does not significantly impact the orienting ability of the aligned substrate. This process is commonly referred to as graphoepitaxy. 

The most extensive studies of incommensurate systems have used a quartz-crystal microbalance (QCM) to measure the friction between adsorbed layers and crystalline substrates. The QCM is usually used to determine the mass of an adsorbed layer from the decrease in resonance frequency of the quartz crystal. Krim and collaborators [2,4,130] have shown that the increase in the width of the resonance can be used to determine the amount of dissipation due to sliding. The crystal is cut so that the applied voltage drives a shear mode, and the dissipation is studied as a function of drive amplitude. 

Measurements have been made on a wide variety of molecules adsorbed on Au, Ag, or Pb surfaces [3,4,131,132]. The phase of the adsorbed layer changes from fluid to crystal as the density is increased. As expected, motion of fluid layers produces viscous dissipation that is, the friction vanishes linearly with the sliding velocity. The only surprise is that the ratio between friction and velocity, called the drag coefficient, is orders of magnitude smaller than would be implied by the conventional no-slip boundary condition. When the layer enters an incommensurate phase, the friction retains the viscous form. Not only does the incommensurate crystal shde without measurable static friction, the drag coefficient is as much as an order of magnitude smaller than for the liquid phase ... 

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