Inter-lamellar bonding

This result is interesting and unexpected, since it suggests that inter-lamellar friction within a crystal is higher due to some form of bonding, and that this bonding is reduced by the presence of some contaminant material when the lamellae are at the surfaces. Most evidence suggests that the inter-lamellar bonding within a crystal is at an irreducible minimum and is increased by any known contaminants. 

Although a lamellar crystal structure is favourable for solid lubrication, the inter-lamellar spacing and the nature of the inter-lamellar bonding are of major importance in determining the resistance to inter-lamellar shear, and therefore the sliding friction, of lamellar compounds. 

BOX 1,1 Demonstration of the Nature of Inter-lamellar Bonding within Double Layer Aluminosilicates... 

There may also be more specific effects of certain liquids, such as the softening of the binders in bonded films suggested by Hopkins and Campbell . Any adsorption of a liquid on the surface basal planes, or any intercalation between lamellae would interfere with the ideal charge distributions and inter-lamellar spacing found with uncontaminated molybdenum disulphide. These effects would in fact be similar to the adverse effects of water vapour and other vapours on the friction of burnished films. 

The lubricating performance of the lamellar crystal structure is strongly dependent on the inter-lamellar separation and bonding, and these can be modified by the intercalation of other substances between the lamellae. The general subject of intercalation will therefore be discussed before proceeding to descriptions of the individual lamellar solid lubricants. 

Strong electronic (covalent or electrovalent) bonding is desirable within a crystal lamella, to provide structural strength and to ensure that when shear forces are applied to the crystal, shear takes place between lamellae, and not within them. Conversely, strong bonding between lamellae is undesirable, as it leads to high inter-lamellar shear resistance and high friction. Ideally, the inter-lamellar forces are limited to weak van der Waals forces, and the inter-lamellar space is often called the "van... 

It has been known since the middle of the nineteenth century that atoms or molecules can be introduced between the carbon layers in graphite, and more recently it has been recognised that many other lamellar crystalline materials will behave in the same way. This phenomenon of "intercalation" results in modification of many of the properties of the basic crystalline material. In particular it leads to an expansion of the inter-lamellar gap, but the effect on inter-lamellar shear resistance will also depend on any effects on bonding which may arise. 

In the case of the equivalent niobium compounds, the same electronic effects are not present. He postulated that in pure stoichiometric niobium disulphide this results in poor lubrication. When good lubrication behaviour is observed, it is probably caused by additional niobium atoms intercalated between the lamellae, which contribute non-bonding electrons. On the basis of this theory, non-bonded atoms intercalated between the lamellae can increase the inter-lamellar spacing, whereas bonded intercalated atoms increase the resistance to inter-lamellar shear, and therefore the friction. However, an alternative interpretation is that certain intercalated atoms alter the interaction between the niobium atoms, allowing rearrangement to the 2H structure of molybdenum disulphide, and it is the favourable structure which provides good lubrication performance. 

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