Physical Properties of the Lamellar Solid Lubricants

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. 

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. 

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. 

Elements Whose Dichalcogenides Have Lamellar Crystal Structures and Good Lubricating Properties 

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Table 14.1 Physical Properties of the Lamellar Solid Lubricants...

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