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A lubricant is a substance introduced between contacting surfaces to reduce resistance to relative motion, wear or both. In fluid-film operation, the surfaces are separated by a layer whose viscous stress transmits the load and motion. A converging gap can generate lubrication pressure that supports a normal load while the solids remain apart.

When the film is too thin to prevent microscopic contact, surface interactions also matter; a low-viscosity fluid alone does not guarantee low friction. A simple comparison is to slide a loaded block over the same surface before and after applying a controlled film and measure the required tangential force. Changing load, speed and temperature can change the operating regime. The lubricant still dissipates mechanical energy through shearing and other losses, so reduced dry contact does not imply zero resistance.

In crystal polymorphism, the same chemical composition has different crystal structures; for an chemical element, this is a form of allotropy. Diamond has a three-dimensional tetrahedral network of covalent bonds, giving high hardness and a wide band gap. Graphite has strongly bonded planar sheets, with weaker interactions between sheets. Sliding between sheets makes it a useful solid lubricant; mobile electronic states give good electrical conduction within the sheets. Their different properties come from different bonding and structures, despite both being carbon.