= Lubricant
{wiki}
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.
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