When there is relative motion between two contacting bodies, frictional forces that resist motion
always come into play. Frictional resistance to rotation of the drill string is called torque,
and is especially enhanced when drilling a deviated hole. Many different materials, such as
graphite, fine mica, and diesel or crude oil, have been used as mud additives to improve lubricity.
Since evaluation of the various materials cannot realistically be done on the drilling rig, a
lubricity test was designed to simulate the speed of rotation of the drill pipe and the pressure
with which the pipe bears against the wall of the hole.