How do coatings or platings affect the torque-tension relationship?
A coating or plating changes the surface of the fastener. In general, most coatings increase the lubricity of the fastener over the clean and dry condition. This reduces the amount of friction or surface roughness and means that when the fastener is tightened, more energy goes into stretching the bolt (preload) than is lost to friction between the mating threads or the bearing surface of the bolt or nut. A lower torque will now achieve the same preload in the fastener before the coating was applied. If coatings or platings are employed, the installation torque should use a K-factor appropriate for the internal and external thread conditions.
