Oftentimes coatings are specified to achieve a certain nut factor or k-factor for the purposes of establishing a predictable torque-tension relationship.  But what is this k-factor and what roles does it play in the torque-tension relationship of a bolted joint? 
 
The k-factor comes from the short-form torque equation T = KDF where D is the nominal diameter of the fastener, F is the desired preload, and K is the k-factor.  This equation is a simplified version of a longer, more complex, and generally impractical equation that relates how the torque applied to a fastener creates a tensile force that keeps a joint clamped together.  The k-factor is used because it is impossible to accurately define and account for all the variables that can affect the torque required to achieve the specified preload in a joint.  Variables such as geometric tolerance differences, friction conditions between the threads and bearing areas, tightness and concentricity of through-holes, plastic deformation of threads, torsional and bending stresses, and many others can all have significant effects on the torque-tension relationship.  The k-factor represents the "fudge factor" that takes all these undeterminable variables into account and allows them to be quantified by one value that can be determined through testing. 
 
The k-factor is not to be confused with friction coefficients as they are not the same values and do not have the same meaning.  A coefficient of friction is the ratio of the force keeping two objects in contact to the force required to move the objects relative to each other.  The k-factor takes into account the frictional coefficients between the mating threads and under the bearing surface of the nut or bolt head, as well as the numerous other variables mentioned above.       
 
K-factors can be calculated based off empirical data collected in a lab.  Typical k-factors for some common thread conditions are k = 0.15 for lubricated threads, k = 0.17 for zinc plated threads, and k = 0.20 for clean and dry bare steel threads.  RIE Coatings has the ability to help you select a coating system that achieves a specific k-factor range or they can perform testing to provide you with the calculated k-factor for use in specifying an appropriate installation torque to achieve the desired joint preload.