¹⁾ The coefficient of friction between two materials in relative sliding may depend on contact pressure, surface roughness of the relative harder contact surface, temperature, sliding velocity and the type of lubricant whether the level of contamination. It's the reason that the data found in the many reference tables avaiable may show a large variation.

http://www.carbidedepot.com/formulas-frictioncoefficient.htm
http://www.engineershandbook.com/Tables/frictioncoefficients.htm
http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm
http://hypertextbook.com/facts/2005/steel.shtml
http://www.diracdelta.co.uk/science/source/f/r/friction/source.html
http://nl.wikibooks.org/wiki/Fysica/Dynamica
http://en.wikibooks.org/wiki/Physics_Study_Guide/Frictional_Coefficients
http://encarta.msn.com/media_461544499/Table_of_Friction_Coefficients.html

More reliable data of friction coefficients is obtained from measurement in specific applications. For details consult the tables in the book