A more surface sensitive measurement of hardness than for the regular Rockwell scales. This technique is useful for samples with hardness gradients at the surface, to test small areas, and for thin samples. Superficial Rockwell hardness scales are N and T for metals and W, X, and Y for non-metallic materials and coatings. The Superficial Rockwell hardness test method consists of indenting the test material with a diamond cone (N scale) or hardened steel ball indenter. The indenter is forced into the test material under a preliminary minor (29.43N) and the conditions are allowed to settle. Then the indicating device that follows the movements of the indenter is set to a datum position. An additional major load, is applied with resulting increase in penetration. The conditions are allowed to settle again and then the additional major load is removed. 

The permanent increase in depth of penetration, resulting from the application and removal of the additional major load is used to calculate the Rockwell Superficial hardness number.

HR = E - e

e = permanent increase in depth of penetration due to major load F1, measured in units of 0.001 mm
E = a constant of 100 units for diamond and ball indenters
HR = Rockwell hardness number
D = diameter of steel ball