Surface Mechanical Properties: Roughness and Coefficient of Friction

A very important surface characteristics is the surface profile and the coefficient of friction. It has direct applications in the fields of sensitive instruments, computer data storage, automotive and biomedical components.

Surface roughness measurement requires a calibrated means of determining the peaks and valleys of a surface profile. These peaks and valleys can be measured using non-contact equipments, like lasers, proximity sensors or contact type tools that can ride over the peaks and valleys. The instrument output after calibration is converted in terms of micro-inches or micro-meters.

One of the commonest units of roughness measurement is Ra – or the Average Roughness. The average roughness is defined as the area between the roughness profile and its mean line, or the integral of the absolute value of the roughness profile height over the evaluation length. Accordingly,

Where, L is the evaluation length of the surface profile and Z is the profilometer reading in micro-inches or micro-meters.
The following table from ISO1392 gives the grades of roughness: