Calo tester
Coatings with thicknesses typically between 0.1 to 50 micrometres such as PVD coatings or CVD coatings are used in many industries to improve the surface properties of tools and components. The Calo tester is a quick, simple and inexpensive piece of equipment used to measure the thickness of these coatings. The Calo tester also known as a ball craterer or coating thickness tester is also used to measure the amount of coating wear after a wear test carried out using a pin on disc tester.
The Calo tester consists of a holder for the surface to be tested and a steel sphere of known diameter that is rotated against the surface by a rotating shaft connected to a motor whilst diamond paste is applied to the contact area.
The sphere is rotated for a short period of time (less than 20 seconds for a 0.1 to 5 micrometre thickness) but due to the abrasive nature of the diamond paste this is sufficient time to wear a crater through thin coatings.
Calculating coating thickness using the Calo tester
An optical microscope is used to take two measurements after the Calo test across the crater and the coating thickness is calculated using a simple geometrical equation.
Where t = coating thickness, d = diameter of the sphere
Other coating testing equipment
Scratch tester - Measures adhesion
Pin on disc tester - Measures friction and wear