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control, analysis, and testing THICKNESS TESTING BY NORBERT SAJDERA KOCOUR CO., CHICAGO; www.kocour.net Coatings are applied to base materials to provide properties not inherent in the base. These include, but are not limited to, corrosion protection, wear resis- tance, conductivity, color, reflectivity, and solderability. The amount of coating applied is critical to the final product's utility and cost. The determination of the amount of coating is, therefore, important in apprais- ing its utility and assessing its cost. Thickness is the most commonly used word to describe the amount of coating. A few of the methods used measure the linear depth of the coating directly. These include the micrometer, with variations using styluses attached to sensitive mechanical and electronic amplifiers, and the microscope, with var- ious methods to expose the coating layers for measurement. More commonly, gauges estimating the weight per measured area are used. The thickness is then calculated using the following equation: T = m 10/A d, where T = thickness (µm), m = mass of coating (mg), A = area tested (cm2 d = density (g/cm3 ). The instruments using the weight per unit area as the basis for their mea- surements are beta backscatter, coulometric, and X-ray. The magnetic and eddy-current methods compare the magnetic and electrical properties of the base and coating materials to calibrated standards with similar properties. The drop test is based on the rate of attack of certain chemical solu- tions. With such a diversity of methods, it is useful to use the summary in Table I to help choose a measuring system for a particular requirement. The gravimetric, microscopic, and X-ray are not included in Table I, because they apply to almost all of the coating combinations listed. A convenient conversion table for differ- ent systems of units is given in Table II. BETA BACKSCATTER If a stream of beta particles is directed at matter, it will collide with the atoms in the matter. This results in a reduction of speed and a change in direction of the particles. Those particles that leave the matter through the same surface from which they entered are said to be backscattered. The number of particles backscat- tered is proportional to the number of atoms per unit area and, therefore, to the atomic weight. The penetration depth of the beta rays is dependent on the ener- gy level of the radioisotope used as their source. The backscatter can be measured with a Geiger-Müller counter placed in its path. In a measuring system, a radioisotope is placed between the Geiger-Müller counter and the coating to be measured. A stream of particles is directed from the isotope through an aperture and then to the coating. The backscatter radiates back through this stream, passes the radioisotope, and then is measured with the Geiger-Müller counter. The isotope is chosen on the basis of its maximum energy and half-life. As the 484 (1) ), and