Simple Optical Measurements of Pre-treated Surface Roughness for Thermal Spray

For achieving satisfactory performance of coating layers obtained using the thermal spray technique, pre-treatments are carried out by roughening the smooth surfaces of metals to more than a few tens of μm. Although a surface roughness tester is reliable for quantitatively measuring the surface roughness, it requires expertise and a long measuring time. In the present article, a simple optical method is presented which the authors hope will replace the above surface roughness tester. The proposed method relies on first projecting a narrow beam of light from a light-emitting diode obliquely over the roughened surface, and then on measuring the distortion of the light as observed perpendicular to the surface. Proof-of-principle experiments were performed first against a surface roughness comparator, and then against actual blasted surfaces using both the optical method and the surface roughness tester, yielding the two methods to be in good agreement.