Application of Periodic Laser Heating and Phase-Sensitive Thermography for Thickness Gaging of Coatings

The method of phase-sensitive laser thermography has high sensitivity and allows for monitoring the uniformity and thickness of coatings made of various materials. The use of robotic manipulators as scanning devices enables thorough automated inspection of surfaces of complex-shaped test objects. The article provides information on a prototype of a robotic complex for laser phase-sensitive thermography based on a five-axis robotic manipulator, a laser with a power of up to 8 W and a wavelength of 450 nm, as well as a COX CG640 thermal imager. Methods for processing experimental data to determine the thickness of coatings made of low thermal conductivity materials are proposed. To test the approach, calibration blocks made of aluminum oxide with a polypropylene coating in the range of 40 to 500 μm were manufactured. It has been found that the nonuniformity of the coating is best determined by the distribution of the phase of temperature oscillations with a frequency of 0.1–1 Hz.