Mechanical properties measurement of chromium coatings based on laser ultrasonic technology in high-temperature conditions

The rapid development of the laser ultrasonic technology has promoted non-contact in situ testing of the mechanical properties of materials in high-temperature environments, pursuing enhanced efficiency, precision, and applicability. This paper proposes a laser ultrasound-based technique for characterizing the mechanical properties of materials at high temperatures, and it was applied to determine the variation in the elastic properties of a coating-substrate structure (chromium-Inconel 625) with the temperature. The determination of the mechanical properties of the substrate at different temperatures relies on accurate measurements of the longitudinal and surface wave velocities. The longitudinal wave velocities were modified by considering the effect of thermal expansion on the material thickness during temperature changes, and the mechanical properties of the substrate (Inconel 625) were accurately inverted in the temperature range of 25–1200 °C. It is particularly essential to obtain the mechanical properties of the substrate to measure the elastic constants of the coating. An improved method for inverting the elastic constants of coatings based on dispersion curves is proposed, and a procedure to determine the coating properties is developed based on the Fast Fourier Transform and Green Function Method. The elastic constants of the coating (chromium) at temperatures ranging from 25 °C to 500 °C were accurately measured using the scanning laser source method. This study provides a significant detection scheme for the in situ measurement of mechanical parameters under high-temperature conditions.