Principle and method of ultrasonic testing (UT) coating thickness

1. Propagation characteristics of ultrasonic waves in thin media

The propagation characteristics of ultrasonic waves in thin media play a key role in the measurement of coating thickness. The figure shows the propagation of ultrasonic waves between air and thin-layer media, involving parameters such as acoustic impedance, sound velocity and thin-layer thickness. When ultrasonic waves are incident on the air/thin layer medium interface, multiple reflection echoes will be generated, forming a series of characteristic waveforms. These waveforms contain information related to the thickness of the coating. By measuring the difference in arrival time of the reflected echoes, the time it takes for the ultrasonic wave to travel back and forth in the thin medium can be known.

2. Principles and methods of ultrasonic thickness measurement

The principles of ultrasonic thickness measurement mainly include resonance method and pulse reflection method.

• Resonance method:

By continuously transforming the sine wave electrical signal within a certain frequency range, the piezoelectric chip is excited to generate sound waves and form resonance. The thickness of the specimen can be deduced by measuring two adjacent resonance frequencies. However, the resonance method has certain limitations on the parallelism of the specimen and the thickness variation range, etc., and is not suitable for thermal spray coatings with micron thickness.

• Pulse reflection method:

The coating thickness is calculated by measuring the round-trip time of short ultrasonic pulses. This method is not limited by the geometry of the specimen and is suitable for metallic and non-metallic materials with fine-grained structures. The pulse reflection method is affected by material sound attenuation and surface roughness in practical applications, but it is more accurate for coating thickness measurement.

3. Advantages and scope of application of pulse reflection method

The pulse reflection method has some advantages in measuring surface coating thickness: it is not limited by the geometry of the material being measured, and is suitable for the measurement of flat and curved materials. Compared with the resonance method, the requirements for the parallelism and thickness change of the specimen are lower. It is suitable for metallic and non-metallic materials with fine grain structure and can be used for the measurement of thermal spray coatings. However, for ultra-thin composite materials, coarse-grained materials, and coatings with large surface roughness, the measurement accuracy of the pulse reflection method may be affected to a certain extent.

4 Conclusion

Ultrasonic non-destructive testing technology has broad application prospects in coating thickness measurement. This article focuses on the principles of ultrasonic thickness measurement, with special attention to the advantages and scope of application of the pulse reflection method. In industrial production, choosing an appropriate coating thickness detection method is crucial to ensuring product quality and production efficiency.