Ultrasonic thickness gauge for detecting thick coatings and composite coatings.

Principle

Ultrasonic thickness gauges measure thickness based on the principle of ultrasonic pulse reflection. When the ultrasonic pulse emitted by the probe enters the measured material through the couplant, it travels inside the material and is reflected when encountering interfaces with different acoustic impedances (such as the interface between the substrate and the coating, and the interface between the coating and the air). The instrument calculates the thickness by measuring the time it takes for ultrasonic waves to travel through a coating or material, combined with the known speed of propagation of ultrasonic waves in the material (speed of sound). Its basic calculation formula is:

d = (v × t) / 2

Among them, d represents the thickness, v represents the speed of ultrasonic sound in the material, and t represents the propagation time of ultrasonic waves once back and forth in the material. For thick or composite coatings, the key is to accurately identify the echo signals at the interface of each layer and apply the corresponding sound velocity to calculate them separately.

Thick coating vs. composite coating

Compared with homogeneous single-layer materials, the detection of thick coatings and composite coatings faces several technical challenges. Firstly, the coating material itself may have the characteristics of large sound attenuation, unknown sound velocity, or anisotropy, affecting signal strength and measurement accuracy. Secondly, the composite coating is composed of multiple layers of different materials, and the difference in acoustic impedance of each layer, thickness, and interlayer bonding state will affect the propagation and reflection of ultrasonic signals. If the interlayer bonding is poor, additional interfacial echoes may be generated, interfering with judgment. Finally, for coatings that are too thick, ultrasonic waves may not penetrate the substrate due to excessive attenuation, resulting in measurement failures.

Detection technology

To address these challenges, modern ultrasonic thickness measurement technology has developed a variety of adaptive methods. Sound velocity calibration is fundamental and must be calibrated using a standard block of known thickness as the coating material to be tested to obtain accurate sound velocity values. For composite coatings, instruments with high resolution and high signal-to-noise ratio are required to clearly resolve the interface echoes of each layer that are very close in time. Pulsed echo is the most commonly used method and is measured by analyzing one or more bottom echoes. For thicker coatings with strong acoustic attenuation, multiple echo techniques or lower frequency probes can be used to enhance penetration. Additionally, some instruments support dual crystal or delay block probes to optimize near-surface resolution or accommodate specific shape workpieces.

Operation process

A standardized testing process is a prerequisite for ensuring the reliability of results. Before operation, select the appropriate probe and instrument settings based on the type of coating material, estimated thickness, and workpiece shape. The surface to be tested should be properly cleaned to ensure that it is flat and free of rust or loose attachments. The couplant should be applied evenly to remove air from the probe and the coating. When measuring, the probe should be kept stable and vertically pressed, and multiple measurements should be taken in the same area to average the value. For composite coatings, the sound velocity of each layer of material needs to be set in advance in the instrument, and the corresponding echo signal of each layer should be carefully identified. If there are abnormal fluctuations in the measurement results, check the coupling status, surface condition, or recalibrate the sound velocity.

Application and standard reference

This technology is widely used in industrial applications where the thickness of protective or functional coatings needs to be monitored. For example, in marine and marine engineering, it is used to measure the thickness of anti-corrosion paint coatings; In the petrochemical field, it is used to detect the thickness of the anti-corrosion lining or weld overlay layer of the inner wall of the pipeline; In the aerospace sector, it is used to evaluate the thickness uniformity of thermal barrier coatings or wear-resistant coatings. Relevant testing activities usually refer to widely recognized technical standards at home and abroad, which provide normative guidance on instrument accuracy, calibration methods, measurement steps, and result reporting.

Reference Standards

The following table lists some relevant domestic and foreign technical standards for reference.

Please note that the latest version of the standard in force shall prevail when applying. The magnetic method standard is included for method comparison reference.

Summary

Ultrasonic thickness gauges are an effective tool for detecting the thickness of thick coatings and composite coatings. Its successful application relies on a deep understanding of the principles of ultrasonic propagation, the correct selection of techniques for the characteristics of the object under test and strictly regulated operating procedures. By overcoming challenges such as complex material acoustic properties and multi-layer interface signal recognition, this technology can provide key data support for quality control and condition evaluation of industrial products.