Coatings for surface protection and decoration of materials, such as coatings, coatings, coatings, overlays, chemically formed films, etc., are referred to as coatings in relevant national and international standards.

Coating thickness measurement has become an important part of the processing industry and surface engineering quality testing, and is an indispensable means for products to achieve superior quality standards. In order to internationalize the products, China's export commodities and foreign-related projects have clear requirements for the thickness of the coating.

The coating thickness measurement methods mainly include: wedge cutting method, optical cutting method, electrolysis method, thickness difference measurement method, weighing method, X-ray fluorescence method, β-ray backscattering method, capacitance method, magnetic measurement method and eddy current measurement. Law and so on. The first five of these methods are lossy detection, and the measurement methods are cumbersome and slow, and are more suitable for sampling inspection.

The X-ray and beta-ray methods are non-contact non-destructive measurements, but the device is complicated and expensive, and the measurement range is small. Users must comply with radiation protection regulations due to radioactive sources. The X-ray method can measure extremely thin plating, double plating, and alloy plating. The beta ray method is suitable for coatings with coatings and substrates with an atomic number greater than 3. The capacitance method is only used when thickness measurement is applied to the insulating coating of a thin conductor.

With the advancement of technology, especially after the introduction of microcomputer technology in recent years, the thickness gauges using magnetic method and eddy current method have taken a step toward micro, intelligent, multifunctional, high-precision and practical. The resolution of the measurement has reached 0.1 micron, and the precision can reach 1%, which has been greatly improved. It has a wide range of applications, wide measuring range, easy operation and low cost. It is the most widely used thickness measuring instrument for industrial and scientific research.

The non-destructive method does not damage the coating or damage the substrate, and the detection speed is fast, so that a large amount of inspection work can be carried out economically.

Measuring principle and instrument

First, the principle of magnetic attraction measurement and thickness gauge

The amount of suction between the permanent magnet (probe) and the magnetically permeable steel is proportional to the distance between the two, which is the thickness of the coating. Using this principle to make a thickness gauge, measurement can be performed as long as the difference in magnetic permeability between the cladding and the substrate is sufficiently large. Magnetic coating thickness gauges are the most widely used, as most industrial products are stamped from structural steel and hot rolled cold rolled steel. The basic structure of the coating thickness gauge consists of magnetic steel, relay spring, scale and self-stop mechanism. After the magnetic steel is attracted to the object to be tested, the measuring spring is gradually elongated and the pulling force is gradually increased. When the pulling force is just greater than the suction force, the thickness of the coating layer can be obtained by recording the pull-down force at the moment of the magnetic steel detachment. The new product automates this recording process. Different models have different ranges and applications.

The instrument is characterized by simple operation, ruggedness, no power supply, no calibration before measurement, and low price, which is very suitable for on-site quality control in the workshop.

Second, the principle of magnetic induction measurement

When the principle of magnetic induction is employed, the thickness of the cladding is measured by the magnitude of the magnetic flux flowing from the probe through the non-ferromagnetic coating into the ferromagnetic substrate. It is also possible to measure the magnitude of the reluctance corresponding thereto to indicate the thickness of the coating. The thicker the coating, the larger the magnetic resistance and the smaller the magnetic flux. In the case of a thickness gauge using the principle of magnetic induction, in principle, the thickness of the non-magnetically permeable cladding on the magnetically permeable substrate can be obtained. The substrate is generally required to have a magnetic permeability of 500 or more. If the cladding material is also magnetic, the difference in magnetic permeability from the substrate is required to be sufficiently large (eg nickel plating on steel). When the probe around the coil on the soft core is placed on the sample to be tested, the instrument automatically outputs the test current or test signal. Early products used a pointer-type meter to measure the magnitude of the induced electromotive force, which was amplified by the instrument to indicate the thickness of the coating. In recent years, circuit design has introduced new technologies such as frequency stabilization, phase lock, and temperature compensation, and uses magnetoresistance to modulate measurement signals. The patented integrated circuit is also introduced, and the microcomputer is introduced to greatly improve the measurement accuracy and reproducibility. The modern magnetic induction thickness gauge has a resolution of 0.1um, an allowable error of 1% and a range of up to 10mm.

The magnetic principle thickness gauge can be applied to accurately measure the paint layer on the steel surface, porcelain, enamel protective layer, plastic, rubber coating, various non-ferrous metal plating layers including nickel-chromium, and various anti-corrosion coatings for chemical oil. Floor.

Third, the principle of eddy current measurement

The high frequency AC signal generates an electromagnetic field in the probe coil, and a vortex is formed therein when the probe is close to the conductor. The closer the probe is to the conductive substrate, the larger the eddy current and the greater the reflection impedance. This feedback action characterizes the distance between the probe and the conductive substrate, that is, the thickness of the non-conductive coating on the conductive substrate. Since such probes are specifically designed to measure the thickness of a coating on a non-ferromagnetic metal substrate, they are often referred to as non-magnetic probes. Non-magnetic probes use high-frequency materials for coil cores, such as platinum-nickel alloys or other new materials. Compared with the principle of magnetic induction, the main difference is that the probes are different, the frequency of the signals is different, and the signal size and scale relationship are different. Like the magnetic induction thickness gauge, the eddy current thickness gauge achieves a resolution of 0.1 um, a tolerance of 1%, and a high level of 10 mm.

Thickness gauges using the principle of eddy currents can in principle be measured on non-conductor coatings on all conductors, such as aerospace aircraft surfaces, vehicles, appliances, aluminum alloy doors and windows and other aluminum surface paints, plastic coatings and Anodized film. The cladding material has a certain conductivity and can also be measured by calibration, but it is required that the ratio of the conductivity of the two is at least 3-5 times (such as chrome plating on copper). Although the steel substrate is also an electrical conductor, such tasks are more suitably measured using magnetic principles.

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