About the classification and measurement principle of coating thickness gauges
The covering layer formed on the surface protection and decoration of materials, such as coating, plating, coating, sticking layer, chemically generated film, etc., is called coating in relevant countries and standards.
The coating thickness measurement has become an important part of the quality inspection of the processing industry and surface engineering, and it is the best means for the product to reach the excellent quality standard. In order to make products into products, my country's export commodities and foreign-related projects have clear requirements for the thickness of the cladding.
The measurement methods of coating thickness mainly include: wedge cutting method, optical interception 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, etc. The first five of these methods are destructive testing, the measurement methods are cumbersome and the speed is slow, and they are mostly suitable for sampling inspection.
X-ray and beta-ray methods are non-contact and non-destructive measurements, but the devices are complex and expensive, and the measurement range is small. Due to the presence of radioactive sources, users must comply with radiation protection regulations. X-ray method can measure ultra-thin coating, double coating and alloy coating. The β-ray method is suitable for the measurement of coatings and substrates with atomic numbers greater than 3. The capacitance method is only used when measuring the thickness of insulating coatings of thin conductors.
With the advancement of technology, especially after the introduction of microcomputer technology in recent years, the thickness gauge using magnetic method and eddy current method has taken a step forward in the direction of miniaturization, intelligence, multi-function, high precision and practicality. The resolution of the measurement has reached 0.1 micron, and the accuracy can reach 1%, which has been greatly improved. It has a wide range of applications, wide measuring range, easy operation and low price, and is the most widely used thickness measuring instrument in industry and scientific research.
The non-destructive method neither damages the coating nor the substrate, the detection speed is fast, and a large number of detection work can be carried out economically.
【Principle of Magnetic Induction Measurement】
When using the principle of magnetic induction, the thickness of the coating is measured by the size of the magnetic flux flowing into the ferromagnetic substrate from the probe through the non-ferromagnetic coating. The size of the corresponding magnetoresistance can also be measured to express the thickness of the coating. The thicker the coating, the greater the magnetoresistance and the smaller the magnetic flux. The thickness gauge using the principle of magnetic induction can in principle have the thickness of the non-magnetic conductive coating on the magnetic conductive substrate. Generally, the magnetic permeability of the substrate is required to be above 500. If the cladding material is also magnetic, the difference in permeability from the base material is required to be large enough (eg nickel plating on steel). When the probe with 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, and the instrument amplifies the signal and then indicates the thickness of the coating. In recent years, the circuit design has introduced new technologies such as frequency stabilization, phase locking, temperature compensation, etc., and uses magnetoresistance to modulate the measurement signal. The designed integrated circuit is also used and the microcomputer is introduced, so that the measurement accuracy and reproducibility have been greatly improved (almost an order of magnitude). The modern magnetic induction thickness gauge has a resolution of up to 0.1um, the allowable error is up to 1%, and the range is up to 10mm.
The magnetic principle thickness gauge can be used to measure the paint layer on the steel surface, the protective layer of porcelain and enamel, the coating of plastic and rubber, the electroplating layer of various non-ferrous metals including nickel-chromium, and the various anti-corrosion coatings of chemical and petroleum industries. .
【Principle of Eddy Current Measurement】
The high-frequency AC signal generates an electromagnetic field in the probe coil, and eddy currents form in the conductor as the probe approaches. The closer the probe is to the conductive substrate, the greater 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. Because these probes are designed to measure the thickness of coatings on non-ferromagnetic metal substrates, they are often referred to as non-magnetic probes. Non-magnetic probes use high-frequency materials as coil cores, such as platinum-nickel alloys or other new materials. Compared with the principle of magnetic induction, the main difference is that the probe is different, the frequency of the signal is different, and the size and scale relationship of the signal are different. Like the magnetic induction thickness gauge, the eddy current thickness gauge also achieves a high resolution of 0.1um, an allowable error of 1%, and a range of 10mm.
The thickness gauge using the eddy current principle can measure non-conductor coatings on all conductors in principle, such as paint on the surface of aerospace aircraft, vehicles, household appliances, aluminum alloy doors and windows and other aluminum products, plastic coatings and Anodized film. The cladding material has a certain conductivity, which can also be measured by calibration, but the ratio of the conductivity of the two is required to be at least 3-5 times different (such as chrome plating on copper). Although the steel matrix is also an electrical conductor, it is more suitable to use the magnetic principle to measure this kind of task.
