Coating thickness measurement has become an important part of the quality inspection of processing industry and surface engineering , and is an essential means for products to reach excellent quality standards. In order to internationalize products, China has set clear requirements for coating thickness in export commodities and foreign-related projects.
The main methods for measuring coating thickness are: wedge cutting method, light 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 method, etc. The first five of these methods are destructive testing, with cumbersome measurement methods and slow speed, and are mostly suitable for sampling inspection.
X-ray and β-ray methods are non-contact and non-destructive measurements, but the devices are complex and expensive, and the measurement range is small. Because of the radioactive source, the user must comply with the radiation protection regulations. The X-ray method can measure extremely thin coatings, double coatings, and alloy coatings. The β-ray method is suitable for measuring coatings and substrates with atomic numbers greater than 3. The capacitance method is only used to measure the thickness of the insulating coating of thin conductors.
With the advancement of technology, especially the introduction of microcomputer technology in recent years, the thickness gauges using magnetic method and eddy current method have taken a step forward in the direction of miniaturization, intelligence, multi-function, high precision and practicality. The resolution of measurement has reached 0.1 micron, and the accuracy can reach 1%, which has been greatly improved. It has a wide range of applications, a wide range, simple operation and low price. It is the most widely used thickness gauge .
The non-destructive method does not damage the coating or the substrate, and the detection speed is fast, which can enable a large number of detection work to be carried out economically.
1. Magnetic force measurement principle and thickness gauge
The force of attraction between the permanent magnet (probe) and the magnetic steel is proportional to the distance between them, which is the thickness of the coating. Using this principle, a thickness gauge can be made. As long as the difference in magnetic permeability between the coating and the substrate is large enough, it can be measured. Since most industrial products are stamped with structural steel and hot-rolled and cold-rolled steel plates, magnetic thickness gauges are the most widely used. The basic structure of the thickness gauge consists of magnetic steel, relay spring, scale and automatic stop mechanism. After the magnetic steel is attracted to the object to be measured, the measuring spring is gradually stretched behind it, and the tension gradually increases. When the pulling force is just greater than the suction force, the magnitude of the pulling force is recorded at the moment when the magnetic steel is separated to obtain the coating thickness. New products can automatically complete this recording process. Different models have different ranges and applicable occasions.
The characteristics of this instrument are easy operation, durability, no need for power supply, no need for calibration before measurement, and low price, making it very suitable for on-site quality control in workshops.
2. Magnetic induction measurement principle and thickness gauge
When the magnetic induction principle is used, the thickness of the coating is measured by the size of the magnetic flux flowing from the probe through the non-ferromagnetic coating and into the ferromagnetic substrate. The corresponding magnetic resistance can also be measured to indicate the coating thickness. The thicker the coating, the greater the magnetic resistance and the smaller the magnetic flux. In principle, a thickness gauge using the magnetic induction principle can measure the thickness of a non-magnetic coating on a magnetic substrate. The magnetic permeability of the substrate is generally required to be above 500. If the coating material is also magnetic, the difference in magnetic permeability with the substrate is required to be large enough (such as nickel plating on steel). When the probe with a coil wound around the soft core is placed on the sample to be tested, the instrument automatically outputs a test current or test signal. Early products used a pointer-type meter to measure the size of the induced electromotive force, and the instrument amplified the signal to indicate the coating thickness. In recent years, circuit design has introduced new technologies such as frequency stabilization, phase locking, and temperature compensation, and uses magnetic resistance to modulate the measurement signal. Patented integrated circuits and microcomputers are also used to greatly improve the measurement accuracy and reproducibility (almost by an order of magnitude). Modern magnetic induction thickness gauges have a resolution of 0.1um, an allowable error of 1%, and a measuring range of 10mm.
The magnetic thickness gauge can be used to accurately measure the paint layer on the steel surface, porcelain and enamel protective layer, plastic and rubber coating, various non-ferrous metal electroplating layers including nickel and chromium, and various anti-corrosion coatings in the chemical and petroleum industries.
3. Eddy current measurement principle and thickness gauge
The high-frequency AC signal generates an electromagnetic field in the probe coil. When the probe is close to the conductor, eddy currents are formed in it. The closer the probe is to the conductive substrate, the greater the eddy currents and the greater the reflected impedance. This feedback action represents the distance between the probe and the conductive substrate, that is, the thickness of the non-conductive coating on the conductive substrate. Since this type of probe is specifically used to measure the coating thickness on non-ferromagnetic metal substrates, it is usually called a non-magnetic probe. Non-magnetic probes use high-frequency materials as coil cores, such as platinum-nickel alloys or other new materials. Compared with the magnetic induction principle, 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 has also achieved a high level of 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-conductive coatings on all conductive bodies in principle, such as paint, plastic coatings and anodized films on the surfaces of aerospace and aircraft, vehicles, household appliances, aluminum alloy doors and windows and other aluminum products. The coating material has a certain conductivity and can also be measured through calibration, but the conductivity ratio of the two must be at least 3-5 times different (such as chrome plating on copper). Although the steel substrate is also a conductor, it is more appropriate to use the magnetic principle to measure this type of task.
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