Bonding strength of coating including the cohesive strength between the coating particles (spray coating bonding strength itself) and the bonding strength between coating and substrate materials (adhesion), generally the coating cohesion strength is greater than the bonding strength of coating and matrix. Usually only the binding strength between the coating and the substrate is tested. Binding strength refers to the force required for the coating to peel off the bonding surface of the matrix material per unit area between the coating and the substrate. It is a very important index to detect coating performance. If the binding strength is too small, light will cause the coating life to decrease, causing early failure, heavy will cause the coating partial peeling, peeling can not be used. Coating quality is directly related to the success of the application of surface engineering technology, related to the parts, and even the whole life of equipment and safety performance, so the surface of coating quality testing is very necessary. There are three basic methods for the quality inspection of coating surface: appearance inspection, non-destructive inspection and destructive sampling inspection.

The purpose of using thermal spray coating for the purpose of appearance inspection is to protect the base material and achieve some functional indexes, and almost all of them require certain appearance quality, and no obvious defects are allowed. In addition, to some extent, the appearance inspection can also infer the defects in the coating process. Therefore, the appearance inspection of coating is the most basic and common inspection content. If the appearance is unqualified, it is not necessary to test other items.

Inspection of surface coatings with the naked eye or a low power magnifying glass is one of the most basic methods. For some thicker thermal coatings, surface defects such as unevenness, porosity, oxidation, peeling, splash, surface crack and peeling can be observed with the naked eye.

Thermal spray coatings can also be tested by nondestructive testing (NDT). NDT technology is under the condition of no damage is detected objects, using materials internal structural abnormalities or defects caused by the heat, sound, light, the change of the reaction, such as electric and magnetic, to detect all kinds of engineering materials, parts, components of the internal and surface defects, and the type of defect, nature, quantity, shape, location, size, distribution and its changes in judgment and evaluation.

There are various kinds of NDT technology of ultrasonic testing, radiographic testing, eddy current testing, magnetic particle testing, penetrant testing is one of five most commonly used and most mature detection method, usually referred to as the conventional nondestructive testing technology. Besides the five conventional NDT technology, application and some more other NDT techniques, such as neutron gamma ray detection technology, X-ray computer tomography technology, acoustic emission testing technology, noise detection diagnosis technology, the magneto-elastic detection technology (magnetic Barkhausen noise testing, acoustic emission testing), leakage magnetic field detection technology, industrial endoscope inspection technology, laser holographic technology, microwave detecting technology and leakage detecting technology, etc. NDT methods has its applicable scope, each other can't replace each other, no one is everything, any kind of detection method is impossible to give all the information you need. Therefore, one or two NDT techniques must be selected according to different defect types.

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