1. What’s The Internal Stress?
When an object is deformed due to an external cause (stress, humidity, etc.), an internal force interacts with each other within the object to resist the effect of the external cause, and it tries to restore the object from the deformed position to the position before the deformation. The internal force per unit area at a point in the cross-section surveyed is called stress. The perpendicular to the same cross-section is called positive stress or normal stress, and the one that is tangential to the cross-section is called shear stress. Stress increases with the increase of external forces. For a certain material, the increase of stress is limited, and the material must be destroyed above this limit. The limit that stress can reach is called the ultimate stress of the material. The ultimate stress value is determined by the mechanical test of the material. Appropriately reduce the measured ultimate stress to specify the maximum stress that the material can work safely, which is the allowable stress. If the material is to be used safely, the stress of it should be lower than its ultimate stress. Otherwise the material will be destroyed.
There is a kind of stress called internal stress, that is, when there is no external force, the internal stress in the object. There is internal stress in the PVD coating, and the internal stress is very harmful, so the magnitude and degree of internal stress should be mastered in production. The main use of PVD coatings is as a wear-resistant protective film, which requires a certain thickness and service life, but the internal stress in the coating limits the thickness of the coating. If the elastic energy per unit volume due to the internal stress exceeds the breaking energy per unit area, the coating will peel off, so that the thickness of the deposited coating is limited. In general, the internal stress, elastic modulus and hardness of the coating are directly proportional to each other. Therefore, during the production process, the stress of the coating, especially the internal stress, should be controlled by the physical vapor deposition process conditions. After process, the internal stress of the coating needs to be detected to ensure it is in an acceptable range.
2. The Test Method of Stress
The stress test methods mainly include X-ray and electron diffraction methods, sample deformation analysis methods and optical interference methods.
◆ X-ray and electron diffraction method
When the coating is under stress, its crystal lattice will be distorted and the lattice constant will change. Therefore, the variation of the diffraction line width of the lattice constant can be measured by X-ray and electron diffraction methods, and then the stress of coatings can be calculated according to a certain formula.
◆ Sample deformation analysis method
One end of a very thin rectangular glass piece is fixed in the vacuum chamber of the coating equipment, and then coating on a surface of the sheet, the stress of the coating will deform the free end of the sheet. The displacement of the free end of the sheet is measured with a telephoto measuring microscope, and then the stress of the coating is calculated according to the mechanical formula of the material.
◆ Light interference method
At first, measure interference fringe between the standard flat plate and the uncoated substrate by interferometer. Then the coating is applied on the substrate, and the substrate surface is deformed due to the coating stress. Then, the interference fringes between the standard flat plate and the coated substrate is measured on the interferometer. According to the variation of the interference fringe, the coating stress can be calculated by the mechanical formula of the material.