1. Stress introduction
When the object is deformed due to external cause (stress, humidity, etc.), internal forces interacting between parts of the object to resist the effects of this external factor, and restore the object from the position after deformation to the position before deformation. The internal force of the area at a point in the cross section is called stress. The normal stress is perpendicular to the cross section, and the shear stress is tangent to the cross section. Stress will increase as external forces increase. There is a limit to the growth of stress for a particular material, and above this limit, the material will be destroyed.
For some materials, stress may reach this limit is called the ultimate stress of materials. The ultimate stress value is determined by the mechanical test of the material. The measured force should be appropriate to reduce the limit, the maximum stress a material capable of a predetermined safety, which is the allowable stress. If the material is to be used safely, the stress within it use should be lower than its limit stress, otherwise the material will be damaged when it is used.
There is a kind of stress called internal stress, that is, when there is no external force, the internal stress in the object is the internal stress. And there is internal stress in PVD coating. The damage of internal stress is very large, so the size and degree of internal stress should be well mastered in production. The main use of PVD coatings is as a wear-resistant protective coating, 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 of the unit volume produced by the internal stress exceeds the fracture energy per unit area, the coating will fall off, so the thickness of the deposited coating is limited. In general, the internal stress, modulus of elasticity and hardness of the coating are proportional to each other. Therefore, during the production process, the stress of the coating, especially the internal stress, should be controlled by physical vapor deposition process. After coating, the internal stress of the coating should be detected, so that it can be accepted within the acceptable range.
2. The stress test method
(1) X-ray and electron diffraction
When the coating is in stress state, the lattice will distort, and the lattice constant will be changed. Therefore, the change of the diffraction line width of the lattice constant can be measured by X-ray and electron diffraction. The stress of the coating can be calculated according to a certain formula.
(2) The specimen deformation analysis
One end of a very thin rectangular glass piece is fixed in the vacuum chamber of the coating apparatus, and then coated on one surface of the sheet. The coating stress causes deformation of the free end of the sheet, and the sheet is measured with a telephoto measuring microscope. The amount of displacement at the free end is then calculated based on the mechanical formula of the material.
(3) The optical interferometry
Using optical interferometry to measure the coating stress interference fringes formed between the standard plate and non coated substrate in the interferometer, and then coated on the substrate due to the coating stress effect, substrate surface deformation, then the measurement of interference fringes formed between the standard plate and coated substrate in the interferometer, based on interference fringes changes, calculate the coating stress by using the formulas of material mechanics.
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