Internal stress and measurement of PVD vacuum coating
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1. Stress profile
When an object deforms due to external causes (force, humidity change, etc.The internal force per unit area at a point in the section under investigation is called the stress.Those that are perpendicular to the cross section are called normal stress or normal stress, and those that are tangent to the cross section are called shear stress or shear stress.Stress can grow as the increase of external force, to some kind of material, the growth of stress has limitation, exceed this limit, material will be destroyed.The limit at which stress can be reached for a material is called its ultimate stress.The ultimate stress is determined by mechanical tests of the material.The allowable stress is defined as the maximum stress that the material can work safely after the measured ultimate stress is reduced appropriately.If the material wants to be used safely, the stress in it should be lower than its limit stress, otherwise the material will be destroyed in use.
There is a kind of stress called internal stress, that is, when there is no external force, the internal stress of the object is the internal stress.Internal stress exists in PVD coating.The harm of internal stress is very big, so the magnitude and degree of internal stress should be mastered in production.PVD coating is mainly used 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 elasticity per unit volume due to internal stress exceeds the fracture energy per unit area, the coating falls off and the thickness of the deposited coating is limited.In general, the coating's internal stress, modulus of elasticity and hardness are proportional to each other.Therefore, in the production process, physical vapor deposition process conditions should be used to control the stress of the coating, especially the internal stress. After the coating, the internal stress of the coating should be tested to make it acceptable
2. Stress test method
Stress testing methods mainly include X-ray and electron diffraction, sample deformation analysis and optical interferometry.
(1) X-ray and electron diffraction
When the coating is in a state of stress, its lattice will produce distortion, so that the lattice constant changes, so the X-ray and electron diffraction method measured the lattice constant diffraction spectral line width changes, can be calculated according to a certain formula coating stress.
(2) Sample deformation analysis
One end of a very thin flat rectangular glass sheet is fixed in the vacuum chamber of the coating equipment, and then a coating is applied on the surface of the thin sheet. The coating stress will deform the free end of the thin sheet. The free end displacement of the thin sheet is measured with a long focal length measuring microscope, and then the stress of the coating is calculated according to the material mechanics formula.
(3) Optical interferential method
Using optical interferometry measuring coating stress is to measure the standard tablet on interferometer with interference fringes formed between coating and substrate, and coating on the substrate, because the coating stress the role of the substrate surface deformation, and then measure the standard tablet on interferometer with interference fringe, we have formed between coating and substrate according to the variation of the interference streak on a distant, with material mechanics formula to calculate the coating stress.