I. Technical Characteristics
(1) First generation anti-wear film technology
The anti-wear film began in the early 1970s, when it was thought that optical lenses were not easy to wear because of their high hardness, while organic lenses were too soft to wear. Therefore, quartz material is coated on the surface of organic lenses in vacuum to form a very hard anti-wear film. However, because its thermal expansion coefficient does not match the substrate material, it is easy to remove the film and brittle crack of the film, so the anti-wear effect is not ideal.
(2) Second generation anti-wear film technology
Since the 1980s, researchers have found that the mechanism of wear is not only related to hardness, but also has the dual characteristics of "hardness/deformation". Some materials have higher hardness but smaller deformation, while others have lower hardness but larger deformation. The second generation of anti-wear film technology is to deposit a high hardness and non-brittle material on the surface of organic lenses by immersion process.
(3) The third generation of anti-wear film technology
The third generation of anti-wear film technology has been developed since 1990s, mainly to solve the wear resistance of organic lenses after anti-reflection coating. Because the hardness of the organic lens base and the hardness of the antireflective film are very different, the new theory holds that there is a layer of anti-wear film between the two, so that the lens can play a buffer role in the grit friction, and not easy to scratch. The hardness of the third generation anti-wear film material is between the hardness of the anti-reflection film and the lens base, and its friction coefficient is low and it is not easy to crack.
(4) Fourth generation anti-wear film technology
The fourth generation of anti-wear film technology uses silicon atoms, such as TITUS from Ivy Road, France, which contains both organic matrix and inorganic ultrafine particles including silicon elements in hard liquor, so as to make the anti-wear film tough and improve its hardness. The most important method of modern anti-wear coating technology is to use immersion method, that is, after multi-channel cleaning, the lens is immersed in the hard liquid, and lifted at a certain speed after a certain time. This velocity is related to the viscosity of the hard fluid and plays a decisive role in the thickness of the anti-wear film. After lifting, the coating was polymerized in an oven at about 100 degree C for 4-5 hours, and the thickness of the coating was about 3-5 microns.
II. Testing methods
The most fundamental way to judge and test the wear resistance of wear-resistant film is to use it clinically, let the wearer wear it for a period of time, and then observe and compare the wear of the lens with a microscope. Of course, this is usually the method used before the formal promotion of this new technology. At present, the more rapid and intuitive testing method we commonly use is:
(1) Grinding test
The lenses are placed in the propaganda articles containing gravel (specifying the grain size and hardness of gravel) and rubbed back and forth under certain control. After that, the diffuse reflectance of light before and after friction was measured with a nebulometer, and compared with standard lenses.
(2) Steel wool test
With a specified kind of steel wool, under a certain pressure and speed, the number of times of rubbing on the lens surface is determined. Then, the diffuse reflectance of light before and after rubbing is measured with a nebulometer, and compared with the standard lens. Of course, we can also manually rub two lenses with the same pressure for the same number of times, and then observe and compare with the naked eye.
The results of these two methods are close to those of long-term wearers.
(3) The relationship between antireflective film and anti-wear film
The antireflection coating on the lens surface is a very thin inorganic metal oxide material (less than 1 micron in thickness), hard and brittle. When coated on optical lenses, it is relatively difficult for the film to scratch because of the hard substrate and the grit scratched on it. However, when the anti-reflection film is coated on organic lenses, the film is easy to scratch because of the soft substrate and the grit scratched on the film.
Therefore, optical lenses must be coated with anti-wear film before anti-reflection coating, and the hardness of the two coatings must match.