Glass was first used for decorative items in Egypt and the Near East in 3000 BC. Later, people invented molding to make glassware. Nowadays, glass is widely used in agriculture, transportation, electronics, aviation and aerospace and other fields because of its good optical transmission performance, low manufacturing cost, simple process control and easy processing. However, the inherent brittleness and low strength of glass limits its further development. Strength refers to the ability of a material to resist damage or failure. From a mechanical perspective, strength refers to the maximum stress value when a material fails under a certain load. For brittle materials, fracture strength best reflects its mechanical properties. Fracture must overcome the cohesive force of the solid, and the atomic bonds must be broken. The theoretical strength of the material is exactly a reflection of the atomic bond energy. Calculated based on the bonding strength of chemical bonds, the theoretical strength of glass is on the order of E /IO. Based on this calculation, the strength of glass should be about 7000 MPa. However, in real applications, the actual strength of glass is only 8 0 to 10 0 MPa, which is 2 to 3 orders of magnitude lower than the theoretical strength. The huge gap between actual and theoretical strength is caused by the presence of microcracks in the glass. There are many factors that affect the actual strength of glass: such as storage environment (such as temperature, humidity, atmosphere, storage time, etc.), surface machining, sample size, loading speed, mechanical scratches, and internal unevenness (bubbles, stones), etc. Among them, the existence of surface microcracks has the greatest impact on the actual strength of glass. Since many applications require high-strength glass, improving the strength of glass is the key to solving the problem. To improve the mechanical properties of glass, researchers have explored many different methods. Among them, surface treatment, such as physical tempering, chemical tempering, acid treatment and coating, are the most common methods. 2 Methods to Improve the Strength of Glass 2. 1 Physical Tempering The method of prefabricating a compressive stress layer on the glass surface using physical principles is called physical strengthening method or physical tempering. The glass is heated to a temperature above the temperature, and then the hot glass surface is cooled evenly and rapidly. The thermal structure of the surface is frozen. When the interior of the glass gradually cools down, the outer surface layer that cools first will restrict the internal shrinkage, so in the glass Compressive stress is generated on the surface and tensile stress is formed inside the glass. The advantages of physical enhancement are low cost, large output, high mechanical strength, thermal shock resistance (the maximum safe operating temperature can reach 28 7 . 7 8 ℃) and high heat resistance gradient (can withstand 20 4 . 4 4 ℃), but there are certain requirements for the thickness and shape of the glass. Glass samples below 2 degrees Celsius cannot be tempered, and complex components cannot be processed. At the same time, there is also the problem of glass deformation during the tempering process, and it cannot be used in places with high optical quality requirements. Application in the field. In addition, physically tempered glass cannot be cut or processed, and may self-explode. 2. 2Chemical Tempering 2 6 7 Proceedings of the 2009 China Glass Industry Annual Conference and Technical Seminar The method of prefabricating a compressive stress layer on the glass surface using chemical methods is called chemical strengthening method, also known as ion exchange strengthening method. The chemical enhancement method was first applied for a British patent in 1960 by Rese arehCorporation. The principle of ion exchange enhancement is: According to the mechanism of ion diffusion, the surface composition of the glass is changed. At a certain temperature F, the glass is immersed in high-temperature molten salt. The alkali metal ions in the glass and the alkali metal ions in the molten salt are dissolved due to diffusion. Mutual exchange occurs, resulting in a "crowding" phenomenon, which causes compressive stress on the surface of the glass, thus increasing the strength of the glass. Ion exchange enhancement technology is divided into two types: high temperature type and low temperature plow. Low-temperature ion exchange refers to the exchange of small-radius alkali metal ions Na + in the glass with large-radius alkali metal ions K + in the molten salt at a temperature lower than the glass strain point, resulting in a crowding phenomenon that enhances the glass surface. In 1962, Kistler first conducted research on K'-N a' ion exchange enhancement using silicate glass as raw material. High-temperature ion exchange is the exchange of large-radius alkali metal ions Na + and K + in the glass with small-radius alkali metal ions Li + in the molten salt to produce a low-expansion surface layer to achieve the purpose of enhancement.
Since the human body's glass contains soda glass, many studies focus on the principles and applications of low-temperature ion exchange. Ion exchange reinforced glass is characterized by high strength, uniform stress, good stability, no self-explosion phenomenon, can be cut and processed, does not deform, does not produce optical distortion, and is suitable for the reinforcement of glass products with complex shapes and small thicknesses. So far, ion exchange enhancement is the only effective method to strengthen special-shaped thin glass below 3 mm. Ion exchange reinforced glass has excellent performance and is mainly used in high-tech fields such as spacecraft, military aircraft, high-speed trains, combat vehicles, ship windshields and side nests. Although single-step ion exchange can improve the strength of glass, the dispersion of strength is still relatively large. In addition, ion exchange enhancement is only applicable to alkali metal-containing glasses, and this method cannot be used to enhance other glasses. The disposal of waste potassium nitrate salt used in ion exchange is detrimental to the environment. In addition, cleaning ion-exchange glass also requires a large amount of water. Therefore, the cost is high and it is not conducive to strengthening glass for general purposes. ’2. 3 In addition to stress enhancement treatment, acid corrosion can also be used to remove surface micro-cracks. The principle of acid etching is to remove the crack layer on the glass surface or passivate the crack tip through acid etching to reduce stress concentration and restore the inherent high-strength properties of the glass. Since pickling is to remove micro-cracks on the surface, it is necessary to choose an acid with strong corrosive ability, such as hydrofluoric acid. However, it is not easy to obtain a smooth surface by using hydrofluoric acid alone. The salts produced after corrosion are all attached to the surface of the glass. In order to remove the salts, strong acids such as sulfuric acid, phosphoric acid and nitric acid need to be added to the hydrofluoric acid. After acid etching, the strength of flat glass can reach 8001000 MPa. However, the surface of acid-treated glass is extremely fragile and is easily corroded by the external environment. The surface hardness is reduced and the strength cannot be effectively maintained. In addition, acid-etched glass is not resistant to high-temperature treatment, and its strength drops sharply after high-temperature corrosion. 2. 4 Coating Protection As energy costs increase, the use of coatings to strengthen glass is an effective and economical and energy-saving method. People have used different methods to prepare different coatings, such as sol-gel silicone coating, organic-inorganic composite coating, epoxy resin coating and silicone modified coating. These coatings can improve the mechanical properties of glass. Researchers have developed different theories to explain the coating's reinforcing effect. Some people have proposed the filling of cracks on the glass surface and the strengthening mechanism of sol-gel coating during partial filling. In addition, the Poisson suppression effect is considered as the enhancement mechanism of epoxy coating. However, the latest research believes that the closure stress generated by the difference in thermal expansion coefficients of glass and coating is a reasonable model to explain the increase in glass strength. Although the manufacturing process of the coating is simple and the cost is low, the coating is easily affected by the external environment. Once the coating is damaged, the strength of the glass will decrease significantly. This is one of the reasons restricting the development of coatings. 3 Conclusion The improvement of the strength of ordinary glass has always been the focus of glass deep processing researchers. For different uses of glass, different strengthening methods can be adopted according to design requirements. In addition, for special-purpose glass, the strength can be improved by combining traditional strengthening processes.