Judging from the sealing devices used in industry at present, most of them have defects such as poor sealing performance, easy corrosion, short service life, and so on. The reason is that most of the seals in these devices are made of metal materials. When used for fluid sealing, the chemicals in the fluid medium will scour and erode the seals, resulting in the decline of their functions and failure. In the leakage of mechanical seal, 80% ~ 95% is caused by the sealing end face 0. Therefore, the selection of auxiliary materials for mechanical seals is very important. The hard ring materials of most mechanical seals usually use cemented carbide, stainless steel, titanium alloy, ceramics, etc. due to their poor corrosion resistance, metal materials can not be used under the conditions of corrosion, high speed, and high pressure. Ceramic materials, such as AI, O, ZrO, SiC, etc., are mostly materials with strong ionic and covalent bonds. Compared with metals and polymers, they have the advantages of high melting point, good corrosion and oxidation resistance, high hardness, and high-temperature strength, It can effectively overcome many defects of metal sealing devices and is becoming more and more popular in the sealing industry. In particular, silicon carbide sealing materials have developed very rapidly in recent years. Silicon carbide sealing rings (or rings) are widely used in mechanical seals in many fields, such as machinery, automobile, petroleum, chemical industry, and so on.
1 Performance requirements of mechanical seal on sealing materials
The mechanical seal ring is the general name of a moving ring and static ring (the same below), which is the most important component of a mechanical seal. The ring seal largely determines the service performance and service life of the mechanical seal. To ensure the normal operation of the seal ring in the mechanical seal device, the seal ring is often configured into a pair of hard rings and soft rings with different hardness from the aspects of wear reduction, corrosion resistance, and bite prevention. The performance requirements for the hard ring materials are as follows:
1) Physical and mechanical properties. Large elastic modulus, high mechanical strength, low density, good thermal conductivity, low coefficient of thermal expansion, good heat crack resistance, and thermal shock resistance;
2) Chemical properties. Good corrosion resistance, swelling resistance, and aging resistance; The sealing ring with good corrosion resistance can resist the corrosion and corrosion wear of fluid and prolong its working life. Especially in corrosive media such as acid and alkali used in the chemical industry, attention should be paid to the selection of corrosion-resistant materials;
3) Tribological properties. Good self-lubrication, low friction coefficient, can withstand short-term dry friction, good wear resistance, and good compatibility. Due to the rotation of the sealing face of the friction pair, it is not enough to only have the good wear resistance of their respective materials, but also consider the compatibility of the friction pair of materials. When two materials with poor compatibility form a friction pair, adhesive wear is easy to occur. Only the materials with good compatibility can get good self- lubricating and wear resistance;
2 Performance of silicon carbide sealing material
Silicon carbide products are usually made by sintering. In the preparation process of silicon carbide sealing material, the most important link is sintering. The sintering methods of silicon carbide include atmospheric pressure sintering, reaction sintering, hot pressing sintering, hot isostatic pressing sintering, and recrystallization sintering. For silicon carbide for sealing, three methods are adopted: atmospheric pressure sintering, reaction sintering, and hot pressing sintering. The three methods have their characteristics, so the properties of sintered silicon products are also different. But on the whole, the sealing performance of silicon carbide material is superior and can be used as sealing material.
1) Mechanical properties of silicon carbide sealing materials
The mechanical properties of silicon carbide sealing materials are mainly explained by the parameters of density, hardness, bending strength, elastic modulus, coefficient of thermal expansion, and thermal conductivity.
2) Corrosion resistance
The corrosion resistance of silicon carbide ceramics prepared by three sintering methods in some common strong acids and alkalis is shown in Table 2. The values in the table are the reduction of corrosion resistance quality of silicon carbide in solution.
3) Wear resistance
As a sealing auxiliary material, silicon carbide has its unique tribological properties, such as good thermal conductivity, self-lubrication, high hardness, and so on, so it can meet the sealing requirements of many industries. Self-lubricity is mainly because silicon carbide contains a certain amount of small graphite or carbon particles. When the sealing surface rotates, these small graphite particles will quickly form a layer of graphite film on the end face, which plays the role of solid lubricant. When silicon carbide is paired with carbon graphite , the friction coefficient is very small in all cases, which is mainly due to the lubrication of graphite. When silicon carbide material is matched with other materials, to effectively reduce the friction coefficient, the sealed liquid is usually used to form a liquid film between the sealing end faces, and the film pressure effect is used to lubricate; ② Prepare the silicon carbide sealing ring with a spherical pore on the sealing end f ace, and the spherical pore size is micron; ③ Using laser technology and other means, a certain amount of micropores are processed on the silicon carbide seal face, and the pore diameter and hole depth are micron size.