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Engineering ceramics is one of the indispensable friction pair materials for mechanical seals for corrosion resistance. If it is not selected properly, many problems will arise.The following introduces failure forms using engineering ceramics are prone to and correspondent material selection methods.
1. Fracture
Brittleness is a common disadvantage of all ceramics. Especially edges and corners are easily cracked under impact load. Try to adopt simple shapes in design to avoid sharp corners. Be extremely careful during installation, disassembly, storage and transportation, and there should be no impact load. Overtightening assembly is main reason for fracture of ceramic ring (its strength is 1/4 of that of cemented carbide) of mechanical seal.
Thermal shock is also a common cause of ceramic ring rupture. The mechanical seal may run dry for a short period of time causing temperature rise. When liquid suddenly enters the sealing interface, the ceramic ring suddenly cools down, causing unbearable thermal stress and damages occur.
Therefore, no matter what kind the said ceramics is, we must consider its resistance to sudden temperature changes which often occurs during operation of mechanical seals. Under high temperature conditions, if sudden cooling often occurs, it is best not to use alumina ceramic. Alumina ceramics with a purity of 95% are easy to crack. Higher-purity alumina ceramic ring with a finer particle size is not easy to break.
In order to make up for the shortcoming of low strength, an inlay structure can be used. In order to prevent the ring from falling off the mechanical seal, titanium alloy materials are often used as ring seat, and interference is generally selected in range of 0.05~0.08mm. In this way, the ceramic ring is prevented from breaking or falling off, and the sameness of the two materials in terms of corrosion resistance can be maintained.
Although mechanical strength of ceramic materials is low, if structure design is reasonable, and use and operation are proper, service life is still long under given working conditions.
2. Pair performance
Silicon nitride ceramics are selective as materials for friction pairs of mechanical seals. For example, when it is paired with glass-fiber-filled PTFE in dilute sulfuric acid, it has a large abrasion, and effect is better when paired with carbon graphite;The best effect is when chrome corundum ceramics are paired with glass fiber filled PTFE. The combination of silicon nitride ceramics vs silicon nitride ceramics not only has large wear, but also has large leakage, so only suitable for use under low load conditions.The combination of silicon carbide ceramic and carbon graphite can be used in high load conditions.
1. Fracture
Brittleness is a common disadvantage of all ceramics. Especially edges and corners are easily cracked under impact load. Try to adopt simple shapes in design to avoid sharp corners. Be extremely careful during installation, disassembly, storage and transportation, and there should be no impact load. Overtightening assembly is main reason for fracture of ceramic ring (its strength is 1/4 of that of cemented carbide) of mechanical seal.
Thermal shock is also a common cause of ceramic ring rupture. The mechanical seal may run dry for a short period of time causing temperature rise. When liquid suddenly enters the sealing interface, the ceramic ring suddenly cools down, causing unbearable thermal stress and damages occur.
Therefore, no matter what kind the said ceramics is, we must consider its resistance to sudden temperature changes which often occurs during operation of mechanical seals. Under high temperature conditions, if sudden cooling often occurs, it is best not to use alumina ceramic. Alumina ceramics with a purity of 95% are easy to crack. Higher-purity alumina ceramic ring with a finer particle size is not easy to break.
In order to make up for the shortcoming of low strength, an inlay structure can be used. In order to prevent the ring from falling off the mechanical seal, titanium alloy materials are often used as ring seat, and interference is generally selected in range of 0.05~0.08mm. In this way, the ceramic ring is prevented from breaking or falling off, and the sameness of the two materials in terms of corrosion resistance can be maintained.
Although mechanical strength of ceramic materials is low, if structure design is reasonable, and use and operation are proper, service life is still long under given working conditions.
2. Pair performance
Silicon nitride ceramics are selective as materials for friction pairs of mechanical seals. For example, when it is paired with glass-fiber-filled PTFE in dilute sulfuric acid, it has a large abrasion, and effect is better when paired with carbon graphite;The best effect is when chrome corundum ceramics are paired with glass fiber filled PTFE. The combination of silicon nitride ceramics vs silicon nitride ceramics not only has large wear, but also has large leakage, so only suitable for use under low load conditions.The combination of silicon carbide ceramic and carbon graphite can be used in high load conditions.
