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What is the Mechanical seals
Mechanical seals, also known as end-face seals, are mainly used for sealing the rotating shafts of pumps, compressors, hydraulic drives, and similar equipment. The basic structure of a mechanical seal includes a rotating ring, a stationary ring, auxiliary seals (O-rings), and elastic elements (springs). The function of a mechanical seal can be described as follows: as a type of dynamic seal, it minimizes the inevitable leakage that occurs across the dynamic sealing surface.
Table of Contents
The performance requirements for sealing materials in mechanical seals.
The mechanical seal ring is a general term for the rotating ring and stationary ring, which are the main components of a mechanical seal. The sealing ring largely determines the operational performance and lifespan of the mechanical seal. To ensure the normal operation of the sealing ring in the mechanical seal device, considerations such as wear resistance, corrosion resistance, and prevention of biting are taken into account. Sealing rings are often configured as a pair of hard and soft rings with different hardness to address these considerations. The performance requirements for hard ring materials typically include:
- Physical and mechanical properties: Large elastic modulus, high mechanical strength, low density, good thermal conductivity, low thermal expansion coefficient, good resistance to thermal cracking and thermal shock.
- Chemical properties: Good corrosion resistance, resistance to swelling and aging; Sealing rings with good corrosion resistance can resist fluid corrosion and corrosion wear, extending their working life. Especially when used in corrosive media such as acids and alkalis in the chemical industry, it is crucial to choose corrosion-resistant materials.
- Mechanical properties: Good self-lubrication, low friction coefficient, able to withstand short-term dry friction, good wear resistance, and good compatibility. Due to the rotating nature of the friction pair on the sealing end face, it is not enough for each material to have good wear resistance alone. Compatibility issues of the material pair in the friction pair must also be considered. Poor compatibility between two materials in the friction pair can lead to adhesive wear. Only material pairs with good compatibility can achieve good self-lubrication and wear resistance.
- Easy processing and manufacturing: The sealing ring should have good forming and processing performance, and high precision is required for the sealing end face.
What types of materials are commonly used for mechanical seals？
From the perspective of currently used sealing devices in industry, most of them suffer from defects such as poor sealing performance, susceptibility to corrosion, and short working life. The root cause is that the sealing components in these devices are mostly made of metal materials. When used for fluid sealing, the chemical substances in the fluid medium will flush and erode the sealing components, leading to a decrease in their functionality and eventual failure. In the leakage of mechanical seals, 80% to 95% is attributed to the sealing end face. The selection of materials for mechanical face seals is extremely crucial for sealing performance. The basic criterion is that the hardness of the sealing surface must be greater than the hardness of solid particles present in the pumped medium. Tungsten-drill chrome alloys and any grade of steel are not suitable. Only alumina, silicon carbide, and hard alloys can meet the hardness requirements for external sealing components (as shown in the following figure).
Performance Comparison of Common Mechanical Sealing Materials
1.Comparison of the Physical and Mechanical Properties of Common Seal Materials
The mechanical properties of sealing materials are mainly explained through parameters such as density, hardness, flexural strength, elastic modulus, coefficient of thermal expansion, and thermal conductivity. Silicon carbide has a higher thermal conductivity than silicon nitride and carbon graphite, approximately 10 times that of alumina, and similar to hard alloys. The coefficient of thermal expansion of silicon carbide is relatively small and similar to that of hard alloys, increasing with temperature. The thermal shock resistance coefficient (oK/Ea) of silicon carbide is much higher than that of alumina ceramics, second only to carbon graphite and hard alloys.
The mechanical properties of sealing materials are mainly explained through parameters such as density, hardness, flexural strength, elastic modulus, coefficient of thermal expansion, and thermal conductivity. Silicon carbide has a higher thermal conductivity than silicon nitride and carbon graphite, approximately 10 times that of alumina, and similar to hard alloys. The coefficient of thermal expansion of silicon carbide is relatively small and similar to that of hard alloys, increasing with temperature. The thermal shock resistance coefficient (oK/Ea) of silicon carbide is much higher than that of alumina ceramics, second only to carbon graphite and hard alloys (the table below provides the thermal shock resistance coefficients of several materials).
2.Comparison of Chemical Properties (Corrosion Resistance) of Common Seal Materials
Corrosive wear is the primary cause of failure in friction materials. This corrosive effect is extremely strong and can directly or rapidly lead to changes in sealing performance. Therefore, the choice of sealing materials should be based on the characteristics of the sealing medium.
SiC reacts with strong oxidizing substances only at high temperatures (500-600°C), and within the typical range of mechanical seal usage, it is resistant to virtually all acids and bases. Silicon carbide products are usually obtained by sintering. In the preparation process of silicon carbide sealing materials, the most crucial step is sintering. There are several sintering methods for silicon carbide, including atmospheric pressure sintering, reaction sintering, hot-press sintering, hot isostatic pressing, and recrystallization sintering. For silicon carbide used in seals, three sintering methods are employed: atmospheric pressure sintering, reaction sintering, and hot-press sintering.
Pressureless sintered silicon carbide and hot-pressed silicon carbide are virtually resistant to corrosion by all acids and bases at room temperature. They are currently the most chemically stable materials in friction pairs. Moreover, they exhibit excellent oxidation resistance at high temperatures (>1000°C) due to the formation of a protective silicon oxide film on the silicon carbide surface under high oxygen pressure. In contrast, reaction-sintered silicon carbide contains a certain amount of free silicon. As a result, free silicon can be corroded by certain strong oxidants such as hydrofluoric acid, fuming sulfuric acid, strong alkalis, leading to seal leakage.
Each sintering method has its own characteristics, and as a result, the performance of sintered silicon carbide products varies. The table below shows the corrosion resistance performance of silicon carbide under different methods.
Overall, the sealing performance of silicon carbide material is superior to that of alumina and hard alloys.
3.Comparison of Mechanical Properties of Common Seal Materials
Mechanical seals operate under high pressure, and the sealing end face needs to withstand significant loads. This requires the sealing material to have high strength and stiffness; otherwise, the sealing end face may deform under pressure, leading to leaks or rapid wear.
Silicon carbide has extremely high hardness, second only to diamond and cubic boron nitride, and is comparable to hot-pressed boron carbide. Due to its light weight, silicon carbide has high specific strength. The flexural strength of reaction-sintered silicon carbide and pressureless sintered silicon carbide is higher than that of alumina ceramics, 1 times higher than reaction-sintered silicon nitride but 2/3 to 1/2 lower than hard alloys. Among them, hot-pressed silicon carbide has the highest flexural strength, reaching above 7000 kg/cm. The high-temperature strength of hot-pressed silicon carbide and pressureless sintered silicon carbide remains essentially unchanged up to 1400°C and even slightly increases compared to room temperature. The high-temperature strength of reaction-sintered silicon carbide is correspondingly lower due to defects such as free silicon.
In addition, silicon carbide, as a sealing material, has its unique properties, such as excellent thermal conductivity, self-lubrication, high hardness, which enables it to meet the sealing requirements of various industries. The self-lubrication is mainly due to the presence of small amounts of micro graphite or carbon particles in silicon carbide. When the sealing surface rotates, these small graphite particles quickly form a layer of graphite film on the end face, acting as a solid lubricant. When silicon carbide is paired with carbon graphite, the friction coefficient is very low (around 0.1) in various situations, mainly due to the lubricating effect of graphite. When silicon carbide is paired 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, utilizing the membrane pressure effect for lubrication. Additionally, silicon carbide, with its low friction coefficient and less friction heat, combined with its high thermal conductivity, can avoid thermal cracking, a major cause of mechanical seal failure. The table below provides the friction performance of various materials under dry friction and sealing with hot water.
How to Select Mechanical Sealing Materials
The mechanical seal end face is generally composed of a pairing of hard and soft materials. Regarding the wear or failure of the entire friction pair, it primarily depends on the wear of the soft material. Therefore, the friction characteristics of the hard material can be inferred from the wear amount of the soft material. Additionally, the quality of the hard material can be determined by the depth and quantity of scratches on its surface. There are many factors that can cause damage to the sliding surface of rotating mechanical seals, such as excessive mechanical load, high thermal load, chemical corrosion, and severe mechanical wear. Therefore, the material of the friction ring in mechanical seals should be selected based on the most challenging conditions the sealing components may encounter during operation.
Production plants and users should reasonably select silicon carbide materials based on factors such as batch size, structure or conditions, installation capacity, and cost-effectiveness. For large quantities, pressureless sintered or reaction-sintered silicon carbide is suitable. For products with complex shapes, it is preferable to use easily moldable pressureless sintered silicon carbide, which also has a lower cost. If the medium is a strong oxidant, pressureless sintered and hot-pressed silicon carbide are suitable. For use at high PV values, hot-pressed silicon carbide should be chosen. If the medium contains solid particles, silicon carbide itself or a combination of silicon carbide and hard alloys should be selected as the friction pair material.
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