carbon vs silicon carbide mechanical seal manufacturer

Selection of the proper seal face materials is essential for the successful operation of the mechanical seal. In fact, it could be argued that selection of materials is the most important decision to be made by the seal designer.

In evaluating materials for seal faces both the properties of the individual materials and the combination of the tribological pair must be considered. In general, dissimilar materials are used for seal faces. These materials are frequently thought of as the “soft face” and the “hard face” although sometimes two “hard faces” are used.

Mechanical seal design would be considerably simplified if the “perfect” seal face material could be found. With such a material, the designer would not be concerned about balance ratio, face widths, heat generation, flushing, corrosion, etc. Therefore there is a tremendous incentive to develop improved seal face materials.

Even though a perfect seal face material is not likely, the ideal face seal material can be described based on our experiences and problems with existing materials. This ideal material would have the following characteristics:

Leakage is probably more a result of the seal design rather than a property of the material but good face materials can certainly promote low leakage seal designs. In most seals, the actual face separation is strongly related to the surface finish of the materials. Therefore, materials which have and maintain smooth surfaces generally leak less than those with rough surfaces.

Leakage is also related to the compliance, or ability of the seal faces to conform to each other. Compliance is generally thought of as a function of the seal shape; however, it is strongly influenced by the modulus of elasticity. Materials with a low modulus, such as carbon, are more easily made into compliant shapes than materials such as tungsten carbide.

Mechanical seal calculations are considerably simplified through the use of a coefficient of friction. Unfortunately, this coefficient of friction is not a constant and ranges from around .03 to .3. Naturally, the coefficient of friction is a function of the tribological material pair but it also depends on the fluid being sealed. To make matters worse, it turns out that the coefficient of friction also depends on the seal face load and is reduced when the seal leaks.

In spite of these limitations, the coefficient of friction is a useful means of comparing seal face materials, especially when tests are done under similar conditions. Table II shows coefficients of friction for various face combinations.

As shown in Table II, there is a considerable variation in coefficient of friction for various materials. Even when specific material formulations are tested, the coefficient of friction depends on the fluid being sealed, the seal load and aspects of the seal design such as face distortion.

A good mechanical seal material must not only be strong enough to resist the stresses of normal operation, it must also be strong enough to survive the manufacturing process, storage and the rigors of installation.

The strength, hardness and rigidity of carbon graphite based materials is generally an order of magnitude less than that of metals and ceramics such as steel, tungsten carbide or silicon carbide. This means that more design effort is normally directed toward the component which is manufactured from carbon graphite. The primary reason for the use of carbon graphite in mechanical seals is it self lubricating qualities — not its strength.

Tungsten carbide is at the other extreme from carbon graphite. Tungsten carbide has a very high compressive and tensile strength, is very hard and has a high modulus of elasticity.

Silicon carbides are even harder than tungsten carbides but are much more brittle and greater care must be taken during installation and removal. These difficulties in handling have caused many users to prefer tungsten carbide in spite of the low frictional characteristics of silicon carbide.

The thermal aspects of mechanical seals are a major factor in seal performance and reliability. Two of the major material properties are thermal conductivity and thermal expansion.

The thermal shock characteristics of materials have already been discussed. Although thermal conductivity enters into the thermal shock parameters R2 and R3 directly, its effect on seal face temperature is probably more important.

Carbon graphite materials generally have a thermal conductivity of around 5 to 8 Btu/hr ft F; metal filled carbons are somewhat higher. In contrast, tungsten carbides and silicon carbides have thermal conductivities ranging from 40 to 100 Btu/hr ft F. This means that, in a typical seal with carbon versus tungsten carbide or silicon carbide faces, the major of the heat transfer takes place through the non-carbon element.

Stainless steels, Stellite and alumina have much lower thermal conductivities than tungsten carbide and seals using these materials will run considerably hotter than one using tungsten carbide or silicon carbide.

The thermal expansion of seal face materials is related to both the seal face temperature and the coefficient of expansion of the material. In order to minimize the effects of face temperature on distortion, a low coefficient of expansion is desired.

The coefficient of expansion of carbon graphites, tungsten carbides and silicon carbides is similar. This is fortunate and allows for some degree of substitution in seal face materials within the same design family. Alumina is higher and stainless steels still higher.

Any differences in coefficient of expansion become especially important when a seal is manufactured by shrink fitting components made from different materials. In this case, if the operating temperature is sufficiently different from the manufacturing temperature, the seal faces may become distorted. In an extreme case, the components may become loose.

Corrosion of carbon graphites is usually more related to the binder than the carbon graphite. Metal filled carbon are especially subject to corrosion but a suitable resin filled carbon can usually be found for most services. Carbon graphites are not recommended for aqua regia, oleum or perchloric acid. Resins in common use are attacked by lithium hydroxide, potassium hydroxide, sodium metophosphate, anhydrous ammonia, sodium diphosphate and sodium cyanide.

Alumina has good corrosion resistance and high purity alumina is very good. Before the introduction of silicon carbide, alumina was the preferred corrosion resistant material in many mechanical seal services.

The two most common variations of tungsten carbide are cobalt bound and nickel bound. Nickel bound tungsten carbide is the more corrosion resistant although the cobalt bound tungsten carbide is more than adequate for most services. Neither is as good as alumina.

The chemical resistance of silicon carbide is excellent. The two most common variations of silicon carbide are reaction bonded and alpha sintered. Of the two, the alpha sintered is the more corrosion resistant but even reaction bonded silicon carbide is very resistant to chemical attack. Both are generally better in corrosion resistance than nickel bonded tungsten carbide. The “free silicon” in reaction bonded silicon carbide can be attacked by strong oxidizing chemicals. Alpha sintered silicon carbide has no free silicon; it is considered to be the most corrosion resistant of all the seal face materials.

Many of the desirable material qualities for a seal face are not so desirable during the manufacturing process of that component. In particular, the hardness and high strength of many materials make manufacturing very difficult. A common approach is to mold the “green” material into a near finished shape before completing the manufacturing process.

Carbon graphites are typically molded to a rough shape before being impregnated with resin or metal binder. Some simple shapes with small cross sections may be machined from cylindrical stock. The final shape is machined. Faces are always lapped.

Seal components made of very hard materials such as tungsten carbide and silicon carbide are frequently repairable. The repair process consists of chemical and mechanical cleaning and relapping. Caution must be used to assure that dimensional tolerances are maintained.

Softer materials, such as carbon graphites, frequently are not reused, especially if they have been in service for an extended period of time. These softer components generally have more extensive face damage than the hard component and are also less expensive to replace. In the case of carbon graphites, there may also be a concern about chemical attack of the binder.

The cost of seal components is generally related to the hardness and chemical resistance of the material. This cost is normally considered to be a small fraction of the total cost of removing the pump from service and the labor involved in changing out the seal parts. For this reason, most seal users prefer to use the best available materials in their mechanical seals. Currently, the most popular material combination is a premium resin filled carbon graphite versus silicon carbide.

The additional cost of tungsten carbides and silicon carbides is somewhat offset by the fact that components made from them can frequently be repaired – meaning cleaned and re-lapped.

You must consider the “environment” the seal will be exposed to when selecting the design, and importantly, the material of your mechanical seal. The saying “pay me now, or pay me later” very much applies to seals as not selecting the right material will cost more in the long run.

For all environments the material used for the seal face must be stable, be able to conduct heat, be chemically resistant and deliver good wear resistance. However, certain environments will need these properties to be stronger than in others.

Abrasive and harsh environments mean that the material selected must be able to withstand this, which can be more expensive. However the cost will be returned to you over time as poor material grade selection will only result in costly shut downs, repairs, refurbishments or replacements of the seals once again.

Various materials can be used for seals depending on the requirements and environment they will be used for. By looking at material properties such as hardness, stiffness, thermal expansion, wear and chemical resistance, you are able to find the ideal material for your seal.

When mechanical seals first arrived, seal faces were often made from metals such as hardened steels, copper and bronze. Over the years, more exotic materials have been utilised for their property advantages, including ceramics and various grades of mechanical carbons.

Silicon carbide faces in mechanical seal assemblies result in improved performance, increased seal life, lower maintenance costs, and lower running costs for rotating equipment such as turbines, compressors, and centrifugal pumps. CoorsTek seal faces help reduce the possibility of leakage and catastrophic failure to safeguard the environment from the risk of fugitive emissions. They also lower energy consumption with reduced friction on startup and shutdown, as well as reduced wear and erosion during operation.CoorsTekseal faces are exceptionally durable and help increase the mean time between failures,resulting in greater productivity and lower total cost of ownership for processing equipment.

Distributor of industrial tools & equipment. Products include abrasives, adhesives, bushings, coolants, boring bars, broaches, hand tools, cutters, tool holders, coatings, cleaners, reamers, lubricants, drills, fasteners & gauges. Positioners, knives, brushes, crimpers, cylinders, adhesive dispensers, drill presses, acutators, seals, bulbs, lamps, lifts, pins, polishers, fittings, pumps, punches, saws, sealants & wrenches are also available. Capabilities include sharpening, kitting, repairing, vending, fabricating, heat treating, outsourced storeroom management, reverse engineering, band saw welding, calibration, hardening, bar coding, inventory & process consulting & outsourced procurement. Kan Ban programs.

GRAPHITAR carbon is an ideal material for use as a sliding face within a mechanical seal configuration, operating successfully against counterfaces in materials such as Silicon Carbide , Tungsten Carbide and other ceramics. GRAPHITAR is a very hard material and extremely resistant to wear. It is lubricated by most fluids so that the medium being sealed can serve as the seal lubricant. GRAPHITAR’s permeability can be controlled to allow the lubricating medium to be metered to the sealing surface or to be impervious and fully “Leak Tight” to 6 BAR pressure of nitrogen.

Seal faces can be manufactured as fully machined components, lapped to 3 light bands, as semi finished parts or as finished parts pressed to size parts.

Carrying on with our series on sanitary mechanical seals, the following will provide a good overview of the mechanical seal face materials we see in sanitary pump applications.

Note that we are only discussing seals and seal material for sanitary pumps.  Sanitary pumps have unique characteristics in that they have to be hygienic or cleanable.  Because of that, many commercial mechanical seals and packing type seals that do an excellent job from a sealing standpoint, are not suitable for sanitary pumps.  So, we are keeping our comparisons to sanitary mechanical seals only.

First, it’s important to think about what makes a good seal face material. Few materials, in fact, are suitable for use in seals. As discussed in our previous blog post, to keep leakage to a minimum, the seal gap must be very small. As a result, the lubricating film is very thin. Consequently, the seal faces need to be able to withstand rubbing together at high speeds. And what also comes when things rub together at high speeds? Friction. Heat. So we also need to select materials that are able to withstand this heat. In sum, the best seal face materials have low friction, high hardness, good corrosion resistance, and high heat conductivity.

Carbon seals. The old standby. Carbon seals offer the greatest economy and lubricity for sealing non-abrasive products. It’s good for clean, abrasive free materials. It self-lubricates to reduce heat and extend service life. It works great with all other seal materials.

Ceramic is much more resistive to abrasive materials than carbon. It’s a great all around seal material. It has good hardness and stiffness. It’s wear resistant, corrosion resistant, and cheap. Despite the material’s stiffness, it struggles in applications with thermal shock. It is most commonly paired with carbon and is Waukesha’s standard seal material in its sanitary positive displacement pumps.  It’s the “can do” seal material in the “can do” sanitary pump.

Silicon carbide, specifically reaction bonded and self-sintered silicon carbide, offers superior strength, abrasion resistance, and thermal conductivity to alumina ceramic.

Reaction bonded silicon carbide is made by infiltrating compacts made of mixtures of SiC and carbon with liquid silicon.  The silicon reacts with the carbon forming silicon carbide.  The reaction product bonds the silicon carbide particles.  Any excess silicon fills the remaining pores in the body and produces a dense SiC-Si composite.

Sintered silicon carbide is a premium version of silicon carbide. It contains no free graphite or silicon. Sintered silicon carbide is Waukesha’s hardest seal face for sanitary pumps.  It contains no silicon that can leach into the process, is excellent in an oxidizing environment, and has good thermal shock resistance due to low thermal expansion coefficient and high thermal conductivity.

In summary, silicon carbide’s combination of hardness, strength, and temperature resistance gives it excellent capabilities for services in a wide range of applications where high speeds, high pressures, and chemical and abrasion resistance are required.

Commonly known as Purebide, siliconized graphite is technically a silicon carbide. It is made from specially formulated graphite whose surface has been converted to silicon carbide.  This provides surface characteristics similar to silicon carbide but is less costly to produce than solid silicon carbide.  Free graphite in the silicon carbide layer serves a dry lubricant, reducing friction and improving performance. Purebide is used exclusively in the Waukesha 200 series sanitary pumps.

There is nothing pure about this material. It is a composite. In order to hold the tungsten carbide particles together, a binder is required. Nickel is common in food applications due to its corrosion resistance and compatibility with food products. The metallic binder provides “toughness” to the seal material, making it perfect for heavy-duty applications where high starting torques or shock loading is present. It’s ideal for products that are sticky or set up, high temperature applications, and high loads. It is not inexpensive and should only be specified when absolutely necessary. Tungsten carbide mates with carbon, silicon carbide or itself.

Chrome oxide is an interesting material because most of the seal is actually machined from stainless steel. The base part’s seal face is then coated with chrome oxide. The stainless steel provides a strong base that resists high starting torques, impact loading, and rough handling.  The chrome oxide coating provides a hard, wear resistant face that provides performance similar to solid ceramic.

To close, we hope is that this gives you a good idea of what seal face materials are available for sanitary pumps and when to consider them. A future post will expound on seal face combinations, focusing on specific applications when they should be applied, but for now, hopefully this table serves as a good recap:

Due to their advanced performance characteristics, Morgan silicon carbide materials are used across many challenging industrial applications. Their robust properties make them ideal for use as mechanical components and wear parts that are specified in applications such as but not limited to:

Mechanical seals are a collection of components whose aim it is to prevent leakages from a vessel or machine. Seals serve a wide range of functions, and plug an array of liquids. Therefore, they need to be manufactured from a diverse range of materials. Within a mechanical seal, there are two opposing faces pushing against each other, one attached to the outer vessel wall and the other attached to the rotating shaft. The two faces of a mechanical seal do not, however, have to be made from the same material; for example, you may wish to have a seal constructed with one Silicon Carbide face and one Carbon face. The pairs of faces generally require low friction and a high tolerance for prolonged contact with each other, to ensure smooth movement as one face is constantly rotating against the other. The microscopic gap between the faces allows for lubrication to reduce the effect of friction between the pair; however, certain materials are self-lubricating in themselves and, therefore, do not need external lubricants. Selecting the correct material for your mechanical seal will help its longevity and aid in preventing unnecessary leakages and repairs.

- Silicon Carbide is chemically similar to ceramic, but it has improved lubrication potential, to allow for the smooth movement between the two faces within a mechanical seal

- Mechanical seals made from Silicon Carbide can be refurbished multiple times to expand their lifespan, and so could be a more cost-effective choice for your business in the long term

- Mechanical seals made from Tungsten Carbide can be refurbished multiple times in order to expand their lifespan, so could be more cost-effective for your business in the long term

Tekhniseal’s specialised team is ready to help you decide what material would be most appropriate for your mechanical seal faces. We can also recondition your pre-existing mechanical seals into a new seal of the highest quality to be the best fit for sealing a completely different liquid, or within a different vessel. Our team are experts in finding cost-effective solutions to your manufacturing leakage problems. Our on-site workshop means that we can offer a quick turnaround, often a same or next day delivery depending on the quality of the original part and its condition. Contact us today to learn about our next-day delivery of mechanical seals.

Tungsten carbide seal faces and mating faces are used especially in the waste water sector or in applications where high pressures and demands are made on the stability of a seal face. We supply tungsten carbide faces up to high dimensional ranges. Please contact us if you have any questions or are interested.

We manufacture carbon graphite seal faces for many applications. We have approx. 3 tons of graphite materials in stock. A wide range of diameters, up to 570 mm with different infusions enables us to offer shortest delivery times for the benefit of our customers. You can also benefit from our stock of tested seal faces. We offer carbon seal faces with synthetic resin infusion for food applications (FDA-compliant) up to 250°C. For higher stability of the face material, we also offer carbon seal faces with antimony infused.