carbon vs silicon carbide mechanical seal free sample

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.

 Sintered / Reaction-Bonded Silicon Carbide - Silicon carbide is an advanced ceramic material. The earliest type of silicon carbide available for use in mechanical seals was reaction bonded and developments have made a number of variations available. Silicon carbide is extremely hard, being highly wear resistant and with good mechanical properties. It has high temperature strength and thermal shock resistance, maintaining its high mechanical strength at temperatures as high as 2550°F (1400°C). Above 2570°F (1410°C) the free silicon melts and strength decays. These advanced ceramics are routinely used in midstream applications as typically the mating face pair with a carbon ring in light hydrocarbon or finished products due to the exceptional PV characteristics of the material pairs. The ceramic materials are typically run against one another (dissimilar grades) in high viscosity applications such as crude oil. Robust drive designs for seal rings of this material are recommended to avoid potential hang-up when contacting the comparatively softer metal components of the seal retainer.

 Tungsten Carbide - Cemented tungsten carbides are derived from a high percentage of tungsten carbide particles bonded together by a ductile metal. The common binders used for seal rings are nickel and cobalt. The resultant properties are dependent upon the tungsten matrix and percentage of binder (typically 6 to 12% by weight per volume). Tungsten carbide is an extremely tough material with good wear resistance, with Nickel bound being the most common material used in midstream pipeline applications. Seal rings in this material offer improved protection against mechanical or thermal shock, but will be limited in PV characteristics and are more susceptible to heat checking damage when compared against advanced ceramics.

Silicon Carbide / Graphite Composites - These are sintered silicon carbide composite containing free graphite. The free graphite reduces friction, improving dry run survivability and better thermal shock resistance than conventional sintered materials. Grades are also available with a network of non-interconnecting pores, which entrap fluid to support hydrodynamic lubrication. These materials offer exceptional PV characteristics when paired with a corresponding advanced ceramic material. Seal rings manufactured from graphite / silicon carbide composite materials are typically used in crude oil or finished products and light hydrocarbon service especially when operating pressures may exceed the limits of conventional metal-filled carbon grades.

Machine lapping is the production method of lapping Mechanical Seal Faces. Kemet Lapping Systems are used in both manufacturing and reconditioning processes. The following techniques are applicable to Bench Mounted and Free Standing Machines, as well as Kemet Hand Lapping Systems.

The machine size to be used would normally be determined by the largest seal to be lapped. For example, a Kemet 15 machine could lap seals up to approximately 125mm diameter (5”), whereas a Kemet 24 machine would be required if seals up to 200mm (8”) diameter were to be lapped. In most cases, if the following procedures are adopted, it should be possible to produce high quality reflective surfaces flat to less than 2LB.(0.0006mm)

Many materials used for the manufacture of mechanical seals are now too hard to be lapped using old fashioned conventional abrasives like aluminium oxide and silicon carbide. A Kemet composite lapping plate with a diamond slurry will generate a good cutting rate, and a highly reflective surface finish meaning a secondary polishing stage is not needed.

Kemet Flat Lapping Systems are available to fit most Lapping Machines up to 72” diameter, but for the majority of seal repair workshops 15” or 24” machines are adequate.

Flat Mechanical Seals can only be produced on flat lapping plates. It is therefore essential to regularly check plate flatness. This is the most important skill to master when operating a flat lapping machine.

Seals can then be lapped by placing them inside the machine’s control rings and applying a pressure. If multiple parts are to be lapped inside the same conditioning ring, it is essential that they are the same thickness (+/- 0.5mm). To keep the parts in position during lapping either a work-holder, (sometimes called a nest), should be manufactured. This is typically a 3mm disc of Tufnell with the profile of the parts to be lapped machined into it. Alternatively the pressure weights themselves can be faced with a Kemet Facing Kit. This special material has non-skid properties that hold the parts in position during lapping without the need for a work-holder.

Single seals may be allowed to run freely inside a control ring, but it is important to apply a weight to obtain optimum stock removal. This weight should be balanced. An ideal weight is a large steel ball that can simply be placed on top of the seal. It is also recommended that a rubber band be placed around single seals as this gives them drive within the control rings, thus avoiding tracking on the lapping plate. .

If a large seal is to be lapped on a relatively small lapping machine, then it is possible to remove a control ring and substitute the seal, which will then run in the Roller Yoke Assembly.

Parts were lapped for 10 minutes on the diamond lapping machine and then lapped by hand on the Kemet Copper plate until they were polished. Hand lapping took up to 30 seconds. As the parts were spring loaded the parts had to be put on top of an optical flat to get a light band reading. rather than putting an optical flat on top of the seal.

Materials used for the manufacture of mechanical seal faces. These materials must meet the tribological demands unique to mechanical seals. Industry has evolved primarily to the use of mechanical carbons and ceramics for these components. A mechanical seal (Fig. 1) is by its very nature a collection of components that are designed to control leakage of a fluid through a sealing interface. These components serve a wide variety of functions and correspondingly require a wide range of material properties. Many of these properties, such as mechanical strength, thermal conductivity, or corrosion resistance, are similar to considerations made for many other devices. Some properties, however, are unique to mechanical seals, such as coefficient of...

S. Chinowsky, Friction and wear of carbon-graphite materials, in ASM Handbook, Friction, Lubrication, and Wear Technology, vol. 18, ed. by P. Blau (1992)

M.B. Huebner, Material selection for mechanical seals, in Proceedings of the 21st International Pump User’s Symposium, Turbomachinery Laboratory, Texas A&M University, College Station, Texas (2005)

Editors and AffiliationsDepartment of Mechanical Engineering and Center for Surface Engineering and Tribology, Northwestern University, Evanston, IL, USA