Mechanical Seal Face Material Combinations

Mechanical seals rely on two very flat mating faces – one usually attached to the rotating shaft and one stationary – to prevent fluid leakage. The choice of face material combination is critical for seal longevity and performance.

These combinations are often categorized as “soft vs hard” (a softer, self-lubricating face against a hard face) or “hard vs hard” (two hard faces). Soft/hard pairs (typically carbon against a hard ceramic) are common when some face contact or poor lubrication is expected, because the carbon provides a sacrificial, lubricious surface.

Hard/hard pairs (such as two carbides) are used when a robust fluid film can be maintained (e.g. in abrasive or high-viscosity services) so that face contact is minimized. In general, one face is often made harder than the other so that “something has to wear” – having one sacrificial face prevents severe damage to both.

Carbon vs. Ceramic (Alumina)

This pairing consists of a carbon-graphite face running against a technical ceramic (usually high-purity aluminum oxide). Carbon-graphite seal faces are soft, porous composites made from amorphous carbon and graphite, often impregnated with resin or metal to improve strength and impermeability. Alumina ceramic is a hard, brittle material with good wear resistance but limited shock tolerance.

Recommended Media & Use Cases:

• ✅ Clean or Treated Water: Ideal for fresh water, cooling water, potable water systems, and heating/cooling loops.
• ✅ Light Oils and Coolants: Suitable for lubricating oils, kerosene, and glycol-based coolants/antifreeze.
• ❌ Abrasive or Slurry Fluids: Not recommended for wastewater, slurries, or any fluid with significant solids (sand, silt, etc.).
• ❌ Strong Acids/Alkalis: Avoid in acidic or caustic chemical service.
• ❌ High-Speed or High-Pressure Pumps: In high PV (pressure-velocity) conditions, this combination runs hot.

Carbon vs. Silicon Carbide

This extremely popular combination pairs a carbon-graphite face with a silicon carbide face. Silicon carbide (SiC) is a ceramic revered for its exceptional hardness (almost as hard as diamond) and wear resistance. It exists in variants – reaction-bonded SiC (contains ~10% free silicon) and sintered α-SiC (almost pure SiC) – the latter being more chemically inert. The carbon face in this pairing is typically a high-grade, resin or antimony-impregnated carbon graphite, often “premium” grade to resist blistering. This soft carbon provides the lubricity against the super-hard SiC counter face.

Recommended Media & Use Cases:

• ✅ Chemicals and Corrosive Fluids: A top recommendation for acids, caustics, and solvents.
• ✅ Hydrocarbons and Light Oils: Suitable for pumps moving gasoline, diesel, crude oil, and other hydrocarbons.
• ✅ Hot Water and Light Slurries: Used in boiler feed water pumps (the hardness of SiC resists the slight abrasiveness of boiler scale, and chemical inertness handles water chemistry), provided the pump design manages thermal shock.
• ❌ Large Particle Slurries / Severe Abrasion: For mining slurries, drilling mud, or grit-filled fluids, carbon vs. SiC may wear out too fast because the carbon face will erode.
• ❌ Services with High Vibration or Shock: If the pump environment is very rough (e.g. significant shaft vibration, frequent start/stop hammering), the SiC face could crack.
• ❌ Dry Gas or Poor Lubrication Conditions: If the sealed fluid is a gas or extremely low viscosity liquid (providing almost no lubrication), a soft/hard pair might not suffice.

Carbon vs. Tungsten Carbide

This combination uses a carbon-graphite face against a tungsten carbide (WC) face. Tungsten carbide is a sintered composite material: very hard tungsten carbide particles cemented with a metal binder (historically cobalt, now often nickel for better chemical resistance). WC is sometimes called “hard metal” and is nearly as hard as SiC, but with one key difference – it has a higher fracture toughness (it’s “tougher” and less brittle due to the ductile metal binder). The carbon is typically similar grades as used with SiC (resin or antimony impregnated, etc.).

Recommended Media & Use Cases:

• ✅ Hydrocarbon Oils and Fuels: Great for crude oil, lubricating oil, diesel, etc.
• ✅ Moderate Dirty or Abrasive Fluids: Used in produced water, sour water, and some mining slurries that have low to moderate solid content.
• ✅ High-Pressure or High-Speed Pumps: In high-pressure boiler feed or charge pumps, carbon/WC faces are often selected for their strength.
• ❌ Strong Acid or Caustic Services: Not recommended for acids like HCl, H₂SO₄, or strong caustic soda.
• ❌ Poorly Lubricating Fluids / Dry Gas: In services like hot water with very low viscosity or pumps that might run dry, carbon/WC is still risky.
• ❌ Extremely Abrasive Slurries: In very abrasive slurry (e.g., heavy mineral slurry, sand slurry), even tungsten’s wear resistance may not suffice (and carbon will wear quickly).

Silicon Carbide vs. Silicon Carbide

Both seal faces are silicon carbide, often the same grade (commonly both are direct-sintered SiC, or one may be a graphite-loaded SiC for a bit of lubrication). This is a hard-on-hard combination: two extremely hard, wear-resistant ceramic surfaces running together. Because neither face is self-lubricating, SiC/SiC seals rely on a robust fluid film or ancillary face treatments (like hydrodynamic grooves or diamond-like coatings) to avoid dry friction.

Recommended Media & Use Cases:

• ✅ Aggressive Chemicals (Acids, Caustics, Solvents): For pumps handling fuming acids, strong nitric/hydrofluoric mixes (with caution for HF), or high-purity chemicals, SiC/SiC is often the only viable choice for long life.
• ✅ Abrasive Slurries (with Flush): In mining (e.g., pumping mineral slurry or bottom ash), SiC/SiC with a flush plan is a top-tier solution to resist abrasion.
• ✅ High-Temperature Oil or Non-Aqueous Service: In some specialized high-temp pumps (thermal oil, molten salt, etc.), carbon might not be suitable due to oxidation risk.
• ❌ Services with Risk of Dry or Mixed Lubrication: If the application cannot guarantee a full liquid film (for instance, if pumping a near-saturated liquid that might flash to vapor across the seal, or if intermittent dry running is possible), SiC/SiC is not recommended without special design considerations.
• ❌ High Vibration or Impact Environments: Avoid SiC/SiC in equipment with significant vibration, cavitation, or potential for impact on the seal.

Tungsten Carbide vs. Tungsten Carbide

Both the rotating and stationary faces are tungsten carbide, typically the same grade (often nickel-bound for chemical service). This is a hard-on-hard metal-carbide combination. Tungsten carbide vs tungsten carbide (often abbreviated WC/WC) yields an extremely hard interface, similar in hardness class to SiC/SiC, but with some metal present in each face. Each face is very robust mechanically, and together they form one of the most wear-resistant pairs available (short of exotic options like diamond-faced seals).

Recommended Media & Use Cases:

• ✅ Heavy Slurries and Abrasives: Best suited for mining slurries (rock, tailings, sand), dredging, and fly ash or grit pumping.
• ✅ Viscous Fluids: For high-viscosity oils, syrups, or polymers where fluid film is thick and carbon faces might overheat (blister), WC/WC can be a solution.
• ✅ Some Wastewater Applications: In sewage pumps, some manufacturers offer WC/WC faces as standard or options.
• ❌ Strongly Corrosive Fluids: Do not use WC/WC in acid, bleach, or any service requiring high corrosion resistance. It will fail by corrosion or pitting.
• ❌ Poor Lubrication / Intermittent Dry Conditions: If you cannot guarantee a fully wetted seal (e.g., pump might run dry, or fluid can flash), WC/WC is unsafe.

Tungsten Carbide vs. Silicon Carbide

One face is tungsten carbide (WC) and the other is silicon carbide (SiC) – a hybrid hard-on-hard combination. Typically, the rotating face might be tungsten carbide (for toughness), and the stationary face silicon carbide (for hardness and corrosion resistance), though it can be configured either way. This pairing mixes the attributes of WC and SiC: it’s unique in that the two hard materials have slightly different properties, potentially offering a balance of toughness and hardness.

Recommended Media & Use Cases:

• ✅ Gritty or Sandy Water, Wastewater: For pumps in dirty water service – e.g. sewage with sand, storm water with grit – WC/SiC is often recommended.
• ✅ Mixed Phase Fluids: In oil & gas, pumps moving mixed oil, water, and gas (such as certain multiphase pumps or crude transfer pumps with sand) benefit from WC/SiC.
• ✅ Moderate Chemical Slurries: In chemical processes where there’s a slurry of solids in a chemical solution – say a crystallizer pump with solid crystals in a mother liquor that’s somewhat acidic – WC/SiC would handle the abrasion of crystals and the chemical to a point.
• ❌ Strong Acids or Ultra-Pure Chemicals: If chemical attack on WC is a concern, avoid this combo.
• ❌ Cases Where Lubrication is Unreliable: If flush water might fail or the fluid is prone to phases (liquid to vapor) that could deprive lubrication, WC/SiC will suffer (though arguably not as fast as WC/WC).

Seal Face Combination	Relative Durability (Wear Life)	Relative Cost	Friction & Heat Generation	Chemical Compatibility & Corrosion Resistance
Carbon vs Ceramic (Al₂O₃)	Low in abrasive service (best for clean fluids only). Can last long in clean water, but solids quickly cause wear.	Lowest (very economical initial cost). Ceramic and carbon are inexpensive common materials.	Moderate friction: Carbon provides lubrication, but ceramic’s poor thermal conductivity can cause higher face heat vs carbon/SiC. Generally smooth in water service.	Limited: Suitable for neutral fluids (water, light oil, glycols). Not for strong acids/alkalis (ceramic is attacked by aggressive chemicals). Carbon impregnants (resins) also limit chemical use.
Carbon vs Silicon Carbide	Medium-High: Great wear resistance in most services. SiC’s hardness gives long life, though hard slurries can still erode the carbon face over time. Far longer life than carbon/ceramic in abrasives.	Medium: Higher cost than C/Ceramic (SiC is costly), but cheaper than two hard faces. Often justified by longer life and energy savings.	Low friction: Excellent tribological pairing – self-lubricating carbon + ultra-smooth SiC yields low friction and minimal heat. Runs cooler and more efficient than carbon/ceramic.	Broad: Handles a wide range of chemicals. SiC is highly inert (resists acids, bases, solvents). Carbon is generally inert; choose proper grade for strong acids (e.g. use resin-free in acid). Good for water, hydrocarbons, chemicals. Avoid HF acid (attacks SiC).
Carbon vs Tungsten Carbide	Medium-High: Very durable in many applications. WC’s hardness protects against moderate abrasives, though carbon will wear under heavy solids. Can greatly extend life vs carbon/ceramic in dirty service.	Medium: Similar range to carbon/SiC. WC is costly, but one face is still carbon. Price often justified for more demanding services.	Low friction: Carbon provides a lubricating film, and WC’s high thermal conductivity helps dissipate heat. Typically runs smoothly. Slightly higher friction than carbon/SiC (WC has a bit higher coefficient), but still low.	Moderate: Compatible with water, oils, and mild chemicals. Not for strong acids/oxidizers unless using Ni-bound WC (even then limited). Binder corrosion is the weak point. Overall chemical resistance is good but not as universal as SiC.
Silicon Carbide vs Silicon Carbide	Very High: Exceptional wear life in abrasives – both faces extremely hard. In slurry service with proper lubrication, this combo greatly outlasts any with carbon (reference: 5× life vs C/Ceramic in abrasives).	High: Two premium SiC faces make it expensive. Used only when needed for its strengths.	High friction if boundary contact: Lubricated SiC/SiC has low friction, but zero self-lubrication means any face contact can generate significant heat. Requires full fluid film; otherwise faces may overheat/crack.	Excellent: Nearly universal chemical resistance – both faces are ceramic and inert to almost all fluids (acids, caustics, etc.). No metal exposure (if mounted properly), so ideal for corrosives. Thermal shock can be an issue, but corrosion is not.
Tungsten Carbide vs Tungsten Carbide	Very High (in ideal conditions): The most wear-resistant pair for severe abrasion. Can handle years of service in slurry if kept lubricated. However, without proper lubrication, faces can fail despite hardness.	High: Two WC faces are very costly. Typically justified only for critical abrasive services.	High friction tendency: No soft film – metal-carbide on metal-carbide can run hot without full lubrication. Prone to face squeal or heat checking if lubrication is marginal. Requires flush or sufficient fluid film to run cool.	Limited: Avoid corrosives. Both faces have metal binder susceptible to acids/sea-water. Best in non-corrosive fluids (oils, clean or slurry water). Chemical compatibility is inferior to SiC/SiC – corrosion of binder is a concern in wrong service.
Tungsten Carbide vs Silicon Carbide	Very High: Excellent wear in abrasive/erosive service. Two hard faces ensure long life, and pairing WC with SiC mitigates brittleness – less risk of cracking than SiC/SiC, and less wear than WC/WC in corrosive grit. Known to perform well in gritty wastewater and slurries (with flush).	High: Both faces are hard materials (expensive), on par with other hard/hard combos. Often warranted by improved reliability in challenging services.	Medium-High friction: Still a hard/hard interface, but dissimilar materials reduce stick-slip. Many find WC/SiC runs cooler than WC/WC (SiC lowers friction). Nonetheless, must maintain lubrication; can overheat if run dry.	Broad (minus extremes): SiC side gives corrosion protection; WC side is the limiting factor. Tolerates moderately corrosive fluids better than WC/WC (only one face can corrode). Not for very acidic/chlorine (WC face would attack). Great for slurry in mildly corrosive water, etc.