The difference between sintered and reaction-bonded silicon carbide that matters for a seal face is free silicon. A reaction-bonded face carries 8 to 12 percent silicon metal between its carbide grains, and caustics, other high-pH fluids and hydrofluoric acid attack that silicon; a sintered face has none and resists almost any fluid a centrifugal pump handles.
On hardness, strength and thermal shock the two grades are close. Where they do differ, reaction-bonded is the grade for the higher pressures and speeds. In practice the pumped fluid is the basis of the choice: sintered for caustic and chemical service, reaction-bonded for hydrocarbons, water and refinery duty.
What Is Sintered Silicon Carbide
Sintered silicon carbide is silicon carbide powder fused to itself in the furnace, with no second material between the grains. Fine SiC powder with small additions of non-oxide sintering aids, such as boron, carbon or aluminium, is formed to shape and fired in an inert gas at 2,000 to 2,200 °C.
The grade goes by several names that all mean the same material: sintered SiC (SSiC), direct-sintered SiC (DSSiC), alpha-sintered SiC (αSiC) and pressureless-sintered SiC.
What Is Reaction-Bonded Silicon Carbide
Reaction-bonded silicon carbide is silicon carbide grains bonded by new silicon carbide formed in the furnace, with the remaining pore space filled by silicon metal. SiC particles and carbon are pressed to shape and exposed to molten silicon in an inert atmosphere.
In current seal-face grades, free silicon makes up 8 to 12 percent of the finished reaction-bonded composition, and it runs through the face as interconnected passages between the grains.
The same material is sold as reaction-bonded SiC (RBSiC) and as silicon-infiltrated SiC (SiSiC).
The Difference Between Sintered and Reaction-Bonded Silicon Carbide
The two grades differ most in chemical resistance.
Chemical Resistance
Sintered SiC resists almost any fluid a centrifugal pump handles; reaction-bonded SiC has to be kept out of caustics, other high-pH fluids and hydrofluoric acid. Any fluid that attacks elemental silicon attacks a reaction-bonded face through its interconnected silicon.
Typical compatibility ratings by fluid class:
| Fluid class | Reaction-bonded SiC | Sintered SiC |
|---|---|---|
| Light hydrocarbons | Excellent | Excellent |
| Heavy hydrocarbons and oil | Excellent | Excellent |
| Acids | Fair | Excellent |
| Bases (caustics) | Poor | Excellent |
These ratings cover general classes of chemicals, so confirm the specific fluid before ordering.
Acids are a narrower risk for reaction-bonded SiC than its “fair” rating suggests. In a 1994 corrosion study, both grades stayed passive in hydrochloric, nitric and sulfuric acid. That study’s summary does not give acid concentrations or temperatures, so for a hot or concentrated acid we still specify sintered.
No pH cut-off quoted for reaction-bonded SiC traces to an independent test, so treat any caustic or high-pH fluid as sintered duty.
Hardness and Wear
Sintered SiC is the harder grade, at a representative 3,000 Vickers against 2,400 Vickers for reaction-bonded SiC, but both grades rate excellent in abrasive service. Representative values vary by vendor and specific grade.
For the harshest slurries, SiC often goes on both faces, and the choice between the two grades still comes down to the fluid’s chemistry.
Strength and Chipping
Sintered SiC is the stronger grade in representative values, with a transverse strength of about 65,000 psi against 40,000 psi for reaction-bonded SiC. Neither grade has been shown to chip more readily.
Thermal Shock
The two grades resist thermal shock equally well: both rate excellent, where alumina rates poor.
Switching grade does not cure thermal shock cracking in a silicon carbide face; the cause lies in how the seal is run.
Pressure and Speed Limits
Reaction-bonded SiC is the grade for the higher pressures and speeds.
Supplier pages often credit sintered SiC with the higher limits. The Fluid Sealing Association states the reverse: sintered SiC lacks some of reaction-bonded SiC’s tribological characteristics and cannot run at its higher duties. No independent PV figure separates the two grades, so the difference is relative.
Cost
Neither grade is reliably the cheaper one. Price the two grades for your face size and quantity. When the fluid rules reaction-bonded SiC out, the price gap no longer matters.
Size and Shape
Reaction-bonded SiC is the easier grade for very large parts, because it does not shrink when the silicon infiltrates and comes out of the furnace at the size it was formed. At the ring sizes of most pump seals both grades are standard.
How to Choose the Right One
Choose by the fluid: sintered SiC for caustics, other high-pH fluids, hydrofluoric acid and hot or concentrated acids; reaction-bonded SiC for hydrocarbons, oil, water and the higher-pressure, higher-speed services. If a plant pumps a range of chemicals, or the fluid may change, sintered is the safe default for its chemical pump seals.
API 682 4th edition makes premium carbon against either grade the default face pair and leaves the grade to the seal maker and the user, but for caustic service it recommends sintered SiC whatever the seal category.
Temperature is not a reason to choose sintered: the ceiling sometimes quoted for reaction-bonded SiC, about 1,380 °C, set by the melting point of its silicon, is far above any pump seal chamber.
To tell which grade an installed ring is, check the seal datasheet. In the API 682 4th edition seal code, face letters N and R mean carbon against reaction-bonded and sintered SiC respectively, and P and Q mean reaction-bonded or sintered SiC on both faces. Without paperwork, the free silicon of a reaction-bonded face shows between the carbide grains under 100x magnification.
A third option, graphite-loaded SiC in either grade, lowers friction and extends how long the faces survive running dry; API 682 allows it with the purchaser’s approval.