A ceramic seal face is aluminum oxide (alumina), and a silicon carbide face is a harder ceramic that outperforms it on wear, heat transfer and thermal shock. Alumina costs less to make into a face and still does well on clean liquid at low pressure, low surface speed and mild temperature. In chemical service, the grade of each face matters more than which of the two it is.
What Is a Ceramic Seal Face
A ceramic seal face is a ring of aluminum oxide, or alumina, pressed and sintered from powder and then lapped flat. It comes in grades from about 85 % to 99.8 % pure, so “ceramic” on a seal code names the material but not the grade.
Alumina is one of the four materials most seal faces are made from, with carbon, tungsten carbide and silicon carbide.
What Is a Silicon Carbide Seal Face
A silicon carbide seal face is a ring of silicon carbide, a non-oxide ceramic made in several grades: reaction-bonded, sintered, porous sintered, and grades loaded with graphite. Silicon carbide is a ceramic too, but seal catalogs keep the word “ceramic” for alumina and list silicon carbide as a separate face material.
The two common grades differ in what the finished ring contains. Reaction-bonded SiC keeps some free silicon from the molten silicon it is made with, while sintered SiC is fired from silicon carbide powder with small additions of boron and carbon.
The Difference Between Ceramic and Silicon Carbide Seal Faces
Alumina and silicon carbide are about equally strong, stiff and brittle, so the differences that matter in a pump lie in wear, heat transfer, thermal shock, chemical resistance and cost.
Wear Resistance
Silicon carbide lasts longer when the fluid carries abrasive solids, because it is about twice as hard as alumina.
The chart shows NIST reference hardness for sintered alumina of 99.5 % purity or higher and for sintered silicon carbide.
Heat Dissipation
Silicon carbide carries frictional heat away from the seal faces better, with about three and a half times the thermal conductivity of alumina.
Charted below are NIST’s room-temperature values for 99.5 % alumina and sintered silicon carbide.
For dry running, though, a face-material rating table in a Texas A&M pump symposium paper rates alumina at least as well as either silicon carbide grade against common mating faces.
Thermal Shock Resistance
Silicon carbide survives a sudden temperature change far better than alumina: quenched side by side in a published test, it withstood a temperature drop almost twice as large before losing strength.
Chemical Resistance
Chemical resistance depends more on the grade than on the material: high-purity alumina and sintered silicon carbide both resist most process fluids, while low-purity alumina and reaction-bonded silicon carbide are the weak grades.
Some strong acids attack even high-purity alumina, so for strong acids, strong caustics or hydrofluoric acid, specify sintered silicon carbide. For other fluids, put the alumina purity or the silicon carbide grade on the order and check it against the fluid in a chemical compatibility chart.
Cost
Alumina remains cost-effective to form into a seal face, while silicon carbide is difficult to manufacture in seal shapes, so a silicon carbide face generally costs more for the same seal.
On a duty where an alumina face wears out or cracks early, the saving on the face is smaller than the cost of replacing the seal again.
How to Choose the Right One
Silicon carbide is the right hard face for most pump duties; alumina against carbon remains a sound choice only in mild, clean-liquid service with no risk of thermal shock.
- Abrasive solids or slurry: silicon carbide. In the harshest slurry duties, both faces are silicon carbide.
- Hot liquid, temperature swings, or liquid returning to a face that ran dry: silicon carbide, which survives the quench that cracks alumina.
- Refinery and petrochemical pumps: reaction-bonded silicon carbide against carbon, the industry’s standard hard face and a default pair in API 682.
- Chemical process pumps: sintered silicon carbide, the standard hard face for chemical pump seals.
- Clean, cool water at low pressure and low surface speed: alumina against carbon, the usual pair in pool pump seals. Silicon carbide also works there, but nothing in that duty needs its extra hardness or heat resistance.
The failed alumina face indicates whether silicon carbide will help. A switch to silicon carbide prevents wear grooves from solids and cracks from a quench after a hot run; if the carbon face wore out while the alumina seat stayed intact, the hard face was not the problem.
Silicon carbide does not chip less than alumina. Chipped edges therefore point to handling, installation or vibration, not to the face material.
A silicon carbide seat can be made to the same dimensions as the alumina seat it replaces, so moving from a carbon/ceramic to a carbon/silicon carbide face pair is a part swap.
