SUBJECT: Selecting the correct
hard face seal material 5-2
A good mechanical seal should run leak free until
the carbon/graphite seal face wears away. This is the same way we
decide if we're getting good life with our automobile tires. The
tires should not go flat, or the sidewalls "blow-out". The tire tread
should wear at a rate that is consistent with our driving
An inspection of your used seals will show that
85% or more of mechanical seals fail long before the faces wear out.
The seal starts to leak and an inspection shows that there's plenty
of wearable face visible. Some of these failures are caused by the
wrong choice of seal face materials, so we have to be knowledgeable
about those materials that are available to us. The ideal hard face
material would incorporate the following features:
- Excellent corrosion resistance.
- It would be self lubricating.
- High strength in compression, shear and
- High modulus of elasticity to prevent face
- Good heat conductivity.
- Good wearing characteristics
- Low friction.
- High temperature capability.
- Temperature cycling capability.
- Easy insertion into a metal holder
- Low coefficient of friction.
- The ability to be molded in thin cross
- And of course, low cost
Needless to say all of these characteristics are
not available in the same face material. The idea is to get as many
of them as you can in a properly chosen face combination.
With just a few exceptions, seal companies
purchase hard face materials from outside vendors. Be sure the face
component you chose is identified by material, type and grade so that
you can check out the "physicals". Some companies change the generic
name of the material to confuse you. Make sure you know exactly what
you're purchasing, or you will never be able to trouble shoot a seal
failure caused by a wrong material selection.
Here is some information about the common hard
face materials we use in the seal business:
bonded silicon carbide
- Reaction bonded silicon carbide is produced by
adding molten silicon to a mixture of silicon carbide and carbon.
A reaction between the silicon and carbon bonds the structure
while the excess silicon metal fills the majority of the pits left
in the resultant material. There is almost no shrinkage during the
- The silicon content is about 8% to 15%. High
pH chemicals such as caustic can attack this grade of silicon
- As of this writing, carbon/ graphite vs.
reaction bonded silicon carbide has been demonstrated to have the
best wear characteristics of all the possible face
- Reaction bonded silicon carbide is difficult
to insert into a metal holder so it is usually supplied in a solid
rather than a composite configuration.
There are many manufacturers of reaction bonded
silicon carbide. They include:
Shunk and Hoechst of West Germany are also
manufacturers of reaction bonded silicon carbide .
- Reaction bonded silicon carbide has proven to
be more chip resistant than the sintered version
- Avoid the following chemicals when using
reaction bonded silicon carbide :
- Sodium Hydroxide
- Potassium Hydroxide
- Nitric Acid *
- Green Sulfate Liquor *
- Calcium Hydroxide *
- Hydrofluoric Acid
- Caustics and strong acids
- Most high pH chemicals
* Results vary with temperature and concentration.
These chemical can leach out the silicon leaving a weakened structure
that can act like a grinding wheel against the softer carbon
sintered silicon carbide (sometimes called
direct sintered or pressure less sintered)
- This material begins as a mixture of silicon
carbide grains and a sintering aid which is pressed and
subsequently sintered as its name implies. Unlike Reaction bonded
SiC, there is no free silicon present These direct sintered
materials have no metal phase and are therefore more resistant to
- There are two grain shapes available to the
manufacturer. Alpha (Hexagonal Structure), and Beta (Cubic
Structure). There does not appear to be any difference in the
chemical resistance, wear or friction of these two grain
- These self sintered materials will not be
attacked by most process chemicals.
- Here are a few of the bigger
EKasicD (the standard)
Tribo 2000 (Controlled
Tribo 2000-1 (Controlled porosity +
- Sintered silicon carbide is impossible to
shrink into a metal holder.
- Self sintered silicon carbide carries a slight
price premium compared to the reaction bonded version.
- Although the preferred seal face material, it
is sometimes too brittle for some designs.
- The manufacturing process uses a permeable
form of carbon graphite that is reaction sintered in silicon at
elevated temperature. This forms an outer layer of silicon carbide
on the graphite base.
- A resin impregnate is added to increase the
- Cobalt and nickel are the common binders. Each
is susceptible to selective chemical attack of this metallic
binder that will leave a skeletal surface structure of tungsten
- Galvanic corrosion can take place between a
passivated stainless steel shaft, or seal face holder and the
active nickel in the nickel base tungsten carbide seal face. This
can be a real problem in caustic and other high pH fluids. The
temperature at the seal face is higher than the temperature of the
sealing fluid so the attack takes place quicker.
- The metallic binders in tungsten carbide are
also subject to galvanic attack near copper, brass or
- Tungsten carbide is easy to insert into a
metal holder so it is the most common material used in metal
bellows and other hard face&endash;metal composite
some additional thoughts about hard seal faces:
- Many sales people promote two hard faces as
the ideal face combination for slurry and similar services. Keep
in mind that solids cannot penetrate between seal faces unless
they open. Seal faces are lapped to a flatness of less than one
micron (three helium light bands), and as long as they stay in
contact solids are filtered out. Here are some of the main
disadvantages of using two hard faces in a seal
- Higher cost compared to using carbon as a
- If either face is "out of flat" it is
almost impossible for the faces to lap them selves back
- Carbon graphite provides an additional
lubricating film if you are sealing a poor or non lubricating
fluid. It should be noted that many fluids fall into that
category. It takes a film thickness of at least one micron at
operating temperature and face load to be classified as a
- Carbon graphite can easily be inserted into
a metal holder.
- In the event the equipment is run dry,
carbon/ graphite is self lubricating.
- Use two hard faces in the following
applications. or any place carbon is not acceptable:
- If you are sealing hot oil or almost any
hot hydrocarbon. Most oils coke between the seal faces and
can pull out pieces of carbon , causing fugitive emissions
- If the product tends to stick the faces
- Some DI water applications can attack
any form of carbon.
- Halogens can attack all forms of carbon.
These chemicals include:
- If the product you are sealing is an
that will attack all forms of carbon, including black
- If you are pumping a slurry and you
cannot keep the two lapped faces together by flushing,
suction recirculation, a large diameter stuffing box or some
other method usually employed to seal a large percentage of
- If nothing black is allowed in the
system because of a possible color contamination of the
product you are pumping.
- Plated or coated faces can "heat check" and
crack due to the differential expansion of the coating and the
- PV factors as a design tool are unreliable
because carbon is sensitive to "Pressure" but not to
- Water can cause cracking problems with both
85% and 99.5% grade ceramic. The cause is not fully understood,
but hydrogen embrittlement is suspected as the culprit. Cracks
have been observed after seven to eight temperature
Unfilled carbon should be your first choice for a
material to run against the above mentioned hard faces. Use an
unfilled carbon in all applications except an oxidizing
agent, halogen, cryogenic fluid, or if
color contamination is a potential problem. See another paper in this
site for details about how
carbon/graphite seal faces are manufactured.
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