BEARINGS | CERAMICS
Full ceramic bearings can
accommodate the many challenging
conditions present during the stages
of semiconductor production; from
furnace temperatures that can reach
close to 1400°C, to clean-room 1 air
quality. Suddenly, the increased costs
are clearly justified.
Zirconia or silicon nitride?
There are a number of commercially
available ceramic bearing types, all
which offer many advantages over
traditional bearing elements. Typical
ceramics that are used as bearing
materials are silicon nitride (Si3N4),
and zirconia (ZrO2).
Silicon nitride is a very hard but
also very light material. Boasting
an excellent resistance to water,
saltwater and many acids and alkalis,
it also has a very wide temperature
range and is suitable for use in high
vacuum applications. The extreme
hardness of silicon nitride also means
greater brittleness, so shock or
impact loads should be minimised to
avoid the risk of cracking.
Silicon nitride has been deployed
as the main material in several
aerospace applications. Notably,
NASA’s space shuttles were
originally made with steel bearings
within their turbo pumps — not a
good combination when the space
shuttle, and particularly its engines,
experienced immense loads and
temperatures. Because of these
extreme loads, NASA engineers
upgraded the bearings to the
silicon nitride equivalent, due to its
superiority in vacuum environments.
Impressively, according to the NASA
analysis report, Si3N4 bearings
showed a 40% gain in runtime
compared to their steel bearing
counterparts.
Ceramic bearings made from
ZrO2, or zirconium dioxide, is a
tough ceramic material with very
similar expansion properties to steel,
although they are 30% lighter. This
is an advantage when considering
shaft and housing fits for higher
temperature applications, where
bearing expansion could mean the
shaft no longer fits.
Although they are usually referred
to as ZrO2 bearings, they are
actually made from ZrO2 stabilised
with yttrium oxide, which gives
the material greater strength and
fracture resistance at room
temperature. They are also
extremely water resistant,
meaning that they are
often used in marine
applications, particularly
where the equipment is
fully submerged, or where
traditional steel bearings
will not cope with the load or
speed.
Using a hybrid
bearing combination
allows for higher speeds
than full ceramic options, as
the less brittle metal rings
are not as prone to sudden
catastrophic failure
Weighing up whether a Si3N4
or a ZrO2 bearing is the right option
is a complex decision, but generally
speaking, ZrO2 bearings are more
commonly specified due to their
extreme corrosion resistance and
tougher properties.
Full ceramic or hybrid ceramic?
When most people think of ceramic
bearings, they are usually referring
to hybrid versions. Hybrid bearings
sit in the middle of ceramic and steel,
typically including stainless steel
races or rings, and ceramic balls.
The steel inner and outer rings of a
hybrid bearing can be machined to
very close tolerances, meaning that
they are best suited for applications
such as electric motors, laboratory
equipment and machine tooling.
On a grinder machine for example,
25% higher revolutions per minute
(RPM) can be achieved by adding
in ceramic hybrid bearings and
synthetic grease lubricant due to the
reduced friction. Grinding
spindles with hybrid
ceramics may run 4,000
hours without problems,
compared to 3,000 hours
with steel bearings.
Hybrid bearings can also
lower temperatures by
nearly 50%. In a horizontal
machining centre, switching
from traditional bearings to the
hybrid alternative has been shown
to drop bearing temperature from
60°C to 36°C at 12,000 RPM.
Using a hybrid bearing
combination allows for higher speeds
than full ceramic options, as the less
brittle metal rings are not as prone
to sudden catastrophic failure under
high speed or load. That said, a
hybrid bearing’s corrosion resistance
pales in comparison to a full ceramic
equivalent.
Specifying the right bearings for
extreme environments has always
been a complex issue. But just as
Zirconia clarified the origin of life,
asking these three questions will help
with the decision-making process. !
More information on SMB Bearing’s
product lines can be found on
the company’s website and its
dedicated whitepaper, ‘Ceramic
bearings: Your essential guide to
ceramic bearings’, which can be
downloaded for free.
under high speed
or load
24 WWW.EUREKAMAGAZINE.CO.UK | MAY 2020
/WWW.EUREKAMAGAZINE.CO.UK