Back in 2000, scientists discovered zircon in rocks that showed life might have started 500 million years earlier than previously thought. This incredible compound has made a huge impact, as has its oxide, zirconia (ZrO2), used to make full ceramic bearings. Here, Chris Johnson, managing director of specialist bearing supplier, SMB Bearings, explains the three most overlooked factors concerning full ceramic bearings.
There are a number of commercially available ceramic bearing types, all of 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 minimized 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 Notably, NASA’s space shuttles were originally made with steel bearings within their turbopumps — not a good combination when the space shuttle, and particularly its engines, experienced immense loads and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, Si3N4bearings showed a 40 per cent 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.
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.