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Designing with ceramic bearings

Erick Sloan of Carter Manufacturing highlights the pros and cons of using ceramic bearings, outlines the common ceramic bearing materials and touches upon a few of the possible applications.

It is very important to understand the advantages and disadvantages of ceramic bearings before designing them into your application. One of the key benefits of course is their strength. Ceramic microstructures enjoy covalent bonding inherent between non-metal elements. This means they share electrons. This atomic co-operation yields a very strong attraction force and because of this, ceramics offer a series of benefits in comparison to metals.

They normally have a very high hardness and elastic, or Young’s, modulus. This means they are resistant to shape change when loads are applied along with improved wear characteristics. Further, ceramic bearings usually behave in a stable manner at high temperatures meaning there is less thermal expansion. In addition, ceramics are non-metallic, non-ferrous materials; their high degree of corrosion resistance allows them to perform excellently in wet and chemically-corrosive environments.

Another important benefit is that ceramic bearings can run lubrication-free. This is because ceramic materials don’t micro-weld. Micro-welding happens, typically with metals, when the surface imperfections on the rolling element and raceway interact with one another causing an electric arc. This degrades the surface and substantially reduces the bearing life. Ceramic materials do not have this issue which makes them suitable for various applications which require a lube-free environment.

Many engineering ceramics also have a low density leading to improvements in bearings’ operational speeds, which is due to low centripetal forces and reduced friction. And they are non-magnetic and excellent insulators. Also, as non-metallic, non-ferrous materials, they don’t corrode in the same way as metals when exposed to water and other hazardous chemicals.

But there are a few disadvantages to ceramics. The first is that they are substantially more expensive than their metal counterparts. There are many reasons for this, not least being the high energy and processing costs associated with the massive energy needed to reach the required temperature for the sintering process of high grade raw materials. Also, since ceramics are so hard, the machining and grinding costs add up quickly when manufacturing precision bearings. And ceramics are incredibly sensitive to impurities in their pores so any contaminates could cause premature failure. As the size increases the price also increases exponentially because of the requirement of high cost processing methods.

Further, ceramic bearings have lower load capacities in comparison to metals and are sensitive to thermal shock, which results in an internal stress. This stress can exceed the strength of the material thus forming a crack. It is also more difficult to achieve a high quality surface finish with ceramics

Common ceramic bearing materials include silicon nitride, zirconia and silicon carbide. Silicon nitride combines the retention of high strength and creep resistance with oxidation resistance. It has better high temperature capabilities than most metals and its low thermal expansion coefficient gives a better thermal shock resistance in comparison with most ceramic materials.

High speed applications

Silicon nitride is black in colour and the material of choice for vacuum and high speed applications. It’s 58% lighter than traditional steel causing a reduction in centripetal force generated by the rolling elements, which significantly increases fatigue life time. Unlike other ceramic materials, Silicon nitride can hold similar loads to bearing steel; however, it is unsuitable for the race design in any application with shock loading.

Zirconia was developed in the 1960s and 70s to produce a thermal barrier on the external tiles of a space shuttle in order to allow the shuttle to re-enter the Earth’s atmosphere without disintegrating. Since then, Zirconia has become the material of choice for high temperature and highly corrosive applications. The density and thermal expansion of Zirconia is more similar to steel than that of any other ceramic material; therefore it does not have the same weight saving and thermal shock resistance enjoyed by other ceramic materials. However, it has a high fracture toughness. Zirconia is white in colour.

Silicon carbide offers the best heat and corrosion resistance of all the ceramic materials, although it is less frequently used that other ceramic materials due to its raw materials cost and difficulty to machine. Silicon carbide is best used under low loads and in highly corrosive environments.

Applications for ceramic bearings include space and satellites, chemical and medical, and scientific instrumentation. Space exploration exhibits extreme loads and turbulent environments while demanding strict weight constraints and vacuum requirements.

Ceramic bearings are able to fulfil these requirements as many of them are lightweight and vacuum compatible. Unlike their steel equivalents, ceramic bearings are able to run unlubricated which not only stops possible contamination of delicate components in the surrounding applications but also reduces weight as there is no need for heavy greases. They also don’t experience cold welding unlike their steel counterparts.

In chemical and medical applications where contamination can be potentially life threatening, ceramic bearings provide the best solutions. Standard steels can succumb to the effect of strong acids and alkalis, and can rust when washed with solutions and result in particulate contamination. Ceramic bearings are not chemically reactive to corrosive materials and will not release harmful by-products. Further, because they don’t need lubrication, there is no additional microbiology to worry about.

Some highly specialised instrumentation may require a fully non-magnetic system. The magneto-optical phenomenon called the Faraday Effect showcases the interaction between light and a magnetic field in a medium. If light is being measured or utilised in an instrument, a standard steel bearing must be avoided. Ceramic bearings are perfect for situations when magnetic resonance is an issue.

We can see, then, that ceramic bearings exhibit a vast range of advantages for engineering applications but also have disadvantages that must be taken into consideration. They are extremely hard, corrosion-resistant and have a high elastic modulus. They are able to run without lubrication, have low thermal expansion, are normally low density and have non-magnetic qualities. However, they are expensive, have low load capacities, are sensitive to thermal shock and are difficult to achieve a high quality of surface finish on.

Whether you are using Silicon Nitride, Zirconia or even Silicon Carbide, ceramic bearings are available for a wide range of applications such as space, chemical, medical, and scientific instrumentation.

Carter Bearings Ltd