Ceramic bearings in medicine
Chris Johnson, managing director of SMB Bearings, discusses some key factors when considering ceramic and alternative bearings for medical environments.
Back in 2000, scientists discovered zircon in rocks that revealed life might have started 500 million years earlier than previously thought. This incredible compound continues to make a huge impact, as does its oxide, zirconia (ZrO₂), routinely used to make full ceramic bearings.
Advancements in medical equipment and care have created demanding new environments for manufacturers of medical-grade bearings. With each medical application having its own set of required properties and performance criteria.
Applications that require high-precision or high-speed capabilities, are more suited to steel bearings. For example, medical robots require high-precision components to undertake keyhole surgery and centrifuges must spin at high speeds to separate substances that have different weights.
In contrast, medical and surgical devices that need to withstand harsh chemicals during the sterilisation process, require ceramic bearings. In this environment, steel bearings can become susceptible to corrosion, so hard-wearing ceramic bearings are ultimately a better choice to extend the service life of the device. Ceramic bearings are equally suited to clean rooms, as they do not require lubrication to run, therefore reducing the risk of contamination.
Materials selection
Traditionally, bearings in medical instruments have been created from high-purity metals such as martensitic stainless steel. Stainless steel miniature bearings are the most common type used in handheld medical equipment, but some high-value equipment applications are far more suited to ceramic materials.
Full ceramic bearings are harder than steel and offer superior corrosion and heat resistance, higher dimensional stability and lower density. However, this comes at a cost. Ceramic bearings are significantly more expensive than their steel counterparts.
So, when does investing in ceramic bearings become worth it? High-value applications, like lab equipment, have exact requirements that need to be met every time the application is used. Using the wrong components in such equipment can contaminate study conditions, which could cause the study to cease altogether.
Strict regulations in the medical and pharmaceutical industries are in place to prevent contamination. Choosing the correct bearing for these stringent environments, coupled with other design considerations such as the need for high speed, low noise and non-magnetic properties, is key.
Plastic ball bearings are also completely nonmagnetic when fitted with polymer, glass or ceramic balls. They are ideal for use in MRI equipment or sensors where there cannot be any magnetic distortion. The rings can also be made from different plastics, such as polyether ether keton (PEEK) or polytetrafluoroethylene (PTFE).
Hospitals want to ensure these machines run effectively without high maintenance parts, so quality ceramic bearings prove vital in these situations. Full ceramic bearings do not require a lubricant to operate, whereas steel alternatives may need routine maintenance to relubricate the part.
In addition, if a bearing requires a lubricant, it must be carefully matched to the bearing material and application environment. In a cleanroom for example, where the cleanliness of a room is classified from ISO 1 (strictest) to ISO 9 (room air), based on the ISO 14644-1 criteria, an inert cleanroom grease is vital as most greases will vaporise to an unacceptable level, contaminating the environment.
In contrast, for expensive medical devices such as surgical robots, steel bearings are favoured over ceramic options that cannot compete with steel’s speed and precision. It is estimated that the medical robots and computer-assisted surgery field will grow to US$6.8bln by 2021. For this growing area of medical technology, the need for accurate and precise bearing performance has never been higher.
Like all bearings for robotics, surgical robot bearings must deliver the highest possible rotational accuracy, while also being small enough to fit into these often compact and portable hospital devices. Steel thin section bearings or miniature ball bearings, which have an inner diameter of under 10mm, are most commonly selected for their space saving potential.
The considerations when choosing bearings for medical robots and MRI scanners are very different. It is important to review each individual application and the cost associated with the entire component lifespan, instead of solely focusing on the initial investment. For high-value equipment like MRI scanners for instance, which require non-magnetic properties, the increased costs of ceramic bearings are justified.
Zirconia or silicon nitride?
There are a number of commercially available ceramic bearing types, all of which offer many advantages over traditional bearing elements. Typical ceramics used as bearing materials are silicon nitride (Si₃N₄) as well as ZrO₂.
Silicon nitride ceramic bearings are created using a hot isostatic pressing process which uses multi-axial pressing to ensure a uniform component. Compared to standard alumina, which was historically the ceramic bearing material of choice, it provides good corrosion resistance and is far stronger due to its microstructure.
Silicon nitride is a very hard but also very light material. It has 40% the density of steel, resulting in a bearing that is 58% lighter. With a high resistance to water, saltwater and many acids and alkalis, it also has a wide temperature range and is suitable for use in vacuum applications. Ceramic bearings in silicon nitride have a maximum operating temperature of 900°C.
The extreme hardness of silicon nitride also means greater brittleness, so shock or impact loads should be minimised to avoid the risk of cracking. These bearings are suitable for some medical instruments, but for high-speed use, such as dental handpieces, high-precision steel bearings with ceramic balls are used.
Ceramic bearings made from ZrO₂ are tough with similar expansion properties to chrome steel and 440 stainless steel, although they are 30% lighter. This is an advantage when considering shaft and housing fits for higher temperature applications. Great care must be taken with silicon nitride bearings mounted on a steel shaft as the shaft will expand much more than the bearing inner ring, potentially leading to a cracked inner ring. This is not a risk with ZrO₂ bearings.
Although 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 water-resistant, meaning that they can cope with regular washdowns.
Full ceramic versus hybrid ceramic
When most people think of ceramic bearings, they are usually referring to hybrid versions. Hybrids sit in the middle of full ceramic and steel, typically including stainless steel races or rings, and ceramic balls. The steel inner and outer rings of a hybrid bearing can also be machined to close tolerances.
Using a hybrid bearing combination allows for much 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.
Ceramic bearings might be more expensive than steel, but provide reduced friction and weight, and have the potential to last much longer than the traditional stainless-steel offering, particularly in many harsh medical environments. Specifying the right bearings for medical and pharmaceutical environments has always been a complex issue. But understanding the material foundations of bearings before use in any medical application can inform the decision-making process.
