Ceramic bearings are mostly made using ceramic silicon nitride (Si3N4) which is both lighter and harder than traditional steel bearings, although zirconia is also sometimes used. Steel bearings have a hardness of around 30 million lbs per square inch. Ceramic bearings by comparision measure 47million lbs per square inch.
Thanks to silicon nitride being less dense than steel the bearings are up to a third lighter than comparable components made from steel. In addition due to the extra fine finish they are less prone to friction and therefore create less heat. They also require less lubrication.
One of the main obsessions of the serious cyclist is of course weight so any component that reduces the weight of the bicycle is naturally welcome. However, the performance benefits of ceramic bearings are not chiefly down to the weight savings they offer, but rather from their hardness and surface quality. These combine to create reduced levels of friction. "Ceramic balls are smoother, so they roll with less friction. Most companies offering hybrid-ceramic bearings are using a harder steel alloy for the races (compared to all-steel bearings), which will polish into a smoother surface and compliment the hardness of the ceramic balls."
This reduced friction means that the bearings offer less resistance and so require less energy to roll.
Riders using ceramic bearings can also expect a smoother riding experience. Fitting ceramic bearings will make the bike feel like it is rolling smoother. If a cyclist has ridden a bike fitted with ceramic bearings and then returns to steel bearings, it is likely that slightly more vibration will become noticeable. Jonathan Day of Strada Wheels, says that whilst diffcult to quantify, a possible benefit to this reduced vibration could be less fatigue felt in the legs over the course of a long ride or race.
The relative hardness of ceramic bearings results in increased durability over comparable steel bearings. Ceramic material does not rust, unlike steel, meaning exposure to moisture is less of a concern, particularly for full ceramic bearings. They require less lubrication and do not suffer from pitting.
However, there is a risk of chipping, particularly where they are located in areas that are susceptible to water or grit ingress. So whilst ceramic bearings can last between 5 and 20 times longer than steel bearings it is important that they are fitted and maintained correctly. If so, Jonathan Day says that the increased wear resistance means that an initial investment in ceramic bearings should be paid back over time.