The Ceramic Revolution

For more than two decades, SKF has been unlocking the secrets of hybrid bearing technology and now the company is bringing this advanced solution to the mainstream.

At the heart of this journey are hybrid bearings, which combine advanced ceramic materials with traditional steel components to deliver unmatched performance for high demand applications. Charlotte Vieillard, a materials scientist, has played a pivotal role in this effort. With a background in non-metallic materials, she joined SKF’s R&D facility in the Netherlands as a graduate researcher, focusing on the development of hybrid bearings.

While SKF is renowned for its high-quality steel bearings, Vieillard and her colleagues have turned their attention to the remarkable properties of ceramics. Hybrid bearings, which combine steel raceways with ceramic rolling elements made of silicon nitride, offer unique advantages.

"This ceramic is 60 per cent lighter than steel," says Vieillard. "That weight reduction is especially valuable in larger bearings. Lightweight rolling elements also significantly minimize the centrifugal forces that typically burden bearings at high speeds, enabling hybrid bearings to operate successfully at much higher speeds."

Enhanced durability

These attributes not only extend the life of the grease and bearing but also improve performance at the contact points between ceramic and steel. "Silicon nitride is much harder and stiffer than steel, allowing it to handle contamination and imperfections in the raceway surface better," explains Vieillard.

Ceramic rolling elements can even flatten raised portions of dents and imperfections in steel raceways, preventing damage from escalating. Additionally, because steel and ceramic resist seizing, hybrid bearings are exceptionally durable.

The use of steel rings ensures hybrid bearings remain user-friendly for both manufacturers and end users, as they can be installed like conventional steel bearings. This makes them a versatile 'plug-and-play' solution.

Over the years, Vieillard says, hybrid bearings have found their way into many other products. These range from the unique Thrust Supersonic Car or Space Shuttle to widely used fans in hospitals, air conditioners in buildings, wind turbines, and industrial pumps and compressors.

Currently, she adds, the biggest growth area is in electric motors, which are replacing internal combustion engines in cars, motorcycles and other mobility applications. These applications benefit from another key advantage of ceramic materials: they are excellent electrical insulators, so the bearings are not susceptible to damage from the stray currents that can pass through them in high-frequency electrical machines.

Hard materials, harder to make

There's only one thing standing in the way of wider adoption of hybrid bearings: ceramic rolling elements are difficult and costly to make. "It's a challenge to make routinely and systematically high-quality silicon nitride rolling elements that can withstand the highly loaded point contacts of a bearing," says Vieillard. "You need a really good material, and because ceramics are more brittle than steel, you have to secure a high toughness and strength by developing a specific microstructure."

Ceramic components are made by a sintering process and similar processes than in powder metallurgy. Fine powders of silicon nitride and other additives are first milled together and compacted into a shape, then heated under high pressure until the material “fuses” or sinter into a solid and dense "blank". The blanks are then ground and superfinished into precision balls and rollers. Production requires tight control over multiple parameters at each manufacturing step to achieve the desired final structure and quality.

"Many companies produce silicon nitride parts, but few achieve today the quality level and consistency we need for bearing components production," says Vieillard. 

Costly processes were maybe acceptable for successful and early hybrid bearing implementation. Customers in the aerospace industry, for instance, needed relatively small volumes of product, and they could absorb the higher cost of hybrids in exchange for the performance benefits they offered.

For Vieillard, a key focus, right from her early years of working with hybrids, was ensuring that customers received these benefits in every bearing SKF delivered, by understanding the performance of these materials and finished components as well as the whole hybrid bearing behaviour, the associated detailed components specifications and inspection or characterisation processes. This required relentless attention to quality control and inspection processes to ensure that each batch of bearings met all the specifications.

From niche to mainstream

As hybrid bearings move into mainstream applications such as automotive powertrains, Vieillard and her team face a greater challenge. SKF has invested in a full in-house powder to finished product value chain in parallel to strategic suppliers and has developed larger production systems for this new bearing generation. 

So, not only will the work continue to unveil new performance understanding and life prediction capability of such product performance, striving for fit for purpose specifications at best cost, but now more detailed work on manufacturing step optimization and impact on final product quality is crucial.

SKF’s ceramic community, consisting of researchers, designers, developers and production staff, is now tasked with reducing cost by exploring all process improvements and innovations across the entire value chain. This spans from raw powder selection to finished product inspection techniques, including automation and improved near net shape compaction or sintering methods.

Further research is needed to uncover and understand many of these opportunities.

Charlotte Vieillard, Senior Ceramic Technologist at SKF.

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