Whether for kitchen hand mixers, rotating machine components, magnetic drives for agitators or pumps in the construction of chemical plants and equipment – sliding bearings often cope with billions of revolutions in the course of their life. Together with roller bearings in machine and equipment construction, sliding bearings are among the most commonly used types of bearings. Their functional principle is based on contactless movement with a tiny gap between shaft and impeller with minimal friction – during industrial manufacturing processes they’re exposed to enormous temperature and pressure fluctuations. Nevertheless, sliding friction always causes resistance, so there’s also a measure of wear when the sliding surfaces touch the contact surfaces. Constant lubrication with oil, grease or the conveyed medium itself is therefore important – and combining low-friction materials is crucial in this pairing.
Sliding bearings made of silicon carbide (SiC) have proven themselves extremely effective in pumps and drive systems when compared to those made of metal.
Safe operation in harsh industrial environments
“Sliding bearings made of silicon carbide (SiC) have proven themselves extremely effective in pumps and drive systems when compared to metals,” explains Georg Victor, Manager of Product & Application Development at 3M Technical Ceramics. Ceramic material – which in its crystalline structure resembles diamond – has a higher degree of hardness than conventional steel materials, combined with excellent dimensional stability and very good wear resistance. Because of this, the largely maintenance-free life of the bearings is extended many times over – and life cycle costs are subsequently reduced. With their low weight and correspondingly low centrifugal forces, SiC ceramic sliding bearings are well-suited to high-speed and space-saving applications.
In chemical or process plants it’s often the case that the only lubrication available to the sliding bearings is the media being processed. These often contain aggressive acids and alkalis, corrosive fluids, hot and dirty water or even abrasive suspensions, often combined with high-temperature or thermal shock loads. It’s exactly under these conditions that silicon carbide reveals its full potential, impressively maintaining its outstanding tribological properties. It’s therefore even possible to install SiC sliding bearings directly in the area of the conveyed media. This has the advantage of making space-saving constructions and hermetic applications particularly feasible. In addition, SiC sliding bearings can even be operated in a mixed friction environment for a longer period of time – without fusing or seizing up.
The functional properties of ceramic materials can be adjusted to exact requirements.
Tailor-made properties of silicon carbide
SiC is by no means always just the same SiC. “The functional properties of ceramic materials can be adjusted to exact requirements,” says Georg Victor, engineer in metallurgy and materials technology. A distinction is made between porous SiC, dense SiC and graphite-containing SiC. The structural grain sizes also vary. Pressureless and densely sintered silicon carbide is often used as an inexpensive all-round SiC material for sliding bearings. Not only is it super hard, it also offers good chemical resistance. The SSiC manufactured via solid-phase sintering is particularly suitable for use in sliding bearing components. Hot isostatic pressing after sintering can reduce the porosity from approx. 3% to below 1%. Ceramic pump shafts, for example, can easily be made from fine-grained SiC grades as this almost pore-free structure has the highest flexural strength – up to 510 MPa. Being resistant to chemical corrosion, a coarser grained, sintered (S)SiC material would for example be a good choice for pump applications, especially in areas where critical chemical media is conveyed. When high-strength miniature bearings are required, non-pressurised liquid-phase sintered silicon carbide (LPSSiC) is ideal. It has a fine-grained, virtually pore-free structure and shines with a high degree of fracture toughness. If there’s a risk of hot water corrosion, it’s advisable to use coarse-grained SSiC that includes graphite. This has the advantage that the properties of the self-lubricating graphite included in this composite keep the temperatures low, even if it occasionally runs dry or with insufficient lubrication.
Nature as a model for perfect sliding
As paradoxical as it may sound at first, completely smooth does not necessarily equal perfect sliding properties. We know this from studying shark skin, which has special scales that create less drag. Exactly the same happens with bearing surfaces made of SiC. On these surfaces, microscopic ripples that were introduced by means of 3-D micro-laser structuring work miracles by improving the flow properties of the lubricant in the sliding gap, resulting in enhanced sliding behavior. There’s also a useful side effect in that the resulting uniform lubricating film works like a pillow, helping to cope with spikes in axial thrust in the bearing system. This means that the critical load limit of a ceramic sliding bearing can be increased more than ten times. “A good practical example of this is a hermetically sealed, magnetically coupled tank drainage pump which was constantly breaking down when simple bearings were used,” says Victor. “When it was retrofitted with a silicon carbide-graphite thrust bearing ring with microstructure pockets, no further breakdowns that could be ascribed to bearings were experienced with this type of pump.”