Increasing demands are being placed on bearings, so it is often necessary for bearing manufacturers to combinations of coatings to suit applications.
Dr. Tim Hosenfeldt, head of the Surface Technology Competence Centre at Schaeffler comments: “Our customers expect us to provide holistic, consistent support – starting with technical advice and the development of coating systems, through to delivery and quality assurance. We offer customers a range of coatings from which to select the most suitable solution. Our Competence Centre uses the most innovative PVD [Plasma Vapour Deposition], PACVD [Plasma Assisted Chemical Vapour Deposition] and electroplating technologies.”
He continues: “Even for extreme applications – which are becoming more frequent these days – combined coating methods can be implemented. In these types of application, we are exploring the very edges of physical and chemical thresholds. We’re not just talking about nanotechnology, but also manufacturing and delivering corresponding products in high volumes. Customers also expect us to manufacture prototypes quickly and to develop these through to volume production.”
Surface technology is enabling the ‘customisation of surfaces for a specific purpose’, including the provision of built-in ‘intelligence’ by adding sensory properties; and in 2011, Schaeffler invested several million euros in new surface coating machinery at its automotive engine component manufacturing plant in South Wales.
“The latest developments give rise to multifunctional and sensor technical coatings,” Hosenfeldt adds. “For example, the development of the novel Triondur DLC coating systems from Schaeffler, which can be adapted to the respective conditions such as high-additive, low viscous engine lubricating oils, has enabled a reduction in friction levels of up to 50%.”
For industrial rolling bearing applications, many different types of coatings are available, and corrosion or fretting corrosion can be prevented by applying a zinc-iron alloy or zinc-nickel coating to the bearing components (inner rings, outer rings, rolling elements, etc.). For high quality cathodic anti-corrosion protection, zinc-nickel alloys with thick layer passivation (or electroplated layer) can be used. For even higher levels of anti-corrosion protection, zinc lamellar coatings can be used.
Many precision rolling bearings require protection from wear, friction or slippage. Here, a variety of coatings can be applied to bearing components, including mixed iron oxide coatings for improved running-in behaviour and which prevent slippage. If fretting corrosion is expected, zinc phosphate coatings can be used. Manganese phosphate coatings are applied when improvements in sliding or running-in behaviour are required. Application examples include inner ring bores in crankshaft bearings, sheet steel cages, adapter and withdrawal sleeves.
For bearings that require high wear resistance (high hardness), particularly at the axial contact running points or on surfaces where fretting corrosion is expected, columnar hard chromium coatings are recommended.
Dark grey or black in colour, PTFE coatings are used to improve the sliding capacity of the bearing outer ring. Another advantage is the reduction in the friction factor to less than 0.1 under axial displacement of the bearing rings. Application examples include main rotor bearings on wind turbines and spherical roller bearings for cement mills and paper mills.
If the bearings need to be insulated from the passage of electric current, oxide ceramic coatings can be applied. These normally take the form of a ceramic coating comprising aluminium oxide with sealant. For surfaces that are subjected to high tribological stresses with lubricant starvation, carbon coatings can be used.