However, neither friction nor wear constitute intrinsic material properties, these rather being system parameters which, in addition to the frictional partners, depend on ambient influences and the stress profile involved. Very different materials are therefore utilised, depending on the particular application, specification and environment. Functional coatings are frequently applied to surfaces subject to frictional wear if the bearing material cannot be replaced. Multilayer systems or composite layers are also utilised. However, the improvement of the system subject to tribological stress through suitable material design is most frequently attempted. When addressing these aspects, carbon materials and, increasingly, ceramic materials are, next to metals and alloys, the primary objects of consideration when selecting bearing and seal materials in contemporary applications. Carbon and ceramic materials embody excellent self-lubrication properties, practically eliminating the possibility of contramotional or contrarotational partner seizure or fusion.
The type of pairing depends heavily on the operational conditions involved. Hard/soft pairings are, for example, recommended for sealing gaseous media. Practical examples in this context are pumps in dry-running operation. Excellent self-lubricating characteristics are required in this case. Carbon materials and composites demonstrate their superiority to other materials in terms of dry-running suitability, chemical and temperature resistance and thermal conductivity, depending on the operating temperature, corrosiveness of media or rotation speed. Wear resistant hard/hard pairings, on the other hand, are required in abrasive media. Modified mixed ceramics, such as SiC30 (a mixture of silicon carbide and graphite), are especially suitable in terms of slippage, with the pairing of SiC30 against SiC30 demonstrating particularly unique emergency running properties. Metals and carbon are suitable materials if, for example, the transmission of electrical current is also required in an application. However, the selection should be carefully weighed up, as transmission of currents can lead to increased material erosion at the contact points in the case of metal alloys.