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The tribological performance of a hydrogenated amorphous carbon coating on Ti6Al4V against ceramic balls was checked by simple reciprocating sliding tests and by more complicated tests in a hip joint simulator. 3 of 4 joints failed during hip simulator testing already after approximately 100 × 10³ cycles and possibly due to coating failures caused by insufficient polishing of the metallic acetabular cups prior to coating deposition. Since such failures occurred only occasionally, the problem was not revealed by reciprocating sliding tests which are stressing a much smaller surface area compared to the tests in the hip joint simulator. Another type of failure, pits of 12 µm in diameter distributed randomly at the coating surface, was frequently observed by Scanning Electron Microscopy and analyzed comprehensively by Focused Ion Beam technique in combination with Transmission Electron Microscopy in one case. Although not necessarily affecting wear, such small failures might cause long-term problems in vivo, by providing access of body fluid to the substratecoating interface making it susceptible to corrosion.
The tribological behaviour of different monolithic and composite ceramics was evaluated in the temperature range between room temperature and 750°C. The test method was oscillating sliding with a ball-on-disk arrangement in an SRV machine. Alumina balls were used as counter body. The friction behaviour was determined on-line, and the wear behaviour was determined from calculations on the basis of wear scar dimensions and profilometric measurements. The friction depends on temperature and shows an increase for most materials for increasing temperature; the smallest friction at all temperatures is found for monolithic TiC. The wear behaviour shows different trends for the different materials. In tests against SiC a maximum of wear is found at 500°C, for TiC at 200°C and for TiB2 at 750°C. The composite ceramics suffer the smallest wear of all materials in the range from 200°C to 500°C.
The tribological behaviour under dry sliding conditions of Magneli phase Ti4O7 and e-phase Ti2Cr2O7 with α-alumina as a sliding partner has been characterised at room temperature by oscillating sliding. The mechanical properties as well as Vickers hardness measurements do not reveal big differences between the materials, and 4-point bending strengths are around 200 MPa for both materials. Although the friction coefficients are in the medium range of 0.6 and do not differ much between the materials, the wear behaviour is very different. At room temperature, the wear coefficient of Ti2O7 is strongly dependent on relative humidity, and wear is highest under dry conditions when relative humidity is low as it is typically found for many ceramic materials. Ti2Cr2O7 shows remarkably different wear behaviour with the lowest wear in dry conditions. The measured coefficients of wear at low relative humidity of 4% are more than 2 orders of magnitude lower than those for Ti2O7.
Stribeck curve
(2014)
The importance of lubricants in virtually all fields of the engineering industry is reflected by an increasing scientific research of the basic principles. Energy efficiency and material saving are just two core objectives of the employment of high-tech lubricants. The encyclopedia presents a comprehensive overview of the current state of knowledge in the realm of lubrication. All the aspects of fundamental data, underlying concepts and use cases, as well as theoretical research and last but not least terminology are covered in hundreds of essays and definitions, authored by experts in their respective fields, from industry and academic institutes.