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- Zeitschriftenartikel (19) (entfernen)
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- Fretting (5)
- Contact pressure (3)
- Roughness (3)
- Wires (3)
- Atomic force microscopy (2)
- Coating (2)
- Dry sliding (2)
- Wear (2)
- A-C:H (1)
- AFM (1)
Main focus was on the correlation of the coefficient of friction (COF) with the geometrical parameters of the surface during early stage of the running-in period. TiN, TiAlN and AlTiN coatings were prepared by using rotating cathode arc evaporation unit. The roughness of the WC-Co substrate Ra ranged from 0.05 to 0.2 µm and the coating thickness was 2.3 ± 0.2 µm. Part of the samples were subjected to the surface modification by the drag grinding finishing. Geometrical parameters of the surface were evaluated by means of atomic force microscopy (AFM) and fretting tests were conducted with 10 mm diameter alumina ball under 2 N load, 100 µm slip distance and the frequency of 10 Hz. It was found that the distribution of macroparticles strongly influences the geometrical parameters of the surface and the number of macroparticles increases with increasing Al content in the coating. In the case of the positive skewness Ssq of the coating surface, the kurtosis Sku provides a statistically significant effect on the COF, namely the COF depends inversely on the kurtosis Sku. The kurtosis value for TiN was the highest among as-deposited coatings, subsequently, the slowest increase rate in the COF was observed for the TiN coating.
After having checked in a previous study the influence of stroke and normal load in laboratory fretting tests accomplished with thin steel roping wires, in this paper the effect of the contact pressure (test configuration), environmental conditions (relative humidity) and reduction of the friction coefficient (lubricants) on the wear behaviour were analysed. The wires were 0.45 mm in diameter and the material was a cold-drawn eutectoid carbon steel (0.8% C) with a Tensile Strength over 2800 MPa. The tests were performed on an oscillating test rig and the volumetric wear of both specimens of the tribocouple was determined separately from three-dimensional topographies of the wear scars acquired by means of a confocal profiler, and a calculation algorithm developed in using MATLAB®. The strong effect of humidity and lubrication on wear behaviour was proved, whereas a slight effect of contact pressure was derived.
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.
Zero wear (Null Verschleiß)
(2013)
Different fretting tests of thin steel wires under different conditions (loads, strokes, number of cycles and mean pressures) were performed. The wires used had 0.45 mm in diameter and the material was a cold-drawn eutectoid carbon steel (0.8% C) with a tensile strength over 2800 MPa. The tests were carried out with 90° "crossed cylinders" configuration varying the stroke, normal load and testing time (number of cycles). Afterwards the volumetric wear was obtained using surface profilometry with diamond stylus.
With a classical study of the coefficient of wear with respect to the sliding distance, the results show that the wear coefficient increases with normal load and stroke, consequently, for the same sliding distance, the wear produced was higher when the load and/or stroke is increased. In this work, the coefficient of wear is decreasing continuously as the test is running. In order to analyze better the wear behaviour, another method, in which the volumetric wear is compared with respect to the Archard's loading factor, is proposed. As result of this method, two distinctive behaviours were detected; one corresponds to the running-in period with a more aggressive wear behaviour and the second one to the stable steady state period. In all the tests the running-in period was similar with regard to the wear rates but in the stable period the wear rates were increasing with higher loads or strokes.
The tribological behaviour of unfilled thermoplastic polyurethane (TPU) and a polymer sphere filled (TPUG) thermoplastic polyurethane have been studied under oscillating sliding condition against cast iron as a counterpart. In the case of unfilled TPU, the wear mechanisms are dominated by particle detachment and roll formation. In principle, TPUG also showed a similar wear mechanism as that of unfilled TPU; in addition, particle pull-out and delamination are also observed. Wear volume of TPUG was significantly higher than that for the unfilled TPU and this is attributed to the different material removal processes taking place in the material during sliding. The polymer spheres as a filler material deteriorated the wear resistance of TPU because of improper adhesion and bonding of filler in the TPU matrix and therefore it contributed to more wear. In case of TPU the friction behaviour was strongly dependant on the temperature and surface roughness of the counter body. The results showed that below the glass transition temperature higher friction values are observed with higher counter body surface roughness. However, above the glass transition temperature, higher friction values are observed with a smoother surface roughness of the counter body. In case of TPUG, the friction behaviour was not significantly dependent on surface roughness of the counter body.
The wear behaviour of thin steel wires has been analyzed under oscillating sliding conditions in crossed cylinders contact geometry. The focus of this analysis was the influence of the crossing angle between the wires on the wear. The wires used had 0.45 mm in diameter and the material was cold-drawn eutectoid carbon steel (0.8% C) with a tensile strength higher than 2800 MPa. Two different types of tests were carried out, the first one representing the influence of the crossing angle for a constant load and the second one representing the influence of the crossing angle with constant contact pressure. In the first type of tests it was seen that as the contact angle decreases the contact pressure decreases too and hence less energy specific wear resistance is observed. As a consequence less wear is produced, thus increasing the life of the wires. In the second type of tests it was seen that with constant contact pressure but different crossing angles, nearly the same energy specific wear resistance was observed. This points at an identical wear behaviour in both type of tests but with a running-in and a steady state period as two different wear periods. The tests showed that the running in period may play an important role in the overall wear particle generation and hence the wear occurring in the steady state period is rather mild.
The high temperature tribological performance of tetrahedral amorphous carbon coatings has been analyzed at elevated temperatures up to 250 °C in air against three different counterbody materials-steel 100Cr6, a-alumina and silicon nitride. The results show that the counterbody material influences the friction and wear behavior and therefore coating life time strongly. This effect is well known for these coatings at room temperature under dry environmental conditions, equivalent to conditions above 100 °C when water molecules desorb from the surface. However, the sharp difference in tribological performance between silicon nitride on the one hand and alumina and steel on the other hand cannot be understood in this context. Analyzing the friction behavior during the running-in phase, it is evident that only alumina and steel form a stable interface with constant low friction and relatively low wear rates. Silicon nitride forms an unstable interface with fluctuating COF and relatively high wear rates due to its own inherent tendency to tribo-oxidation.