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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 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.
An extended study on tribological properties of carbon nitride coatings under oscillating sliding conditions has been carried out. The coatings were prepared by reactive magnetron sputtering with graphite target using nitrogen as reactive gas in an argon/nitrogen atmosphere during the deposition process to deposit carbon nitride, CNx. Coating thickness was about 2 µm. A 200300 nm thick Ti interlayer was used to enhance adhesion. The nitrogen flow during the deposition was varied to control the amount of nitrogen in the coating. Maximum amount of N-content was about 28 at%, achieved at a maximum N2 flow rate. Other deposition parameters were target to substrate distance, substrate temperature during deposition and bias voltage. The results show a large variation of hardness depending mainly on the nitrogen content and bias voltage. Higher bias voltage produced harder coatings as did lower nitrogen content. The tribological properties were tested unlubricated and lubricated by paraffin oil under gross slip fretting conditions at room temperature against alumina as counter material. Most coatings, but not all, showed a more or less pronounced dependence of wear on relative humidity with wear rates from 10-5 to 10-6 mm3/Nm. The best coatings with regard to a low wear rate and the least dependence upon relative humidity were carbon nitride coatings deposited at elevated substrate temperatures up to 350 °C. The wear resistance of these coatings was increased under the most severe conditions by about one order of magnitude. The results are discussed on the basis of a transfer layer formation. The results show clearly that wear is not significantly correlated to the coating hardness which itself is strongly dependant on the nitrogen content. However, under lubricated conditions with paraffin oil, wear rates were generally very low with wear coefficients at about 10-8 mm3/Nm.
A new method for sampling wear particles directly from the lubricant reservoir has been developed and applied successfully for analyzing wear particles by high-resolution scanning electron microscopy in transmission mode having coupled energy-dispersive X-ray spectroscopy. The lubricated tribological testing was carried out with fully formulated as well as with non-formulated synthetic base oil. It was possible to analyze individual particles with dimensions as small as about 5–30 nm which are likely the 'primary' wear particles. A majority of the particles, however, are agglomerated and, thus, lead to the formation of larger agglomerates of up to a few micrometers. Chemical analysis led to the conclusion that most of the observed particles generated in formulated oil, especially the larger ones, are composed of the additives of the lubricant oil. In non-formulated base oil, the primary particles are of similar dimensions but contain only iron, chromium and oxygen, but most likely stem from the mating materials. This finding points to the fact that the main wear mechanism under lubricated conditions with fully formulated oil is more like a continuous shearing process rather than a catastrophic failure with the generation of larger primary particles. When the oil is non-formulated, however, several wear mechanisms act simultaneously and the wear rate is increased significantly. Generated larger primary particles are milled down to the nanoscale. When the oil is fully formulated, wear mainly takes places at the additive layer or tribofilm; thus, the steel surface is protected.
The influence of temperature and counterbody material on the tribological properties of a-C:H coatings deposited on Cronidur 30 steel has been investigated in a lubricated ball on disk contact situation. The results show, that the wear volumes of the system increase exponentially with increasing temperature. Two different wear mechanisms seem to have a major influence: First, the abrasive action due to materials hardness and second, the tribooxidation when silicon nitride is counter material.
The influence of temperature and counterbody material on the tribological properties of a-C:H
coatings deposited on Cronidur 30 steel has been investigated in a lubricated ball on disk
contact situation with alpha-alumina and silicon nitride as counterbodies. The results show, that the wear volumes of the systems increase exponentially with increasing temperature, for alpha-alumina more than for silicon nitride. Two different wear mechanisms seem to have a
major influence: First, the abrasive action due to materials hardness and second, the tribo-oxidation when silicon nitride is counter material.