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- Wear (12)
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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.
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.
Wear mechanisms, as adhesion, abrasion, fatigue and tribochemical wear, are complex in their physical and chemical nature. A theoretical description and prediction of wear are in most cases still far from reality. It is, therefore, important to use test rigs before a practical application of a given material combination under tribological loading.
On the other hand, model equations can be helpful for wear description, if a single wear mechanism is dominant.
Under oscillating and continuous sliding contact conditions, equations for wear calculation are presented to describe running-in and stationary wear behaviour of metals and ceramics. By using shear energy density, real area of contact, flash temperature, activation energy and numerical simulations with the method of movable cellular automata (MCA), wear data were calculated and compared to experimental laboratory results.
Ceramic particulate composites in the system SiC-TiC-TiB2 sliding against SiC and Al2O3 under water
(1999)
The tribological behaviour of SiC, SiC-TiC and SiC-TiC-TiB2 was determined in oscillating sliding against SiC and -Al2O3 in water at room temperature. The tribo-systems with the composite materials containing TiC and TiB2 differ significantly from the systems with the single phase SiC: The wear is reduced and the friction is increased. The wear reduction up to a factor of 10 is mainly due to the formation of an oxide film containing titanium oxides which is soft, stable in water and well adhering to the bulk material. This oxide film is transferred to the alumina ball but not to the silicon carbide ball.
Die Dispergiereigenschaften von verschiedenen nanoskaligen Pulvern mit unterschiedlicher Kornfraktion im Bereich von 10 bis 250 nm werden in verdünnten und konzentrierten keramischen Schlickern mit der Ultraschallspektroskopie untersucht. Alle stabilen Suspensionen weisen im entsprechenden pH-Bereich eine monomodale Partikelgrößenverteilung auf. Durch die Charakterisierung der Suspensionen mit der Ultraschallspektroskopie kann eine verbesserte Bewertung hinsichtlich Homogenität und Agglomeratbildung auch für nanoskalige Pulver erhalten werden. Dies ermöglicht eine bessere Qualitätskontrolle prozessrelevanter Schritte bei der Verarbeitung.
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.