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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.
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.
Zeta potential as measured with the electrokinetic sonic amplitude (ESA) method has been compared to streaming potential as measured with the particle charge detection (PCD) method. The measurements have been carried out on identical suspensions of a-Al2O3, SiC and Si3N4 with a solids volume fraction of 1% and 10%. Zeta potential and streaming potential are related by a linear function. The correlation coefficients for this relationship are better than 0.99 for the suspensions with 1% solids loading. For the suspensions with 10% solids loading, correlation coefficients are slightly lower due to the influence of increasing viscosity. Regardless of lower correlation at the higher solids loading, the isoelectric pH is constant within the limits of accuracy of the pH measurements.
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 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.
Tribological properties of ultrahigh-molecular weight polyethylene (UHMWPE) and cross-linked polyethylene (XLPE) were studied in two different wear modes. Firstly, reciprocating sliding wear studies under non-conformal contact investigated the effects of counterface surface roughness (polished, lapped and ground) of Ti6Al4V on the friction and wear of the polyethylenes. Secondly, two-body abrasive wear studies in conformal contact against different abrasive grit size papers were also carried out to ascertain the wear sensitivity of the polyethylenes under these adverse conditions. Wear mechanisms were studied using optical and scanning electron micrographs. The results of the reciprocating sliding wear studies showed that surface roughness of the counterface influenced friction and wear characteristics although no correlation was found between the coefficient of friction and specific wear rate. XLPE demonstrated wear sensitivity, particularly under severe abrasive wear condition. The results indicated that the performance of the polyethylenes greatly depends upon the tribological system under which it is operating.
Tribological properties of neat polyetherimide (PEI), glass, carbon fiber, and solid lubricants filled PEI composites are presented in this article. The aim of this study was to investigate the friction and wear properties of these composites under dry oscillating sliding condition at room temperature (RT) as well as at elevated temperature (120 °C). The polymer specimens were made to oscillate against steel cylinder as a counterpart. The friction and wear properties of PEI and composites were strongly influenced by the temperature. Incorporation of carbon fiber in the PEI matrix has increased the wear rate at RT, while at elevated temperature this trend was opposite. Abrasive action of carbon fibers has severely damaged the counterpart and resulted in accelerated wear of the composite at RT. Solid lubricants filled (PTFE, MoS2, graphite) along with glass fiber is beneficial in improving the friction and wear performance of the PEI composite at RT, whereas at elevated temperature wear performance was deteriorated. Tribological performance of neat PEI and glass fiber composite was similar with each other at RT. Scanning electron micrographs and optical micrographs of the worn polymer specimens and the steel cylinders was used to study the possible wear mechanisms. The present test results were also compared with data available on the reciprocating wear of PEI and composites in the literature and trends have been reported.