M. Kalin

Comparison of different theoretical models for flash temperature calculation under fretting conditions

The wear and friction properties of tribological interfaces depend significantly on the contact temperature, and its determination is therefore important for each tribological application. Temperature calculation methods available in the literature use quite different physical, dynamic and geometrical assumptions. Furthermore, the assumptions necessary for temperature calculations also include various interfacial properties, which are usually unknown due to many difficulties in their exact determination. It is therefore important to know the possible differences between several frequently used models for flash temperature calculation and also the effect of these pre-assumed input parameters. In the present work the effects of the tribological interface between silicon nitride and steel under dry and boundary lubricated fretting conditions were studied. Effects of the change of thermal properties, as well as the coefficient of friction and the real contact area on the calculated flash temperature are presented. Ten different theoretical models were selected for the purposes of this investigation. The results show crucial differences between the various models and the significant importance of the tribological interface properties on the calculated temperatures. Based on these calculations, supported by experimental evidence, it is clear that their severe limitations must be considered and care in the interpretation of the results taken when such models are used.

Use of equations for wear volume determination in fretting experiments

In every tribological application, the extent of damage or surface deterioration is of interest. There are several methods of evaluating the wear volume/loss, which can be roughly classified into weighing, topographical analysis, and 2D analysis by means of empirical equations. In tribological research where many specimens need to be analysed, a simple and fast procedure is desirable for wear volume/loss determination. Methods with time-consuming specimen preparation or measuring procedures are not appropriate in these cases. Sometimes, this is accompanied by very low wear volumes/losses, as is the case in most fretting experiments, which also limits the adequacy of some methods. The use of empirical equations is by far the easiest, fastest and cheapest way to obtain the wear volume in a tribological test, but it has one major disadvantage, i.e., its accuracy may be poor. In this paper we compare calculated values from three different equations with stylus-tip profilometry determination of wear volume, based on the results of a comprehensive study of fretting wear of steel and ceramics. The effect of different material combinations, amplitude of oscillation, size of the wear scar and lubrication on the accuracy of these equations is presented. Justification for the use of 2D analysis in terms of empirical equations in research with many specimens used and low-extent wear is discussed.

Wear and friction behavior of alumina ceramics in aqueous solutions with different pH

It is well known that the wear and friction behavior of ceramics can be significantly improved by using them in water or humid air rather then a dry atmosphere. Accordingly, various ceramics have found many water-lubricated applications. In spite of this, the effect of the pH of the aqueous media on the wear and friction behavior has not been investigated in detail. In this study, we have investigated the wear behavior of alumina ceramics in different water-lubricated conditions with a range of pH values from 0.85 to 13. Based on the results of reciprocating sliding tests, we found that the wear can vary by as much as one order of magnitude and the coefficient of friction between 0.2 and 0.6, depending on the conditions. We also observed that significantly different wear surfaces are generated for different pH values, and these surfaces have a diverse effect on the wear and friction behavior. Wear mechanisms were established by employing surface topography analyses and scanning electron microscopy (SEM). The chemical and electrochemical effects under the selected tribological conditions are discussed to help explain the observed behavior. Our findings suggest that by varying the pH of a solution we can obtain low-wear and/or high-wear of alumina ceramics to suit the requirements of the process.

Wear mechanisms in oil-lubricated and dry fretting of silicon nitride against bearing steel contacts☆

Abstract The wear and friction behaviour of silicon nitride against bearing steel was investigated under lubricated and dry fretting conditions as a function of amplitude and test duration. Tests were performed on a high frequency fretting tester. Silicon nitride bearing balls were used as the upper oscillating specimens while the lower stationary flats were standard specimens of bearing steel. Amplitudes in the intermediate 5 to 50 μm range and a test duration from 10 to 360 min were studied. In lubricated conditions a commercial lubricant. ISO VG 220, was used. Light microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Auger spectroscopy (AES) and transmission electron microscopy (TEM) were employed to determine the wear mechanisms. Under lubricated conditions transition from high to low wear volumes was recognised with increasing amplitude. At lower amplitudes and in the early stage of fretting tests at moderate amplitudes, mechanical wear dominated. Cracks on the stick-slip boundary and spalling of a thin tribolayer was observed. Under these conditions the highest wear in lubricated fretting was obtained. In the final stage of fretting tests at moderate amplitudes, and from the beginning at higher amplitude, tribochemical wear is suggested as the dominant wear form. A 0.2 μm thick tribolayer was observed on the contact, containing inclusions with different Fe and Si contents. A very high concentration of carbon, formed by oil degradation, was also determined in this layer, confirming the critical influence of oil on the wear behaviour. Quite a different wear mechanism is proposed for dry fretting conditions. Results of AES analysis showed a layer an order of magnitude thicker than in lubricated fretting, also having a remarkably different chemical composition. TEM analysis confirmed that the reaction layer consisted of a silica-rich amorphous phase containing small inclusions of Fe2O3 and Fe3O4. In contrast to lubricated conditions, where the layer created was ductile, in the case of dry fretting the layer was brittle. The continuous process of forming and spalling the brittle tribolayer caused much higher wear rates and wear losses than under lubricated fretting conditions. No transition in wear behaviour was observed as was the case in lubricated fretting.

How anion and cation species influence the tribology of a green lubricant based on ionic liquids

A group of halide-free ionic liquids involving two different anions (methyl sulfate and methyl sulfonate) and four types of cations (short-chain tetraalkylammonium, dialkylpyrrolidinium, choline, and methoxycholine) were investigated as 2.5 wt% additives in glycerol as a model base fluid, yielding highly biodegradable polar lubricants for study of ionic liquid interaction with a substrate. The results were compared to the behavior of conventional bis(trifluoromethylsulfonyl)imide ([Tf2N]) ionic liquids with identical counter-ions. The neat ionic liquids (100 wt%) were tested in identical manner and compared to the behavior when they operate as additives. Tribotests were performed in a ball-on-disc configuration under boundary conditions, by lubricating steel–steel couples at room temperature and at 100°C. Wear reduction was achieved for all temperatures, and the results were strongly anion-dominated, with good results for methyl sulfates and the [Tf2N] references. Particularly for higher temperatures, ionic liquids were also able to reduce friction by a substantial amount, with a clear order between the individual anions, and the lowest values were again obtained for methyl sulfates. Cationic influence on the test results was found to be subordinate for both temperatures. It could be recognized that at elevated temperatures, the newly formulated lubricants containing an ionic liquid as an additive behaved similarly to neat ionic liquids in terms of friction and wear reduction. Using X-ray photoelectron spectroscopy analyses, the formation of a beneficial iron sulfide film was detected, with the sulfur originating from the sulfate of the ionic liquid, presumably as a result of a redox reaction with metallic iron. For this mechanism, a hypothesis for possible reaction pathways was developed.

High temperature phase transformations under fretting conditions

Different and sometimes contradictory assessments of the importance of thermal and mechanical effects on tribochemical reactions in fretting have been reported in the literature. In our study of fretting wear between steel and silicon nitride ceramics, tribochemical reactions were determined to play the critical role under both non-lubricated and oil-lubricated conditions. Various possible effects on the resulting tribological interfaces are discussed based on the experimental evidence of microstructural changes, phase transformations, various thermodynamic and flash temperature calculations, and interaction couple experiments in which chemical wear was isolated by controlled mechanical and thermal parameters. It is shown here that a very high temperature must be considered as the most important factor influencing in tribochemical reactions and various phase transformations, even under conditions which are usually considered to be low speed-low temperature conditions.

Chemical aspects of wear of alumina ceramics

Abstract In our investigation, the effects of the tribochemically-induced dissolution of alumina ceramics and modulation of near-surface forces (surface charge) within the tribocontact were studied. The wear and friction behaviors of alumina were investigated using a reciprocating sliding test in different chemical environments. The samples for the tests were hemispherical pins and plates of polished alumina, both prepared by a near-net-shaping method. The sliding tests were conducted in water-based liquids with different pH values or with the addition of a polyelectrolyte to control the surface charge at solid surfaces. The coefficient of friction was continuously recorded during the tests and the wear-loss was subsequently determined for all samples. The results show a significant effect of the chemical agents on the coefficient of friction as well as on the material-removal rate in different aqueous media. The results are discussed in terms of the chemical and electrochemical properties of the materials in the tribocontact.

Pyrrolidinium sulfate and ammonium sulfate ionic liquids as lubricant additives for steel/steel contact lubrication

In this work, we report on the BuMepyr-MeSO4 and Et3MeN-MeSO4 ionic liquids that were synthesized and used as additives in a glycerol model lubricant for steel/steel contacts. Tests were performed with three different ionic liquid concentrations, i.e. 0.625 wt%, 2.5 wt% and 8 wt%, as well as in glycerol without any ionic liquid (neat glycerol) and in neat ionic liquids (100%) at 100 °C. The wear and friction were measured and the worn surfaces were examined with scanning electron microscopy and atomic force microscopy. The results show a reduction of the wear and friction with the use of ionic liquids as additives, when compared to the neat glycerol. With an increasing ionic-liquid concentration in the glycerol, the friction was observed to decrease and the wear to increase. In this work, however, the results obtained for neat ionic liquids represent the lowest values in terms of both friction and wear.

Wear Behavior of Deep-Cryogenic Treated High-Speed Steels at Different Loads

Tools for the cold-working applications are typically made from the high-speed steels. However, due to wear and plastic deformation their performance in several applications is not adequate and should be further improved. By using appropriate combination of vacuum heat-treatment in conjunction with a deep-cryogenic treatment (duplex treatment) the microstructure of high-speed steel matrix can be substantially changed and the hardness and fracture toughness can be modified and optimised. In the present work we have investigated the effect of four different tempering temperatures of vacuum and cryogenically treated ESR AISI M2 high-speed steel on the resulting combinations of microstructure, hardness and toughness and their effect on the wear mechanisms at different loads. The results showed that at relatively high loads the different treatments resulted in an order-of-magnitude difference of wear resistance, while at low loads the selected treatments were efficient enough to keep the wear within the mild wear regime and small variations between the samples. However, the overall wear transition did not occur at any load used or any sample treatment, although some small differences in wear mechanisms can be seen, primarily depending on the fracture toughness of the samples.

Effect of counterface roughness on abrasive wear of hydroxyapatite

The present study was undertaken to evaluate the effect of surface roughness of glass-infiltrated alumina on the abrasive wear of hydroxyapatite using a pin-on-disk tribometer. Hydroxyapatite was used as a model material to simulate tooth enamel. The wear tests were conducted in distilled water at room temperature using a constant sliding speed and three loads to model the normal occlusal contact conditions. The wear volume of polished hydroxyapatite pins increased by more than 20-fold as the average roughness of the alumina disks increased from 14 to 649 nm Ra. No measurable wear was detected on the alumina specimens. The wear surfaces were viewed in a scanning electron microscope (SEM) and were analyzed with X-ray dispersion spectroscopy to determine the extent of surface damage and the wear mechanisms. It is suggested that polished ceramic restorations will cause relatively low enamel wear, while increased roughness could severely abrade and damage the tooth enamel.