J. Vižintin

Tribological properties of plasma nitrided and hard coated AISI 4140 steel

Abstract In the present study, samples made of AISI 4140 steel pre-treated with plasma nitriding and coated with different PVD coatings (TiN, TiAlN and ta-C) were investigated in terms of their microhardness, surface roughness, scratch adhesion and dry sliding wear resistance. Wear tests, in which duplex-treated pins were mated to hardened ball bearing steel discs, were performed with a pin-on-disc machine. To examine the influence of the nitrided zone on the performance of the coating–substrate composite, coatings were deposited on hardened as well as on plasma nitrided samples, prepared under different nitriding conditions. The results of the investigation showed improved mechanical and wear properties of the plasma nitrided hard-coated specimens compared to the uncoated and pre-hardened ones. Furthermore, the compound layer was found to act as an intermediate hard layer leading to superior sliding wear properties of the composite.

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.

Tribological properties of an environmentally adopted universal tractor transmission oil based on vegetable oil

Universal tractor transmission oil (UTTO) is multipurpose tractor oil formulated for use in transmissions, final drives, wet brakes and hydraulic systems of tractors employing a common oil reservoir. In the present work, the results of a series of tests performed to evaluate vegetable based oils are presented. Tribological properties of formulated rapeseed and high oleic sunflower based UTTO were investigated in the standard and non-standard test procedures and compared with the properties of commercially available synthetic and mineral UTTO. Performances of test oils were demonstrated by using SRV high frequency test device, four-ball test rig, and FZG spur gear test rig. For final tests a laboratory hydraulic system and a spur gear test rig were 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.

Tribological properties of plasma and pulse plasma nitrided AISI 4140 steel

Abstract Plasma nitriding is usually used for ferrous materials to improve their surface properties. Knowledge of the properties of thin surface layers is essential for designing engineering components with optimal wear performance. In our study, we investigated the microstructural, mechanical and tribological properties of plasma- and pulse plasma-nitrided AISI 4140 steel in comparison to hardened steel. The influence of nitriding case depth as well as the presence of a compound layer on its tribological behaviour was also examined. Plasma and pulse plasma nitriding were carried out using commercial nitriding processes. Nitrided samples were fully characterised, using metallographic, SEM microscopic, microhardness and profilometric techniques, before and after wear testing. Wear tests were performed on a pin-on-disc wear testing machine in which nitrided pins were mated to hardened ball bearing steel discs. The wear tests were carried out under dry conditions where hardened samples were used as a reference. The resulting wear loss as well as the coefficient of friction was monitored as a function of load and test time. Several microscopic techniques were used to analyse the worn surfaces and wear debris in order to determine the dominant friction and wear characteristics. Results showed improved tribological properties of AISI 4140 steel after plasma and pulse plasma nitriding compared to hardening. However, the compound layer should be removed from the surface by mechanical means or by decreasing the amount of nitrogen in the nitriding atmosphere, to avoid impairment of the tribological properties by fracture of the hard and brittle compound layer followed by the formation of hard abrasive particles.

Tribological behaviour of WC/C coatings operating under different lubrication regimes

Abstract The aim of the present work was to investigate the tribological behaviour of oil-lubricated WC/C coated surfaces, operating under different lubrication regimes. Tests were performed in a pin-on-disc and a load-scanning test rig, where WC/C coated samples were loaded against coated and uncoated ones, while the steel–steel combination was used as a reference. All material combinations were tested under starved, boundary and elastohydrodynamic (EHD) lubrication conditions, using four lubricants; a pure poly-alpha-olefin (PAO) oil, PAO mixed with commercial sulfur-based extreme-pressure additive or zincdithiophosphate-based anti-wear additive and a fully formulated gearbox oil. The results of this investigation show that application of WC/C coating, especially when using a WC/C–steel combination, gives faster and smoother running-in behaviour and improved tribological properties of the contact surfaces running under starved or boundary lubrication conditions. However, the combination of two coated contact surfaces leads to increased wear and high friction under EHD lubrication conditions.

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.

Wear resistance of pulse plasma nitrided AISI 4140 and A355 steels

In this study, the wear resistance of pulse plasma nitrided AISI 4140 and A355 steels was evaluated under dry and lubricated sliding conditions and compared with conventional plasma nitrided samples. After surface characterisation using metallographic, microhardness and surface examination techniques, sliding wear tests were performed on a pin-on-disc machine in which hardened ball bearing steel discs were used as the counterface. The influence of surface treatment, steel composition, and testing conditions on the wear resistance of the surface treated pins was determined. The test results showed that the wear resistance of the investigated steels could be improved by means of plasma and pulse plasma nitriding. However, compared with conventional plasma nitriding, pulse plasma nitriding lead to very similar wear properties of the nitrided steel surface. Also, the steel composition has an important effect on the properties and wear resistance of the plasma nitrided steel surface. Addition of Al, an alloying element with a high affinity to nitrogen, increased the hardness and residual stresses, and improved the wear resistance of the nitrided steel surface.

Wear and friction behaviour of duplex-treated AISI 4140 steel

Abstract In this study samples of AISI 4140 steel were pretreated by plasma nitriding and coated with two different physical vapour deposited coatings (TiN and TiAlN). A hardened AISI 4140 sample and a coated sample were also included in the investigation. To examine the influence of the nitrided zone on the performance of the coating–substrate composite, two different nitriding conditions — a conventional 25% N2 and an N2-poor gas mixture — were used. The specimens were investigated with respect to their microhardness, surface roughness, scratch adhesion and dry sliding wear resistance. Wear tests in which the duplex-treated pins were mated to hardened ball bearing steel discs were performed in a pin-on-disc machine under dry sliding conditions. Metallography, scanning electron microscopy and profilometry were used to analyse the worn surfaces in order to determine the dominant friction and wear characteristics of the samples investigated. The results show improved wear properties of the plasma-nitrided hard-coated specimens compared with uncoated and pre-hardened ones. Although previous investigations showed a negative effect of the compound layer, it was found that a precisely controlled plasma nitriding process can lead to a dense, uniform and highly adherent compound layer with a positive effect on the wear properties of pre-nitrided and hard-coated AISI 4140 steel.