Cyclic loading effect on tribological tests during intermetallic contacts of long chained hydrocarbons

Abstract

High value of coefficient of friction was obtained for dry friction between AISI 304 stainless steel specimen and counter body made from 100Cr6 steel. Aliphatic hydrocarbon liquid films of dodecane (C₁₂ H₂₆ ) and monounsaturated oleic acid (C₁₈ H₃₄ O₂ ) were used as lubricating layers between the above two contacting pairs. This was carried out to evaluate the effect of alkane and alkene liquid lubricants on the evolution of coefficient of friction. Furthermore, the cyclic loading effect of progressive and regressive loading on coefficient of friction was also evaluated. Sliding speed of the contacting bodies also varied. The loading and sliding effects resulted in interruption of hydrodynamic lubrication, which caused breaks in the superlubricity of these hydrocarbons. Such circumstances led to manifestation of higher coefficient of friction. Frictional behaviour of these contacting bodies during cyclic loading procedure has been correlated with deformation induced morphologies of wear tracks.     

The effect of surface chemistry in lubrication on the tribological behaviour of steel rollers under rolling-sliding contacts

Experiments were carried out on a wear test machine utilising a gearcam adapter to simulate line-contact lubrication. Due to the formation of a surface adsorption layer on the steel substrate, the tribological performance was detected by measuring the voltage for a lubricant with various additive concentrations. The roller wear rate was found to be strongly dependent upon both the rising rate of voltage in the wear process and the time period needed to create a positive voltage. The antiwear effect of differing additive concentrations was evaluated using the measurements of voltage for various operating conditions. An increase in rotational speed, with a lubricating oil with a low additive concentration, decreases the wear rate. A lubricating oil with a high additive concentration does little to decrease the wear rate at low rotational speeds.     

Investigation of the influence of specimen geometry on quench behaviour of steels by X-ray determination of surface residual stresses

Considerable residual stresses may form during quenching due to the differential cooling and the increase in volume accompanying the phase transformations. The design of a part may be entirely responsible for the formation of residual stresses at a critical level and even cracking during quenching. Furthermore, a certain design may be perfectly safe for one type of steel, or one type cooling conditions, and unsafe for another.In this study, an experimental procedure to investigate the influence of specimen geometry on the evolution residual stresses is proposed. The cylindrical specimens with 30 mm outer diameter were prepared from C60 and 90MnCrV8 steel bars. First, solid cylinders were quenched according to different procedures. Then, the treatment giving the minimum residual stress was applied to the hollow cylinders having various hole diameter and degree of eccentricity. By changing the position of holes in the cross-section of the specimens, a thickness gradient as a function of the eccentricity ratio was obtained. Thus, for a given transformation behaviour and quenching conditions, the effect of shape becomes more discriminating on the eccentrically drilled holes. The tangential residual stresses were determined at the specified points along the circumference of the cylinders by X-ray diffraction and d-sin² Ψ technique. The microstructures of the specimens were determined by metallographic investigation also using hardness values and respective CCT-diagrams. The results were discussed considering the microstructural evaluation of the specimens.     

The influence of residual stresses induced by plastic deformation on rolling contact fatigue

A general method is first described for finding the optimum residual stress distribution. Cylindrical test specimens were subjected to a prestress cycle which was able to induce a favourable residual stress distribution, all the specimens being fatigue tested after that. The 10% fatigue life of the prestressed test specimens was more than twice that of the baseline group. Fatigue tests carried out on four groups of ball-bearings also confirm both the existence of the optimum residual stress distribution and the possibility of inducing this distribution by a prestress cycle.     

A study of the influence of steam treatment on the tribological behaviour of steel specimens

The influence of steam treatment (ST) on the tribological behaviour of steel was assessed by Falex, Schwingungsreibverschleiss (SRV) and Amsler tests performed either under dry conditions or using solid MoS₂, graphite, polytetrafluoroethylene (PTFE) or grease lubricants. ST produced a thin layer of magnetite containing small quantities of haematite. MoS₂ was found to be the best lubricant in Falex tests with normal test pieces. Only greaselubricated Amsler couples withstood the full testing cycle. The SRV tests were inconclusive because the endurance time was too short, only a few seconds with MoS₂ lubrication.The influence of ST varied in the Falex tests. No improvement was found with grease-lubricated journals and the load-carrying capacity of MoS₂-lubricated test pieces was decreased. However, it was beneficial for PTFE lubrication. In the Amsler tests the grease-lubricated disks were crazed and magnetite splinters were removed. The performance of the graphite lubricant was lowered by ST and the median lives of MoS₂- or PTFE-lubricated couples were increased although their reliability was impaired. In SRV tests, ST improved the performance of grease and MoS₂-lubricated test pieces. The test results do not confirm the reported improved behaviour of ST components, particularly cutting tools, in workshop practice.     

On the flow of emulsions in tribological contacts

There are numerous instances of technical importance in which multi-component lubricants are utilized either by design or by necessity. In many of these cases one of the components is a liquid while the other component is a gas, or both components are liquids. These mixtures do not exhibit Newtonian behavior even when their components themselves are Newtonian, and thus classical lubrication theory is inapplicable to bearings lubricated with mixtures. Our objective here is to extend hydrodynamic lubrication theory to lubrication with liquid-liquid and liquid-gas mixtures. The extended theory is then applied to journal bearings, roller bearings and to lubrication in cold rolling. We are able to predict several experimentally observed phenomena such as oil-pooling ahead of an elastohydrodynamic lubrication conjunction.     

FTIR spectroscopy and nanotribological comparative studies: influence of the adsorbed water layers on the tribological behaviour

Under ambient conditions, a water film is always present on a silica substrate and generates additional capillary forces between the nanotip and the studied surface. In the present paper, we report AFM measurements of pull-off and friction forces as a function of the temperature and a comparative FTIR spectroscopy study. The AFM results show a net decrease of the forces as the temperature increases, while the IR spectroscopy indicates that the liquid film is removed at high temperature. Consequently, we deduce that a liquid neck is created between the tip and the surface and that the forces measured are mostly capillary forces. The present work shows that temperature studies with AFM can be a useful way to probe the influence of the capillary force in turn to characterize surface properties.     

The tribological behaviour of electroless Ni-P-Gr-SiC composite

The tribological behaviour and wear mechanism of Ni-P-Gr (graphite)-SiC (manufactured by electroless plating) is surveyed in this paper. The worn surface, wear debris and the compositional changes that take place during wear were characterized using scanning electron microscopy (SEM) and energy-dispersive analysis of X-ray (EDAX). By comparison with Ni-P-Gr and Ni-P-SiC, the results indicate that hybrid Ni-P-Gr-SiC composite presented well anti-friction and wear resistance which is resulted from a graphite-rich mechanical mixed layer (GRMML) formed on the contact surface. Hard SiC particles mixed with GRMML played a load-bearing role at high load when relative motion occurs. After heat treatment at 400 °C for 1 h, the wear rate of the hybrid composite decreased with an increase in microhardness. During sliding, the temperature of the hybrid composite occured less change than Ni-P-SiC coating and further guaranteed the stable state of the whole wear process.     

The tribological behaviour of glass filled polytetrafluoroethylene

Polymers and polymer composites are steadily gaining ground over metals in the field of engineering applications in tribology. Laboratory wear tests were carried out under ambient temperatures with no lubricant as well as in distilled water at an average sliding velocity of 0.2 m/s and contact pressures of 2.6–6.4 MPa. Three forms of glass viz. glass fibres, glass beads and glass flakes, each with a content of 25% weight were used in this study. Both hollow and solid glass beads were used. The sliding wear of the different glass filled PTFE composites was dependent on their ability to form transfer films on the counterface. The glass bead filled PTFE showed comparatively thicker films and higher wear rates than other forms of glass filled grades. The glass fibres and solid glass beads showed the lowest wear whilst hollow beads showed the highest under both low and high pressures due to crumbling and crushing of the beads during the sliding process. The glass flake filled PTFE showed relatively high but stable wear results up to 4.5 MPa above which the wear rate increased dramatically. A marginal increase in wear was achieved by using high aspect ratio glass fibres to the PTFE matrix. No correlation between the size of glass reinforcement and wear rate was established. The addition of a lamellar solid lubricant to the glass fibres reduced both the wear and friction of PTFE. The study of the transfer film growth by means of an optical microscope revealed that it was due to the mechanical interlocking of the polymer fragments into the metal asperity valleys. The compositional changes in the transfer film were studied by XPS which, among other things, showed presence of metal fluoride on the metal counterface.     

Formation of tangential pre-stress as a residual stress and its influence on the tribological performance of nodular cast irons

Since any state of residual stress influences the service behavior of a material, it is of particular interest for engineers and designers to know its benefits to machine parts, and where it can be utilized successfully. From this point of view, mechanical tangential pre-stress as circumferential compressive residual stress has been investigated for wear performance. For this purpose, a thick walled cylinder specimen model was established as a tribo-element, and the force, which will form the desired tangential pre-stress representing residual stress, has been calculated by the rules of elasticity theory. In order to compare the effect of residual stress on the wear performance under dry and lubricated conditions, 0.8 R_e (R_e: unidirectional yield strength) were applied to the nodular cast iron (DDK-40, DDK-60) specimens. Wear test results have been evaluated in terms of two different stress levels, 0 R_e and 0.8 R_e, formed on the specimen.     

Distributions of load stresses and residual stresses at notches

The fatigue strengh reduction factor K, can be mitigated or eliminated by suitable surface treatments. Analysis of these affects requires the knowledge of the distributions of load stresses and of residual stresses below the surface of notches. This paper describes a simple, approximate formula to determine load stress distributions and residual stress distributions at notches. The load stress distributions by the present approach were compared with finite element analysis under tension, bending and torsion loading. Residual stress distributions by the simple formula were compared with measured date by shot peening. An example of optimization in surface treatments by such analysis is shown.     

The influence of oxidation on the tribological performance of diester lubricant

The oxidative degradation test of di‐2‐ethylhexyl sebacate (DEHS) was carried out by a sealed stainless steel reactor. Fourier transform infrared spectroscopy and gas chromatography–mass spectrometry (GC–MS) were utilised to characterise the chemical composition and molecular structure of the oxidation products. The results revealed that 2‐ethyl‐1‐hexanol and mono(2‐ethylhexyl)sebacate were major degradation products in the liquid phase. Tribological behaviour was measured using a four‐ball tester. Scanning electron microscopy was employed to investigate the morphologies of the worn surface. Tribological results showed that anti‐wear and friction‐reducing performances of DEHS were highly dependent on the chemical composition of the oxidation products generated during the test. The hydroperoxides formed during the initial oxidation process could deteriorate lubrication properties of DEHS. However, certain amount of highly polar degradation products (acids and alcohols) generated could significantly improve the friction‐reducing performance of DEHS by the formation of boundary adsorption films on the rubbing surfaces. Copyright © 2013 John Wiley & Sons, Ltd.     

Study of the influence of residual stresses on the fatigue strength of samples made of titanium and nickel alloys

In this paper, we compare the results of measurements of residual stresses and study the influence of residual stresses on the fatigue strength of samples made of titanium and nickel alloys after various types of treatment: turning with the use of various lubricating-cooling tools, grinding, and surface plastic strain operation.     

Residual stresses in TiN, DLC and MoS2 coated surfaces with regard to their tribological fracture behaviour

Thin hard coatings on components and tools are used increasingly due to the rapid development in deposition techniques, tribological performance and application skills. The residual stresses in a coated surface are crucial for its tribological performance. Compressive residual stresses in PVD deposited TiN and DLC coatings were measured to be in the range of 0.03-4 GPa on steel substrate and 0.1-1.3 GPa on silicon. MoS₂ coatings had tensional stresses in the range of 0.8-1.3 on steel and 0.16 GPa compressive stresses on silicon. The fracture pattern of coatings deposited on steel substrate were analysed both in bend testing and scratch testing. A micro-scale finite element method (FEM) modelling and stress simulation of a 2 μm TiN-coated steel surface was carried out and showed a reduction of the generated tensile buckling stresses in front of the sliding tip when compressive residual stresses of 1 GPa were included in the model. However, this reduction is not similarly observed in the scratch groove behind the tip, possibly due to sliding contact-induced stress relaxation. Scratch and bending tests allowed calculation of the fracture toughness of the three coated surfaces, based on both empirical crack pattern observations and FEM stress calculation, which resulted in highest values for TiN coating followed by MoS₂ and DLC coatings, being K_C = 4-11, about 2, and 1-2 MPa m^1/2, respectively. Higher compressive residual stresses in the coating and higher elastic modulus of the coating correlated to increased fracture toughness of the coated surface.     

A comparison of the tribological behaviour of steel/steel, steel/DLC and DLC/DLC contacts when lubricated with mineral and biodegradable oils

Diamond-like carbon (DLC) coatings, which can nowadays be applied to many highly loaded mechanical components, sometimes need to operate under lubricated conditions. It is reasonable to expect that in steel/DLC contacts, at least the steel counter body will behave according to conventional lubrication mechanisms and will interact with lubricants and additives in the contact. However, in DLC/DLC contacts, such mechanisms are still unclear. For example, the “inertness” of DLC coatings raises several questions about whether they are able to provide real boundary “lubrication” or whether they are just a “passive” member in these contacts. On the other hand, biodegradable oils, in particular vegetable base oils, possess a good lubricating ability, often much better than mineral or conventional synthetic oils as a result of the large amount of un-saturated and polar components that can promote the lubricity of DLC coatings. Accordingly, in this study, we present the results of experiments under severe boundary-lubrication conditions during reciprocating sliding. We look at the effect of the type of mating surfaces - steel/DLC, DLC/DLC and steel/steel - and the type of oil on the tribological performance of DLC coatings. We compare the wear and friction behaviours of two types of DLC coatings, i.e., a “pure” non-doped a-C:H DLC coating (denoted as a-DLC) and a WC-containing multilayer coating (denoted as W-DLC) tested with a mineral oil and a biodegradable vegetable oil. These oils, which have very different chemical compositions, were used as base oils and also with mild AW and strong EP additives. Among other things, the results confirm the following: (1) coating/coating lubricated contacts can resemble metal-lubrication mechanisms; (2) additives reduce wear in coating/coating contacts by up to 80%; (3) better wear and friction performance are obtained with oils that contain large amounts of polar and un-saturated molecules; (4) a coating/coating combination generally results in less wear than a steel/coating combination.     

The tribological behaviour of mineral oils additivated with polyethylene

The paper investigates the elastohydrodynamic lubrication of concentrated contacts in a rolling bearing operating at moderate speeds. Two base oils with different characteristics, additivated with low-density polyethylene were used as lubricants. Rheological and tribological tests were performed to study the influence of temperature, polymer concentration, and bearing speed. The influence of the addition of a polymer component on the film-forming properties and on wear under high pressure was examined in a four-ball test rig. Experimental data show that the polymer concentration added to the base oil plays an important role in obtaining greater film thicknesses and in preventing wear in elastohydrodynamic contacts.     

A study of the tribological behaviour of perfluoro-polyethers

The tribological behaviour of perfluoropolyethers (PFPEs) was investigated by performing four-ball extreme-pressure (EP), Falex, four-ball wear, Reichert, SRV (both point-plane and plane-plane contacts), Amsler and TE 77 determinations on four commercially available Fomblin Y fluids, which cover the entire viscosity range. For comparison a petroleum-based hydraulic oil was subjected to the same tests.In load-carrying capacity measurements (four-ball EP and Falex tests) the PFPEs also surpassed the hydraulic oil as an EP gear lubricant.The performances of the PFPEs in determinations of their wearpreventing characteristics were variable. They were better than the hydraulic oil in Reichert tests but behaved in a roughly identical way in four-ball wear determinations and were inferior to it in the SRV (point-plane contact) method.The irregular behaviour is due to the different severity of operating conditions. The harsh strains exerted in four-ball EP, Falex and Reichert tests led to elevated contact temperatures, thus causing the decomposition of fluorinated macromolecules. The subsequent reactions of decomposition products originated the fluoride films to which the relevant performances of PFPEs are ultimately due. As the stresses to be endured in the SRV (point-plane contacts) tests had not been so extreme, the temperature at which the thermal decomposition of PFPEs would have begun was not attained.The more fluid the grade, the better was its performance in four-ball wear tests, since a lower viscosity led to a more abundant supply of lubricant at the rubbing spots and thus to a smaller wear rate.Lubrication was hydrodynamic in SRV (plane-plane contact), Amsler and TE 77 tests, and hence no damage took place in them. Friction coefficients, which depended on both testing machine and test conditions, were slightly higher for PFPEs than for the hydraulic oil.     

The effect of substrate properties on tribological behaviour of composite DLC coatings

The unique features of DLC coatings in providing low friction and low wear and, at the same time, causing low wear to the counterface make them very attractive in industrial applications, in improving tribological performance of mechanical components on various substrates. In this study, composite DLC coatings have been deposited on sintered ferrous alloy, M42 tool steel, 2618 aluminium alloy, and 6063 aluminium extrusion substrates using the combined CFUBMS–PACVD technique. The effect of mechanical properties of substrate materials on tribological behaviour of the composite DLC coatings has been investigated at various loads on a ball-on-disk wear machine in dry air. A transition load was usually observed for coatings on the various substrates except for the aluminium extrusion; above the transition load the coating was completely destroyed via some spallation/fragmentation process after 2 h sliding, and the wear rate increased dramatically with further increase in load. The coating system on sintered ferrous alloy substrate exhibited the highest transition load among the four types of substrates studied. This is considered to have resulted from the combined effects of the lower elastic modulus of the porous sintered ferrous alloy substrate, which decreases the stress concentrations in the contact region, and the surface roughness and porosity, which enhance the bonding strength between the coating and the substrate under multi-contact conditions. The high elastic modulus of the tool steel substrate leads to tensile stress conditions in the sliding contact region and therefore makes coatings deposited on such a substrate more prone to breakdown/fragmentation, resulting in a transition load close to that for coatings on the soft 2618 aluminium alloy substrate. For coatings on the 6063 aluminium extrusion substrate, significant plastic deformation occurred in the substrate at loads above 1.5 N. However, despite the heavy deformation in the substrate, coatings on this substrate were not scraped off, as were coatings on the 2618 aluminium alloy substrate, even at a load as high as 20 N. The specific wear rate increased continuously with load, no apparent transition load being explicitly identifiable. This study shows that hard DLC coatings can be applied on both hard and soft substrates for improvement of the tribological behaviour of mechanical components.     

The influence of current load on fretting of electrical contacts

The fretting corrosion behavior of tin coated brass contacts is studied at various current loads (1, 2 and 3 A). The typical characteristics of the change in contact resistance with fretting cycles are explained. The fretted surface is examined using scanning electron microscope, laser scanning electron microscope and energy dispersive analysis of X-rays to assess the surface morphology, extent of fretting damage, extent of oxidation, surface profile and elemental distribution across the contact zone. The degradation of contacts at high and low values of current is explained with reference to the thermal and electrical phenomena occurring at the contact interface. The results showed that irrespective of the current loads under study, the contact resistance is maintained at 1.0±0.02 Ω where the oxide debris formation and the electrical breakdown of oxide particles competed with each other maintaining the equilibrium. The number of cycles to failure of the contacts is delayed at lower current. The fretting corrosion degradation of tin coated contacts occurs much faster at higher currents as it generates more accumulation of oxide wear debris at the contact zone. The observed surface morphology and the tin profile of the fretted surface are in agreement with the experimental results.