Unlubricated Contact Research Articles

Tribological behaviour of alumina sliding against Ti6Al4V in unlubricated contact

Tribological behaviour of alumina balls (99.5%) sliding against a Ti6Al4V disc over a range of loads (5–80 N) and speeds (0.0625–1 m s −1) has been investigated using a pin-on-disc tribometer under unlubricated conditions. The maximum wear coefficient was observed to be several orders of magnitude higher than the reported value for alumina against alumina or alumina against steel counterfaces. When the load or speed increased, the wear rate of the alumina ball increased initially and then decreased, showing typical transition features. On the other hand, the friction coefficients for the Ti6Al4V/alumina tribosystem were found to increase inversely with the applied loads or the sliding speeds. The wear mechanisms and the wear transition were investigated based on examinations of worn surfaces as well as debris using SEM, XRD and XPS, and it was revealed that a tribochemical mechanism accounted for the observed high wear rate of alumina sliding against the titanium alloy.

Lubrication of High Rolling Speed Ceramic Contact with Two Percent Slip at 815°C (1500°F)

A selection of liquid lubricants was vaporized and deposited on a hot, 705°C (1300°F), ceramic surface. Friction was then evaluated in low speed, 7 MPa (1000 psi), sliding contact. The ceramic materials included aluminum oxide, silicon aluminum oxynitride (SiAlON), silicon nitride, and silicon carbide. Each specimen was mated with an equally hot specimen of the same material. Deposits reduced friction significantly compared to the unlubricated contact. Two superior lubricants were then evaluated in continuous lubrication of silicon nitride using a high speed ball-on-disk contact. Rolling speed was 2.5 m/sec at a Hertzian stress of 2 GPa (300 ksi). Operating temperatures were varied between 621° C (1150°F) and 815°C (1500°F). With the slip level maintained at 2%, the traction coefficient recorded at 815°C was 0.05, comparable to current metallic bearings in aerospace applications. Wear volume was reduced by an order of magnitude compared to the unlubricated case. Temporarily stopping the lubricant flow resulted in a steady rise of the traction coefficient, while resuming the lubricant flow reduced the traction to the original level. Presented at the 51st Annual Meeting in Cincinnati, Ohio May 19–23, 1996

Wear behaviour of alumina toughened zirconia materials

This study presents correlations between, on the one hand, wear rates and wear mechanisms and, on the other, toughness and microstructure of highly reactive alumina-based materials containing 0 to 24% partially stabilized zirconia using 1.5% yttria (Y-PSZ). All samples were sintered to 97.5% of their theoretical density (±0.5%). A 4 × 4 Latin square design of experiments was followed. Wear resistance was measured using a ring-on-disc experimental set-up that was conceived to measure volumetric wear and dynamic coefficient of friction under constant sliding speed of an unlubricated contact. Results obtained at a speed of 0.26 m s −1 within a range of applied loads of 50 to 200 N showed that the optimal values of Y-PSZ content needed to minimize the wear rate were a function of these applied loads; the optimal amount of Y-PSZ required to obtain the maximum wear resistance, within the boundaries of the present study, varied from 8% at 200 N to 16% at 50 N. The main wear mechanism changed abruptly from abrasion to chipping above 100 N. As expected, toughness was not the only criterion to predict wear resistance; microstructure control was also important.

The wear and friction of short glass-fibre-reinforced polymer composites in unlubricated rolling-sliding contact

The wear and friction behaviour of short glass-fibre-reinforced polyamide 66 composites running against each other, unlubricated, in non-conformal, rolling-sliding contact has been investigated. Both a wide range of loads and slip ratios and a range of samples with different fibre concentration and different crystallinity have been examined. Short glass-fibre reinforcement makes the polyamide 66 exhibit unique tribological behaviour. There is a high resistance to wear and friction which results from a significant “self-lubricating” property. A thin film layer exists on the contacting surfaces when two discs run against each other within the range of the test conditions. It is this thin film that plays a dominant role in the “self-lubricating” property of the composite. The formation of the thin film and the life of the composite depend on a complex of interactions between structure, strength and fibre concentration, and the specific conditions of load and slip ratio imposed. Under identical loading conditions, either lower fibre concentration or lower crystallinity cause the thin film to form continuously during the wear process so that the life of the composite may reach 6×106−107 cycles. It is suggested that the “self-lubricating” property may be used in the working period of engineering components rather than only during the temporary running-in period of machine elements.

Friction mechanisms in hydrogenated amorphous carbon coatings

Abstract The friction behaviour of unlubricated contacts between a hydrogenated amorphous carbon coating and corundum counterbodies was investigated under reciprocating sliding conditions in ambient air. Tests were performed under various normal forces at controlled relative humidity. The coefficient of friction is shown to decrease as the normal contact force increases. At low (5%) and medium (50%) relative humidities, linear correlations between the coefficient of friction and the inverse of the hertzian contact pressure were observed. A physical interpretation for this behaviour is proposed by considering interfacial shear mechanisms accommodating the sliding motion.

Oxidational wear of TiN coatings on tool steel and nitrided tool steel in unlubricated fretting

The fretting wear behaviour of PVD TiN coatings against corundum has been investigated in unlubricated contacts. Analysis of the debris indicated that titanium oxides of different composition are being generated. The volumetric wear on the TiN coatings is shown to be directly proportional to the dissipated friction energy measured on-line during the fretting tests. Furthermore, the effect of oxide debris present in the vibrating contact on the velocity accommodation and, consequently, on the coefficient of friction is discussed. The hardness of some of the ASP 23 tool steel substrates was increased with a plasma nitriding treatment prior to the coating deposition. It was found that the coefficient of friction and the fretting wear rate of the TiN coatings remain unaltered by the plasma nitriding treatment. After perforation of the TiN coating, however, wear proceeds faster on nitrided tool steel substrates.

The influence of surface finish and strain hardening on near-surface residual stress and the friction and wear behavior of A2, D2 and CPM-10V tool steels

The effect of surface finishing method on near-surface residual stress, relative cold work and sliding friction and wear behavior of commercially available air hardening A2, D2, and CPM-10V tool steels was investigated. Microstructural characterization was accomplished using X-ray diffraction and scanning electron and optical microscopy combined with macrohardness measurements. The wear tests were performed using low load-slow speed reciprocating sliding conditions with tool steel flats in unlubricated contact with a 52100 steel spherical slider. The coefficient of friction and mass loss were measured during the course of wear testing and residual stress measurements were made before and after wear testing. Surface grinding and polishing conditions had a significant effect on surface residual stress but had little effect on tool flat sliding wear rates. Residual stress measurements performed after wear testing showed a decrease in compressive residual stresses as wear rate increased. Tool steel wear rates were a linear function of sliding distance except for the initial run-in period. The low wear rates, the run-in friction behavior, and the constancy of near-surface mechanical properties indicate that during run-in surface finish was wear rate controlling and that sample microstructure and composition controlled long-term sliding friction and wear behavior. The low wear rates and the post wear residual stress measurements also indicate that oxidative wear was the primary mechanism responsible for tool flat material attrition. CPM-10V steel flats exhibited the lowest coefficient of friction and the highest resistance to wear which can be attributed to the high volume fraction and homogeneity of V-Cr-C carbides.

Sliding wear of ceramic-ceramic, ceramic-steel and steel-steel pairs in lubricated and unlubricated contact

Sliding wear tests were carried out on different materials in air, water and oil at ambient temperatures by using a pin-on-ring tribometer. The tests in water and oil were run under conditions of boundary lubrication. The investigated ceramics could be divided into oxides (Al 2O 3, ZrO 2, Al 2TiO 5) and non-oxides (SiSiC). Steels with 0.2–0.9 wt.% C were heat treated to obtain spheroidized, normalized, and quenched and tempered structures. The steels and ceramics were studied with different metallic, ceramic and ceramic-metallic combinations. Scanning electron microscopy and energy-dispersive X-ray analysis revealed different wear mechanisms, depending on the materials and experimental conditions. Plastic deformation, material transfer, tribochemical reaction, grooving and microfracture were observed on the worn surfaces. The results show that ceramics may be more wear resistant than steels but their tribological behaviour is strongly influenced by operating conditions.

Wear tests of grey cast iron against ceramics

The rolling wear of grey cast iron, partially stabilized zirconia, SiC and Si 3N 4 against themselves in unlubricated contact was performed at room temperature. The wear behaviour of grey cast iron against each of the above ceramics was also examined, both unlubricated and lubricated with methanol at room temperature. It was found that the wear life, defined as the onset of severe wear, was highest for the wear of silicon carbide against itself and was approximately ten times that compared with the other materials in this study. The hardness of the ceramics had a strong influence on the wear of grey cast iron in unlubricated contact. The thermal conductivity of the ceramics also influenced the wear of grey cast iron in combination with ceramics when lubricated with methanol.

Metallography of adhesive wear in aluminum alloys

The wear characteristics of two aluminum alloys, 1100 and 3004, were studied using a pin-on-disc wear test with a normal load of 1 kg and a sliding velocity of 0.25 m/s. Measurements were made of the coefficient of friction and wear rate for unlubricated contact between the alloys and a spherical tungsten carbide slider. A two-surface metallographic technique was developed for simultaneous observation of the wear surface topography and the microstructure of the polished section perpendicular to this surface. The observations indicate the following: 1. 1. Large local strains and strain gradients are present adjacent to the wear surface. 2. 2. Second-phase particles provide nucleation sites for surface cracks and decohesion of material from the surface. 3. 3. Adhesive wear appears to involve a process of cumulative damage due to void growth in the stress field beneath the sliding indenter.

Experimental studies of fatigue cracks in sphere-flat contact without macro slip. (1st report The effects of load on crack initiation)

It has become clear that the damage on the contact surface is much affected by the tangential force. However, it seems that its influence over the wide range of contact conditions still remains quantitatively indefinite. In this study, some experiments were performed under various unlubricated contact conditions of a hard metal sphere against a hard metal surface subjected to the oscillating tangential force with several static normal loads. The occurence of damage (especially cracking) on the surface in the contact field without any macro slip has been observed. The effects of the normal load and the range of the oscillating tangential force on the occurence of surface cracking and its advance have been investigated. It was also found that these effects can be explained using stress field.

Tribological properties of MoNx coatings in contact with copper

In metal forming processes, coatings can be used to prevent adhesion between the tool and the workpiece in order to increase the lifetime of the tool. A new promising coating material for copper forming tools is MoNx, which has low galling tendency in an unlubricated contact with copper. MoNx coatings were deposited in a magnetron sputtering system. Tribological tests were carried out in a pin‐on‐disk tribometer. Also the adhesion and structure of the coatings were characterized. The lowest coefficient of friction and the lowest wear rate against copper was measured when the nitrogen concentration in the coating was 15 at. %–18 at. % and the Vickers microhardness about 2000 HV at a load of 10 g.

Forces in a Heavy-Duty Drive Chain during Articulation

A rigid-body analysis is presented of the forces which occur in a chain bearing during articulation. Coulomb frictional effects due to unlubricated contact are included, as these provide the energy source for deleterious wear processes and affect the force distribution. The analysis differentiates between the two types of chain bearing—open end leading or open end trailing—and the results show that the force characteristics in each are significantly different. Experimental verification of the chain link tension is good.

The Thermal Control of Friction at High Sliding Speeds

It is proposed that thermal effects govern friction in high speed unlubricated contact. The general principle can be found from a manipulation of Blok’s flash temperature theory to give the coefficient of friction as a dependent parameter. The principal factors controlling friction, provided the conditions are sufficiently severe, appear to be the decomposition temperature, hardness and thermal properties of the materials. The same mechanism appears from a numerical model of thermal effects in sliding. The model is compared with several sets of experimental results in different configurations.

Boundary lubrication of thermoplastic polymers in model fluids

Organic polymers, particularly thermoplastics, have traditionally been used in unlubricated contacts because of what is termed their ‘self lubricating’ character. Significant improvement in bearing performance may often be obtained by conventional liquid lubrication but a number of subtle interfacial phenomena are sometimes noted some of which are deleterious to good operation. A particular type of problem arises when the fluid migrates to a significant depth into the matrix of the polymer and causes a premature form of scuffing failure under intense levels of energy dissipation. This paper reviews the scuffing behaviour of several commercial polymeric matrices and speculates upon the fluid-polymer interactions which can occur at the onset of lubrication failure.

Magnetron sputtering of non-ferrous aerospace alloys for wear protection

The light-weight non-ferrous alloys (titanium, aluminum and magnesium alloys) used in aerospace applications are prone to severe wear in small- displacement and large-displacement unlubricated contacts. Few metallurgical means are available for the surface protection of this class of materials. Deposition of hard compounds by sputtering provides a simple means of protecting such surfaces. Hard coating and tribological studies of coated samples were therefore carried out. The results obtained are presented in this paper. It is shown that magnetron- sputtered TiN hard coatings do provide the required wear protection.

Macroscopic effects of delamination wear

Measurement using a pin-on-disc machine of periodic changes in the energy parameters of sliding surfaces under conditions of unlubricated contact has provided experimental evidence for the delamination theory of wear. The experimental procedure is suitable for the investigation of lubricated frictional contact.

The assessment of wear-resistant coatings for arduous conditions of rolling contact

A brief review is presented of surface treatments available to combat wear. Using an accelerated service simulation test, various surface treatments have been assessed and compared with sintered tungsten carbide, hot pressed silicon nitride and conventional ball bearing materials under arduous conditions of rolling contact. Chemical vapour deposited titanium carbide and soft nitriding appear to be potentially suitable for surface treatments for wear resistance under conditions of unlubricated contact. Use of in situ lubrication from a solid cage material is beneficial.

Lubrication of sliding point contacts of AISI 52100 steel — the influence of curvature

The lubrication mechanism of fully submerged sliding point contacts of non-oxidized AISI 52100 steel has been studied as a function of radius of curvature in the contact area. The results of experiments, performed with hemispherically tipped pins with radii of curvature R 1 ranging from 1 mm to 23 mm in contact with the curved surfaces of rings of 76 mm diameter, corroborate the existence of three well-defined lubrication regimes, i.e. a regime of thin oil film lubrication, a regime of boundary lubrication and a regime of virtually unlubricated contact. At small radii of curvature ( i.e. R 1 = 1 mm and 5 mm), the contacts derive their load-carrying capacity almost entirely from the presence of a boundary lubricant film. At R 1 = 23 mm, lubrication is mainly due to the presence of a thin oil film.

Problems of friction, wear and lubrication : Vol. 9. Published by Panstwowe Wydawnictwo Naukowe, Warszawa, 1971. (Book)

Electroless nickel (EN) coatings are used in a wide range of applications concerning their excellent mechanical and tribological properties. The incorporation of solid lubricants, such as polytetrafluoroethylene (PTFE), could even improve the properties of the EN coatings. Above all, we can achieve a film with self-lubricating and excellent anti-sticking characteristics. These uses of electroless nickel coatings are widespread in dry contacts. However, it is well known that in the mechanical components there are a large number of applications that require especially low friction, usually not compatible with the use of unlubricated contacts. Moreover, nowadays, there is a general concern with the environmental impact in the use of lubricants. The tendency is to use fluids with small environmental impact, reducing the contamination made by lubricants of mineral origin. Hence, the aim of the present work was to investigate and improve the understanding of the behaviour of EN and EN + PTFE coatings under lubricated contacts using biolubricants.Tests of coated cylinders were done in lubricated contact against hard AISI 52100 steel on a crossed cylinder sliding tester. The tests were carried out varying widely both the sliding speed and the normal applied load. To allow the comparison of different lubricants, Stribeck curves were used. The influence of the different lubricants under study on the wear amount was investigated by measuring the wear scar on the end of each test. Scanning electron microscopy was used to identify the wear mechanisms.