Maximum Hertzian Contact Pressure Research Articles

Peculiarities of tribological behavior of composites based on polyether ether ketone (PEEK) and whisker carbon nanotubes

Herein, the internal lubrication of polyether ether ketone (PEEK) with added whiskered carbon nanotubes (Wh-CNTs) was investigated under point-contact dry sliding conditions with various loads and velocities. The wear rate of the composite was reduced by 86.99% at a maximum Hertzian contact pressure of 344.69 MPa, which was attributed to the addition of additives with excellent mechanical properties and high-temperature stability. Wh-CNTs could be uniformly distributed in the PEEK to form a three-dimensional (3D) network structure with load-bearing capacity, thus the load could be effectively borne to prevent crack propagation on the PEEK surface. Besides, the filler was induced to form a micron-sized 3D spherical structure with a ball bearing effect, which promotes lubrication more effectively than the fragment structure.

Refining the mechanistic understanding of microstructural decay during rolling contact fatigue in 52100 bearing steel tempered at high temperature

Subsurface rolling contact fatigue (RCF) failure occurs beneath heavily loaded hard contacts like gears, bearings, and cams. This study investigates microstructural decay beneath a RCF-tested surface in AISI/SAE 52100 bearing steel tempered at 240 ℃. RCF tests were conducted at 100 ℃ with a maximum Hertzian contact pressure of 4 GPa for four stress cycles. Microstructural characterization utilized scanning electron microscopy, electron backscatter diffraction, transmission Kikuchi diffraction, and transmission electron microscopy. Due to high tempering temperature, white etching bands (WEBs) were observed without preceding dark etching regions. The microstructural decay sequence involved: (1) formation of elongated ferrite and ferrite microbands, (2) complete dissolution of tempered carbides and partial dissolution of residual cementite, (3) formation of WEBs composed of nano-sized ferrite grains (100–300 nm) transformed from ferrite microbands, and (4) appearance of lenticular carbides. Within the WEBs, most nano-sized grains had high-angle grain boundaries, while the fraction of low-angle grain boundaries increased in later stages of RCF. Lenticular carbides formed alongside elongated ferrite and coalesced nano-sized ferritic grains.

Nano and micro-indentation driven characterisation of asperity and bulk plasticity at the surface of modern premium rail steels

The plastic deformation behaviour of rail steels due to cyclic rail-wheel contacts is important to understand due to its connection to wear and rolling contact fatigue (RCF) damage initiation in service. Simulation models such as the ‘Layer’ and ‘Brick’ model have previously been developed to estimate the accumulation of plastic damage in a rail steel; however, the data available to drive these models is currently sparse, with limited applicability to modern rail steel grades. This paper presents the research examining the shear stress-strain curve relationships of rail steels derived from plastic shear strain and shear yield stress data collected from twin-disc test samples. A combination of microhardness and nanohardness testing was used to derive the shear yield stress data, whereas the plastic shear strain was acquired from optical microscopy. Six different conditions were investigated for this research for the purpose of examining how shear stress-strain curve relationships compared between the standard R260 and the premium HP335 and R350HT rail steels and how this compares to wear damage data. The influence of the maximum Hertzian contact pressure on the shear stress-strain curve relationships of R260 between 600 and 1500 MPa contact pressure was also investigated. The wear rate results derived from the mass loss in interrupted twin-disc tests showed HP335 wearing the least, followed by R350HT and then R260 for 1500 MPa, dry contact conditions. However, the highest shear yield stress achieved was for R350HT, then HP335, and R260. The results show that the shear stress-strain curve relationships by themselves are insufficient to determine rail steel wear performance in a laboratory environment. The shear stress-strain curve relationships for R260 collected under different contact pressures showed the results are near independent of the contact pressure within the range explored.

Rolling-sliding contact fatigue failure and associated evolutions of microstructure, crystallographic orientation and residual stress of AISI 9310 gear steel

In the present study, rolling-sliding contact fatigue (RSCF) failures of AISI 9310 gear steel were analyzed systematically, in terms of characterizing the fatigue life, evolution of microstructure and crystallographic orientation, and variation of microhardness and residual stress distributions. The RSCF experiments were conducted using a twin rollers fatigue test rig. The RSCF life of AISI 9310 steel roller decreased continuously with increasing the maximum Hertzian contact pressure, however, it could be effectively improved by additional shot peening process owing to the enhanced microhardness and compressive residual stress near the roller surface. The main fatigue damage characteristics near the roller surface were consisted of inclined cracks, micropitting, spallation and plastic deformation. Although the compressive residual stress was reduced by RSCF, there was an obvious increase in the microhardness which should be attributed to the refined microstructure, increased dislocation density as well as phase transformation from retained austenite to martensite. In addition, the accumulated plastic strain during RSCF resulted into development of several preferred crystallographic orientations corresponding to ideal shear texture components of body-centered cubic (BCC) materials that has been seldom reported.

Discussion of “Contact Unloading Behaviors of Elastic-Power-Law Strain Hardening Material Considering Indenter Elasticity Effect” (Chen, C., Wang, Q., Wang, H., Ding, H., Hu, W., Xie, W., Weng, P., Jiang, L., and Yin, X., 2022, ASME J. Tribol., 144(12), p. 121501)

Discussion of “Contact Unloading Behaviors of Elastic-Power-Law Strain Hardening Material Considering Indenter Elasticity Effect” (Chen, C., Wang, Q., Wang, H., Ding, H., Hu, W., Xie, W., Weng, P., Jiang, L., and Yin, X., 2022, ASME J. Tribol., 144(12), p. 121501)

Tribological performances of a-C films tested on dry nitrogen environment with various Hertz pressures: Failure mechanisms and in-situ formation graphene

The tribological properties of hydrogen-free amorphous carbon (a-C) films are affected by the test environment and load condition. The wear mechanisms of a-C films sliding against Si3N4 ball under inert atmosphere exhibits different mode during different sliding cycles. The results show that the tribological behaviors for a-C films exhibit adhesive wear and it did not wear out even after sliding test 40,000 cycles when the normal load is 1 N. As the normal load increases, the wear mechanisms mainly show fatigue wear and three-body wear induced by fatigue wears due to the increasing of alternating stress. The emergence of three-body wear will drastically change the local stress distribution of a-C film. The position obtaining maximum Von-Mises stress shifts from the subsurface of substrate to subsurface of a-C film, and the maximum Von-Mises stress and Hertzian contact pressure increases sharply. The structures of in-situ formation graphene originated from the transformation of amorphous carbonaceous structure are detected on the wear scars. The formation of carbonaceous transfer film is beneficial to facilitate the graphitization transition with increasing Hertz pressure from 0.65 GPa to 1.35 GPa. On the other hand, the transition temperature of graphitization is sharply reduced to the flash temperature of contact zone due to the emergence of three-body wear obtaining the instantaneous Hertz pressure reached to 29.5 GPa. These conclusions possibly provide valuable references for designing film thicknesses, reveal tribological mechanism for a-C films under dry inertness atmosphere and broaden industrial application.

Self-lubricating coating with zero weight loss performance on additively manufactured Ti-6Al-4V

The poor tribological performance of Ti-6Al-4V alloy severely limits its application as moving components under high contact pressure or long-term sliding conditions. In this work, a novel self-lubricating coating consisted of honeycomb structured Ti-TiN and Polytetrafluoroethylene (PTFE) was developed on the surface of additively manufactured Ti-6Al-4V. The Ti-TiN/PTFE coating exhibited an excellent tribological performance under harsh conditions (ultrahigh maximum Hertzian contact pressure of 1619 MPa, 15 Hz), and with a stable low friction coefficient of 0.12 and a nearly zero weight loss performance after 1 × 106 sliding cycles. The superior tribological performance can be attributed to the robust lubricant film spontaneously generated at the frictional interface, which can be replenished by the continuous release of PTFE due to the capture of abrasive debris. This design strategy could guide the design of novel low friction and wear resistance coatings for additively manufactured metallic components.

Effect of MoP additive on wear properties of cemented carbide

This paper describes the effect of MoP lubricant additive on the wear properties of cemented carbide. In the wear tests, the reciprocating sliding tests were conducted under about 7 GPa of maximum Hertzian contact pressure, and MoP, MoA, MoDTC and ZnDTP were used as additives. The results showed that in tests using Mo type additives the amount of wear in MoP was smaller than that in MoA and MoDTC, and that the film was formed on the contact surface of cemented carbide after the tests. In addition, the wear in ZnDTP was greater than that in MoP, implying that Mo and P elements in the additive affect the formation of anti-wear film on the cemented carbide surface. Moreover, it was also shown by EDX analysis that in the rubbed surface of cemented carbide in the test using MoP, Mo, P, O was detected respectively, and that the concentration of Mo was greater than that in the test using MoA. These results suggest that in the test using MoP the molybdenum trioxide and the phosphorus film were formed o the cemented carbide, resulting that the film formed by MoP should prevent the decohesion of WC grains.

Normal Load and Counter Body Size Influence the Initiation of Microstructural Discontinuities in Copper during Sliding

Near the interface of two contacting metallic bodies in relative motion, the microstructure changes. This modified microstructure leads to changes in material properties and thereby influences the tribological behavior of the entire contact. Tribological properties such as the friction coefficient and wear rate are controlled by the microstructure, while the elementary mechanisms for microstructural changes are not sufficiently understood. In this paper, the influence of the normal load and the size of the counter body on the initiation of a tribologically induced microstructure in copper after a single sliding pass is revealed. A systematic variation in the normal load and sphere diameter resulted in maximum Hertzian contact pressures between 530 MPa and 1953 MPa. Scanning electron microscopy, focused ion beam, and transmission electron microscopy were used to probe the subsurface deformation. Irrespective of the normal load and the sphere diameter, a sharp line-like feature consisting of dislocations, the so-called dislocation trace line, was identified in the subsurface area at depths between 100 nm and 400 nm. For normal loads below 6.75 N, dislocation features are formed below this line. For higher normal loads, the microstructure evolution directly underneath the surface is mainly confined to the area between the sample surface and the dislocation trace line, which itself is located at increasing depth. Transmission Kikuchi diffraction and transmission electron microscopy demonstrate that the misorientation is predominantly concentrated at the dislocation trace line. The results disclose a material rotation around axes roughly parallel to the transverse direction. This study demonstrates the generality of the trace line phenomena over a wide range of loads and contact pressures and the complexity of subsurface processes under a sliding contact and provides the basis for modeling the early stages in the microstructure evolution.

Experimental Analysis of Frictional Characteristics of the Interface between Gantry Crane Wheels and Rails under Submerged Rail Conditions

The purpose of this paper is to clarify the influence of rail submergence that occurs at the quay on the frictional characteristics of the interface between gantry crane wheels and rails. The characteristics serve as a basis for the dynamic simulation analysis of cranes that run away due to wind and crane designs. An experimental analysis system developed uniquely by the authors was used to carry out this research. Wheel was pressed to submerged rail, then the rail was slid. The friction force generated between the wheel and the rail was measured. This experimental analysis was conducted for contact pressure conditions that the ratios of the maximum Hertzian contact pressure to the yield stress of the material of the rail are approximately 1.76, 2.12 and 2.39. Under these various contact pressures, the ratio of the maximum static friction force under submergence conditions to the same friction force under dry conditions increased slightly. In contrast, when contact pressure conditions were nearly identical, the dynamic friction force under submergence and dry conditions was about same. Finally, the ratio of the dynamic friction coefficient to the static friction coefficient as a reference for designing cranes was shown and discussed.

Tribocorrosion behaviour of Ti6Al4V under various normal loads in phosphate buffered saline solution

Abstract Tribocorrosion tests were conducted on Ti6Al4V against alumina in phosphate buffered saline solution under normal loads of 3−30 N (corresponding to the maximum Hertzian contact pressures of 816−1758 MPa) using a ball-on-disk tribometer. Nano-hardness measurements revealed the formation of work-hardened layers on the pure wear and tribocorrosion surfaces. As the normal load increased from 15 to 30 N during the pure wear, the surface hardness was increased by about 100%. However, a lower generation of wear debris resulted in a lower wear rate under a normal load of 30 N. The presence of corrosion caused an increase in the wear rates by 28%−245% under various normal loads. The corrosion current density acquired from polarization curves was increased by three orders of magnitude and the open circuit potential (OCP) shifted to more negative potentials during tribocorrosion compared with the stagnant condition. The successive formation and removal of tribofilms, which consisted of oxygen and phosphorous compounds, resulted in peaks in the OCP trend and lower fluctuations in coefficient of friction under normal loads higher than 3 N.

Studies on the Influence of a Counterpart on Fretting Wear of Cold-Rolled High Strength Steel

This article investigates the influence of a counterpart on fretting wear of cold-rolled high strength steel for automotive applications. Fretting wear tests are conducted using a ball-on-flat test apparatus. The friction forces between cold-rolled high strength steel plates and various counterparts are measured. The worn profile of a cold-rolled high strength steel specimen is determined to calculate the wear rate. Experimental results show that the ratio of the maximum tangential to the normal force at a contact surface varies with a counterpart material. Measured worn surface profiles indicate that the cross-sectional area of a wear scar is enlarged as the number of cycles increases. The effects of counterpart hardness and the maximum Hertzian contact pressure are identified on the wear rate of cold-rolled high strength steel. From measurement and analysis results, ZrO2 counterparts are found to be more adequate than the commercial ones for the cold-rolled high strength steel rails.

Possibility of elasto-hydrostatic evolved-gas bearing as one of the mechanisms of superlubricity

In this article, firstly the experiments previously reported have been reviewed. That is, in the experiments conducted earlier it was seen that the friction coefficient reduced to the friction-tester noise-level of 0.0001, and only a small amount of sliding marks was seen on the tribofilm by scanning electron microscopic observation even though the maximum Hertzian contact pressure was 2.6 GPa. Based on these measurements, the possibility of elasto-hydrostatic gas bearing as the mechanism of super-low friction coefficient, where gas is not supplied from outside of the contact area but evolved from the contact surface, is investigated in the present report. Assuming the evolution of hydrocarbon gas mixture by ZrO2 catalytic reaction, which is represented by C2H4 in this report, the hydrostatic pressure distribution at the contact area and the evolved gas amount required for supporting a heavy applied load of 60 N are calculated.

Analysis of fluorinated interactions in plain ZDDP and fully formulated oils using design of experiment (DOE) and chemistry characterization of tribofilms in boundary lubrication under extreme loading conditions

PurposeThe purpose of this paper is to describe very reproducible boundary lubrication tests, conducted as part of design of experiment (DOE) to study the behavior of fluorinated catalyst mixed with polutetrafluoroethylene or Teflon (PTFE) in developing environmentally friendly (reduced P and S) anti‐wear additives for future engine oil formulations. The paper presents both the statistical and experimental solution to the fluorinated interactions in fully formulated (F‐fully formulated) and plain ZDDP (F‐plain) oils.Design/methodology/approachThe wear performance of fluorinated additive in combination with conventional commercial oil and ZDDP plain oil were investigated using DOE software. Several chemistry combinations were prepared and tested under extreme boundary lubrication (385 N with maximum Hertzian contact pressure of 2.72 GPa). Wear and frictional properties were evaluated using DOE and the interactions of fluorinated mix with minimum phosphorus were studied and compared with respect to fully formulated and plain ZDDP oils.FindingsThe optimized desirability shows the best condition that leads to more consistency in the breakdown of the tribofilm for a fixed contact load and fixed amount of fully formulated ZDDP oil. The influence of catalyst and PTFE fluorinated mix were examined. Scanning electron microscopy with chemistry analysis was developed. Hardness of the tribofilms, X‐ray of the wear track and Auger spectroscopy confirm the present of fluorine and phosphorus.Originality/valueTo ensure the reliability of the model, two original tests were conducted on the fully formulated and plain oil with minimum fluorinated mix.

Wear resistant electrically conductive Au–ZnO nanocomposite coatings synthesized by e-beam evaporation

The tribological and electrical behavior of e-beam codeposited Au–ZnO nanocomposite films were investigated for compositions in the range 0.1–28.0vol% of ZnO. A 2.0vol% ZnO film did not exhibit measurable wear sliding against a commercial gold alloy rider. Electron backscatter diffraction (EBSD) analysis on film surfaces of varying composition revealed a significant reduction in grain size for a 0.1vol% ZnO film contrasting with a pure Au film. Electrical resistivity measurements of films in the range 0.0–28.0vol% ZnO exhibited a linear increase in resistivity from 2.73μΩcm to 39.88μΩcm. The friction, wear, and electrical contact resistance of 2.0 and 28.0vol% ZnO films sliding against commercially available hardened Au riders (72Au–14Cu–8Pt–5Ag by weight) were investigated. Friction coefficient and electrical contact resistance data were in the range μ=0.3–0.5 and ECR=40mΩ and 500–2000mΩ, respectively, for 2μm thick films deposited on conductive substrates in unidirectional sliding at 1mm/s under a 185MPa maximum Hertzian contact pressure (100mN contact force). The presence of ZnO transfer to the rider was observed via energy-dispersive X-ray spectroscopy (EDS). Significant improvements in wear resistance and friction behavior were observed for Au–ZnO composite films with volume fractions of ZnO commensurate with the metal species codeposited in traditional hardened gold coatings.

Slip‐rolling resistance of ta‐C and a‐C coatings up to 3,000 MPa of maximum Hertzian contact pressure

AbstractThe slip‐rolling resistances of hard and stiff thin films under high Hertzian contact pressures can be improved by optimizing the “coating/substrate systems”. It is known from former investigations that the so‐called “egg‐shell” effect is no general hindrance for high slip‐rolling resistance of thin hard coatings. The coating stability depends more on specific deposition process and coating/substrate interface design. In this article it is experimentally shown, that pure amorphous carbon thin films with hardness between 15 and 63 GPa can be slip‐rolling resistant several million load cycles under a maximum Hertzian contact pressures of up to 3.0 GPa. Whereas all coatings were stable up to 10 million load cycles in paraffin oil at room temperature, reduced coating lifetime was found in SAE 0W‐30 engine oil at 120°C. It was shown how the coating hardness and the initial coating surface roughness influence the running‐in process and coating lifetime. No clear correlation between coating hardness and coating lifetime could be observed, but friction coefficients seem to be reduced with higher coating hardness. Very low friction down to ˜0.03 in unmodified engine oils was found for the hardest ta‐C film.

Frictional Voltammetry with Copper

This paper presents an experimental study correlating frictional behavior with in situ voltammetry for a unidirectional sliding contact between a hemispherical tipped alumina probe and a flat rotating copper counterface (maximum Hertzian contact pressure of 68 MPa and sliding speed of 10 mm/s). The contact was immersed in an aqueous 0.1 M Na2CO3 solution (pH ∼11) where the copper counterface acted as the working electrode in a potentiostat controlled three-electrode cell; a coiled Pt wire was used as the counter electrode and a saturated calomel electrode (SCE) as the reference. Clear and reproducible trends were found between friction coefficient and published data suggesting the onset of particular redox reactions, graphically presented in a frictional voltammetry plot. At anodic potentials primarily associated with the formation of copper(I) oxide (Cu2O) (V vs SCE ∼−0.25), the measured friction coefficient was in the range μ ∼0.4–0.5. At cathodic potentials primarily associated with the formation of CuO, Cu(OH)2, and CuCO3 (V vs SCE ∼−0.10), the friction coefficient transitions to the range μ ∼0.7–1.0. At sustained cathodic potentials associated with reduction of the native copper oxide, Cu2O, (V vs SCE ∼−0.65), the friction coefficient is observed to fluctuate between μ ∼0.2 and 0.5, arguably a result of exposure of bare copper due to non-uniform reduction (fractional coverage) of Cu2O.

The preparation of double-walled carbon nanotube/Cu composites by spark plasma sintering, and their hardness and friction properties

Double-walled carbon nanotube (DWCNT)/copper composite powders were prepared by a rapid route involving freeze-drying without oxidative acidic treatment or ball-milling. The DWCNTs are not damaged and are homogeneously dispersed in the matrix. Dense specimens were prepared by spark plasma sintering. The Vickers microhardness is doubled, the wear against a steel or an alumina ball seems very low and the average friction coefficient is decreased by a factor of about 4 compared to pure copper. The best results are obtained for a carbon loading (5 vol%) significantly lower than those reported when using multi-walled carbon nanotubes (10–20 vol%). Maximum Hertzian contact pressure data could indicate that the surface DWCNTs and bundles of them are deformed and broken, possibly resulting in the formation of a graphitized lubricating tribofilm in the contact.

Fluorinated FeF3 catalyst interactions in three different oil formulations using design of experiment optimization and chemistry characterization of tribofilms

ABSTRACTThe influence of fluorinated FeF3 catalyst on the formation and properties of tribofilms was examined using design of experiment (DOE) under extreme boundary lubrication (385 N or maximum Hertzian contact pressure of 2.72 GPa). A closed loop boundary condition test was developed to examine the behaviour of lubricants under boundary conditions. The reduction of phosphorus in engine oil was examined using two different plain oils and one fully formulated oil. Results indicate the formation of a thicker tribofilm in plain fluorinated oil when compared with fluorinated fully formulated oils.Several chemistry combinations were prepared and tested. Wear and frictional properties were evaluated using DOE, and the interactions of fluorinated FeF3 catalyst with minimum phosphorus were studied and compared with respect to fully formulated and plain zinc dialkyl dithiophosphate oils. Tribofilms with thickness ranging from 150 to 350 nm were developed during wear tests and were analysed for fluorinated plain and fully formulated oils that target reducing phosphorus. Three specific tests with optimised 0.6% FeF3 catalyst concentrations were used with 0.05% phosphorus in plain and fully formulated oil to verify the model optimised conditions with respect to wear and time to full break down. Scanning electron microscopy, hardness of the tribofilms and Auger spectroscopy confirm the presence of fluorine and phosphorus. These tests confirm the optimised prediction of the DOE model. Copyright © 2011 John Wiley & Sons, Ltd.

Sliding Friction Phenomena of a Ball-on-Disc Contact under Different Lubricating Conditions

Experiments were carried out to measure the friction of a ball-on-disc contact in a simple sliding experiment under different lubricating conditions using a Universal Micro Materials Tester (UMT-2). The measurements were also taken for the direct contact between the ball and the disc. The sliding speed was between 5.0 × 10–6 m/s and 1.0 × 10–2m/s. The load of the contact ranged between 1 N and 40 N giving the nominal maximum Hertzian contact pressures of 0.59 GPa to 2.03 GPa. It was found that when the sliding speed was below 1.0 × 10–5m/s, the contact is in boundary lubrication and the measured friction coefficients for all the lubricants could be considered as independent of the sliding speed. It is suggested that the boundary film-contact interfacial slippage in the loaded zone of the asperity contact between the two surfaces is responsible for this friction phenomenon and the friction coefficient is determined by the boundary film-contact interfacial shear strength. Under the same load condition, the boundary film-contact interfacial shear strength for the 68 mechanical oil was found to be the highest; that for the 32 mechanical oil was higher than that for the PB2400 oil; while that for the purified and deionized water was the lowest and significantly lower than that for the other three lubricants. The friction coefficients measured for the direct contact between the ball and the disc were found to be close to those for water lubrication. When the sliding speed was above 1.0 × 10 –5m/s, the friction phenomena, taking into account sliding speed variation, was found to be quite different for the different lubricants. For the applied loads, the measured friction coefficients can be considered as independent of the load. From this phenomenon, it is suggested that the asperity contacts formed between the coupled surfaces in the experiment are almost in full plastic deformation mode.