Hertzian Contact Pressure Research Articles

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.

Static and dynamic friction coefficient in low loads and sliding speed conditions

Using an unidirectional oscillator including a mass-spring system, the authors evidenced the important variations of the static friction coefficient both for very low contact pressure (0.002 MPa) and for high contact pressure (386 MPa).The experiments were realised by the CETR UMT-2 Tribometer with a variation of the sliding speed between 0.02 mm/s to 8 mm/s. The oscillator mass have 0.242 grams and the stiffness of the elastic spring had 77 N/m. The contact surface of the oscillator mass have a roughness Ra = 0.1 μm and the opposite plane surfaces have roughness of Ra = 0.1 μm and Ra = 2.5 μm. Two types of experiments were realized: the sliding between two plane surfaces with a nominal pressure of 0.002 MPa and the sliding between a plane surface and three steel balls of 6 mm with a maximum Hertz contact pressure of 386 MPa. For very low pressure the obtained static friction coefficient varied between 0.08 to 0.3 and the dynamic friction coefficient varied between 0.08 to 0.16. For high pressure the obtained friction coefficient varied between 0.2 to 0.4 and the dynamic friction coefficient varied between 0.15 to 0.2. To simulate the sliding speed the model of Zuleeg [5] for dynamic friction coefficient has been adapted to the experimental parameters and the dynamic equation of mass displacement as function on time has been numerical integrated.

Stick-Slip Phenomena and Acoustic Emission in the Hertzian Linear Contact

AE detection and analysis usually requires a specific, costly platform due to its particular burst nature and high-frequency content. This experimental study investigates the relationship between low-demand acoustic emission parameters (AE) and the occurrence of stick–slip (SS) at the Hertzian linear contact. Hence, the correlation of basic AE characteristics (amplitude, energy, and evolution in time) with stick–slip characteristics (static and kinetic friction coefficients, amplitude, energy, and evolution in time) is pursued. Tribological tests were conducted on cylinder–plane specimens under dry friction conditions with different loads at different low driving speeds and Hertzian contact pressures at a constant stiffness. The AE, normal, and friction forces were recorded simultaneously on the experimental stand. At the cylinder–plane interface, the jumps specific to the stick–slip phenomenon (friction coefficient—COF) were followed after a few milliseconds by AE jump peaks. The results of the experiments show that the amplitude and energy generated by AE were sensitive to the occurrence of the stick–slip phenomenon, while the AE and COF energies in the stick and slip phases had the same law of variation based on the driving velocities. The results show that the amplitude and energy of the sampled low-frequency AE signals were enough to detect the friction in SS and demonstrate the potential of AE as a tool for detecting and monitoring the tribological behaviour of SS at the linear Hertzian contact.

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.

Impact of surface texturing on the tribological behaviour of aluminium-silicon (Al-Si/Al2O3) advanced composite under dry and lubricating conditions

In the present work, modification of aluminum-silicon (Al-Si) alloy based advanced composite surface is performed using laser texturing (LT). Surface texturing (ST) with different patterns, i.e., dimple (T1), square (T2), triangular (T3), and line hatched (T4) textures are done on the alloy surface and its impact on the mechanism to reduce friction and wear are investigated. The tribo tests are conducted at 10 N load, Hertzian contact pressure (Pmax) 708.7 MPa, 1 mm stroke length, frequency 50 Hz and sliding distance (SD) up to 450 m. The tribological behavior of the non-textured surface (NTS) and textured surface (TS) are examined under dry sliding conditions (DSC) and two lubricating sliding conditions (LSC) comprising of virgin PAO-4 (Lube-1) and PAO-4 + 1 wt% graphene nanoplatelets (Lube-2). T2 texture reveals a 49.53%, 69.21% and 44.91% decrease in friction coefficient(COF) and 58.1%, 43.74% and 83.86% decrease in wear volume (WV) than the NTS for DSC, Lube-1 and,Lube-2, correspondingly. Results show that ST improved friction efficiency, and shortened the running-in period. The current study results help to provide in-depth interpretation of micro-texturing and its relationship w.r.t. tribological characteristics.

Identification of Plastic Properties through Spherical Indentation

Indentation has become a promising tool to characterize the mechanical parameters of materials. In this work, herein, spherical indentations on elastic‐perfectly plastic materials and strain‐hardening materials are investigated through finite‐element method (FEM) simulations. A new method to extract the yield strength and strain‐hardening exponent from spherical indentation is proposed. The deviation of nominal contact pressure from Hertzian prediction is used as an indicator to determine the plastic parameters. The difference between Hertzian contact pressure and real contact pressure is also investigated for the estimation of contact radius during indentation process. Herein, a more convenient approach is provided in this analysis to directly determine the mechanical parameters of elastic–plastic materials from spherical indentation tests. Validation of this method is presented in the form of a comparison between analytical stress–strain relationships and corresponding curves obtained via iterative FEM simulations of the indentation process. No experimental data are involved in this validation.

Design architecture of colorful Si-DLC/PLC nanostructured multilayer films for robust superlubricity at high contact stress in dry N2 atmosphere

Diamond-like carbon (DLC) enables the design of novel lubricant structures to realize macroscale superlubricity in harsh working conditions. In this work, a series of nanostructured multilayer Si-DLC/PLC films with colorful appearances were developed to provide a lubrication state with near-zero friction and wear for responding the challenge of lubrication requirement in high-load operation environment. The results reveal the crucial role of structural parameter, namely the thickness of a bilayer period λ (i.e., 9–324 nm), in determining the construction architecture, mechanical properties and anti-friction lubricity of the multilayered films. Specifically, a bilayer period of 324 nm (total film thickness of 1.53 μm) endows the film with a superlow friction coefficient of 0.003 and a near-zero wear rate of 4.06 × 10−9 mm/Nm under a peak Hertz contact pressure of 1.76 GPa. The relationship between the load-bearing capacity, nano-clustering transformation of the sliding interface and the steady state of superlubricity is investigated in detail to clarify its high-stress adaption and robustness of the anti-friction features. The design architecture of well-tailored multicomponent and nanostructured multilayer films indeed offers an effective theoretical and technical solution to the long-term lubrication and protection of mechanical moving parts under heavy load conditions.

Wear and friction behavior of CrTiN/TiCN and CrTiN/CrCN multilayer composite coatings

Multi-layer CrTiN/CrCN and CrTiN/TiCN coatings were deposited on 2379 and SLD steels by cathodic arc physical vapor deposition (arc-PVD) in different thicknesses. Coatings are mechanically, physically, and chemically characterized by nanoindentation, optical microscope, atomic force microscope (AFM), and scanning electron microscope/energy-dispersive X-ray (SEM/EDX) analyses. The crystal structure of the coatings was investigated by X-ray diffraction (XRD) analysis. Wear and friction characterizations were performed via reciprocating tribometer tests under 2.4 and 3.0 GPa Hertzian contact pressures at the unlubricated conditions. Results showed that CrTiN/CrCN coatings demonstrated excellent wear resistance and friction coefficient (COF) at elevated contact pressures both on 2379 and SLD steels.

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.

Relative contact width evaluation of end hemispherical cavities rollers as an indication of Hertzian contact pressure

In a recent investigation, the end hemispherical cavities (EHC) rollers exhibited better strength against fracture than hollow rollers. Furthermore, EHC rollers looked promising from a higher fatigue life aspect than conventional solid rollers in a simulation study. Therefore, it necessitated further exploration of the EHC roller concept and to this end, in the present investigation, the contact widths of EHC rollers were relatively evaluated to judge its contact stresses' behavior with respect to the solid roller because the contact stresses are responsible for the fatigue life of rolling bearings. In the experiments, the contact footprints were obtained by forcing specimens of rollers against chemically etched surfaces and were examined by a microscope for measurement of contact widths. The experimental trials were performed with individual roller-on-plate tests and also with full-bearing samples. The etch correction factor was used to correct anomalies of real and observed contact widths due to etching film thickness. The parabolic relationships were established for roller variants which yielded constants signifying their relative ranks. The contact semi-widths, thus derived from corrected experimental results of individual roller-on-plate tests, demonstrated good agreement (<5%) with those derived from simulation results. The results of full-bearing sample tests for roller variants also ranked same as individual roller-on-plate tests. The encouraging results of contact semi-width assuredly favor the prospects of relatively higher fatigue life in case of EHC rollers.

Stress- and Time-Dependent Formation of Self-Lubricating In Situ Carbon (SLIC) Films on Catalytically-Active Noble Alloys

Low shear strength (30 MPa) organic films were grown in situ on Pt0.9Au0.1 surfaces via cyclic sliding contact in dry N2 with trace concentrations of ambient hydrocarbons. We present a systematic investigation of the stress- and time-dependent film formation. Steady-state friction coefficients were found to be as low as µ ~ 0.015 and inversely proportional to contact pressure, revealing non-Amontonian behavior. Above a Hertzian contact pressure of ~500 MPa, shear strength dropped, indicating an activated process. Raman spectroscopy identified non-uniformity in areal coverage and relative order with contact pressure. Regions of steady-state low-friction behavior exhibited spectra similar to DLC coatings. Atomic force microscopy was used to study the formation and growth of films at the nanoscale. Stress- and time-dependent measurements suggested a sublinear increase of film volume with time, and a transition from growth to wear at a Hertzian contact pressure of ~1.2 GPa.

Enhanced lubricity of CVD diamond films by in-situ synthetization of top-layered graphene sheets

In the present study, we demonstrated a novel process of in-situ fabrication of graphene sheets by adopting the chemical vapor deposition (CVD) method. Fabricating such a layer of vertical graphene sheets (VGs) on the surface of microcrystalline diamond (MCD) films could decrease the stable coefficient of friction (COF) by 20–30%. Besides, such lubricity showed excellent load capacity and durability. With normal loads ranging from 3 N to 9 N (Hertz contact pressure of 2.1–3.1 GPa), the MCD-VGs films exhibited COFs stabilized at 0.105 and wear rates less than 5 × 10−6 mm3 N−1 m−1. Additionally, the produced VGs displayed an excellent service lifetime with 72,000 sliding cycles. During the friction tests, an equilibrium graphene self-mated contact formed on the sliding interface as graphene fragments adhered onto the counterpart ball and resulted in the reduction of friction and low wear rates. Additionally, the mixtures of Si3N4/SiO2 particles and graphene sheets residual within the wear tracks would promote the transformation of the sliding regime to three-body abrasion, beneficial for friction reduction. The findings presented in this study provide a novel and effective approach to enhance the lubricity of any engineering diamond surface.

Self-lubricating triboactive (Cr,Al)N+Mo:S coatings for fluid-free applications

Within this study, self-lubricating and triboactive (Cr,Al)N+Mo:S coatings were developed and investigated for the deposition on components in a low-temperature physical vapor deposition (PVD) hybrid process. Therefore, direct current magnetron sputtering (dcMS) and high power pulse magnetron sputtering (HPPMS) PVD were combined by using an industrial coating machine. Hereby, it was possible to deposit dense and smooth triboactive, self-lubricating nitride coatings with different chemical compositions and architectures on 16MnCr5E samples. Two coating architectures, a matrix monolayer and a graded coating structure, were developed to evaluate the effect on the tribological behavior. The morphology and coating thickness were analyzed by means of scanning electron microscopy (SEM). Furthermore, the indentation hardness and modulus of indentation as well as the compound adhesion between substrate materials and coating were analyzed. Tribological analyses of (Cr,Al)N+Mo:S-coated and uncoated samples were conducted under fluid-free friction regime at room temperature T = (20 ± 3) °C, a velocity v = 0.1 m/s and a distance s = 1000 m by varying the Hertzian contact pressure from 400 MPa ≤ pH ≤ 1300 MPa against steel counterparts, 100Cr6, in a pin-on-disk (PoD) tribometer. The graded coating architecture of (Cr,Al)N+Mo:S enabled a significant wear and friction reduction. Furthermore, Raman analyses prove the formation of solid lubrication tribofilm containing MoS2, MoO3 MoO2 and MoxOy at the toplayer of a graded (Cr,Al)N+Mo:S coating, which are responsible for the improved tribological behavior.

Fretting wear behaviour and frictional force mapping of Al2O3 based thermal barrier coatings

Efficiency and structural stability of gas-turbine engines were acheived with compliant thermal barrier coatings (TBCs) on structural components. In TBCs, top ceramic coatings encountered with other structural parts in a relative motion (amplitude of 100–500 μm and frequency of 1–5 Hz) was leading to failure, namely fretting wear. So, a high hardness material, Al2O3 based ceramic matrix composite (CMC) coatings were utilized to improve the tribological properties of the engineered surface. At this juncture, fretting wear behaviour of atmospheric plasma sprayed Al2O3–3 and 8 mol% Y2O3 stabilized ZrO2 (3YSZ and 8YSZ)‑carbon nanotube (CNT) based CMC coatings were investigated. The fretting wear rate of Al2O3 based CMC coatings against WC ball was observed as 0.97–1.87 × 10−6 mm3N−1 m−1. The least in wear resistance of Al2O3-CNT (1.87 × 10−6 mm3N−1 m−1) among the composite coating is attributed to low hardness and high Hertzian contact pressure (1.65 GPa) on the coatings. Among the composite coatings, Al2O3-3YSZ and Al2O3-8YSZ-CNT showed an improved wear resistance (~40.1% and ~34.5%, respectively) with wear rate of 0.97 × 10−6 mm3N−1 m−1 and 1.06 × 10−6 mm3N−1 m−1, respectively. It is attributed to improved mechanical properties, altered surface by reciprocating motion and the presence of wear debris, which reduces the wear rate by acting as a third body. So, there is an overestimation of wear rate (4.1–6.6 times) compared to that of experimentally calculated wear rate. Further, the Al2O3-YSZ-CNT composite showed fatigue crack on the fretting wear surface due to low plasticity index (62.0–109.3 MPa) of the coating. Also, the gradual change of running-in regime to the steady-state regime is achieved in the CNT reinforced composite coatings due to high resistance by the coatings which possess commendable mechanical properties. The frictional force mapping during the reciprocating motion confirms the rapid and gradual transition of two different regimes in the composite coatings.

Slip-rolling resistant steel alloys up to P0max of 3,920 MPa

Downsizing (power-to-weight ratio) and higher speeds lead to a rise in Hertzian contact pressures in combination with an increase in surface or oil temperatures. Under such conditions, commonly used bearing steels, such as 100Cr6, reach their limits, creating a demand for alternative slip-rolling resistant steel alloys. The present work therefore compares the slip-rolling performance of various steel types with Maraging- and PM-type steel alloys such as e.g. CSS-42L™, ASP2012, BIMAX42+, in the Hertzian contact pressure range up to P0max of 4 GPa. Through-hardened 100Cr6H (AISI 52100), case-hardened 20MnCr5 (AISI 5120H) and nitrogen alloyed Cronidur30 (AMS 5898) still compete in terms of slip-rolling and wear resistance and load carrying capacity, whereas Maraging- and PM-type steel alloys offer superior strength and toughness properties.

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.

Ambient and Nitrogen Environment Friction Data for Various Materials & Surface Treatments for Space Applications

A multivariate tribological evaluation of candidate materials, surface treatments and dry film lubricants is needed for design of moving mechanical components that function reliably in extreme conditions, including for long-duration space missions. In this study, linear reciprocating or unidirectional sliding friction data was collected using ball-on-flat tests. The balls were hardened 440C stainless steel (either uncoated or sputtered with MoS2) and flat surfaces were 440C stainless steel, Nitronic 60 stainless steel or Ti6Al4V titanium alloy with various surface treatments and/or dry film lubricants. Surface treatments included anodizing, nitriding and electrical discharge machining. The dry film lubricants included Microseal 200-1, sputtered MoS2 and a nano-composite coating i-Kote. The data contains applied normal load, measured friction force, calculated coefficient of friction, ball position, ambient temperature and relative humidity during testing. Tests were performed at different peak Hertzian contact pressure conditions ranging from 300 to 2000 MPa. Data is also available for flat surfaces that were vacuum baked at 150 °C after surface treatment and dry film coating as well as samples tested in inert gas (nitrogen) environment. This data can be used both to fundamentally understand the tribological properties of different material systems as well as to enable design of components for specific applications, conditions and duty cycles.Graphical Abstract

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.

Deflection calculation of cam profile due to a Hertzian contact pressure

AbstractThe bending deflection of cam profile was analyzed for three paths of contact and distinct Hertzian contact pressure. The impact happened between the disc cam and roller follower based on the contact parameters. The contact parameters are contact body stiffness, sliding contact velocity, exponent and penetration. A disc cam and roller follower system were discussed and analyzed for the dynamic response of the follower and bending deflection of the cam profile. The objective of this paper was to study the effect of contact load on the bending deflection. A system with spring stiffness (k) and viscous damping coefficient (c) at the end of the follower stem was used to reduce the bending deflection on the cam profile. The theory of circular plate was applied to derive the analytic solution of the bending deflection. The dynamic response of the follower had been determined by using the SolidWorks software based on the contact parameters. The experimental setup was done through an infrared camera device. Finite-element analysis was used to calculate the bending deflection of the cam profile numerically. Finite-element analysis was carried out by using the ANSYS version 19.2 package. The analytic and simulation results are checked and verified for bending deflection at the point of contact. The reduction rate for bending deflection was 73.425% for path no. (1), 85.925% for path no. (2) and 61.467% for path no. (3).

Test Modes for Establishing the Tribological Profile under Slip-Rolling

The complex nature of slip-rolling contacts in many applications such as gear tooth flanks, rolling bearings, and heavy machinery often makes determining the friction and wear properties, as well as the fatigue resistance, of tribosystems difficult. The establishment of the tribological profile of a tribocouple under high Hertzian contact pressure and under slip-rolling will allow for the measurement and comparison of friction and wear coefficients as well as slip-rolling resistance by continuously monitoring the wear rate, coefficient of friction, temperature, oil film thickness, and/or electrical contact resistance using high-resolution signal analysis (HRA). A twin disc system can provide insight into the adhesive behavior of material and lubricant products such as alternative base oils and additives, ceramics, alloys, and thin film coatings. The strength and endurance of these products are often characterized through fatigue and resistance tests, which apply high Hertzian contact pressures to the rolling contact until seizure or failure is obtained. The further observation of the formation of tribofilms on the surface of contact yields information about the reactivity and thermochemical properties of additives. This review aims to illustrate how the implementation of different screening methodologies can be used as a meaningful tool for assessing the aforementioned tribological profile properties for the development of slip-rolling tribosystems.