Rolling Contact Fatigue Performance Research Articles

Fatigue life and mechanical behaviors of bearing steel by nitrogen plasma immersion ion implantation

Rolling contact fatigue (RCF) performance and mechanical characteristics of nitrogen plasma immersion ion implantation (PIII) on AISI52100 bearing steel surface has been investigated using a conventional three ball-on-rod rig. Testing investigations include optical microscopy (OM), friction and wear behavior, rolling contact fatigue life, and nano-indentation measurements. Results indicate that the rolling elements failed at the surface or near-surface layer. Moreover, an appreciable amount of surface wear was observed on the surface of the rolling elements after RCF tests. The maximum microhardness of treated samples is nearly twice as that of substrate. The friction coefficient of the treated samples is decreased from 0.90 to 0.15. The L 10 life of the treated specimens increase by 99.6% and L 50 life enhance by 236.3% at a Hertzian stress level of 5.1 GPa and 90% confidence level, respectively. OM morphology results indicate that the surface roughness, processing parameters, and inner defects play an important role in surface and near-surface initiated RCF. This improvement of fatigue and mechanical performance is attributed to form a combination of nitrides phase structure and residual compressive stress during the nitrogen-PIII process.

Statistical Analysis of Nonmetallic Inclusions for the Estimation of Rolling Contact Fatigue Range and Quality Control of Bearing Steel

Abstract The cleanliness of bearing steels is a critical factor that influences directly the rolling contact fatigue performance and other mechanical properties of bearings. Hence, the evaluation of nonmetallic inclusions is a very important issue for steelmakers, bearing producers, and users. The material qualification procedure includes classical inclusion rating and rolling contact fatigue tests as acceptance tests for evaluating steel suppliers and ensuring the required bearing performance. The evolution of steel cleanliness had led to an increase in fatigue life so that running time of rolling contact fatigue tests have been considerably extended, until they became too time and cost consuming for a realistic qualification. In this paper, quantitative metallographic evaluation of the inclusion cleanliness used to collect data on the size, type, and expected number of inclusions in small samples is presented. The data were analyzed using Statistics of Extreme Values (SEV) and the results were used to extrapolate in larger samples representative of actual volumes of material involved in rolling contact fatigue tests and were related to the fatigue life of bearing steels from different suppliers. This method gives useful and reliable statistical data on inclusions for evaluating the performance level under rolling contact fatigue and defines the required cleanliness characteristics for bearing steels qualification. Other information of this work is the ability of the statistical inclusion rating to serve as a substitute for rolling contact fatigue tests to forecast the performance level of bearing steels.

Subsurface propagation of partial ring cracks under rolling contact: Part II. Fracture mechanics analysis

Surface ring crack defects are normally found on the surfaces of ceramic balls. These ring cracks are circular extending for approximately one quarter of circumference of a circle and these cracks can significantly reduce rolling contact fatigue life. In this study, a boundary element analysis was carried out to identify the main factors governing the propagation of sub-surface cracks. The calculations of the main stress intensity factors (SIF), K I and K II, were made to determine which factors were important for crack propagation. Various geometries of the ring crack were selected to investigate their effect on the subsurface propagation of the ring crack and calculated results showed the most important factor of crack geometry was shown to be the subsurface crack length with crack radius. Crack angle and arc length was shown to have relatively little effect. The K II values were mainly affected by load, crack length and crack face friction. Increasing the load or crack length resulted in an increase in absolute K II values, but high crack face friction led to lower K II. Lubricants with less ability to penetrate a crack and thereby increase crack face friction should therefore give a better rolling contact fatigue performance. The numerical results were verified by a comprehensive experimental study [20], which show that present predictions of subsurface crack growth are consistent with the experimental observations.

New Methodology to Evaluate the Rolling Contact Fatigue Performance of Bearing Steels With Surface Dents: Application to 32CrMoV13 (Nitrided) and M50 Steels

A new methodology is proposed to evaluate the rolling contact fatigue (RCF) performance of bearing steels in presence of surface dents. The experimental procedure consists of denting the raceway of test specimens with a hardness machine using spherical diamond tips of different radii (i.e., 200, 400, and 600μm) and with an applied normal load ranging from 5to50daN. Analysis of various dent geometries yield an analytical law with five parameters useful for fitting experimental profiles for contact simulation. Fatigue tests are conducted using a two-disk machine to study the effect of different operating conditions on RCF and to compare the performances of nitrided 32CrMoV13 steel versus M50 reference steel. A numerical investigation is conducted to analyze experimental result. Initially, the local residual stresses and plastic strains around the dent are obtained through finite element simulations of the indentation process. Second, the overrolling of the dent is simulated with a contact code. Finally, an indent-based endurance limit, called H1I, is proposed and comparisons are made with test results. Both RCF tests and numerical simulations show improved performance with nitrided 32CrMoV13 steel when compared to the M50 reference steel. The dominating role of sliding is also experimentally highlighted and two areas of damage initiation are identified. The effects of normal load and hoop stress are less marked.

Contact fatigue failure evaluation of post-treated WC–NiCrBSi functionally graded thermal spray coatings

Functionally graded thermal spray coatings applied to industrial components, which are subjected to contact fatigue or repeated impact loading, can reduce components weight and internal stresses whilst improving the adhesive strength to combat surface and sub-surface crack propagation. However, defects within the coating microstructure, which cannot be removed by the functionally graded approach, can compromise components reliability in high stress tribological applications. Post-treatments such as vacuum heating and Hot Isostatic Pressing (HIPing) have been shown in scientific studies to improve the coating microstructure, however, the influence of post-treatment on thermal spray coatings in rolling/sliding contacts have been seldom reported. This paper reports the rolling contact fatigue (RCF) analysis of functionally graded WC–NiCrBSi coatings deposited by a JP5000 system and subjected to post-treatment. HIPing was carried out at two different furnace temperatures of 850 and 1200 °C, whilst vacuum heating was performed at the elevated temperature of 1200 °C. The rate of heating and cooling was kept constant at 4 °C/min. RCF tests were conducted using a modified four-ball machine under various tribological conditions of contact stress and configuration, in both full film and mixed elasto-hydrodynamic lubrication (EHL). Test results reveal that the performance of coatings was highly dependent on the changes within the coating microstructure. Coatings HIPed at 1200 °C displayed relatively improved RCF performance over the as-sprayed coatings at stress levels of 2 and 2.7 GPa in full film lubrication. Improvement in RCF performance was attributed to the densification and homogeneity within the coating microstructure.

Rolling contact fatigue of post-treated WC–NiCrBSi thermal spray coatings

The aim of this experimental study was to comprehend the relative rolling contact fatigue (RCF) performance and failure modes of functional graded WC–NiCrBSi thermal spray coatings in the as-sprayed and post processed state, by means of Hot Isostatic Pressing (HIPing) and vacuum heating. Functional graded WC–NiCrBSi coatings were deposited by a JP5000 system. HIPing was carried out at two different furnace temperatures of 850 and 1200 °C, while vacuum heating was performed at the elevated temperature of 1200 °C. The rate of heating and cooling was kept constant at 4 °C/min. Rolling contact fatigue tests were conducted using a modified four ball machine under various tribological conditions of contact stress and configuration, in full film elasto hydrodynamic lubrication. Results are discussed in terms of the relative RCF performance of the as-sprayed and post-treated coatings, and also surface and sub-surface examination of rolling elements using scanning electron microscope (SEM), light microscope and surface interferometry. Test results reveal that performance of the coating was dependant on the microstructural changes due to post-treatment. Coatings heat-treated at 1200 °C displayed superior performance in RCF testing over the as-sprayed coatings at all stress levels (2, 2.3, 2.7 GPa) with emphasis on RCF performance at lower stress load of 2 GPa, where no failure occurred. Improvement in RCF performance was attributed to the diffusion between the carbides and matrix resulting in improved strength. At higher levels of contact stress, failure was surface initiated, and was attributed to initiation and propagation of micro-cracks at the edge of rolling contact region which led to coating delamination.

Boundary Lubrication RCF Performance of 4 cSt Gas Turbine Engine Oils at 204 °C

The rolling contact fatigue (RCF) and wear performance of three qualified formulations of MIL-L-7808K (4 cSt at 100 °C) were evaluated using AISI VIM-VAR M50 steel. The RCF tester differentiated the fatigue life and wear performance of gas turbine engine lubricants in boundary lubrication regime. Tests were conducted using a ball-on-rod type RCF tester at a maximum Hertzian stress of 4.8 GPa and temperature of 204 °C. Although all the three lubricants meet the military specification, the RCF results suggest that life and wear in the boundary lubrication regime are significantly affected by the formulation. Changes in physical and chemical properties of post test lubricants such as viscosity, acid number and additive concentration were minimal. However, the lowest Fe concentration and COBRA reading, which is a measure of the electrical property of the lubricant, correlated with the highest RCF life and lowest wear.

Development of bainitic rail steels with potential resistance to rolling contact fatigue

ABSTRACTRails are a major capital and maintenance cost for railways in North America. While manufacturers produce clean steels with high quality, most rails made today retain the basic carbon–manganese chemistry of traditional pearlitic rails. This paper describes the development of a bainitic rail steel with potential additional resistance to rolling contact fatigue damage. It is shown that rails can be produced in bainitic steel without the need for complex heat treatments after rolling, and that bainitic rails can have higher hardness and fracture toughness than pearlitic rails. Although small‐ and full‐scale tests indicate that the wear performance of bainitic steel depends considerably on test conditions, the indication is that bainitic steel rails can have significantly better rolling contact fatigue performance compared to pearlitic rails. Reasons for the superior fatigue performance are not fully understood, although a number of hypotheses exist. A conclusion is that continued research would be useful to understand quantitatively the physics and metallurgy of wheel/rail contact.

Prevention of RCF damage in curved track through development of the INFRA‐STAR two‐material rail

ABSTRACTResults from the European 5th frame research project ‘INFRA‐STAR’ are presented. The goal of the project is to prevent rolling contact fatigue (RCF) and to reduce squeal noise in curves by applying an additional surface layer material on the top of the railhead, resulting in a two‐material rail. In INFRA‐STAR, a dynamic train–track interaction model is used to provide the contact forces. Wheel‐rail profiles, wheel‐rail friction, vehicle data, track data and operating conditions are included to calculate the wheel‐rail contact forces and spin moments, contact positions and load distributions in the contact patch. The contact pressure, friction coefficient, coating thickness, material properties of the coating and the rail material are used in finite element calculations and shakedown theory to calculate shakedown limits, which are then used to predict the RCF performance of the system. The paper details the work on theoretical modelling, twin disc testing, metallurgical research and field testing completed to date (August 2002, the project just passed midterm). The development of the surface layer application methods that are used, and the further objectives of the INFRA‐STAR project, are discussed.

Contact fatigue failure modes in hot isostatically pressed WC-12%Co coatings

The objective of this experimental study was to investigate the influence of the post-treatment, Hot Isostatic Pressing (HIPing), on the Rolling Contact Fatigue (RCF) performance of thermal spray (WC-12%Co) coatings. Thermal spray coatings were deposited using a JP5000 High Velocity Oxy Fuel (HVOF) system in three different thicknesses on the surface of 440-C steel substrate cones to vary the depth of the shear stress within the Hertzian stress field. The furnace temperature during the HIPing process was varied at 850 °C and 1200 °C. RCF tests were conducted using a modified four ball machine under identical tribological conditions of contact stress, configuration and lubrication. Surface observations were made using Scanning Electron Microscopy (SEM) and Light Microscopy. Results of this preliminary study, which is the first of its kind in published literature to evaluate the RCF of HIPed cermet coatings, indicate that variation in HIPing temperature can have a significant influence on a coating's delamination resistance. These results are discussed to comprehend the performance and ascertain the fatigue failure modes in HIPed HVOF coated rolling elements. Apart from comparing the failure modes between HIPed and As-Sprayed coatings, results indicate that by increasing the HIPing temperature to 1200 °C, and maintaining full film lubrication, it is possible to achieve a fatigue life in excess of 70 million stress cycles without failure in relatively thin (50 μm) cermet coatings. Coating failure was attributed to maximum shear stress occurring either at the coating/substrate interface or within the coating microstructure, resulting in delamination due to cyclic loading.

Rolling contact fatigue of surface coatings—a review

The aim of this review is to survey the state of the art relating to the rolling contact fatigue (RCF) investigation of various overlay coatings and also, to ascertain the influence of design parameters such as the type of deposition process, coating material and thickness on the RCF performance. Rolling contact fatigue is a significant factor in the failure of components in rolling/sliding contact. Although, sintered ceramics have provided improvements in RCF life of components in rolling/sliding contact, e.g. hybrid ceramic bearings, the economic and technological constraints associated have so far limited their use to specialist applications. Physical and chemical vapor deposition (PVD, CVD) as well as thermal spraying are methods of depositing overlay coatings. The designer must thus choose a deposition method based on economic and technical flexibility, e.g. material choice, functional grading, etc. Amongst this family of overlay coatings, PVD coatings are already finding commercial use whilst others are at a research and development stage. The available literature on the RCF testing of various types of overlay coatings is considerable, but it is generally difficult to synthesize all of the results to obtain a comprehensive understanding of the parameters which can have a significant effect on a coating’s resistance to rolling contact fatigue. This review thus compares the RCF performance of these overlay coatings and discusses the results in terms of coating processes, materials, thickness, residual stress and tribological conditions of contact stress and lubrication.

Numerical Analysis of the Effect of Coating Microstructure on Three-Dimensional Crack Propagation in the Coating Under Rolling Contact Fatigue Conditions

Hard protective coatings produced by the physical vapor deposition (PVD) processes can potentially be used to prevent rolling contact fatigue (RCF) failures in gears and rolling bearings. Although TiN appears to be an attractive material for such applications, the RCF performance of PVD TiN coatings is limited by their columnar microstructure. One possible solution is to use interlayers interrupting the column growth in TiN to achieve a more equiaxed grain morphology. In this paper, the effect of the coating layer structure on the propagation of a three-dimensional crack through the coating thickness is studied theoretically. Numerical simulations of both planar and kinked three-dimensional cracks under cyclic contact loading are performed using a new numerical approach recently developed by the authors, which is based on a combination of the fast Fourier transform (FFT) technique, the eigenstrain theory, and the conjugate gradient method. The simulation results indicate that high-endurance TiN-based coatings can in principle be produced by alternating relatively thick TiN layers with much thinner interlayers made of another material.

Rolling contact fatigue performance of HIPed Si 3N 4 with different surface roughness

Accelerated rolling contact fatigue four-ball tests were performed on hot isostatically pressed (HIPed) silicon nitride/steel contacts under lubricated condition using a PLINT TE92/HS microprocessor controlled rotary tribometer at a maximum compressive stress of 6.58 GPa and at a speed of 10,000 rpm for 135–200 million stress cycles. Rolling track surfaces were examined by optical microscopy, SEM, 3-D surface analysis, atomic force microscopy and interference profilometry. Experimental results show that with an initial roughness value R a from 0.016 to 0.094 μm, the surfaces became smoother during testing and surface pitting (10–20 μm) occurred. For the surfaces with a roughness value R a from 0.002 to 0.008 μm, the surfaces became rougher and lubricant oil residues were deposited at the edges of rolling track. Some small and shallow surface pitting 2–3 μm in size was also observed. Some scratch marks left on the surface by the previous lapping process were further extended during testing. R a and R q are still most important roughness parameters in relation to rolling contact behaviour. The effects of other surface roughness parameters, such as the shape of the valley, R sk and R ku are not obvious in this study.

Rolling contact fatigue behaviour of Pyrowear 675

This work represents one phase of a comprehensive gear and bearing materials programme, involving both substrates and coatings. The programme is directed at enhancing both the performance and durability of gears and bearings for insertion into military systems. This paper deals specifically with the Pyrowear 675 alloy, a carburising stainless steel recently developed by Carpenter Technology Corp. The rolling contact fatigue (RCF) performance of steel specimens with four different heat treatments is presented. The optimum heat treatment produced a fivefold increase in both B10 and B50 lifetimes compared with M50 steel, the standard turbine engine bearing steel. Thus, the improved RCF performance combined with good corrosion resistance make Pyrowear 675 alloy an attractive candidate for advanced bearing applications.

Effects of Linear PFPAE Decomposition Products on the Rolling Contact Fatigue Performance of M50

In bench tribology tests, the influence of lubricant decomposition products on the lubricated system and on lubricant performance are often overlooked, primarily because testing is performed in vented, or open, systems. However, the lubricant in a gas turbine engine might be expected to more closely follow the principles of interaction that are found in unvented, or closed, systems. Results reported here comparing vented and unvented bearing housings in rolling contact fatigue (RCF) tests with a linear perfluoropolyalkylether (PFPAE) lubricant and VIM-VAR M50 steel at 316°C and a stress of 4.8 GPa clearly show that significantly more wear, corrosion, and fluid breakdown occur in the closed system than in the open system. Under these conditions, PFPAEs catalytically decompose to corrosive products. These corrosive products are partially vented in the open system, but retained in the closed system, causing more extensive corrosion of bearing materials, and significantly affecting the lubricant's tribological performance. Post test lubricant was analyzed for viscosity, acid number, and metals to assess changes in the lubricant's physical and chemical properties. Changes were more severe in the closed system. The nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectra also showed significantly more carboxylic acid buildup in the stressed fluid from the closed system. The films formed in the tribojunction were analyzed using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Organic films formed in the open system reduced wear while inorganic films formed in the closed system resulted in higher wear. Consequently, we conclude that more attention needs to be given to the effect of decomposition products during bench type tribological testing of high temperature lubricants.

Rolling contact fatigue performance of plasma sprayed coatings

This experimental study describes the Rolling Contact Fatigue (RCF) performance and the failure mechanisms of plasma sprayed tungsten carbide cobalt (WC-15%Co) coatings. The advancements of plasma spray coatings due to higher velocity and temperature of the impacting lamella call for investigations into new applications. One possible application is the rolling element bearing. A modified four ball machine which models the configuration of a deep groove rolling element ball bearing was used as an accelerated method to compare the rolling contact fatigue resistance of the test materials. RCF tests were conducted in conventional rolling element steel ball bearing (steel lower balls) and hybrid ceramic bearing (ceramic lower balls) contact configurations. Plasma sprayed coatings were deposited on a bearing steel (440-C) substrate at different thicknesses. The coated rolling element cones were ground and polished to achieve a good surface finish on the rolling elements. RCF tests were conducted under the various tribological conditions of contact stress, lubricant and test configurations. The frictional torque in the cup assembly was measured during the RCF tests with the aid of a force transducer. The speed of the planetary balls was monitored using an accelerometer and fast fourier transformation of the vibration signal. This enabled the experimental measurement of sliding between the coated cone and the driven balls. The failed rolling elements, coating debris and the lower planetary balls were analyzed for surface observations under the Scanning Electron Microscope (SEM), and Electro Probe Microscope Analysis (EPMA). The test results indicate that the coating performance was dependent upon the tribological conditions during the test. The failure modes were observed to be surface wear and the coating delamination.

Fatigue behaviour of HVOF coated M50 steel rolling elements

An experimental approach using a modified four ball machine has been used to investigate the rolling contact fatigue (RCF) performance and failure modes of WC-12Co coated rolling elements deposited by the high velocity oxyfuel (HVOF) process. The rolling elements were in the form of12·7 mm diameter M50 bearing steel rolling element balls. The RCF tests were conducted under various tribological conditions of contact loading in conventional steel ball bearing and hybrid ceramic bearing configurations. Results indicate that the rolling elements failed at contactloads above 2·4 GPa and that the failure mode was coated delamination at the coating/substrate interface. Moreover, an appreciable amount of surface wear was observed on the surface of the rolling elements after the RCF tests. The test results and failure modes are discussed in thecontext of the coating/substrate microhardness results, elastohydrodynamic lubrication (EHL) regimes, surface observation of the failed areas using scanning electron microscopy (SEM), slip analysis, frictional torque measurements, and stress contours developed by finite element modelling ofthe coated rolling element ball. Solutions to resist interfacial delamination are also discussed

Wear of high-velocity oxy-fuel (HVOF)-coated cones in rolling contact

An experimental approach using a modified four-ball machine was used to investigate the rolling-contact fatigue (RCF) performance of tungsten carbide cobalt (WC-12%Co) coatings deposited by the high-velocity oxy-fuel (HVOF) process. The coated rolling-elements were in the geometrical shape of a cone which replaced the upper ball of the modified four-ball machine. The RCF tests simulate the configuration of a deep groove rolling-element ball bearing, and the tests were conducted in a conventional steel ball bearing and hybrid ceramic bearing configurations. The coating thickness, Hertz contact stress and the test lubricants were varied to provide various tribological conditions during the tests. Three different coating thicknesses were investigated and the coatings were ground and polished to attain a good surface finish on the rolling elements. The effect of substrate hardness, coating thickness and fluid film thickness on the RCF performance have been investigated along with the coating failure modes. Coating wear has been analysed using a scanning electron microscope (SEM) and the results discussed with the aid of the coating microstructure, microhardness and talysurf analysis of the wear track. Test results reveal that the performance of the coating is dependent upon the combination of the substrate and the coating properties. Moreover, the coating failure is a function of surface and subsurface mechanisms depending upon the tribological conditions during the test. Thinner coatings did not fail at the interface, indicating a good adhesive strength between the coating and the substrate. The depth of the failure and its comparison with the orthogonal shear-stress depth has been discussed. The coating failure was observed to be a combination of surface wear and subsurface delamination.

Rolling contact fatigue performance of detonation gun coated elements

Rolling contact fatigue performance of thermal spray coatings has been investigated using an experimental approach. A modified four ball machine which simulates a rolling element bearing was used to examine the coating performance and failure modes in a conventional steel ball bearing and hybrid ceramic bearing configurations. Tungsten carbide (WC-15% Co) and aluminium oxide (Al 2O 3) were thermally sprayed using a super D-Gun (SDG2040) on M-50 bearing steel substrate in the geometrical shape of a cone. A coated cone replaced the upper ball that contacts with three lower balls. The rolling contact fatigue (RCF) tests were performed under immersed lubricated conditions using two different lubricants. Fatigue failure modes were observed using a scanning electron microscope. Microhardness measurements of the coating and the substrate and elastohydrodynamic fluid film thickness results are included. The results show the requirement for significant optimization of the coating before use in rolling element bearing applications. The coating was fractured in a delamination mode. Test results show an optimization in coating process is required before these coatings can be used for rolling contact applications. WC-Co coatings perform better than Al 2O 3 coatings in rolling contact.

Rolling contact fatigue behaviour of thermally sprayed rolling elements

An experimental approach with a modified four-ball machine is used to investigate the rolling contact fatigue (RCF) performance and failure mode of thermally sprayed rolling element bearing steel balls. A superdetonation gun (SDG 2040) is used to deposit a tungsten carbide coating (WC-15%Co) on steel substrate balls. This test configuration has been used to simulate traditional steel and ceramic hybrid rolling element ball-bearings. Microhardness of coating and substrate, scanning electron microscopy (SEM) and finite element modelling (FEM) of the contact problem for coated elements are the key tools in the analysis. The results show that coated specimens do not perform well and the failure is through the coating itself and not at the interface. Results from FEM and SEM of failed areas indicate that the failure of coatings is in plain shear at the edge of the contact area. An improvement is required in the coating technique for rolling contact fatigue application. Advanced processes such as high velocity oxy-fuel (HVOF), etc. may provide different results. The mode of failure in the coated specimens is delamination.