Effects of Service Condition on Rolling Contact Fatigue Failure Mechanism and Lifetime of Thermal Spray Coatings——A Review

The service condition determines the Roiling Contact Fatigue（RCF） failure mechanism and lifetime under ascertain material structure integrity parameter of thermal spray coating. The available literature on the RCF testing of thermal spray coatings under various condition services is considerable; it is generally difficult to synthesize all of the result to obtain a comprehensive understanding of the parameters which has a great effect on a thermal spray coating＇s resistance of RCF. The effects of service conditions（lubrication states, contact stresses, revolve speed, and slip ratio） on the changing of thermal spray coatings＇ contact fatigue lifetime is introduced systematically. The effects of different service condition on RCF failure mechanism of thermal spray coating from the change of material structure integrity are also summarized. Moreover, In order to enhance the RCF performance, the parameter optimal design formula of service condition and material structure integrity is proposed based on the effect of service condition on thermal spray coatings＇ contact fatigue lifetime and RCF failure mechanism. The shortage of available literature and the forecast focus in future researches are discussed based on available research. The explicit result of RCF lifetime law and parameter optimal design formula in term of lubrication states, contact stresses, revolve speed, and slip ratio, is significant to improve the RCF performance on the engineering application.

Rail fatigue crack propagation in high-speed wheel/rail rolling contact

To study the wheel/rail rolling contact fatigue of high-speed trains, we obtain the distribution of contact forces between wheel and rail by introducing the strain-rate effect. Based on the finite element simulation, a two-dimensional finite element model is established, and the process of a wheel rolling over a crack is analyzed to predict the crack propagation direction. The statistics of possible crack prop- agation angles are calculated by the maximum circumfer- ential stress criterion. The crack path is then obtained by using the average crack propagation angle as the crack propagation direction according to Weibull distribution. Results show that the rail crack mode of low-speed trains is different from that of high-speed trains. The rail crack propagation experiences a migration from opening mode to sliding mode under the low-speed trains; however, the rail crack mainly propagates in the opening mode under high- speed trains. Furthermore, the crack propagation rate for high-speed trains is faster than that for low-speed trains. The simulated crack paths are consistent with the experimental ones, which proves that it is reasonable to use the average value of possible crack propagation directions as the actual crack propagation direction.

Rolling contact fatigue of silicon nitride bearing balls under pure rolling condition

A newly developed pure rolling fatigue test rig with three contact points for bearing bails was used to perform rolling contact fatigue （RCF） tests. The fatigue properties of GCrl5 steel bails and two kinds of Si3N4 ceramic balls （GSN-200 and NBD-200） produced with different technologies were compared. Ball surfaces were examined after failure with optical microscope and scanning electron microscope （SEM）. It was identified by tests that the failure mode of ceramic balls was surface spail. The main factor of ceramic bail failure was principal tensile stress. Life tests data, summarized in accordance with the Weibull theory, showed that the life of GSN-200 balls was close to that of GCrl5 balls, whereas the life of NBD-200 balls was much longer than those of GSN-200 and GCr15. Under the same working condition, the temperature rise of all ceramic bails was lower than that of steel balls, and their crack propagation rates were slower than that of steel balls.

The Morphology of Rolling Contact Fatigue Fracture of Hardened Steels

Rolling Contact Fatigue(RCF) is a cumulative damage phenomenon when metals are subjected to repeated contact stresses. The fomation of pitting on the contact surface is the result of the rolling contact fatigue. The morphologies of rolling contact fatigue fracture of the har- dened steels (86CrHoV7, 42CrMo) show that strong resemblance in fractuye mechanisms exists between rolling contact fatigue and uni-axial fatigue. Since fatigue striations are hardly observed in hardened steels under uni-axial fatigue, it is interesting to note that the state of stress in rolling contact fatigue is more favor- able to ductile fractures than in uni-axial fatigue.

ROLLING CONTACT FATIGUE BEHAVIOR OF CERAMIC BALLS LUBRICATED BY LUBRICANTS WITH EXTREME PRESSURE ADDITIVES

An experiment is conducted to investigate the effects of lubricant10#, which contains extreme pressure additives T304 and T305, on the rolling contact fatigue （RCF） life of the contact pairs of a Si3N4 ceramic ball and a steel rod. The experimental investigation is carried out using a ball-rod RCF test rig. The results show that the extreme pressure additives increase the anti-contact-fatigue performance of ceramic balls; When the content of the additives varies from 1% to 5%, the increasing gradient of the RCF life curve decreases; And the oil sample with 1% T305 additive corresponds to the maximal gradient of the RCF life curve, with the RCF life being increased by about 10.77 times. The fatigue surface of the ceramic ball is analyzed with scanning electron microscope （SEM） and X-ray electron dispersion analysis（EDAX）, and the physical model of extreme pressure additives＇ increasing the RCF life of the ceramic ball is proposed. It is found that the extreme pressure additives form a corrosive film and a transfer film on the surface of the ceramic ball, which decrease the surface tangential stress, and to increase the surface energy is the most effective means for increasing the RCF life.

The Effects of Different Post-Heat Treatments on Rolling Contact Fatigue Behaviors of Direct Laser Cladding Inconel 625 Coatings

In this paper,the microstructures and rolling contact fatigue behaviors of laser cladding Inconel 625 coatings with or without post-heat treatments were analyzed.The results revealed that the fatigue resistance of the laser cladding coating after any post-heat treatment was worse than that of the as-deposited coating.First,through the finite element analysis,the distribution of stress along the thickness direction of the coating was obtained,and it was concluded that the bonding interface between the coating and the matrix had little effect on the fatigue properties of the coating.Then X-ray diffraction(XRD),scanning electron microscopy(SEM)and energy dispersive spectrometry(EDS)were used to analyze the microstructure and failure morphology.The results revealed that the subsurface failure morphology of the coatings showed a consistent correlation with rolling fatigue property after different heat treatments.The TCP phase and carbides have been shown in the laser cladding coating.The coating after stress relieved annealing exhibited chain-shaped granular carbides on the grain boundaries which could accelerate crack propagation.The aging heat treatment made small amounts of Laves phase dissolved in the coating,while the dispersed phase was precipitated which could result in the formation of pores.And the solution treatment made large amounts of Laves phase dissolved,while the rod-shape brittle phases were generated which was easy to fracture and contribute to crack initiation and spalling.

Bi-Modal Failure Mechanism of Rolling Contact Bearings

The theory of failure of rolling contact bearings is based on fluctuating high level loading and material fatigue. This theory is unimodal, considering only the solid components of the bearing, and ignoring the liquid phase, which is the lubricant. Bearing life is rather dispersed, reaching a ratio of 20 between the extreme values. Since this theory was established, several exceptional phenomena were detected that could not be explained by it, such as: 1) Pitting damage beyond the contact path;2) Detrimental effect of a minute quantity of water in the lubricant on bearing life. 25 ppm of water in the lubricant brought about shorter bearing life by over than 30%. The bimodal failure theory considers both solid and liquid bearing components. The damaging process of the lubricant evolves from its cavitation. During this process vapor filled cavities are formed in low pressure zones. When these cavities reach high pressure zones they implode exothermally. These implosions cause local high pressure pulses reaching 30,000 at accompanied by a temperature rise of about 2000 degrees K [1]. This paper includes cavitation erosion test results on stainless steel samples by vibratory and water tunnel test rigs. Various methods of lubricant dehydration are presented and evaluated. The main conclusion from this analysis is the use of water-free lubricants, for long life of RC bearings and more uniform service life thereof.

Ideal Failure Curve of Rolling Contact Bearings

The prevailing cumulative failure curves of Rolling Contact Bearings (RCB) have two main drawbacks: they begin at the origin and have a large dispersion. The purpose of this study is to develop an ideal failure curve and overcome the present drawbacks. The ideal failure curve of RC bearings is obtained by applying a water-free lubricant to the tested bearings. This eliminates the cavitation erosion from the Bimodal failure mechanism and the synergistic effect with the mechanical failure mode. This new concept considers the fatigue process involved in the failure mechanism and suggests decreasing the dispersion of bearing life.

Analysis and Numerical Simulation of Rolling Contact between Sphere and Cone

In non-conforming rolling contact, the contact stress is highly concentrated in the contact area. However, there are some limitations of the special contact model and stress model used for the theoretical study of the phenomenon, and this has prevented in-depth analysis of the associated friction, wear, and failure. This paper is particularly aimed at investigating the area of rolling contact between a sphere and a cone, for which purpose the boundary is determined by the Hertz theory and the geometries of the non-conforming surfaces. The phenomenon of stick-slip contact is observed to occur in the contact area under the condition of no-full-slip（Q 〈 μ·P）. Using the two-dimensional rolling contact theory developed by CARTER, the relative positions of the stick and slip regions and the distribution of the tangential force over the contact area are analyzed. Furthermore, each stress component is calculated based on the Mc Ewen theory and the idea of narrow band. The stress equations for the three-dimensional rolling contact between the sphere and the cone are obtained by the principle of superposition, and are used to perform some numerical simulations. The results show that the stress components have a large gradient along the boundary between the stick and slip regions, and that the maximum stress is inversely proportional to the contact coefficient and proportional to the friction coefficient. A new method for investigating the stress during non-classical three-dimensional rolling contact is proposed as a theoretical foundation for the analysis of the associated friction, wear, and failure.

Exploring the ultrahigh rolling contact fatigue life of M50 bearing steel by adjusting the cryogenic sequence

The influence of different cryogenic sequences on the rolling contact fatigue(RCF)life of M50-bearing steel has been studied.The results show that direct cryogenic treatment after quenching can effectively improve RCF life.The L_(10)life is strikingly 5 times longer than that with cryogenic treatment after tem-pering.This is caused by the distinct lattice construction of martensite and the transformation of retained austenite.More secondary nanocarbides and fine twins are formed via cryogenic treatment before tem-pering compared with cryogenic treatment after tempering.The improvement in the RCF life of the steel is attributed to the joint effects of the secondary nanocarbides and twin boundaries with a width of 5-13 nm,which delays significantly crack initiation and propagation.This study highlights a common method to improve the service life of high-carbon and high-alloy steels by adjusting the cryogenic se-quence.

Effects of Primary Carbide Size and Type on the Sliding Wear and Rolling Contact Fatigue Properties of M50 Bearing Steel

The influences of primary carbide size and type on the sliding wear behavior and rolling contact fatigue (RCF) properties of M50 bearing steel were systematically investigated under oil lubrication condition. A major breakthrough was achieved in the influence of primary carbide on tribological behavior. The opposite effect brought by primary carbide size on the sliding wear resistance and RCF life of M50 bearing steel was determined. Wear resistance increased with an increase in the studied primary carbide size, whereas RCF life decreased significantly. Compared with the 0 R and R positions with a relatively small carbide size, the wear volume of the 1/2 R position with a large carbide size was the smallest. Compared with the 0 R and R positions, the L10 life of the 1/2 R position decreased by 82.7% and 84.8%, respectively. On the basis of the statistical correlation between primary carbide size and the two tribological properties, a critical maximum carbide size of 5-10 μm was proposed to achieve optimal tribological performance. This research suggests that the equivalent diameter of the primary carbide should be controlled to be smaller than 10 μm, but further decreasing primary carbide size to less than 5 μm is unnecessary. The influence of primary carbide type in M50 bearing steel on sliding wear resistance was also discussed. Results indicate that the MC-type carbides with higher elastic modulus and microhardness exhibit better wear resistance than the M2C-type carbides.

Numerical study on fatigue crack propagation behaviors in lubricated rolling contact

The surface-initiated Rolling Contact Fatigue(RCF)including pitting and micro-pitting is one of the key issues affecting the reliability of tribological components such as gears and bearings used in various devices.In this work,a surface-initiated crack Finite Element(FE)model which considers the effect of lubricant on crack faces was developed to investigate surface-initiated RCF using an automatic crack propagating Python script.Different lubricating states,initial crack parameters and loading conditions were simulated to analyze the evolution of crack propagation and the Stress Intensity Factors(SIFs).The RCF crack propagation path and life were predicted by employing the Maximum Tangential Stress(MTS)criterion coupled with the Paris’s law.A typical RCF failure is predicted in the numerical simulation.Results reveal that the lubricating pressurization dominates the surface-initiated RCF.In addition,the initial crack angle has a significant effect on the RCF crack propagation path and the fatigue life.

Influence of white etching layer on rolling contact behavior at wheel-rail interface

The existence of narrow and brittle white etching layers(WELs)on the rail surface is often linked with the formation of rail defects such as squats and studs,which play the key roles in rail surface degradation and tribological performance.In the present study,a systematic investigation on stress/strain distribution and fatigue life of the WEL during wheel-rail rolling contact was conducted based on a numerical model considering the realistic wheel geometry.This is the first study considering the influence of rail materials,loading pressure,frictional condition,WEL geometry(a/b),and slip ratio(Sr)in the practical service conditions at the same time.The results revealed much higher residual stress in WEL than in rail matrix.Stress changes along the rail depth matched with the previously reported microstructure evolutions.The current work revealed that the maximum difference in contact stress between the wheel passages of rail matrix and the WEL region(noted as stress variation)rises with the increase of loading pressure,the value of a/b,and Sr;but drops with the friction coefficient(μ).In addition,a critical length–depth ratio of 5 for a/b has been found.The fatigue parameter,FP,of the WEL decreased quickly with the length–depth ratio when it was less than 5 and then increased slightly when it was larger than 5.This study also revealed that the fatigue life of the WEL was reduced for high strength head hardened(HH)rail compared with standard carbon(SC)rail.

Numerical analysis of three-dimensional thermo-elastic rolling contact under steady-state conditions

In this study,a three-dimensional thermo-elastic model that considers the interaction of mechanical and thermal deformation is developed using a semi-analytic method for steady-state rolling contact.Creepage types in all directions are considered in this model.For verification,the numerical analysis results of shear traction and temperature increase are compared separately with existing numerical results,and the consistency is confirmed.The analysis results include heat flux,temperature increase,contact pressure,and shear traction.Under severe rolling conditions,the thermal effect changes the behavior of the contact interface significantly.Furthermore,the effects of creepage,rolling speed,and conformity under different rolling and creep conditions are investigated.

Vertical Short Crack Initiation in Medium Carbon Bainitic Steel Under Mild Tractive Rolling Contact

To improve the current grinding procedure of the back-up roll of CVC hot rolling mills so that the back-up roll service life can be extended, the crack initiation and propagation behavior of medium carbon bainitic back-up roll steel was investigated, a kind of asperity-scale, surface originated vertical short cracks occurred at 5 × 10^2 -1 × 10^4 cycles. Theoretical analysis indicated that the maximum tensile stress occurring at the back edge of the contact of asperities keeps at above 1 347. 97 MPa, and ratcheting and cyclic plastic deformation take place at such sites within 1 × 10^4 cycles. The early initiation of the vertical short cracks is caused by the asperity contact. According to the crack initiation mechanism, short crack behavior and preventive grinding strategy, steel consumption can be reduced considerably by decreasing the surface roughness and removing the asperity influenced surface thin layer at about 70%-80% of the surface distress life.

Effects of solid friction modifier on friction and rolling contact fatigue damage of wheel-rail surfaces

In railway network,friction is an important factor to consider in terms of the service behaviors of wheel-rail system.The objective of this study was to investigate the effect of a solid friction modifier(FM)in a railway environment.This was achieved by studying the friction,wear,and rolling contact fatigue(RCF)damage on the wheel-rail materials at different slip ratios.The results showed that when a solid FM was applied,the friction coefficient decreased.After the solid FM was separated from the wheel-rail interface,the friction coefficient gradually increased to its original level.With the application of the solid FM,the wear rates of the wheel-rail decreased.In addition,the thickness and hardness of the plastic deformation layers of the wheel-rail materials were reduced.The worn surfaces of the wheel-rail were dominated by pits and RCF cracks.Without the FM,RCF cracks ranged from 84 to 120μm,and subsurface cracks were generated.However,with the FM,RCF cracks ranged from 17 to 97μm and no subsurface cracks were generated.These findings indicate possible methods of improving the performance of railway rolling stock by managing friction,and reducing wear and permanent RCF damage affecting both the wheels and rails.

Rolling Contact Fatigue Properties of SAE 8620 Steel after Case Carburizing

Rolling contact fatigue（RCF）properties of SAE 8620 steel after case carburizing have been investigated under two contact stresses of 4.0and 5.5GPa.Results show that the RCF life ranges from 2.5×10^6 to 3×10^7 cycles under the contact stress of 5.5GPa,while it can be more than 1×10^8 cycles under the contact stress of 4.0GPa.The rated fatigue life L_（10）（lives with the 10%failure）is also drastically shortened from 9.8×10^6 to 5.4×10^5 cycles when the contact stress is increased from 4.0to 5.5GPa.Theoretical calculations and fractographs show that the maximum shear stress and the contact area increase with increasing the contact stress,making RCF tend to occur earlier.

Accumulated damage process of thermal sprayed coating under rolling contact by acoustic emission technique

The accumulated damage process of rolling contact fatigue （RCF） of plasma-sprayed coatings was investigated. The influences of surface roughness, loading condition, and stress cycle frequency on the accumulated damage status of the coatings were discussed. A ball-on- disc machine was employed to conduct RCF experiments. Acoustic emission （AE） technique was introduced to monitor thc RCF process of the coatings. AE signal characteristics were investigated to reveal the accumulated damage process. Result showed that the polished coating would resist the asperity contact and remit accumulated damage. The RCF lifetime would then extend. Heavy load would aggravate the accumulated damage status and induce surface fracture. Wear became the main failure mode that reduced the RCF lifetime. Frequent stress cycle would aggravate the accumulated damage status and induce interface fracture. Fatigue then became the main failure mode that also reduced the RCF lifetime.

Review of the damage mechanism in wind turbine gearbox bearings under rolling contact fatigue

Wind turbine gearbox bearings fail with the service life is much shorter than the designed life.Gearbox bearings are subjected to rolling contact fatigue(RCF)and they are observed to fail due to axial cracking,surface flaking,and the formation of white etching areas(WEAs).The current study reviewed these three typical failure modes.The underlying dominant mechanisms were discussed with emphasis on the formation mechanism of WEAs.Although numerous studies have been carried out,the formation of WEAs remains unclear.The prevailing mechanism of the rubbing of crack faces that generates WEAs was questioned by the authors.WEAs were compared with adiabatic shear bands(ASBs)generated in the high strain rate deformation in terms of microstructural compositions,grain refinement,and formation mechanism.Results indicate that a number of similarities exist between them.However,substantial evidence is required to verify whether or not WEAs and ASBs are the same matters.

Effects of Lateral Motion on the Creep Forces in Wheel/Rail Rolling Contact

The influences of the lateral motion of a single wheelset running on a tangent railway on the creepages and creep forces between wheel and rail are investigated with numerical methods. The effect of the yaw motion of wheelset is neglected in the analysis, and Kalker’s theory of three dimensional elastic bodies in rolling contact is employed to analyze the creep forces in the wheel/rail rolling contact with Non Hertzian form.