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

CUI Huawei ,CUI XiufangWANG Haidou, *,XING Zhiguo,and JIN Guo

1 Institute of Surface/Interface Science and Technology,Harbin Engineering University,Harbin 150001,China

2 National Key Lab for Remanufacturing,Academy of Armored Forces Engineering,Beijing 100072,China

1 Introduction*

The failure of bearings,gears,and camshafts is mainly due to surface failure,in order to improve the service life of part,thermal spray technology has a widely application in preparing wear resistance,stainless resistance and fatigue resistance coatings of the parts surface[1-4].Rolling Contact Fatigue(RCF)is generally defined as cracking or pitting/delamination caused by cycling load near the contact surface layer of bodies rolling against each other,which leads to breakage and failure of contact bodies[5-10].Contact fatigue failure is a major failure mode of bearings,gears,and camshafts surface coatings under rolling contact,contact fatigue life ultimately determines service life[11-14].There is an increased demand for improve life,reliability and future applications of bearing materials to call for their use in more hostile environments.In this review,the effects of service condition on RCF failure mechanism and lifetime have introduced in terms of lubrication states,contact stresses,revolve speed,and slip ratio.

Heavy load,high speed and complex environment are the features of service condition that coatings face with the rapid development of modern industry.The outside service conditions of coating,such as lubrication conditions,contact stress,revolve speed,and rolling-sliding ratio has a significant effect of coating failures behavior and RCF lifetime.Service condition directly result in different rolling contact fatigue lifetime through different failure behavior,and then limits coatings' service life.Many scholars have done a lot of researches of service conditions on RCF.SONG,et al[15],investigated the contact fatigue resistance of lubricant with nano-diamond particles.The result revealed that increasing viscosity of the lubricant has a significant effect on RCF performance.TAIZO,et al[16],investigated the effect of slip ratio on the rolling contact fatigue property of railway wheel steel,and found that the traction coefficient increased with the increase in the slip ratio and the fatigue strength decreased simultaneously.SANTUS,et al[17],investigated the surface and subsurface rolling contact fatigue characteristic depths and proposal of stress indexes,and found that the different failure mechanism of subsurface and surface is identified by the shear stress.

The RCF failure behavior mainly generalized pitting[18-19],abrasion[20],spalling[21-22]and delamination[23]by a lot of researches of RCF behavior and RCF mechanism within the range of material property and structure.Fig.1 is the different failure behavior morphology under rolling contact fatigue[5,22,24-25].Due to the thermal spray coating failure behavior is complex and indeterminate under different service condition.It is difficult to synthesize understanding the RCF lifetime of coatings and the design parameters of service condition.Thus researching the influence service condition on RCF life and failure mechanism under service condition,and optimizing service condition parameters to improve RCF life of coatings has a great significance.This paper provides a systematic review on the current situation of RCF life study under rolling contact under different service condition and new expectation for future research.

Fig.1.Different failure behavior morphology under rolling contact fatigue

2 Effect of Lubrication Conditions

In order to avoid direct friction in rolling contact,lubrication oil is used to decrease rubbing wear and probability of contact fatigue failure as well as prolong RCF life by form a separating lubricating film.The thickness and viscosity of lubricant film are the major parameters to measure lubrication condition,which can directly affect coatings' RCF life.

The comprehensive interaction between surface roughness and film thickness is the main factors which has affected lubrication condition.If the film thickness value is greater than roughness peak magnitude,the rough surface has covered by the film,and roughness has a little effect on friction pair surface;on the contrary,direct friction has occurred between friction pair when the thickness value is smaller than roughness peak magnitude and the surface roughness peak region gets cross the lubrication film[26].In order to research the effect of lubrication condition on contact fatigue life under the comprehensive interaction between lubrication film thickness and surface roughness,the ratio(λ)of the elasto-hydrodynamic lubricant(EHL)film thickness to the average surface roughness has been used by ZHOU,et al[27].The ratio λ is decided by thickness of elastic-plastic oil film and surface roughness which calculated by

Where Rqdis the surface roughness of sample coating,and Rqpis the surface roughness of standard sample,and Hminis the minimum film thickness.Hmincan be calculatedby

Where U is the dimensionless speed parameter,G is the dimensionless material ellipse parameter,W is the dimensionless load parameter,and k is the dimensionless ellipse parameter[25,28-32].The proposing of the formulation of λ has provided a science quantify parameter for confirming the influence of EHL film thickness on surface roughness,and has put forward a contrast standard of RCF life under different EHL film thickness and surface roughness.

Many scholars researched the influence of EHL thickness on RCF life,the results of impact analysis indicate that when λ＞3,the film is thick enough.There has a full elasto-hydrodynamic lubrication(EHL)status between contact bodies,which has reduced direct asperity contact and prolonged RCF lifetime;when λ＜3,there has a mixed lubrication and boundary lubrication status between contact bodies.The direct contact abrasion has occurred due to not-fully formed film with mixed lubrication and three-body abrasion appears with boundary lubrication.Fig.2 shows the process of coating abrasion failure,insufficient lubrication causes direct contact and friction between the two surfaces[30].In the rolling process,tangential force caused by micro-slip produces tiny fines on the surfaces,which accelerated surface abrasion and decreased RCF life[25,30,33-34].The previous study has revealed that when λ＞3 the RCF lifetime is improved,but it didn't has a range.The other scholar also found out that the value of λ is not the larger the better.RCF life has increased with the increased of ratio(λ)value,and then reduced when the value of λ is smaller than 6(λ＞6)[35].

Viscosity is another important element that affects RCF lifetime in terms of lubrication condition.The high viscosity lubrication oil has significantly prolonged RCF lifetime of coatings[36-37].Lubrication oil is hard to roll and extrude under heavy press if viscosity is high enough,thus consistent oil film between contact surfaces can be ensured,which can help average surface stress and avoid stress concentration.After the fatigue cracks generated,lubrication oil will infiltrate into fatigue cracks and infiltration capacity and velocity has a direct relation with viscosity.The oil will generate a heavy pressure on fatigue cracks under contact stress,which accelerates the expansion of cracks and causes fatigue failure of coatings[30].Further studies have found that this theory only applies to low viscosity oil,and high viscosity oil cannot infiltrate into fatigue cracks[38].AHMED,et al[30],also testified the result through contact fatigue test on WC-15%Co plasma spray coating under full lubrication condition,which indicates that high viscosity oil has a little effect on RCF life due to it has less probability to cause fatigue cracks and crack expansion.In order to confirm the suitable viscosity parameter of lubrication oil,some other researches on the RCF life of gear under different lubrication oil viscosity.Higher viscosity has improved RCF lifetime,but it is not the higher,the better.When viscosity is greater than 0.044,it has increased the maximum shear stress within coating and reduced RCF lifetime[39].

Fig.2.Schematic of coating surface wear process

Therefore,under the certain out condition,high viscosity oil and thicker lubrication films leads to full lubrication on coating surface.It has reduced abrasion which caused by direct contact between rough surfaces and thus increasing RCF lifetime of work pieces.The crack expansion and coating failure which caused by oil infiltrate into cracks has a low probability during fatigue cracks expansion period.So it has less effect on RCF life of coating.The values of λ and lubrication viscosity are not the larger,the better,which should be controlled within a proper range in order to prolong RCF life.

3 Effect of Contact Stress

Contact stress is the load value in contact location of coatings under applied load,which calculated by the Hertz mode.The different contact stress can affect failure mechanism and RCF life directly.The morphology of coatings' contact fatigue failures under different load contact stresses has shown in Fig.3[40].Under heavy load,coatings has prone to spall and delamination failure and RCF life is short;under lower load,wear and pitting failure has occurred and RCF life has a greater increases than heavy load[40-41].

Fig.3.Contact fatigue mechanism under different loads

Contact fatigue lifetime of coatings has affected by different fatigue mechanisms under different contact stresses[42-44].Under lower load,coatings are prone to wear failure;while under heavy load,spalling failure has occurred;under extreme heavy load,delamination failure is the main failure mode.Under lower load,roughness contact and surface defect are the main reasons for surface wear and spalling failure.Due to mirco-contact caused by elastoplasticity deformation between coatings and planetary balls,subsurface wear failure needs a long cycle period,so coatings have a longer contact fatigue life.Under heavy load,delamination failure occurs easily.It is mainly because the depth of the maximum shear stress and the orthogonal stress within coatings has transferred to the interface of substrate and coating[42,45-47].The transfer result of shear stress has shown in Fig.4[45].The interface shear stress caused by high load has resulted in defects within the coating and substrate interface transfer to fatigue cracks during a short period,then has a fast expansion to the coating surface,and that leaded to delamination failure,thus decreasing the RCF life of coatings seriously.

Fig.4.Distributions of orthogonal shear stress and maximum shear stress within the coating and substrate with respect to different contact stresses

Under lower load,some scholars have studied the influence between abrasion rate and RCF cracks propagate rate on RCF life[48-49].It indicates that fatigue cracks are worn off before expansion when the wear speed was greater than the expansion rate of cracks,thus prolonging the RCF lifetime indirectly.The material property has changed by the contact stress under the service process,and it also can affect the RCF life of coating.FINNEY,et al[50],found that under high load,coating surface suffers high temperature due to friction,and that decreases the elastic modulus of materials as well as reduces the wear-resisting performance of coating,thus reducing RCF lifetime remarkably.The above study has expounded the regular pattern of different failure behaviors and RCF lifetime under different loads.

The coating failure is not determined by a single failure behavior.It's the competing active consequence between abrasion failure and fatigue failure.Moreover,In order to get the relationship between contact stress and RCF life,the RCF data under contact stress has analyzed to find out the regular pattern.

Profound researchers have found that the relationship between stress and life accord with inverse power law function[6].Eq.(3)shows the relationship between contact stress(S)and RCF lifetime(N):

C and m are undetermined coefficients[38,51].This empirical equation is acquired by many experimental data,which can be used to predict RCF life of other coatings.There are many parameter estimation methods to estimate undetermined coefficient for built an equation for stress and lifetime.Maximum likelihood method,least square estimation method,linear least variance method,and moment estimator method are the common parameter estimation methods at present[52-53].

The level of load has an important influence on RCF failure mechanism of coatings.Under lower load,smaller shear stress within coating leads to a smaller probability of delamination failure,and surface wear failure is the main failure which caused by rough contact for a long time,so contact fatigue lifetime is relatively long.Under heavy load,the shear stress has leads to fatigue cracks at micro-defect area within coatings.The instability of cracks expansion has accelerated,and results in delamination and spalling failure at a short time.

4 Effect of Revolve Speed

At the same level of contact stress,revolving speed is a decisive factor affect contact fatigue life of coatings.Fig.5 shows that high speed reduces contact fatigue life of coatings dramatically[50].

Fig.5.Contact stresses vs fatigue of nano-composite at different rolling speed

Different cycle load speed has caused different repeat loading at per unit time of coating.The shakedown map has presented by Johnson,which divided the response of materials into contact fatigue failure at different cycle load speed into four stages based on Hertz theory.EKBERG applied this theory to the prediction of fatigue failure behavior[54].The four stages are(a)elastic behavior,(b)elastic shakedown,(c)plastic shakedown,and(d)ratcheting[55-56].Fig.6 shows that material surface transforms from elastic behavior to cumulative damage with the increase of revolving speed,thus reducing contact fatigue life of coating[55].The alternating frequency of alternating load increases,the number of load at per unit time has increased dramatically for a regular point on coating.Then the coating does not have enough time to recover but suffers continual load and accelerated damage speed,and contact fatigue life has reduced[38].The shakedown map explains that speed changing leads to load changing under per unit time of the failure process.It has great significant on the research of coatings' failure and contact fatigue life under changing load.

Fig.6.Four different asymptotic states under cyclic loadings

With different revolving speed,the alternating frequency of shear stress within coatings has affected at the same time.With the increase of revolving speed at per unit time,coatings will suffer greater alternating shear stress.When the speed is fast enough,shear stresses has superimposed with each other and enlarged,RCF lifetime has reduced due to the cracks generated and expensed[38].This concludes has been certificated by other study[50,56-57].Moreover,high revolving speed accelerates wear and fatigue cracks initiation as shown in Fig.7[56],thus reducing RCF lifetime.

The previous study has introduced the influence of revolving speed on failure mechanism and RCF life under the certain outside condition.However,some studies have found that low speed has reduced RCF lifetime by the coupling interaction between maximum lubrication and high roughness.This is due to stress pinning increases the stress near the surface and improves the driving force for contact fatigue failure[58].Therefore,coatings have grinded and polished before applied,and the result has a little influence at earlier stage.

5 Effect of Slip Ratio

Slip ratio within contact area is inevitable with rolling contact[59].Contact region may have different tangential velocities from the axis of rotation,and results in micro-slip between contacting bodies[60-61].Fig.8 is the Multi-step rolling-sliding contact analysis procedure[62].According to China National Standard,slip ratio is defined as the quotient of speed difference between contact bodies and speed of assistant body[63].Slip ratio with rolling contact can be calculated by using the follow Eq.(4)[60]:

Fig.7.SEM photograph of rolling contact fatigue cracks at different speed

Fig.8.Multi-step rolling-sliding contact analysis procedure

Some scholars have defined the positive slip ratio and negative slip ratio in rolling/sliding contact fatigue test process.Slip ratio has defined as positive slip ratio when rolling speed of coating side is greater;otherwise,it is negative ratio[60,64].

Under pure rolling(micro-slip)condition,coating has a longer RCF life than slip ratio presence condition[65].Slip ratio has increased the contact stress,friction torque and abrasion probability,and reduced the RCF lifetime[30,60,64].Slip ratio has significant effect on the area of contact region as shown in Table 1[60].Under the same load,the width of contact region has decreased due to negative slip ratio,and RCF life has reduced sequentially by the stress increase from the width of contact region decrease.Slip ratio will generate tangential friction force under test process,RCF life reduces by 40%-90% due to the high friction torque caused by negative slip ratio[64].The generation of slip ratio will cause micro-defect on surface,which turns the corrosion failure into three-body wear according to RCF test on WC-15%Co coating[30],and thus reducing RCF lifetime.The RCF test on Mo coatings has found that the wear mass with5% slip condition is remarkably greater than pure rolling(micro-slip),and reduces RCF life[66].In a word,the presence of slip ratio has reduced RCF lifetime and accelerated abrasion failure.

Table 1.Effect of micro-slip on contact width under different contact stresses

The presence of slip ratio has reduced RCF lifetime,the positive slip ratio and negative slip ratio have different effects on the RCF life.RCF test on WC coating under rolling/slip condition has found that positive slip ratio has increased RCF lifetime compared with negative slip ratio.The positive slip ratio can prevent coating from breaking off,while the negative ratio leads to the coating's spalling,thus reducing its RCF lifetime[67].Moreover,there is less research for RCF lifetime on the influence of positive slip ratio and negative slip ratio,and the influence of slip ration on real service components such as gear and connecting rod have more study.

6 Conclusions and Expectation

(1)The RCF lifetime has decreased with the load and revolves speed increased.Under the certain load and revolve speed,high viscosity lubrication oil on the coating surface has a full elasto-hydrodynamic lubrication,which have a good RCF performance.

(2)Most of the researches only focus on a certain single influencing factor on RCF life.Coatings failure is the result of interaction between commission condition and construal integrity,which does not catch the attention from most researchers.

(3)There is not a standard for the failure degree of coating,so it is impossible to compare the findings of different researchers.

(4)There are fewer researches on data comparison between accelerated RCF life and commission life in RCF accelerated test with heavy load and high revolving speed.

(5)Both coating condition and outside condition has considered,modify the analogy mode of RCF life based on a big date in order to more accurate forecast RCF life under real commission condition.

(6)Under different failure mechanisms,standard quantification has adopted to coating failure degree by monitoring means so as to provide scientific standard for the judgments of RCF lifetime.

(7)Analysis of failure mechanisms after accelerated RCF test has conducted to compare RCF lives between accelerated test and real condition.Therefore,the real life distribution of coatings will be achieved to improve the effectiveness and correctness of RCF lifetime study.

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