FENG Xu*,ZHOU Jianzhong,MEI Yufen,HUANG Shu,SHENG Jie,and ZHU Weili

School of Mechanical Engineering,Jiangsu University,Zhenjiang 212013,China

1 Introduction

Disc brakes are widely used for reducing automobile velocity.The material used in brake discs should be able to provide a high and stable friction coefficient in order to ensure safety.The value of the coefficient of friction is an important index of tribological performance during the braking process.However,frictional heat generated at the interface of the discs and pads can lead to elevated temperatures.The extreme thermal environments will lead to undesirable effects,such as degradation of the friction coefficient known as heat fade and premature wear[1-3].Consequently,there is an increasing need to raise the friction coefficient as well as its stability of gray cast iron which is traditionally used to manufacture brake discs,in turn,to improve efficiency and reliability of the brake system.

Surface texturing has emerged in the past decades as a viable option of surface engineering,resulting in significant improvement in wear resistance and the friction coefficient of tribological mechanical components[4].Various texturing techniques are employed mainly including machining,etching techniques and laser texturing.For instance,NAKATSUJI,et al[5],utilized a roller with micro-dents indented by a diamond pyramid,to evaluate the effect of texture on lubrication and pitting durability.The results demonstrated that the micro-pockets provided beneficial effects on improving scuffing resistance and suppressing pitting corrosion.ZHOU,et al[6],textured the air-bearing surface of magnetic recording sliders by using magnetron sputtering and ion beam etching method.They investigated the effect of slider texture on the head/disc interface.Results showed that texturing of sliders was a promising way to improve the tribological performance of the head/disc interface.WAKUDA,et al[7],studied the frictional properties of silicon nitride ceramic surfaces in which dimple patterns were fabricated with abrasive jet machining and laser texturing.They verified that surface texturing was an effective method to reduce friction under lubricated conditions and the tribological characteristics depended greatly on the size and density of the micro-dimples rather than the shape of them.

Of all the practical micro-surface patterning methods,laser surface texturing(LST) is probably the most advanced so far because of its short processing times,cleanness to the environment and excellent control of theshape and size of the micro-dimples[4].It allows realization of optimum designs while avoiding build-up of material during processing,compared with mechanical methods[8].Israeli scholar,Etsion,is the pioneer to develop a series of scientific research in this area.Their research group first developed a mathematical model to predict performance of all-liquid noncontacting mechanical seals with regular micro-surface structure in the form of hemispherical pores[9].Then fundamental research work on LST for various tribological applications was carried out by several research groups worldwide.RONEN,et al[10],studied the potential use of micro-surface structure in the form of micro-pores to improve tribological properties of reciprocating automotive components.Results showed that the optimum texturing induced by laser efficiently reduced the friction losses in reciprocating automotive components.Argonne National Laboratory in the USA performed a serious of experiments to investigate the effect of LST on the transition from boundary to hydrodynamic lubrication regime[11].It was demonstrated that LST expanded the range of the hydrodynamic lubrication regime in terms of load and sliding speed and reduced the friction coefficient substantially under similar operating conditions when compared with untextured surfaces.ETSION,et al[12-15],conducted theoretical or experimental studies of the effect of LST on various applications including hydrostatic mechanical seals,parallel thrust bearings,piston pins and piston rings,successively.Results showed that friction reduction,scuffing resistance,pressure capability and surface clearance of these mechanical components were improved substantially by LST in comparison with untextured components.VILHENA,et al[16],presented that textured surfaces of 100Cr6 bearing steel,produced by pulsed Nd:YAG laser,could not only obtain a lower friction coefficient but also result in higher and steady-state friction under different test conditions respectively.

The texturing process of these above researches all used thermal effect of high-energy laser pulses to ablate partial surface regions by rapid melting and transpiration.However,surface area in close proximity to the dimple also undergoes melting and rapid resolidification because of the high intensity of the laser beam pulse.This ‘‘collateral''damage of the surface is unavoidable when using LST process based on thermal effect,leading to increases of tensile residual stress in transverse direction[17],roughness and micro-cracks around the holes[18-20].The fatigue life of the workpiece could be drastically shortened because of this change in surface integrity.

Laser peening(LP),as one of the LST methods,is a proven surface treatment technology which utilizes the laser-shock-induced mechanical effect rather than the thermal effect[21-23].CASLARU,et al[24],studied the surface topography and micro-hardness of micro-dent arrays on Al6061-T6 fabricated by LP.They focused on the effect ofdent geometry on surface tribological performance.The research indicated that a 10% density of micro-dent arrays reduced the friction coefficients compared with untreated surfaces under lubricated sliding conditions.GUO,et al[25],used an LP process to produce micro-dent arrays on the surface of Ti-6Al-4V.They demonstrated that the use of LP along with an automatic XY table proved to be an attractive and reliable method for producing micro-dent arrays with different densities and enhanced surface integrity.ZHANG,et al[26],investigated the effects of LP on the mechanical properties of laser welded ANSI 304 stainless steel joints.They demonstrated that LP process could cause brighter fracture surface without delamination splitting,as well as finer and more uniform dimples.KIM,et al[27],investigated the effects of the geometry and distribution of micro-dimples on the frictional behavior of cast iron surfaces for applications in automotive engines.The results showed that under some lubrication conditions,the effect of the depth over diameter ratio was greater than that of the surface area density ratio.LUO,et al[28],investigated the effects of grooves manufactured by LP on dry sliding wear performance.Results showed that LSP significantly improved the wear resistance of the T9 tool steel surface.Groove spacing was an important factor for improving friction coefficients and mitigating wear mass losses of specimens.KUMAR,et al[29],studied the effects of LP conditions on the fretting wear of Ti-6Al-4V against Type 52100 bearing steel.Friction log plots indicated that wear mechanisms were mostly adhesion,abrasion and delamination.Tribological performances of Ti-6Al-4V were efficiently improved with the assistance of LP.The fatigue life of engineering components was prolonged because of the compressive residual stresses induced by LP.

However,most of the previous studies all utilized surface texturing as lubricant reservoirs to reduce friction and wear in sliding and rolling contact applications.There is a dearth of data about the friction coefficient increment of gray cast iron induced by micro-dimples fabricated by LP.Moreover,the friction coefficient of brake disc surfaces degrades due to the elevated temperature during the braking process[3].It is a key issue as to how to raise the friction coefficient of brake discs,not only at room temperature but also at higher temperatures,especially the stability with hyperthermia.

Dynamic strain aging(DSA) phenomenon is a strengthening phenomenon in metals and alloys,which is caused by the interaction between the diffusive solute atoms and the moving dislocation.This interaction produces a series of strengthening effects and leads to some evident changes of the mechanical behavior of metals and alloys under adscititiousloads[30].Laser peening in DSA temperature regime can lead to the formation of a high density of dislocations and more uniform dislocation arrangement,which contribute to a higher stability of mechanical property during cyclic loading and high temperature[31].

In this paper,the surfaces of gray cast iron widely usedin brake discs are textured with micro-dimples by LP in the DSA temperature regime.Tribological behaviors of samples under dry conditions are investigated by using a UMT-2 test machine.The influence of surface area density and dimple depth over diameter on the friction coefficient of LPed samples is studied systematically at room temperature.The specimen with optimum parameters was singled out to perform variable temperature tests and the results were compared with that of untextured specimen.

2 Mechanism of LP Assisted by DSA

As illustrated in Fig.1,the surface of the metal target is covered by an opaque absorbing layer and a transparent confining layer,which are black tape and K9 glass,respectively.The target is fixed on a hot plate,while a temperature sensor is used to monitor the sample temperature to maintain the working temperature in DSA regime.

Fig.1.Schematic of process

When high-energy(GW/cm2) and short burst(ns) laser pulses irradiate onto the workpiece surface,the absorbing layer instantaneously vaporizes into high-density plasma.On one hand,the vaporization of absorbent material sucks up the heat of the high laser energy to protect the target from thermal effect.On the other hand,plasma expands rapidly in the direction away from the target surface due to the successive absorption of the laser energy.But simultaneously the rapidly expanding plasma is kept within the bounds of the sample and the confining layer,which in turn,creates a high amplitude and short duration pressure pulse,and then propagates into the material as a shock wave[32].When the pressure of the shock wave which is on the order of GPa exceeds the dynamic yield strength of the metal,local plastic deformation is produced on the surface of the metal and a micro-dimple is fabricated.Meanwhile,the process is performed during the DSA temperature regime,and the interactions between dislocations and solute atoms of gray cast iron result in repeated pinning of dislocations and thus lead to enhanced work hardening.Then,not only are pattern samples with certain geometric shapes obtained but also a series ofstrengthening microstructures on the near surface are produced by combining the advantages of laser peening and dynamic strain aging.

3 Experiment

3.1 Specimens and parameters

The specimens were prepared from HT250 cast iron with the chemical composition 3.27 C,1.97 Si,0.85 Mn,0.08 P,0.07 S,0.21Cu,the remainder Fe(all wt.%),by machining them to discs and polishing the discs surface to mirror smoothness(Ra=0.05 μm).The disc size wasφ30×5 mm.Working temperatures[33]for LP were controlled in the DSA temperature regime of gray cast iron by using a hot plate controller and temperature sensor.K9 glass was used as the confining medium because of its high shock impedance and high melting point,making it suitable for LP at elevated temperatures.Black tape was used as an ablative coating material.An Nd:YAG laser with a wavelength of 1064 nm,frequency of 1 Hz and pulse duration of 10 ns full-width at half-maximum(FWHM) with a Gaussian laser beam profile was used to produce micro-dimples on the surface of the specimens.The laser beam size used was 1 mm.Fig.2 shows the experimental setup.Discs were textured with nine patterns of micro-dimples,different in terms of the area density and ratio of the dimple depth over diameter.Table 1 lists the main surface texturing characteristics of the various patterns used in this study.

Fig.2.Experimental setup of LP assisted by DSA

Table 1.Specimen specifications

The surface area density(total dimple area/total surface area) ranged from 10% to 30%.The ratios of the dimple depth over diameter were 0.01,0.02 and 0.03 respectively by adjusting the pulse energy of the laser from 0.4 J to 1.2 J.

3.2 Friction and wear tests

Friction tests on disc samples with different texture parameters were carried out using a ball-on-disc friction machine(UMT-2,USA) in reciprocating sliding under dry conditions at room temperature first.The upper GCr15 steel ball with hardness of 750 HV was loaded against the lower disc specimen.The loading direction was parallel to the axis of rotation.The diameter and initial surface roughness(Ra) of the ball were 9.5 mm and 0.05 μm,respectively.Load and rotation speed were set to 10 N and 200 r/min.The testing time was set to 20 min and the coefficient of friction was recorded as a function of time.The friction tests were repeated with three different samples for each test condition,and the measured coefficients of friction were averaged.Then,a set of parameters resulted in the highest coefficient of friction were singled out to perform variable temperature tests while load and rotation speed were the same as the value of room temperature test.The coefficients of friction of selected specimens and untextured discs were recorded as a function of temperature from 50℃ to 400℃,the interval being 25℃.The balls and discs were cleaned with alcohol prior to each tribological test.The atmosphere surrounding the test was room air of 25±2℃ temperature and 30±5%relative humidity.An analytical balance with a precision of 0.01mg was adopted to measure wear mass loss which was used to evaluate the wear resistance.Microstructure of the worn surface was observed using SEM and the wear mechanism was studied.

4 Results and Discussion

4.1 Texture patterns

Fig.3 shows the surface topography of micro-dimple arrays at 0.8 J laser pulse energy with surface area density of 10%,20%,and 30%,respectively.

Fig.3.Surface topography of micro-dimple arrays with different surface area density

The glossy dimples with hemispherical shape emerge at the treated area after LP in DSA.The surface of the sample becomes accidented because of the impact effect of shock wave.

4.2 Friction behavior at room temperature

4.2.1 Influence of surface area density

Samples with various surface area density ratios(10%,20%,and 30%) and three different depths over diameter ratios(0.01,0.02 and 0.03) were investigated.The friction coefficients of various surface area densities under selected depth over diameter were compared,as shown in Fig.4.Coefficients of friction of all the textured discs are noticeably higher than the untextured discs and they increase nonlinearly with the enlargement of surface area density.At low surface area density values of 10% and 20%,friction coefficients with depth over diameter ratio of 0.01 are lower than that of 0.02,but higher than 0.03.Under a surface area density of 30%,no significant differences are observed between the ratio of 0.01 and 0.02.But the value of coefficient is up to 0.351 at the ratio of 0.03,which is the highest of all specimens.

Fig.4.Coefficients of friction of specimens vs.surface area density for different dimple depth over diameter

4.2.2 Influence of dimple depth over diameter

Fig.5 shows the friction coefficient for different dimple depth over diameter under given surface area density varied from 10% to 30%,respectively.

Fig.5.Coefficients of friction of specimens vs.dimple depth over diameter for different surface area density

With the ever-increasing of depth over diameter ratio,the coefficient of specimens with 10% and 20% surface area density first increase and then decrease,while for surface area density of 30%,the value of the friction coefficient always rises.Under the same depth over diameter ratio,the coefficients enhance when surface area density enlarges.

It can be seen from Fig.5 and Fig.6 that the specimen with dimple depth over diameter of 0.03 and surface area density of 30% has the highest coefficient of friction of all specimens,which dramatically rises up to 1.33 times that of untextured specimen.

4.3 Friction behavior at variable temperature

Fig.6 shows the friction coefficient of two kinds of specimens under variable temperature from 50℃ to 400℃,the interval being 25℃.One is the textured specimen with selected parameters of surface area density(30%) and depth over diameter ratio(0.03) which resulted in the highest coefficient of friction at room temperature.The other is an untextured specimen.The load and rotation speed are the same as the values of the room temperature test.It can be observed that the friction coefficients of the two kinds of specimens increase almost linearly with the increase in temperature from 25℃ to 125℃,then grow slowly and reach their respective maximum values at the temperature of 200 ℃(textured) and 225 ℃(untextured).When the temperatures vary from 250℃ to 400℃,the friction coefficient of the untextured specimen decreases sharply while that of the textured specimen declines slowly.More important is that the friction coefficients of textured specimens are always higher than those of untextured specimens at each test temperature,and the trend along with temperature is more gentle and stable.It is noteworthy that the friction coefficient of the untextured specimen is only 0.2441 when the test temperature reaches 400℃,but that value of the textured specimen is up to 0.330 5,1.35 times that of the former.

Fig.6.Friction coefficients of specimens vs.variable temperature

Friction tests explore the effect of dimple properties on friction increase.Results suggest that under dry conditions,textured specimens have the higher friction coefficient compared to untreated specimens,both at room temperature and elevated temperature.Potential explanation of this observation might be speculated as,the surfaces with the dimples are subjected to higher contact pressure because of reduction of actual contact surface.And uneven surfaces owing to dimple patterns result in the variation of surface roughness.HWANG,et al[34],demonstrated that the value of static and kinetic coefficient of friction increased with increasing contact pressure.And at a given value of normal load,the kinetic coefficient of friction increased with increasing surface roughness.This may explain why pattern samples of 30% surface area density give the best friction increase.Gentler and stabler coefficient trends of textured specimens may be attributed to the strengthening phenomenon of DSA.

4.4 Analyses of wear mass loss

Fig.7 presents the wear mass losses comparison of untextured specimens and textured specimens(density of 30% and ratio of 0.03) with different temperatures under dry friction.It can be seen that the wear mass losses of the two kinds of specimens all increase with temperature,while the losses of LPed specimens are lower than those of untreated specimens all along.On one hand,the texturing allows an easier wear debris escape from the fretted zone into the micro-dimples,thus improving the fretting fatigue resistance[4].On the other hand,LP is better as a post-processing technique to improve mechanical properties of the surface layer because it induces compressive residual stress with a depth of more than 1 mm and enhances surface microhardness[28].Consequently,scuffing resistance and thermal fatigue life are improved noticeably.

Fig.7.Wear mass losses of specimens vs.variable temperature

4.5 Analyses of worn surfaces

Owing to wear resistance of material used in brakediscs under elevated temperature being particularly critical,typical micrographs of worn surfaces of the two kinds of specimens under the highest temperature(400℃) are shown in Fig.8 and Fig.9,respectively.There are three typical morphologies on all of the micrographs.The first is characterized by ploughing grooves and aggregated wear debris,which is typical abrasive wear.The second morphology appears as adhered materials and plastic deformation.This is typical adhesive wear.The third is a common morphology in all dry sliding wears.Smooth regions and delaminated regions are found on the worn surfaces.Moreover,elevated temperature increases the wear of the material and favors oxidation[35].Delamination and exfoliation of oxide film occur easily along crack paths to accelerate the wear.

Fig.8.Micrograph of the worn surface of untextured sample

It is noticeable that the wear degree of LPed sample is obviously slighter than that of untreated sample.Massive exfoliation and deeper furrows can be observed in Fig.8.But the wear behaviors do not appear so rough,seen from Fig.9.It indicates that the textured specimen has better wear resistance compared with the untextured one.

Fig.9.Micrograph of the worn surface of textured sample

5 Conclusions

(1) Under dry conditions at room temperature,friction coefficients of all the LPed specimens are noticeably higher than those of untextured ones.The specimen with dimple depth over diameter of 0.03 and surface area density of 30% has the highest coefficient of friction,which dramatically rises up to 1.33 times that of untextured one.

(2) Under dry conditions at elevated temperature,the friction coefficients of specimens with selected parameters(density of 30% and ratio of 0.03) are always higher than that of untextured specimens at each test temperature and the trend along with temperature is more gentle and stable.It is noteworthy that the friction coefficient of untextured specimen is only 0.244 1 when the test temperature reaches 400℃,but that value of textured one is up to 0.330 5,1.35 times that of the former.

(3) For dry sliding,the wear mass losses of untextured specimens and textured specimens(density of 30% and ratio of 0.03) all increase with temperature,while the losses of LPed specimens are lower than those of untreated specimens all along.The surface of textured specimen shows better wear resistance compared with that of untreated one from the observation of typical micrographs under the highest temperature(400℃).Wear mechanism includes abrasive wear and adhesive wear,accompanying oxidation wear as well.

[1]CHICHINADZE A V.Processes in heat dynamics and modelling of friction and wear(dry and boundary friction)[J].Tribology International,1995,28(1):55-58.

[2]GAO C H,LIN X Z.Transient temperature field analysis of a brake in a non-axisymmetric three-dimensional model[J].Journal of Materials Processing Technology,2002,129(1):513-517.

[3]BELHOCINE A,BOUCHETARA M.Thermal behavior of full and ventilated disc brakes of vehicles[J].Journal of Mechanical Science and Technology,2012,26(11):3643-3652.

[4]ETSION I.State of the art in laser surface texturing[J].Journal of Tribology,2005,127(1):248-253.

[5]NAKATSUJI T,MORI A.The tribological effect of mechanically produced micro-dents by a micro diamond pyramid on medium carbon steel surfaces in rolling-sliding contact[J].Meccanica,2001,36(6):663-674.

[6]ZHOU L,KATO K,VURENS G,et al.The effect of slider surface texture on flyability and lubricant migration under near contact conditions[J].Tribology International,2003,36(4):269-277.

[7]WAKUDA M,YAMAUCHI Y,KANZAKI S,et al.Effect of surface texturing on friction reduction between ceramic and steel materials under lubricated sliding contact[J].Wear,2003,254(3):356-363.

[8]FRIEDRICH C R.Micromechanical machining of high aspect ratio prototypes[J].Microsystem Technologies,2002,8(4-5):343-347.

[9]ETSION I,BURSTEIN L.A model for mechanical seals with regular microsurface structure[J].Tribology Transactions,1996,39(3):677-683.

[10]RONEN A,ETSION I,KLIGERMAN Y.Friction-reducing surface-texturing in reciprocating automotive components[J].Tribology Transactions,2001,44(3):359-366.

[11]KOVALCHENKO A,AJAYI O,ERDEMIR A,et al.The effect of laser texturing of steel surfaces and speed-load parameters on the transition of lubrication regime from boundary to hydrodynamic[J].Tribology Transactions,2004,47(2):299-307.

[12]ETSION I,HALPERIN G.A laser surface textured hydrostatic mechanical seal[J].Tribology Transactions,2002,45(3):430-434.

[13]ETSION I,HALPERIN G,BRIZMER V,et al.Experimental investigation of laser surface textured parallel thrust bearings[J].Tribology Letters,2004,17(2):295-300.

[14]ETSION I,HALPERIN G,BECKER E.The effect of various surface treatments on piston pin scuffing resistance[J].Wear,2006,261(7):785-791.

[15]ETSION I,SHER E.Improving fuel efficiency with laser surface textured piston rings[J].Tribology International,2009,42(4):542-547.

[16]VILHENA L M,PODGORNIK B,VIZINTIN J,et al.Influence of texturing parameters and contact conditions on tribological behaviour of laser textured surfaces[J].Meccanica,2011,46(3):567-575.

[17]IORDANOVA I,ANTONOV V,GURKOVSKY S.Changes of microstructure and mechanical properties of cold-rolled low carbon steel due to its surface treatment by Nd:glass pulsed laser[J].Surface and Coatings Technology,2002,153(2):267-275.

[18]ABBOUD J H,FIDEL A F,BENYOUNIS K Y.Surface nitriding of Ti-6Al-4V alloy with a high power CO2laser[J].Optics &Laser Technology,2008,40(2):405-414.

[19]VILHENA L M,SEDLACEK M,PODGORNIK B,et al.Surface texturing by pulsed Nd:YAG laser[J].Tribology International,2009,42(10):1496-1504.

[20]LIU Z,ZHANG X,XUAN F,et al.Effect of laser power on the microstructure and mechanical properties of TiN/Ti3Al composite coatings on Ti6Al4V[J].Chinese Journal of Mechanical Engineering,2013,26(4):714-721.

[21]MONTROSS C S,WEI T,YE L,et al.Laser shock processing and its effects on microstructure and properties of metal alloys:a review[J].International Journal of Fatigue,2002,24(10):1021-1036.

[22]ZHANG W,YAO Y L.Micro scale laser shock processing of metallic components[J].Journal of manufacturing Science and Engineering,2002,124(2):369-378.

[23]YAOZ Q,LAWRENCE Y Y,FEI W.Progress in advanced laser assisted manufacturing technology[J].Chinese Journal of Mechanical Engineering,2003,39(12):57-61.

[24]CASLARU R,SEALY M P,GUO Y B,et al.Fabrication and characterization of micro dent array produced by laser shock peening on aluminum surfaces[J].Trans.NAMRI/SME,2009,37:159-166.

[25]GUO Y B,CASLARU R.Fabrication and characterization of micro dent arrays produced by laser shock peening on titanium Ti-6Al-4V surfaces[J].Journal of Materials Processing Technology,2011,211(4):729-736.

[26]ZHANG Y K,ZHANG L,LUO K Y,et al.Effects of laser shock processing on mechanical properties of laser welded ANSI 304 stainless steel joint[J].Chinese Journal of Mechanical Engineering,2012,25(2):285-292.

[27]KIM B,CHAE Y H,CHOI H S.Effects of surface texturing on the frictional behavior of cast iron surfaces[J].Tribology International,2014,70:128-135.

[28]LUO K Y,WANG C Y,LI Y M,et al.Effects of laser shock peening and groove spacing on the wear behavior of non-smooth surface fabricated by laser surface texturing[J].Applied Surface Science,2014,313:600-606.

[29]KUMAR D,AKHTAR S N,PATEL A K,et al.Tribological performance of laser peened Ti-6Al-4V[J].Wear,2015,322:203-217.

[30]QIAN K W,LI X,XIAO L,et al.Dynamic strain aging phenomenon in metals and alloys[J].Journal of Fuzhou University(Natural Sciences Edtion),2001,29(6):8-23.(in Chinese)

[31]YE C,SUSLOV S,KIM B J,et al.Fatigue performance improvement in AISI 4140 steel by dynamic strain aging and dynamic precipitation during warm laser shock peening[J].Actamaterialia,2011,59(3):1014-1025.

[32]TAN Y,WU G,YANG J M,et al.Laser shock peening on fatigue crack growth behaviour of aluminiumalloy[J].Fatigue &Fracture of Engineering Materials &Structures,2004,27(8):649-656.

[33]MOURI H,WUNDERLICH W,HAYASHI M.New aspects about reduced LCF-life time of spherical ductile cast iron due to dynamic strain aging at intermediate temperatures[J].Journal of Nuclear Materials,2009,389(1):137-141.

[34]HWANG D H,ZUMGAHR K H.Transition from static to kinetic friction of unlubricated or oil lubricated steel/steel,steel/ceramic and ceramic/ceramic pairs[J].Wear,2003,255(1):365-375.

[35]DING T,CHEN G X,ZHU M H,et al.Influence of the spring stiffness on friction and wear behaviours of stainless steel/copper-impregnated metallized carbon couple with electrical current[J].Wear,2009,267(5):1080-1086.

Biographical notes

FENG Xu,born in 1981,is currently a PhD candidate atSchool of Mechanical Engineering,Jiangsu University,China.Her research interests include laser surface texturing process and laser surface strengthen process.

Tel:+86-13921 581192;E-mail:feng_xu2009@hotmail.com

ZHOU Jianzhong,born in 1964,is currently a professor atJiangsu University,China.His research interests include laser shock peening and laser direct manufacturing,digital mould design and rapid manufacturing,CAD/CAE Technology,anti-fatigue manufacturing mechanism based on laser shock waves.

Tel:+86-13906 100256;E-mail:zhoujz@ujs.edu.cn

MEI Yufen,born in 1990,is a master candidate atSchool of Mechanical Engineering,Jiangsu University,China.Her research interests include laser surface modification and tribological properties.

Tel:+86-18361 810056;E-mail:meiyufen1990@sina.com