An Investigation of the Influence of Initial Roughness on the Friction

materials

Article An Investigation of the Inﬂuence of Initial Roughness on the Friction and Wear Behavior of Ground Surfaces

Guoxing Liang 1,*, Siegfried Schmauder 2, Ming Lyu 1, Yanling Schneider 2, Cheng Zhang 1 and Yang Han 1 1 Shanxi Precision Machining Key Laboratory, Taiyuan University of Technology, Yingze west street No. 79, Taiyuan 030024, China; [email protected] (M.L.); [email protected] (C.Z.); [email protected] (Y.H.) 2 Institute for Materials Testing, Materials Science and Strength of Materials (IMWF) University of Stuttgart, Pfaffenwaldring 32, Stuttgart D-70569, Germany; [email protected] (S.S.); [email protected] (Y.S.) * Correspondence: [email protected]; Tel.: +86-0351-6018582

Received: 21 November 2017; Accepted: 29 January 2018; Published: 4 February 2018

Abstract: Friction and wear tests were performed on AISI 1045 steel specimens with different initial roughness parameters, machined by a creep-feed dry grinding process, to study the friction and wear behavior on a pin-on-disc tester in dry sliding conditions. Average surface roughness (Ra), root mean square (Rq), skewness (Rsk) and kurtosis (Rku) were involved in order to analyse the inﬂuence of the friction and wear behavior. The observations reveal that a surface with initial roughness parameters of higher Ra, Rq and Rku will lead to a longer initial-steady transition period in the sliding tests. The plastic deformation mainly concentrates in the depth of 20–50 µm under the worn surface and the critical plastic deformation is generated on the rough surface. For surfaces with large Ra, Rq, low Rsk and high Rku values, it is easy to lose the C element in, the reciprocating extrusion.

Keywords: surface roughness; friction; wear; plastic deformation

1. Introduction AISI 1045 steel groove components, with a depth to width ratio greater than two and a width of no more than 4 mm, are widely applied in small and miniaturized vane pump rotors [1]. Friction and wear usually occur in the contact zone between the vane and side profiles of the groove, which results in the failure of parts and extremely limits the performance of the pump. Higher friction coefficients means more pumping energy needed and higher operating costs. For most surfaces involved in the relative motion of contact pairs, the contact area of a rough surface is much lower than that of a smooth one. However, the contact pairs with rough surfaces usually lead to the speeding up of the damage process due to the friction and wear [2]. Hence, it is important to weaken the adverse impacts caused by the friction and wear. To do so, there are two common means, i.e., improving the surface integrity, especially decreasing the initial roughness value, and adopting special materials with excellent wear resistance. Between these two methods, the former is preferred thanks to its applicability for diverse machining processes and its lower costs [3–5]. Concerning the friction coefficient of AISI 1045 steel, Rech et al. [6] set up a friction model and described the friction coefficient at the interface of dry cutting of AISI 1045 steel with TiN coated carbide tools. The results showed that the friction coefficient has a strong dependence on the sliding velocity. Applying AISI 1045 steel, Iqbal et al. [7] studied the evolution of flow stress and the interface friction distribution. It was subsequently found that pure sliding is often an appropriate friction scheme for conventional machining at higher cutting speeds. B. Abdelali et al. [8] investigated the friction coefficient at the tool–chip–workpiece interface during the dry cutting of AISI 1045 and found that the sliding velocity is the most influential parameter compared to the contact pressure. In researching the

Materials 2018, 11, 237; doi:10.3390/ma11020237 www.mdpi.com/journal/materials Materials 2018, 11, 237 2 of 22 friction and wear characteristics of AISI 1045 [9], it was found that the wear rate increases when the contact pressure and the sliding velocity increase. The wear mechanisms of AISI 1045 steel are mainly abrasion and adhesive wear. For the same material, Yang et al. [10] also studied the wear behavior and mechanisms using a pin-on-disk high wear apparatus. With increasing temperature, the microstructure of the worn surface was softened while the wear resistance rapidly decreased. Previous studies by Kubiak [11] explained the influence of surface roughness on the contact interface and evaluated the surface finishing as a factor in the friction and wear damage process. Wang et al. [12] studied the material formation on a worn surface layer during dry rubbing and found that the sliding friction triggers the shear deformation and heat in the contact zone. Hughes et al. [13] found a dislocation substructure and a mechanically mixed layer at different depths from the surface in sliding copper tests. Hughes et al. [14] also found that a structural description based on individual dislocations or dislocations arranged in smaller groups (tangles and cell walls) is valid at low strains. Meanwhile, a gradual transition in the scaling behavior is observed in the transition from small to large strain behavior and quantitative measurements are emphasized in the paper [15]. In addition, the asperity deformation model can explain the variation of friction coefficients and the stress concentration induced by friction. Two different patterns in a deformed surface imply the localized material flows. Lu et al. [16] investigated the plastic deformation on the surface by means of a surface mechanical attrition treatment and found that as more dislocations are activated, the grain size decreases gradually near the surface [17]. Other publications reported similar deformations caused by plastic flow in the surface of steels, such as the ball impact test on plastic steels [18], and steel cutting operations [19–21]. It was also found that the surface texture influences the plastic deformation in ductile materials after sliding [22]. M. Sedlacek et al. [23] investigated the correlation between surface roughness parameters and friction and found that the surfaces with higher kurtosis and negative skewness values tend to reduce friction. Meanwhile, they [24] investigated the influence of surface preparation on roughness parameters and the correlation between roughness parameters and friction and wear. A. Dzierwa [25] found that the initial surface topography has a significant influence on friction and wear levels under dry sliding conditions. W. L. Lu et al. [26] implied that the friction torque varies nonlinearly with the increase of surface roughness and surface roughness shows the opposite influence on wear under two different texture directions. Moreover, different topographies can also affect the tribological behavior due to the various contact mechanics. In studies of contact mechanics [27–29], the contact area rose linearly with the applied load and was inversely proportional to the RMS (root-mean-square) gradient of the surface profile. B. N. J. Persson et al. derived the boundary conditions for the stress probability distribution function for elastic, elastoplastic and adhesive contact between solids. Meanwhile, they studied the distribution of interfacial separation in the contact region between two elastic solids with randomly rough surfaces [30,31]. Hyun et al. [32] provided a method for determining contact geometry as a function of load on a constructed surface. Most previous studies emphasized the appearance of the worn grooves, the ridged features, the material delamination and wear after sliding tests [33–35]. These focused on the origins of the occurrence other than the material plastic deformation in the wear layer. Bumps, tears and crack-like features had also been observed in sliding tests [36]. The present observations also showed that wear particles can be developed during sliding in even only 1–2 sliding passes according to the folding-type mechanism. It suggested a mechanism for the initial steady wear that does not require chip formation by cutting due to the interaction of asperity surfaces [37,38]. Different initial roughness parameters result in different wear behavior, the important factors of which also include the initial steady wear transition time, the wear loss and the deformation due to shear strain in the subsurface. Until now, these influencing aspects seem to be neglected in revealing the wear behavior. The major proposition of the current work is to investigate the friction and wear behavior of AISI 10415 steel with different initial roughness parameters. Average surface roughness (Ra), root mean Materials 2018, 11, 238 3 of 22

MaterialsThe2018 major, 11, 237proposition of the current work is to investigate the friction and wear behavior3 of of 22 AISI 10415 steel with different initial roughness parameters. Average surface roughness (Ra), root mean square (Rq), skewness (Rsk) and kurtosis (Rku) were used to characterize the surface square (Rq), skewness (Rsk) and kurtosis (Rku) were used to characterize the surface roughness. One roughness. One emphasis of our work is to explain variations in wear behavior owing to the emphasis of our work is to explain variations in wear behavior owing to the above-mentioned factors. above-mentioned factors. 2. Experiments 2. Experiments 2.1. Material 2.1. Material Commercially available AISI 1045 steel (HRC 34, Taiyuan Iron and Steel (GROUP) Co., Ltd., Taiyuan,Commercially China) was available machined AISI to six 1045 workpieces steel (HRC with 34, the Taiyuan dimensions Iron ofand100 Steel mm (GROUP)× 10 mm ×Co.,20 Ltd., mm. TheTaiyuan, grinding China) process was machined induced 25 to mmsix workpieces deep grooves with in the each dimensions specimen. of Each 100 mm groove × 10 was mm formed × 20 mm. in oneThe grinding process travel. Theinduced chemical 25 mm composition deep grooves of AISIin each 1045 specimen. steel is given Each ingroove Table was1. The formed initial in metallographicone grinding travel. microstructures The chemical of the composition workpiece material of AISI before1045 steel machining is given was in observed Table 1. by The scanning initial electronmetallographic microscope microstructures (SEM) and the of results the workpiec are showne material in Figure 1before. machining was observed by scanning electron microscope (SEM) and the results are shown in Figure 1. Table 1. Chemical compositions of AISI 1045 steel (wt. %). Table 1. Chemical compositions of AISI 1045 steel (wt %). C P Si Ca Mn Mo Ni Cr W Fe C P Si Ca Mn Mo Ni Cr W Fe 0.46 0.03 0.31 0.4 0.65 0.08 0.39 0.04 <0.01 Balance 0.46 0.03 0.31 0.4 0.65 0.08 0.39 0.04 <0.01 Balance

Figure 1. Microstructure of the investigated of AISI 1045 steel (etched with HNO3 2.5%). Figure 1. Microstructure of the investigated of AISI 1045 steel (etched with HNO3 2.5%). It can be seen that AISI 1045 steel is composed of Ferrite–Pearlite and lamellar Pearlite with a proximalIt can ratio be seen of 1:1. that The AISI shape 1045 of steel ferritic is composed and Pearlite of Ferrite–Pearlitegrains is similar and to polygonal lamellar Pearlite and equiaxed with a proximalgrains. The ratio size of of 1:1. Ferritic The shape grains of range ferritic from and Pearlite20 to 30 grainsμm mixed is similar with to same polygonal size pearlitic and equiaxed grains grains.approximately. The size All of features Ferritic show grains that range the fromworkpiece 20 to is 30 suitableµm mixed for grinding. with same size pearlitic grains approximately. All features show that the workpiece is suitable for grinding. 2.2. Grinding-Induced Variety of Surface Roughness 2.2. Grinding-Induced Variety of Surface Roughness The sample preparation is formed by the creep-feed dry grinding process (CFDG) on a MV-40 The sample preparation is formed by the creep-feed dry grinding process (CFDG) on a MV-40 computer numerical control (CNC) precision vertical machining center (Wintec Precision Machinery computer numerical control (CNC) precision vertical machining center (Wintec Precision Machinery Co., LTD.,Taiwan, China) and a spindle revolution between 10 and 10,000 r/min. The variety of Co., LTD.,Taiwan, China) and a spindle revolution between 10 and 10,000 r/min. The variety of surface surface roughness was induced by using different grinding parameters in the CFDG. Thermal roughness was induced by using different grinding parameters in the CFDG. Thermal damage is not damage is not allowed on the ground surface. The grinding setup is shown in Figure 2a. The allowed on the ground surface. The grinding setup is shown in Figure2a. The grinding wheel used in grinding wheel used in the whole experiment was an electroplated monolayer CBN (Cubic Boron the whole experiment was an electroplated monolayer CBN (Cubic Boron Nitride) with a diameter of Nitride) with a diameter of 220 mm and a thickness of 2.0 mm, as shown in Figure 2b. The bond 220 mm and a thickness of 2.0 mm, as shown in Figure2b. The bond agent is Nickel–Fe alloy. The CBN agent is Nickel–Fe alloy. The CBN abrasive with a grain size of 100–120 meshes was employed and abrasive with a grain size of 100–120 meshes was employed and its distribution on the wheel is shown its distribution on the wheel is shown in Figure 2c. in Figure2c. The detection instruments used for the micrograph are the TESCAN scanning electron The detection instruments used for the micrograph are the TESCAN scanning electron microscope microscope CSM-100X (SEM) (TESCAN CHINA, Ltd., Shanghai, China), M-800X optical microscope CSM-100X (SEM) (TESCAN CHINA, Ltd., Shanghai, China), M-800X optical microscope (Shanghai (Shanghai Institute of Optics and Fine Mechanics. CAS, Shanghai, China) and Think Focus SM-1000 Institute of Optics and Fine Mechanics. CAS, Shanghai, China) and Think Focus SM-1000 3D scanning 3D scanning system (Sixian Photoelectric technology (Shannghai) Co., Ltd. Shanghai, China). system (Sixian Photoelectric technology (Shannghai) Co., Ltd. Shanghai, China). Oxford energy

Materials 2018, 11, 237 4 of 22 Materials 2018, 11, 238 4 of 22 dispersiveOxford energy spectrometer dispersive X-act spectrometer (EDS) (Oxford X-act Instruments, (EDS) (Oxford Abingdon, Instruments, UK) was appliedAbingdon, to measure UK) was the weightapplied percentages to measure ofthe elements. weight percentages of elements.

Figure 2. The detailed information of experiment setup: (a) The experiment setup; (b) the Figure 2. The detailed information of experiment setup: (a) The experiment setup; (b) the electroplated electroplated monolayer CBN; (c) CBN abrasive distribution. monolayer CBN; (c) CBN abrasive distribution. The design requirements must be fulfilled for the groove geometry, i.e., the width of 2 mm, the depthThe of 25 design mm and requirements the fillet radius must no be more fulfilled than for 0.3 the mm. groove The feature geometry, of th i.e.,e microstructure the width of on 2 mm, the theground depth surface of 25 mm should and thenot fillet be radiuschanged no in more CFDG. than 0.3Figure mm. 3a The shows feature the of theamplified microstructure photographic on the groundstructure surface of the should groove not cross-section be changed near in CFDG. to the Figure sample3a showsbottom. the Three amplified positions photographic of the groove structure are ofpresented. the groove Position cross-section 1 denotes near the to groove the sample side, bottom.position Three2 denotes positions the groove of the bottom groove and are presented.position 3 Positiondenotes 1the denotes groove the fillet. groove After side, the position grinding 2 denotes process, the Figure groove 3b–d bottom show and the position corresponding 3 denotes themetallographic groove fillet. microstructures After the grinding at three process, different Figure positions.3b–d show The the common corresponding characteristic metallographic of these microstructures atlies three in differentthe consistency positions. of The the common ferrite and characteristic the pearlite. of these A microstructurescomparison of liesthe inmicrostructures the consistency before of the and ferrite after andthe CFDG, the pearlite. the subs Aurface comparison textures of showed the microstructures in Figure 3b–d before present and aftergood theagreement CFDG, the with subsurface that of texturesFigure 1. showed Furthermore, in Figure and3b–d as present desired, good no agreement phase transformation with that of Figureappears1. during Furthermore, the grinding and as process. desired, The no phase results transformation confirm that the appears CFDG during does not the introduce grinding process.thermal Thedamage results into confirm the subsurface that the CFDGof ground does specimens. not introduce It may thermal be concluded damage intothat thethe subsurfaceCFDG is a oftechnique ground specimens.with the potential It may beto concludedimprove the that grinding the CFDG performance is a technique during with machining the potential components to improve with the grindingnarrow–deep performance groove structures. during machining components with narrow–deep groove structures. Sectioning along along the the groove groove longitudinal longitudinal direction, direction, the theside side profiles profiles of the of ground the ground surface surface were wereobtained obtained for preparing for preparing the samples. the samples. The size The of size each of eachsample sample is 25 ismm 25 mm× 10 ×mm10 × mm 10 ×mm.10 mm.The Thesamples samples with with 3D topography, 3D topography, SEM SEM microstructure microstructure an andd roughness roughness parameters parameters are are listed listed in in Table Table 22.. The samples werewere conductedconducted withwith variousvarious grindinggrinding feedfeed ratesrates ((vvww)) whenwhen thethe grindinggrinding speed is equal to 93 m/s. Each Each type type of of roughness roughness parameter parameter listed listed in in the the Table Table 22 isis anan averagedaveraged valuevalue measuredmeasured onon fivefive different locations. From Table2 2,, it it cancan bebe seenseen thatthat thethe texturestextures ofof thethe groundground surfacesurface areare quitequite differentdifferent fromfrom eacheach other, and that the initial roughness parameters are not obvious asas thethe feedfeed raterate decreases.decreases. The feed rate has observable effects on the surfacesurface texture andand roughnessroughness valuesvalues duringduring CFDG.CFDG. It can alsoalso bebe seenseen thatthat similarsimilar surfacessurfaces havehave veryvery differentdifferent standardstandard roughness parameters, as shown in Table 2 2.. TheThe parameterparameter ofof Ra Ra describes describes the the height height variations. variations. TheThe parameterparameter ofof RqRq hashas aa higher sensitivity to deviations from the main lineline than Ra. The The parameter parameter of of Rsk can be used to describe occasionaloccasional deep deep valleys valleys or or high high peaks. peaks. The The parameter parameter of Rku of describesRku describes the probability the probability density sharpnessdensity sharpness of the profile of the [ 24profile]. The [24]. roughness The roughn parameteress parameter results of Ra,results Rq, of Rk Ra, and Rq, Rsk Rk are and collected Rsk are in Figurecollected4. in Figure 4.

Materials 2018, 11, 237 5 of 22 Materials 2018, 11, 238 5 of 22

Figure 3. Microphotography and subsurface metallographic texture of ground specimen: (a) Figure 3. Microphotography and subsurface metallographic texture of ground specimen: Microphotograph of specimen; (b–d) subsurface metallographic texture on groove side-surface, on (a) Microphotograph of specimen; (b–d) subsurface metallographic texture on groove side-surface, on groove bottom and in groove transition fillet area, respectively. groove bottom and in groove transition ﬁllet area, respectively.

Materials 2018, 11, 237 6 of 22

MaterialsMaterials 2018 2018, 11, ,11 238, 238 6 of6 of22 22 Materials 2018, 11, 238 Table 2. Detailed information on the roughness and topography of the samples. 6 of 22

MaterialsMaterialsMaterials 2018 2018, 201811, ,238 11, 11, 238, 238 6 of 622 6of of 22 22 Samples 3D Topography of GroundTableTable Surfaces 2. 2.Detailed Detailed information information on SEM on the the ofroughness roughness Ground and Surfaces and topography topography of ofthe the samples. samples. Ra (µ m) Rq (µm) Rsk Rku Table 2. Detailed information on the roughness and topography of the samples. RaRa (μ (m)μm) Rq Rq (μ (m)μm) Rsk Rsk Rku Rku TableTableTable 2. Detailed 2. 2. Detailed Detailed information information information on theon on theroughness the roughness roughness and and topographyand topography topography of the of of thesamples. the samples. samples.Ra (μ m) Rq (μm) Rsk Rku

Ra Ra(μRam) ( μ(μm)m) Rq Rq( Rqμm) ( μ(μm)m) Rsk Rsk Rsk Rku Rku Rku No. 1 1.127 1.52 −0.182 7.63 1.1271.127 1.521.52 −0.182−0.182 7.637.63 1.127 1.52 −0.182 7.63

1.1271.1.127 127 1.521.52 1.52 −0.182−−0.1820.182 7.63 7.63 7.63

No. 2 0.805 0.970 −0.0764 3.24 No. 2 0.805 0.970 −−0.0764 3.24 No.No. 2 2 0.0.805805 0.970 0.970 −0.07640.0764 3.243.24

No.No. No.2 2 2 0.8050.0.805 805 0.9700.9700.970 − 0.0764−−0.07640.0764 3.24 3.24 3.24

No.No. 3 3 0.6890.689 0.8670.867 0.07480.0748 2.812.81 No. 3 0.689 0.867 0.0748 2.81 No. 3 0.689 0.867 0.0748 2.81 No.No. No.3 3 3 0.68900.689 .689 0.8670.8670.867 0.07480.07480.0748 2.81 2.81 2.81

No.No. 4 4 0.4490.449 0.5430.543 0.1310.131 2.702.70 No. 4 0.449 0.543 0.131 2.70

No.No. No.4 4 4 0.4490.0.449 449 0.5430.5430.543 0.1310.1310.131 2.70 2.70 2.70 MaterialsMaterials 2018 2018, 11, 11238, 238 6 of6 22 of 22

TableTable 2. Detailed 2. Detailed information information on onthe the roughness roughness and and topography topography of theof the samples. samples.

RaRa (μ m)(μm) Rq Rq (μ m)(μm) Rsk Rsk Rku Rku

1.1271.127 1.521.52 −0.182−0.182 7.637.63

No.No. 2 2 0.8050.805 0.9700.970 −0.0764−0.0764 3.243.24

Materials 2018, 11, 237 7 of 22

Materials 2018, 11, 238 7 of 22 No.No. 3 3 Table 2. Cont. 0.6890.689 0.8670.867 0.07480.0748 2.812.81 Materials 2018, 11, 238 7 of 22

Samples 3D Topography of Ground Surfaces SEM of Ground Surfaces Ra (µm) Rq (µm) Rsk Rku

No. 5 0.263 0.314 0.169 2.44 No. 5 0.263 0.314 0.169 2.44 MaterialsMaterials 2018 2018, 11No.,, 11238, 4 238 0.449 0.543 0.131 2.707 of7 22of 22 No.No. 4 4 0.4490.449 0.5430.543 0.1310.131 2.702.70

No.No. 5 5 0.2630.263 0.3140.314 0.1690.169 2.442.44 No. 6 0.108 0.140 0.359 2.13 No. 6 0.108 0.140 0.359 2.13

No. 5 0.263 0.314 0.169 2.44

No.No. 6 6 0.1080.108 0.1400.140 0.3590.359 2.132.13

No. 6 0.108 0.140 0.359 2.13 Materials 2018, 11, 238 8 of 22

MaterialsMaterials2018 2018, 11,, 23711, 238 8 of 22 8 of 22

Figure 4. The surface roughness variation trend of different specimens. Figure 4. The surface roughness variation trend of different specimens. Figure 4. The surface roughness variation trend of different specimens. 2.3. Friction and Wear Testing 2.3. Friction and Wear Testing 2.3. FrictionBefore and theWear friction Testing and wear test, the GGr15 and AISI 1045 steel specimens were cleaned in an ultrasonicBefore the cleaning friction machine and wear using test, an the acetone GGr15 bath and. Then AISI the 1045 specimens steel specimens presenting wereon the cleaned ground in an surface were employed for the friction and wear testing on the CFT-1 (Lanzhou Zhongke Kaihua ultrasonic cleaning machine using an acetone bath.bath. Then the specimens presenting on the ground Technology Development Co., Lanzhou, China), multi-function, pin-on-disc machine, as shown in surface were employed for the friction and wear testing on the CFT-1 (Lanzhou(Lanzhou Zhongke Kaihua TechnologyFigure 5. Development During the experiment, Co., Lanzhou, the temperatur China), multi-function,e of the experiment pin-on-disc was 20 °C machine, and the asrelative shown in Technologyhumidity Development was 23–25%. TheCo., cylindrical Lanzhou, pin China), contains multi-function, a 5 mm diameter pin-on-disc ball made machine, from the as GCr15 shown in Figure5. During the experiment, the temperature of the experiment was 20 ◦C and the relative Figurebearing 5. During steel with the a experiment,hardness of 63 the (HRC) temperatur and the chemicale of the compositions experiment are was listed 20 in°C Table and 3. the relative humidity waswas 23–25%.23–25%. The The cylindrical cylindrical pin pin contains contains a 5 mma 5 diametermm diameter ball made ball frommade the from GCr15 the bearingGCr15 bearingsteel with steel a hardness with a hardness of 63 (HRC) of 63 and (HRC) the chemicaland the chemical compositions compositions are listed are in listed Table3 in. Table 3.

Figure 5. The pin-on-disc tester.

Table 3. Chemical compositions of GGr15 steel (wt %). C Si FigureMn 5. TheNi pin-on-disc P tester. S Mo Cr Cu Figure 5. The pin-on-disc tester. 0.95~1.05 0.15~0.35 0.20~0.40 ≤0.03 ≤0.027 ≤0.02 ≤0.10 1.3~1.65 ≤0.025 Table 3. Chemical compositions of GGr15 steel (wt %). The nylon cage wasTable utilized 3. Chemical to hold the compositions ball and avoid of GGr15 the vibration steel (wt and %). noise. An external load of 10 N wasC applied alongSi the axisMn direction Ni of the cylinder P pressing S on Mo the aforementioned Cr Cu ball in the sliding0.95~1.05C test. The0.15~0.35 Sistroke of0.20~0.40 the Mn table was Ni≤0.03 set as≤ P50.027 mm and S≤0.02 the spindle Mo≤0.10 rotation1.3~1.65 Cr was ≤ Cu8000.025 r/min. Meanwhile,0.95~1.05 the 0.15~0.35friction frequency0.20~0.40 and≤ 0.03amplitude≤0.027 were≤0.02 recorded≤0.10 by 1.3~1.65an acceleration≤0.025 sensor (JHK-ZHJ-3)The nylon cage (Beijing was utilizedCentrwin to Technology. hold the ball Co., and Ltd. avoid Beijing, the vibration China.) andtogether noise. with An theexternal data load of 10 N was applied along the axis direction of the cylinder pressing on the aforementioned ball in The nylon cage was utilized to hold the ball and avoid the vibration and noise. An external theload sliding of 10 N test. was The applied stroke along of the the table axis direction was set ofas the5 mm cylinder and the pressing spindle on rotation the aforementioned was 800 r/min. ball Meanwhile,in the sliding the test. friction The stroke frequency of the tableand wasamplitude set as 5 mmwere and recorded the spindle by rotationan acceleration was 800 r/min.sensor (JHK-ZHJ-3)Meanwhile, the (Beijing friction Centrwin frequency andTechnology. amplitude Co., were Ltd. recorded Beijing, by anChina.) acceleration together sensor with (JHK-ZHJ-3) the data

Materials 2018, 11, 237 9 of 22

Materials 2018, 11, 238 9 of 22 (Beijing Centrwin Technology. Co., Ltd. Beijing, China.) together with the data acquisition instrument (RP-3500)acquisition (Rigol instrument Technologies (RP-3500) INC, (Rigol Suzhou, Technologies China). The INC, samples Suzhou, with China). various The surface samples roughness with parametersvarious surface were roughness tested to obtain parameters the friction were coefﬁcient, tested to theobtain friction the frequency,friction coefficient, the friction the amplitude, friction thefrequency, wear width, the friction the wear amplitude, depth, the the weight wear width, and the the volume wear depth, loss, the the friction weight coefﬁcient and the volume and then loss, to observethe friction the coefficient material deformation and then to inobserve the subsurface. the material deformation in the subsurface.

3. Results and Discussion

3.1. Friction Coefficient Coefficient The evolution ofof thethe frictionfriction coefficientcoefficient versus versus time time was was obtained obtained for for a runninga running time time of of 1800 1800 s, ass, shownas shown in Figurein Figure6. The 6. initial The stageinitial wearstage transition wear transition time, the time, friction the coefficient friction coefficient oscillation frequencyoscillation (Freq) and its mean amplitude (E ) value are also noted in the figures. frequency (Freq) and its mean amplitudeAmpl (EAmpl) value are also noted in the figures.

(a) (b)

(e) (f)

Figure 6. Time dependencies of the friction coefﬁcients:coefficients: (a–f) No. 1–No. 6 coefficient coefﬁcient as a function of the sliding time.

Figure 6 shows the evolution of the friction coefficients of the each sample as a function of the sliding time. One can conclude that the measured curves are characterized by the oscillations of the friction coefficients regardless of the values of amplitudes and frequencies. A similar phenomenon has been presented in the literature [39]. The vertical red dot line presents the start point of the stable

Materials 2018, 11, 237 10 of 22

Figure6 shows the evolution of the friction coefficients of the each sample as a function of the sliding time. One can conclude that the measured curves are characterized by the oscillations of the friction coefficients regardless of the values of amplitudes and frequencies. A similar phenomenon has been presented in the literature [39]. The vertical red dot line presents the start point of the stable friction time. As a criterion of distinguishing the initial wear from the steady wear, there is a flat tendency with a sudden reduction in amplitude occurring on the friction coefficient curves and the transition of the initial steady stage is identifiable. The friction coefficient linear fitting is also marked with a blue dashed line and a pink arrow line to denote the tendency of the friction coefficient in different stages. For sample No. 1, the friction coefficient fluctuates greatly (Figure6a). The average friction coefficient is 0.76. The value of EAmpl is 0.53 and Freq is 22.9. In the initial stage, the friction coefficient experiences a relatively steep rise. After the time of 1308 s, it shows a slightly declined tendency and smooth pattern. A similar pattern occurs with sample No. 2, with a friction coefficient with a drastic fluctuation in the initial stage, and almost equivalent values of average friction coefficient (0.72), EAmpl (0.44) and Freq (22.7). The clear distinction information is the transition time of 1017 s (Figure6b). In Figure6c, it can be observed that the fluctuation mode of the friction coefficient is different from the curves displayed in Figure6a,b. The friction coefficient performs similar fluctuation modes during the sliding test. It does not seem to burst the sudden reduction of the amplitude in Figure6c. According to the results monitored by the data acquisition instrument, the frequency of the friction coefficient decreased rapidly at the time of 952 s, and then became flat. The values of the friction coefficient (0.65), EAmpl (0.37) and Freq (19.4) can also be obtained in the sliding test. In Figure6d, greater fluctuation of the friction coefficient is also distributed in the initial stage. The distinction of amplitude is much evident. It is obvious that the curve becomes smooth behind the transition time of 680 s. The values of the friction coefficient (0.63), EAmpl (0.35) and Freq (20.1) were found in the test. Comparing the results in the aforementioned four figures, a prominent sudden tendency of friction coefficients occurred at time of 417 s (Figure6e), and 66 s (Figure6f). Analogously, the values of friction coefficient (0.57), EAmpl (0.24) and Freq (12.4) were found in sliding sample No. 5. The values of friction coefficient (0.53), EAmpl (0.13) and Freq (11.4) were found in sliding sample No. 6. The results of the curves show that the transition time at the end of the initial stages is different from each sample in the sliding test.The time at that point was marked with the red dot line in each graphy. After this point, the trends of the friction coefficient remains relatively steady and flat, as shown by the pink arrow lines marked in Figure6. According to the fitting line, the friction coefficient of all samples has a slightly increasing tendency, although the testing results are very unstable at the initial stage. After the transition time, the gross trend of the curve becomes flat and tends to be smooth, which indicates that the transition from the initial wear to the steady wear takes place. Furthermore, wave modes of the friction coefficient present diversity in frequency and oscillation amplitude as well as the transition time of the initial stage. According to the description mentioned above, the largest EAmpl of 0.53 and the largest Freq of 22.9 Hz were monitored in sliding sample No. 1. On the contrary, the smallest EAmpl of 0.13 and the smallest Freq of 11.4 Hz were detected in testing sample No. 6 (Figure6f). Overall, the E Ampl and the Freq show a declining tendency, with a decrease of the initial roughness values. This performance, appearing in pin-on-disk testing, has been declared in other publications [40]. Besides, a similar decline also happens in the initial steady transition time in the sliding test. Focusing on the complete results shown in Figure6, the initial roughness parameters play a crucial role in determining the friction coefficient and the initial steady transition time directly, while their gross tendencies are shown in Figure7. Materials 2018, 11, 237 11 of 22 Materials 2018, 11, 238 11 of 22

(a) (b)

Figure 7. VariationsVariations of friction coefficient coefﬁcient and initialinitial-steadysteady transition time with respect to initial roughness parameters: ( a–d) corresponding to Ra, Rq, Rsk and Rku, respectively.

In Figure 7a,b, the averaged friction coefficient decreases with the increasing of the initial In Figure7a,b, the averaged friction coefficient decreases with the increasing of the initial roughness values of Ra and Rq, while the initial steady wear transition time behaves in the opposite roughness values of Ra and Rq, while the initial steady wear transition time behaves in the opposite manner. The lower the friction coefficient is, the longer the transition period will be. Under this manner. The lower the friction coefficient is, the longer the transition period will be. Under this condition, for a surface with high roughness values of Ra and Rq, the friction and the wear behavior condition, for a surface with high roughness values of Ra and Rq, the friction and the wear behavior will reach their own stable stage more quickly and remain at the flattened level until the end of the will reach their own stable stage more quickly and remain at the flattened level until the end of the test. test. Hence, the wear transition mode is easy to control in the running components. Analogously, Hence, the wear transition mode is easy to control in the running components. Analogously, smaller smaller initial roughness values of Ra and Rq lead to a higher friction coefficient, which is in initial roughness values of Ra and Rq lead to a higher friction coefficient, which is in agreement with agreement with the results reported by other authors [41]. In the case of samples with larger initial the results reported by other authors [41]. In the case of samples with larger initial roughness values of roughness values of Ra and Rq, a longer period would be taken to form the new surface in the Ra and Rq, a longer period would be taken to form the new surface in the sliding track. Consequently, sliding track. Consequently, specimens with an asperity surface need much more sliding time to specimens with an asperity surface need much more sliding time to reach the stable stage. The wear reach the stable stage. The wear transition will come into the stable stage when the initial rough transition will come into the stable stage when the initial rough surface is removed thoroughly. surface is removed thoroughly. In Figure7c, the initial roughness value of Rsk also has an effect on the average coefficients. With In Figure 7c, the initial roughness value of Rsk also has an effect on the average coefficients. the value of Rsk increment, the friction coefficient increases and the initial steady wear transition time With the value of Rsk increment, the friction coefficient increases and the initial steady wear shortens. The parameter of Rsk with a negative value means that deep valleys and few high peak are transition time shortens. The parameter of Rsk with a negative value means that deep valleys and generated on the sample in the CFDG process, which looks like a plateau smooth surface. Therefore, few high peak are generated on the sample in the CFDG process, which looks like a plateau smooth a small tangential force will be induced during sliding over each valley and a smaller friction coefficient surface. Therefore, a small tangential force will be induced during sliding over each valley and a is found on the rough surface in sliding tests, such as for sample No. 1 and No. 2. Besides this, the new smaller friction coefficient is found on the rough surface in sliding tests, such as for sample No. 1 surface in the sliding track will be generated within a short time. and No. 2. Besides this, the new surface in the sliding track will be generated within a short time. In Figure7d, it can be seen that the averaged friction coefficient first increases and then becomes In Figure 7d, it can be seen that the averaged friction coefficient first increases and then flat with a rising Rku, and the initial steady wear transition time tendency introduces the opposite case. becomes flat with a rising Rku, and the initial steady wear transition time tendency introduces the The surface has some high peaks and deep valleys when the value of Rku exceeds three. Negative Rsk opposite case. The surface has some high peaks and deep valleys when the value of Rku exceeds and Rku exceeding three occurred in samples No. 1 and No. 2, in which deep valleys and high peaks three. Negative Rsk and Rku exceeding three occurred in samples No. 1 and No. 2, in which deep formed in the CFDG. One can also find that samples with larger Rku result in smaller average friction valleys and high peaks formed in the CFDG. One can also find that samples with larger Rku result in smaller average friction coefficients. The larger the Rku, the higher the cantilever between the valley and peak will be. Then the strength of the cantilever decreases. Consequently, the tangential

Materials 2018, 11, 237 12 of 22

Materials 2018, 11, 238 12 of 22 coefﬁcients. The larger the Rku, the higher the cantilever between the valley and peak will be. Then forcethe strength generated of the in cantilever the sliding decreases. test declines Consequently, as well as the the tangential friction forcecoefficient. generated Nevertheless, in the sliding more test slidingdeclines travel as well can as realize the friction new surface. coefﬁcient. Nevertheless, more sliding travel can realize new surface. The results noted in Figure 66 alsoalso showshow thatthat thethe initialinitial roughnessroughness parametersparameters alsoalso affectaffect thethe oscillation Freq andand the oscillation EAmpl, ,as as given given in in Figure Figure 8.8.

(a) (b)

Figure 8.8.The The friction friction parameters parameters of Freqof Freq and EAmpland EA asmpl a function as a function of the initial of roughnessthe initial parameters:roughness (parameters:a–d) corresponding (a–d) corresponding to Ra, Rq, Rsk to andRa, Rku,Rq, Rsk respectively. and Rku, respectively.

In Figure 8a,b, it can be seen that a larger roughness of Ra and Rq lead to a higher frequency In Figure8a,b, it can be seen that a larger roughness of Ra and Rq lead to a higher frequency and and amplitude of the friction coefficient. Fitting lines indicate that the Freq and EAmpl of friction amplitude of the friction coefficient. Fitting lines indicate that the Freq and E of friction coefficients coefficients present an incremental tendency when the surface becomes rougher.Ampl If the texture along present an incremental tendency when the surface becomes rougher. If the texture along the horizontal the horizontal surface is severely uneven, the oscillation Freq will become high. If the texture surface is severely uneven, the oscillation Freq will become high. If the texture perpendicular to the perpendicular to the surface is intensely bumpy, the oscillation EAmpl will be greater. Therefore, the surface is intensely bumpy, the oscillation E will be greater. Therefore, the rougher surface will rougher surface will induce a larger fluctuationAmpl for the tangential forces along the sliding direction. induce a larger fluctuation for the tangential forces along the sliding direction. When the ball slides When the ball slides over the surface, the friction coefficient curve will present various vibration over the surface, the friction coefficient curve will present various vibration modes and the friction modes and the friction coefficient is not stable on the rough surface. Furthermore, it is reported in coefficient is not stable on the rough surface. Furthermore, it is reported in [42] that diverse surface [42] that diverse surface textures could be a reason inducing the oscillation in sliding tests. textures could be a reason inducing the oscillation in sliding tests. In Figure 8c, the Freq and the EAmpl of the friction coefficient present the decline tendency as the In Figure8c, the Freq and the E of the friction coefficient present the decline tendency as the Rsk increases. A larger positive Rsk Amplmeans a sample surface with high peaks and shallow valleys. It Rsk increases. A larger positive Rsk means a sample surface with high peaks and shallow valleys. It is reported in publication [23] that positive Rsk can provide a greater actual contact area and a large is reported in publication [23] that positive Rsk can provide a greater actual contact area and a large number of peaks between sliding ball and surface. As a result, the smaller Freq and EAmpl of the number of peaks between sliding ball and surface. As a result, the smaller Freq and E of the friction coefficient can be noted on the samples with large Rsk in sliding tests. Ampl friction coefficient can be noted on the samples with large Rsk in sliding tests. On the contrary, it can be seen in Figure 8d that both Freq and EAmpl of the friction coefficient On the contrary, it can be seen in Figure8d that both Freq and E of the friction coefficient first increases and then becomes flat with the increase in Rku. The sampleAmpl with the large Rku results first increases and then becomes flat with the increase in Rku. The sample with the large Rku results in a high Freq and EAmpl friction coefficient due to a lot of high peaks and deep valleys on the sample surface.

Materials 2018, 11, 237 13 of 22

in a high Freq and EAmpl friction coefﬁcient due to a lot of high peaks and deep valleys on the sample surface. Materials 2018, 11, 238 13 of 22 3.2. The Cross-Section Proﬁle of Worn Surface Analysis 3.2. The Cross-Section Profile of Worn Surface Analysis To understand well the friction and wear behavior of AISI 1045 steel, the worn grooves and To understand well the friction and wear behavior of AISI 1045 steel, the worn grooves and their cross-section and wear loss were studied after the sliding test. The representative worn surface their cross-section and wear loss were studied after the sliding test. The representative worn surface topographytopography is shownis shown in in Figure Figure9. 9.

Figure 9. Microphotographs of the worn surface: (a) microphotograph of the worn groove surface; Figure 9. Microphotographs of the worn surface: (a) microphotograph of the worn groove surface; (b,c) microphotographs of the left-side and right-side ridge of the worn groove respectively; (d) (b,c) microphotographs of the left-side and right-side ridge of the worn groove respectively; (d) section section profile of the worn groove. profile of the worn groove. An enlarged view of the worn groove is shown in Figure 9a. The ground surface is worn out alongAn enlargedthe sliding view direction of the wornand a groove typical is wear shown mark in Figure with 9a a.width The groundof about surface 2 mm is can worn be outclearly along theobserved. sliding direction On both sides and aof typical the worn wear grooves, mark two with st aripes width of ridges of about were 2 produced mm can be due clearly to the observed.plastic Ondeformation both sides of in the the worn sliding grooves, test, as two indicated stripes in of Figure ridges were9b,c. Such produced ridges due could tothe be plasticattributed deformation to the in theplastic sliding flow. test, When as indicatedthe GCr15 in ball Figure slides9b,c. against Such the ridges surface, could the bedisplacement attributed toin thethe plasticsliding flow.direction When thecauses GCr15 the ball tangential slides against force the to surface,induce the the material displacement plastic in deformation the sliding directionwhich mainly causes flows the tangentialin two forcedirections, to induce i.e., the the material sliding plasticdirection deformation and the perpen whichdicular mainly direction flows into twothe sliding directions, track. i.e., Along the slidingthe sliding direction, the material was partly pulled up and bumped ahead of the ball, then was direction and the perpendicular direction to the sliding track. Along the sliding direction, the material compressed in the groove or removed from the worn surface. Shown in Figure 9d, compressed was partly pulled up and bumped ahead of the ball, then was compressed in the groove or removed bumps that are not removed are helpful for debris to grow in the reciprocal sliding interaction. The component force perpendicular to the sliding track is caused by the normal force, which compresses the material flow to two sides of the groove, then the ridges formed. Two-dimensional

Materials 2018, 11, 237 14 of 22 from the worn surface. Shown in Figure9d, compressed bumps that are not removed are helpful for debris to grow in the reciprocal sliding interaction. The component force perpendicular to the sliding Materialstrack is 2018 caused, 11, 238 by the normal force, which compresses the material ﬂow to two sides of the groove,14 of 22 then the ridges formed. Two-dimensional cross-sectional proﬁles of the groove are obtained in the cross-sectionalmiddle of the track,profiles as of shown the groove in Figure are 10obtained, and the in shapesthe middle of the of ridgesthe track, on bothas shown sides in of Figure the worn 10, andgroove the areshapes also of presented. the ridges In on each both graph, sides theof the depth worn of groove the worn are groove also presented. and the height In each of graph, the ridges the depthwere measured. of the worn groove and the height of the ridges were measured.

(a) (b)

(e) (f)

Figure 10. Cross-section profiles proﬁles of worn grooves for mentioned six specimens: ( a–f)) cross-section profile,proﬁle, the ridges on both sides of the worn grooves in the middle of the sliding track.track.

For the cross-section profile of sample No. 1 (Figure 10a), the groove has a profile with dimensions of 177.21 μm depth and 1.92 μm width. On both sides of the groove, two ridges can be observed. The height of the left-side ridge is 4.47 μm and the right-side ridge is 4.63 μm. For the profile of sample No. 2 (Figure 10b), the groove with a depth of 153.6 μm and a width of 2.08 μm appears shallower than that of sample No. 1. The height of left-side ridge is 8.52 μm and the right-side ridge is 6.64 μm. For sample No. 3 (Figure 10c), the groove profile shape has a similar geometry as aforementioned. A profile with a depth of 179.30 μm and a width of 1.64 μm was

Materials 2018, 11, 237 15 of 22

For the cross-section profile of sample No. 1 (Figure 10a), the groove has a profile with dimensions of 177.21 µm depth and 1.92 µm width. On both sides of the groove, two ridges can be observed. The height of the left-side ridge is 4.47 µm and the right-side ridge is 4.63 µm. For the profile of sample No. 2 (Figure 10b), the groove with a depth of 153.6 µm and a width of 2.08 µm appears shallower than that of sample No. 1. The height of left-side ridge is 8.52 µm and the right-side ridge is 6.64 µm. For sample No. 3 (Figure 10c), the groove profile shape has a similar geometry as aforementioned. A profile with a depth of 179.30 µm and a width of 1.64 µm was obtained. No. 4 (Figure 10d) indicates a groove profile of 153.26 µm depth and 2.02 µm width. The two ridges are of 2.16 µm and 4.45 µm depth. In Figure 10d, the ridge height (left-side, 16.51 µm and right-side, 16.77 µm) on both sides of the groove increases. The groove profile presents 134.37 µm depth and 1.31 µm width (Figure 10e). As for the sample No. 6 (Figure 10f), the depth of the worn groove is 147.05 µm. The ridge height (left-side, 20.83 µm and right-side, 15.94 µm) and groove width (1.38 µm) can also be measured after the sliding test. Overall, the depth of the grooves varies with respect to the initial roughness value and there is an ascending trend of depth with the increment of the initial roughness value. It may be possible to conclude that the larger initial surface roughness value of the specimens results in a longer running-in duration [43]. Furthermore, the longer running-in duration causes the longer initial wear stage. In this stage, the rough surface may lead to rapid damage by the sliding ball in the contact zone and a large amount asperity material has been flattened or removed. Hence, it is reasonable to conclude that the depth of the groove increases as the initial surface roughness values of Ra and Rq become larger. The cross-section profiles also show that the plastic deformation occurred on the sides of the worn grooves. Furthermore, there is no obvious regular change in the height of the ridges when the initial roughness value varies. Another interesting phenomenon is that the ridges are almost very rough. For sample No. 5 (Figure 10e) and sample No. 6 (Figure 10f), higher ridges occurred in the samples with initial roughness parameters of smaller Ra, lower Rq, larger Rsk and smaller Rku. According to the topography of the samples listed in Table2, a rougher surface has a higher rib-like uplift than a smoother surface. It looks like the ribs adhere to the steel plate. A rougher surface with larger Ra and Rq, especially with higher Rsk, means a larger height of the ribs, which increases the strength in a certain area close to the surface. When the ball is sliding over the surface, the plastic deformation perpendicular to the sliding track becomes more difficult than that along the sliding track. Then, little plastic deformation is produced perpendicular to the sliding track and high ridges can hardly be found in samples with a rough surface. The samples were completely cleaned with anhydrous alcohol and weighted using a digital balance with an accuracy of ±0.1 mg. By comparison with the sample mass weighted before the wear tests, one obtains the weight loss. The results of the volume loss were derived by evaluating the mean area of three measured cross-sections and multiplying by the length of the wear track. These results also indicate a trend for the wear rates for the different surfaces. For a better understanding of the wear behaviour of AISI 1045 steel specimens with various roughness values, the weight/volume loss versus the initial roughness is plotted in Figure 11. In Figure 11, the scale of the volume loss is from 0 to 14 mm3 and the scale of the weight loss is from 0 to 109.9 mg. When one divides 109.9 by 14, a ratio of 7.85 mg·mm−3 (the density of specimen) is achieved. Normally, the slopes of two fitting lines should be consistent. It is obvious that the slope of the weight loss is smaller than that of the volume loss, which can be explained by the fact that the side ridge volume was not taken into consideration when calculating the volume loss. However, the material of the ridge on both sides of the groove comes from the material deformed due to the plastic deformation, as mentioned before. The trends of linear fitting lines increase with the increment of the initial roughness value Ra and Rq (Figure 11a,b). Inversely, the wear loss decreases with the increment of Rsk (Figure 11c). In the case of the dry sliding, the dominant wear mechanism is abrasion [23]. The peaks on the surface with a high Rku value will easily get worn away and become Materials 2018, 11, 238 15 of 22

obtained. No. 4 (Figure 10d) indicates a groove profile of 153.26 μm depth and 2.02 μm width. The two ridges are of 2.16 μm and 4.45 μm depth. In Figure 10d, the ridge height (left-side, 16.51 μm and right-side, 16.77 μm) on both sides of the groove increases. The groove profile presents 134.37 μm depth and 1.31 μm width (Figure 10e). As for the sample No. 6 (Figure 10f), the depth of the worn groove is 147.05 μm. The ridge height (left-side, 20.83 μm and right-side, 15.94 μm) and groove width (1.38 μm) can also be measured after the sliding test. Overall, the depth of the grooves varies with respect to the initial roughness value and there is an ascending trend of depth with the increment of the initial roughness value. It may be possible to conclude that the larger initial surface roughness value of the specimens results in a longer running-in duration [43]. Furthermore, the longer running-in duration causes the longer initial wear stage. In this stage, the rough surface may lead to rapid damage by the sliding ball in the contact zone and a large amount asperity material has been flattened or removed. Hence, it is reasonable to conclude that the depth of the groove increases as the initial surface roughness values of Ra and Rq become larger. The cross-section profiles also show that the plastic deformation occurred on the sides of the worn grooves. Furthermore, there is no obvious regular change in the height of the ridges when the initial roughness value varies. Another interesting phenomenon is that the ridges are almost very rough. For sample No. 5 (Figure 10e) and sample No. 6 (Figure 10f), higher ridges occurred in the samples with initial roughness parameters of smaller Ra, lower Rq, larger Rsk and smaller Rku. According to the topography of the samples listed in Table 2, a rougher surface has a higher rib-like uplift than a smoother surface. It looks like the ribs adhere to the steel plate. A rougher surface with larger Ra and Rq, especially with higher Rsk, means a larger height of the ribs, which increases the strength in a certain area close to the surface. When the ball is sliding over the surface, the plastic deformation perpendicular to the sliding track becomes more difficult than that along the sliding track. Then, little plastic deformation is produced perpendicular to the sliding track and high ridges can hardly be found in samples with a rough surface. The samples were completely cleaned with anhydrous alcohol and weighted using a digital balance with an accuracy of ±0.1 mg. By comparison with the sample mass weighted before the wear

tests,Materials one2018 obtains, 11, 237 the weight loss. The results of the volume loss were derived by evaluating16 ofthe 22 mean area of three measured cross-sections and multiplying by the length of the wear track. These results also indicate a trend for the wear rates for the different surfaces. For a better understanding of theparticles. wear behaviour The generated of AISI particles 1045 steel together specimens with with the sliding various ball roughness intensify va thelues, rubbing the weight/volume effect, which lossgives versus rise to the larger initial wear roughness loss generated is plotted on thein Figure sample 11. with a high Rku value, as shown in Figure 11d.

Materials 2018, 11, 238 16 of 22 (a) (b)

FigureFigure 11.11. TheThe volume volume and and weight weight losses losses as a function as a function of the initial of roughnessthe initialvalue: roughness (a–d) correspondingvalue: (a–d) correspondingto Ra, Rq, Rsk andto Ra, Rku, Rq, respectively. Rsk and Rku, respectively.

In Figure 11, the scale of the volume loss is from 0 to 14 mm3 and the scale of the weight loss is In fact, surfaces with large roughness values of Ra and Rq, together with high Rku and more from 0 to 109.9 mg. When one divides 109.9 by 14, a ratio of 7.85 mg·mm−3 (the density of specimen) negative Rsk, imply more high peaks and deep valleys existing on the samples. In the initial stage, is achieved. Normally, the slopes of two fitting lines should be consistent. It is obvious that the slope the size of the worn particles is generally between 10 and 100 µm. Those particles cannot get into the of the weight loss is smaller than that of the volume loss, which can be explained by the fact that the traps between the peaks and valleys on the surface and begin to cut the material as a tool driven by side ridge volume was not taken into consideration when calculating the volume loss. However, the the sliding ball. Therefore more material in the contact zone will be removed. material of the ridge on both sides of the groove comes from the material deformed due to the plastic A smoother surface has a lower volume and weight loss than a rougher one. As for the friction deformation, as mentioned before. The trends of linear fitting lines increase with the increment of and wear, a possible detail found in the experiment is that the loss is approximately on the same level the initial roughness value Ra and Rq (Figure 11a,b). Inversely, the wear loss decreases with the when the initial roughness is below a certain limit (about Ra 0.3–0.5 µm, Rq 0.1–0.4 µm). These results increment of Rsk (Figure 11c). In the case of the dry sliding, the dominant wear mechanism is indicate that decreasing the roughness values of Ra and Rq does not involve too much beneﬁt in terms abrasion [23]. The peaks on the surface with a high Rku value will easily get worn away and of improving the wear resistance when the surface roughness value is low enough. become particles. The generated particles together with the sliding ball intensify the rubbing effect, which3.3. Analysis gives ofrise Material to larger Plastic wear Deformation loss generated in the on Worn the Layer sample with a high Rku value, as shown in Figure 11d. InWear fact, debris surfaces and with particles large have roughness been observed values onof Ra the and worn Rq, surfaces together after with sliding highthe Rku sample and more with negativea rough surface,Rsk, imply as shown more inhigh Figure peaks 12 a.and The deep wear valleys debris existing may be explainedon the samples. by the In fretting the initial wear stage, mode theoccurring size of inthe a localizedworn particles zone [is44 generally]. When thebetween ball slides 10 and over 100 the μm. ridges Those between particles two cannot stripes get on into the therough traps surface, between oscillation the peaks with and a small valleys amplitude on the surface appears and only begin in a localizedto cut the zone material between as a thetool ball driven and bythe the rough sliding surface. ball. Then,Therefore oscillatory more material tangential in the vibrations contact arezone generated will be removed. in the relative sliding motion. SuchA a smoother phenomenon surface seems has to a be lower caused volume by the and fretting weight wear loss process. than a Frettingrougher wear one. inducesAs for the damage friction to and wear, a possible detail found in the experiment is that the loss is approximately on the same level when the initial roughness is below a certain limit (about Ra 0.3–0.5 μm, Rq 0.1–0.4 μm). These results indicate that decreasing the roughness values of Ra and Rq does not involve too much benefit in terms of improving the wear resistance when the surface roughness value is low enough.

3.3. Analysis of Material Plastic Deformation in the Worn Layer Wear debris and particles have been observed on the worn surfaces after sliding the sample with a rough surface, as shown in Figure 12a. The wear debris may be explained by the fretting wear mode occurring in a localized zone [44]. When the ball slides over the ridges between two stripes on the rough surface, oscillation with a small amplitude appears only in a localized zone between the ball and the rough surface. Then, oscillatory tangential vibrations are generated in the relative sliding motion. Such a phenomenon seems to be caused by the fretting wear process. Fretting wear induces damage to pits in which powder particles are usually filled, as shown in Figure 12b,c. Furthermore, wear fringes, grooves as well as surface micro-cracks are produced in the localized zone.

Materials 2018, 11, 237 17 of 22 pits in which powder particles are usually ﬁlled, as shown in Figure 12b,c. Furthermore, wear fringes, groovesMaterials 2018 as well, 11, 238 as surface micro-cracks are produced in the localized zone. 17 of 22

Materials 2018, 11, 238 17 of 22

(a) (b)(c)

FigureFigure 12. 12. SEM(SEMa graphs) graphs of worn of wornsurface: surface: (a–c) the ( asimilar–c) the(b characteristics)( similar characteristics of fretting ofwear fretting in localizedc) wear zone. in localized zone. FigureIn order 12. SEM to graphsobtain ofa wornmore surface:thorough (a– cunderstandi) the similar characteristicsng of the initial of fr ettingroughness wear ineffect localized on the zone. wear behavior, SEM was used to trace the material plastic deformation. The specimens to be examined In order to obtain a more thorough understanding of the initial roughness effect on the wear wereIn sectioned order to perpendicularobtain a more to thorough the worn understandi surface andng along of the the initial sliding roughness track. The effect features on theobserved wear behavior, SEM was used to trace the material plastic deformation. The specimens to be examined were behavior,in the cross SEM section was ofused the toworn trace samples the material are shown plas ticin Figuredeformation. 13. The specimens to be examined sectionedwere sectioned perpendicular perpendicular to the to worn the surfaceworn surface and along and along the sliding the sliding track. track. The features The features observed observed in the crossin the section cross section of the wornof the samplesworn samples are shown are shown in Figure in Figure13. 13.

Figure 13. SEM graphs of plastic deformation under the worn surface: (a–f) present the plastic flow

microstructures of No. 1–No. 6 specimens, the red rectangular area denotes the detection position of Figurethe EDS. 13.13. SEMSEM graphsgraphs ofof plasticplastic deformationdeformation under the wornworn surface:surface: (a–f) present the plasticplastic ﬂowflow microstructures ofof No.No. 1–No. 6 specimens, the red rectangularrectangular area denotes the detection position of thetheFor EDS.EDS. the samples No. 1–No. 4, some extreme plastic deformation was generated in the subsurface layer (Figure 13a–d) with a thickness ranging from 10–50 μm. Specimen No. 5 has a mild deformedFor the layer samples with a No.thickness 1–No. of 4, about some 20 extreme μm, and plastic little plastic deformation deformation was isgenerated observed in in thethe subsurfacesubsurface layer layer (Figure of specimen 13a–d) wiNo.th 6.a thicknessStructurally, ranging this fromdeformation 10–50 μ m.layer Specimen may be No. divided 5 has ainto mild a deformedfragmentation layer zone with (close a thickness to the wornof about surface) 20 μ m,and and a textured little plastic grain deformation zone (under is the observed fragmentation in the subsurfacezone). It is clearlayer thatof specimen a larger deformation No. 6. Structurally, is generated this indeformation the fragmentation layer may zone. be One divided can see into that a fragmentationthe strain gradually zone (close grows, to thestarting worn fromsurface) the and dee apest textured layers grain of the zone text (underured thegrain fragmentation zone to the zone). It is clear that a larger deformation is generated in the fragmentation zone. One can see that the strain gradually grows, starting from the deepest layers of the textured grain zone to the

Materials 2018, 11, 237 18 of 22

For the samples No. 1–No. 4, some extreme plastic deformation was generated in the subsurface layer (Figure 13a–d) with a thickness ranging from 10–50 µm. Specimen No. 5 has a mild deformed layer with a thickness of about 20 µm, and little plastic deformation is observed in the subsurface layer of specimen No. 6. Structurally, this deformation layer may be divided into a fragmentation zone (close to the worn surface) and a textured grain zone (under the fragmentation zone). It is clear that a larger deformation is generated in the fragmentation zone. One can see that the strain gradually grows, starting from the deepest layers of the textured grain zone to the fragmentation zone. The deformation of the grain boundaries is too critical to be observed besides a few stripes of distorted residual boundaries remaining in the subsurface layer. It looks like a phenomenon of material flowing below the subsurface layer, although the material is not mobile in accordance with the rheology theory. In the textured grain zone, the grains are slightly deformed by the shear stress coming from the fragmentation zone. The critical deformation in the subsurface layer is presented in Figure 13a (sample No. 1). However, a slight deformation is observed in the sample No. 6 (Figure 13f). This can be explained by the fact that the initial roughness parameters with higher Ra, Rq and larger Rku induce larger tangential forces along the sliding track when the normal force is constant. The larger tangential force causes the greater shear stress and deformation generated in the subsurface layers. The severe shear deformation induces the grain boundary to be destroyed. With a high enough strain, one can expect the plastic deformation in the sliding testing. In this case, the wear debris is trapped and accumulated in the traps of the oscillating surfaces. The rougher the surface is, the stronger the tendency to form debris will be, especially the surface with a high Rku value. The wear particles are generated as the ball ploughs the surface. Detached wear particles experience micro-cutting, adhesive junctions and spalling to form the new surface. Simultaneously, nucleation voids will develop into the cracks on the surface and subsurface layers by further sliding. The aforementioned cracks hasa length of about 10 µm in the subsurface of sample No. 1 (Figure 13a) during the sliding process. Additionally, subsurface cracks will be nucleated, and then the debris and the particles will be generated due to microstructural defects in the material. Generally, the void nucleation and crack development are always generated in the fragmentation zones and the textured grain zone, especially in the former. Depending on the mechanisms of the void nucleation [45], the dislocation pile-up and the grain boundary slip contribute to the void nucleation due to the large plastic strain caused by the sliding. In sliding tests, the higher tangential force leads to a larger plastic strain in the subsurface layer when the ball slides over a rougher surface. Therefore, the voids are liable to be nucleated under a rougher surface in sliding tests. According to the experimental results, it is not difficult to infer that a higher friction coefficient will lead to a thicker plastic deformed layer, which is accompanied by a higher plastic strain under the worn surface. EDS was employed to confirm that the elements change in the fragmentation zone. The results are shown in Figure 14. The peaks of ferrite (Fe), carbon (C), silicon (Si), manganese (Mn) and oxygen (O) in the fragmentation zones (marked in Figure 13) are shown together with the EDS spectra (Figure 14). It appears that the chemical compositions are different from each other. The largest weight percentage of Fe is 100%, which was detected in the marked area of specimen No. 1. No other elements were detected except the Fe element. The largest weight percentage of the C element was found in specimen No. 6 after the sliding test, as displayed in Figure 14f. It can be pointed out that a transition was generated in the sliding tests. The results also implied that little cementite was observed and the grains were refined in the fragmentation zone, which was explained with the mechanism of the ultrafine structure in the ductile material [13]. In order to study the C element in the subsurface, EDS measurements performed for the six specimens were derived from three different positions at the same depth of 10 µm under the worn surface. The mean values of the C element content under the worn surfaces are shown in Figure 15. As presented in the Figure 15, minor weight percentages of C are detected in samples with smaller Ra and Rq values, as well as low Rsk and high Rku values. This implies that the initial roughness parameters affect the loss of the C element. To be confirmed, a surface with high Ra, high Rq, low Rsk Materials 2018, 11, 238 18 of 22 fragmentation zone. The deformation of the grain boundaries is too critical to be observed besides a few stripes of distorted residual boundaries remaining in the subsurface layer. It looks like a phenomenon of material flowing below the subsurface layer, although the material is not mobile in accordance with the rheology theory. In the textured grain zone, the grains are slightly deformed by the shear stress coming from the fragmentation zone. The critical deformation in the subsurface layer is presented in Figure 13a (sample No. 1). However, a slight deformation is observed in the sample No. 6 (Figure 13f). This can be explained by the fact that the initial roughness parameters with higher Ra, Rq and larger Rku induce larger tangential forces along the sliding track when the normal force is constant. The larger tangential force causes the greater shear stress and deformation generated in the subsurface layers. The severe shear deformation induces the grain boundary to be destroyed. With a high enough strain, one can expect the plastic deformation in the sliding testing. In this case, the wear debris is trapped and accumulated in the traps of the oscillating surfaces. The rougher the surface is, the stronger the tendency to form debris will be, especially the surface with a high Rku value. The wear particles are generated as the ball ploughs the surface. Detached wear particles experience micro-cutting, adhesive junctions and spalling to form the new surface. Simultaneously, nucleation voids will develop into the cracks on the surface and subsurface layers by further sliding. The aforementioned cracks hasa length of about 10 μm in the subsurface of sample No. 1 (Figure 13a) during the sliding process. Additionally, subsurface cracks will be nucleated, and then the debris and the particles will be generated due to microstructural defects in the material. Generally, the void nucleation and crack development are always generated in the fragmentation zones and the textured grain zone, especially in the former. Depending on the mechanisms of the void nucleation [45], the dislocation pile-up and the grain boundary slip contributeMaterials 2018 to, 11 the, 237 void nucleation due to the large plastic strain caused by the sliding. In sliding19 tests, of 22 the higher tangential force leads to a larger plastic strain in the subsurface layer when the ball slides over a rougher surface. Therefore, the voids are liable to be nucleated under a rougher surface in slidingand Rku tests. values According will generate to the aexperimental larger plastic results, deformation it is not in difficult a dry sliding to infer test. that Such a higher a deformation friction coefficientleads to the will abnormal lead to straina thicker misfit plastic dislocation deformed at the layer, grain which boundaries is accompanied under the by worn a higher surface plastic layer. strainThe cementite under the was worn lost duesurface. to the EDS unfavourable was employed performance to confirm in the that reciprocating the elements extrusion, change which in the is fragmentationan explanation zone. from The another result points are of shown view ofin theFigure voids 14. generated under a rougher surface as well.

Materials 2018, 11, 238 19 of 22 (a) (b)

(e) (f)

FigureFigure 14. EDS measurement results in the fragmentation zones: ( (aa––ff)) EDS EDS spectrums spectrums analysis analysis and and thethe elements elements compositions compositions in in the the fragmentation fragmentation zo zonesnes taken taken from from the the squared zones 1–6 marked in FigureFigure 13,13, respectively.

The peaks of ferrite (Fe), carbon (C), silicon (Si), manganese (Mn) and oxygen (O) in the fragmentation zones (marked in Figure 13) are shown together with the EDS spectra (Figure 14). It appears that the chemical compositions are different from each other. The largest weight percentage of Fe is 100%, which was detected in the marked area of specimen No. 1. No other elements were detected except the Fe element. The largest weight percentage of the C element was found in specimen No. 6 after the sliding test, as displayed in Figure 14f. It can be pointed out that a transition was generated in the sliding tests. The results also implied that little cementite was observed and the grains were refined in the fragmentation zone, which was explained with the mechanism of the ultrafine structure in the ductile material [13]. In order to study the C element in the subsurface, EDS measurements performed for the six specimens were derived from three different positions at the same depth of 10 μm under the worn surface. The mean values of the C element content under the worn surfaces are shown in Figure 15.

Materials 2018, 11, 237 20 of 22 Materials 2018, 11, 238 20 of 22

FigureFigure 15. 15. TheThe weight weight percentages percentages of of the the C C element. element.

As presented in the Figure 15, minor weight percentages of C are detected in samples with 4. Conclusions smaller Ra and Rq values, as well as low Rsk and high Rku values. This implies that the initial roughnessBased parameters on the discussion affect the in theloss present of the C work, element. the following To be confirmed, five points a surface can be with concluded: high Ra, high Rq, low Rsk and Rku values will generate a larger plastic deformation in a dry sliding test. Such a (1) AISI 1045 steel surfaces with higher initial roughness values of Ra, Rq and Rku result in a larger deformation leads to the abnormal strain misfit dislocation at the grain boundaries under the worn average friction coefficient and a longer initial steady wear transition period in the sliding friction surface layer. The cementite was lost due to the unfavourable performance in the reciprocating tests. Surfaces with low Rsk and high Rku values have little benefit for improving the wear extrusion, which is an explanation from another point of view of the voids generated under a resistance and tribological properties in the dry sliding test. rougher surface as well. (2) For the AISI 1045 steel in dry sliding tests, it was found that the weight and the volume loss 4. Conclusionskeep approximately the same level for specimens with initial roughness values below a certain limit (Ra 0.3–0.5 µm, Rq 0.1–0.4 µm). In this case, this is not preferable for improving the wear Basedresistance on the by discussion further decreasing in the present Ra and work, Rq. the following five points can be concluded: (3) The plastic deformation mainly concentrates in the depth of 20–50 µm under the worn surface. (1) AISI 1045 steel surfaces with higher initial roughness values of Ra, Rq and Rku result in a Critical plastic deformation is generated in the samples with surface roughness parameters of larger average friction coefficient and a longer initial steady wear transition period in the higher Ra, Rq and Rku. sliding friction tests. Surfaces with low Rsk and high Rku values have little benefit for (4) The Fe element weight percentage measured in the fragmentation zones decreases with increasing improving the wear resistance and tribological properties in the dry sliding test. initial roughness values of Ra and Rq, whereas a 100% content was found in the sample with the (2) For the AISI 1045 steel in dry sliding tests, it was found that the weight and the volume loss lowest initial roughness values of Ra and Rq. This indicates that the refinement of the grains took keep approximately the same level for specimens with initial roughness values below a certain place during the sliding. limit (Ra 0.3–0.5 μm, Rq 0.1–0.4 μm). In this case, this is not preferable for improving the wear (5) The initial roughness parameters affect the loss of the C element in the worn surface. Surfaces resistance by further decreasing Ra and Rq. with large Ra, Rq, low Rsk and high Rku values easily lose the C element in dry sliding. (3) The plastic deformation mainly concentrates in the depth of 20–50 μm under the worn surface. Critical plastic deformation is generated in the samples with surface roughness parameters of Acknowledgments:higher Ra, Rq andThis Rku. work was supported by the National Natural Science Foundation of China (grant number 51575375). (4) The Fe element weight percentage measured in the fragmentation zones decreases with Authorincreasing Contributions: initial Guoxingroughness Liang values carried of outRa literatureand Rq, search,whereas data a acquisition100% content and was manuscript found editing.in the Guoxing Liang and Yang Han performed manuscript review. All authors have read and approved the content of the manuscript.Siegfriedsample with the Schmauder lowest initial and Ming roughness Lyu carried values out of the Ra concepts, and Rq. design, This indicates definition ofthat intellectual the refinement content, literatureof the search, grains data took acquisition, place during data analysisthe sliding. and manuscript preparation. Yanling Schneider, Cheng Zhang (5)and YangThe Haninitial provided roughness assistance parameters for data affect acquisition, the loss data of analysis the C element and statistical in the analysis. worn surface. Surfaces Conflictswith oflarge Interest: Ra, Rq,We low declare Rsk thatand wehigh have Rku no values financial easily and lose personal the C relationships element in dry with sliding. other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position Acknowledgments: This work was supported by the National Natural Science Foundation of China (grant presented in, or the review of, the manuscript entitled. number 51575375).

Author Contributions: Guoxing Liang carried out literature search, data acquisition and manuscript editing. Guoxing Liang and Yang Han performed manuscript review. All authors have read and approved the content of the manuscript.Siegfried Schmauder and Ming Lyu carried out the concepts, design, definition of intellectual content, literature search, data acquisition, data analysis and manuscript preparation. Yanling

Materials 2018, 11, 237 21 of 22

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