Article Experimental Analysis of Grease Friction Properties on Sliding Textured Surfaces
Xijun Hua 1, Julius Caesar Puoza 1,*, Peiyun Zhang 1, Xuan Xie 1 and Bifeng Yin 2 1 School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, China; [email protected] (X.H.); [email protected] (P.Z.); [email protected] (X.X.) 2 School of Automotive and Traffic Engineering, Jiangsu University, Zhenjiang 212013, China; [email protected] * Correspondence: [email protected]; Tel.: +86-156-0610-2187
Received: 4 October 2017; Accepted: 24 October 2017; Published: 27 October 2017
Abstract: There is comprehensive work on the tribological properties and lubrication mechanisms of oil lubricant used on textured surfaces, however the use of grease lubrication on textured surfaces is rather new. This research article presents an experimental study of the frictional behaviours of grease lubricated sliding contact under mixed lubrication conditions. The influences of surface texture parameters on the frictional properties were investigated using a disc-on-ring tribometer. The results showed that the friction coefficient is largely dependent on texture parameters, with higher and lower texture density resulting in a higher friction coefficient at a fixed texture depth. The sample with texture density of 15% and texture depth of 19 µm exhibited the best friction properties in all experimental conditions because it can store more grease and trap wear debris. The reduction of friction is mainly attributable to the formation of a stable grease lubrication film composed of oil film, transfer film and deposited film, and the hydrodynamic pressure effect of the surface texture, which increases the mating gap and reduces the probability of asperity contact. This result will help in understanding the tribological behaviour of grease on a textured surface and in predicting the lubrication conditions of sliding bearings for better operation in any machinery.
Keywords: grease lubrication; laser surface texturing; friction reduction; texture density; texture depth; mixed lubrication
1. Introduction With the rapid development of industrialization, friction and wear reduction between two sliding bodies requires the constant presence of a lubricant within the contact area in order to reduce and control friction and wear, thus reducing energy consumption, extending service life of mechanical systems and enhancing their reliability and safety [1–3]. In many situations the lubricant is automatically fed into the contact area by the relative movement of the friction pair itself. However, there are limitations that constrain the feeding of lubricants in some situations. For example: (i) when the load is high, the surfaces deform and adapt to each other and the interface between them becomes very tight; (ii) when the sliding distance is small relative to the extension of the contact area, especially in reciprocated sliding and fretting, the lubricant is not effectively fed into the interface; (iii) greases are not able to readily flow back (returning) into a contact zone in the same way as an oil lubricant due to their relatively low fluidity (high viscosity), in comparison with lubricating oil [4]. Laser surface texturing, which was pioneered by Etsion et al. [5,6] as a surface-engineering technique, has emerged as a viable technology to keep grease where it is needed and to maintain lubrication for longer periods of time in order to address these concerns [7]. Applications of laser surface texturing on contacting parts such as piston ring-cylinder liners [8], cams/tappets [9], bearings [10], seals [11] and cutting tools [12,13] can result in a significant reduction in friction and wear. Generally, surface texture may be positive, in that it protrudes out of the surface, or negative,
Lubricants 2017, 5, 42; doi:10.3390/lubricants5040042 www.mdpi.com/journal/lubricants Lubricants 2017, 5, 42 2 of 11 for example dimples or holes. The dimpled surface texture is more popular due to its advantages in terms of micro-lubrication and ease of manufacturing. The role of surface texturing in improving the tribological properties varies based on the contact conditions—whether they are hydrodynamic (full), mixed, boundary or dry conditions. In the case of full hydrodynamic and mixed lubrication conditions, the micro-craters serve as micro-hydrodynamic bearings that increase the hydrodynamic pressure due to asymmetric pressure distribution, which results in greater hydrodynamic lift or larger fluid film thickness [14,15]. In mixed lubrication conditions, this additional lift in hydrodynamic pressure alters the balance between hydrodynamic and boundary lubrication, and consequently, the number of asperities in contact decreases, and friction and wear likewise decrease [16]. In boundary and dry contact lubrication conditions, these craters (dimples or grooves) act as lubricant micro-reservoirs for the continuous retention and supply of lubricant [17], or as micro-containers to capture wear debris and contamination particles so that plowing decreases [18,19]. Consequently, abrasive wear and friction are reduced. Sudeep et al. [7] experimentally compared the tribological and vibration behaviours of textured point contacts of bearing steel lubricated with oil and grease under starved conditions. It showed that the textured surface lubricated with grease yielded reductions in the friction coefficient and specific wear rate (SWR) of the balls as compared to the oil lubricated textured contacts. Additionally, reduction in the amplitudes of vibrations (at normal contact resonance frequencies) was observed with the grease lubricated textured surface in comparison with the corresponding oil lubricated cases. They further studied the vibration of lubricated textured point contacts of steel bearings due to surface topographies by simulation using fractals surface characterization method and experiment [20]. Their findings revealed that in the presence of surface textures, vibration reduced due to the enhanced value of damping in comparison with the contacts having smooth surfaces. Tang et al. [21] experimentally investigated the effects of surface texture parameters and roughness parameters on the frictional properties of grease lubricated spherical plain bearings under mixed lubrication conditions. The results showed that higher dimple depths and lower dimple densities would result in a distinct improvement of the friction coefficients. A maximum reduction of 55% was gained for the textured sample under running conditions, as compared with the un-textured one. Wang et al. [22] investigated the friction and wear behaviour of laser-textured surfaces sliding against a pin under the lubrication of polyurea grease containing various additives using a Universal Mechanical Tester (UMT-2MT) reciprocating friction tester. Results showed that the dimple patterns had lower friction coefficients than the un-textured surfaces, at a lower sliding speed of 0.015 m/s. However, the groove patterns had higher friction coefficients than those of the un-textured surfaces at all sliding speeds. In the literature of previous studies, there is very little or no open research on the tribological performance of textured surfaces of mechanical components under grease lubrication as compared to that of oil. This is because grease behaviour is complicated and difficult to predict under operation due to the dual phases of its structure, namely solid and liquid. Secondly, the frictional performance of surface textures used on mechanical components under grease lubrication conditions in sliding motions is still not clear and needs further investigation. The basic objective of this research is to comprehensively establish the effects of surface texture parameters (texture density and texture depth) on friction in grease lubricated contacts. It is hoped that the understanding of the tribological behaviour of grease on a textured surface will help in predicting the lubrication conditions of sliding bearings for better operation in any machinery.
2. Materials and Methods
2.1. Specimen The friction properties of laser textured surfaces were studied using a ring-on-washer configuration. In this respect, a stationary ring on a rotating disc were prepared as experimental specimens from commercially available alloys of copper (ZCuPb30) and bearing steel (GCr15) Lubricants 2017, 5, 42 3 of 11 Lubricants 2017, 5, 42 3 of 11 with respectively.a normal hardness The ring of was 56 54 HRC. mm inIts outer contact diameter, surface 38 was mm intextured. inner diameter The disc and was 10 mmun-textured in thickness on the with a normal hardness of 56 HRC. Its contact surface was textured. The disc was un-textured on the contact surface. Its outer diameter was 53 mm, while the inner diameter was 39 mm and the thickness contact surface. Its outer diameter was 53 mm, while the inner diameter was 39 mm and the thickness was 5 mm, with a hardness of 60–65 HRC. The picture and 2D diagram of the friction pair used in was 5 mm, with a hardness of 60–65 HRC. The picture and 2D diagram of the friction pair used in this this researchresearch areare shown shown Figure Figure1. Prior 1. Prior to surface to surface texturing, texturing, the frontal the surfacesfrontal surfaces of each specimen of each werespecimen wereground ground and and polished polished to obtain to theobtain desired the surface desired roughness surface Ra roughness of 0.05 µm usingRa of a metallographic0.05 μm using a metallographicspecimen polishing specimen machine. polishing machine.
Figure 1. Picture and 2D diagram of friction pair. Figure 1. Picture and 2D diagram of friction pair.
In orderIn order to observe to observe the the effects effects of of texture texture density density andand depthdepth on on different different loads loads and and sliding sliding speed speed conditions, the reference grease should possess a high capacity for friction and wear, and it should conditions, the reference grease should possess a high capacity for friction and wear, and it should be widely used in industrial applications. In this research, lithium-based high-quality multipurpose be widely used in industrial applications. In this research, lithium-based high-quality multipurpose grease, which is often adequate for the lubrication of plain and rolling bearings, was used for the grease,tribometer which test.is often The detailedadequate information for the lubrication of the reference of plain grease and is givenrolling in Tablebearings,1. was used for the tribometer test. The detailed information of the reference grease is given in Table 1. Table 1. Grease properties. Table 1. Grease properties. Grease type NLGI a Grade Thickener Type Dropping Point ◦C Base Oil Viscosity Grease type NLGI a Grade Thickener Type Dropping Point °C Base Oil Viscosity @ 40 ◦C @ 100 ◦C LGMT b 2/(pack size) 2 Lithium >180 ◦C 110 @ 40 °C @ 11100 °C b LGMT 2/(pack size) a: National2 LubricatingLithium Grease Institute; b: Grease>180 label. °C 110 11 a: National Lubricating Grease Institute; b: Grease label. 2.2. Laser Surface Texturing 2.2. Laser Surface Texturing Round micro-dimples in different texture densities and depths were manufactured on the flat sideRound of the micro-dimples copper rings using in different high energy texture laser pulsesdensiti ines order and todepths ablate were the material manufactured by rapid melting on the flat side ofand the vaporizing. copper rings A green using light high Q-switched energy laser CEO® pulsesREA series in order Nd: to YAG ablate laser the machine material with by extra rapid double melting and cavity,vaporizing. output A wavelength green light of 532Q-switched nm, pulse widthCEO® of REA≤70 nm,series laser Nd: power YAG of ≤laser15 W, machine spot diameter with of extra double60 µcavity,m and output convex lenswavelength with focal of length 532 nm, of 60 pulse mm waswidth used of for≤ the70 texturing nm, laser process. power Comparedof ≤15 W, to spot diameter of 60 μm and convex lens with focal length of 60 mm was used for the texturing process. Compared to the 1064 nm wavelength or continuous wave laser, this laser source produces less heat effect on the metallic material surface. Hence, the generation of recast layers is restrained effectively. The laser beam was reflected by three reflecting mirrors and then focused on the surface of the specimen by the convex lens during fabrication to optimize light-matter interaction. The specimens were clamp in a three key jaw fixture mounted on a motor controlled two-dimensional stage during
Lubricants 2017, 5, 42 4 of 11 the 1064 nm wavelength or continuous wave laser, this laser source produces less heat effect on the metallic material surface. Hence, the generation of recast layers is restrained effectively. The laser beam was reflected by three reflecting mirrors and then focused on the surface of the specimen by the convex lensLubricants during 2017 fabrication, 5, 42 to optimize light-matter interaction. The specimens were clamp in a three4 of key 11 jaw fixture mounted on a motor controlled two-dimensional stage during the laser beam irradiation. Thethe laser dimple beam depths irradiation. were controlled The dimple by changing depths were the lasercontrolled power by and changing the duration the laser of the power laser and beam’s the ablatingduration on of thethe surface,laser beam’s while ablating the dimple on the diameters surface, were while kept theconstant dimple diameters at 60 µm. Thewere dimples kept constant on the surfaceat 60 μm. of The the ringdimples specimen on the were surface uniformly of the ring arranged specimen in a rectangular were uniformly array arranged as shown in in a Figure rectangular2 and array as shown in Figure 2 and the design texture density (T ) calculated based on the formula the design texture density (Td) calculated based on the formulad A π 2 A=×d =πDD2 × Td d 100% 100% (1) Td = AKL× 100% = × × 100% (1) At t K × L
2 where T is the texture area ratio (texture density); A is the area of the dimple ( A = π2D ); A is where Tddis the texture area ratio (texture density); Ad isd the area of the dimple (Ad =dπD ); At ist the =× areathe area of the ofmicro the micro dimple dimple unit A unitt = KAKLt× L; D is; theD diameter is the diameter of the dimple; of the dimple;K and L areK theand sides L are length the ofsides the length micro-dimple of the micro-dimple unit or spacing unit between or spacing dimples, between as shown dimples, Figure as 2shown. The dimple Figure parameters2. The dimple of theparameters laser textured of the surfacelaser textured for the tribologicalsurface for testthe intribol thisogical research test arein this shown research in Tables are2 shownand3 while in Tables the samples2 and 3 while are depicted the samples in Figures are depicted3 and4 .in This Figures dimple 3 and size 4. wasThis chosen dimple based size was on pastchosen research based byon thepast authorsresearch [ 23by] forthe effectiveauthors grease[23] for lubrication. effective grease The bulges lubrication. or burrs The formed bulges around or burrs the dimpleformed rimsaround during the thedimple laser rims texturing during process the laser were texturing removed process by a gentle were removed polishing by process. a gentle It haspolishing been experimentally process. It has demonstratedbeen experimentally that these demonstrated solidified bulges that these or burrs soli arounddified bulges the edges or burrs of the around dimples the have edges a negative of the effectdimples on have the tribological a negative performance effect on the oftribological contacting pe surfacesrformance [24]. of Finally, contacting all specimens surfaces [24]. were Finally, measured all inspecimens order to were obtain measured 3D topography in order and to obtain 3D roughness 3D topography parameters and 3D by roughness using a noncontact parameters 3D by surface using profilera noncontact (Wyko-NT1100, 3D surface Veeco,profiler New (Wyk York,o-NT1100, NY, USA). Veeco, New York, NY, USA).
Figure 2. Diagram of the micro-dimple array.
Table 2. Texture densitydensity parametersparameters ofof laserlaser surfacesurface texturingtexturing (LST)(LST) dimples.dimples. Parameter T-1 T-2 T-3 T-4 T-5 Parameter T-1 T-2 T-3 T-4 T-5 Texture density (%) 5 10 15 20 25 Texture density (%) 5 10 15 20 25 Dimple depth (H/μm) 12 12 12 12 12 Dimple depth (H/µm) 12 12 12 12 12 DimpleDimple diameter diameter (D/µm) (D/μ 60m) 60 6060 6060 60 6060 60
Table 3. Texture depth parameters of LST dimples. Table 3. Texture depth parameters of LST dimples. Parameter H1H2H3H4H5 ParameterTexture density (%) H1 15 H2 15 H3 15 15 H4 15 H5 TextureDimple density depth (%) (H/μm) 15 6 15 12 1519 24 1530 15 DimpleDimple depth (H/diameterµm) (D/μ 6m) 60 12 60 1960 60 2460 30 Dimple diameter (D/µm) 60 60 60 60 60
Lubricants 2017, 5, 42 5 of 11 Lubricants 2017,, 5,, 4242 5 of 11
FigureFigure 3. 3.SurfaceSurfaceSurface topographiestopographies topographies ofof differentdifferentdifferent texture texturetexture densities. densities.densities.
FigureFigure 4. 4.SurfaceSurfaceSurface topographiestopographies topographies ofof differentdifferentdifferent texture texturetexture depths. depths.depths.
2.3. Friction2.3. Friction and andWear Wear Test Test The tribological experiment was carried out in order to observe the friction behaviour of the The tribological experiment was carried out in order to observe the friction behaviour of the slidingsliding pair pair on an on anMMW-1A MMW-1A tribotester tribotester (Ji’nan (Ji’nan Yihua FrictionFriction Testing Testing Technology Technology Co., Co. Ltd., Ltd., Ji’nan, Ji’nan, slidingChina). pair Theon tribotesteran MMW-1A was composedtribotester of (Ji’nan a driving Yihua spindle, Friction driving Testing disc holder, Technology stationary Co. disc Ltd., holder, Ji’nan, China). The tribotester was composed of a driving spindle, driving disc holder, stationary disc holder, China).spindle The drivetribotester system, was operation composed panel, of force a driving sensor spin (inside),dle, torquedriving sensor disc holder, (inside) andstationary microcomputer disc holder, spindledata acquisitiondrive system, system, operation as depicted panel, in Figure force5a andsensor the test (inside), rig in Figure torque5b. Thesensor test geometry(inside) and microcomputerwas a rotating data disc acquisition on ring (flat-on-flat) system, with as depicted Kingsbury-type in Figure lubricant 5a and entryways the testto rig ensure in Figure adequate 5b. The test geometrydistribution was of grease a rotating along disc thesliding on ring surface (flat-on for-flat) proper with lubrication. Kingsbury-type The disc waslubricant mounted entryways on the to ensuredriving adequate disc holder distribution which was of grease connected along to the the driving sliding spindle, surface while for proper the textured lubrication. ring was The mounted disc was mountedon the on stationary the driving ring holder.disc holder The disc which slid was along conne the axiscted and to rubbedthe driving against spindle, the fixed while textured the ring,textured ring was mounted on the stationary ring holder. The disc slid along the axis and rubbed against the fixed textured ring, and the sliding direction was perpendicular to the micro-dimples for all experiments. During the course of the test, grease was being added to the contact surface through the entryways. The friction coefficient between the disc and the ring samples was automatically and digitally recorded by the computer, while the force sensor and torque sensor measured the load and friction torque respectively. The coefficient of friction was measured against time, applied loads and
Lubricants 2017, 5, 42 6 of 11 and the sliding direction was perpendicular to the micro-dimples for all experiments. During the course of the test, grease was being added to the contact surface through the entryways. The friction coefficient between the disc and the ring samples was automatically and digitally recorded by the Lubricantscomputer, 2017 while, 5, 42 the force sensor and torque sensor measured the load and friction torque respectively.6 of 11 The coefficient of friction was measured against time, applied loads and speeds. The range of loads speeds.was from The 100 range to 500of loads N with was a from contact 100 pressureto 500 N with of 0.46 a contact to 2.31 pressure MPa and of the0.46 range to 2.31 of MPa speeds and wasthe rangefrom 100–500of speeds rpm. was In from this 100–500 research, rpm. 5 different In this loadsresearch, and 5 speeds different were loads input and to speeds observe were the frictionalinput to observebehaviour the infrictional different behaviour conditions. in Fordifferent checking conditio the repeatabilityns. For checking of the the results, repeatability all experiments of the results, were allconducted experiments three were times conducted with complete three sets times of new with contact complete pairs, sets and theof new maximum contact variation pairs, and between the maximumthe experimental variation values between were controlledthe experimental within ±va5%.lues The were data controlled presented within in this report±5%. The represent data presentedthe average in ofthis the report test results.represent All the the average tests were of the conducted test results. at atmospheric All the tests conditionswere conducted (ambient at atmospherictemperature conditions of 23 ± 2 ◦ C(ambient and relative temperature humidity of of23 50–60%) ± 2 °C and relative humidity of 50–60%)
Figure 5. MMW-1A tribotester (a) and test-rig (b). Figure 5. MMW-1A tribotester (a) and test-rig (b).
3. Results and Discussion 3. Results and Discussion Under the present experimental conditions, the frictional behaviour of grease under textured Under the present experimental conditions, the frictional behaviour of grease under textured surfaces in sliding conditions can be explained with the properties of grease and the influences of surfaces in sliding conditions can be explained with the properties of grease and the influences of surface texture parameters. surface texture parameters.
3.1.3.1. Effect Effect of of Texture Texture Density Density on on Coefficient Coefficient of Friction TheThe variations variations in in friction friction coefficients coefficients of of the the disc disc and and ring ring contact contact versus versus time time for for texture texture density density areare plotted plotted in in Figure Figure 6a.6a. A A 200 200 N N load load and and 100 100 rpm rpm sliding sliding speed speed were were applied applied for for a duration a duration of 30 of min30 min during during the thetest. test. The The results results of ofthe the average average steady steady state state friction friction coefficient coefficient of of the the three three repeated repeated experimentsexperiments are are plotted plotted in in Figure Figure 6b6b at at a atexture texture depth depth of ofH2 H2 = 12 = 12μm.µ Inm. the In initial the initial phase phase of the of test, the the friction coefficients of the five samples were very high, of which texture density 5% and 25% were test, the friction coefficients of the five samples were very high, of which texture density 5% and 25% the highest. This is because grease exhibits semi solid colloidal characteristics, which makes it difficult were the highest. This is because grease exhibits semi solid colloidal characteristics, which makes to form a uniform stable lubrication film on the contact surface, resulting in a relatively poor it difficult to form a uniform stable lubrication film on the contact surface, resulting in a relatively lubrication effect. Secondly, the thixotropic properties of grease make the viscosity very high, and the poor lubrication effect. Secondly, the thixotropic properties of grease make the viscosity very high, thickness contributed friction resistance to the initial stage of the test. After the initial stage, the and the thickness contributed friction resistance to the initial stage of the test. After the initial stage, friction coefficients were greatly reduced and they reached a relatively stable state after a short period the friction coefficients were greatly reduced and they reached a relatively stable state after a short of time. This can be attributable to the formation of a more stable lubricating film on the contact period of time. This can be attributable to the formation of a more stable lubricating film on the contact surface by the relative movement of the friction pair and the even distribution of grease from the surface by the relative movement of the friction pair and the even distribution of grease from the dimples. Additionally, the thixotropic properties of grease showed that the viscosity decreased with dimples. Additionally, the thixotropic properties of grease showed that the viscosity decreased with an increase in shear time, thus resulting in more fluidity and less resistance to friction. In Figure 6b, an increase in shear time, thus resulting in more fluidity and less resistance to friction. In Figure6b, it can be seen that the texture density of 15% displayed the lowest friction coefficient of 0.16191 as it can be seen that the texture density of 15% displayed the lowest friction coefficient of 0.16191 as compared to texture densities 5% (0.21272), 10% (0.19068), 20% (0.17032) and 25% (0.20861) under the compared to texture densities 5% (0.21272), 10% (0.19068), 20% (0.17032) and 25% (0.20861) under the experimental conditions. The reason for this is that the contact area between the disc and ring was experimental conditions. The reason for this is that the contact area between the disc and ring was sufficiently reduced but could still support the contact load with enough grease lubricant retained on sufficiently reduced but could still support the contact load with enough grease lubricant retained the contact surface by the dimples [25]. This enabled the formation of a stable grease boundary film on the contact surface by the dimples [25]. This enabled the formation of a stable grease boundary through mechanical entrapment and surface deposition of grease fibres on the contact surface. Additionally, the hydrodynamic action of the base oil from the dimples generated additional pressure to separate the sliding pair, thus reducing the friction. However, surface texture with texture densities of 5% and 25% had the highest coefficient of friction. This can be explained as a lack of sufficient grease lubricant on the contact surface to generate hydrodynamic pressure lift on the part of texture density 5%, due to the small number of dimples. For the texture density 25%, although there was more grease lubricant retained on the contact surface, the space between each dimple acted like a peak, which affected the film distribution by making it easy for it to flake away from the contact
Lubricants 2017, 5, 42 7 of 11
film through mechanical entrapment and surface deposition of grease fibres on the contact surface. Additionally, the hydrodynamic action of the base oil from the dimples generated additional pressure to separate the sliding pair, thus reducing the friction. However, surface texture with texture densities of 5% and 25% had the highest coefficient of friction. This can be explained as a lack of sufficient grease lubricant on the contact surface to generate hydrodynamic pressure lift on the part of texture density 5%, due to the small number of dimples. For the texture density 25%, although there was Lubricants 2017, 5, 42 7 of 11 more grease lubricant retained on the contact surface, the space between each dimple acted like a peak, which affected the film distribution by making it easy for it to flake away from the contact surface, surface, resulting in a higher friction. Secondly, the load bearing area was drastically reduced, with resulting in a higher friction. Secondly, the load bearing area was drastically reduced, with the side the sideeffect effect far exceedingfar exceeding the grease the grea lubricationse lubrication effect oneffect the on contact the contact surface. surface. Thirdly, Thirdly, the thickener the thickener was was ineffective, as as it it was was unable unable to beto absorbedbe absorbed on the on contactthe contact surface surface to form to a form boundary a boundary film and film was and was mostly foundfound inside inside the the dimples. dimples. Finally, Finally, the friction the fr coefficientiction coefficient decreased decreased with an increase with an in increase texture in texturedensity density up toup a certainto a certain point, point, after which after itwhich began it rising began gradually rising gradually as depicted as in depicted Figure6b. in Figure 6b.
FigureFigure 6. Friction 6. Friction coefficient coefficient versus versus time time (a ()a )and and steady steady state frictionfriction coefficient coefficient (b )(b for) for texture texture density. density.
3.2. Effect3.2. Effect of Texture of Texture Depth Depth on onCoefficient Coefficient of of Friction Friction FigureFigure 7a shows7a shows the the friction friction coefficient coefficient versus versus time time for for texture texture depth depth of specimens of specimens H1, H2, H1, H3, H2, H4 H3, H4 andand H5 H5 with aa texture texture density density of 15%of 15% (T-3), (T-3), while wh Figureile 7Figureb depicts 7bthe depicts average th steadye average state steady friction state frictioncoefficient. coefficient. A constant A constant load and load speed and ofspeed 200 N of and 200 100 N rpm,and 100 respectively, rpm, respectively, were applied were for a applied duration for a durationof 30 of min. 30 Themin. results The showresults that show the texture that the depth text hasure an depth effect onhas friction. an effect The on friction friction. coefficient The friction of coefficientspecimen of specimen H5 was higher H5 was than higher the other than specimens. the other This specimens. is because This the depthis because of dimples the depth in specimen of dimples H5 was so deep that both the grease base oil and the thickener stayed in the bottom of the dimple and in specimen H5 was so deep that both the grease base oil and the thickener stayed in the bottom of were very difficult to extrude on to the contact surface. This prevented the replenishment of the lost the dimple and were very difficult to extrude on to the contact surface. This prevented the oil, which hindered the generation of hydrodynamic pressure and the formation of a boundary film. replenishmentThis made theof filmthe thicknesslost oil, which decrease hindered sharply, andthe thegeneration lubrication of state hydrodynamic of the friction pressure pairs changed and the formationfrom fluidof a boundary lubrication film. to boundary This made lubrication. the film thickness This revelation decrease is contrary sharply, to and the the research lubrication finding state of theof Tangfriction et al.pairs [21] wherechanged higher from dimple fluid depths lubrication resulted to in boundary a lower friction lubrication. coefficient. This Conversely, revelation is contraryfor sample to the H1, research the dimple finding depth of was Tang too et shallow al. [21] and where the textured higher surface dimple seemed depths to beresulted close to ain plain a lower frictionsurface. coefficient. This resulted Conversely, in less greasefor sample storage H1, by the dimplesdimple anddepth led was to a weaktoo shallow micro-hydrodynamic and the textured surfaceeffect seemed which to increased be close the to frictiona plain coefficientsurface. This [26]. resulted However, in specimen less grease H3, storage with the by depth the ofdimples 19 µm, and led tohad a theweak lowest micro-hydrodynamic friction coefficient. Thiseffect shows which that in specimencreased H3the hadfriction a moderate coefficient depth [26]. which However, was specimenable toH3, form with a stablethe depth lubrication of 19 μ filmm, had composed the lowest of oil friction film, transfercoefficient. film This and depositedshows that film specimen on the contact surface in order to obtain a good lubricating effect. Secondly, the depth-to-diameter H3 had a moderate depth which was able to form a stable lubrication film composed of oil film, ratio (H3/d = 19/60 = 0.31) of specimen H3 was near to the optimum depth-to-diameter ratio range transfer film and deposited film on the contact surface in order to obtain a good lubricating effect. (0.2–0.28) reported by Zavos and Nikolakopoulos for a minimum friction coefficient [27]. Additionally, Secondly,it can betterthe depth-to-diameter capture and store the rati wearo (H3/d debris, = thus19/60 reducing = 0.31) frictionof specimen and wear H3 between was near the to contact the optimum pair. depth-to-diameter ratio range (0.2–0.28) reported by Zavos and Nikolakopoulos for a minimum friction coefficient [27]. Additionally, it can better capture and store the wear debris, thus reducing friction and wear between the contact pair.
Lubricants 2017, 5, 42 8 of 11 Lubricants 2017, 5, 42 8 of 11
FigureFigure 7. 7.FrictionFriction coefficient coefficient versus versus time time ( (aa)) and and steady steady statestate frictionfriction coefficient coefficient (b ()b for) for texture texture depth. depth.
3.3.3.3. Effect Effect of ofLoad Load on on Friction Friction Coefficient Coefficient TheThe effects effects of of load load on on friction friction behaviour behaviour of thethe SKFSKF (Svenska(Svenska KullagerfabrikenKullagerfabriken AB) AB) grease grease lubrication were investigated by using the control variable method. Table4 shows the parameters of lubrication were investigated by using the control variable method. Table 4 shows the parameters of the specimens selected to be investigated. The specimens that exhibited the lowest friction coefficient the specimens selected to be investigated. The specimens that exhibited the lowest friction coefficient among the specimens presented in Sections 3.1 and 3.2 were selected for this series of experiments. among the specimens presented in Sections 3.1 and 3.2 were selected for this series of experiments. Table 4. Variable control method texture parameters of specimens. Table 4. Variable control method texture parameters of specimens.
TestTest Number Number V1 V1 V2 V2 V3 V3 TextureTexture density density (%) (%) 10% 10% 15% 15% 15% Texture depth (H/µm) 12 19 24 Texture depth (H/μm) 12 19 24
FigureFigure 8 8shows shows the the effect effect of of load load on the frictionfriction coefficientcoefficient of of the the specimens specimens at at a fixeda fixed speed speed of of 100100 rpm rpm and and a aduration duration of of 20 20 min. min. It It can can be be seen thatthat thethe frictionfriction coefficientscoefficients of of the the specimens specimens are are reducedreduced by by increasing increasing the the normal normal load. load. At At the the initia initiall load ofof 100100 N,N, the the friction friction coefficient coefficient of of all all the the threethree specimens specimens was was high, high, with with that that of of the the texture texture density ofof 10%10% (V1) (V1) being being relatively relatively smaller smaller than than that of the texture density of 15% (V2 and V3). This indicates that at low contact pressure, the surface that of the texture density of 15% (V2 and V3). This indicates that at low contact pressure, the surface roughness had a certain influence on the friction properties, since higher texture density led to higher roughness had a certain influence on the friction properties, since higher texture density led to higher surface roughness, and therefore a higher friction coefficient was more easily obtained. As the load surface roughness, and therefore a higher friction coefficient was more easily obtained. As the load increased, the friction coefficients of the three specimens were sharply reduced. This is attributable to increased, the friction coefficients of the three specimens were sharply reduced. This is attributable the large quantity of oil being constantly released from the grease and the dimples into the sliding to contactthe large surface. quantity This of oil oil participated being constantly in the releas lubricationed from process the grease and achieved and the a dimples stable lubrication into the sliding film contactdue to surface. the squeezing This oil effect participated of the higher in the load. lubrication Additionally, process as the and load achieved increased, a stable the shear lubrication stress and film duethe to apparent the squeezing grease viscosityeffect of the also higher increased load. at aAddi constanttionally, sliding as the speed. load This increased, increase the the shear thickener stress andconcentration the apparent on grease the contact viscosity surface—which also increased facilitated at a theconstant formation sliding of aspeed. protective This film increase and the the thickenersmoothing concentration phenomena—are on the believed contact to surface—which be responsible for facilitated the reduction the information friction. However, of a protective when the film andload the increased smoothing past phenomena—are a certain point, the believed friction coefficients to be responsible tended to for be stable.the reduction This is because in friction. the However,excessive when load damagedthe load theincreased grease fibrepast structure,a certain makingpoint, the the friction grease fail, coefficients thereby affecting tended theto be friction stable. Thiscoefficient. is because At the this excessive stage, the load specimen damagedV1 exhibitedthe grease the fibre highest structure, friction making coefficient the grease while fail, specimen thereby affectingV2 exhibited the friction the lowest. coefficient. This is becauseAt this specimenstage, theV1 specimenhad a relatively V1 exhibited smaller texturethe highest density friction and coefficientdepth, with while a low specimen total storage V2 exhibited capacity, resulting the lowest. in less This storage is because of the grease specimen lubricant V1 had in the a dimplesrelatively smalleras compared texture todensity specimens and V2depth,and V3with. This a low led total to a relatively storage capacity, poor lubrication resulting effect, in less thus storage the friction of the greasecoefficient lubricant of specimen in the dimplesV1 was slightlyas compared higher. to For specimens specimens V2V2 andand V3., althoughThis led theyto a relatively had the same poor lubricationtexture density, effect, thethus depth the friction of the dimples coefficient in specimen of specimenV2 (19 V1µ wasm) was slightly relatively higher. shallow. For specimens This more V2 andeasily V3, dispersedalthough thethey grease had fromthe same the dimples texture by density, the compression the depth effect of the of thedimples load,resulting in specimen in a lower V2 (19 μm)friction was coefficient.relatively shallow. This more easily dispersed the grease from the dimples by the compression effect of the load, resulting in a lower friction coefficient.
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FigureFigure 8. 8.Friction Friction coefficientcoefficient versus versus applied applied load. load. Figure 8. Friction coefficient versus applied load. 3.4.3.4. Effect Effect of Speedof Speed on on Friction Friction Coefficient Coefficient 3.4. Effect of Speed on Friction Coefficient FigureFigure9 shows 9 shows the the plots plots of of friction friction coefficient coefficient versusversus sliding sliding speed speed at at different different texture texture depths depths for a duration of 20 min. The textured copper surfaces with texture depth of H2, H3, and H4 were for a durationFigure 9 of shows 20 min. the Theplots textured of friction copper coefficien surfacest versus with sliding texture speed depth at different of H2, H3,texture and depths H4 were selected for investigation and comparison. As can be seen from the diagram in Figure 9, the friction selectedfor a duration for investigation of 20 min. and The comparison. textured copper As cansurf beaces seen with from texture the depth diagram of H2, in FigureH3, and9, H4 the were friction coefficientselected for was investigation low at the and initial comparison. stage and, As as canthe bespeed seen increased, from the diagramthe friction in Figurecoefficient 9, the rose friction first coefficientand then was decreased. low at the This initial phenomenon stage and, can as thebe speedcategorized increased, into three the friction lubrication coefficient regimes: rose (1) first the and thencoefficient decreased. was This low phenomenon at the initial stage can be and, categorized as the speed into increased, three lubrication the friction regimes: coefficient (1) the rose boundary first boundaryand then decreased.lubrication This regime; phenomenon (2) the mixed can be film cate lubricationgorized into regime; three lubricationand (3) the regimes: hydrodynamic (1) the lubrication regime; (2) the mixed film lubrication regime; and (3) the hydrodynamic lubrication regime. lubricationboundary lubricationregime. In theregime; low speed(2) the range mixed (100–20 film 0lubrication rpm), there regime; was enough and (3) grease the hydrodynamicon the contact In the low speed range (100–200 rpm), there was enough grease on the contact surface at 100 rpm surfacelubrication at 100 regime. rpm for In effectivethe low speedlubrication, range hence(100–20 th0e rpm),low friction there wascoefficient. enough However, grease on as the the contact speed forstartedsurface effective toat lubrication,increase,100 rpm forsome effective hence of the the lubrication,grease low frictionon the hence contac coefficient. thet lowzone friction However,was easily coefficient. thrown as the speedHowever, out. At started this as point,the to speed increase, the somegreasestarted of the supplyto grease increase, directly on some the affected contact of the the zonegrease surface was on the easilylubricat contac throwniont zone performance. out. was At easily this The point,thrown dynamic the out. grease pressureAt this supply point, effect directly the of affectedthegrease surface the supply surface texture directly lubrication was affected relatively performance. the weak surface and lubricat the The lubr dynamicionicant performance. film pressure thickness The effect was dynamic ofvery the thin.pressure surface This texture effectdirectly of was relativelyincreasedthe surface weak the texture and shearing the was lubricant relativelyof the contact film weak thickness surfaces, and the was lubrsinceicant very the thin.filmlubrication thickness This directly film was had very increased little thin. effect This the shearinganddirectly the of thefrictionincreased contact coefficient surfaces, the shearing sinceof the theof specimen the lubrication contact rose surfaces,at film this had stage littlesince (region effectthe lubricationI). and In the the high friction film speed had coefficient range little (200–400effect of the and specimenrpm), the rosethefriction at grease this coefficient stage gradually (region of exhibitedtheI). specimen In the a higher highrose athydrodynamic speed this st rangeage (region (200–400 pressure I). In rpm),effectthe high created the speed grease by range the gradually influence (200–400 exhibitedofrpm), the a higherdimplesthe grease hydrodynamic to graduallylift the specimen. exhibited pressure Thus, a higher effect shearing createdhydrodynamic between by the contact pressure influence surfaces effect of became the created dimples smaller, by the to influenceand lift the friction specimen. of the coefficient decreased. At this stage, an extensive interaction of surface materials occurred, and the Thus,dimples shearing to lift between the specimen. contact Thus, surfaces shearing became between smaller, contact and surfaces the friction became coefficient smaller, and decreased. the friction At this mixed film played a significant role in decreasing the friction coefficient (region II). In the speed range stage,coefficient an extensive decreased. interaction At this ofstage, surface an extensive materials interaction occurred, of and surface the mixed materials film occurred, played a and significant the ofmixed 500 rpmfilm andplayed above, a significant the friction role coefficient in decreasing again th rosee friction slightly coefficient for specimens (region H3 II). andIn the H4, speed due rangeto the rolelarge in decreasing amount of the hydrodynamic friction coefficient pressure (region and a II).negligible In the speed sharing range of surfaces. of 500 rpm In this and case, above, the thecontact friction coefficientof 500 rpm again and roseabove, slightly the friction for specimens coefficient H3again and rose H4, slightly due to for the specimens large amount H3 and of H4, hydrodynamic due to the surfaceslarge amount were ofmainly hydrodynamic separated pressure by the grease and a flnegligibleuid film, sharingand the of friction surfaces. was In governed this case, completelythe contact pressure and a negligible sharing of surfaces. In this case, the contact surfaces were mainly separated bysurfaces the shearing were mainly of the separated lubricant byfluid the film.grease The fluid results film, show and thethat friction under wasthe presentgoverned experimental completely by the grease fluid film, and the friction was governed completely by the shearing of the lubricant fluid conditions,by the shearing specimen of the H3 lubricant with texture fluid depth film. ofThe 19 resultsμm had show the lowest that under friction the coefficient present experimentalachievable at film.allconditions, The sliding results speeds, specimen show as that depicted H3 underwith texturein the Figure present depth 9. experimentalof 19 μm had the conditions, lowest friction specimen coefficient H3 with achievable texture depthat of 19allµ slidingm had speeds, the lowest as depicted friction in coefficient Figure 9. achievable at all sliding speeds, as depicted in Figure9.
Figure 9. Friction coefficient versus speed for texture depth. FigureFigure 9. 9.Friction Friction coefficientcoefficient versus speed speed for for texture texture depth. depth.
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4. Conclusions In this research, a series of experiments were conducted to explore the effect of surface texture parameters on frictional properties of lithium-based grease under sliding condition. The circular dimples on the contact surface of the ring were designed with different dimple depths and densities. The experiments facilitated an understanding of the tribological behaviours of grease on textured surfaces. The conclusions drawn, based on the results under the present experimental conditions, are: i. In general, the friction coefficient is largely dependent on texture parameters, with lower and higher texture densities resulting in a higher friction coefficient at a fixed texture depth. ii. The sample with texture density of 15% (T-3) and texture depth of 19 µm (H3) exhibited the best friction properties, experimentally, in all conditions because it could store more grease and trap wear debris. iii. Higher load and speed increases the grease shear stress, apparent viscosity and thickener concentration, which facilitates the formation of a protective film and a smoothing phenomena, which are believed to be responsible for the reduction in friction. iv. The reduction in friction by the surface texture and the main mechanism may be attributed to the hydrodynamic pressure effect, which increases the mating gap and reduces the probability of asperity contact.
Acknowledgments: The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by the National Natural Science Foundation of China (Nos. 51375211 and 51375213) and Primary Research & Development Plan of Jiangsu Province (BE2017122). Author Contributions: Julius Caesar Puoza conceived and designed the experiments with Xuan Xie under the supervision of Xijun Hua and Peiyun Zhang; Julius Caesar Puoza and Xuan Xie performed the experiments; Julius Caesar Puoza and Xuan Xie analyzed the data and discussed the results with Xijun Hua and Peiyun Zhang; Xijun Hua, Peiyun Zhang, Bifeng Yin and Julius Caesar Puoza contributed with reagents/materials/analysis tools; Julius Caesar Puoza wrote the paper; Xijun Hua, Peiyun Zhang, Bifeng Yin revised the paper. Conflicts of Interest: The authors declare no conflict of interest.
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