Investigation of the Tribological Properties of Different Textured Lead Bronze Coatings Under Severe Load Conditions

lubricants

Article Investigation of the Tribological Properties of Different Textured Lead Bronze Coatings under Severe Load Conditions

Adolfo Senatore 1 , Giacomo Risitano 2 , Lorenzo Scappaticci 3 and Danilo D’Andrea 2,*

1 Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano, SA, Italy; [email protected] 2 Department of Engineering, University of Messina, Contrada di Dio (S. Agata), 98166 Messina, SA, Italy; [email protected] 3 Sustainability Engineering Department, Guglielmo Marconi University, via Plinio 44, 00193 Rome, SA, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-393-020-9246

Abstract: The purpose of this paper is to investigate the variation in the coefﬁcient of friction (CoF) and also the wear in a lead bronze coating under different texture conditions. The tribological tests were performed using a tribometer with pin on disk conﬁguration. Several kinds of textures, realised by a surface laser texturing, were tested by varying the diameter, depth, and density of the dimples under severe working conditions. The innovative aspect concerns the behaviour of the textured lead bronze coating and the lubrication conditions when the sample is subjected to extreme load conditions. Confocal microscopies and SEM (Scanning Electron Microscopy)/EDS (Energy Dispersive X-Ray Spectroscopy) analyses were performed to evaluate the texture behaviour and also the surface deterioration of the coating. The results show that the application of texture processing

leads to an improvement in the tribological properties of the coating. By analysing separately the variation of the different geometric parameters of the dimples, it has been shown that the best results µ µ Citation: Senatore, A.; Risitano, G.; are obtained with a diameter of 50 m, a density of 5%, and a depth of 5 m. Scappaticci, L.; D’Andrea, D. Investigation of the Tribological Keywords: laser surface texturing; wear; friction Properties of Different Textured Lead Bronze Coatings under Severe Load Conditions. Lubricants 2021, 9, 34. https://doi.org/10.3390/lubricants 1. Introduction 9040034 Mechanical components, and in particular hydraulic components, are often subject to sliding contacts and high contact pressure, which cause several failure modes such as Received: 27 February 2021 delamination, seizure, noise, vibrations, and fatigue crack due to friction and wear [1–3]. Accepted: 22 March 2021 The efficiency improvement in piston pumps and motors is one of the most important Published: 26 March 2021 goals in the development of these machines [4,5]. A small improvement in pump efficiency in terms of percentage points (%) has a significant impact in terms of noise, efficiency, Publisher’s Note: MDPI stays neutral and reduced consumption in many technologies, such as CVT (Continuously Variable with regard to jurisdictional claims in Transmission) power split [6]. published maps and institutional affil- The reduction in both friction and wear is very important to ensure high efficiency and iations. extension of the component life. Typically, the problem of friction and wear is dealt with by using lubricants, but in some cases such as a low rotation speed, lubrication is insufficient. For this reason, we use coatings capable of guaranteeing dry lubrication, for example lead bronze [7–9]. However, solid lubricants such as lead are now being decommissioned Copyright: © 2021 by the authors. because of their toxicity. Licensee MDPI, Basel, Switzerland. The increasing request for high efficiency anti-friction coatings from modern industries This article is an open access article has pushed researchers towards the development of new technologies, such as PVD-TiN distributed under the terms and coating [10,11] or a super-hydrophobic/super-oleophobic nanostructured surface [12–14]. conditions of the Creative Commons Another technology studied to enhance the tribological property of materials is surface Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ texturing. Surface texturing has been known for a long time, as shown by the numerous 4.0/). studies reported in the scientific literature [15–18]. The use of texture to improve the

Lubricants 2021, 9, 34. https://doi.org/10.3390/lubricants9040034 https://www.mdpi.com/journal/lubricants Lubricants 2021, 9, x 2 of 15

Lubricants 2021, 9, 34 2 of 14 surface texturing. Surface texturing has been known for a long time, as shown by the numerous studies reported in the scientific literature [15–18]. The use of texture to improve the performance of mechanical systems and reduce friction and wear is particularly used in performancethe automotive of mechanicalfield, as demonstrated systems and by reduce numerous friction scientific and wear studies is particularly [19–23]. Borghi used in et the al.automotive [24] have studied field, as the demonstrated effects of surface by numerous modification scientific by studieslaser texturing [19–23]. on Borghi tribological et al. [24 ] performanceshave studied of the nitriding effects steel of surface for high-perfo modificationrmance by engine laser texturing applications. on tribological Vladescu et perfor- al. [25]mances have investigated of nitriding the steel effect for high-performance of surface texture engineon friction applications. for a convergent–divergent Vladescu et al. [25 ] bearing,have investigated operating under the effect different of surface lubrication texture regimes. on friction However, for a convergent–divergent almost all the scientific bear- worksing, operatinganalysed investigate under different the tribological lubrication regimes.behaviour However, of texturing almost subjected all the scientificto loads worksvar- yinganalysed between investigate 5 and 100 theN [26–28]. tribological behaviour of texturing subjected to loads varying betweenThe novelty 5 and 100of this N [ 26paper–28]. is the use of extremal working condition to analyse the applicationsThe novelty of surface of thistexturing paper on is hydraulic the use of pumps extremal and working motors, conditionwhich represent to analyse some the applications of surface texturing on hydraulic pumps and motors, which represent some of the components most studied for the application of this technology [29–32]. Another of the components most studied for the application of this technology [29–32]. Another innovative aspect is the use of surface texturing on a lead bronze alloy, which already has innovative aspect is the use of surface texturing on a lead bronze alloy, which already has anti-friction characteristics. anti-friction characteristics. To reduce friction and wear, different shapes of texture can be used, such as dimples, To reduce friction and wear, different shapes of texture can be used, such as dimples, holes, cavities or asperities, but the most used is the micro-dimples; moreover, the effect holes, cavities or asperities, but the most used is the micro-dimples; moreover, the effect of the spatial arrangement of the dimples could be evaluated. There are many techniques of the spatial arrangement of the dimples could be evaluated. There are many techniques to achieve surface texturing including etchant, ion beam texturing, embossing, and laser to achieve surface texturing including etchant, ion beam texturing, embossing, and laser surface texturing (LST), which is the most applied technique [33] and has been used in surface texturing (LST), which is the most applied technique [33] and has been used in this work. this work. Texturised surfaces have numerous advantages in terms of mechanical and tribolog- Texturised surfaces have numerous advantages in terms of mechanical and tribological icalbehaviour. behaviour. In In fact, fact, the the presence presence of micro-incisions,of micro-incisions, allows allows an improvementan improvement of theof tribo-the tribologicallogical behavior behavior in in the the case case of of starved starved lubrication, lubrication, as as the the dimples dimples act act as as oil oil reservoir reservoir [34 ]. [34].While, While, in thein the case case of hydrodynamicof hydrodynamic or or mixed mixed lubrication, lubrication, the the micro-dimples micro-dimples behave behave like likehydrodynamic hydrodynamic bearings bearings [35 [35].]. Another Another important important feature feature of texturised of texturised surfaces surfaces is the is ability the abilityto reduce to reduce abrasive abrasive wear, wear, as micro-engravings as micro-engravings behave behave like traps like traps for debris. for debris. In Inthis this paper paper different different types types of textures of textures were were tested, tested, varying varying the diameter, the diameter, depth, depth,and densityand density of the dimples, of the dimples, using the using pin on the disk pin te onchnique. disk technique. After the Aftertribological the tribological tests, failure tests, analysisfailure was analysis performed was performed in order to in evaluate order to th evaluatee impact the of impactthe texture of the on texture the lead on bronze the lead coating.bronze coating.

2. 2.Materials Materials and and Methods Methods TheThe tribological tribological tests tests were were performed performed by by using using reciprocating reciprocating tester. tester. The The tribo-system tribo-system consistsconsists of ofthe the stationary stationary sample sample (counter-specimen) (counter-specimen) pressed pressed at atthe the required required load load against against thethe cylinder cylinder sample sample (specimen) (specimen) performing performing reciprocating reciprocating motion. The coefficientscoefficients of of friction fric- tionand and wear wear were were calculated calculated using using a UMT-3 a UMT-3 tribometer tribometer in the in configurationthe configuration pin-on-disc pin-on-disc (CETR (CETRLtd., USA),Ltd., USA), according according to ASTM to G99-05ASTM G99-05 (2010). A(2010). cylinder A cylinder pin (Figure pin1a) (Figure of 100Cr6Steel 1a) of 100Cr6Steel(diameter 6.0(diameter mm, length 6.0 mm, 20.0 mm)length is placed20.0 mm) in contact is placed with in a contact known andwith regulated a known vertical and regulatedload (constant) vertical onload the (constant) surface of on the the discs surface (a 70.0 of mmthe discs diameter (a 70.0 with mm a bronze diameter thickness with a of bronze0.8 mm) thickness (Figure of1b). 0.8 From mm) the(Figure measurement 1b). From of the the measurement lateral force, itof is the possible lateral to force, obtain it theis possiblecoefficient to obtain of friction the coefficient by the ratio of between friction theby the lateral ratio and between vertical the force. lateral and vertical force.

(a) (b)

Figure 1. (a) The cylinder pin of a Steel 100Cr6 (b) bi-metal disk.

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Figure 1. (a) The cylinder pin of a Steel 100Cr6 (b) bi-metal disk.

TheThe tribometertribometer isis shownshown inin FigureFigure2 .2

FigureFigure 2.2. TribometerTribometer systemsystem (CETR(CETR UMT-3).UMT-3).

TheThe tribologicaltribological teststests havehave beenbeen performedperformed withwith specimensspecimens fullyfully immersedimmersed inin thethe lubricantlubricant bath.bath. TheThe oiloil lubricantlubricant usedused waswas LI-HIVLI-HIV 4646 (viscosity(viscosity indexindex 175).175). ItIt isis usedused inin hydraulicshydraulics machines,machines, suchsuch asas pumpspumps andand motors.motors. TableTable1 1shows shows the the characteristics characteristics of of the the lubricatinglubricating oil.oil.

TableTable 1. 1.LI-HIV LI-HIV 46 46 oil oil parameters. parameters.

Parameters ValueValue 3 Density atat 20 20◦ C°C 46 46 kg/m kg/m3 ViscosityViscosity at at 40 40◦ C°C 46 46 cStcSt ViscosityViscosity atat 100 100◦ C°C 9 9 cStcSt ViscosityViscosity Index Index 175 175 Freezing ◦C −35 Freezing °C −35 Flammability ◦C 210 Flammability °C 210

TheThe temperaturetemperature systemsystem isis composedcomposed byby aa thermocouplethermocouple immersedimmersed inin thethe oiloil bath.bath. TheThe testtest parametersparameters areare shownshown inin TableTable2 .2.

TableTable 2. 2.Tribological Tribological test test parameters. parameters.

Parameters ValueValue Vertical load load 706.8 706.8 N N ContactContact pressurepressure 25 25 MPaMPa Rotation speed 2000 rpm Rotation speed 2000 rpm Oil temperature =∼ 70 ◦C Oil temperatureTime 10≅ 70 min °C LubricationTime 10 full min Lubrication full The micro-structured surface on bi-metal cylinder disk samples have been realised usingThe surface micro-structured laser texturing surface technique. on bi-metal Nine cylinder typologies disk of samples texture, have varying been diameter, realised depth,using andsurface density laser of texturing the dimples technique. on the surface Nine typologies have been produced.of texture, Thevarying features diameter, of the different kind of samples are reported in Table3.

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depth, and density of the dimples on the surface have been produced. The features of the depth,differentdepth, andand kind densitydensity of samples ofof thethe dimplesaredimples reported onon thethe in surfacTablesurfac e3.e have have beenbeen produced.produced. TheThe featuresfeatures ofof thethe differentdepth,depth, andand kind densitydensity of samples ofof thethe dimplesaredimples reported onon thethe in surfacTablesurfac 3.ee have have beenbeen produced.produced. TheThe featuresfeatures ofof thethe Lubricants 2021, 9, 34 depth,different and kind density of samples of the dimplesare reported on the in surfacTable e3. have 4 of been 14 produced. The features of the differentdifferent kindkind ofof samplessamples areare reportedreported inin TableTable 3.3. differentTable 3. Information kind of samples of texture are parameters. reported in Table 3. Table 3. Information of texture parameters. Table 3. Information of texture parameters. Table 3. Information of texture parameters. 2 Sample Name Table Typology 3. Information of Texture of texture parameters. Density Area (mm2) Sample Name Table Typology 3. Information of Texture of texture parameters. Density Area (mm 2) TableSample 3. Information Name of texture parameters. Typology of TextureTexture Density Density AreaArea (mm(mm2)) SampleA Name Typology As produced of Texture Density Not considered Area (mm 2) SampleA Name Typology As produced of Texture Density Not considered Area (mm 22) SampleSampleA Name Name Typology Typology As produced of of Texture Density Area (mm2 Density) Not considered Area (mm ) AA As As producedproduced Not Not consideredconsidered A As produced Not considered 2 B50 A As produced Not considered10% of total area (334.8 mm2) B50 10% of total area (334.8 mm2) B50 10% of total area (334.8 mm2)) B50 10% of total area (334.8 mm2) B50 10% of total area (334.8 mm 2) B50B50 10%10% ofof total2total areaarea (334.8(334.8 mmmm2)) B50 10% of total area (334.8 mm )

2 B100 10% of total area (334.8 mm2) B100 10% of total area (334.8 mm2) B100 10% of total area (334.8 mm2)) B100 10% of total area (334.8 mm2) B100 10% of 2total area (334.8 mm 2) B100B100 10% of total area (334.810% mm of total) area (334.8 mm2 ) B100 10% of total area (334.8 mm )

2 B150 10% of total area (334.8 mm2) B150 10% of total area (334.8 mm2) B150B150 10%10% ofof totaltotal areaarea (334.8(334.8 mmmm2)) B150B150 10% of total area (334.810% mm of 2total) area (334.8 mm2) B150B150 10%10% ofof totaltotal areaarea (334.8(334.8 mmmm22))

2 C5 5% of total area (167.4 mm2) C5 5% of total area (167.4 mm2) C5 5% of total2 area (167.4 mm2)) C5 C5 5% of total area (167.45% mmof total) area (167.4 mm2) C5 5% of total area (167.4 mm22) C5 5% of total area (167.4 mm )

2 C15 15% of total area (502.2 mm2) C15 15% of total area (502.2 mm2) C15 C15 15% of total area (502.215% mm of 2total) area (502.2 mm2)) C15 15% of total area (502.2 mm2) C15 15% of total area (502.2 mm22) C15 15% of total area (502.2 mm )

2 C20 20% of total area (669.6 mm2) C20 20% of 2total area (669.6 mm2) C20 C20 20% of total area (669.620% mm of total) area (669.6 mm2)) C20 20% of total area (669.6 mm2) C20 20% of total area (669.6 mm22) C20 20% of total area (669.6 mm )

2 D5 10% of total2 area (334.8 mm2) D5 D5 10% of total area (334.810% mmof total) area (334.8 mm2) D5 10% of total area (334.8 mm2)) D5 10% of total area (334.8 mm2) D5 10% of total area (334.8 mm22) D5 10% of total area (334.8 mm )

2 2 D15D15 10% of total area (334.810% mmof total) area (334.8 mm2) D15 10% of total area (334.8 mm2) D15 10% of total area (334.8 mm2)) D15 10% of total area (334.8 mm2) D15 10% of total area (334.8 mm 2) D15 10% of total area (334.8 mm2 ) D15 10% of total area (334.8 mm )

In total, 36 samples were realised;In total, they 36 can samples be classified were realised; in four classes they labelledcan be classified with in four classes labelled with the letters A, B, C, D. As shownthe letters inInIn Table total,total, A,3 ,3636B, the samplesC,samples samples D. As werewereshown A is “as realised;realised; in produced”, Table theythey 3, the withoutcancan samples bebe classifiedclassified any A is “asinin fourfour produced”, classesclasses labelled labelledwithout withwith any the lettersIn total, A, 36B, samplesC, D. As wereshown realised; in Table they 3, the can samples be classified A is “asin four produced”, classes labelled without with any kind of texture; the samplesthekindthe in B lettersletters (FigureInInof total,total,texture; A,A,3a–c) 36 36B,B, samples wereC,thesamplesC, D.D. samples produced AsAs were wereshownshown in realised;byrealised; B inin changing(Figure TableTable theythey 3,33, thea–c) thethe cancan diameter weresamples samples bebe classifiedclassified produced of AA isis “as in“asin by fourfour produced”,produced”, changing classesclasses labelled labelledthe withoutwithout diameter with with anyany the dimples, keeping a fixedkindthe depth letters of and texture; A, density B, C,the of D. samples dimples; As shown thein B samplesin (Figure Table in 3,3 Ca–c) the (Figure weresamples4a–c produced) A is “as by produced”, changing thewithout diameter any thekindofkindthe the lettersletters ofof dimples, texture;texture; A,A, B,B, C,keepingtheC,the D.D. samplessamples AsAs a shownshown fixed inin BB indepth in (Figure(Figure TableTable and 3,33,3a–c)a–c) thedensitythe sampleswereweresamples of producedproduced dimples; AA isis “as“as by by the produced”,produced”, changingchanging samples the thewithoutinwithout C diameterdiameter (Figure anyany were made by changing theof density,kind the of dimples, keepingtexture; an keepingthe unchanged samples a fixed depthin B depth (Figure and diameter and 3a–c) density of were dimples. of produced dimples; by the changing samples thein Cdiameter (Figure kindof4a–c)ofkind thethe of ofwere dimples,dimples, texture;texture; made thekeepingkeepingthe by samples sampleschanging aa fixedfixed inin the B B depth depth (Figure (Figuredensity, andand 33a–c) a–c) k densitydensityeeping werewere anof of producedproduced dimples;unchangeddimples; by by thethe changingdepthchanging samplessamples and the thein indiameter CC diameterdiameter (Figure(Figure of The latter kind of samples in4a–c)of D the (Figure were dimples,5a,b) made were keeping by produced changing a fixed by the changing depth density, and only k densityeeping the depth anof of dimples;unchanged the depth samples and in diameter C (Figure of ofdimples.4a–c)of thethe were dimples,dimples, The made latter keepingkeeping by kind changing aaof fixedfixed samples the depthdepth density, in andDand (Figure k densitydensityeeping 5a,b) ofanof dimples; unchangeddimples;were produced thethe depth samplessamples by and changing in indiameter CC (Figure(Figure only of the dimples. The surface laser4a–c)dimples. texturing were The made was latter carriedby kind changing out of onsamples leadthe density, bronze in D (Figure specimens keeping 5a,b) an (EN unchanged were produced depth by and changing diameter only of 4a–c)thedimples.4a–c) depth werewere The of mademade the latter dimples. byby kind changingchanging ofThe samples surface thethe density,density, in laser D (Figure ktexturingkeepingeeping 5a,b) anan was unchanged unchangedwere carried produced out depthdepth on bylead andand changing bronze diameterdiameter spec- only ofof CC496K). This material is typicallydimples.the depth used The of for the latter anti-friction dimples. kind ofThe coating samples surface in the in laser hydraulicD (Figure texturing industry. 5a,b) was were carried produced out on bylead changing bronze spec-only dimples.imensthethedimples. depthdepth (EN The The ofof CC496K). thethe latterlatter dimples.dimples. kindkind This of ofTheThematerial samplessamples surfacesurface is intypicallyin laserlaser DD (Figure(Figure texturingtexturing used 5a,b)5a,b) for waswas anti-friction werewere carriedcarried producedproduced outout coating onon by byleadlead changinginchanging bronzebronze the hydrau- spec-spec- onlyonly In this case, it is used as a coatingtheimens depth for (EN valve of CC496K). the plates dimples. and This cylinder Thematerial surface blocks is intypically laser axial texturing piston used pumps. for was anti-friction carried out coating on lead in bronze the hydrau- spec- A failure analysis wasthe performedlicimensimensthe industry. depthdepth (EN(EN oftoof CC496K).CC496K). characterise theInthe this dimples.dimples. case, ThisThis the it The The materialmaterialis influence used surfacesurface as isis of a typicallytypically laser laser thecoating dimples texturingtexturing forusedused andvalve forfor was was the anti-friction anti-frictionplates carriedcarried and outout cylinder coatingcoating onon leadlead blocksinin bronzebronze thethe hydrau-hydrau- in spec-spec-axial imenslic industry. (EN CC496K). In this case, This it material is used as is atypically coating usedfor valve for anti-friction plates and cylinder coating inblocks the hydrau- in axial microstructure of the bronzeimenslicpistonlicimens alloy industry.industry. (EN subjected(ENpumps. CC496K). CC496K). InIn thisthis to the case,case, This This tribological itit material materialisis usedused test as asis is throughatypically atypically coatingcoating the usedforforused use valvevalve for offor an anti-friction anti-friction platesplates andand cylinder cylindercoating coating in blocksinblocks the the hydrau- hydrau- inin axialaxial pistonliclic industry.industry. pumps. InIn thisthis case,case, itit isis usedused asas aa coatingcoating forfor valvevalve platesplates andand cylindercylinder blocksblocks inin axialaxial optical microscope (OM), alicpiston scanning industry. pumps. electronic In this microscope case, it is used (SEM) as equipped a coating with for anvalve EDS plates and cylinder blocks in axial probe, and a confocal microscope.pistonpiston pumps.pumps. The optical microscopy was carried out by means of piston pumps. a LEICA stereomicroscope (LEICA Microsystems GmbH, Wetzlar, Germany), while the SEM microscopy was carried out by TM3030PLUS (HITACHI, Tokyo, Japan). Confocal microscopies (Leica DCM 3D) were performed with a 100× lens whose characteristics are shown in the Table4.

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(a) (b) (a) (b)

(c) (c) Figure 3. (a) B50 sample, (b) B100 sample, (c) B150 sample. FigureFigure 3. ( 3.a)( aB50) B50 sample, sample, (b (b) )B100 B100 sample, sample, ( (cc)) B150 B150 sample.sample.

(a) (b) (a) (b)

(c) (c) Figure 4. (a) C5 sample, (b) C15 sample, (c) C20 sample. FigureFigure 4. ( 4.a)( aC5) C5 sample, sample, (b (b) )C15 C15 sample, sample, ( (cc)) C20 C20 sample.

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(a) (b) Figure 5. (a) D5 sample, (b) D15 sample.

A failure analysis was performed to characterise the influence of the dimples and the microstructure of the bronze alloy subjected to the tribological test through the use of an optical microscope (OM), a scanning electronic microscope (SEM) equipped with an EDS probe, and a confocal microscope. The optical microscopy was carried out by means of a

LEICA stereomicroscope(a) (LEICA Microsystems GmbH, Wetzlar,(b) Germany), while the SEM microscopy was carried out by TM3030PLUS (HITACHI, Tokyo, Japan). Confocal FiguremicroscopiesFigure 5. (a 5.) D5(a )sample, D5 (Leica sample, ( bDCM) D15 (b) 3D) D15sample sample.were. performed with a 100× lens whose characteristics are shown in the Table 4. A failureTable analysis 4. Characteristic was performed parameters to characte of the 100rise× lens. the influence of the dimples and the microstructureTable 4. Characteristic of the bronze parameters alloy of subjected the 100x lens to the. tribological test through the use of an optical microscope (OM),Characteristic a scanning Parameters electronic microscope (SEM) equipped with an EDS Objective NAprobe, (Numerical and a Aperture) confocalCha microscope.racteristic WD (Working Parameters The optica Distance) l microscopy Pixel was Size carried (µm) out Min.by means Range of (µ m)a Objective NA (numerical Aperture) WD (Working Distance) Pixel Size (μm) Min. Range (μm) 100× LEICA 0.9stereomicroscope (LEICA Microsyste 0.27ms GmbH, Wetzlar, 0.16 Germany), while 4 the 100× SEM0.9 microscopy was carried ou 0.27t by TM3030PLUS (HITACHI, 0.16 Tokyo, Japan). Confocal4 microscopies (Leica DCM 3D) were performed with a 100× lens whose characteristics are shown3.3. Results Results in the and Table and Discussions Discussion 4. ToTo evaluate evaluate the the behavior behavior of of the the bronze bronze al alloyloy coating coating subject subject to to texture texture processing, processing, Tabletribologicaltribological 4. Characteristic tests tests were parameters were carried carried of theout out 100x according according lens. to to the the specifications speciﬁcations shown shown in in Table Table 22.. It It should be noted that very high loads have been specifically chosen, compared to the loads should be notedCha thatracteristic very high Parameters loads have been speciﬁcally chosen, compared to the loads generally used in the literature, to simulate the severe contact conditions to which the Objective NA (numerical Aperture)generally used in WD the (Working literature, Distance) to simulate the Pixel severe Size (μ contactm) conditions Min. Range to (μ whichm) the 100× 0.9 rotatingrotating parts parts of of the the axial axial piston piston 0.27 pumps pumps are are subjected subjected 0.16 in in conditions conditions of of poor poor 4lubrication. lubrication.

3.1.3.1. Friction Friction and and Wear Wear 3. Results and Discussions FigureFigure 66 shows shows the the variation variation of of the the friction friction coefficient coefﬁcient as as a a function function of of time, time, while while in in To evaluate the behavior of the bronze alloy coating subject to texture processing, TableTable 55 thethe averages averages of of the the friction friction coefficients coefﬁcients are shown. tribological tests were carried out according to the specifications shown in Table 2. It should be noted that very high loads have been specifically chosen, compared to the loads generally used in the literature, to simulate the severe contact conditions to which the rotating parts of the axial piston pumps are subjected in conditions of poor lubrication.

3.1. Friction and Wear Figure 6 shows the variation of the friction coefficient as a function of time, while in Table 5 the averages of the friction coefficients are shown.

FigureFigure 6. 6. ComparisonComparison of of the the different different coefficients coefﬁcients of of friction friction..

Table 5. Averages of coefﬁcients of friction.

Averages of Coefﬁcients of Friction A B50 B100 B150 C5 C15 C20 D5 D15 0.066 0.023 0.029 0.045 0.010 0.053 0.043 0.019 0.039 Figure 6. Comparison of the different coefficients of friction.

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Lubricants 2021, 9, 34 Table 5. Averages of coefficients of friction. 7 of 14 Averages of coefficients of friction A B50 B100 B150 C5 C15 C20 D5 D15 0.066By analysing0.023 the0.029 trend of0.045 the friction0.010 coefficients 0.053 shown0.043 in Figure0.0196 , it0.039 is possible to notice that the non-textured specimen maintains a higher friction coefficient compared to theBy texturedanalysing samplesthe trend whenof the friction subjected coeffici toents a load shown of 706.8in Figure N. 6, The it is frictionpossible coefficientto notice that the non-textured specimen maintains a higher friction coefficient compared to decreases by producing the dimples on the surface of the samples; however, by changing the textured samples when subjected to a load of 706.8 N. The friction coefficient decreases theby producing characteristics the dimples of the on texture, the surface the response of the samples; of the coatinghowever, in by terms changing of friction the char- coefficient alsoacteristics changes, of the which texture, does the notresponse always of the represent coating in an terms improvement of friction comparedcoefficient also to the non- texturedchanges, which surface. does In not fact, always Figure represent6 shows an that improvement at the end ofcompared the test to for the some non-textured types of profiles, thesurface. friction In fact, coefficient Figure 6 increasesshows that reaching at the end values of the similar test for tosome the types unprocessed of profiles, specimen. the As highlightedfriction coefficient by the increases values reaching shown invalu Tablees similar5, sample to the C5 unprocessed maintains specimen. an average As COF of 0.010,highlighted which by remains the values constant shown in for Table almost 5, sample the entireC5 maintains tribological an average test andCOF representsof 0.010, a net improvementwhich remains inconstant the tribological for almost behaviourthe entire tribological when compared test and with represents the untextured a net im- sample A,provement which showsin the tribological an average behaviour value of wh theen compared COF equal with to the 0.066. untextured An interesting sample A, analysis regardswhich shows the influencean average assumed value of the by COF the equal variation to 0.066. of aAn single interesting geometric analysis parameter regards on the frictionthe influence coefficient. assumed by the variation of a single geometric parameter on the friction coefficient. 3.2. Effect of Dimples Diameter 3.2. Effect of Dimples Diameter Figure7 shows the trend of the friction coefficient as a variation of the diameter of Figure 7 shows the trend of the friction coefficient as a variation of the diameter of thethe dimples.

FigureFigure 7. 7. VariationVariation of of the the friction friction coefficient coefﬁcient as a asfunction a function of the ofdiameter the diameter of dimples of dimples..

The curve curve shows shows that that as asthe the diameter diameter of the of dimples the dimples increases, increases, the friction the frictioncoefficient coefﬁcient increasesincreases [36]. [36]. This This behaviour behaviour of ofthe the coating coating may may be due be due in part in part to the to formation the formation of pile- of pile-up, Lubricants 2021, 9, x i.e.,up, i.e., the the accumulations accumulations of of bronze bronze alloy du duee to to the the melting melting generated generated by the by processing the processing 8 of of 15

laserof laser texturing texturing (Figure (Figure8 8).).

FigureFigure 8.8. ConfocalConfocal analysisanalysis ofof thethe surfacesurface ofof samplesample B150,B150, whichwhich highlightshighlights thethe presencepresence ofof pile-up.pile-up.

The greater the dimples diameter, the greater the height of the pile-up, and therefore the greater the friction generated on contact with the counter-specimen. In fact, Figure 9 shows the profile of the surface along the x axis. It is possible to notice that the pile-up assumes dimensions of even 20 μm.

Figure 9. Profile analysis of specimen B150.

The increase in friction coefficients can also be explained by the decrease in lubrication during the test. In fact, by increasing the size of the dimples, the oleophobicity of the coating decreases and the oil remains trapped inside the dimples. By comparing the wear track in the case of specimen B50 and in the case of specimen B150 (Figure 10a,b), it can be underlined that in the second case an evident phenomenon of seizure was generated in part due to poor lubrication, and in part due to the formation of debris, favoured by the presence of pile-up as highlighted by the SEM analyses in Figure 11.

Lubricants 2021, 9, x 8 of 15

Lubricants 2021, 9, 34 8 of 14 Figure 8. Confocal analysis of the surface of sample B150, which highlights the presence of pile-up.

The greater the dimples diameter, the greater the height of the pile-up, and therefore the greaterThe greater the friction the dimples genera diameter,ted on contact the greater with the the height counte of ther-specimen. pile-up, and In therefore fact, Figure 9 the greater the friction generated on contact with the counter-specimen. In fact, Figure9 shows the profile of the surface along the x axis. It is possible to notice that the pile-up shows the proﬁle of the surface along the x axis. It is possible to notice that the pile-up assumesassumes dimensions dimensions of even 2020µ μm.m.

Figure 9. ProfileFigure analysis 9. Proﬁle of analysisspecimen of specimenB150. B150.

TheThe increase increase in frictionfriction coefﬁcients coefficients can can also al beso explained be explained by the by decrease the decrease in lubrication in lubrica- during the test. In fact, by increasing the size of the dimples, the oleophobicity of the tion during the test. In fact, by increasing the size of the dimples, the oleophobicity of the coating decreases and the oil remains trapped inside the dimples. By comparing the wear coatingtrack in decreases the case of and specimen the oil B50 remains and in trapped the case ofinside specimen the dimples. B150 (Figure By comparing10a,b), it can the be wear Lubricants 2021, 9, x trackunderlined in the case that of in thespecimen second B50 case and an evident in the phenomenoncase of specimen of seizure B150 was (Figure generated9 of 15 10a,b), in it can bepart underlined due to poor that lubrication, in the second and incase part an due evident to the formationphenomenon of debris, of seizure favoured was by generated the in presencepart due of to pile-up poor lubrication, as highlighted and by in the part SEM due analyses to the information Figure 11 .of debris, favoured by the presence of pile-up as highlighted by the SEM analyses in Figure 11.

(a) (b)

Figure 10. (a) Wear trackFigure of 10. sample(a) Wear B50, track (b of) samplewear track B50, (b of) wear sample track B150 of sample. B150.

Since the bronze alloy is very malleable, the pile-up in contact with the pin (100Cr6) Since the bronzeis partiallyalloy is removedvery malleable, with the formationthe pile-up of debris in contact and partially with the “spread” pin (100Cr6) on the dimples is partially removedcausing with the a partial formation occlusion. of debris and partially “spread” on the dimples causing a partial occlusion.

Figure 11. The SEM image shows the presence of bronze alloy debris inside the dimples.

3.3. Effect of Dimples Density Figure 12 shows the trend of the friction coefficient as a variation of the density of dimples.

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(a) (b) Figure 10. (a) Wear track of sample B50, (b) wear track of sample B150.

Lubricants 2021, 9, 34 Since the bronze alloy is very malleable, the pile-up in contact with the pin (100Cr6)9 of 14 is partially removed with the formation of debris and partially “spread” on the dimples causing a partial occlusion.

FigureFigure 11.11. TheThe SEMSEM imageimage showsshows thethe presencepresence ofof bronzebronze alloyalloy debrisdebris insideinside thethe dimples.dimples. 3.3. Effect of Dimples Density 3.3. Effect of Dimples Density Lubricants 2021, 9, x Figure 12 shows the trend of the friction coefﬁcient as a variation of the density10 of 15 Figure 12 shows the trend of the friction coefficient as a variation of the density of of dimples. dimples.

Figure 12.12. Variation ofof thethe frictionfriction coefﬁcientcoefficient asas aa functionfunction ofof thethe densitydensityof ofdimples. dimples.

As shown by the the trend trend of of the the orange orange curve curve in in Figure Figure 12, 12 by, by varying varying the the density density of the of thedimples dimples on the on surface the surface of the of sample, the sample, the fricti theon friction coefficient coefficient also varies. also varies. The effect The shown effect shownby the tribological by the tribological tests is unexpected tests is unexpected with respect with to respect the predicted to the predictedtrend of the trend coefficient of the coefficientof friction of(CoF). friction As the (CoF). area As of the the area surface of the occupied surface occupiedby the dimples by the increases, dimples increases, a decrease a decreasein CoF is in expected CoF is expected due to the due increase to the increase in the surface in the surface oleophobicit; oleophobicit; i.e., the i.e., contact the contact angle angleincreases increases as the surface as thesurface roughness roughness increases. increases. This observation This observation is partly true, is partly but as true, demon- but asstrated demonstrated also by [18] also et by al., [18 the] et increase al., the increasein density in can density lead canto an lead increase to an increase in the friction in the frictioncoefficient coefficient due to an due increase to an increase in roughness in roughness and to the and pile-up, to the pile-up,whose increase whose increase is propor- is proportionaltional to the quantity to the quantity of dimples of dimples present present on the surface. on the surface. What can What be noticed can be noticedis that the is thatparameter the parameter Sa (Table Sa 6), (Table which6), represents which represents the average the average roughness roughness of the analysed of the analysed surface, surface,increases increases with the with density the densityof the dimples. of the dimples. In particular, In particular, for the C5 for specimen the C5 specimen with a density with a densityof 5% there of 5% will there be willan average be an average roughness roughness Sa equal Sa equalto 1.08 to μ 1.08m; forµm; the for C15 the specimen C15 specimen with µ witha density a density of 15% of there 15% therewill be will an be average an average roughness roughness Sa equal Sa equal to 2.13 to μ 2.13m; form; the for C20 the spec- C20 µ specimenimen with with a density a density of 20% of 20%there there will willbe an be average an average roughness roughness Sa equal Sa equal to 2.50 to2.50 μm. m.

Table 6. Surface parameters obtained with confocal microscopy of an area of 636 x 427 μm2 with a resolution of 10−4 μm.

Characteristic Parameters Name Max Sa C5 8.4017 μm 1.08 μm C15 8.3067 μm 2.13μm C20 10.582 μm 2.50 μm

The surface deterioration due to the tribological test is evident by analysing Figure 13a,b which show the confocal scans of the C20 specimen before and after the test.

Lubricants 2021, 9, 34 10 of 14

Table 6. Surface parameters obtained with confocal microscopy of an area of 636 x 427 µm2 with a resolution of 10−4 µm.

Characteristic Parameters Name Max Sa C5 8.4017 µm 1.08 µm C15 8.3067 µm 2.13µm C20 10.582 µm 2.50 µm

Lubricants 2021, 9, x 11 of 15 The surface deterioration due to the tribological test is evident by analysing Figure 13a,b which show the confocal scans of the C20 specimen before and after the test.

(a)

(b) FigureFigure 13. 13.(a) ( Confocala) Confocal scan scan of C20 of specimen C20 specimen untested untested surface; (b )surface; confocal ( scanb) confocal of C20 specimen scan of C20 specimen afterafter the the tribological tribological test. test. From Figure 13b it can be detected that on the wear track there are microgrooves, whichFrom indicate Figure the presence 13b it of can abrasive be detected particles. that In Figure on the13a, wear the pile-ups track arethere instead are microgrooves, verywhich evident, indicate highlighted the presence in white of in abrasive the image. particles. A more detailed In Figure analysis 13a, of the the pile-ups wear are instead trackvery was evident, carried highlighted out with the SEM in white microscope in the and imag thee. EDS A more probe. detailed In Table7 analysisthe values of the wear track calculatedwas carried according out with to ISO the 25178 SEM are microscope reported. The and mean the EDS values probe. of the In roughness Table 7 ofthe values calcu- untested and tested specimens are, respectively, 2.68 µm and 1.65 µm. lated according to ISO 25178 are reported. The mean values of the roughness of untested and tested specimens are, respectively, 2.68 μm and 1.65 μm.

Table 7. Summary of roughness parameters.

Height Parameters Parameter Untested Tested Sq (μm) 3.70 3.13 Ssk (μm) −1.42 −3.03 Sku (μm) 5.68 12.70 Sp (μm) 10.38 10.05 Sv (μm) 20.65 18.33 Sz (μm) 31.04 28.38 Sa (μm) 2.68 1.65

3.4. Effect of Dimple Depth Figure 14 shows the trend of the friction coefficient as a variation of the depth of the dimples.

Lubricants 2021, 9, 34 11 of 14

Table 7. Summary of roughness parameters.

Height Parameters Parameter Untested Tested Sq (µm) 3.70 3.13 Ssk (µm) −1.42 −3.03 Sku (µm) 5.68 12.70 Sp (µm) 10.38 10.05 Sv (µm) 20.65 18.33 Sz (µm) 31.04 28.38 Sa (µm) 2.68 1.65

3.4. Effect of Dimple Depth Lubricants 2021, 9, x 12 of 15 Figure 14 shows the trend of the friction coefﬁcient as a variation of the depth of the dimples.

Figure 14.14.Variation Variation of of the the friction friction coefﬁcient coefficient as a functionas a function of the depthof the ofdepth the dimples. of the dimples.

ItIt isis possible possible to to highlight highlight in Figurein Figure 14 the 14 frictionthe friction coefficient coefficient increases increases with increasing with increasing depth of the dimples [37]. This behaviour could be due to an excessive “reservoir” effect of depth of the dimples [37]. This behaviour could be due to an excessive “reservoir” effect the dimples, which with increasing depth tend to catch the oil and do not allow correct lubrication.of the dimples, Additionally, which with in increasing this case, the depth increase tend in to the catch depth the ofoil the and dimples do not couldallow correct leadlubrication. to a decrease Additionally, in the surface in this oleophobicity case, the increase and therefore in the depth a poor of lubrication. the dimples After could lead separatelyto a decrease analsing in the the surface effect of oleophobicity the diameter, density,and therefore and depth a poor of the lubrication. dimples on After the sepa- frictionrately analsing coefficient, the it effect is possible of the to diameter, confirm that den asity, lower and density depth associated of the dimples with a on lower the friction depthcoefficient, provides it is a visiblepossible improvement to confirm on that the a friction lower coefficient density associated compared towith the changea lower depth inprovides diameter. a visible improvement on the friction coefficient compared to the change in diameter. 3.5. Wear Track As mentioned in the previous paragraphs, the presence of the pile-up can trigger 3.5. Wear Track abrasive wear, as shown by the microgrooves in Figure 13b (previous paragraph). To analyseAs thementioned state of the in wearthe previous track in moreparagraphs, detail, microscopies the presence were of performedthe pile-up with can trigger theabrasive aid of wear, the SEM. as shown The B150 by the was microgrooves chosen as the specimenin Figure to13b be (previous studied, as paragraph). shown in To an- Figurealyse the 15, state in which of the the wear diameter track of in the more dimples detail, is measured, microscopies which were is around performed 150 µm, with as the aid perof the specifications SEM. The inB150 Table was3. chosen as the specimen to be studied, as shown in Figure 15, in which the diameter of the dimples is measured, which is around 150 μm, as per specifications in Table 3.

Figure 15. SEM image at 600× of the dimple of the B150 specimen.

Analysing the wear track (Figure 16), one should note the presence of the microgrooves due to the action of an abrasive particle, which could be associated to the action

Lubricants 2021, 9, x 12 of 15

Figure 14. Variation of the friction coefficient as a function of the depth of the dimples.

It is possible to highlight in Figure 14 the friction coefficient increases with increasing depth of the dimples [37]. This behaviour could be due to an excessive “reservoir” effect of the dimples, which with increasing depth tend to catch the oil and do not allow correct lubrication. Additionally, in this case, the increase in the depth of the dimples could lead to a decrease in the surface oleophobicity and therefore a poor lubrication. After separately analsing the effect of the diameter, density, and depth of the dimples on the friction coefficient, it is possible to confirm that a lower density associated with a lower depth provides a visible improvement on the friction coefficient compared to the change in diameter.

3.5. Wear Track As mentioned in the previous paragraphs, the presence of the pile-up can trigger abrasive wear, as shown by the microgrooves in Figure 13b (previous paragraph). To analyse the state of the wear track in more detail, microscopies were performed with the aid Lubricants 2021, 9, 34 of the SEM. The B150 was chosen as the specimen to be studied, as shown in Figure12 15, of 14in which the diameter of the dimples is measured, which is around 150 μm, as per specifications in Table 3.

Figure 15. SEM image at 600× of the dimple of the B150 specimen. Lubricants 2021, 9, x Figure 15. SEM image at 600× of the dimple of the B150 specimen. 13 of 15

AnalysingAnalysing the the wear wear track track (Figure (Figure 16), 16), one one should should note thenote presence the presence of the microgroovesof the micro- Lubricants 2021, 9, x 13 of 15 duegrooves to the due action to the of anaction abrasive of an particle,abrasive which particle, could which be associatedcould be associated to the action to the of debris action thatof debris was formedthat was by formed detachment by detac ofhment a portion of a ofportion pile-up, of pile-up, or it could or it be could debris be duedebris to pindue deterioration.to pin deterioration. of debris that was formed by detachment of a portion of pile-up, or it could be debris due to pin deterioration.

FigureFigure 16.16. SEMSEMFigure imageimage 16. atat SEM 250250×× image ofof microgroov microgrooves at 250× of microgrooves on on a a bronze bronzees on alloy a bronze surface surface. alloy. surface.

ToTo conﬁrmconfirm thetheTo compositionconfirmcomposition the composition ofof thethe debris,debris, of the aa “pointdebris,“point aandand “point shot”shot” and analysisanalysis shot” analysis withwith the thewith EDSEDS the EDS probeprobe waswas performed.performed.probe was Asperformed. cancan bebe seenseenAs can inin be FigureFigu seenre in1717, Figu, thethere analysisanalysis 17, the analysis ofof thethe debrisdebrisof the debris returnsreturns returns thethe the characteristiccharacteristiccharacteristic peak of Fe at peak 6.46.4 of keV.keV. Fe atSinceSince 6.4 keV. therethere Since isis nono there ironiron is presencepresenceno iron presence inin thethe compositionincomposition the composition ofof of thethe bronzebronze alloy,alloy,the thebronze the particles particles alloy, thatthe that particles generated generat thated the generat the microgrooves microgroovesed the microgrooves certainly certainly comecertainly come from comefrom the pinfromthe the pin deterioration. deterioration.pin deterioration.

FigureFigure 17. 17.EDS EDS analysis analysis shows shows the presence of of Fe Fe th throughrough the the characteristic characteristic peak peak at 6.4at keV. 6.4 keV.

Figure 17. EDS analysis shows4. Conclusions the presence of Fe through the characteristic peak at 6.4 keV. In order to increase the efficiency of components that work under reciprocating slid- 4. Conclusionsing, the application of the surface texture on an antifriction coating used in the automotive and hydraulic fields was tested. By using a low economic impact technique such as surface In order to increase the efficiency of components that work under reciprocating slid- laser texturing, it is possible to decrease the friction coefficient of a surface and at the same ing, the applicationtime increase of the surfacethe oleophobicity texture on and an therefore antifriction improve coating lubrication, used in theespecially automotive in starved and hydrauliclubrication fields was conditions. tested. By Theusing improvement a low economic in lubrication impact technique due to surface such texturingas surface influ- laser texturing,enced it is thepossible COF value to decrease which remainedthe friction low coefficient for almost of all a surfacetypes of andtextures, at the even same under time increaseextreme the oleophobicity load conditions. and The therefore study was improve carried lubrication, out on a lead especially bronze alloy in which,starved owing lubrication conditions.to the lubricating The improvement characteristics ofin lead, lubrication maintains due a low to frictionsurface coefficient. texturing Three influ- types enced the COFof specimensvalue which were remained tested by varyinglow for the almost geomet allric types characteristics of textures, of the even dimples, under i.e., di- extreme loadameter conditions. (class B),The density study (classwas carriedC), and outdepth on (class a lead D). bronze It has been alloy demonstrated which, owing that by to the lubricatingdecreasing characteristics the diameter of oflead, the dimples,maintains the a Co lowF also friction decreased. coefficient. Additionally, Three bytypes increasing the depth of the dimples, the CoF increases. On the other hand, increasing the surface of specimens were tested by varying the geometric characteristics of the dimples, i.e., di- density of the dimples increases the CoF. The results obtained are very interesting and ameter (class B), density (class C), and depth (class D). It has been demonstrated that by decreasing the diameter of the dimples, the CoF also decreased. Additionally, by increasing the depth of the dimples, the CoF increases. On the other hand, increasing the surface density of the dimples increases the CoF. The results obtained are very interesting and

Lubricants 2021, 9, 34 13 of 14

4. Conclusions In order to increase the efficiency of components that work under reciprocating sliding, the application of the surface texture on an antifriction coating used in the automotive and hydraulic fields was tested. By using a low economic impact technique such as surface laser texturing, it is possible to decrease the friction coefficient of a surface and at the same time increase the oleophobicity and therefore improve lubrication, especially in starved lubrication conditions. The improvement in lubrication due to surface texturing influenced the COF value which remained low for almost all types of textures, even under extreme load conditions. The study was carried out on a lead bronze alloy which, owing to the lubricating characteristics of lead, maintains a low friction coefficient. Three types of specimens were tested by varying the geometric characteristics of the dimples, i.e., diameter (class B), density (class C), and depth (class D). It has been demonstrated that by decreasing the diameter of the dimples, the CoF also decreased. Additionally, by increasing the depth of the dimples, the CoF increases. On the other hand, increasing the surface density of the dimples increases the CoF. The results obtained are very interesting and show how the application of the surface texture affects the decrease in the friction coefficient, which assumes a lower value for all textured samples than untextured, which demonstrates how the alloy used already has high anti-friction properties due to the presence of lead which acts as a solid lubricant. The future challenge is to eliminate lead from the bronze alloy, due to its toxicity, and to use the texture to keep the friction coefficient low.

Author Contributions: Conceptualization, G.R. and L.S.; methodology, D.D. and A.S.; validation, G.R., L.S. and A.S.; formal analysis, D.D.; investigation, D.D.; data curation, D.D.; writing—original draft preparation, D.D.; writing—review and editing, D.D. and A.S.; visualization, D.D. and A.S.; su- pervision, G.R. and L.S. All authors have read and agreed to the published version of the manuscript. Funding: The author(s) received no financial support for the research, authorship, and/or publication of this article. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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