materials Article Surface Damaging of Brass and Steel Pins when Sliding over Nitrided Samples Cut by Finishing and Roughing EDM Conditions Vitaliy Martynenko 1,2 , Daniel Martínez Krahmer 1,2 , Amelia Nápoles Alberro 3 , Amado Cabo 4 , Daniela Pérez 1,2 , Enrique E. Zayas Figueras 3 , Hernán A. Gonzalez Rojas 3 and Antonio J. Sánchez Egea 3,5, * 1 Center for Research and Development in Mechanics, National Institute of Industrial Technology (INTI), Avenida General Paz 5445, Buenos Aires 1650, Argentina; vmart@inti.gob.ar (V.M.); dmartinez@inti.gob.ar (D.M.K.); danielap@inti.gob.ar (D.P.) 2 Faculty of Engineering, Universidad Nacional de Lomas de Zamora, Juan XXIII y Camino de Cintura, Buenos Aires 1832, Argentina 3 Department of Mechanical Engineering, Universitat Politècnica de Catalunya, C. Jordi Girona, 1–3, 08034 Barcelona, Spain; amelia.napoles@upc.edu (A.N.A.); enrique.zayas@upc.edu (E.E.Z.F.); hernan.gonzalez@upc.edu (H.A.G.R.) 4 IONAR S.A. Avenida Arias, 342, Ciudad Autónoma de Buenos Aires C1430CRB, Argentina; cabo@ionar.com.ar 5 Department of Mechanical and Metallurgical Engineering, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Región Metropolitana 7820436, Chile * Correspondence: antonio.egea@upc.edu  Received: 26 June 2020; Accepted: 15 July 2020; Published: 17 July 2020  Abstract: In the forging industry, surface quality and surface treatments of dies are crucial parameters to extend their life. These components are usually machined by milling or by Electrical Discharge Machining (EDM), and the final surface roughness depends on the machining techniques and operational conditions used in its fabrication. After milling, a nitriding treatment is widely applied to extend its service life. Nevertheless, no scientific report that informs about nitriding after EDM has been found. Accordingly, this work focuses on the wear and friction behavior of pins made of brass and medium carbon steel sliding over AISI H13 discs, made by wire EDM in the conditions of finishing and roughing. The discs are plasma nitride, and their effect on the wear during pin-on-disc tests is evaluated. In this sense, the analysis of the surface damage for the different pins will help us to understand the service life and wear evolution of the forging dies. The results show that plasma nitride reduces the friction and prevents the degradation of the pin, independently of the material of the pin, when sliding over finishing and roughing EDM conditions. Keywords: plasma nitriding; EDM; pin-on disc test; surface quality; wear and friction 1. Introduction Forging is a metal forming process widely used in industries, such as automotive, aerospace, railway, naval, oil, mining, and health [1,2]. Several parts are involved in this process: press, dies, material to be forged, lubrication system, and type of lubricant. Press and dies have to bear fatigue stresses, and dies are commonly manufactured with H series tool steel, where AISI H13 grade is the most universally used. This tool steel is hard to machine because of the high cutting tension due to its alloy content [3]. Replacing dies represent between 10% and 30% of the cost of a forged part [4], which enforces the need to decrease this percentage. In that sense, a proper forging process must take into account how the die is manufactured and during the forging process, protect and lubricate. Materials 2020, 13, 3199; doi:10.3390/ma13143199 www.mdpi.com/journal/materials Materials 2020, 13, 3199 2 of 10 The die’s surface quality is relevant in the forging process to manufacture the forged part without surface damage or scratches. Accordingly, different options exist to manufacture dies, and depending on the option, different surface features can be achieved. One option is milling the dies. Here, parameters such as type of tool and its geometry [5–8], the machining strategy [9,10] and the operational conditions [11,12] play an essential role on its final surface properties, and these parameters are tightly related to the service efficiency of the die (for example, fatigue resistance). Another option is the Electrical Discharge Machining (EDM) of the dies [13]. EDM is capable of making complex shapes and deep cavities; the only requirement is to work with an electrically conductive material. The necessary conditions to take into account in the EDM process that can affect the EDM removal rates and surface finishing of dies are the following: the material of the electrodes [14], the frequency of pulses and depths [15] and the addition of external powders [16] to modify surface properties and minimize the roughness by filling pores and cracks. Once the dies are manufactured, different surface treatments are used to extend their service life [17]; the most used treatment is the diffusion of nitrogen (nitriding), which achieves a significant increase of surface hardness and increase up to 125% in fatigue life [18]. The plasma nitriding is a modern technology that depends mainly on: gas composition, voltage and duty cycle, pressure, time, and temperature. For example, Solis-Romero et al. [19] used pin-on-disk tests at room temperature with several axial loads. In all cases, the friction coefficients were reduced for nitrided samples. A similar nitriding procedure was performed by Leite et al. [20] at 400 ◦ C for 4 h, 9 h, 16 h, and 36 h. Then, during the ball-on disk test, a decrease in wear rate was found by up to 50% when the nitriding time passed from 4 to 36 h. Other experiments were made, modifying the internal pressure at the plasma nitriding chamber [21]. These authors found that the friction coefficients went from 0.55 to 0.30 for nitrided samples with pressures of 200 to 300 Pa, respectively. Besides the technology of manufacturing the dies and the type of their surface treatment, it is necessary to consider the medium or lubricant used during the forging operation. A graphite-based lubricant is commonly used due to the high temperature reached in hot and warm forging processes. Dual-phase lubricants, spreading over the surface, reduce the friction coefficient during the forging process. Previous works [22–24] analyzed the behavior of several commercial graphite-based lubricants in hot forging, employing wear tests like pin-on-disc and friction tests like ring compression test, performed in different surface conditions. The experience shows that the size of the embedded graphite particles, the graphite concentration, and the lubricant’s kinematic viscosity are parameters that influence the friction coefficient at the interface, independent of the testing temperature. This work aims to study the initial surface properties to describe their influence on surface wear. These surface properties of the discs are defined by wiring EDM in finishing and roughing conditions and using plasma nitride. Then pin-on-disc tests are performed to compare the friction coefficient and wear rate for pins made of brass and medium carbon steel, which slide over discs made of AISI H13. 2. Methodology This section is divided into several subchapters: characteristics of the lubricant, machining conditions and surface treatment of the discs, and pin-on-disc tests at room temperature. Throughout these subsections, the experimental protocols, equipment, and facilities used to investigate the friction and wear of steel and brass pins on EDMed discs with different types of surfaces conditions are described. 2.1. Characteristics of the Lubricant The graphite-based lubricant was diluted up to 5% in water, which corresponds to a typical average lubricant used by Argentine forging companies. This lubricant had a density between 1.10 and 1.20 g/cm3 . A scanning electron microscopy (FEI Model: QUANTA 250 FEG, FEI, Eindhoven, the Netherlands) was used to determine the elemental chemical composition of the lubricant and the size of the graphite particles. Also, the kinematic viscosity was determined with an oscillating rheometer (Anton Paar Physica Model MCR301, Anton Paar, Ostfildern, Stuttgart, Germany). Figure 1a Materials 2020, 13, x FOR PEER REVIEW 3 of 10 Materials 2020, 13, 3199 3 of 10 of the graphite particles. Also, the kinematic viscosity was determined with an oscillating rheometer (Anton Paar Physica Model MCR301, Anton Paar, Ostfildern, Stuttgart, Germany). Figure 1a shows shows the as-received graphite lubricant matrix as supplied, and Figure 1b presents the kinematic the as-received graphite lubricant matrix as supplied, and Figure 1b presents the kinematic viscosity viscosity curve for the water diluted lubricant. Table 1 quantifies the average size of the graphite curve for the water diluted lubricant. Table 1 quantifies the average size of the graphite particles, particles, which are obtained from 20 measurements. Also, the average percentage of the weight of which are obtained from 20 measurements. Also, the average percentage of the weight of each each chemical element corresponds to the result of five measurements. chemical element corresponds to the result of five measurements. Figure 1. Figure 1. (a) (a) Graphite-based Graphite-based lubricant lubricant matrix matrix and and (b) the kinematic (b) the kinematic viscosity viscosity curve curve of of the the water water diluted lubricant. diluted lubricant. Table 1. Chemical Table 1. Chemical composition composition and and particle particle size size of of the the graphite graphite particles. particles. Na Al Al Si Si Graphite Lubricant CC (wt.%) OO (wt.%) Na Lubricant SS(wt.%) Graphite Size (wt.%) (wt.%) (wt.%) Size (µm) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (μm) Average 80.6780.67 11.7811.78 2.36 2.36 Average 0.920.92 3.25 3.25 0.290.29 5.63± ±2.27 5.63 2.27 2.2. 2.2. Discs and Pins Preparation A Promecor Promecor SMT SMT 19/500 19/500 numerical numerical control control lathe (Promecor, (Promecor, Córdoba, Córdoba, Argentina) Argentina) was was usedused to to machine machine 16 AISI H13 steel discs with an external diameter of 63 mm, an internal diameter of 19 mm, and and 8 mmmm of of thickness. thickness.All Alldiscs discs were were hardened hardened at 1040 °C ◦and at 1040 C and tempered tempered for 2for 2h h at 570at°C. 570Both◦ C. Both faces faces of theofdiscs the discs werewere grounded grounded on on a tangentialgrinding a tangential grindingmachine machine(Davonis (Davonis model model SGS-1230AHR, Davonis, Davonis, Billinghurst, Buenos Aires, Argentina), using an A46I10V A46I10V grinding grinding wheel wheel withwith an an average average grain grain size of 0.38 mm. A hardness tester (INSTRON WOLPERT, Model: Model: S8-233971, Rockwell C Scale, Instron Instron Corporation, Corporation, Canton, Canton, MA,MA, USA) USA) was was used used toto measure measure the the hardness hardness of of the the disc disc before before the wire wire EDM EDM and nitriding nitriding treatment, treatment,being 51.6± ±1.51.5 beingofof51.6 HRC HRC forfor finishing finishing condition condition andand 51.151.1 ± 2.1± 2.1 HRC HRC for for roughing roughing condition. condition. Finally, Finally, the the surfaces surfaces of theof discs the discs werewerewirewire EDM EDM (Novick (Novick AR 35 ARMA) 35 MA)using using two two operational operational conditions: conditions: half half of themof them in a finishing in a finishing condition condition μs;=toff (ton = 5(ton 5 µs; μs;=T34= µs; = 34toff 39 μs;T =f 39= 25.6 µs; = 25.6 fHz; ton/THz; ton /TV==0.13; = 0.13; = 4 the 4 V),Vand V), and otherthe other half in ahalf in a roughing roughing conditioncondition (ton = 50(tμs; on =t 50 off = 180 µs; t μs; off = 180 T = 230 µs; = f230 Tμs; = 4.3 f =ton µs;Hz; 4.3 /T Hz; ton /T = 0.22; V ==60.22; V = 6these V). From V). From these two two groups, halfgroups, half of of the discs weretheplasma discs were plasma nitriding by nitriding by IONAR S.A. The nitriding conditions were: of temperatures ◦ C, exposure time of of 500time IONAR S.A. The nitriding conditions were: temperatures 500 °C, exposure of 13 h, protective 13 gash,made protective of 20% gasNmade 2 + 80% of H 20% N2 + 2, and 80% H2 , and a pressure of 6 ahPa. pressure of 6ahPa. Finally, Finally,section transversal a transversal of the section nitride of the nitride combined withcombined finishing with finishing and and roughing EDMroughing conditions EDMwasconditions analyzed in was analyzed a Vickers in a Vickers microhardness microhardness tester (Shimadzu tester (Shimadzu HMV-2000, HMV-2000, Shimadzu Shimadzu Corporation, Corporation, Kyoto, Kyoto, Japan). Japan). Regarding Regarding the pins, they come the pins, from they drawncomebars from drawn of 6.35 mmbars of 6.35 mm in diameter. in diameter. Sixteen (16) pinsSixteen were (16) made pins of were brass made ASTMofB16 brass ASTM hardness B16 HRBhardness HRB 76, 16 76, and another and another pins were 16 pinsofwere made steelmade AISI of steel 1045 AISI 1045 hardness HRB hardness 87. AllHRBof them 87. All were of made them were made with hemispherical with hemispherical tips 4 mm tips 4 mm diameter diameter at both at both ends that wereends that were polished polishedpapers with abrasive with abrasive of grain papers of grain sizes 100, 600, andsizes 100, 600, and 1000. 1000. 2.3. Pin-on Disk Test at Room Temperature Materials 2020, 13, 3199 4 of 10 2.3. Pin-on Disk Test at Room Temperature The pin-on-discs Materials testsREVIEW 2020, 13, x FOR PEER were carried out with equipment made in-house (Figure 4 of 102) at INTI-Mechanical Center in Argentina. The tangential force during the tests was recorded with The (Vernier a data logger pin-on-discs ModeltestsLabQuest, were carried out with Vernier, equipment Beaverton, OR,made USA),in-house which (Figure 2) at has a load INTI-up to range Mechanical Center in Argentina. The tangential force during the tests was recorded with a data logger 50 N. Pins and disks were fixed in the machine to carry out the tests with the appropriate amount (Vernier Model LabQuest, Vernier, Beaverton, OR, USA), which has a load range up to 50 N. Pins of lubricant. The lubricant was added on the disc’s surface, checking that a uniform dispersion had and disks were fixed in the machine to carry out the tests with the appropriate amount of lubricant. occurred before the test began. The axial loads on the pins were 4.5 N for the brass and 6.5 N for The lubricant was added on the disc’s surface, checking that a uniform dispersion had occurred the steel, to before thereproduce test began.a contact The axialpressure loads onsimilar the pinstowere that4.5 which N foroccurs in aand the brass forging 6.5 N process withtothese for the steel, materials (140 and 200 MPa respectively [25]). The tangential speed in the pin-disk contact reproduce a contact pressure similar to that which occurs in a forging process with these materials was 0.2 m/s, and the (140total and experiment duration 200 MPa respectively of each [25]). test was 20 The tangential minin(equivalent speed the pin-disk to 240 m). contact was These 0.2 m/s,experiments and the were total experiment repeated 32 times duration of each resulting fromtestthe wascombination 20 min (equivalent to 240materials, of two-pin m). These two experiments were discs machining repeated 32 times resulting from the combination of two-pin materials, conditions (finishing and roughing wire EDM), two surface treatment conditions (non-nitrided and two discs machining conditions nitrided discs), (finishing and fourand roughingper repetitions wireeach EDM), two surface condition. treatment From conditions (non-nitrided the experiments, and and friction curves nitrided discs), and four repetitions per each condition. From the experiments, friction curves and weight loss of the pins were determined. The weight difference was measured with a Radwag electronic weight loss of the pins were determined. The weight difference was measured with a Radwag scale, model AS 220.R2 (Radwag, Radom, Poland), with an accuracy of 0.1 mg. Finally, the adhesions of electronic scale, model AS 220.R2 (Radwag, Radom, Poland), with an accuracy of 0.1 mg. Finally, the material on discs were studied to describe the surface properties. The pin-on-disc tests were performed adhesions of material on discs were studied to describe the surface properties. The pin-on-disc tests at 21 ± ◦ and relative humidity of 50 ± 10%. were C 3 performed at 21 ± 3 °C and relative humidity of 50 ± 10%. Figure 2. Pin-on-disk equipment used in this work and data for different experiments. Figure 2. Pin-on-disk equipment used in this work and data for different experiments. 2.4. Surface 2.4. Surface Roughness Roughness As the EDM process does not have a predominant surface roughness direction [26], the surface As the EDM process does not have a predominant surface roughness direction [26], the surface roughness measurements were made in random (multidirectional) directions. A portable roughness roughness measurements were made in random (multidirectional) directions. A portable roughness meter (Taylor Hobson Surtronic 3+, Taylor Hobson, Leicester, England) measured the arithmetic metermean (Taylor Hobson Surtronic 3+, Taylor Hobson, Leicester, England) measured the arithmetic mean surface roughness (Ra) and the total height of the roughness profile (Rt), where the cut-off and surface roughness (Ra) and the evaluation length weretheset total at 0.8 height mm andof the respectively. 4 mm roughness profile (Rt), The AISI H13where thewere steel discs cut-off and the turned evaluation length were set at 0.8 mm and 4 mm respectively. The AISI H13 steel discs quenched, tempered and finally grounded up to a Ra of 0.51 ± 0.08 µ m. Table 2 shows average values were turned quenched, and thetempered standardand finallyofgrounded deviation the surfaceup to a Ra of 0.51 roughness ± 0.08 the discs µm. Table measured on the 2 shows average surfaces obtained values from and the finishingdeviation standard and roughing of theEDM with roughness surface and withoutofthe theplasma nitriding treatment. discs measured A total of on the surfaces 96 obtained from measurements finishing andwere made to roughing EDMdetermine arithmetic with and mean without thesurface plasma roughness nitriding(Ratreatment. ) and the total A height total of 96 of the roughness profile (R t). measurements were made to determine arithmetic mean surface roughness (Ra ) and the total height of the roughness profile (Rt ). Materials 2020, 13, 3199 5 of 10 Materials 2020, 13, x FOR PEER REVIEW 5 of 10 Table 2. Surface Table roughness Surface andand roughness material hardness material for the hardness fordifferent EDM conditions the different with andwith EDM conditions without and nitriding.nitriding. without RRaa ±±SD SD Rt ±SD Rt ± SD Hardness Hardness Sample Sample (μm±±SD) (µm SD) (μm ± SD) (µm ± SD) (HV ±SD) (HV ± SD) Finishing Finishing EDM (substrate) EDM (substrate) 3.74±±0.38 3.74 0.38 26.8526.85 ± 3.39 556.0 ± 25.2 ± 3.39 (*)± 25.2 (*) 556.0 Roughing EDM (substrate) Roughing EDM (substrate) 7.88±±1.51 7.88 1.51 54.9554.95 ± 8.93 ± 8.93 (*)± 35.4 (*) 543.1 543.1 ± 35.4 Nitrided finishing Nitrided EDM EDM finishing 2.31±±0.40 2.31 0.40 18.0918.09 ± 2.601130.3 ± 1130.3 ± 2.60 71.1 ± 71.1 Nitrided roughing EDM 5.43 ± 0.76 43.07 ± 6.08 853.3 ± 26.4 Nitrided roughing EDM 5.43 ± 0.76 43.07 ± 6.08 853.3 ± 26.4 * The substrate was hardened and tempered to record the hardness similar than [20]. * The substrate was hardened and tempered to record the hardness similar than 20. The finishing The finishingand androughing roughing EDM conditions EDM have significant conditions differences have significant in the surface differences in theroughness, surface being higher for the roughing condition of EDM. The nitrided specimens presented roughness, being higher for the roughing condition of EDM. The nitrided specimens presented lower lower values of Ra and Rt,of values as Ra well as lower and Rt, as dispersion. well as lower Similar trends Similar dispersion. were also foundwere trends by Solis alsoRomero found by et al. [19]. Solis Figureet3 Romero shows the surface morphologies for finishing and roughing EDM surfaces without and al. [19]. Figure 3 shows the surface morphologies for finishing and roughing EDM surfaces without with the plasma nitriding treatment. Regarding the material’s hardness, the specimen treated and with the plasma nitriding treatment. Regarding the material’s hardness, the specimen treatedin a finishing EDM in condition presented a hardness 32.5% higher than the roughing EDM condition. a finishing EDM condition presented a hardness 32.5% higher than the roughing EDM condition. Nitrided finishing EDM specimens Nitrided finishingreached similar hardness EDM specimens reached values to [18]. similar Regarding hardness valuesthe to EDM surface morphology 18. Regarding the EDM (Figure surface 3a,c), the results morphology present (Figure thethe 3a,c), same features results as the present thework samereported featuresinas[13], wherereported the work the sample was in [13], EDMed with short pulses and a long duration. where the sample was EDMed with short pulses and a long duration. Figure 3. (a) Surface morphology of EDM in roughing condition; (b) Surface morphology of EDM in roughing condition and nitriding; (c) Surface morphology of EDM in finishing condition; (d) Surface morphology of EDM in finishing condition and nitriding. 3. Results and Discussion 3. Results and Discussion This section analyzes the friction curves to describe the influence of the surface roughness and the This section analyzes the friction curves to describe the influence of the surface roughness and nitriding treatment on the friction coefficients. Besides, the weight loss of the pins after the pin-on-disk the nitriding treatment on the friction coefficients. Besides, the weight loss of the pins after the pin- tests is analyzed to measure the wear of tool tips for the pins made of brass and steel. on-disk tests is analyzed to measure the wear of tool tips for the pins made of brass and steel. Materials 2020, 13, 3199 6 of 10 Materials 2020, 13, x FOR PEER REVIEW 6 of 10 3.1. Pin-on Disk Tests 3.1. Pin-on Disk Tests The experimental tests are repeated five times per each surface condition to analyze the friction The experimental tests are repeated five times per each surface condition to analyze the friction coefficient Materialsat the13,stationary 2020, x FOR PEER phase. REVIEW Figure 4 exhibits the boxplot of the friction coefficients for 6 ofthe 10 two coefficient at the stationary phase. Figure 4 exhibits the boxplot of the friction coefficients for the two surface conditions and with the presence or not of the nitriding treatment. surface 3.1.conditions Pin-on Disk Testsand with the presence or not of the nitriding treatment. The experimental tests are repeated five times per each surface condition to analyze the friction coefficient at the stationary phase. Figure 4 exhibits the boxplot of the friction coefficients for the two surface conditions and with the presence or not of the nitriding treatment. (a) (b) Figure 4. Figure 4. Boxplots of the friction coefficients at the stationary phase for the different surface surface conditions conditions with and with and without without the (a) the nitriding nitriding treatmentfor treatment for(a) (a)pins pinsmade madeofofsteel steeland (b)pins and(b) (b) pinsmade madeofofbrass. brass. Figure 4. Boxplots of the friction coefficients at the stationary phase for the different surface conditions Regarding with andthe Regarding the pins made pinsthe without made of of brass, nitridingbrass, the friction the for treatment friction coefficient (a) pinscoefficient made of steel shows and (b)no shows no significant significant pins changes changes with made of brass. with and and without withoutthe plasma the plasmanitriding treatment. nitriding However, treatment. as expected, However, the lowerthe as expected, friction lowercoefficient frictioncorresponds coefficient to correspondsRegarding the specimens to the the pins made processed of brass,EDM by finishing specimens processed thebyfriction comparedcoefficient finishing toEDM shows roughing no EDM. compared significant On thechanges contrary, to roughing with EDM.theand friction On the without coefficient of the pins plasma made nitriding of steel is treatment. noticeably However, reduced as when expected, the the surface contrary, the friction coefficient of pins made of steel is noticeably reduced when the surface lower is friction nitrided, in coefficient particular, 7.6%is corresponds to the specimens processed by finishing EDM compared to roughing EDM. On the for the finishing nitrided, EDM and in particular, 7.6%19.4% forfinishing for the roughingEDM EDM. In this and 19.4% sense, Solis Romero for roughing EDM. et In al.this [19]sense, using Solis pins contrary, the friction coefficient of pins made of steel is noticeably reduced when the surface is made Romero of AISI et al.52,100 on AISI [19] using H13 pins discsofpolished made AISI 52,100 with on silicon AISIcarbide H13 discsemery paperswith polished and diamond paste silicon carbide nitrided, in particular, 7.6% for the finishing EDM and 19.4% for roughing EDM. In this sense, Solis to obtain emery mirrorand papers finish, found a paste diamond decrease to of 33% of obtain friction mirror coefficient finish, foundwhen they treated a decrease Romero et al. [19] using pins made of AISI 52,100 on AISI H13 discs polished with silicon carbide of 33% the of discs with friction plasma nitriding. coefficient when Consequently, they treated the the lowest discs friction with coefficient plasma resulted nitriding. in an Consequently, emery papers and diamond paste to obtain mirror finish, found a decrease of 33% of friction averagethe of 0.22, lowest for both friction types of materials, coefficient coefficient whenwith resulted anand inthey without average treated nitriding of discs the 0.22, for both with in finishing plasmatypes EDM of materials, nitriding. condition. with and Consequently, As alowest reference, thewithout similar nitriding friction in friction finishing values EDMwere coefficient found condition. resulted in anbyaverage As Leite etofal. a reference, [20], 0.22, forwhen similarboth testing friction types Si3 N4 were values of ballswith materials, on foundpolished andby surfaces Leite without et al. of nitrided [20], nitriding inwhen discs testingmade Si3Nof finishing AISIon EDM 4 balls H13 steel.As surfaces condition. polished a reference, of similar nitrided friction discs values made were found of AISI H13 bysteel. Leite et al. [20], when testing Si3N4 balls on polished surfaces of nitrided discs made of AISI H13 steel. 3.2. 3.2. Weight Weight Loss Loss of of the the Pins Pins 3.2. Weight Loss of the Pins During During the the pin-on-disc pin-on-disc test, test, the the pin pin isis expected expected toto wear wear because because thethe material material of of the the disc disc is is harder harder than the During pin. the pin-on-disc Accordingly, test,5 the Figure pin is the exhibits expected to wear weight loss because of the the material different pins of andthesurface disc is harder conditions than the pin. Accordingly, Figure 5 exhibits the weight loss of the different pins and surface than the pin. Accordingly, Figure 5 exhibits the weight loss of the different pins and surface after the pin-on-disc conditions afterafter tests. thethe A cleaning procedure pin-on-disc of 15 min of ultrasonic bath ofwith isopropanol was conditions pin-on-disctests.tests. AA cleaning procedure cleaning procedure ofof15 15 min min ultrasonic of ultrasonic bath bath with with performed in each sample before measuring the weight loss. isopropanol was performed in each sample before measuring the weight loss. isopropanol was performed in each sample before measuring the weight loss. Figure Figure 5. Weight 5. Weight lossloss of the of the pins pins madeofofbrass made brass and and steel steelafter afterthe thepin-on-discs testtest pin-on-discs in the in following the following surface surface Figure 5. conditions: conditions: finishing finishing Weight loss (left) of the (left) and roughing and roughing pins made (right) of brass (right) EDM EDM and steel conditions afterconditionswith and without with andtest the pin-on-discs innitriding. without the nitriding. following surface conditions: finishing (left) and roughing (right) EDM conditions with and without nitriding. The weight loss values show that the average weight loss is about 1.22 to 2.32 times higher for brass and steel when comparing non-nitrided discs versus nitrided ones. The lowest surface The weight loss values show that the average weight loss is about 1.22 to 2.32 times higher for brass and steel when comparing non-nitrided discs versus nitrided ones. The lowest surface Materials 2020, 13, 3199 7 of 10 The weight loss values show that the average weight loss is about 1.22 to 2.32 times higher for brass and steel when comparing non-nitrided discs versus nitrided ones. The lowest surface degradation was found for a nitrided and roughing EDM condition, whereas the worst surface was denoted for Materials 2020, 13, x FOR PEER REVIEW 7 of 10 non-nitrided and finishing EDM conditions. Das et al. [27] reported wear tests of non-nitrided and nitrided AISI H13was degradation discs prepared found by standard for a nitrided metallographic and roughing EDM condition,methods, whereasand the pins worstmade surfaceofwasalumina with a denoted semi-sphere morphology. for non-nitrided They found and finishing EDM that the weight conditions. Das etloss was al. [27] about wear reported threetests times higher for of non- non-nitrided nitrideddiscs. Also, Sarkar and nitrided AISI H13et discs al. [28] performed prepared pin-on-disc by standard tests with metallographic pins ofand methods, brass pinsover made discs of of alumina with a semi-sphere morphology. They found that the weight loss steel. They found a comparable weight loss to our results. In particular, they describe weight losswas about three times values higher lower forthan non-nitrided 100 mg fordiscs. Also, Sarkar a traveling et al.of distance [28] performed pin-on-disc approximately 150 m (240testsmwith pinswork). in this of brassTable 3 over discs of steel. They found a comparable weight loss to our results. In particular, they describe shows the average weight loss and its standard deviation in increasing order for the different surface weight loss values lower than 100 mg for a traveling distance of approximately 150 m (240 m in this conditions work).(finishing Table 3 showsor roughing) the averageand treatments weight loss and(with and without its standard nitriding). deviation in increasing order for the different surface conditions (finishing or roughing) and treatments (with and without nitriding). Table 3. Average weight loss and its standard deviation in increasing order to identify which surface conditions Tableand treatments 3. Average present weight lower loss and wear. deviation in increasing order to identify which surface its standard conditions and treatments present lower wear. Brass Steel Configuration\Sample Configuration\Sample (mgBrass ± SD) Steel (mg ± SD) (mg ±SD) (mg ±SD) Nitrided & finishing 1st 4.1 ± 0.2 Nitrided & finishing 1st 4.1 ± 0.2 Finishing 2nd 5.1 ± 0.3 Finishing 2nd 5.1 ± 0.3 Nitrided & roughing 3rd 15.0 ± 0.7 Nitrided & roughing 3rd 15.0 ± 0.7 Nitrided & finishing 16.8 ± 2.7 4th Nitrided & finishing Finishing 24.8 ± ±10.4 16.8 2.7 4th 5th Roughing Finishing 24.8 ± 6th 10.4 5th 34.8 ± 5.0 Roughing Nitrided & roughing 192.7 6th ± 64.4 34.8 ± 5.0 7th Nitrided Roughing & roughing 192.7±±63.5 236.1 64.4 7th 8th Roughing 236.1 ± 63.5 8th 3.3. Surface Damage 3.3. Surface of theof Tool Damage TipTip the Tool As expected, the pins made of brass show higher tip degradation compared to the steel pins. As expected, the pins made of brass show higher tip degradation compared to the steel pins. Besides, the surface Besides, roughness the surface presents roughness a different presents a differentinfluence influence ononthe the type type of material. of material. A smooth A smooth surfacesurface favors the degradation favors of theofbrass the degradation pin, while the brass lowerlower pin, while degradation is found degradation is foundfor for the the steel pin.pin. steel TheThe contrary happens when happens contrary the coarse whensurface roughness the coarse surfaceis present on roughness the surface is present on theof the discs. surface of theFigure 6 exhibits discs. Figure 6 the exhibitsofthe SEM images theSEM tipsimages of the tips after sliding after slidingtest) (pin-on-disc (pin-on-disc test) over over nitride discs nitride withdiscs with a finishing a finishing EDM process. EDMshow The results process. Thethe that results show lowest that the weight lowest loss wasweight foundlossforwas pinsfound made forofpins made steel withof the steelfinishing with the EDM finishing EDM process, sliding over nitrided discs. On the opposite, the highest weight loss was process, sliding over nitrided discs. On the opposite, the highest weight loss was found for pins made found for pins made of brass sliding over non-nitrided discs in the roughing EDM condition. It is of brass sliding over non-nitrided discs in the roughing EDM condition. It is noticeable how after noticeable how after 20 min of sliding testing, the hemispherical morphology of the tips made of brass 20 minare of sliding testing, the hemispherical morphology of the tips made of brass are wholly removed. wholly removed. (a) (b) Figure Figure 6. Pin6.tips Pinafter tips after sliding sliding on on nitrideddiscs nitrided discsmade made by byEDM EDMwith withfinishing condition. finishing (a) Surface condition. (a) Surface damage of a pin made of steel; (b) Surface damage of a pin made of brass. damage of a pin made of steel; (b) Surface damage of a pin made of brass. Materials 2020, 13, 3199 8 of 10 Finally, an EDS analysis (Energy Dispersive X-rays Spectroscopy) was performed on the discs to determine if material adhesion was on its surface during the sliding tests. Table 4 shows the EDS values to estimate Cu and Zn on the surface of the discs tested with brass pins. The EDS analysis shows similar adhesion values when comparing finishing and roughing surfaces. However, a significant difference is observed when comparing the adhesion of Cu and Zn between nitrided and non-nitrided surfaces. Note that the amount of adhesion of copper-zinc significantly increased in the non-nitrided discs. This behavior can be justified by the compatibility chart of Rabinowicz et al. [29] to justify the adhesion of Cu and Zn on the discs of steel. Zn and Cu have soluble values between 0.1 and 1% and, consequently, have a high tendency to adhere to iron. Table 4. EDS values of Cu and Zn on the surface of the wore discs for the four type of surface conditions and brass pins. Sample Nitrided & Roughing Roughing Nitrided & Finishing Finishing Cu (wt.%) 0.38 1.46 0.28 2.02 Zn (wt.%) - 0.89 0.22 1.10 4. Conclusions The present study shows a thorough comparison of the friction and wear capability between two different EDM surfaces (finished and roughened) with and without nitriding treatment. Accordingly, some conclusions can be drawn: • Nitriding significantly reduced the roughness, considering the arithmetic mean surface roughness (Ra ) and the total height of the roughness profile (Rt ). The roughing EDM process showed an increase of the surface roughness of 2.1 times in respect to the finishing EDM process. The nitriding treatment decreases 31% of the surface roughness on average. • The material hardness of the nitrided disc machined with a finishing EDM was 32.5% higher than for the same disc machined with a roughing EDM condition. • With respect to the pin weight loss, nitrided discs reduce the pin degradation at least between 18.4% and 19.6% for brass and steel, respectively. • The friction coefficient exhibits lower values for nitride finishing surfaces and higher values for non-nitrided roughing surfaces, independently of pin material. However, for brass pins, significant differences are found for the surface condition (finishing and roughing EDM), and no significant differences are denoted in the disc treated or not with plasma nitriding. Author Contributions: Conceptualization: D.M.K. and A.J.S.E. Data curation: V.M., A.N.A. and D.P. Formal analysis: D.P. and E.E.Z.F. Funding acquisition: D.M.K. and A.J.S.E. Resources: A.C. Methodology: V.M. and D.M.K. Software: D.P. and H.A.G.R. Supervision: E.E.Z.F. and H.A.G.R. Validation: V.M. and A.C. Writing original draft: A.N.A., D.M.K., A.J.S.E. and A.C. Writing, review and editing: A.J.S.E. and D.M.K. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by the National Agency for Research and Development of Chile (ANID)—grant number: 3180006 and the Serra Húnter program (Generalitat de Catalunya)—reference number UPC-LE-304 (2018). Acknowledgments: We are also grateful to Soledad Pereda and Andrea Romano for helping with Electron microscopy, Alejandro Bacigalupe for analyzing the kinematic viscosity, IONAR S.A. for nitriding the discs, Javier Pouton for metallographic tasks, Alberto Forcato for providing colloidal graphite and Nazareno Antunez for wire EDM. Conflicts of Interest: The authors declare no conflict of interest. Materials 2020, 13, 3199 9 of 10 References 1. Egea, A.J.S.; Deferrari, N.; Abate, G.; Krahmer, D.M.; de Lacalle, L.N.L. 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