Tribology of Polymer Blends PBT + PTFE

materials

Article Tribology of Polymer Blends PBT + PTFE

Constantin Georgescu 1,* , Lorena Deleanu 1,*, Larisa Chiper Titire 1 and Alina Cantaragiu Ceoromila 2

1 Department of Mechanical Engineering, Faculty of Engineering, “Dunarea de Jos” University of Galati, 800008 Galati, Romania; [email protected] 2 Department of Applied Sciences, Cross-Border Faculty, “Dunarea de Jos” University of Galati, 800008 Galati, Romania; [email protected] * Correspondence: [email protected] (C.G.); [email protected] (L.D.); Tel.: +40-743-105-835 (L.D.)

Abstract: This paper presents results on tribological characteristics for polymer blends made of polybutylene terephthalate (PBT) and polytetrafluoroethylene (PTFE). This blend is relatively new in research as PBT has restricted processability because of its processing temperature near the degradation one. Tests were done block-on-ring tribotester, in dry regime, the variables being the PTFE concentration (0%, 5%, 10% and 15% wt) and the sliding regime parameters (load: 1, 2.5 and 5 N, the sliding speed: 0.25, 0.5 and 0.75 m/s, and the sliding distance: 2500, 5000 and 7500 m). Results are encouraging as PBT as neat polymer has very good tribological characteristics in terms of friction coefficient and wear rate. SEM investigation reveals a quite uniform dispersion of PTFE drops in the PBT matrix. Either considered a composite or a blend, the mixture PBT + 15% PTFE exhibits a very good tribological behavior, the resulting material gathering both stable and low friction coefficient and a linear wear rate lower than each component when tested under the same conditions.

Keywords: polybutylene terephthalate (PBT); polytetraﬂuoroethylene (PTFE); blend PBT + PTFE; block-on-ring test; linear wear rate; friction coefﬁcient

Citation: Georgescu, C.; Deleanu, L.; Chiper Titire, L.; Ceoromila, A.C. Tribology of Polymer Blends 1. Introduction PBT + PTFE. Materials 2021, 14, 997. Due to a longer sequence of methyl groups in the monomer, the molecular chains of https://doi.org/10.3390/ma14040997 polybutylene terephthalate (PBT) are more ﬂexible and less polar than those of polyethy1ene terephthalate (PET), inducing a lower melting temperature T (224–230 ◦C) and the glass Academic Editor: Sergey Panin transition temperature Tg (22–43 ◦C). This lower Tg allows for a fast crystallization when molding and shorter molding cycles with faster molding speed [1–4]. PBT, a semicrystalline Received: 10 January 2021 engineering polymer, is included in the polyester class of resins and has a set of properties Accepted: 16 February 2021 Published: 20 February 2021 that recommends it in many special applications: rigidity and strength, combined with very good heat aging resistance. PBT-based materials (composites and blends) have better

Publisher’s Note: MDPI stays neutral dimensional stability with uniform shrinkage behavior, stiffness, and heat resistance, low with regard to jurisdictional claims in water absorption and high chemical resistance by incorporating fillers, reinforcing materi- published maps and institutional affil- als, and additives; material properties are tailored for user’s interest. They are processed iations. mainly by injection molding [5]. PBT has restricted processability because of its processing temperature near the degradation one [6]. Lin and Schlarb [7] tested a hybrid material, short carbon fiber-filled PBT, with and without graphite as a solid lubricant, using a pin-on-disc tribotester, in dry regime. PBT with nanoparticles without graphite exhibits very good tribological performance, under Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. moderate and severe load, with better mechanical characteristics as compared to the same This article is an open access article material filled with graphite. Under 3 MPa and 2 m/s, the friction coefficient is 0.18 and −6 3 distributed under the terms and wear rate is 0.8 × 10 mm /Nm. conditions of the Creative Commons Dechet et al. [8] presented a laboratory technology for producing spherical polymer Attribution (CC BY) license (https:// blend particles made of PBT and polycarbonate (PC) for selective laser sintering (SLS), creativecommons.org/licenses/by/ including cogrinding and thermal rounding. The size distribution, shape and morphology 4.0/). of polymer constituents in this PBT + PC blend were analyzed.

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including cogrinding and thermal rounding. The size distribution, shape and morphol- Materials 2021, 14, 997 ogy of polymer constituents in this PBT + PC blend were analyzed. 2 of 19 Materials based on blends of PBT + PET with flame retardant agents (a new formulated agent, expandable graphite (EG), added separately or in a mixture of both) were testedMaterials for determining based on the blends influence of PBT of + PETflame with retardant flame retardant [9]. Results agents demonstrate (a new formulated that in- agent,corporation expandable of the graphitemixture (EG),in PBT added + PET separately blends could or in be a mixturerecommended of both) as were potential tested ap- for determiningplications for the electronic influence household of flame retardantdevices, products [9]. Results and demonstrate automotive that components, incorporation but ofresearch the mixture should in be PBT continued + PET blends for their could mechanical be recommended and tribological as potential behavior. applications for electronicIn machine household design, devices, components products made and exclus automotiveively of components, polytetrafluoroethylene but research should(PTFE) beare continued rare, even for if theirthey mechanicalmaintain their and properti tribologicales at behavior.the initial values, independent of the manufacturingIn machine method design, components[1], including made chemical exclusively properties of polytetrafluoroethylene that remain unchanged (PTFE) for a arelong rare, time even (stability if they in maintainchemically their aggressive properties environments, at the initial insolubility, values, independent weather stability of the manufacturingand anti-adhesion). method The [properties1], including practically chemical unaffected properties are that flexibility remain unchangedat negative fortem- a longperatures, time (stabilitythermal stability, in chemically a low dielectric aggressive constant environments, and high insolubility, resistance at weather electrical stabil- arch ity[10]. and All anti-adhesion). experimental works The propertiesunderlined practically a very low unaffected friction coefficient are flexibility [11,12] at and negative high temperatures,wear rate, especially thermal in stability, dry regime a low [13–15], dielectric but constantadding reinforcements and high resistance as short at electrical fibers or archpowders [10]. reduces All experimental the wear by works a factor underlined of 100 or a veryeven lowmore friction [13], especially coefficient in [ 11lubricated,12] and highcontacts wear [16,17]. rate, especially Producers in and dry users regime prefer [13– 15to ],add but PTFE adding in reinforcementsother polymers asbecause short fibers of its orpoor powders wear resistance, reduces the being wear more by a factor efficient of 100 as orsolid even lubricant more [13 [18]], especially and rarely in lubricatedused as a contactsmatrix [19].[16 ,17]. Producers and users prefer to add PTFE in other polymers because of its poorPolymer wear resistance,blends have being been more developed efficient for as their solid sets lubricant of characteristics, [18] and rarely the used mixture as a matrixresulting [19 in]. several new and improved properties or different from those characterizing each Polymercomponent blends alone have [20]. beenThe notion developed of a poly formeric their setsblend of refers characteristics, to materials the artificially mixture resultingcreated, rationally in several combining new and improved different comp propertiesonents or to different improve from one thoseor more characterizing characteris- eachtics and component to diminish alone those [20]. that The are notion not, ofbase a polymericd on theoretical blend models, refers to laboratory materials artificiallytests and, created,finally, prototype rationally combiningresults. At differentpresent, po componentslymer alloys, to improve polymer one blends or more and characteristics their compo- andsites torepresent diminish over those 80% that (by aremass) not, of basedthe total on polymer-based theoretical models, materials laboratory [21]. tests and, finally,In prototypetribology,results. there are At present,several polymer alloys,blends polymerused for blends their andgood their characteristics, composites representespecially overusing 80% PTFE (by as mass) a solid of thelubricant. total polymer-based Polymer blends materials are attracting [21]. the attention of specialistsIn tribology, through there a areset several of particular polymer blendsproperties, used forsuch their as goodlow characteristics,specific mass espe-and ciallystrength-to-mass using PTFE asand a solid stiffness-to-mass lubricant. Polymer ratios blends that areare attractingsuperior theto attentiontraditional of specialistsmaterials, throughtribological a set properties of particular [22], properties, resistance suchto aggressive as low specific environments, mass and electrical strength-to-mass and thermal and stiffness-to-massproperties, which ratios led thatto the are use superior in the to field traditional of aeronautics, materials, tribologicalshipbuilding, properties electronics, [22], resistancemedical components to aggressive etc. environments, Blends can electricalbe formed and with thermal miscible properties, polymers, which and led tohomoge- the use inneous the fieldpolymer of aeronautics, mixtures up shipbuilding, to the molecular electronics, level medical and with components immiscible etc. polymers, Blends can as bein formedthe PBT with+ PTFE miscible blends. polymers, and homogeneous polymer mixtures up to the molecular level and with immiscible polymers, as in the PBT + PTFE blends. Analyzing the properties presented in Figure 1, one may notice the narrow range for Analyzing the properties presented in Figure1, one may notice the narrow range for the melting temperature and the difference of about 100 °C for this characteristic. As PBT the melting temperature and the difference of about 100 ◦C for this characteristic. As PBT rapidly degrades above the melting temperature, it results in the blends of PBT + PTFE rapidly degrades above the melting temperature, it results in the blends of PBT + PTFE pro- processed by mixing the melt PBT with solid PTFE powder; the dispersion quality de- cessed by mixing the melt PBT with solid PTFE powder; the dispersion quality depending pending on processing parameters [23]. on processing parameters [23].

Density (g/cm3) Elongation at break Ultimate tensile (%) strength (MPa) 2.5 300 150 2 250 200 100 1.5 150 1 100 50 0.5 50 0 0 0 PBT PTFE PBT PTFE PBT PTFE (a) (b) (c)

Figure 1. Cont.

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Flexural modulus Melting temperature Flexural strength (GPa) (ºC) (MPa) 10 350 250 300 8 200 250 6 200 150 4 150 100 100 2 50 50 0 0 0 PBT PTFE PBT PTFE PBT PTFE

(d) (e) (f)

FigureFigure 1. A comparison 1. A comparison between between characteristics characteristics for PBT for and PBT PTFE: and PTFE: (a) density; (a) density; (b) elongation (b) elongation at break; at (break;c) ultimate (c) ultimate tensile strength;tensile (d) ﬂexuralstrength; modulus; (d) (flexurale) melting modulus; temperature; (e) melting (f) ﬂexural temperature; strength. (f Mechanical) flexural strength. characteristics Mechanical are given characteristics for ambient are temperature given for [24]. The darkambient color temperature in each column [24]. represents The dark thecolo minimumr in each valuecolumn and represents the light colorthe mini in eachmum column value and is for the the light maximum color in value each of the column is for the maximum value of the material characteristic. material characteristic.

As PTFEPTFE hashas a a very very low low reactivity, reactivity, when when it is it added is added in a harderin a harder polymer, polymer, as polyethereth as poly- erketoneetheretherketone (PEEK), (PEEK), PBT or polyamide PBT or polyamide (PA), it exhibits (PA), it anexhibits immiscible an immiscible character, character, but, depend- but, ingdepending on the processing on the processing characteristics, characteristics, the morphology the morphology of the blends of the obtained blends may obtained vary frommay alternatingvary from alternating microzones microzones of PTFE and of microzonesPTFE and microzones of the other of polymer the other to polymer a fine dispersion to a fine ofdispersion droplets. of droplets. Burris and Sawyer Sawyer tested tested blends blends of of PEEK PEEK + +PTFE PTFE [25,26]. [25,26 ].PEEK PEEK has has good good wear wear re- resistancesistance and and higher higher work work temperature temperature as as compared compared to to other other thermoplastic polymers, aa friction coefficientcoefficient µ ~0.4 (dry regime) and a low thermal conductivity. Even if the recipes for polymericpolymeric blendblend with with PTFE PTFE recommend recommend 5–20% 5–20% PTFE PTFE [18 ,[18,27,28],27,28], Burris Burris and and Sawyer Sawyer [25] reported[25] reported that thethat polymeric the polymeric blend blend with ~20%with ~20% (vol) PEEK(vol) PEEK had a had wear a intensitywear intensity 26 times 26 lowertimes lower as compared as compared to that forto that PEEK, for andPEEK, 900 and times 900 lower times than lower that than exhibited that exhibited by PTFE. by PTFE.Briscoe and Sinha [19,29,30] made samples of PTFE with PEEK, using from 0% to 100% PEEK.Briscoe Their and results Sinha point [19,29,30] out a monotonous made samples increase of PTFE of the with wear PEEK, rate and using a monotonous from 0% to decrease100% PEEK. of the Their friction results coefficient point out as a the monotonous PTFE concentration increase of increases. the wear Therate differencesand a mo- betweennotonous the decrease results reportedof the friction in [31 ]coefficient and [25,26 ]as are the related PTFE to concentration material quality, increases. their manu- The facturingdifferences process, between microstructure the results reported and testing in [31] conditions. and [25,26] Bijwe are related et al. [ to31 ]material tested abrasive quality, resistancetheir manufacturing of PEEK + PTFEprocess, blends, microstructure with PTFE concentration and testing upconditions. to 30% wt. Bijwe Using et a al. pin-on- [31] disctested tester, abrasive single resistance pass condition of PEEK against + PTFE abrasive blends, paper, with lowPTFE sliding concentration velocity (vup = to 0.05 30% m/s), wt. underUsing a loads pin-on-disc of (6, 8, 10tester, and single 12 N) andpass acondition very short against sliding abrasive distance, paper, L = 3.26 low m, sliding the wear ve- ratelocity was (v = shown 0.05 m/s), to increase under loads with loadof (6, and 8, 10 PTFE and concentration.12 N) and a very short sliding distance, L = 3.26These m, the different wear rate results was onshown blends to incr of PEEKease with + PTFE load underline and PTFE the concentration. idea that friction and wearThese parameters different fail results to obey on any blends mixture of PEEK rule + and PTFE laboratory underline tests; the idea follow-up that friction testing and of actualwear parameters systems is afail necessity. to obey any mixture rule and laboratory tests; follow-up testing of actualResearch systems studiesis a necessity. on tribological behavior of PBT as matrix with glass beads or short aramidResearch fibers werestudies reported on tribological in Georgescu behavior et al.of PBT [32] andas matrix Botan with et al. glass [33, 34beads], only or short for a slidingaramid distancefibers were of 5000 reported m. in Georgescu et al. [32] and Botan et al. [33,34], only for a slidingAn distance interesting of 5000 tribological m. study was reported by Jozwik et al. [35], comparing several tribologicalAn interesting characteristics tribological study for several was reported polymeric by materials,Jozwik et includingal. [35], comparing PET + PTFE, sev- PTFEeral tribological + bronze and characteristics PTFE + graphite. for several Tests polymeric were done materials, on a ball-on-disk including system, PET + withPTFE, a sliding velocity of 0.8 m/s, the disk being made of polymeric material and the ball of PTFE + bronze and PTFE + graphite. Tests were done on a ball-on-disk system, with a aluminum oxide (Al O ), for a sliding distance of 1000 m. The temperature was measured sliding velocity of 0.82 3m/s, the disk being made of polymeric material and the ball of near the contact. The blend PET + PTFE (80/20) had the most convenient curve of tem- aluminum oxide (Al2O3), for a sliding distance of 1000 m. The temperature was measured perature during the test, being characterized by a stable and low contact temperature, not near the contact. The blend PET + PTFE (80/20) had the most convenient curve of tem- exceeding 29 ◦C, under F = 30 N. The friction coefficient was 0.11 for F = 10 N and lower perature during the test, being characterized by a stable and low contact temperature, not (0.07) for a higher load (F = 30 N). In addition, mass loss as a wear parameter was the exceeding 29 °C, under F = 30 N. The friction coefficient was 0.11 for F = 10 N and lower smallest for a disk made of a PET + PTFE blend. Taking into account the similarity between (0.07) for a higher load (F = 30 N). In addition, mass loss as a wear parameter was the the chemistry of PET and PBT, as members of the polyester polymers, the results presented

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smallest for a disk made of a PET + PTFE blend. Taking into account the similarity between the chemistry of PET and PBT, as members of the polyester polymers, the results presented in that study, even if the sliding distance seems too short for a comprehensive evaluation towards actual applications, is an inducement for testing PBT blends.

Materials 2021, 14, 997 Research reports on the blends with both PBT and PTFE polymers4 of 19are rare in literature, even if large polymer-producing companies [36] are using blends with PTFE for wear resistance applications. in thatA study, very even important if the sliding aspect distance in polymer seems too blends short for is athe comprehensive nature of components, evaluation these could betowards miscible, actual partial applications, miscible is an inducementor even immiscible. for testing PBT blends. ResearchThe main reports aim on of the this blends research with both is PBTto emphas and PTFEis polymersthe influence are rare of in literature,PTFE concentration in PBTeven on if large tribological polymer-producing characteristics companies in dry [36 ]regime. are using blends with PTFE for wear resistance applications. A very important aspect in polymer blends is the nature of components, these could 2.be Materials miscible, partial and miscible Methods or even immiscible. TheThe main polymer aim of this(PBT) research and the is to blends emphasis (PBT the inﬂuence + PTFE) of formulated PTFE concentration for this in tribological re- PBT on tribological characteristics in dry regime. search were processed by die molding, resulting in bone specimens of type 1A, as rec- ommended2. Materials and by Methods SR EN ISO 527-2:2012 [37], at Monofil SA (Neamt, Romania). The com- mercialThe polymergrade of (PBT) PBT and used the was blends Crastin (PBT +6130 PTFE) NC010 formulated (as supplied for this tribological by DuPont, Bucharest, Romania)research were [38]. processed by die molding, resulting in bone specimens of type 1A, as recommendedThe commercial by SR EN grade ISO 527-2:2012 of PTFE [ 37was], at NFF Monoﬁl FT-1-1T SA (Neamt,® Flontech Romania). (Ospitaletto, The Brescia, It- commercial grade of PBT used was Crastin 6130 NC010 (as supplied by DuPont, Bucharest, aly),Romania) with [38 an]. average particle size of ~20 µm [39]. The dispersion of immiscible polymers is importantThe commercial in obtaining grade of PTFE good was results NFF FT-1-1T (dimen® Flontechsion stability, (Ospitaletto, mechanical Brescia, Italy), and thermal, in- withcluding an average impact, particle tribological size of ~20 characteriµm [39].stics, The dispersion chemical of resistance immiscible etc.). polymers is importantFor inthis obtaining study, good the resultsrole of (dimension PTFE as stability, an added mechanical material and thermal,used in including the following recipes impact, tribological characteristics, chemical resistance etc.). was in concentrations of 5, 10 and 15% wt. For this study, the role of PTFE as an added material used in the following recipes was in concentrationsThe parallelepiped of 5, 10 and block 15% wt. (dimensions of 16.5 mm × 10 mm × 4 mm) was obtained by cuttingThe parallelepipedparts from the block central (dimensions bone specimens. of 16.5 mm × The10 mm other× 4 mm)component was obtained of the friction pair wasby cutting the external parts from ring the centralof the bonerolling specimens. bearing The KBS other 30202 component (Kedron of theBearings friction Services LLC, Frankfort,pair was the externalKY, USA), ring ofhaving the rolling a diameter bearing KBS of Ø35 30202 mm (Kedron and Bearings a width Services of 10 LLC,mm, made of steel Frankfort, KY, USA), having a diameter of Ø35 mm and a width of 10 mm, made of steel grade 100Cr6, 100Cr6, with with 60 −6062 − HRC62 HRC and and Ra = Ra 0.8 =µ m.0.8 Theµm. shapes The shapes and dimensions and dimensions of the of the frictionfriction couple couple (Timken-type) (Timken-type) are are presented presented in Figure in Figure2. 2.

Load

16.5 Sample 4 10

5 ?

Rotating ring 10 Figure 2. 2.Dimensions Dimensions (in mm) (in mm) and shape and ofshape the tribotester of the tribotester elements. elements. The materials code and the average values for mechanical characteristics of the poly- mericThe blends materials tested are presentedcode and in Tablethe average1. values for mechanical characteristics of the polymeric blends tested are presented in Table 1.

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Table 1. Average values for mechanical characteristics of the polymeric blends tested [23].

Material Code Composition, % wt Characteristic Young Modulus Ultimate Tensile Elongation at Energy at PBT PTFE (MPa) Strength (MPa) Break (%) Break (N·m) PBT 100 - 1923 41.5 9.4 17.6 PF5 95 5 1826 46.4 5.1 6.8 PF10 90 10 2202 36.9 2.4 1.8 PF15 85 15 1867 43.2 4.1 5.0

For the forces selected, the Hertz pressure in contact was calculated with the help of the Hertz contact calculator, included in UMT-2, UMT Test Viewer software (Version 2.14 Build 77, CETR, Campbell, CA, USA) [40], and the results are given in Table2. Taking into account the elastoplastic characteristic of the block contact, these calculated values are estimated. A more realistic evaluation could be done with the help of the ﬁnite element method by introducing an adequate constitutive model of the block material. Values for Poisson coefﬁcient were taken from literature and the values for the elasticity modulus (Young modulus) are average values experimentally determined by Georgescu from tensile tests. For all tests, the length of the contact between the block and steel ring was 4 mm.

Table 2. Hertz pressure for linear contact of the tested blocks made of PBT and PBT + PTFE blends.

Hertz Pressure (MPa) Material Code F = 1 N F = 2.5 N F = 5 N PBT 3.1 5.0 7.1 PF5 3.1 4.9 7.0 PF10 3.4 5.3 7.6 PF15 3.1 4.9 7.0

The parameters of the block-on-ring test, for one material, are presented in Table3. After a literature survey [7,41–44], each test was repeated twice, and the authors mentioned if the plots present one test or an average of the two tests.

Table 3. Test parameters.

Sliding Distance (m) Sliding Velocity Revolution Speed Normal Force (N) 2500 5000 7500 (m/s) (rpm) * Testing Time 0.25 136 2 h 46 min 40 s 5 h 33 min 20 s 8 h 20 min 1 0.50 273 1 h 23 min 18 s 2 h 46 min 40 s 4 h 10 min 0.75 409 55 min 33 s 1 h 51 min 7 s 2 h 46 min 40 s 0.25 136 2 h 46 min 40 s 5 h 33 min 20 s 8 h 20 min 2.5 0.50 273 1 h 23 min 18 s 2 h 46 min 40 s 4 h 10 min 0.75 409 55 min 33 s 1 h 51 min 7 s 2 h 46 min 40 s 0.25 136 2 h 46 min 40 s 5 h 33 min 20 s 8 h 20 min 5 0.50 273 1 h 23 min 18 s 2 h 46 min 40 s 4 h 10 min 0.75 409 55 min 33 s 1 h 51 min 7 s 2 h 46 min 40 s * as an integer value in the test program.

The friction coefﬁcient (COF) was monitored using a Universal UMT-2 (CETR®, Camp- bell, CA, USA) tribometer that had a transducer capable of measuring in actual time the friction force and calculating COF as a ratio between the normal force and the friction force, in any moment t of the test. The tribometer software [40] allows for viewing measured and calculated parameters. The scanning electron microscope Quanta 200 3D from the Faculty of Mechanical Engineering (Technical University “Gheorghe Asachi” of Iasi, Iasi, Romania) and the Materials 2021, 14, x FOR PEER REVIEW 6 of 19

friction force, in any moment t of the test. The tribometer software [40] allows for viewing Materials 2021, 14, 997 measured and calculated parameters. 6 of 19 The scanning electron microscope Quanta 200 3D from the Faculty of Mechanical Engineering (Technical University “Gheorghe Asachi” of Iasi, Iasi, Romania) and the scanning electron microscope FEI Quanta 200 (“Dunarea de Jos” University of Galati, Galati,scanning Romania) electron were microscope used for investigating FEI Quanta 200the (“Dunareaworn surfaces. de Jos” University of Galati, Galati, Romania) were used for investigating the worn surfaces. 3. Results 3. Results Figure 3 presents the evolution of the friction coefficient for one test done for all in- Figure3 presents the evolution of the friction coefﬁcient for one test done for all vestigatedinvestigated materials, materials, L = L 7500 = 7500 m mand and F = F =5 N, 5 N, the the longest longest test test as as concerning concerning the the sliding sliding distance.distance. At At the the lower lower velocity velocity (v (v= 0.25 = 0.25 m/s), m/s), the neat the neatpolymer polymer has a haslower a lower value valueof COF of forCOF a test, for abut test, the but second the second test recorded test recorded higher highervalues valuesthan those than of those the blends. of the blends.For higher For velocities,higher velocities, the blends the PBT blends + PTFE PBT have + PTFE lower have values, lower except values, for except PF10 forin the PF10 last in third the lastof thethird test. of Short the test. time Short oscillations time oscillations of COF record of COFed recordedfor the blends for the may blends be generated may be generated by local disturbanceby local disturbance in the components’ in the components’ dispersions. dispersions. It is important It is important to mention to mentionthat this thatstabili- this zationstabilization of COF of characterize COF characterize forces F forces = 2.5 FN = and 2.5 F N = and 5 N. F = 5 N.

0.45 0.45 0.4 v = 0.25 m/s 0.4 v = 0.5 m/s PBT PF5 0.35 0.35 PF10 0.3 0.3 PF15 0.25 0.25 COF 0.2 COF 0.2 0.15 PBT 0.15 0.1 PF5 0.1 PF10 0.05 0.05 PF15 0 0 0 2000 4000 6000 8000 0 2000 4000 6000 8000 Sliding distance (m) Sliding distance (m) (a) (b) 0.45 0.4 v = 0.75 m/s PBT 0.35 PF5 PF10 0.3 PF15 0.25

COF 0.2 0.15 0.1 0.05 0 0 2000 4000 6000 8000 Sliding distance (m) (c)

FigureFigure 3. 3. EvolutionEvolution of of friction friction coefficient coefﬁcient (COF) (COF) in time in time for fortested tested materials, materials, F = 5 F N, = 5L N,= 7500 L = m: 7500 (a) m: v =( 0.25a) v m/s; = 0.25 (b m/s;) v = 0.5 (b ) m/s;v = 0.5(c) m/s;v = 0.75 (c) m/s. v = 0.75 m/s.

InvestigationInvestigation of of the the worn worn surfaces surfaces by by sca scanningnning electron electron microscopy microscopy revealed revealed deeper deeper andand fringier fringier grooves, grooves, with with larger larger rolled rolled wear wear particles particles (see (see Figure Figure 4a)4a) as compared to the samesame aspects aspects after after testing testing at athigher higher velocity, velocity, v = v 0.75 = 0.75 m/s m/s (see (seeFigure Figure 4b). 4Forb). a For higher a higher load (Fload = 5 (FN), = the 5 N), aspect the aspectof worn of surfaces worn surfaces is similar is similar(see Figure (see 4c,d) Figure with4c,d) several with several deep traces, deep buttraces, there but is thereno evidence is no evidence of tearing-off of tearing-off a grea at volume great volume of polymer of polymer as wear as weardebris. debris. In the In scanningthe scanning electron electron microscope microscope (SEM) (SEM) images images of ofworn worn surfaces, surfaces, the the arrow arrow indicates indicates the the slidingsliding direction. direction.

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

(c(c) ) (d(d) ) FigureFigure 4. 4. Appearance AppearanceFigure of of worn worn 4. surfaces Appearancesurfaces for for the the of block wornblock made made surfaces of of PBT, PBT, for L theL = = 5000 block5000 m: m: made ( a(a) )F F = of= 1 1 N, PBT,N, v v = =L 0.25 0.25 = 5000m/s; m/s; (m: b(b) )F (F a= )= 1 1 F N, N, = v 1v = = N, 0.75 0.75 m/s;m/s; ( c(c) )F F = = 5 5 N, N, v v = = 0.25v 0.25 = 0.25m/s; m/s; m/s; ( d(d) )F F( b= = )5 5 F N, N, = 1v v N,= = 0.75 0.75 v = m/s. 0.75m/s. m/s; (c) F = 5 N, v = 0.25 m/s; (d) F = 5 N, v = 0.75 m/s.

AnalyzingAnalyzingAnalyzing the same the the wornsame same worn surfacesworn surfaces surfaces at higher at at higher higher magniﬁcation magnification magnification (Figure (Figure (Figure5), the 5), 5), lighterthe the lighter lighter re- re- regimegimegime (F = (F 1(F N,= = 1 v1 N, =N, 0.25 vv = = 0.25 m/s)0.25 m/s) m/s) produced producedproduced worse worseworse damage damagedamage (deeper (deeper (deeper grooves, grooves, grooves, larger larger larger wear wearwear de-de- debris).bris).bris). A higher A A higher higher velocity velocity velocity increases increases increases the temperature the the temper temperature inature the in superﬁcialin the the superficial superficial layer andlayer layer the and and cracks the the cracks cracks inducedinducedinduced in the polymerin in the the polymer polymer are shorter are are shorter shorter and the and and wear the the debris wear wear debris aredebris smaller are are smaller smaller and less and and numerous. less less numerous. numerous. This isThis anThis observation is is an an observation observation characterizing characterizing characterizing PBT sliding PBT PBT sliding againstsliding against steel.against No steel. steel. analogy No No analogy analogy could be could could done be be done done with otherwithwith polymers other other polymers polymers without without without investigations. investigations. investigations.

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

Figure 5. Cont.

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Figure 5. DetailsFigure of the worn5. DetailsFigure surface of 5. the Detailsfor worn the block of surface the made worn for surfaceofthe PBT, block forL =made the 5000 block ofm: PBT, ( madea) F L = of=1 5000N, PBT, v m:= L 0.25 = (a 5000) Fm/s; = m: 1 (N,b (a) ) vF F == = 0.251 1 N, N, m/s;v v = = 0.75 0.25(b) F m/s; = 1 N, v = 0.75 m/s; (c) F = 5 N, vm/s; = 0.25 (c) m/s;F = (5b (N,)d F) Fv = = 1 50.25 N, N, v m/s;v = = 0.75 0.75 (d) m/s; m/s.F = 5 ( cN,) F v = = 50.75 N, vm/s. = 0.25 m/s; (d) F = 5 N, v = 0.75 m/s.

AddingAdding PTFE PTFE inAdding in PBT, thePTFE aspect in PBT, of worn the aspect surfacessurfaces of worn changed surfaces (Figure changed6 6).). NotedNoted (Figure werewere 6). Noted were surfacessurfaces with with local localsurfaces agglomeration, agglomeration, with local agglomeration, like like that that for for PF5 like and that local for zones PF5 and depleted local zones in PTFE depleted as in PTFE as thosethose for for PF10 PF10 thoseand PF15. for PF10 The The andzones zones PF15. rich rich Thein PTFE zones alternate rich in PTFE with depleted alternate zones, with depleted meaning zones, meaning thatthat the the molding that process the molding could be process modifiedmodified could for be improving modified for the improving dispersion. the The dispersion. flakes flakes of The flakes of PTFEPTFE embedded embeddedPTFE in in the theembedded tribolayer in seem seemthe tribolayer to to ha haveve lower seem size to ha than ve lower 20 µm,µm, size meaning than 20 thatµm, themeaning that the mixingmixing procedure proceduremixing separate procedure the initial separate particles the initial from theparticles smaller from ones, the which smaller is ones, beneficialbeneficial which is beneficial forfor the the tribological tribologicalfor the behavior. behavior. tribological behavior.

PF5 PF5 PF10 PF10 PF15 PF15

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

(d) (d) (e) (e) (f) (f) Figure 6. DetailsDetailsFigure of of the the worn6. worn Details surface surface of the for for worn the the block surface block made made for ofthe ofPF5, block PF5, PF10 PF10made and and of PF15, PF5, PF15, LPF10 = L 5000 = and 5000 m PF15, and m and v L = = 0.25v 5000 = 0.25m/s: m and m/s:(a) PF5,v (=a 0.25) F PF5, = 1m/s: (a) PF5, F = 1 N;F = (b 1) N; PF10, (b) PF10,F = 1N; N F ( =b; () 1c PF10,) N; PF15, (c) F PF15, F= =1 1N FN; ; =( c( 1d) PF15,) N; PF5, (d) FF PF5, == 15 N; FN; = ((d 5e) N; PF10,PF5, (e) F PF10,F = = 5 5 N; N F (=ande) 5 PF10, N (f and) PF15, F (=f) 5 PF15, FN = and5 N. F =( f) 5 PF15, N. F = 5 N.

PBTPBT has has average averagePBT values values has averageof of COF COF invalues in the the narrowest of narrowest COF in range, the range, narrowest with with greater greaterrange, average with average greater values values averagefor values for testsfor tests done done with tests witha sliding done a sliding distance with distancea sliding of L = 7500 distance of L m. = 7500The of L increase m.= 7500 The m. of increase Theaverage increase of COF average of may average result COF COF from may may result from theresult elimination from the theof elimination relativelyelimination oflarger relativelyof relatively wear debris larger larger, wearcharacteristic wear debris, debrischaracteristic for, characteristic this polymer. for for thisThe thispolymer. average polymer. The average values of the frictionvalues coefficient of the friction are under coefficient 0.2 for are tests under under 0.2 Ffor = 5tests N and under all slidingF = 5 N velocities. and all sliding velocities.

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The average value of COF has a decreasing tendency for polymeric blends PBT + PTFE, at the sliding velocity of v = 0.75 m/s (Figure 7). At load F = 5 N, the polymeric blends have the friction coefficient lower for v = 0.5 m/s and v = 0.75 m/s, probably because at lower sliding velocity, PTFE is detaching in microribbons, especially when it is added in higher concentration (15%), ensuring, due to the lamination and the transfer of Materials 2021, 14, 997 9 of 19 PTFE, a reduced friction. A similar wear process for PTFE and its composites are described by Gong et al. [45]. Jones et al. [46] pointed out high values of COF (over 0.6), for three polymeric balls sliding against a steel disk. The averageFigure 7 valuespresents of thethe frictionaverage coefﬁcient values obtain are undered from 0.2 two for tests tests for under the Ffriction = 5 N andcoeffi- all cient.sliding Except velocities. for results obtained for the testing regime characterized by F = 1 N and v = 0.25 m/s,The averagethe value value for COF of COF is has0.15–0.18. a decreasing This means tendency that for the polymeric tribological blends characteristicPBT + PTFE is, lessat the sensible sliding to velocity regime ofparameters v = 0.75 m/s (sliding (Figure velocity7). At in load the Frange = 5 N, of the0.5–0.75 polymeric m/s and blends load inhave the therange friction F = 2.5–5 coefﬁcient N), but lower also to for PTFE v = 0.5concentration. m/s and v =This 0.75 conclusion, m/s, probably based because on the plotsat lower shown sliding in Figure velocity, 7, means PTFE is that detaching investigation in microribbons, should extend especially the parameters when it is toward added higherin higher loads, concentration for the same (15%), materials. ensuring, Adding due toPTFE the laminationdoes not change and the this transfer parameter of PTFE, too mucha reduced for the friction. same regime A similar ranges wear of inputs. process As for for PTFE the influence and its composites of sliding distance, are described COF isby keptGong in eta narrow al. [45]. rangeJones for et al.L = [ 462500] pointed m and outwith high a larger values spread of COF for (overthe longer 0.6), distance, for three butpolymeric the average balls values sliding still against remain a steel in an disk. acceptable range for practical use.

0.5 L = 2500 m 0.4 F = 1 N 0.3 F = 2.5 N

COF 0.2 F = 5 N 0.1 0 0.25 0.5 0.75 0.25 0.5 0.75 0.25 0.5 0.75 0.25 0.5 0.75 PBT PF5 PF10 PF15 Velocity (m/s)

(a) 0.5 L = 5000 m 0.4 F = 1 N 0.3 F = 2.5 N COF 0.2 F = 5 N 0.1 0 0.25 0.5 0.75 0.25 0.5 0.75 0.25 0.5 0.75 0.25 0.5 0.75 PBT PF5 PF10 PF15 Velocity (m/s) (b) 0.5 L = 7500 m 0.4 F = 1 N 0.3 F = 2.5 N

COF 0.2 F = 5 N 0.1 0 0.25 0.5 0.75 0.25 0.5 0.75 0.25 0.5 0.75 0.25 0.5 0.75 PBT PF5 PF10 PF15 Velocity (m/s) (c)

Figure 7. Average values of friction coefﬁcient (COF), with standard deviation for tested materials, calculated for different sliding distances: (a) L = 2500 m; (b) L = 5000 m; (c) L = 7500 m.

Figure7 presents the average values obtained from two tests for the friction coefﬁcient.

Except for results obtained for the testing regime characterized by F = 1 N and v = 0.25 m/s, the value for COF is 0.15–0.18. This means that the tribological characteristic is less sensible to regime parameters (sliding velocity in the range of 0.5–0.75 m/s and load in the range F = 2.5–5 N), but also to PTFE concentration. This conclusion, based on the plots shown in Figure7, means that investigation should extend the parameters toward higher loads, for Materials 2021, 14, 997 10 of 19

the same materials. Adding PTFE does not change this parameter too much for the same regime ranges of inputs. As for the inﬂuence of sliding distance, COF is kept in a narrow range for L = 2500 m and with a larger spread for the longer distance, but the average values still remain in an acceptable range for practical use. Myshkin et al. [47] presented trends of COF for polymeric materials sliding on steel, depending on sliding regime, including the plateau type, which is very advantageous for tribosystems functioning in dry conditions. As the tribometer used for testing this class of polymeric materials is very accurate for measuring the linear wear, the authors calculated a linear wear rate of the block material, Wl, which gave the following relationship:

∆Z Wl = (µm/(N · km)) (1) F · L where ∆Z(µm) is the approaching distance between the steel ring (considered rigid) and the block, recorded at the end of each test, F (N) is the normal force and L (m) is the sliding distance. Mapping the tribological characteristics for parameters of interest as a function of a set of parameters is important in evaluating experimental results, as this mathematical model- ing could reveal domains with optimum values for a certain set of testing parameters [48]. Maps allows for a panoramic view of the dependence of a tribological characteristic (here, the linear wear rate) on two variables [49]. The shape of a map could point out a change in wear mechanisms, a zone with optimal values for wear, even if tests were not done precisely for those indicated values. Maps in Figures8 and9 were drawn using a double spline technique using the software MATLAB R2009b (R2009b, MathWorks, Santa Clara, CA, USA) with each map representing the linear wear rate (Wl) as a function of the sliding velocity and PTFE concentration, with the help of a cubic interpolation. Map surfaces are “obliged” to pass through points given by experimental data (sliding velocity, PTFE concentration and linear wear rate). This methodology makes the map surface wavy, but the influence of one or a set of parameters is well evidenced. Comparing maps for different loads (Figure8 for F = 1 N and F = 2.5 N), note that the greatest values are obtained for the lowest tested force (F = 1 N), meaning that there is an intense abrasive process (like a microcutting) as the superficial layer is not sufficiently compressed and the metallic counterpart (even with a high-quality texture) rasps the polymer and the polymer blends. This process is more intense for a high concentration of PTFE (15% wt) for low velocity (v = 0.25 m/s). It is interesting to note that for F = 1 N and L = 7500 m, the neat polymer has a greater linear wear rate, especially for high velocities (0.5 m/s and 0.75 m/s) meaning that wear processes were qualitatively modified. It is possible that high velocity makes the tribolayer soften and the polymer detachment becomes easier. The high linear wear rate for the blends with around 10% wt PTFE can be explained by several causes, including the existence of PTFE agglomeration that is torn off in larger microvolumes than those from blends having a better dispersion of PTFE in smaller microvolumes. When the load increases from F = 1 N to F = 2.5 N, the linear wear rate decreases very much, underlining the idea that contact polymer (or polymer blend)–steel functions better when there is a sufficient load that does not allow for tearing off the polymer or the softer polymer, in the case of PBT + PTFE blends. MaterialsMaterials 20212021,, 1414,, 997x FOR PEER REVIEW 1111 ofof 1919

(a) (b)

FigureFigure 8.8.Maps Maps ofof linearlinear wearwear rate:rate: ((aa)) FF == 11 N;N; ((bb)) FF == 2.5 N.

Figure9 9 presentspresents the maps of of linear linear wear wear rate rate for for the the highest highest load load F = F 5 = N. 5 Note N. Note that thatthe map the mapscale scalebecomes becomes smaller smaller when when the slidin the slidingg distance distance is increased. is increased. Analyzing Analyzing Figure Figure9, the following9, the following conclusions conclusions may be may formulated be formulated for each for map: each in map: Figure in 9a, Figure for L9a, = for2500 Lm, = the 2500 blends m, the behave blends better behave for betterhigher for velocity higher (v velocity = 0.75 m/s) (v = 0.75and m/s)the lowest and thevalues lowest for valueslinear wear for linear rate wearare obtained rate are for obtained PF5, but for also PF5, for but PF15. also forThis PF15. could This result could from result hard from as- hardperities asperities and polymeric and polymeric material. material. Obviously, Obviously, droplets droplets of PTFE of PTFEare more are morerapidly rapidly and pref- and preferentiallyerentially transferred. transferred. For ForFigure Figure 9b, 9Lb, = L5000 = 5000 m/s, m/s, the best the bestresults results were were obtained obtained for PBT for PBTat v at= 0.25v = 0.25m/s m/sand andfor PF15 for PF15 for forall alltested tested velocities. velocities. Values Values for for PF10 PF10 are are close close to thosethose obtained for shortershorter sliding distance.distance. In FigureFigure 99c,c, forfor thethelongest longest testtest (L (L = =7500 7500 m), m), the the map shapeshape isis similar similar to to that that for for L =L 2500 = 2500 m but m withbut with lower lower values values (almost (almost three timesthree lowertimes

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lowerfor PBT for at PBT v = 0.75at v = m/s 0.75 and m/s twoand timestwo times lower lo forwer PF15). for PF15). The The blocks blocks made made of PF10 of PF10 have have the thehighest highest linear linear wear wear among among the PBTthe PBT + PTFE + PTFE blends. blends. Supplementary Supplementary tests tests and and investigations investiga- tionsare needed are needed for explaining for explaining this maximumthis maximum or to or check to check if a possibleif a possible poor poor PTFE PTFE dispersion dispersion(the presence(the presence of agglomerates) of agglomerates) could could be the be cause. the cause. The increaseThe increase of linear of linear wear wear rate for rate PF10 for PF10could could be justiﬁed be justified by nonuniform by nonuniform dispersion dispersion of PTFE, of as PTFE, revealed as revealed by SEM images.by SEM For images. these Forblocks, these agglomeration blocks, agglomeration of PTFE was of foundPTFE was in the found superﬁcial in the layer,superficial which layer, was detachedwhich was as detachedlarger wear as debris.larger wear debris.

(a) (b)

(c)

FigureFigure 9.9. MapsMaps ofof wearwear raterate forfor testedtested loadload FF == 55 N:N: ((aa)) LL == 2500 m; (b) LL == 5000 m; ( c) L = 7500 7500 m. m.

4. Discussion Discussion on Tribological Processes Wear mechanisms for polymericpolymeric materials have been discussed in in Dasari et al. [50], [50], Stachowiak and Batchelor [51] [51] and Deleanu et al. [52], [52], the main topics being abrasion, erosion, adhesion, transfer, fatigue,fatigue, tribocorrosion and delamination as particular types associated with polymeric triboelements. The actual wear process is the result of synergic actions implying particularparticular rubbingrubbing pair pair of of materials materials and and several several wear wear mechanisms mechanisms acting act- ingat the at the same same time. time. Detailed Detailed description description of of these these processes processes is is given given inin StachowiakStachowiak and Batchelor [51]. [51]. Figure 10 shows evidence that wear mechanisms are present simultaneously on the worn surface of PBT. Each letter is writtenwritten near the microzone where a certain wear mechanism is is evident: evident: A—fatigue A—fatigue cracks cracks that that are are almost almost perpendicular perpendicular to the to thesliding sliding direction;direction; B—abrasion B—abrasion trace trace with with small small depth, depth, without without rising rising edges edges and and without material removal, typicaltypical forfor aa polymer polymer in in normal normal regime; regime; C—deep C—deep groove groove resulting resulting from from abrasive abra- siveploughing, ploughing, also characterizingalso characterizing the polymer the poly slidingmer sliding against against steel, withsteel, rising with edgesrising aboveedges abovethe initial the surface,initial surface, repeatedly repeatedly deformed; deform D—adhesioned; D—adhesion wear, which wear, resulted which fromresulted trapping from trappingand embedding and embedding of PBT wear of PB debris,T wear previously debris, previously detached; anddetached; E—lateral and lipsE—lateral and cracks, lips

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generatedand cracks, due generated to the viscoplastic due to the natureviscoplastic of the nature polymer of whenthe polymer hard asperities when hard slide asperities against theslide polymer. against the polymer.

Figure 10. Wear mechanisms,Figure identified 10. Wear on the mechanisms, worn surface identified of the block on the made worn of surfacePBT (F = of 5 theN, v block = 0.25 made m/s, L of = PBT5000 (Fm). = 5 N, v = 0.25 m/s, L = 5000 m). Transfer films generated on polymer–metal rubbing contacts help for a gradual transitionTransfer from films transient generated to onsteady-state polymer–metal wear rubbingprocesses. contacts The transfer help for amechanisms gradual transi- for tionPTFE from and transient PTFE composites to steady-state were explained wear processes. and argued The transfer by experimental mechanisms studies for PTFE by Gong and PTFEet al. [45] composites and Tomescu were explained [13]. Gong and et al. argued presents by experimentala model of adhesive studies bywear Gong for PTFE et al. [and45] andPTFE Tomescu composite [13 with]. Gong particles et al. sliding presents against a model hard of bodies adhesive (as those wear made for PTFE of steel), and PTFEin dry compositeregime, with with mechanical particles sliding processes against only, hard as bodies PTFE (asis thosean almost made inert of steel), material in dry from regime, the withpoint mechanical of view of processeschemical reactions only, as PTFEwith the is an contacting almost inert materials material (solids from or the lubricants). point of viewTomescu of chemical presented reactions images withof adhesion the contacting and transfer materials of PTFE (solids and or PTFE lubricants). composites Tomescu when presentedsliding in water images against of adhesion steel; the and adhesion transfer an ofd PTFE transfer and process PTFE composites is diminished when very sliding much infor water lubricating against contacts steel; theand adhesion it preferentially and transfer involved process PTFE is and diminished less for the very harder much mate- for lubricatingrial, when dealing contacts with and PTFE it preferentially composites. involved PTFE and less for the harder material, whenFor dealing PBT, withthe adhesion/transfer PTFE composites. on steel counterbody has a lumpy character and the transferredFor PBT, debris the adhesion/transfer are thicker and lumpy. on steel For counterbody the blend PBT has + a PTFE, lumpy the character transfer and implies the transferredsmaller debris debris of PBT, are thicker more PTFE and lumpy. and the For agglomerations the blend PBT of + wear PTFE, debris the transfer occur in implies larger smallerwear particles debris of that PBT, are more pressed PTFE in and the the steel agglomerations surface texture. of wearThese debris attached occur agglomerations in larger wear particlescan explain that the are oscillations pressed in of the friction steel surface coefficient, texture. higher These for attachedlow loads; agglomerations when these parti- can explaincles arethe not oscillations sufficiently of pressed friction coefficient,and flattene higherd, they for become low loads; rolled when (as theseshown particles in Figures are not12c sufficientlyand 15c). pressed and flattened, they become rolled (as shown in Figures 12c and 15c). TheThe abrasionabrasion of of PBT PBT blocks blocks is evidencedis evidenced by scratchby scratch traces traces of uneven of uneven depth anddepth width, and butwidth, with but less with evidence less evidence of detaching of detaching the polymer; the thispolymer; explains this the explains very good the tribological very good behaviortribological of thisbehavior polymer. of this Due polymer. to the viscous–plastic Due to the viscous–pl nature ofastic the polymer, nature of the the grooves polymer, in the sliding direction generated by the metallic asperities have wavy edges, with lips due to the grooves in the sliding direction generated by the metallic asperities have wavy edges, the viscous flow and intermittent tears, with an oblique direction to sliding (Figure 11). with lips due to the viscous flow and intermittent tears, with an oblique direction to sliding (Figure 11).

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

(d(d) ) (e) ((ff)) FigureFigure 11. 11. WearWear Wear mechanisms mechanisms for for PBT PBT slidinslidingslidingg against against steel, steel, L L = = 5000 5000 m: m: (a (a)) )FF F == = 11 1N,N, N, vv v== =0.250.25 0.25 m/s;m/s; m/s; ( (bb)) (F bF )= = F1 1 =N, N, 1 v N,v = = 0.75 v 0.75 = 0.75m/s; m/s; m/s; ( c()c ) F = 1 N, v = 0.25 m/s; (d) F= 5 N, v = 0.25 m/s; (e) F = 5 N, v = 0.75 m/s; (f) F = 5 N, v = 0.25 m/s. (cF) = F 1 = N, 1 N,v = v 0.25 = 0.25 m/s; m/s; (d) (F=d) 5 F= N, 5 v N, = 0.25 v = 0.25m/s; m/s;(e) F (=e 5) FN, = v 5 = N, 0.75 v = m/s; 0.75 (f m/s;) F = 5 (f N,) F v = = 5 0.25 N, v m/s. = 0.25 m/s.

WearWear debris debris shown shown in in Figure 1212 areare different different in in shape shape and and size: size: in in Figure Figure 12a, 12a,12a, wear wear particleparticleparticle generated generated fromfrom the the neat neat polymer polymer th thatatat waswas was trappedtrapped inin in aa deepdeep wearwear groove; groove; in in FigureFigureFigure 12b, 1212b,b, two two smallsmall agglomerations agglomerations of of PTFE PTFE particlesparticles particles thatthat are are are very very likely likely likely to to be be be detached detached detached ififif the the movementmovementmovement were werewere to toto continue; continue; in in Figure Figure 12 c, 12c,12c, a conglomerate aa conglomerateconglomerate of small ofof smallsmall wear wear wear debris debris debris made madeofmade PTFE, ofof PTFE,PTFE, that adhered thatthat adheredadhered and bonded and bonded one to one another, toto another,another, being pressedbeingbeing pressedpressed and rolled andand repeatedlyrolledrolled re- re- peatedlyinpeatedly contact—the inin contact—thecontact—the presence presence of such particlesof such part couldiclesicles be couldcould the becausebe thethe ofcause cause high of of oscillationshigh high oscillations oscillations of the offrictionof thethe frictionfriction coefﬁcient, coefficient,coefficient, especially especially under lowerunder loads;lower inloads;loads; Figure inin Figure12Figured, rolled 12d, 12d, wearrolled rolled debris wear wear withdebris debris an withextremelywith an an extremelyextremely high concentration highhigh concentration of PTFE; of in PTFE; Figure inin 12 FigureFiguree, a droplet 12e,12e, a a of droplet droplet deformed of of deformed deformed PTFE torn PTFE PTFE from tornitstorn “bed” fromfrom of itsits PBT “bed”“bed” (up) ofof and PBTPBT another (up) and round another volume roundround of PTFE,volumevolume covered ofof PTFE,PTFE, by coveredcovered a thin PBT by by a bridge,a thin thin PBTprobablyPBT bridge,bridge, formed probablyprobably by the formed wide spreadingby the wide of spre a smalladingading volume ofof aa smallsmall of PBT; volumevolume in Figure of of PBT; PBT; 12 f,in in at Figure Figure higher 12f,sliding12f, atat higherhigher velocity, slidingsliding the wear velocity,velocity, debris the made wear of debris PTFE havemademade a ofof butterﬂy PTFEPTFE havehave aspect, a a butterfly butterfly but they aspect, aspect, are smaller but but theyandthey rolled. areare smallersmaller andand rolled.rolled.

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

Figure 12. Cont.

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(d) (e) (f) FigureFigure 12. Wear 12. Wear debris debris after Lafter = 5000 L = m:5000 (a )m: PBT, (a) FPBT, = 1 N,F = v 1 = N, 0.25 v = m/s; 0.25 (m/s;b) PF5, (b) FPF5, = 5 F N, = v5 =N, 0,25 v = m/s;0,25 m/s; (c) PF10, (c) PF10, F = 1 F N, = 1 v N, = 0.25 v = m/s; (d) PF10,0.25 Fm/s; = 1 ( N,d) vPF10, = 0.25 F = m/s; 1 N, ( ev) = PF15, 0.25 m/s; F = 5 (e N,) PF15, v = 0.25 F = m/s;5 N, v (f )= PF15,0.25 m/s; F = 5(f N,) PF15, v = 0.75F = 5 m/s. N, v = 0.75 m/s.

Better dispersion dispersion was was noticed noticed for for the the block block ma madede of ofPF15 PF15 (Figure (Figure 13), 13 tested), tested at L at= 7500L = 7500 m. Worn m. Worn surfaces surfaces are not are gold not goldcoated coated before before SEM SEMinvestigation, investigation, but for but even for evenpoor qualitypoor quality images, images, the dispersion the dispersion can canbe beseen, seen, and and the the worn worn surface surface presents presents only only small wear traces in depth and width.

(a) (b) (c)

Figure 13. AspectsAspects of of worn worn surfaces surfaces for for blocks blocks made made of of PBT PBT + 15% + 15% PTFE, PTFE, tested tested under under 5 N, 5 N,for fora sliding a sliding distance distance of L of = 7500 m: (a) v = 0.5 m/s; (b) v = 0.25 m/s and (c) v = 0.25 m/s, worn surface with rolled conglomerate debris and thin debris L7500 = 7500 m: ( ma):( v a=) 0.5 v = m/s; 0.5 m/s;(b) v (=b )0.25 v = m/s 0.25 and m/s (c and) v = ( c0.25) v =m/s, 0.25 worn m/s, surface worn surface with rolled with conglomerate rolled conglomerate debris debrisand thin and debris thin reattached and pressed on the surface. debris reattached and pressed on the surface.

The transfer on the metallic ring is very different, as revealedrevealed byby comparingcomparing SEM imagesimages obtainedobtained afterafter testing testing PBT, PBT, PBT PBT + PTFE + PTFE and and PTFE PTFE (Figure (Figure 14): in 14): Figure in Figure14a, lumpy, 14a, lumpy,refragmented refragmented wear debris wear deposit debris on deposit the steel on ring; the insteel Figure ring; 14 inb aFigure rolled and14b presseda rolledwear and pressedparticle fromwear theparticle PTFE from block, the the PTFE folding block, of the the this folding wear debrisof the resultingthis wear from debris consecutive resulting fromprocesses consecutive of laminating, processes adhering of laminating, and rolling; adhering in Figure and rolling; 14c other in Figure wear debris 14c other from wear the debrisblock madefrom ofthe PTFE block is pressedmade of in PTFE the steel is pre texture;ssed thisin the process steel wastexture; identiﬁed this process (less intense was identifiedas thickness (less and intense area) foras thickness the PBT + and PTFE area) blends. for the PBT + PTFE blends.

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(a) (b) (c) Figure 14. TransferTransfer on on the the metallic metallic disk disk from from tests tests done done in in dry dry regi regime,me, on on a ablock-on-ring block-on-ring tester, tester, PTFE, PTFE, tested tested under under 5 N, 5 N, v =v 0.75 = 0.75 m/s, m/s L ,= L 7500: = 7500: (a) (lumpy,a) lumpy, refragmentated refragmentated wear wear deposit deposit on on the the steel steel ring; ring; (b (b) )rolled rolled and and pressed pressed wear wear particle particle from from thethe PTFE block; ( c) microzone with better adhered PTFE.

PBTPBT has has a a different different transfer transfer process process (Figure 15):15): in in Figure Figure 15a15a wear debris debris are rare; in FigureFigure 1515b,b, wearwear debris debris transferred transferred on on the the steel steel ring ring as lumpy as lumpy islands, islands, without without being rolled.being rolled.Figure Figure15c presents 15c presents wear debris wear expelleddebris expelle from thed from contact the nearcontact the near friction the pathfriction on thepath steel on thering, steel when ring, running when arunning disk made a disk of PF10;made wearof PF10; particles wear areparticles made are almost made of almost PBT and of these PBT andare robust,these are not robust, rolled, not shown rolled, in shown darker in gray, darker and gray, wear and particles wear madeparticles almost made of almost PTFE are of PTFEwhite, are thinner white, and thinner rolled, and many rolled, partially many bondedpartially to bonded one another. to one another.

(a) (b) (c) Figure 15. Worn surface of the PBT block, tested under 5 N, L = 7500: ( a) abrasive wear and a wear particle reattached to the friction surface, v = 0.75 m/s; (b) v = 0.25 m/s abrasive wear located next to the friction path of the steel ring; (c) wear thethe friction surface, v == 0.75 m/s; ( (bb)) v v = = 0.25 0.25 m/s m/s abrasive abrasive wear wear located located next next to to the the friction friction path path of of the the steel steel ring; ring; ( (cc)) wear wear debris expelled from the contact near the friction path on the ring the block made of PF10. debris expelled from the contact near the frictionfriction path on the ring the block made of PF10.

TheThe blends blends PBT PBT + PTFE PTFE have have a a transfer on on the steel triboelement less intense as comparedcompared to to that that of of neat PTFE (Figure 16a).16a). The The wear wear particles particles are are smaller and not so agglomerated.agglomerated. The The two two particles particles in in Figure Figure 16b16b have have different different aspects; aspects; the the bottom is more compact,compact, very very likely likely containing containing more more PBT, PBT, with with several several microvolumes microvolumes of of PTFE PTFE (white). (white). TheThe other other is intensely white, meaning its composition is consistent in PTFE. The aspect is rolled,rolled, and it isis obviousobvious thatthat thethe agglomerated agglomerated particle particle was was generated generated by by smaller smaller adhering adher- ingwear wear particles. particles. The The presence presence of more of more PTFE PTFE is revealed is revealed by the by high the degreehigh degree of deformation. of deformation.Figure 16 Figurec shows 16c the shows details the of details two wear of two debris wear particles debris that particles could bethat considered could be extreme:consid- eredthe particle extreme: in the the particle bottom ofin thethe SEMbottom images of the is SEM robust, images thicker is androbust, not thicker rolled(the and gray not rolledshade (the characterizing gray shade PBT); characterizing the particle PBT); in thethe upperparticle right in the corner upper of right the same corner image of the is samemostly image made is of mostly PTFE, butmade also of contains PTFE, abut small also volume contains of PBTa small (gray volume shade). of Both PBT particles (gray shade).are conglomerates Both particles formed are conglomerates by the bonding formed and adhering by the bonding of initially and small adhering wear of debris. initially small wear debris.

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(a) (b) (c)

Figure 16. Wear debrisdebris fromfrom PBTPBT ++ 15%15% PTFEPTFE blocksblocks onon steelsteel counterparts,counterparts, testedtested underunder 55 N,N, vv= = 0.250.25 m/s,m/s, L = 7500 m: ( a)) wear debris with different concentrations of PTFE (PTFE is bright white, PBT is gray); ( b) two different wear particles; ( (c)) detail of the particles in the previous image.

5. Conclusions Conclusions This relatively new entry in the family of polymer blends, PBT + PTFE, is promising inin tribological tribological applications, applications, at at least least for for the the pa parametersrameters tested. tested. By Byadding adding PTFE PTFE in PBT, in PBT, the frictionthe friction coefficient coefficient is kept is kept in narrow in narrow range range F = 2.5–5 F = 2.5–5 N, v N, = 0.25–0.75 v = 0.25–0.75 m/s m/sand is and less is sen- less sitivesensitive to PTFE to PTFE concentration concentration if the if thedispersion dispersion is of is good of good quality. quality. Local Local agglomerations agglomerations of PTFEof PTFE detach detach from from the thePBT PBT matrix matrix more more easily easily and generate and generate higher higher wear wearrates and rates oscil- and lationsoscillations of the of friction the friction coefficient. coefficient. Results for testing PBT + PTFE blends in dry conditions revealed that tribological characteristics are influenced influenced by PTFE concentration and the test regime (load and sliding velocity). Components Components made made of of these these blends blends can can lower lower the power loss and enlarge the durability by re reducingducing wear characteristics. Linear wear rate have better values for longer sliding distance, meaning that wear is more intense at the beginning of sliding; th thee transfer process and the plastic deformation of the superficialsuperficial layerlayer allow allow for for reducing reducing friction friction and and wear. wear. The The presence presence of PTFE of PTFE reduces re- duceswear, especiallywear, especially for 5% andfor 5% 15%. and For 15%. 10%, For this 10%, parameter this parameter increased, increased, but the SEM but investiga-the SEM investigationtion revealed revealed poorer dispersion poorer dispersion of PTFE. of This PTFE. decrease This decrease in wear ratein wear is more rate obviousis more ob- for vioushigher for velocities higher andvelocities loads, meaningand loads, that meaning the polymeric that the material polymeric has tomaterial be compressed has to be to compressedmake it less proneto make to it be less scratched prone to and be tornscratched off by and metallic torn texture.off by metallic texture. InIn order to underline the better wear resistance of blendsblends PBTPBT ++ PTFE, the authorsauthors testedtested blocks blocks made made of of neat neat PTFE PTFE for for the sliding distance of of L = 7500 7500 m m and and load load F F = = 5 5 N. N. When comparing to the results obtained for the blend PBT + 15% PTFE (PF15), it was in thethe favor of the blend: a. Fora. v =For 0.25 v m/s, = 0.25 Wl m/s,PTFE =Wl 19.995PTFE = µ19.995m/(N µm/(N·km), approx.∙km), approx. 70 times 70 largertimes thanlarger that than of PF15, (WlthatPF15 of= PF15, 0.283 (Wlµm/(NPF15 = ·0.283km)); µm/(N∙km)); b. Forb. v =For 0.5 m/s,v = 0.5 Wl m/s,PTFE Wl=PTFE 20.629 = 20.629µm/(N µm/(N·km),∙km), approx. approx. 85 times85 times larger larger than than that that of PF15, (Wlof PF15PF15,= 0.240(WlPF15µ m/(N= 0.240·km)); µm/(N∙km)); c. Forc. v =For 0.75 v m/s, = 0.75 Wl m/s,PTFE =Wl 16.648PTFE = µ16.648m/(N µm/(N·km), approx.∙km), approx. 77 times 77 largertimes thanlarger that than of PF15, (WlthatPF15 of= PF15, 0.216 (Wlµm/(NPF15 = ·0.216km)). µm/(N∙km)). The experimental results,results, especially especially for for the th longe long sliding sliding distance distance against against steel steel (7.5 km),(7.5 km),recommend recommend using using a PBT a +PBT 15% + PTFE15% PTFE polymer polymer blend blend as a replacementas a replacement for components for compo- nentsfor tribological for tribological applications applications made onlymade of only PTFE of in PTFE household in household appliances appliances and automotive and au- tomotivecomponents. components. Components Components made of thesemade blendsof these could blends lower could the lower power the loss power and loss enlarge and enlargethe durability the durability by reducing by reducing wear characteristics. wear characteristics.

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Author Contributions: Conceptualization, C.G. and L.D.; methodology, C.G. and L.C.T.; software, C.G.; validation, L.D. and C.G.; formal analysis, L.C.T.; investigation, A.C.C.; resources, C.G.; writing— original draft preparation, C.G.; writing—review and editing, L.D. and L.C.T.; visualization, A.C.C.; supervision, L.D. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Faculty of Engineering, “Dunarea de Jos” University of Galati, Romania. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Acknowledgments: The authors thanks to Doina Constantinescu, from Monofil Savinesti SA, for technical support in obtaining the samples. Conflicts of Interest: The authors declare no conflict of interest.

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