Graphene nanoflakes and zirconium dioxide (ZrO 2) were added to polyamide 66- based composites with glass fibre in order to improve their self -lubrica ting properties and make them suitable for high- performance bearing applications. The influence of texturing the surfaces w as also investigated. The t ribological behaviour was analysed for different combinations of hybrid polymeric composites, percentages of additive , and texturing geometr ies. The r esults show that the lowest low friction coefficients are obtained for PA66+30GF with 5% graphene . Worn surfaces , analysed by scanning electron microscopy and optical microscopy confirm the enhancement of the tribological properties with the addition of graphene . Texturing results under dry lubrication are not relevant for tribological purposes. Keywords : ZrO 2, graphene , friction , texturing . 3 1. INTRODUCTION It is estimated that almost 30% of the energy consumed worl dwide is lost in the form of heat due to friction [1]. Traditionally, tribological properties such as friction, wear , and lubrication have been investigated by focusing on the development of new surface treatments [2, 3], e.g., nitriding a nd carburizing, grease lubricants , and hard coatings. Nitriding and carburizing significant ly increase the hardness , which leads to better tribological behaviour [4–7]. Owing to their composition [8] and industrial applications [9], new gr ease lubricants are continuously investigated to optimis e the tribological performance. H ard coatings such as nanocomposites of silver containing carbon films (a-C:H/Ag ) [10], Molibdenum dysulfide with mixture of titanium and tungsten (MoS 2TiW) [11], (Zr,Ti)N deposited via physical vapo ur deposition ( PVD) [12], and diamond films deposited via chemical vapo ur deposition (CVD) [13, 14] , as well as plasma spr ay techniques [15, 16] , are used to reduce heat generat ion. Some friction and wear phenomena are controlled by processes at the atomic or molecular scale, and others are controlled by processes at a scale of roughness of approximately 1–20 μm [17]. Consequently , one of the chall enges that tribology research faces with regard to new surface treatments based on coatings is the difficulty of studying microscopic phenomena resulting i n macroscopic wear and friction . Additionally , the interacting forces between the coating and substrate surface determine the adherence properties and coating detachment. Thus, new self -lubrica ting materials with optimal properties for each application without coating requirements are a good solution, as they allow the elimination of external lubricants, simplify ing the design and reducing maintenance costs. Polymers are extensively used in active engineering components such as gears and cams , and their self -lubricating properties are exploited to avoid the need for oil or grease lubrication, which present the problems of sealing and possible contamination [18]. Bearings are one of the most widely used mechanical components in industry and it is one of the components to which tribology research can be most directly applied. New trends in bearings are focused on obtaining high -performance bearings capable of operating under high- speed and high -temperature conditions [ 19, 20 ]. These features are directly related to the friction during bearing performance caused by rolling and/or sliding between rolling elements and the bearing cage [ 21–23]. Cages for small - to medium -size bearings can be made of metallic materials or polymer composites , which 4 minimis e the inertial effects [24]. Different polymers have been studied from a tribological view point , such as polyether ether ketone ( PEEK )/polytetrafluoroethylene (PTFE ) [25] and polyamide (PA), w hich is the most promising material for this purpose [26–30]. PAs are usually mixed with glass fibre (GF) to improve their mechanical properties. The t ribological behaviour of these composites under sliding conditions was reported in [ 31-32] long ago , indicating the ability of PA to form a thin and uniform transfer film while sliding against steel counterparts , which helps to reduc e the coefficient of friction (COF) . The t ribological behaviour of the composite is affected by the different contents of GF in the PA matrix during sliding , as described in [33]. While few investigation s of the behaviour of PA under rolling conditions have been reported, studies have revealed the negative effect of GF on wear [34-36]. Therefore , new bearing applications demand new composites combining the good mechanical properties of GF, good processability of polymers , and good wear behaviour that can be achieved by adding nano -particles to the polymer matrix . Carbon nanotubes provide effective reinforcement for polymers , improving the mechanical properties [37] and reducing the friction and specific wear rate for a range of loads [ 38]. Additionally , alumina has been added to P TFE to reduce the COF and wear, and titanium dioxide ( TiO 2) has been proven to reduce the COF, especially under high-load/speed conditions [ 27]. Grap hene, which was first obtained from graphite in 2004 [39], is one of the most promising additives owing to its special bidimensional layered structure , whose atom ic thickness is 0.1 nm. The most relevant properties of graphene are its high conductive capacity, hardness , and strength as a very thin layered structure. Regarding the tribological behaviour , it has been dispersed on an oil lubricant to reduce the wear and friction of mechanical elements immersed in th e lubricant [ 40] and applied to silicon dioxide ( SiO 2) substrate s [41]. Literature concerning the tribological behaviour of graphene applied to polymers is limited to the study of PEEK [ 42], acrylonitrile butadiene rubbers (NBRs) [43], PTFE [ 44], and polyphenylene sulphide (PPS) [45], with graphene used for coating purposes . ZrO 2 particles are also a promising material owing to their high hardness and stiffness , as well as their optimum thermal stability. When they are applied to coatings for metallic materials , a higher working temperature can be reached [ 46], and the mechanical properties are significantly improved [ 47]. The modification of the 5 tribological properties of ZrO 2 itself was analysed in [48], as well as its influence on ceramic metal materials ( CerMet ) materials [ 49], but few investigations have focused on the improvement of the tribological properties of polymers doped with ZrO 2 [50]. A technique that has not been thoroughly explored for improving the self -lubric ating properties is surface modification by texturing. In the past few years , new mechanical [51], pulsed air arc [ 52], and energetic beam [ 53] techniques for surface modification have led to innovat ive surface treatments that can replace the current ones for improving the tribological properties. One of the most promising techniques is the micromach ining of geometrical shapes on lubricated surfaces , which act as small lubricant reservoirs [54]. Laser micromachining is the most advanced technique for surface modification in tribological applications and is a clean, rapid , and highly accurate process. Even when micromachining is combined with other surface techniques, the best tribological results are due to the micromachined geometry [ 55–57]. However, studies show that the tribological improvement depends strongly on the main micromachining parameters , such as the shape, dimensions , and density of the motif. The t ypical micromachined geometry suitable for improving the tribological properties consists of dimples that act as cells of hydrodynamic pressure to en sure heat removal in the case of wet lubrication [58] or as reservoirs for preventing the accumulation of the wear material in contact areas [ 59, 60] in the case of dry lubrication. These dimples also allow the modif ication of the width and height of the limit lubrication area achieving lower friction values that are close to hydrodynamic lubrication values [ 61]. Another advantage of this kind of micromachining is the non -interference with the surface macro -geometry produced by material roughness [ 62]. Most of the literature regarding the impro vement of the tribolog ical behaviour via surface geometry modification is focused on metals . In the present study, the effect of graphene and ZrO 2 additivation on the friction and wear behaviour of a GF PA66 composite is investigated . A second strategy for improvement involves texturing the surfaces of the additived samples exhibiting the best tribological behaviour with dimples having two different geometries in order to analyse their influence on the COF and wear results . 2 EXPERIMENTAL 2.1 M aterials 6 Polymer n ano-reinforced composites based on PA 66 + 30% GF from INVIST A (TORZEN® G3000 NC01) [63] were analysed . Glass fibre is sized 10 µm diameter and an average length of 150 µm. The ad ditives used were ZrO2 and graphene . The g raphene used was Avangraphen Plat 40, which is a two -dimensional c arbon atom structure obtained via the mechanical exfoliation of graphite, with a n oxygen content (X-ray photoelectron spectroscopy ) of <0.5% , a hydrogen content of <1%, a nitrogen content of <1% , an average thickness of 10 nm , a side size (L aser Diffraction ) of 40 µm, a surface area (Brunauer –Emmett– Teller) of 282 m2/g, and an average number of layers of <30. It is specially designed to be mixed with polymeric materials , and a low dosage is required to reach the percolation level . The other add itive used was general -purpose ZrO 2 supplied by TORRECID [64] , sized 750nm and with a tensile strength of 450 MPa. 2.2. Specimen preparation Five different nan o-reinforced composite material s were prepared , as shown in T able 1, via extrusion compounding in a co- rotating twin screw extruder ( Coperion ZS26K ) equippe d with two gravimetric feeders ( Brabender ). The maximum torque was 100 Nm , maximum screw speed was 1,200 rpm, and maximum pressure was 180 bar. A screw diameter of 25 mm and a length /diameter ratio of 40 w ere used i n the different test procedures . The o ptimum level of ad ditive dispersion into the polymeric matrix was determined using an ultrasonic device specially designed for this purpose , fully described in [65, 66] . The morphology of the samples was analysed via scanning electron microscopy ( SEM , Hitachi S -3400 N ) and energy -dispersive X -ray spectroscopy ( EDX, Röntec XFlash de Si(Li) ), at an acceleration voltage of 15 kV and a working distance of 17.6 mm. The s amples were previously sputtered with a thin layer of gold in vacuum before observation. Scanning electron images of the microstructure of the resulting PA66GF30 composites are shown in F igure 1. Figure 1a shows the morphology of the samples without additives. Figures 1b and 1c show samples doped with ZrO 2, and figures 1 d and 1e shows samples doped with graphene. It can be observed for samples doped with low percentages (1%) in figures 1b and 1 d that lamin ar morphology of these samples do not change significantly regarding to morphology of sample 1a without doping. When percentages of additive are increased up to 5% graphene (figure 1e) and 10 ZrO 2 (figure 1c), a change in sample morphology is clearly appreciated. In case of samples doped with ZrO 2 an alveolar structure is induced into the polymer due to the spherical shape of the ZrO 2. In case of samples doped with graphene, a flaked structure is induced into the polymer matrix due to the 7 laminar structure of the graphene. All the samples doped with any of the additives do not exhibit aggregates which could imply a good dispersion of the additives for both low and high percentages of additive Composite Base polymer additive PA66GF30 PA 66 + 30% GF -- PA66GF30ZrO 21 PA 66 + 30% GF 1% ZrO 2 PA66GF30ZrO 210 PA 66 + 30% GF 10% ZrO 2 PA66GF30G1 PA 66 + 30% GF 1% graphene PA66GF30G5 PA 66 + 30% GF 5% graphene Table 1: Hybrid composites tested Samples of 45 × 45 × 3 mm3 from the resulting composite materials were injected using an electric injection machine ( JSW 85 EL II ) with a clamping force of 85 tons, a maximum injection pressure of 1,960 bar, a maximum injection volume of 97 cm3, and a screw diameter of 32 mm. The p rocess conditions were as follows: an in jection temperature profile of 240/260/280 °C, a mould temperature of 70 °C, an injection time of 1 s, a packing pressure of 50 MPa for 5 s, and a cooling time of 7 s. 2.3 Friction and wear test Ball-on-disk w ear experiments were conducted using a tribometer (Falex Multispecimen) according to the ASTM G99 standard. Because the purpose of the study was to obtain a self -lubric ating material for bearing cages, the ball specimen was an AISI 52100 100Cr6 ball with hardness of HRC 60 and roughness of R a = 0.010 µm, directly obtained from rolling bearings, with a diameter of 12.7 mm. The counterface material was obtained from the different composite -injected samples at a sliding speed of 249 mm/s at 80 °C. A test normal load of F N = 22.5 N w as applied for 1,800 s in an attempt to avoid plastic deformation on the polymeric disc surface. The following results were recorded: the COF evolution over time, the roughness measured using a MARSURF XCR 20 device , and the wear determined both by the weight difference and the wear track measured via optical microscopic technique s. The m icroscope used was a SEM EDX Hitachi S -3400 N , and the mass loss of the samples was weighed using an electric balance in order to calculate the specific wear rate via th e following equation: 8 𝑤𝑤𝑠𝑠=∆𝑚𝑚 𝜌𝜌𝐹𝐹𝑁𝑁𝐿𝐿 �𝑚𝑚𝑚𝑚3 𝑁𝑁𝑚𝑚�, (1) where F N is the normal force applied during the test, ρ is the material density, L = 447.63 m is the sliding distance according to the sliding speed and test duration, and ∆ m is the mass loss obtained experimentally from the weight difference of the sample before and after the test. Density values ( ρ) are calculated from equation: ρ= (% matrix*ρmatrix) +(% additive *ρadditive ) (2) Where ρ is density for the sample material, % additive is mass percentage of additive (ZrO 2 or graphene) added to the sample composite, % matrix is mass percentage of polymer material (PA66 + 30% glass fiber), ρmatrix is density of polymer, and ρadditive is density of additive. 2.4 Texturi ng Surface microstructuring was performed by using a picosecond -pulse Nd:YVO 4 laser, which was integrated in a micromachining workstation (3D MICROMAC ). Dimples were created on the material by applying 355 nm wavelength. Two different dimple configurations were obtai ned by varying the frequency, the overlapping distance between laser pulses, energy , and number of laser passes. The first dimple configuration, which was called T1, was generated by applying a frequency of 10 kHz, an overlapping distance between pulses of 100 µm, an energy of 50 µJ, and five passes, leading to the formation of dimples 20 µm in diameter and 15 µm in depth. The second dimple configuration —T2 —was obtain ed by applying a frequency of 250 kHz, an overlapping distance between pulses of 2 µm, an energy of 1.5 µJ, and 12 passes, yielding dimples 300 µm in diameter and 60 µm in depth. The l aser beam was focused onto the sample by a focusing lens placed in the a ir. The lens had a focal length of 103 mm for 355 nm -wavelength light. Figure 2 shows the distribution of the circular dimples on the samples obtained by SEM. 9 3. RESULTS 3.1 Friction and wear Figure 3 show s the evolution of the COF during the wear test for the hybrid composite materials described in T able 1 . The r esults exhibit an increasing trend at times under approximately 60 s for the running- in stage of all composites , except for PA66 GF30 ZrO 21, whose increasing trend lasts for 400 s and exhibits instability until the test was half -complete. Then, once the transfer film has been developed and remains intact, the curve reaches a steady stage, exhibiting a constant value. PA66 GF30 has an average COF of 0.3. The addition of ZrO 2 has a negativ e effect , increasing the COF up to 0.5 and 0.6 for ZrO 2 concentrations of 1% and 10% , respectively . The l ower average COFs resulted from the addition of graphene to PA66GF30: 0.15 in the case of 1% graphen e and 0.1 in the case of 5% graphene. The results show more stable behaviour during the test with gr aphene than during the test with ZrO 2. Table 2 summaris es the average values of the COF , which were calculated by averaging the experimental values fro m the steady -state stage in F igure 3. It also indicates the specific wear rate ωs calculated using E quation (1) , the width of the wear track , and the roughness induced by wear. Material µ ρ (g/cm3) ws 10-3 (mm3/Nm) Ra (µm) Before test Ra (µm) After test Width of Wear track (mm) Average Standard deviation PA66GF30 0.309 0.016 1.37 1.289 0.167 0.219 1.186 PA66GF30+1% graphene 0.157 0.024 1.3783 0.757 0.149 0.257 1.001 PA66GF30+5% graphene 0.098 0.008 1.4115 0.527 0.153 0.489 1.072 PA66GF30+1% ZrO 2 0.519 0.069 1.4123 0.853 0.109 0.126 1.240 PA66GF30+10 % ZrO 2 0.606 0.063 1.793 0.941 0.114 0.128 1.122 Table 2: Summary of wear test results for hybrid composites According to Figure 4, the m ass loss is reduced from 17.8 mg for the samples without additives to 10.5 mg for the samples with 1% graphene and to 7.5 for the samples with 10 5% graphene. When PA66GF30 is doped with ZrO 2, a smaller reduction in the mass loss is achieved : up to 12 mg for 1% ZrO 2 samples and up to 17 mg for 10% ZrO 2 samples . It can be stated that the PA66 GF30+ graphene exhibit s better tribological behaviour than PA 66GF30, with lower COFs and a lower wear rate. The b est results are achieved by adding 5% graphene to PA66GF30, reducing the COF from 0.3 to 0.098 and reducing the wear rate from 1.54 to 0.65. C onsequently, a shallow wear track is obtained, as indicated by T able 2 and Figure 5. As stated previously , the addition of ZrO 2 has a negative effect on the COF , yielding values approximately twice those for PA66GF30, although the wear rate is reduced , especially for low fractions of ZrO 2. The friction and wear results become worse when high percentages of zirconia (10%) are added . In spite of the low friction values achi eved for the samples doped with graphene , the roughness values remain higher than those for the non-doped samples (0.25 µm for PA66GF30G1 and 0.489 µm for PA66GF30G5 ). The h igh roughness values are attributed to material deposition onto the sharpest polymer peaks during sliding , similar to the phenomenon reported in [ 67]. 3.2. Charact erisation of worn surface The w ear track in PA66GF30, with a width of 1.080 mm, is clearly observed i n Figure 5a. Samples 5d and 5e , which were doped with 1% and 5% graphene , respectively, exhibit a shallower wear path owing to their smaller width of 1.001 and 0.809 mm , respectively . The s amples doped with ZrO 2 exhibit a wider we ar path , as show n in Figures 5b and 5c , with values of 1.105 mm for 1% ZrO 2 and 1.202 mm for 10% ZrO 2. Using optical microscopy, images of the worn surface were obtained to better analyse the generated track s (Figure 6) . The worn surface exhibit s a wavy morphology for PA66GF30, as shown in F igure 6a , indicating the presence of a transfer film with PA66 debris on top of the sample due to plastic softening . This is caused by local plastic deformation , which indicates the viscous flow of the polymer induced by the high flash temperature occurring in the real contact area and the stress concentration near the sample borders . The s tress concentration is mainly caused by the GF interface, although the optical image cannot identify the typical shiny spots of GF needles disrupting the transfer film [ 33]. This 11 softening of the polymeric material is reduced in F igure s 6d and 6e , showing less debris for samples doped with 5% graphene . Owing to the graphene nanoflakes , the COF is reduced and the debris is eliminated under a normal process of fibre thinning, fibre fracture, or fibre peeling- off. Therefore, the fibres can effectively undertake the load and be gradually removed via a normal process. [ 27]. The PA66GF30ZrO 210 samples in Figure 6c exhibit a worn surface with slight grooves parallel to the sliding direction, indicating mechanical interaction. This suggests that the wear process is partially governed by an abrasive wear mechanism. In this case, the polymer generally obtains a high wear rate, depending on the original roughness of the harder counterpa rts and the contact pressure [ 27]. The w ear rate for these samples is higher than that for other additive d samples , as shown in T able 2, but hardly lower than the val ues obtained for non- doped ones. This is probably because of the positive effect s of both the additives , which reduced the temperature in the contact area owing to the high thermal conductivity , and the low ball counterpart roughness (Ra = 0.010 µm), which compensated for the effect of the abrasive wear mechanism . 3.3. Results for micromachined samples According to the aforementioned results, PA66GF5G exhibit s the best tribological behaviour . Therefore , the surface s of these samples were textured with two different dimple geometries , as described in S ection 2.3. The r esults for the COF are shown in Figure 7, indicating an increasing trend until reaching a steady state . The steady state is reached quickly , except for the textured T2 samples , which require 500 s and exhibit instability during the running -in stage. The T1 and T2 s amples ha ve average COF s of 0.15 and 0.38, respectively . Micromachining does not appear to have a positive effect; the results are even worse than those for the non -doped texturis ed T2 samples. Table 3 summarises the average values of the COFs, which were calculated by averaging the experimental values from the steady -state stage in Figure 7, and the wear resul ts for the specific wear rate ωs calculated using Equation (1). All the micromachined samples exhibit worse tribological behaviour than the non- textured samples with the same composition. The COFs are 1.5 and 3.8 times higher for T1 and T2, respectively, and the wear rate is almost twice as high for both textured cases compared with the non - textured ones. 12 Material µ ws 10-3(mm3/Nm) PA66GF30 0.309 1.549 PA66GF30+5% graphene 0.098 0.653 PA66GF30+5% graphene T1 0.153 1.167 PA66GF30+5% graphene T2 0.38 1.211 Table 3: Summary of wear test results for the textured hybrid composites The w orn surfaces analysed via SEM are shown in Figure 8 . They exhibit a wavy morphology , indicating plastic softening. The SEM images show dimples filled by the plasticised material detached during rolling wear, especially for the T1 samples , where the dimples are smaller and closer . For the T2 samples , the refilling of the dimples with debris and detached material is clearly observed . 4. DISCUSSION 4.1. Wear mechanism The p olymer selected for the research has very good tribological properties to begin with. It also forms a good transfer film on the counterfaces during sliding , which is generally a prerequisite for go od tribological properties. Thus, i t is a challenge to improve its tribological properties further. The m ain cause of rolling wear is adhesion, which lea ds to the formation of a t ransfer film. Figure 9 a shows an optical image of the ball and figure 9b shows SEM image of the ball where transferred polymer can be observed. An EDX analysis has been carried out to confirm this presence of polymer. Figure 1 0c shows EDX spectrum (named i n the figure as spectrum 5) for a local area of the ball where sample material has not been transferred. Materials detected in this area were Fe (89.66 % weight), Cr (1.31 % weight) and C (9.03% weight). Figure 11d shows EDX spectrum (named in the figure as spectrum 1) for a local area of the ball where polymer sample material has been transferred. Materials detected in this area were Fe (26.06 % weight), Cr (0.54 % weight), O (9.34 % weight and C (64.04% weight). High presence of C confirms the presence of polymeric material transferred from the sample to the ball. The w ear adhesion mechanism is as follows: owing to the low load- carrying capacity of polymers , the cohesive forces inside the polymer are weaker than the adhesion power between the steel counterpart and the polymer . This leads to the development of a transfer film on the counterface in the transient wear state , through the 13 microfragmentation of the polymeric material , because of the interaction with metal asperities . Owing to the low er wear resistance of the polymeric composite, small wear particles are rapidly removed from the soften ed polymeric surface by the hard asperities of the metallic counterpart surface almost from the beginning of the wear test. As stated in [68], the fricti on component resulting from adhesion is equal to the product of the real contact area and the shear strength of the polymer for the sliding contact of polymers against metals. The adhesion ability of polymers can differ significantly because of the specifi c surface characteristics of the material , which are expressed by its surface energy. PA has one of the highest surface energ ies in the literature [ 69] owing to the strongly increased polar component caused by its chemical structure and the strong interactions (hydrogen bonds) between adjacent polymer chains , which is correlated with high adhesion work between PA contacts. The f ormation and growth of this transfer film is usually followed by the moderate stabilis ation of the COF , once the sharp as perities of the hard counterpart cease to abrade, after being covered by the fragments of the polymer [ 70, 71]. Once the transf er film is formed on the counterpart , wear arises from the loss of transfer -film fragments during repetitive sliding and the occasional separation of the material from the sample surface, which causes it to escape from the interface rather than building up the transfer film. At the transfer film, shearing is accompanied by the detachment of fragments of polymer, with a large amo unt of broken polymer s ticking to the metal counterpart . Figure 3 shows the increasing trend of the COF values during the running -in stage, which is probably caused by the increase of the real contact area and the contact temperature due to frictional heating [ 27]. The d uration of this stage is determined by the speed of the formation of the stable transfer film, owing to the accelerated generation of wear debris and the easier formation of the transfer film . 4.2. Influence of nanoadditives Under dry sliding, the temperature of the polymer surface increases owing to the accumulation of frictional heat , and a softening point is reached because of the relaxation of polymer molecular chains . Therefore, the Young’s modulus decreases , leading to higher plastic deformation and thus an increased real contact area, making the predominant adhesive mechanism friction and increasing the COF until the total formation of the transfer film [68]. 14 Samples based on PA + 30% GF are studied . Although GF has the advantage of a high thermal conductivity , providing good heat dissipation, i t has a negative effect on the friction behaviour because it prevents the formation of a good transfer film , forms abrasive tracks on the counterpart surfaces , and increases the contact temperature. The good results obtained with samples containing graphene are expected because this additive significantly improves the dissipation of accumulated frictional heat owing to its high thermal conductivity of 3,000 W/mK (compared with 0.5 W/mK for PA66 GF30), thereby reducing the influence of the adhesive friction mechanism. Although a better heat dissipation in the tribocontact seems to have a key for the decreasing of coefficient of friction, morphology of the graphene seems to play an important role in the tribological behaviour of the composite doped with it as well. Layers of graphene act as a lubricant allowing soft sliding instead of wear [ 72-73]. All of this contributes to the reduced COF for PA66GF 30 doped with graphene . Besides , Young’s modulus (E) is improved by the addition of graphene and ZrO 2, as described in Table 4 , which le ads to a higher wear resistance and a lower wear rate, as shown in Table 2. E (MPa) Tensile strength (MPa) Average Standard deviation Average Standard deviation PA66GF30 9800 975 195 9.75 PA66GF30G1 10687 1050 189 9.072 PA66GF30G5 11711 1208 169 8.52 PA66GF30ZrO 21 9996 990 189 9.45 PA66GF30ZrO 210 10030 998 182 9.24 Table 4: Summary of mechanical properties for the samples tested On the other hand, during wear test, the increasing of temperature in the contact area leads to a reduction of Young modulus resulting into an increasing of contact area, and so and higher coefficient of friction. In case of samples doped with graphene, improvement of mecha nical properties due to graphene addition is dominant compared to the effect of reducing Young modulus due to temperature and contact area increasing, therefore, a lower coefficient of friction is achieved. In case of samples doped with ZrO2, improvement of mechanical properties due to additive presence is not dominant compared to the effect of reducing Young modulus due to temperature and contact area increasing, so a higher coefficient of friction is induced. 15 A second consequence of the increasing of temperature in the contact area is the reduction of the shear strength (τ), which theoretically leads to lower COFs . Thus , depending on the polymer composition, one of these two effects , reducing Young modulus or reducing shear strength, can be predominant leading to opposite effects . For the samples doped with ZrO 2, reduction of Young modulus seems to be dominant , leading to higher value s of the COF . 4.3. Worn surfaces Composites containing GF exhibit a worn surface with shear lips owing to the softening and stress concentrations near the sample borders and at the interfaces between the PA 66 matrix and the GF , where crack growth initiation at the interfaces between particles o f GFs and the polymeric matrix is observable , as described in [ 34]. The cracks do not grow straight ; rather, they join together —thus, wea r debris is formed and removed. The PA66GF30, PA66GF30G1 , and PA66GF30ZrO 21 materials exhibit a wrinkled and wavy surface morphology owing to the heating e ffect of friction , which soften s the polymer surface. In GF composites without nanoadditives , all the effect s of the counterpart asperities are usually loaded on the fibre , and there is no interface damage , because of the major interfacial bonding of the PA matrix and the GF . The t ransfer film is disrupted in areas under which there is an accumulation of GF . PA is removed in large flakes , and GF is detached as separate needles [29]. With the addition of nanoparticles , the real contact area is reduced by the three bodies of contact instead of surface contact ; thus, a lower adhesive force is achieved . Consequently , the frictional coefficient is effectively reduced , limiting the increas e of the contact temperature. This reduction enhances the load -carrying capacity of the composite. On the other hand, according to [ 27, 74] , in three -component composites, the rolling effect appears , replacing the sliding and preventing the fibre from being detached ; rather, it is only grooved, which reduces the shear stress, frictional coefficient, contact temperature, and matrix damage. In spite of this , the COF can be increased for high percentages of nanoadditives owing to the ir abrasive effect on the composite , as observed for PA66GF30ZrO 210. When nanocomposite particles slide against a smoother counterpart, the nanoparticles cannot provide the aforementioned rolling effect , because the particle dimensions should agree with the gap between the mating surfaces , which is mostly determined by the roughness of the steel counterpart . In this 16 case, the COF can even be increased , as observed for the PA66GF30ZrO 21 and PA66GF30ZrO 210 samples. 4.4. Micromachined samples Dimple micromachining appears to be a more efficient way of improving the tribological properties under wet lubrication. The dimples play the role of lubricant reservoirs , which act as hydrodynamic cells , ensuring efficient heat dissipation. In the case of dry lubrication , this advantage cannot be exploited to improve the tribological behaviour . On the other hand, the presence of intercalated dimples along the wear track can hinder the development of a homogeneous and continuous transfer film that allows easy detachment of the polymeric material. 5. CONCLUSIONS The i mprovement of the tribological behaviour of hybrid pol ymer composites based on PA + 30% GF via two strategies was analysed . The first strategy employs additive composites containing 1% and 5% graphene and 1% and 10% ZrO 2. Wear test s reveal that the composites doped with graphene exhibit lower values of the COF and wear rate, especially those doped with 5% graphene, which achiev e reductions of 30% and 35% in the COF and wear rate, respectively compared to non-doped samples . This improvement is attributed to the high thermal conductivity of graphene , which contributes to better dissipation of frictional heat, and to its high Young’s modulus and tensile strength, which reduce the wear rate. The addition of ZrO 2 does not improve the we ar performance of the composite. In this case, the increase of the real cont act area appears to be dominant compared with the reduction of the shear strength due to the increase of the frictional temperature, leading to worse tribological behaviour . The second strategy involves microtexturing the surface of the best hybrid nanocomposite, i.e., PA66G30 + 5% graphene. Two different configurations of dimples were laser -micromachined, but no significant improvement was observed in the tribological results. 17 Acknowledgement s This work was supported by the Regional Government of Aragon, Spain (INNOVA -A1- 008-15) and the University of Zaragoza (BOA10- OTRI 2016/0172). The authors are members of the I+AITIIP (DGA -T08_17R) research group of the FEDER 2014- 2020 “Construyendo Europa desde Aragón” program, which is recogni sed by the Regional Government of Aragon. The authors acknowledge the AITIIP Foundation in Spain for providing facilities for sample preparation and the TEKNIKER Foundation in Spain for the loan of equipment for tribological measure ments .