Tribological and Mechanical Behavior of Graphite Composites of Polytetrafluoroethylene (PTFE) Irradiated by the Electron Beam

This research investigated the effect of irradiation with an electron beam energy of 10 MeV in doses of 26–156 kGy on polytetrafluoroethylene (PTFE) with a 15% and 20% graphite additive. The research has shown that mechanical (compression strength, hardness, and Young’s modulus) and sclerometric (coefficient of wear micromechanism and coefficient of resistance to wear) properties improve and tribological wear decreases as graphite content increases. Electron beam irradiation increases the degree of crystallinity of both materials to a similar extent. However significant differences in the improvement of all examined properties have been demonstrated for PTFE with higher (20%) graphite content subjected to the electron beam irradiation. This polymer is characterized by higher hardness and Young’s modulus, reduced susceptibility to permanent deformation, higher elasticity, compression strength, and above all, a nearly 30% reduction in tribological wear compared to PTFE with a 15% graphite additive. The most advantageous properties can be obtained for both of the examined composites after absorbing a dose of 104 kGy. The obtained results hold promise for the improvement of the operational life of friction couples which do not require lubrication, used for example in air compressors and engines, and for the possibility of application of these modified polymers. In particular PTFE with 20% graphite content, in the nuclear and space industry.


Introduction
Polytetrafluoroethylene (PTFE) has favorable properties like high chemical and biological tolerance, thermal stability, as well as its excellent dielectric, antifriction, and antiadhesive properties, polytetrafluoroethylene (PTFE) is a preferable material for the manufacture of articles in different industries and engineering [1][2][3][4][5][6]. For tribological applications, PTFE is also an expedient polymer material that is widely available. The structure of PTFE molecules causes transfer of material thin film onto the surface during friction improving value of the friction coefficient [7][8][9]. It is a known fact that during tribological interaction pure PTFE without technological defects resulting from the

Materials and Methods
The research material was polytetrafluoroethylene with 15% and 20% graphite filling (SM-G15 and SM-G20, Inbras, Tarnów, Poland). The rods were made by sintering from PTFE suspension grounded and granulated. The additive used was electrographite with a maximum grain size of 63 µm, a minimum carbon content of 95%, including 80% in the graphite structure. Test specimens were cut out in the form of pins with a diameter of 5 mm (tribological tests) and cylinders with a diameter of 20 mm (other tests). The finished material was subjected to irradiation with an electron beam on an accelerator with an energy of 10 MeV and a power of 10 kW (NPO Torij, Moscow, Russia). Dosages from 26-156 kGy (2.6-15.6 Mrad) were used. The process was carried out at room temperature in a vacuum. Then the samples were annealed at 200 • C for 4 h and slowly cooled to room temperature.

Examination of Thermal Properties of PTFE-Graphite Composites
The degree of crystallinity of the tested composites was determined by differential scanning calorimetry (DSC) on a Mettler-Toledo DSC 1 device (Mettler-Toledo GmbH, Greifensee, Switzerland). Samples were about 0.015 g and they were cut from the central part of the cylinder and sealed in Polymers 2020, 12,1676 3 of 13 aluminum cells. The heating rate was 0.167 • C/S. Thermograms were registered for the melting process at temperatures from −40 to 400 • C, and for the recrystallization process from 400 • C to −40 • C. Registered data allowed to determine the degree of crystallinity χ c , using Formula (1) [25]. The tests were carried out on polymers in the initial state and after exposure to an electron beam.
where: ∆H c -heat of phase transition from a DSC thermogram [cal/g]; ∆H f -heat of crystal phase transition of PTFE (19,585 cal/g). The heat ∆H c , using Formula (2), allows one to calculate the number average molecular weight M n of polytetrafluoroethylene-graphite composites [26]:

Compressive Strength Tests of PTFE-Graphite Composites
Uniaxial compression of polytetrafluoroethylene composites with 15% and 20% graphite content was performed on Instron 5982, with a constant deformation rate of 4.2 × 10 −3 s −1 . The deformation was performed up to half the height of the samples, then the system was unloaded without supporting and the maximum stress was determined σ max = P max /A 0 , where A 0 is the area of the initial cross-section of the tested samples, and P max is the maximum force recorded at 50% compression. The measurements were performed at a temperature of 21 ± 1 • C, for each tested variant 5 repetitions were performed.

Tests of Micromechanical Properties
Microindentation tests were performed on a Micron-Gamma device (manufactured by the Aviation Faculty, Technical University of Kiev, Kiev, Ukraine). For measurements, a Berkovich indenter with a pyramid shape and an angle between the center line and each wall equal to 65.3 • was used. Measurement parameters were as follows: maximum load-1 N and holding time-15 s. A load-unload curve was recorded in real time, which allows the determination using the Oliver-Phare method [27], instrumental hardness H, instrumental Young E module, and allowed analysis of the work of indentation (total deformation-W tot , plastic deformation-W pl , and elastic deformation-W sp ). Six impressions were made for all variants. A self-leveling table was used to increase the accuracy of measurements.

Surface Scratch Tests
The scratches were made using the Revetest device (Anton-Paar, Graz, Austria). A Berkovich indenter with a diameter of 200 µm was used in this study. The scratch test parameters were as follows: maximum load 4 N, crack length 4 mm, scratch speed 5.4 mm·min −1 . Three scratches were made for each the tested variants. The profilographic measurements allowed to determine the surface of the groove and elevation (A i and B i ) and on their basis calculate the coefficient of wear mechanism β and the wear resistance coefficient W β [28][29][30][31]: where:

Polytetrafluoroethylene-Graphite Composite Wear Tests
Tribological studies of polymers were carried out using a T-01 tribometer (manufactured by ITeE Radom, Poland). Polytetrafluoroethylene composites with 15% and 20% graphite content were tested both at the initial state and after electron beam irradiation. Three pins were prepared. The tests were performed in a pin-on-disk configuration. AISI 321 acid resistant steel (1H18N9T) and titanium grade 2 were used as discs. The surfaces of the discs were ground on 360-1200 grades to obtain a uniform surface roughness of Ra = 0.2 µm. This treatment was performed deliberately to allow faster formation of a thin film from the tested composites during the friction process, limiting the coefficient of friction. Tribological test conditions were: load 20 N (1 MPa), slip speed 10 cm/s, friction path 1 km, friction path diameter 2.4 cm. Ambient conditions in accordance with VAMAS (Versailles Project on Advanced Materials and Standards) guidelines and ASTM G-99 standard (Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus) [32]. Stereometric analysis of wear traces on the pins was made using a profilograph, and then the surfaces were visualized in 3D using Talymap and Matlab software.

Thermal Studies
It has been determined based on DSC runs that electron-beam irradiation of PTFE with a 15% and 20% graphite additive induced a change in thermal properties of composites ( Figure 1, please find the original data in Supplementary Materials). The calculations were made on the basis of peaks shown on DSC scans illustrating the melting enanthropicity and crystallization of polyterafluoroethylene and graphite composites. On DSC scans recorded for polytetrafluoroethylene, two main peaks can be seen: the first at a temperature of about 10-40 • C and the second at a temperature of 320-340 • C. The first is related to crystal form transitions (they are most probably attributable to triclinic/hexagonal and hexagonal/pseudo-hexagonal transitions of the crystalline part of the polymer). The other is related to polymer melting. Irradiation with electron flux affected both changes in molecular weight and degree of crystallinity. The results are shown in Figures 1-5.
First of all, it is clear that for a polymer with higher graphite content the heat of crystallization ∆H c was approximately 20% lower in the initial state than for a composite with 15% graphite content ( Figure 2). Electron beam irradiation gradually increases ∆H c for both of the examined composites. The heat of crystallization did not change significantly with increase of the dose absorbed by the composites above 52 kGy.
As it results from Formula (2), the polymer composites' heat of crystallization is closely related to their molecular weight M n ( Figure 3). The reduction in molecular weight observed for both PTFE composites with 15% and 20% graphite content along with the increase in the absorbed dose of electron beam is influenced by the PTFE chain disruption reactions. A polymer with higher graphite content is characterized by significantly higher average molecular weight in its initial state and, consequently, a more intensive decrease in M n after electron beam irradiation.
A significant increase in the crystallinity degree of PTFE with 15% and 20% graphite as a function of electron beam irradiation is shown in Figure 4. As mentioned above, when PTFE-graphite composites absorb high doses of radiation, the molecular weight decreases due to the process of chain splitting [33]. It induces due to greater mobility and lower entanglement between polymer particles, which are much more favorable conditions for the formation of new crystallites. The authors of the papers reached similar conclusions [19,34].
With increasing absorbed radiation dose, a linear increase in melting point T m of PTFE was observed for both graphite content variants ( Figure 5). Furthermore, a polymer with 20% graphite content was characterized by lower temperature T m in its initial condition and higher intensity of growth after electron beam irradiation. Authors of paper [34], who examined PTFE with no additives, obtained similar results after irradiating the polymer with an electron beam of energy of 1.5 MeV and 10 kGy doses. First of all, it is clear that for a polymer with higher graphite content the heat of crystallization ΔHc was approximately 20% lower in the initial state than for a composite with 15% graphite content ( Figure 2). Electron beam irradiation gradually increases ΔHc for both of the examined composites. The heat of crystallization did not change significantly with increase of the dose absorbed by the composites above 52 kGy. As it results from Formula (2), the polymer composites' heat of crystallization is closely related to their molecular weight Mn (Figure 3). The reduction in molecular weight observed for both PTFE composites with 15% and 20% graphite content along with the increase in the absorbed dose of electron beam is influenced by the PTFE chain disruption reactions. A polymer with higher graphite content is characterized by significantly higher average molecular weight in its initial state and, consequently, a more intensive decrease in Mn after electron beam irradiation. As it results from Formula (2), the polymer composites' heat of crystallization is closely related to their molecular weight Mn (Figure 3). The reduction in molecular weight observed for both PTFE composites with 15% and 20% graphite content along with the increase in the absorbed dose of electron beam is influenced by the PTFE chain disruption reactions. A polymer with higher graphite content is characterized by significantly higher average molecular weight in its initial state and, consequently, a more intensive decrease in Mn after electron beam irradiation. A significant increase in the crystallinity degree of PTFE with 15% and 20% graphite as a function of electron beam irradiation is shown in Figure 4. As mentioned above, when PTFE-graphite composites absorb high doses of radiation, the molecular weight decreases due to the process of chain splitting [33]. It induces due to greater mobility and lower entanglement between polymer particles, which are much more favorable conditions for the formation of new crystallites. The authors of the papers reached similar conclusions [19,34].  With increasing absorbed radiation dose, a linear increase in melting point Tm of PTFE was observed for both graphite content variants ( Figure 5). Furthermore, a polymer with 20% graphite content was characterized by lower temperature Tm in its initial condition and higher intensity of growth after electron beam irradiation. Authors of paper [34], who examined PTFE with no additives, obtained similar results after irradiating the polymer with an electron beam of energy of 1.5 MeV and 10 kGy doses. On this basis, it can be assumed that the effect of electron beam irradiation on thermal properties  With increasing absorbed radiation dose, a linear increase in melting point Tm of PTFE was observed for both graphite content variants ( Figure 5). Furthermore, a polymer with 20% graphite content was characterized by lower temperature Tm in its initial condition and higher intensity of growth after electron beam irradiation. Authors of paper [34], who examined PTFE with no additives, obtained similar results after irradiating the polymer with an electron beam of energy of 1.5 MeV and 10 kGy doses. On this basis, it can be assumed that the effect of electron beam irradiation on thermal properties will have a direct influence on PTFE's mechanical properties and wear resistance of both polymers' (PTFE with 15% C and PTFE with 20% C). On this basis, it can be assumed that the effect of electron beam irradiation on thermal properties will have a direct influence on PTFE's mechanical properties and wear resistance of both polymers' (PTFE with 15% C and PTFE with 20% C).

Mechanical Properties of PTFE
In spite of a lack of large differences in thermal properties for both of the examined composites subjected to irradiation, significant differences are noticeable in mechanical properties of PTFE with 20% carbon content compared to PTFE with 15% C. Electron beam irradiation of both examined composites causes changes in hardness H and Young's modulus E, which increase proportionately to the absorbed radiation dose (Figure 6a,b). Changes in the hardness and Young's modulus of PTFE are the consequence of the increase in the crystallinity. It was also found that a polymer with higher graphite content was characterized by 15-25% higher hardness compared to PTFE with a 15% graphite additive. As far as Young's modulus is considered, differences between the composites are minor. However, greater increase in the elasticity modules can be observed due to the effect of the electron beam on a polymer with 20% graphite content. Both parameters (H, E) were subject to a considerable increase, especially in the range of 52-156 kGy, as irradiation i-multiplicity increased. 20% carbon content compared to PTFE with 15% C. Electron beam irradiation of both examined composites causes changes in hardness H and Young's modulus E, which increase proportionately to the absorbed radiation dose (Figure 6a,b). Changes in the hardness and Young's modulus of PTFE are the consequence of the increase in the crystallinity. It was also found that a polymer with higher graphite content was characterized by 15-25% higher hardness compared to PTFE with a 15% graphite additive. As far as Young's modulus is considered, differences between the composites are minor. However, greater increase in the elasticity modules can be observed due to the effect of the electron beam on a polymer with 20% graphite content. Both parameters (H, E) were subject to a considerable increase, especially in the range of 52-156 kGy, as irradiation i-multiplicity increased. The course of load-unload curves, especially the analysis of the area under the curve allows to determine the parameters of the indentation work. The material's resistance to deformation affects the value of the indentation work determined by the depth, volume and surface of the impressions. The total indentation work Wtot is the sum of the plastic deformation works Wpl and the elastic deformation. The area calculations were made using Matlab software.
The performed tests have shown (Figure 7a-c) that introducing additional graphite content reduces the value of total work of indentation. Further decrease in Wtot is observed as the electron beam irradiation dose absorbed by both of the examined polymers increases. Similar dependences occur in the case of work of plastic (Wpl) and elastic deformation (Wel), which reflects hardness increasing along with the growing radiation dose. Furthermore, lower susceptibility to plastic deformation ( Figure 7b) and superior elastic properties (Figure 7c) can be observed in the case of a composite with higher graphite content than in PTFE with 15% graphite content. The course of load-unload curves, especially the analysis of the area under the curve allows to determine the parameters of the indentation work. The material's resistance to deformation affects the value of the indentation work determined by the depth, volume and surface of the impressions. The total indentation work W tot is the sum of the plastic deformation works W pl and the elastic deformation. The area calculations were made using Matlab software.
The performed tests have shown (Figure 7a-c) that introducing additional graphite content reduces the value of total work of indentation. Further decrease in W tot is observed as the electron beam irradiation dose absorbed by both of the examined polymers increases. Similar dependences occur in the case of work of plastic (W pl ) and elastic deformation (W el ), which reflects hardness increasing along with the growing radiation dose. Furthermore, lower susceptibility to plastic deformation (Figure 7b) and superior elastic properties (Figure 7c) can be observed in the case of a composite with higher graphite content than in PTFE with 15% graphite content. Polymers 2020, 12, x; doi: FOR PEER REVIEW 9 of 14 Differences in graphite content also directly influence PTFE's compression strength Rc ( Figure  8). Polytetrafluoroethylene with 20% graphite content was characterized 25%-35% higher strength than PTFE with 15% C. Electron beam irradiation causes intensive growth of Rc of both of the examined composites up to the size of the absorbed dose (104 kGy); absorption of a dose higher than 104 kGy caused deterioration of mechanical properties of the composites. At a high radiation dose (>104 kGy), strength of a polymer decreases due to chain scission in the presence of air, consequently, Rc decreases as the radiation dose increases. This may also be caused by the occurrence of radiationinduced oxidation on the surface of the polymers in the presence of air [35]. During irradiation at high doses the oxide layer diffuses into the bulk of the polymer and reduces mechanical properties. Differences in graphite content also directly influence PTFE's compression strength R c (Figure 8). Polytetrafluoroethylene with 20% graphite content was characterized 25%-35% higher strength than PTFE with 15% C. Electron beam irradiation causes intensive growth of R c of both of the examined composites up to the size of the absorbed dose (104 kGy); absorption of a dose higher than 104 kGy caused deterioration of mechanical properties of the composites. At a high radiation dose (>104 kGy), strength of a polymer decreases due to chain scission in the presence of air, consequently, R c decreases as the radiation dose increases. This may also be caused by the occurrence of radiation-induced oxidation on the surface of the polymers in the presence of air [35]. During irradiation at high doses the oxide layer diffuses into the bulk of the polymer and reduces mechanical properties.

The Influence the Addition of Graphite and Irradiation on the Scratch Test Parameters of PTFE
Stereometric analysis of scratch traces provide information on wear mechanism β and resistance to wear W β of both of the examined polymers. According to the obtained data (Figure 9a), polytetrafluoroethylene with 20% carbon content is subject to the machining mechanism to a lesser degree than PTFE with 15% C after electron beam irradiation. This means that a larger part of the furrow material undergoes plastic deformation during the scratch test and is elevated on the edge of the scratch formed. The most advantageous results for both examined composites were obtained for the absorbed dose of 104 kGy. This was corroborated by an increase in wear resistance coefficient W β (Figure 9b), which causes a significant reduction in tribological wear compared to the initial material. Coefficient W β increases intensively, especially in the 26-104 kGy range. However, a dose of 156 kGy causes an intensive reduction in this parameter, confirming the fact that high radiation doses lead to degradation of the plastic.

The influence the Addition of Graphite and Irradiation on the Scratch Test Parameters of PTFE
Stereometric analysis of scratch traces provide information on wear mechanism β and resistance to wear Wβ of both of the examined polymers. According to the obtained data (Figure 9a), polytetrafluoroethylene with 20% carbon content is subject to the machining mechanism to a lesser degree than PTFE with 15% C after electron beam irradiation. This means that a larger part of the furrow material undergoes plastic deformation during the scratch test and is elevated on the edge of the scratch formed. The most advantageous results for both examined composites were obtained for the absorbed dose of 104 kGy. This was corroborated by an increase in wear resistance coefficient Wβ (Figure 9b), which causes a significant reduction in tribological wear compared to the initial material. Coefficient Wβ increases intensively, especially in the 26-104 kGy range. However, a dose of 156 kGy causes an intensive reduction in this parameter, confirming the fact that high radiation doses lead to degradation of the plastic.

The influence the Addition of Graphite and Irradiation on the Scratch Test Parameters of PTFE
Stereometric analysis of scratch traces provide information on wear mechanism β and resistance to wear Wβ of both of the examined polymers. According to the obtained data (Figure 9a), polytetrafluoroethylene with 20% carbon content is subject to the machining mechanism to a lesser degree than PTFE with 15% C after electron beam irradiation. This means that a larger part of the furrow material undergoes plastic deformation during the scratch test and is elevated on the edge of the scratch formed. The most advantageous results for both examined composites were obtained for the absorbed dose of 104 kGy. This was corroborated by an increase in wear resistance coefficient Wβ (Figure 9b), which causes a significant reduction in tribological wear compared to the initial material. Coefficient Wβ increases intensively, especially in the 26-104 kGy range. However, a dose of 156 kGy causes an intensive reduction in this parameter, confirming the fact that high radiation doses lead to degradation of the plastic.

Wear Properties of PTFE
The reduction in tribological wear is the most important effect of radiation modification from the point of view of commercial applications of the examined composites. Figure 10 shows linear wear W L PTFE with a 15% and 20% graphite additive as a function of the absorbed irradiation dose. It is clear that already in the initial state an increase in graphite content causes a reduction in linear wear both in the case of interaction with 1H18N9T steel (by 25%) and grade 2 titanium (by 15%). Electron beam irradiation further reduces wear of both of the examined composites. The most advantageous results were obtained for polytetrafluoroethylene with 20% graphite content after absorbing a dose of 104 kGy. When steel was used as a counterpartner, the reduction in linear wear compared to the initial state was almost four-fold and more than five-fold when compared to a polymer with 15% graphite content, also in its initial state. A 2.5 fold wear reduction was observed, respectively, in the case of tribological interaction with titanium. Irradiation above a dose of 104 kGy led to polymer degradation. This effect was also visible during tribological tests. W L , increased again for a dose of 156 kGy, especially for PTFE with 20% graphite content.
absorbing a dose of 104 kGy. When steel was used as a counterpartner, the reduction in linear wear compared to the initial state was almost four-fold and more than five-fold when compared to a polymer with 15% graphite content, also in its initial state. A 2.5 fold wear reduction was observed, respectively, in the case of tribological interaction with titanium. Irradiation above a dose of 104 kGy led to polymer degradation. This effect was also visible during tribological tests. WL, increased again for a dose of 156 kGy, especially for PTFE with 20% graphite content. Stereometric tests of the friction surface performed with a profilographometer after tribological tests confirmed the reduction in wear. Convexities and concavities, arranged as bands oriented along the motion direction, were found on the friction surface of both examined composites. During the interaction the surface becomes smooth, which smoothness increased with a higher irradiation dose, up to 104 kGy ( Figure 11). Such surface morphology indicates the occurrence of lower plastic deformation as well as reduced transport of the surface material during the tribological process. Polymer degradation which occurs when applying higher radiation doses causes friction properties of PTFE to deteriorate. Stereometric tests of the friction surface performed with a profilographometer after tribological tests confirmed the reduction in wear. Convexities and concavities, arranged as bands oriented along the motion direction, were found on the friction surface of both examined composites. During the interaction the surface becomes smooth, which smoothness increased with a higher irradiation dose, up to 104 kGy ( Figure 11). Such surface morphology indicates the occurrence of lower plastic deformation as well as reduced transport of the surface material during the tribological process. Polymer degradation which occurs when applying higher radiation doses causes friction properties of PTFE to deteriorate. compared to the initial state was almost four-fold and more than five-fold when compared to a polymer with 15% graphite content, also in its initial state. A 2.5 fold wear reduction was observed, respectively, in the case of tribological interaction with titanium. Irradiation above a dose of 104 kGy led to polymer degradation. This effect was also visible during tribological tests. WL, increased again for a dose of 156 kGy, especially for PTFE with 20% graphite content. Stereometric tests of the friction surface performed with a profilographometer after tribological tests confirmed the reduction in wear. Convexities and concavities, arranged as bands oriented along the motion direction, were found on the friction surface of both examined composites. During the interaction the surface becomes smooth, which smoothness increased with a higher irradiation dose, up to 104 kGy ( Figure 11). Such surface morphology indicates the occurrence of lower plastic deformation as well as reduced transport of the surface material during the tribological process. Polymer degradation which occurs when applying higher radiation doses causes friction properties of PTFE to deteriorate.

Conclusions
1. Electron beam irradiation gradually increased crystallization heat ΔHc for both examined composites. The PTFE with higher graphite content was characterized by 20% lower value of ΔHc compared to PTFE+15%C in the initial state. These studies also showed that along with the pattern of absorbed radiation dose and progressive chain cleavage reactions of both PTFEgraphite composites, there is a decrease in the molecular weight of polymers.