HUNGARIAN JOURNAL OF INDUSTRY AND CHEMISTRY Vol. 46(2) pp. 1-11 (2018) hjic.mk.uni-pannon.hu DOI: 10.1515/hjic-2018-0011 TRIBOLOGICAL BEHAVIOUR OF POLYMERS IN TERMS OF PLASMA TREATMENT: A BRIEF REVIEW HAYDER AL-MALIKI ∗1 AND GÁBOR KALÁCSKA1 1Faculty of Mechanical Engineering, Mechanical Engineering PhD School, Szent István University, Páter Károly utca 1, H-2100 Gödöllő, HUNGARY A review to enrich the literature concerning the effect of various plasmas on the tribological behaviour of polymers and monitor the developments of plasma for the modification of polymer surfaces over recent decades using up-to-date data. A comparative study of plasmas was conducted to identify the most useful and efficient ones which facilitate optimal improvements with regard to the characterizations of polymer surfaces and tribological properties. The studies included in this review strongly suggest that (besides Plasma-Immersion Ion Implantation, PIII) atmospheric plasmas (dielectric barrier discharges, DBD) are an effective technique in terms of modifying the characterizations of polymer surfaces thereby enhancing the tribological behaviour of polymers under different operating conditions that extends the operating life of elements within the machine. Keywords: polymer, plasma treatment, surface characterization, tribology 1. Introduction The term tribology originates from two Greek words, namely τριβω (tribo), a verb which means “I rub”; and the suffix -logy which is derived from λωγια (logia) which can be translated as “study of” or “knowledge of”. This word was introduced by the British scientists Bow- den, Tabor [1] and Jost in 1964 [2]. Luke Mitchell, a professor of lubrication, observed the problems caused by increasing the level of friction exerted on machines [3]. Tribology is the science which studies the princi- ples and applications of friction, wear and lubrication [4]. Friction and wear are widespread phenomena in our daily lives. Processes that result in friction occur when moving surfaces are in contact with each other [5]. Fric- tion is a consequence of two main non-interacting com- ponents (adhesion and deformation) and can be consid- ered as an approach for all materials including polymers. Polymers are nonpolar materials that exhibit unique tri- bological behaviour, and some of them possess good self- lubrication characteristics, examples of which are stated in [6, 7]. The rapid development of industry was accom- panied by the urgent need to decrease the size of equip- ment which in turn leads to a higher degree of friction and enhanced rates of wear to be exerted on the elements of machines. This prompted the evolution of several surface- modification techniques, in particular for polymers, to di- minish the disadvantages of friction and wear. The alter- ation of surface properties such as hydrophilicity, chemi- cal composition and roughness lead to inevitable changes ∗Correspondence: haidrlatif@gmail.com in tribological behaviour which provided an opportunity for researchers to govern such behaviour with regard to polymers. Therefore, specific techniques have been de- veloped over the last few years to modify the surfaces of polymers to improve frictional behaviour and adhe- sive bonding performance. A lot of chemical and physical techniques have been developed to treat the surfaces of polymers. Chemical techniques are the processes which deal with wet or chemical reactions on surfaces, for in- stance, wet etching, grafting, acid-induced oxidation and plasma polymerization. Physical techniques deal with the modification of physical surfaces such as corona dis- charge, ion or electron beams, photon beams, plasma dis- charge and oxidizing flames [8]. Experimentally, the chemical treatments of polymer surfaces exhibit some disadvantages such as localized corrosion and environmental pollution. In contrast, phys- ical treatments have been adapted recently to modify the surface of polymers [9]. One of the most effective tech- niques to modify the surfaces of polymers are plasmas which facilitate changes in the properties of polymer sur- faces by several processes, for instance, cleaning, abla- tion, crosslinking and surface chemical functionalization [10]. Recently, plasma surface treatment was referred to as the most accepted method of modifying the surface of polymeric materials due to its remarkable stability con- cerning the enhancement of surface properties compared to conventional techniques, in particular, atmospheric plasma treatment which has the ability to operate under ambient atmospheric conditions (in terms of temperature haidrlatif@gmail.com 2 AL-MALIKI AND KALÁCSKA and humidity) and does not require a vacuum [9,11]. The literature often suggests that plasma treatment may im- prove the hydrophilicity of polymers [12–16], which has also been demonstrated in our previous works [17–19]. Even though plasmas often increase the surface rough- ness of polymers [15, 20, 21], some studies have proven that a decrease in surface roughness might occur after shorter treatment times [16, 22]. The present study attempts to review the tribological aspects of polymeric materials treated by various plas- mas and compared to their pristine surface behaviour by taking into consideration the influence of plasmas on the characteristics of polymer surfaces which vastly governs the tribological behaviour of polymers. Finally, plasma types that may currently be recommended to utilize and reinforce the tribological properties of polymers are high- lighted. 2. Plasma Modification of Plastic Surfaces Plasma can be defined as a chemically active media. The composition of plasmas varies depending on the way they are generated and their working power. Plasmas produce either low or very high temperatures and according to the heat they generate they can be termed as cold or thermal plasmas. Thermal plasmas, especially arc plasmas, were widely industrialized by the aeronautic sector in partic- ular. Cold plasma technologies have evolved in the mi- croelectronics industry, but they have a limited use due to their vacuum equipment. There have been many attempts to transpose plasmas to work under atmospheric pressure without the need for a vacuum. The research has led to various sources that are described in [23]. It is known that plasma is the fourth state of mat- ter and it is more or less an ionized gas that constitutes about 99 % of the universe. Plasma consists of elec- trons, ions and neutrons; these constituents may exist in fundamental or excited states. From a laboratory point of view, plasma is electrically neutral. However, it con- tains some free charge carriers thus is electrically conduc- tive [24, 25]. The degree of plasma ionization can range from small values, e.g. 10−4 − 10−6 for partially ionized gases to 100 % for fully ionized gases. In a laboratory, two types of plasma can be generated; the first are high- temperature plasmas which are also referred to as fusion plasmas, the second are low-temperature plasmas or gas discharges [26]. 2.1 Classification of plasmas Plasmas can be differentiated into several groups depend- ing on the energy supply and amount of energy trans- ferred to them since the properties of a plasma change de- pending on electronic density or temperature as presented in Fig. 1 [23]. There are two major plasma groups - the first are thermal equilibrium plasmas and the second are those not in thermal equilibrium. Plasmas that consist of Figure 1: 2D classification of plasmas (electron tempera- ture versus electron density) [28]. particles (electrons, ions and neutral species) of uniform temperature are known as thermal equilibrium plasmas, for instance, stars and fusion plasmas. Usually they are termed as “local thermodynamic equilibrium plasmas” which is abbreviated to LTE. High temperatures typically ranging from 4,000 K to 20,000 K are required to obtain thermal equilibrium plasmas. Otherwise, plasmas that are not in thermal equilibrium, abbreviated as non-LTEs, will be formed, for example, interstellar plasma matter [25]. LTE and non-LTE plasmas can be distinguished accord- ing to the gas pressure the plasma is subjected to. A high gas pressure is indicative of many collisions in the plasma and it could be argued that just a few collisions occur in the plasma when the gas pressure is low, for ex- ample, dielectric-barrier discharge and atmospheric pres- sure glow discharge plasmas. More details about types, sources and applications of plasmas can be found in [27]. 2.2 Influence of plasmas on the surface char- acteristics of polymers Different types of plasma treatments have exhibited po- tential changes in the physical and chemical surface char- acteristics of polymers. The variation in wettability is a fundamental parameter that controls adhesion, lubrica- tion and/or interactions with molecules. The formation of polar groups at the surface following plasma treat- ments, such as carbonyl, carboxyl and hydroxyl groups, enhances the surface energy. The enhancement of wet- tability following plasma treatment can be a combined effect of surface functionalization and an increase in sur- face roughness. When surface grafting occurs relatively quickly, an increase in surface roughness has mainly been observed following longer treatment times [21]. Depend- ing on the selection of adequate parameters, different types of plasma treatment are used for either enhancing as well as diminishing adhesion or surface hardness. Experimentally, Tóth et al. [29] observed a significant improvement in the surface hardness of ultra-high molec- ular weight polyethylene (UHMWPE) following nitrogen plasma ion implantation (NPII). The hydrogen plasma Hungarian Journal of Industry and Chemistry TRIBOLOGICAL BEHAVIOUR OF POLYMERS 3 Figure 2: F/C atomic ratio vs. voltage. immersion ion implantation (HPIII) treatment of the sur- face of UHMWPE induces amelioration in terms of the scratch resistance, temperature, mean surface roughness, oxygen content, and surface hardness, while the surface slope and elastic modulus often decreased, but it is possi- ble to enhance these characteristics upon treatment. Low values of temperature and mean roughness favour the re- duction of the surface slope [30]. In parallel, the nanome- chanical, chemical and wear properties of the UHMWPE surface have been significantly enhanced following treat- ment with helium plasma immersion ion implantation (HePIII). Over the ranges of the parameters, the surface hardness increased by up to six times, whereas the loss in volume following multipass decreased by up to about 2.5 times [31]. In a related study, the influence of nitrogen plasma immersion ion implantation (NPIII) on the surface of UHMWPE was investigated as well. The results sug- gested a relative increase in surface hardness, macro- scopic temperature and mean surface roughness, while the loss in volume decreased. However, the elastic modu- lus decreased or maybe even increased depending on the actual parameter set applied in the process. According to the parameter range studied, a reduction in wear rate is strongly dependent on thermal effects [32]. A statistical study by Tóth et al. [33] that revised the literature of engi- neering plastics treated by plasma-based ion implantation (PBII) and plasma-based ion implantation and deposition (PBIID) concluded that there is a rapidly trending in the number of related publications. As a consequence, the ef- fect of nitrogen plasma-based ion implantation (NPBII) on poly(tetrafluoroethylene) (PTFE) was discovered by Kereszturi et al. [34]. The study observed that the F/C atomic ratio significantly decreased in an inverse relation- ship with the voltage as shown in Fig. 2. Meanwhile the surface roughness increased inversely with the voltage and correlated directly with fluence as shown in Fig. 3. Figure 3: Mean surface roughness of PTFE vs. fluence and voltage. Although a clear relationship between wear volume and the main process parameters was not observed, in gen- eral this was improved following treatment along with an increase in surface roughness and O/C atomic ratio. Furthermore, the water contact angle recorded increased under low voltages and high fluences. Atmospheric dielectric barrier discharge (DBD) plas- mas enhance the wettability and surface roughness of polypropylene (PP) proportionally to an increase in the duration of plasma exposure [35]. Also, the surface roughness of PP increases linearly with the exposure time of atmospheric DBD plasma due to degradation of the polymer surface and the formation of nodule- like features. These nodules are shaped by highly oxi- dized short fragments of polymer which are referred to as low-molecular-weight oxidized materials (LMWOMs) in the literature [15]. In a subsequent study, Kostov et al. [36] reported that modifications in terms of the sur- face of different engineering plastics, such as polyethy- lene terephthalate (PET), polyethylene (PE) and PP could be achieved by atmospheric pressure plasma jets (APPJ). The primary aim of this research was to identify the op- timal treatment conditions as well as compare the ef- fect of APPJs on another source of atmospheric pres- sure plasmas, e.g., DBD on the characteristics of poly- mer surfaces. As a consequence of APPJ treatment the surface roughness is increased as shown in Fig. 4. How- ever, nodule-like structures were produced as well, but were much smaller compared to those constituted in the previous study when the polymer was treated by DBD. This was attributed to the higher degree of polymer degra- dation during DBD treatment. The adhesion strength of PP and perfluoroalkoxy alkanes (PFAs) has been signifi- cantly enhanced due to atmospheric plasma treatment un- der several gas flows. The characteristics of the surface introduced hydrophilic functional groups where the level 46(2) pp. 1-11 (2018) 4 AL-MALIKI AND KALÁCSKA Figure 4: AFM images of PP (a) untreated; (b) 30 s treated; (c) 30 s treated and washed; (d) 60 s treated; (e) 60 s treated and washed. All treatments were conducted with sample reciprocations using a signal amplitude of 10 kV, frequency of 37 kHz and Ar flow of 1.3 l/min. of improvement changed in proportion to the duration of the treatment as illustrated in Fig. 5 [37]. Although at- mospheric pressure glow (APG) discharge plasma treat- ment is moderate in terms of energy characteristics, APG- derived fluoropolymers exhibit similar surface properties under conventional low-pressure plasmas [38]. Treating metals and polymers by a cold arc-plasma jet under at- mospheric pressure leads to a superficial improvement in hydrophilicity and a decrease in the water contact angle of these materials as shown in Fig. 6 [39]. On the other hand, the literature shows that modifi- cations to the surface of aromatic polymers by dielectric barrier discharge (DBD) can achieve a substantial degree of chemical functionalization on the polymer surface by applying relatively low or intermediate levels of power without suffering severe topographical damage [40]. The surface modification of a PET film by an atmospheric- pressure plasma in combination with different gas flows promptly improved its hydrophilicity and was followed by hydrophobic recovery after longer durations [41]. In parallel, significant changes in the morphology and reac- tivity of PET surfaces have been observed [42,43]. When Figure 5: Effect of gas blowing on wettability of PFA film. Figure 6: Water contact angles of various materials after treatment. exposed to air the processing parameters, such as dis- charge power, processing speed, processing duration, and electrode configurations, affect the nature and scale of changes to the surface. In general, longer durations (low processing speed and a high number of cycles) and high levels of power induced greater changes in the surface wettability of the polymer [44]. Among the various environmental gases studied, air and oxygen yielded the highest levels of hydrophilicity, while argon and nitrogen resulted in lower degrees of hy- drophilicity of the PET surface [45]. Cold atmospheric- pressure plasmas (surface dielectric barrier discharge - SDBD) improve the oxygen and nitrogen contents of PET [46]. In comparison to PET, the effects of plasma treat- ment on polyether ether ketone (PEEK) have been studied less frequently, but also lead to enhanced degrees of hy- drophilicity and adhesion [47] due to the incorporation of functional groups and greater surface roughness follow- ing DBD [48]. The hydrophilicity, which is governed by the oxygenation of PEEK following DBD in the air, also recovered after several months through loss of the struc- turally related functional groups, but it remained more stable than other non-aromatic polymers [49]. Nylon 6 (PA6) exhibited a reduction in surface roughness, and an increase in the O/C and N/C ratios due to diffuse copla- nar surface barrier discharge (DCSBD) plasma treatment in two different gas flows (nitrogen and oxygen). How- ever, the oxygen DCSBD plasma was more efficient in terms of modifying the surface compared to the nitro- gen equivalent [16]. Also, plasma treatment enhances the contact angle and alters the surface topography of Nylon 66 (PA66) [50]. Plasma pre-treatment of the surface of the biopolymer Poly L Lactic Acid (PLLA) leads to the formation of pits in the crystalline phase accompanied by a mild increase in surface roughness [51]. Hungarian Journal of Industry and Chemistry TRIBOLOGICAL BEHAVIOUR OF POLYMERS 5 3. Tribology of Plasma-treated polymer surfaces 3.1 Friction of polymers Many studies have dealt with the tribology of polymers [52, 53]. Hardness and the elastic modulus are governed by the penetration depth, maximum load and strain rate [54]. Microcuttings are a consequence of ploughing, and they may exacerbate the friction force under specific con- ditions. Friction that results from elastic hysteresis is known as the deformation component [55, 56]. “The me- chanical component results from the resistance of the softer material to "ploughing" by asperities of the harder one” [57]. The adhesive bonds generated between the sur- faces in the frictional contact are referred to as the adhe- sion component. The adhesion component is higher than the deformation (mechanical) equivalent [58]. The fric- tion force is equal to the sum of the adhesion and defor- mation components as shown in Eq. 1 [59]. From this, the reason for polymer transferred layers forming on the metal counterface during frictional contact can be deter- mined. The transferred films are an essential factor which must be taken into consideration whilst estimating the tri- bological behaviour of polymers [53]. The effect of load, sliding velocity and temperature on friction are also im- portant parameters that could influence the tribological behaviour of the polymer: Ff=Fa+Fd [N] (1) where Ff represents the friction force, Fa the adhesion component, and Fd the deformation component of fric- tion. Under low loads, Ff ≈ Fa since Fd is far smaller than Fa. 3.2 Wear of polymers Wear is the undesirable removal of layers from the sur- face of a material. Wear occurs when two materials come into contact as a result of movement. Mechanical stress, temperature and chemical reactions directly influence the characteristics of the surface layer. Polymers are sensi- tive to these factors due to their particular structure and mechanical behaviour. The interface temperature can be significantly higher than the temperature of the environ- ment. The wear of some polymers, that were examined by Lancaster [60] whilst being slid against steel, was found to be influenced by temperature with regard to polymers that the pass is minimum at the characteristic tempera- ture. The accurate classification with regard to the wear of a polymer is not a trivial matter due to the highly di- verse nature of such mechanisms [53, 61, 62]. However, abrasion, adhesion and fatigue are the three most com- mon types of polymer wear that occur during the sliding process. Figure 7: Pin-on-disc tribological test system : 1) base frame, 2) pin holder, 3) loading head, 4) positioning rail, 5) rotating steel disc. 3.3 Tribological tests of polymers The tribological behaviour of a specific material is not in- herent of its properties as far as strength, elastic modulus, etc., are concerned. The influence of friction and wear on the overall performance of a polymer is strongly depen- dent on the entire configuration of the test. To evaluate the polymer under operating conditions that occur in actual structures, test configurations have to simulate the main sliding mode as accurately as possible in terms of system pair materials, contact geometry, contact pressure, sliding motion, sliding velocity, environmental conditions, me- chanical stiffness, etc. As a consequence, regular basis tribo-test systems are assembled from test apparatus that has yet to be standardized. Several parameters should be taken into consideration to select the most favourable test system, i.e. the structure of the material (macro- or microstructure), contact con- ditions (whether the contacting bodies possess the same radii of curvature as in a pin-on-disc test or different radii of curvature as in the ball-on-disc test), and energy dis- sipation (sliding temperature) [63]. The real tribosystem of the elements of a polymer is considered to be the most dependable test, however, due to its expense and imprac- tical nature the test cannot be conducted regularly. The scale of tribological tests varies from ‘field tests’ that consist of ‘large-scale simulation tests’ on real com- ponents to ‘laboratory tests’ on artificial samples which are widespread in the field of the tribological research of polymers due to the small apparatus used and their rela- tively low cost and versatility with regard to the testing of various materials under different test conditions, e.g. pin-on-disc Fig. 7). 3.4 Tribological behaviour of the surface of polymers treated by plasmas It is hard to accurately predict the effects of plasma surface treatment on the tribological behaviour of poly- mers, due to the multitude of mechanisms that govern such processes. It can be expected that functionalization, 46(2) pp. 1-11 (2018) 6 AL-MALIKI AND KALÁCSKA (a) (b) (c) Figure 8: Tribological behaviour of pristine and NPIII-treated PA6; (a) dry conditions; (b) water lubrication conditions; and (c) run-out lubrication conditions. crosslinking and chain scission will affect their chemical and mechanical surface properties after plasma treatment, which in turn will alter friction and wear characteristics. However, it cannot be confirmed whether such alterations lead to improved tribological properties or not, this can only be ascertained after the treated surface is tested. The tribological behaviour of rubber treated by plasma has been thoroughly investigated, indicating im- provements in terms of friction and wear resistance [13]. The pin-on-disc test yields the lower friction coefficient of PC and PP, whereas PE and PS exhibit larger coef- ficients of friction after atmospheric plasma treatment compared to their pristine surfaces. Bismarck et al. [64] attributed the significant decrease with regard to the fric- tion coefficient of PC either to the most substantial re- duction in contact angle or the changes in the chemical and simultaneously the physical properties of PC after treatment. On the other hand, the increase in the extent of crosslinking after atmospheric plasma treatment resulted in lower levels of friction and wear of PEEK composites [65], while argon plasma treatment led to a higher friction coefficient of PET [12]. The effects of Nitrogen plasma immersion ion im- plantation (NPIII) on PET indicated improvements with regard to scratch resistance [66]. Kalácska et al. [67] demonstrated that the benefits of sliding tribological properties are strongly dependent on the sliding condi- tions: a lower degree of friction and enhanced wear per- formance of PET treated by PIII only occurred in the presence of low pv factors under dry or water-lubricated sliding conditions. Five minutes of atmospheric plasma treatment is sufficient to reduce the coefficient of fric- tion and enhance the wear properties of a UHMW-PE film under a constant pv factor [68]. In contrast, UHMW-PE treated by atmospheric pressure gas plasma does not ex- hibit a significant change in terms of wear properties after a treatment time of two minutes. Since the level of wear reduced by half after longer treatment times, the duration a polymer is exposed to a plasma plays a major role with regard to the tribological properties of the treated surface, in particular for UHMW- PE [69]. UHMW-PE treated by a cold argon plasma using dielectric barrier discharge (DBD) supports such a find- ing since (under dry conditions) an increase in the friction coefficient with treatment time was observed whereas the wear volume reduced over the treatment time due to mod- ifications of its surface. Under normal saline (N-saline) lubrication conditions, wear volume and friction were re- duced over the treatment time which is indicative of an increase in surface wettability and, therefore, enhanced surface lubrication capability [70]. Sagbas [71] reported an increase in friction and improvement in the wear prop- erties of conventional UHMW-PE after plasma treatment, whereas the friction coefficient was unaffected in terms of Hungarian Journal of Industry and Chemistry TRIBOLOGICAL BEHAVIOUR OF POLYMERS 7 Figure 9: Tribological testing under dry sliding conditions at 0.5; 1; and 2 MPa; including online measurements of coefficients of friction, (specific wear and displacement ∆h), and temperature. vitamin-E-blended UHMW-PE under the same test con- ditions. However, the wear factor slightly decreased com- pared to the significant improvement with regard to the wear properties of conventional UHMW-PE as a conse- quence of the highly cross-linked nature of conventional UHMW-PE. According to the brief review above with regard to the effect of some plasmas on different polymers, the ability of plasmas to alter the surface characterization and thereby the tribological behaviour of polymers is un- questionable. However, the tribological behaviour of the treated surface depends on several factors such as the type of plasma, treatment method, exposure time, operat- ing mechanism and lubrication conditions amongst many other factors. Kalácska et al. [72] studied the tribological behaviour of PA6 treated by nitrogen plasma immersion ion implantation (NPIII) under different conditions (dry, water and oil lubrication) and various pressure velocity factors (pv). The results show that the friction coefficient and specific wear of treated PA6 are lower than that of the untreated one under dry sliding conditions and low pv factors, while the friction coefficient, specific wear and contact temperature of treated PA6 are higher than the pristine equivalent under high pv factors (Fig. 8a). On the other hand, the water lubricant reduces the ad- hesive component of friction after treatment, which was reflected in the fluctuation of the friction behaviour of the treated surface (Fig. 8b). However, under continuous oil lubrication, no difference could be detected between the treated and untreated parameters, therefore, the run-out type was preferred in terms of detecting such a differ- ence. Test conditions in terms of run-out oil lubrication exhibit a lower friction coefficient for treated PA6 than the pristine equivalent in the presence of low pv factors as a consequence of an increase in the dispersive compo- nent (Fig. 8c). In contrast, our last work [73] elaborated on the ef- fect of atmospheric DBD plasma on the tribological be- haviour of PET under various test conditions (3 “dry” normal load and “run-out lubrication” constant normal load conditions). The effect of DBD plasma treatment on the characterization of the surface was interpreted as an enhancement of the surface energy and reduction in the surface roughness due to the melting of surface asperi- ties. However, increasing the surface wettability induces higher coefficients of friction according to Archard’s the- ory of friction [74]. The dry tribological test yielded un- expected behaviour where the coefficient of friction of the treated surface was lower than the pristine equiva- lent under different pv factors. Also, the specific wear and the vertical deformation were enhanced. However, the interface temperature of the treated surface was higher than the untreated one (Fig. 9). The reduction in the co- efficient of friction can be theoretically attributed to the smaller contributions of a deformation component. In a related context, a “run-out” oil lubrication test yielded a lower coefficient of friction for the treated surface due to a rise in the surface energy after treatment, leading to a favourable enhancement with regard to the adsorption of the lubricant as shown in Fig. 10. By using the analy- sis and comparison of the previously reviewed studies, it can be said that atmospheric DBD plasma can be an ef- fective technique nowadays for the treatment of polymer surfaces that aims to reduce friction and improve wear under dry and run-out lubrication conditions under vari- ous pv factors, especially under lower ones compared to other techniques, e.g. NPIII. 4. Conclusion In conclusion, the reviewed research and results have ap- parently shown the capability of plasmas to modify the 46(2) pp. 1-11 (2018) 8 AL-MALIKI AND KALÁCSKA Figure 10: Tribological testing under lubricated sliding and “run-out” conditions at 0.5 MPa. surfaces of polymers which results in a change in the tri- bological properties of surfaces. However, the effect of plasmas depends on several factors such as the sources of plasmas, vacuum, temperature, pressure, etc. The essen- tial points concerning the tribological behaviour of poly- mers that are subjected to plasma treatment are summa- rized below: • Plasmas can efficiently alter the characterization of polymer surfaces by utilizing different sources such as APPJ, PIII, APG, PBII and DBD and various vacuums which results in the enhancement of sur- face wettability and an increase in surface rough- ness with few exceptions. Atmospheric DBD plasma can achieve the optimum surface configuration in a shorter time. • There is a paucity of studies which deal with the tri- bological behaviour of polymers in terms of plasma treatment. Most studies focused on the effect of plas- mas on the tribology of engineering polymers espe- cially PET, PA6, and in some cases PEEK. How- ever, no studies appear to deal with Polyolefins, ex- cept some studies which investigate the tribology of UHMW-PE for medical applications in particular. • UHMW-PE exhibits a higher coefficient of fric- tion and improvements in specific wear after plasma treatment under dry conditions where cross-linking plays a prominent role in terms of controlling the wear rate upon plasma treatment. However, the lu- bricated tribological test of UHMW-PE resulted in a lower coefficient of friction due to an increase in wettability. • The comparative investigation of engineering poly- mers treated by different plasmas under the same conditions (PA6 treated by NPIII and PET treated by atmospheric DBD) can explain the remarkable effect of atmospheric DBD plasma on the tribolog- ical behaviour of PET under dry conditions where the friction coefficient of PET remained lower than the pristine equivalent, whilst subjected to different pv factors. In contrast, PA6 can only sustain low pv factors if a low coefficient of friction is desired in the case of NPIII treatment. • With regard to the aforementioned comparative in- vestigation, atmospheric DBD plasma yielded a unique reduction in the friction coefficient of PET throughout the duration of the test under “run-out” oil lubrication conditions. On the other hand, a slight reduction in the friction coefficient was recorded when PA6 was treated by NPIII under the same test conditions. REFERENCES [1] Field, J.: David Tabor. 23 October 1913-1926 November 2005, Biographical Memoirs of Fellows of the Royal Society, 2008 DOI: 10.1098/rsbm.2007.0031 [2] Jost, P.: Interview with Luminary Professor H. Pe- ter Jost - The Man Who Gave Birth to the Word “Tribology”. (N. C. Jim Fitch, Interviewer), online at: Machinerylubrication.com, accessed on 2017- 05-24 [3] Mitchell, L. (2012). The Fiction of Nonfriction, Ward, J. ed. Popular Science, 2012 281(5), 40 [4] Wikipedia: Tribology, online at: https://en.wikipedia.org/wiki/Tribology#cite_note- 1, accessed on 2017-10-24 [5] Myshkin, N.K.; Petrokovets, M.I.; Kovalev, A.V.: Tribology of polymers: adhesion, friction, wear, and mass-transfer, Tribology International, 2005 38(11- 12), 910-921 DOI: 10.1016/j.triboint.2005.07.016 [6] Briscoe, B.J.: Interfacial Friction of Polymer Com- posites: General Fundamental Principles, Compos- ite Materials Series, 1986 1, 25-59 DOI: 10.1016/B978- 0-444-42524-9.50006-5 [7] Singer, I.L.; Pollock, H. (Eds.): Fundamentals of friction: macroscopic and microscopic processes (Kluwer Academic Publisher, London) 1992 ISBN: 978-94-011-2811-7 [8] Akram, M.; Jansen, K.M.B.; Ernst, L.J.; Bhowmik, S.: Atmospheric plasma modification of poly- imide sheet for joining to titanium with high temperature adhesive, International Journal of Adhesion and Adhesives, 2016 65, 63-69 DOI: 10.1016/j.ijadhadh.2015.11.005 [9] Petrie, E.M.: Handbook of Adhesives and Sealants, Second Edition (McGraw-Hill Companies, Inc., New York) 2007 ISBN: 9780071479165 [10] Noeske, M.; Degenhardt, J.; Strudthoff, S.; Lom- matzsch, U.: Plasma jet treatment of five polymers at atmospheric pressure: surface modifications and the relevance for adhesion, International Journal of Adhesion and Adhesives, 2004 24(2), 171-177 DOI: 10.1016/j.ijadhadh.2003.09.006 Hungarian Journal of Industry and Chemistry https://doi.org/10.1098/rsbm.2007.0031 https://doi.org/10.1016/j.triboint.2005.07.016 https://doi.org/10.1016/B978-0-444-42524-9.50006-5 https://doi.org/10.1016/B978-0-444-42524-9.50006-5 https://doi.org/10.1016/j.ijadhadh.2015.11.005 https://doi.org/10.1016/j.ijadhadh.2015.11.005 https://doi.org/10.1016/j.ijadhadh.2003.09.006 https://doi.org/10.1016/j.ijadhadh.2003.09.006 TRIBOLOGICAL BEHAVIOUR OF POLYMERS 9 [11] Rotheiser, J.: Joining of Plastics, 3rd Edition (Carl Hanser Verlag GmbH & Co. KG, Cincinnati) 1998 ISBN: 978-1-56990-445-9 [12] Leggett, G.J.; Beake, B.D.: Development of surface morphology, local friction, and adhesion in plasma treated poly (ethylene terephthalate) films, Polymer Preprints(USA), 1998 39(2), 1228-1229 [13] Segu, D.Z.: NBR surface modification by Ar plasma and its tribological properties, Industrial Lubri- cation and Tribology, 2016 68(2), 227-232 DOI: 10.1108/ILT-05-2015-0062 [14] Kostov, K.G.; Hamia, Y.A.A.; Mota, R.P.; Dos San- tos, A.L.R.; Nascente, P.A.P.: Treatment of polycar- bonate by dielectric barrier discharge (DBD) at at- mospheric pressure, Journal of Physics: Conference Series, 2014 511(1), 012075. IOP Publishing DOI: 10.1088/1742-6596/511/1/012075 [15] Kostov, K.G.; Nishime, T.M.C.; Hein, L.R.O.; Toth, A.: Study of polypropylene surface modification by air dielectric barrier discharge operated at two dif- ferent frequencies, Surface and Coatings Technol- ogy, 2013 234, 60-66 DOI: 10.1016/j.surfcoat.2012.09.041 [16] Novák, I.; Popelka, A.; Valentín, M.; Chodák, I.; Špírková, M.; Tóth, A.; Kleinová, A.; Sedliačik, J.; Lehockı, M.; Marônek, M.: Surface behavior of polyamide 6 modified by barrier plasma in oxy- gen and nitrogen, International Journal of Polymer Analysis and Characterization, 2014 19(1), 31-38 DOI: 10.1080/1023666X.2014.850907 [17] Károly, Z.; Klébert, S.; Al-Maliki, H.; Pataki, T.: Comparison of NPIII and DBD Plasma Treatment in Terms of Wettability of PTFE and PA6, Scien- tific Bulletin Series C: Fascicle Mechanics, Tribol- ogy, Machine Manufacturing Technology, 2016 30, 47 [18] Al-Maliki, H.; Zoltán, Sz.; Róbert, K.; Kalácska, G.: Shear strength of polypropylene bonded joints using pristine and DBD plasma treated surfaces, Mechanical Engineering Letters R&D, SZIE, 2016 14, 71-77 [19] Al-Maliki, H.; Kalácska, G.: The effect of atmo- spheric DBD plasma on surface energy and shear strength of adhesively bonded polymer, Hungar- ian Agricultural Engineering, 2017 31, 52-58 DOI: 10.17676/HAE.2017.31.52 [20] Ionita, M.D.; Teodorescu, M.; Stancu, C.; Stancu, E.C.; Ionita, E.R.; Moldovan, A.; Acsente, T.; Baza- van, M.; Dinescu, G.: Surface modification of poly- mers at atmospheric pressure in expanding RF plasmas generated by planar dielectric barrier dis- charges, Journal of Optoelectronics and Advanced Materials, 2010 12(3), 777-782 [21] Nastuta, A.V.; Rusu, G.B.; Topala, I.; Chiper, A.S.; Popa, G.: Surface modifications of polymer induced by atmospheric DBD plasma in different configu- rations, Journal of Optoelectronics and Advanced Materials, 2008 10(8), 2038-2042 [22] Hergelová, B.; Homola, T.; Zahoranová, A.; Plecenik, T.; Kováëik, D.: Plasma surface modifica- tion of biocompatible polymers using atmospheric pressure dielectric barrier discharge, WDS’12 Pro- ceedings of Contributed Papers, Part II, pp. 128- 133, 2012 ISBN: 978-80-7378-225-2 [23] Tendero, C.; Tixier, C.; Tristant, P.; Desmai- son, J.; Leprince, P.: Atmospheric pressure plas- mas: A review, Spectrochimica Acta Part B: Atomic Spectroscopy, 2006 61(1), 2-30 DOI: 10.1016/j.sab.2005.10.003 [24] Chapman, B.: Glow Discharge Processes (Wiley, New York) 1980 ISBN: 978-04-7107-828-9 [25] Lieberman, M.A.; Lichtenberg, A.J.: Princi- ples of plasma discharges and materials pro- cessing, MRS Bulletin, 1994 30, 899-901 DOI: https://doi.org/10.1557/mrs2005.242 [26] Conrads, H.; Schmidt, M.: Plasma generation and plasma sources, Plasma Sources Science and Tech- nology, 2000 9(4), 441 IOP Publishing [27] Kalácska, G.; Károly, Z.; Klébert, S.; András, E.: NPIII Treatment towards to Cold Atmospheric Plasma as Surface Modification Technics of Engi- neering Polymers, Scientific Bulletin Series C: Fas- cicle Mechanics, Tribology, Machine Manufactur- ing Technology, 2015 29, 51 [28] Boulos, M.I.; Fauchais, P.; Pfender, E.: Thermal plasmas: fundamentals and applications (Springer Science & Business Media, New York) 2013 [29] Tóth, A.; Bertóti, I.; Szilágyi, E.; Dong, H.; Bell, T.; Juhász, A.; Nagy, P.M.: Surface characteriza- tion of ultrahigh molecular weight polyethylene af- ter nitrogen ion implantation, Surface and Inter- face Analysis, 2000 30(1), 434-438 DOI: 10.1002/1096- 9918(200008)30:1<434::AID-SIA788>3.0.CO;2-W [30] Tóth, A.; Mohai, M.; Ujvári, T.; Bertóti, I.: Hy- drogen plasma immersion ion implantation of ul- trahigh molecular weight polyethylene, Surface and Interface Analysis, 2006 38(4), 898-902 DOI: 10.1002/sia.2197 [31] Tóth, A.; Mohai, M.; Ujvári, T.; Bertóti, I.: Ad- vanced surface modification of ultrahigh molecular weight poly (ethylene) by helium plasma immersion ion implantation, Polymers for Advanced Technolo- gies, 2006 17(1112), 898-901 DOI: 10.1002/pat.788 [32] Tóth, A.; Bertóti, I.; Mohai, M.; Ujvári, T.: Sur- face modification of polyethylene by nitrogen PIII: Surface chemical and nanomechanical properties, Materials Science Forum, 2007 537, 255-262 DOI: 10.4028/www.scientific.net/MSF.537-538.255 [33] Tóth, A.; Kereszturi, K.; Mohai, M.; Bertóti, I.: Plasma based ion implantation of engineering poly- mers, Surface and Coatings Technology, 2010 204(18), 2898-2908 DOI: 10.1016/j.surfcoat.2009.12.004 [34] Kereszturi, K.; Tóth, A.; Mohai, M.; Bertóti, I.; Szépvölgyi, J.: Nitrogen plasma-based ion implan- tation of poly (tetrafluoroethylene): Effect of the main parameters on the surface properties, Applied 46(2) pp. 1-11 (2018) https://doi.org/10.1108/ILT-05-2015-0062 https://doi.org/10.1088/1742-6596/511/1/012075 https://doi.org/10.1016/j.surfcoat.2012.09.041 https://doi.org/10.1080/1023666X.2014.850907 https://doi.org/10.17676/HAE.2017.31.52 https://doi.org/10.17676/HAE.2017.31.52 https://doi.org/10.1016/j.sab.2005.10.003 https://doi.org/10.1016/j.sab.2005.10.003 https://doi.org/https://doi.org/10.1557/mrs2005.242 https://doi.org/https://doi.org/10.1557/mrs2005.242 https://doi.org/10.1002/1096-9918(200008)30:1%3C434::AID-SIA788%3E3.0.CO;2-W https://doi.org/10.1002/1096-9918(200008)30:1%3C434::AID-SIA788%3E3.0.CO;2-W https://doi.org/10.1002/sia.2197 https://doi.org/10.1002/sia.2197 https://doi.org/10.1002/pat.788 https://doi.org/10.4028/www.scientific.net/MSF.537-538.255 https://doi.org/10.4028/www.scientific.net/MSF.537-538.255 https://doi.org/10.1016/j.surfcoat.2009.12.004 10 AL-MALIKI AND KALÁCSKA Surface Science, 2010 256(21), 6385-6389 DOI: 10.1016/j.apsusc.2010.04.021 [35] Nishime, T.M.C.; Tóth, A.; Hein, L.R.O.; Kostov, K.G.: Surface characteristics analysis of polypropy- lene treated by dielectric barrier discharge at atmo- spheric pressure, Journal of Physics: Conference Series, 2012 370(1), 012025 IOP Publishing DOI: 10.1088/1742-6596/370/1/012025 [36] Kostov, K.G.; Nishime, T.M.C.; Castro, A.H.R.; Tóth, A.; Hein, L.R.D.O.: Surface modification of polymeric materials by cold atmospheric plasma jet, Applied Surface Science, 2014 314, 367-375 DOI: 10.1016/j.apsusc.2014.07.009 [37] Tsuchiya, Y.; Akutu, K.; Iwata, A.: Surface mod- ification of polymeric materials by atmospheric plasma treatment, Progress in Organic Coatings, 1998 34(1), 100-107 DOI: 10.1016/S0300-9440(97)00117- 3 [38] Prat, R.; Koh, Y.J.; Babukutty, Y.; Kogoma, M.; Okazaki, S.; Kodama, M.: Polymer deposition us- ing atmospheric pressure plasma glow (APG) dis- charge, Polymer, 2000 41(20), 7355-7360 DOI: 10.1016/S0032-3861(00)00103-8 [39] Toshifuji, J.; Katsumata, T.; Takikawa, H.; Sakak- ibara, T.; Shimizu, I.: Cold arc-plasma jet under at- mospheric pressure for surface modification, Sur- face and Coatings Technology, 2003 171(1), 302- 306 DOI: 10.1016/S0257-8972(03)00290-1 [40] Upadhyay, D.J.; Cui, N.Y.; Meenan, B.J.; Brown, N.M.D.: The effect of dielectric barrier discharge configuration on the surface modification of aro- matic polymers, Journal of Physics D: Applied Physics, 2005 38(6), 922 IOP Publishing DOI: 10.1088/0022-3727/38/6/022 [41] Gotoh, K.; Kobayashi, Y.; Yasukawa, A.; Ishigami, Y.: Surface modification of PET films by atmo- spheric pressure plasma exposure with three reac- tive gas sources, Colloid and Polymer Science, 2012 290(11), 1005-1014 DOI: 10.1007/s00396-012-2600-7 [42] Esena, P.; Riccardi, C.; Zanini, S.; Tontini, M.; Po- letti, G.; Orsini, F.: Surface modification of PET film by a DBD device at atmospheric pressure, Sur- face and Coatings Technology, 2005 200(1), 664- 667 DOI: 10.1016/j.surfcoat.2005.02.188 [43] Rashed, U.M.; Ahmed, H.; Al-Halwagy, A.; Garamoon, A.A.: Surface characteristics and print- ing properties of PET fabric treated by atmospheric dielectric barrier discharge plasma, The Euro- pean Physical Journal-Applied Physics, 2009 45(1), pp.11001-p1–11001-p5 DOI: 10.1051/epjap:2008197 [44] Liu, C.; Brown, N.M.; Meenan, B.J.: Uniformity analysis of dielectric barrier discharge (DBD) pro- cessed polyethylene terephthalate (PET) surface, Applied Surface Science, 2006 252(6), 2297-2310 DOI: 10.1016/j.apsusc.2005.04.016 [45] Onsuratoom, S.; Rujiravanit, R.; Sreethawong, T.; Tokura, S.; Chavadej, S.: Silver loading on DBD plasma-modified woven PET surface for antimi- crobial property improvement, Plasma Chemistry and Plasma Processing, 2010 30(1), 191-206 DOI: 10.1007/s11090-009-9199-6 [46] Novák, I.; Popelka, A.; Luyt, A.S.; Chehimi, M.M.; Špírková, M.; Janigová, I.; Kleinováa, A.; Stopkaf, P.; Šloufe, M.; Vankog, V.; Chodá, I.; Valentin, M.: Adhesive properties of polyester treated by cold plasma in oxygen and nitrogen atmospheres, Sur- face and Coatings Technology, 2013 235, 407-416 DOI: 10.1016/j.surfcoat.2013.07.057 [47] Zhang, S.; Awaja, F.; James, N.; McKenzie, D.R.; Ruys, A.J.: A comparison of the strength of autohesion of plasma treated amorphous and semi-crystalline PEEK films, Polymers for Ad- vanced Technologies, 2011 22(12), 2496-2502 DOI: 10.1002/pat.1791 [48] Luo, H.; Xiong, G.; Ren, K.; Raman, S.R.; Liu, Z.; Li, Q.; Ma, C.; Li, D.; Wan, Y.: Air DBD plasma treatment on three-dimensional braided carbon fiber-reinforced PEEK composites for enhancement of in vitro bioactivity, Surface and Coatings Technology, 2014 242, 1-7 DOI: 10.1016/j.surfcoat.2013.12.069 [49] Upadhyay, D.J.; Cui, N.Y.; Anderson, C.A.; Brown, N.M.D.: Surface recovery and degradation of air dielectric barrier discharge processed poly (methyl methacrylate) and poly (ether ether ketone) films, Polymer Degradation and Stability, 2005 87(1), 33- 41 DOI: 10.1016/j.polymdegradstab.2004.07.006 [50] Labay, C.P.; Canal Arias, J.M.; Navarro Sentanyes, A.; Canal Barnils, C.: Comparison of the effects of corona and low pressure plasma on the release of caffeine from PA66 filaments, 51st Dornbirn Man- made Fibers Congress Lectures CD, pp. 1-6, 2012 [51] Slepička, P.; Michaljaničová, I.; Švorčík, V.: Con- trolled biopolymer roughness induced by plasma and excimer laser treatment, Express Polymer Let- ters, 2013 7(11), 950-958 DOI: 10.3144/expresspolym- lett.2013.92 [52] Kragelskii, I.V.: Friction and Wear (Pergamon Press, Elmsford) 1982 ISBN: 978-00-8027-591-8 [53] Kovalev, N.K.; Myshkin, A.V.: Adhesion and fric- tion of polymers, in: Polymer Tribology, Eds.: Su- jeet, K.S.; Briscoe, B.J. (Imperial College Press, Singapore) 2009 DOI: 10.1142/9781848162044_0001 [54] Kovalev, A.; Shulha, H.; Lemieux, M.; Myshkin, N.; Tsukruk, V.V.: Nanomechanical probing of lay- ered nanoscale polymer films with atomic force microscopy, Journal of Materials Research, 2004 19(3), 716-728 DOI: 10.1557/jmr.2004.19.3.716 [55] Bowden, F.P.; Tabor, D.: Friction and lubrication of solids (Clarendon Press, Oxford) 1964 ISBN: 019851204X [56] Moore, D.F.: The Friction and Lubrication of Elas- tomers (Pergamon Press, Oxford) 1972 ISBN: 978-00- 8016-749-7 Hungarian Journal of Industry and Chemistry https://doi.org/10.1016/j.apsusc.2010.04.021 https://doi.org/10.1016/j.apsusc.2010.04.021 https://doi.org/10.1088/1742-6596/370/1/012025 https://doi.org/10.1088/1742-6596/370/1/012025 https://doi.org/10.1016/j.apsusc.2014.07.009 https://doi.org/10.1016/j.apsusc.2014.07.009 https://doi.org/10.1016/S0300-9440(97)00117-3 https://doi.org/10.1016/S0300-9440(97)00117-3 https://doi.org/10.1016/S0032-3861(00)00103-8 https://doi.org/10.1016/S0032-3861(00)00103-8 https://doi.org/10.1016/S0257-8972(03)00290-1 https://doi.org/10.1088/0022-3727/38/6/022 https://doi.org/10.1088/0022-3727/38/6/022 https://doi.org/10.1007/s00396-012-2600-7 https://doi.org/10.1016/j.surfcoat.2005.02.188 https://doi.org/10.1051/epjap:2008197 https://doi.org/10.1016/j.apsusc.2005.04.016 https://doi.org/10.1007/s11090-009-9199-6 https://doi.org/10.1007/s11090-009-9199-6 https://doi.org/10.1016/j.surfcoat.2013.07.057 https://doi.org/10.1002/pat.1791 https://doi.org/10.1002/pat.1791 https://doi.org/10.1016/j.surfcoat.2013.12.069 https://doi.org/10.1016/j.surfcoat.2013.12.069 https://doi.org/10.1016/j.polymdegradstab.2004.07.006 https://doi.org/10.3144/expresspolymlett.2013.92 https://doi.org/10.3144/expresspolymlett.2013.92 https://doi.org/10.1142/9781848162044_0001 https://doi.org/10.1557/jmr.2004.19.3.716 TRIBOLOGICAL BEHAVIOUR OF POLYMERS 11 [57] Yamaguchi, Y.: Tribology of plastic materials (El- sevier, Tokyo) 1990 ISBN: 978-00-8087-580-4 [58] Bely, V.A.; Sviridenok, A.I.; Petrokovets, M.I.; Savkin, V.G.: Friction and Wear in Polymer-Based Materials (Pergamon Press, Oxford) 1982 [59] Kalácska, G.: An engineering approach to dry fric- tion behaviour of numerous engineering plastics with respect to the mechanical properties, Express Polymer Letters, 2013 7(2), 199-210 DOI: 10.3144/ex- presspolymlett.2013.18 [60] Lancaster, J.K.: Relationships between the wear of polymers and their mechanical properties, Proceed- ings of the Institution of Mechanical Engineers, Conference Proceedings, 1968 183(16), 98-106 [61] Myshkin, N.K.; Kim, C.K.; Petrokovets, M.I.: In- troduction to tribology (Chong Moon Gak, Seoul) 1997 [62] Blau, P.: Friction and wear transitions of materials (Noyes Publication, New York) 1989 ISBN: 978-08- 1551-196-0 [63] Samyn, P.; Schoukens, G.; Quintelier, J.: Scal- ing Effects in Tribotesting of Polymers, in: Poly- mer Tribology, Eds.: Sujeet, K.S.; Briscoe, B.J. (Imperial College Press, Singapore) 2009 DOI: 10.1142/9781848162044_0003 [64] Bismarck, A.; Brostow, W.; Chiu, R.; Hagg Lob- land, H.E.; Ho, K.K.: Effects of surface plasma treatment on tribology of thermoplastic polymers, Polymer Engineering & Science, 2008 48(10), 1971-1976 DOI: 10.1002/pen.21103 [65] Zhang, R.; Häger, A.M.; Friedrich, K.; Song, Q.; Dong, Q.: Study on tribological behaviour of plasma-treated PEEK and its composites, Wear, 1995 181, 613-623 DOI: 10.1016/0043-1648(95)90177-9 [66] Kereszturi, K.; Tóth, A.; Mohai, M.; Bertóti, I.: Surface chemical and nanomechanical alterations in plasma immersion ion implanted PET, Surface and Interface Analysis, 2008 40(34), 664-667 DOI: 10.1002/sia.2643 [67] Kalácska, G.; Zsidai, L.; Kereszturi, K.; Mo- hai, M.; Tóth, A.: Sliding tribological proper- ties of untreated and PIII-treated PETP, Applied Surface Science, 2009 255(11), 5847-5850 DOI: 10.1016/j.apsusc.2009.01.017 [68] Samad, M.A.; Satyanarayana, N.; Sinha, S.K.: Tri- bology of UHMWPE film on air-plasma treated tool steel and the effect of PFPE overcoat, Surface and Coatings Technology, 2010 204(9), 1330-1338 DOI: 10.1016/j.surfcoat.2009.09.011 [69] Perni, S.; Kong, M.G.; Prokopovich, P.: Cold atmo- spheric pressure gas plasma enhances the wear per- formance of ultra-high molecular weight polyethy- lene, Acta Biomaterialia, 2012 8(3), 1357-1365 DOI: 10.1016/j.actbio.2011.12.007 [70] Naresh, K.N.; Yap, S.L.; Khan, M.Z.; Pattela, S.R.S.: Effect of Argon Plasma Treatment on Tribological Properties of UHMWPE/MWCNT Nanocomposites, Polymers, 2016 8(8), 295 DOI: 10.3390/polym8080295 [71] Sagbas, B.: Effect of argon plasma surface mod- ification on tribological behavior of biopolymers, Industrial Lubrication and Tribology, 2016 68(4), 508-514 DOI: 10.1108/ILT-11-2015-0176 [72] Kalácska, G.; Zsidai, L.; Keresztes, R.; Tóth, A.; Mohai, M.; Szépvölgyi, J.: Effect of nitrogen plasma immersion ion implantation of polyamide-6 on its sliding properties against steel surface, Wear, 2012 290, 66-73 DOI: 10.1016/j.wear.2012.05.011 [73] AlMaliki, H.; Zsidai, L.; Samyn, P.; Szakál, Z.; Keresztes, R.; Kalácska, G.: Effects of atmospheric plasma treatment on adhesion and tribology of aro- matic thermoplastic polymers, Polymer Engineer- ing & Science,(Early View) DOI: 10.1002/pen.24689 [74] Archard, J.: Contact and rubbing of flat surfaces, Journal of Applied Physics, 1953 24(8), 981-988 DOI: 10.1063/1.1721448 46(2) pp. 1-11 (2018) https://doi.org/10.3144/expresspolymlett.2013.18 https://doi.org/10.3144/expresspolymlett.2013.18 https://doi.org/10.1142/9781848162044_0003 https://doi.org/10.1142/9781848162044_0003 https://doi.org/10.1002/pen.21103 https://doi.org/10.1016/0043-1648(95)90177-9 https://doi.org/10.1002/sia.2643 https://doi.org/10.1002/sia.2643 https://doi.org/10.1016/j.apsusc.2009.01.017 https://doi.org/10.1016/j.apsusc.2009.01.017 https://doi.org/10.1016/j.surfcoat.2009.09.011 https://doi.org/10.1016/j.surfcoat.2009.09.011 https://doi.org/10.1016/j.actbio.2011.12.007 https://doi.org/10.1016/j.actbio.2011.12.007 https://doi.org/10.3390/polym8080295 https://doi.org/10.3390/polym8080295 https://doi.org/10.1108/ILT-11-2015-0176 https://doi.org/10.1016/j.wear.2012.05.011 https://doi.org/10.1002/pen.24689 https://doi.org/10.1063/1.1721448 Introduction Plasma Modification of Plastic Surfaces Classification of plasmas Influence of plasmas on the surface characteristics of polymers Tribology of Plasma-treated polymer surfaces Friction of polymers Wear of polymers Tribological tests of polymers Tribological behaviour of the surface of polymers treated by plasmas Conclusion