FORMATION OF PHYSICAL INTERACTIONS IN POLYMER-STEEL FRICTION PAIRS

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1 T R I B O L O G I A 153 Jacek PRZEPIÓRKA *, Marian SZCZEREK ** FORMATION OF PHYSICAL INTERACTIONS IN POLYMER-STEEL FRICTION PAIRS KSZTAŁTOWANIE FIZYKALNYCH ODDZIAŁYWAŃ W POLIMEROWO-STALOWYM WĘŹLE TARCIA Key words: friction coefficient, polyethylene, steel Słowa kluczowe: współczynnik tarcia, polietylen, stal Abstract This article presents the results of tribological studies of metal-polymer pairs by taking into account a wide range of the externally applied parameters of friction pairs (pressure, rubbing speed), reaching the limits of polymer flow. Adhesion is the dominant mechanism of the wear of polymer-steel friction pairs; it triggers off the transfer of the polymer onto the metal as a result of local joining between the chains of hydrocarbons and the active centres located on the surface of the steel element. The main reason for this phenomenon is the high oxidation susceptibility of the metal element and, as a result, a tendency for oxide formation on its surface. This is a spontaneous phenomenon that is taking place in a short time, and the degree of over-reaction depends on the duration of * ** Kazimierz Pulaski University of Technology and Humanities, Faculty of Materials Science and Design, ul. Chrobrego, Radom, Poland The Institute for Sustainable Technologies National Research Institute, Tribology Department, ul. Pułaskiego 6/10, Radom, Poland.

2 154 T R I B O L O G I A exposure, humidity, temperature, the presence of impurities on the surface of the metal element, and many other factors. Therefore, limiting the formation of oxides on the surface of the metal element is a key issue related to enhancing the resistance of a polymer-metal friction pair resistance to adhesive wear. INTRODUCTION The wear of a polymer material can follow several mechanisms resulting in a continuous mass decrement of contacting elements being in relative motion. One can identify mechanical wear and interfacial (adhesive) wear. Mechanical wear occurs mainly as an effect of grinding or fatigue cracking and the chipping of material particles. Interfacial wear is a result of the adhesion of collaborating surfaces or the chemical potential of elements of the tribological system. Depending on the dominant process, adhesive wear or corrosive (chemical) wear can be distinguished [L. 1]. The wear of polymer-metal friction pairs is strictly related to the process of polymer transfer onto the surface of the metal; as a result, after a certain time, the collaboration between the metal and the plastic changes into collaboration within a plastic-plastic pair [L. 2 3]. The transfer of the polymer contributes to a slower wear of the metal part and, in some friction conditions, also to a slower wear of the plastic element. However, in this scenario, the heat balance of the pair is markedly impaired due to the insulating nature of plastics. The process of the transfer of polymer material particles is complex due to the possibility of transferred particles going back from the metal surface to the original surface. A key role in the process is attributed to adhesion phenomena [L. 2, 4 7]. Particles of the polymer material can aggregate as wear products and be expelled outside the friction zone. The material is transferred irrespective of the friction conditions. The features of the transferred polymer material, such as the layer thickness, the form and dimensions of transferred particles and the degree of crystallization, strongly depend on the design characteristics of the collaborating elements and the working conditions [L. 8, 11]. The majority of experimental studies described in the literature focus on improving the friction characteristics of polymer-metal friction pairs by modifying the polymers with low friction fillers applied as additives to oils and lubricants used in conventional metal friction pairs. In some conditions, these efforts may decrease the friction force or even polymer wear. However, the prerequisite for a radical improvement in the metal-polymer collaboration is decreasing the impact of the adhesion of the mating surfaces. OBJECT AND METHODOLOGY OF THE STUDY The object of the study were specimens made of ultra-high molecular weight polyethylene made in the form of a rod, and applying treatment parameters that did not cause heating of the material (Figure 1). The counter specimens were

3 T R I B O L O G I A 155 made of two types of steel (Figure 2) subjected to grinding to achieve similar surface smoothness and a repeatable shape, with the height of the microroughness in the range of 1 to 1.4 µm. a) b) Fig. 1. Scanning AFM images of polyethylene: a) surface, b) 3D visualization Rys. 1. Obraz struktury geometrycznej powierzchni próbki z polietylenu uzyskany techniką AFM: a) analizowany obszar próbki, b) wizualizacja 3D a) b) Fig. 2. Scanning AFM images of steel specimens: a) steel, b) N47817 steel Rys. 2. Obraz struktury geometrycznej powierzchni próbek ze stali uzyskany techniką AFM: a) stal C45, b) stal N47817 The tribological study carried out on the T-15 tester (Figure 3) consisted in rubbing the specimen and the counter specimen along a specific friction path in specified working conditions established in preliminary studies (the load and the rubbing speed) in dry friction in the presence of oxygen and an inert gas (argon). The trial was a tribological experiment involving specimens made from the materials under study: UHMW polyethylene and counter specimens made of C45 steel and N47817 (stainless) steel. The parameters adopted for the main study carried out on the T-15 tester, based on an analysis of preliminary study results, were as follows: Contact pressure 0.3 MPa, Rubbing speed 0.2 m/s,

4 156 T R I B O L O G I A Friction path 1,100 m, Temperature of the friction pair environment 21±1 C, Relative humidity of air 30±7% For each material pair, at least three test runs were performed at the specified rubbing speeds of the friction pair. a) b) Fig. 3. Testing rig: a) T-15 view, b) friction pair Rys. 3. Stanowisko badawcze: a) widok ogólny T-15, b) węzeł tarcia The friction characteristics of the combination of materials were determined based on previous studies and measurement data archived in the memory of the measurement system in real time. STUDY RESULTS AND DISCUSSION The results of the tribological study at different rubbing speeds are presented in Figures 4 6. The correlation between the friction force and the type of material collaborating with polyethylene in the presence of oxygen and argon is shown in Figure 4. For C45 steel, the friction force was considerably reduced for the pair working in the presence of argon. As for N47817 steel, the lack of oxygen in the friction area did not contribute to such a significant reduction in friction resistances. An analysis of the degree of total wear of the elements of the friction pair (Figure 5) shows an analogy to motion resistances. For the studied materials, the wear depended on the friction force. The smallest wear was recorded for stainless steel working in an oxygen-free environment. The lowest temperature measured near the friction pair was noted for C45 steel working in the presence of argon (Figure 6). Special attention should be drawn to the fact that, for N47817 steel, which was affected by the smallest wear in collaboration with polyethylene, the temperature of the friction pair did not change when the pair worked in the presence of argon.

5 T R I B O L O G I A 157 Friction in oxygen Friction in argon Friction force [N] Fig. 4. Friction force obtained for the tested pairs in oxygen and in argon Rys. 4. Zestawienie siły tarcia polietylenu PE współpracującego ze stalą w obecności tlenu i argonu Friction in oxygen Friction in argon Wear [µm] Fig. 5. Wear obtained for the tested pairs in oxygen and in argon Rys. 5. Zestawienie zużycia polietylenu PE współpracującego ze stalą w obecności tlenu i argonu

6 158 T R I B O L O G I A Friction in oxygen Friction in argon Temperature [ºC] Fig. 6. Temperatures obtained for the tested pairs in oxygen and in argon Rys. 6. Zestawienie temperatury węzła tarcia polietylenu PE współpracującego ze stalą w obecności tlenu i argonu SUMMARY Comparisons of the results obtained in the tribological studies of pairs working in oxygen and in argon indicate that, among the steel materials working in oxygen, the best tribological characteristics were recorded for stainless steel. However, when the environment was oxygen-free, the material proved a less valuable partner for polyethylene. The opposite situation took place for C45 steel which, when exposed to oxygen, was characterized by high wear, friction force and temperature. However, in the presence of argon, it proved to be the best material. This phenomenon may be attributed to a significant impact of oxides present on the metal surface on the tribological features. C45 steel is easily oxidized and, for this reason, it formed hydrogen bridges with active polyethylene centres in the presence of oxygen. In contrast, stainless steel has low oxidative capacity; therefore, the polar interactions with polyethylene had a smaller impact on the results. On the other hand, during tests carried out in argon, the surface of C45 steel with oxides removed mechanically was not capable of forming lasting bonds with the PE surface. Therefore, the values of the measured correlations were substantially reduced. The situation is different for stainless steel, which oxidizes to a very limited extent even in oxygen. Consequently, the test conducted in argon did not affect a change in the friction and wear characteristics to such a significant extent as for 45 steel. REFERENCES 1. Rymuza Z.: Trybologia polimerów ślizgowych. Wydawnictwo Naukowo- -Techniczne. Warszawa 1986.

7 T R I B O L O G I A Polak A.: Przenoszenie materiału w łożysku ślizgowym stal-tworzywo sztuczne. Monografia, Politechnika Krakowska Ziemiański K.: Zastosowanie Tworzyw sztucznych w budowie maszyn. Politechnika Wrocławska, Wrocław Boizian C.: Molecular engineering of the polymer-metal Bond. Puer App/Chem. Vol. 64, No. 11, 1992, pp Cameron F., Abrams C.F.: Inhomogeneous Coarse-Graining of Polymer and Polymer/Metal Interfaces. NIC series, vol. 23, 2004, pp Tambe N.S.: Nanotribological investigation of materials for nanotechnology application. The Ohio State University, Grundmeier G., Stratmann M.: Adhesion and De-adhesion mechanisms at Polymer/Metal interfaces. Annual Review of Material Research. Max-Plank Institute for Iron Research. Vol. 35, 2005, pp Starczewski L.: Wodorowe zużywanie ciernych elementów maszyn. Monografia. Sulejówek Cameron F., Abrams C.F.: Inhomogeneous Coarse-Graining of Polymer and Polymer/Metal Interfaces. NIC series, vol. 23, 2004, pp Kelsall R., W., Hamley I., W., Geoghegan M.: Nanotechnologie. Wydawnictwo naukowe PWN, Warszawa Capanidis D.: Mechanizm tarcia i zużywania wieloskładnikowych kompozytów na osnowie polioksymetylenu. Oficyna Wydawnicza Politechniki Wrocławskiej, Wrocław Streszczenie W artykule przedstawiono wyniki badań tribologicznych skojarzeń materiałowych stal-polimer, uwzględniając szeroki zakres wymuszeń parametrów badań węzła tarcia (naciski powierzchniowe, prędkość poślizgu), dochodząc do granic płynięcia polimeru. Dominującym mechanizmem zużywania polimerowo-metalowych węzłów tarcia jest adhezja powodująca przenoszenie polimeru na metal na skutek powstawania lokalnych szczepień pomiędzy łańcuchami węglowodorów a aktywnymi centrami zlokalizowanymi na powierzchni elementu metalowego. Głównym powodem występowania tego zjawiska jest duża skłonność elementu metalowego do utleniania się, a w konsekwencji do powstawania na jego powierzchni tlenków żelaza. Jest to zjawisko występujące samorzutnie w bardzo krótkim czasie a stopień przereagowania uzależniony jest od czasu ekspozycji, wilgotności, temperatury, obecności na powierzchni elementu metalowego zanieczyszczeń i wielu innych czynników. Ograniczenie powstawania na powierzchni elementu metalowego tlenków jest więc kluczowym zagadnieniem związanym ze zwiększaniem odporności na zużycie polimerowo-metalowych węzłów tarcia.