14 Tribologie + Schmierungstechnik 63. Jahrgang 2/ 2016 1 Introduction Frictional wear is fatigue wear caused by relative oscillating tangential motion of tiny amplitude, between two surfaces in a tribological system [1]. It can occur under severe boundary friction conditions. Frictional wear creates significant damage at the contact faces. An effective way of reducing or preventing frictional wear is by applying the correct type and balance of white solid lubricants, which enables the surfaces to be separated by chemically reacted surface layers. In formulating such lubricants extensive experiments are required to test the formation of the reaction layer and the interactions of the ingredients of the formulation. 2 Lubricants against frictional wear According to the definition in [1] frictional wear occurs under oscillating motion of tiny amplitude. In a ball-onplate-configuration, for example, there is a high proportion of constantly overlapping contact area (see Figure 1). Aus Wissenschaft und Forschung * Simon Weber M. Eng. Prof. Dr.-Ing. Christian Busch Westsächsische Hochschule Zwickau Fakultät Automobil- und Maschinenbau, 08056 Zwickau Figure 1: With decreasing amplitude, the proportion of continuously contacting surface is higher [2] XPS, ToF-SIMS and tribological studies of the reaction layer of white solid lubricant pastes for preventing frictional wear S. Weber, C. Busch* Eingereicht: 6. 7. 2015 Nach Begutachtung angenommen: 1 .9. 2015 Spezielle reaktive weiße Festschmierstoffe in Pasten bilden Reaktionsschichten, die die Reibpartner effektiv trennen und dadurch Schwingungsverschleiß verhindern können. Es wurde eine Vielzahl an Versuchen durchgeführt, um eine neue Pastengeneration zu entwickeln. Die Kontaktfläche der Kugel-Platte-Tests wurde untersucht und die Reaktionsschicht analysiert. Dadurch ist ein Vergleich der Zusammensetzung der Reaktionsschichten aus konventionellen und neu entwickelten Pasten möglich Schlüsselwörter Schwingungsverschleiß, Reaktionsschicht, Festschmierstoffe, Paste, Oberflächenanalyse Special reactive white solid lubricants in pastes are able to form reaction layers, which can effectively separate the sliding partners and thereby prevent frictional wear. Extensive tests were carried out to develop a new generation of pastes. The contact area of the ball-on-disc-tests are analysed and the reaction layers examined. Therefore a comparison of the composition of the reaction layers from conventional and new pastes was possible. Keywords frictional wear, reaction layer, solid lubricant, paste, surface analysis Kurzfassung Abstract T+S_2_16 05.02.16 14: 23 Seite 14 Tribologie + Schmierungstechnik 63. Jahrgang 2/ 2016 White reactive solid lubricants have been proven to prevent frictional wear. These include inorganic solids, such as certain hydroxides and phosphates of calcium, iron, manganese and zinc. They are often used in synergistic combination to improve the efficiency. With the aid of carrier media in the form of pastes, greases or suspensions, they are applied to the surfaces. At tribo-mechanical excitation these substances form thin, adherent and separating reaction layers on the surface of the base material as a result of tribochemical reactions. The thickness of the reaction layers is about 50 - 150 nm, so the thickness is greater than the layers of oil-soluble AW or EP additives, which are based on phosphorus, sulfur or zinc. [1] It applies to any tribological test, that experiments on the specific machine element provide the best results. However, the required time and cost for the development of new lubricants are too high. Therefore, model systems must be used, especially the ball on disc geometry is common. The tribological performances of the pastes were tested on an Optimol SRV-IV tester. With adjusting the test parameters it is possible to perform rapid screening tests of many paste formulations [2, 3]. Together with further experiments on other test benches, it was possible to develop marketable solid lubricant pastes [3]. 3 Analysis of the reaction layers In the previous publication new paste formulations were tested for their ability to prevent frictional wear [3]. For the analysis of the reaction layers two of these pastes were chosen (see Table 1). Paste A is a conventional paste and B is a new paste development. For this new paste, only unclassified ingredients were used according to the EU-Regulations in contrast to paste A. Following SRV-test conditions were used for the analysis of the reaction layers: - Stroke: 140 µm - Frequency: 50 Hz - Load: 100 N - Temperature: 50 °C - Test duration: 15 min Several analytical methods have been applied in order to investigate the formation of the reaction layers. Light microscope, X-ray photoelectron spectroscopy (XPS) and Time-of-Flight ion mass spectrometry (ToF-SIMS) were the methods of choice. 3.1 Light microscope The use of the light microscope gives a quick overview of the reaction layers. Here it is possible to see the similarities and differences between the layers, which are important for the further analysis. At every reaction layer a more or less distinct thin transparent layer could be detected, which appears colored due to the interference of the light. In addition, there are always areas of the reaction layers that appear black under the light microscope. Figure 2 illustrates the reaction layers that were formed after the ball on disc experiments on the SRV. In these figures, it is clearly visible the different appearance of the reaction layers of the lubricants. While the formed layers using paste A appear rather dark and slightly colorful, the layers of paste B are mainly iridescent and transparent. 15 Aus Wissenschaft und Forschung Table 1: Overview of the tested pastes Formulation Main component of solid lubricant Paste A Calcium hydroxide Paste B Calcium carbonate Figure 2: Picture of the reaction layer of paste A (left) and paste B (right) 3.2 X-ray photoelectron spectroscopy (XPS) The XPS analysis was conducted in cooperation with the Faculty of Physical Engineering at the University of Applied Sciences Zwickau. For that, after the test runs on the SRV the samples were thoroughly cleaned with hexane and then immediately transferred into the vacuum chamber of the XPS apparatus. This should minimize the changes in the reaction layers. Following a brief cleaning with an ion beam to remove the residues of lubricants and cleaning agent, the actual XPS analysis was started. Table 2: XPS analysis of the reaction layer (in atom percentage) C O P Ca Fe Reference 17 19 0 0 64 Paste A 15 55 2 26 2 Paste B 13 58 6 23 0 T+S_2_16 05.02.16 14: 23 Seite 15 16 Tribologie + Schmierungstechnik 63. Jahrgang 2/ 2016 The analysis of the uppermost atomic layers of the reaction layers show that they mainly consist of calcium and oxygen compounds (see Table 2). When compared with the results of the reference surface, it is clearly seen that the iron of the base material of the friction pair is hardly present in the uppermost atomic layers of the reaction. Aus Wissenschaft und Forschung On the substrate surface of the 100Cr6 specimen (reference) about 18 % of each of carbon and oxygen atoms were detected. This percentage is due to lubricant residues and other adsorbents that remained on the surface because of the surface roughness despite the ion beam cleaning. The percentage of the calcium in the layers is nearly equal regardless of the tested lubricant. This leads to the conclusion that the calcium compound present in the reaction layer is similar. It could be assumed that a large part of the reaction layer consists of calcium oxide. Calcium carbonate (CaCO 3 ) and calcium hydroxide (Ca(OH) 2 ) cannot be the main components in the reaction layers, because the proportion of carbon and oxygen atoms are too small. The carbon content and a part of the oxygen content could be attributed to the absorbents. Thus, both calcium carbonate and calcium hydroxide can occur only as a small portion next to calcium oxide. 3.3 ToF-SIMS The analysis using time of flight secondary ion mass spectrometry (ToF-SIMS) was carried out at the University of Münster. This method of analysis examines the uppermost atomic layers of a surface. In this process, complex molecules can be detected in addition to elements. In the first step of the analysis surface scans of the reaction layers were performed. The surface was analyzed point by point according to a predetermined grid. Then, for each ion an image can be created with the local incidence. A selection of figures are shown in Table 3. The results are essentially similar to the XPS analysis. Iron is not present in the reaction layer, but instead primarily calcium and oxygen. For paste B the phosphate ions were still detectable as well as a small fraction of carbonate ions, but both were not detected for paste A. In both lubricants, the phosphate compound appears to act as a „starter“ for the formation of the reaction layer, as its ions were detected in both contact surfaces. The ball on disc test arrangement always starts Table 3: Results of ToF-SIMS Paste A Paste B Light microscope Iron ions (Fe + ) Calcium ions (Ca + ) Oxygen ions (O - ) Carbonate ions (CO 3- ) Phosphate ions (PO 3- ) T+S_2_16 05.02.16 14: 23 Seite 16 Tribologie + Schmierungstechnik 63. Jahrgang 2/ 2016 with larger oscillations, which then quickly adjusts to the nominal value. Hence, the detection of phosphate outside the actual contact area. Apparently, phosphate is the first to react with the surface, and then the formation of the reaction layer begins with calcium oxide. However, there is not enough time for this formation during the brief time with greater stroke. 4 Discussion In order to explain the formation mechanism for the calcium carbonate and calcium hydroxide film, the following path was proposed: The solid lubricant is decomposed to calcium oxide by the high surface pressure and thereby local heating in the surface asperities, then the calcium oxide is deposited as a layer on the substrate surface. For that, the local temperatures must be in the range of 400 °C - 700 °C because at these temperatures calcium hydroxide or calcium carbonate decomposes to calcium oxide (see Figure 3). The formed layer separates the friction partners and prevents the adhesive wear. More lubricant components such as phosphate compounds support the layer formation. The XPS analysis results are shown in Table 4 after the reaction layer of paste B was treated with the chelating agent disodium EDTA. For comparison, the reference measurements and the measurements of the untreated layer are also shown in Table 4. The calcium layer was almost completely removed by the chelating agent, which can be seen in the light microscope images (see Figure 4). The transparent, iridescent regions are no longer visible. The dark regions of the reaction layers remain, which consist mostly of iron and oxygen. This effect occurs with the reaction layers of conventional and new pastes. This indicates that the reaction layer is multi-layered: a layer of iron compounds is formed on top of the base material, which is not homogeneously present throughout the contact surface but is formed randomly and to different degrees (see Figure 2 and Figure 4 for comparison). 17 Aus Wissenschaft und Forschung Table 4: XPS analysis of the reaction layer to determine the effect of disodium EDTA (in atom percentage) C O P Ca Fe Reference 17 19 0 0 64 Paste B 13 58 6 23 0 Paste B + 31 26 1 2 40 disodium EDTA Figure 3: Thermogravimetric analysis of calcium hydroxide and calcium carbonate Figure 4: Pictures of the reaction layers before (left) and after (right) the treatment with disodium EDTA Figure 5: Schematic diagram of possible structure of the reaction layer T+S_2_16 05.02.16 14: 23 Seite 17 18 Tribologie + Schmierungstechnik 63. Jahrgang 2/ 2016 Above that is the calcium oxide transparent layer covering the base material and the formed iron layer. This structure is found for the reaction layer of the conventional paste as well as of the new developed paste. 5 Conclusion Within the research project it was shown that the white solid lubricants form a reaction layer that effectively separates the friction partners and thus can prevent frictional wear. The composition of the reaction layers were examined and it was made an assumption of the structure of these layers. 6 Acknowledgement The authors gratefully acknowledge funding by the German Ministry for Economic Affairs and Energy within the Central Innovation Program SME (Zentrales Innovationsprogramm Mittelstand - ZIM) under grant number KF2341607MF2. 7 Literature [1] Mang, T.: Encyclopedia of Lubricants and Lubrication. 2014, Springer, Heidelberg [2] Junghans, R., Neukirchner, J., Schneider, R.: Bedeutung der Testbedingungen für die Prüfung von Spezialschmierstoffen zur Verhinderung von Schwingungsverschleiß. 14 th International Colloquium Tribology, 13.-15.01.2004, TA Esslingen, Ostfildern [3] Weber, S., Busch, C.: Bedeutung der Testbedingungen für die Entwicklung von Hochleistungsschmierstoffen zur Verhinderung von Schwingungsverschleiß mittels des SRV ® -Prüfstandes nach DIN 51834. Tribologie und Schmierungstechnik, 2015, 62. Jg., Nr. 3 Aus Wissenschaft und Forschung Bestellcoupon Tribologie und Schmierungstechnik „Richtungsweisende Informationen aus Forschung und Entwicklung“ Getriebeschmierung - Motorenschmierung - Schmierfette und Schmierstoffe - Kühlschmierstoffe - Schmierung in der Umformtechnik - Tribologisches Verhalten von Werkstoffen - Minimalmengenschmierung - Gebrauchtölanalyse - Mikro- und Nanotribologie - Ökologische Aspekte der Schmierstoffe - Tribologische Prüfverfahren Bestellcoupon Ich möchte Tribologie und Schmierungstechnik näher kennen lernen. Bitte liefern Sie mir ein Probeabonnement (2 Ausgaben), zum Vorzugspreis von 7 39,-. So kann ich die Zeitschrift in Ruhe prüfen. Wenn Sie dann nichts von mir hören, möchte ich Tribologie und Schmierungstechnik weiter beziehen. Zum jährlichen Abo-Preis incl. Versand von 7 189,- Inland (incl. MwSt.) bzw. 7 198,- Ausland. (In der EU bei fehlender UID-Nr. zzgl. MwSt.). Die Rechnungsstellung erfolgt dann jährlich. 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