1 Introduction In modern heavy industries manifold high temperature applications are standard procedures. Production processes often include operations at elevated temperatures and subsequent treatment of heated goods can be critical to involved components’ surfaces. Machining components which are exposed to high temperatures experience high stresses on the surface and bulk material [1-3]. Additionally to high temperature applications, compacting processes and press operations demand high resilience to plastic deformation [4] of the surface and to adhesion of substrate materials. In general, release agents are used to prevent bonding events and protect the surfaces. A main function of release agents is to represent a barrier and eliminate adhesion between the surface of the mold and the substrate material [5-7]. Adhesion of material can be detrimental to production processes and could cause a substantial loss in production efficiency. The purpose of this work was to investigate the frictional behavior and possible wear reduction abilities of various industrial release agents. Results should indicate the capability of each release agent to act as a barrier between contacting steel surfaces and prevent destructive effects on the surface. Therefore, a ring-on-disc tribometer [8, 9] with steel samples was utilized at two different elevated temperatures. Using the ring geometry allowed for accurate control of the contact area. Besides, continuous unidirectional motion was favored as well in order to best represent application-oriented parameters. Subsequently, coefficient of friction values were evaluated and steel sample surfaces were examined for wear appearances to get an overall picture of the tribological behavior [10]. Aus Wissenschaft und Forschung 21 Tribologie + Schmierungstechnik · 65. Jahrgang · 6/ 2018 Improvement of friction behavior and wear resistance in high temperature applications through release agents L. Widder, A. Nevosad, F. Reichmann, K. Adam* Trennmittel haben großes Potential die Lebensdauer von Komponenten in Hochtemperatur-Verdichtungsprozessen in vielen Bereichen der Schwerindustrie zu erhöhen. Um Abnutzungserscheinungen vorzubeugen, müssen Trennmittel die Reibung vermindern und Oberflächenwechselwirkungen reduzieren. In dieser Arbeit wurden mehrere Trennmittel mit unterschiedlicher Zusammensetzung in einem Ring-Scheibe-Tribometer bei erhöhten Temperaturen untersucht. Zwei verschiedene anwendungsorientierte Testtemperaturen wurden verwendet, um gängige Betriebsparameter zu simulieren. Dabei konnte gezeigt werden, dass sich das Reibungsverhalten nach dem Aufbringen von Trennmitteln auf die Oberflächen signifikant verbesserte, während bei bestimmten Trennmitteln eine starke plastische Verformung sogar gänzlich verhindert werden konnte. Schlüsselwörter Tribologie, Hochtemperatur, Reibung, Verschleiß, Tribotest, Reibkoeffizient, Trennmittel Release agents have high potential to increase component life-time in high temperature compaction processes of many heavy industries. To prevent wear occurrences, release agents need to decrease friction and reduce surface interactions. In this work several release agents with varying composition were investigated in a ring-on-disc tribometer at elevated temperatures. Two different application-oriented test temperatures were used to simulate common operational parameters. It could be shown that the friction behavior significantly improved with the application of release agents onto the surfaces, whereas severe plastic deformation could be prevented even entirely for certain release agents. Keywords Tribology, High temperature, Friction, Wear, Tribotesting, Coefficient of friction, Ring-on-disc, Release agent Kurzfassung Abstract * DDipl.-Ing. Lukas Widder Dipl.-Ing., Dr. Andreas Nevosad AC2T research GmbH, A 2700 Wiener Neustadt Dipl.-Ing. Frank Reichmann Carl Bechem GmbH, D 58089 Hagen Ing. Karl Adam voestalpine Stahl GmbH, A 4020 Linz T+S_6_18.qxp_T+S_2018 29.10.18 17: 05 Seite 21 ments on an Alicona InfiniteFocus G5 were performed. Comparative frictional behavior of the tested release agents was assessed through plots of coefficients of friction (CoF) over test duration. 3 Results and discussion The purpose of this work was to evaluate the frictional behavior and wear protection potential of different release agents. The tests were carried out at two particular temperatures. Representative curves of the resulting CoF values for each release agent are compared in the diagrams of Figure 2. At testing temperatures of 500 °C, significant differences can be observed. To begin with, an initial peak of friction Aus Wissenschaft und Forschung 22 Tribologie + Schmierungstechnik · 65. Jahrgang · 6/ 2018 2 Experimental section Release agents used within the scope of this publication were provided by Carl Bechem GmbH and consisted of three water-based dispersions containing graphite (release agent A), calcium hydroxide (re-lease agent B), or organic additives (release agent C). For all tests the concentrated released agents were diluted with deionized water to solutions with applicationoriented concentrations, as provided by the manufacturing company. For application of the release agent samples onto the surface the steel discs featuring the ring structure were preheated to 200 °C for at least 1 h. Subsequently, the samples were fully immersed into the diluted release agents, removed after 1-2 seconds and subsequently air dried. Tribometric measurements were performed on a high temperature ring-on-disc tribometer previously developed at AC2T research GmbH. For all tribotests C45 steel samples with a diameter of 90 mm were used. For the counterparts discs with a superimposed ring (6 mm in diameter, 1 mm height) were produced. The sample geometry is shown in Figure 1 and was achieved by means of machine turning with original surface roughnesses of Ra 2.0 µm (ring) and Ra 1.8 µm (disc). Test temperatures of 200 °C and 500 °C were used and the applied force was set to 0.74 kN, which corresponds to 1 N/ mm 2 in the contact area. Before starting the test run, the force was applied and the load was kept during the heating procedure until the final test temperature was reached to reduce oxidation processes on the ring surface. The temperature was measured inside the steel samples via temperature sensors. For the rotatory motion of the ring rotational velocities of 0.24 m/ s were applied, i. e. one 360° cycle per second. The duration for gradual increase and attenuation of the ring velocity was set to 1 s, whereas the entire test run at homogeneous speeds lasted 5 s. All tests were performed three times with new samples using equal operating parameters. To evaluate the surface degenerations of used steel samples optical light microscopy (Olympus SZX16) and high resolution optical 3D-topographical focus-variation assess- Figure 1: Sample geometry of used ring (a) and disc (b) reference samples Figure 2: Coefficient of friction diagrams for HT-RRV tribotests at 500 °C (a) and 200 °C (b) T+S_6_18.qxp_T+S_2018 29.10.18 17: 05 Seite 22 values was seen for three release agents. Sample A did not lead to such elevated friction. The highest initial peak was seen for dry reference tests, when no release agent was applied. It is assumed that during the preheating phase surface reactions of the containing calcium hydroxide (release agent B, beginning of dehydration reactions) and organic salts (release agent C, degeneration), respectively, with contacting counter parts occurred. Additional force to break these bonds resulted in described peaks. Subsequently, graphite containing sample A also exhibited lowest friction values over the entire test duration together with sample C, which lead to similar CoF values after ~ 20 % of the test duration. Sample B yielded significantly higher friction values, as did tests only using the dry reference samples, which lead to the highest observed values. At testing temperatures of 200 °C a similar but less pronounced initial peak is seen only for dry reference samples. Using release agents clearly prevented surface reactions for lower temperatures. The dry reference sample also exhibits highest friction values throughout the entire testing duration. Application of release agents consistently lead to substantially lower initial coefficient of friction values below 0.4. Interestingly, when using sample A friction values slightly increased over the course of testing. After tribotesting the release agents in the HT-RRV tribometer the surfaces of the ring samples were investigated using optical microscopy. Representative results of the 500 °C tests are given in Figure 3. Severe plastic surface deformation could be observed for dry reference samples (Figure 3a) and ring samples tested with release agent B (Figure 3c). No surface deteriorations could be detected for release agents A and C. Here, only small debris and agglomerations of the release agent (Figure 3b) or residues of the applied release agent (Figure 3d) could be noticed. Results of microscopic evaluations suggest only low additional wear resistance for application of release agent B, whereas samples A and C could clearly increase the surface protection and prevent significant wear appearances. Besides minor levelling of surface asperities no severe wear occurrences were visible. Furthermore, 3D-analysis of the counterpart disc system topography was performed. Results exhibited similar outcome as ring surface analyses. As can be seen in Figure 4, besides the original grinding grooves from production processes the contacting surface was severely damaged when no release agent was applied. Moreover, the application of release agent B did not show any im- Aus Wissenschaft und Forschung 23 Tribologie + Schmierungstechnik · 65. Jahrgang · 6/ 2018 Figure 3: Representative surface appearances of ring samples after 500 °C HT-RRV tribotests: dry reference sample (a); release agent A (b); release agent B (c); release agent C (d) Figure 4: Representative 3D-images of used disc system topography analyses after 500 °C tribotesting: dry reference sample (a); release agent A (b); release agent B (c); release agent C (d) T+S_6_18.qxp_T+S_2018 29.10.18 17: 05 Seite 23 Acknowledgments This work was funded by the “Austrian Comet-Program” (Project K2, XTribology, no. 849109) and carried out at the “Excellence Centre of Tribology” (AC2T research GmbH). References [1] M., VARGA; H., WINKELMANN; E., BADISCH. Impact of micro-structure on high temperature wear resistance. Procedia Engineering, 10: 1291 - 1296, 2011. [2] K.-H., ZUM GAHR. Microstructure and wear of materials. Else-vier, 10, 1987. [3] S., HERNANDEZ. High temperature wear processes. Doctoral dissertation. Luleå tekniska universitet, 2014. [4] R.L., HEWITT; W., WALLACE; M.C., DE MALHER- BE. Plastic deformation in metal powder compaction. Powder Metallurgy, 17(33): 1-12, 1974. [5] S.L., CLARK. Release Agents. In Handbook of Composites. Springer, 633 - 638, 1982. [6] H., LAMMERTING. Release Agents. In Ullmann’s Encyclopedia of Industrial Chemistry, 2000. [7] M.J., OWEN. Release Agents. In Van Nostrand’s Encyclopedia of Chemistry, 2005. [8] A., TOMALA; S., HERNANDEZ; M.R., RIPOLL; E., BADISCH; B., PRAKASH. Tribological performance of some solid lubricants for hot forming through laboratory simulative tests. Tribology International, 74: 164 - 173, 2014. [9] H., TORRES; M.R., RIPOLL; B., PRAKASH. Selflubricating laser claddings for friction control in press hardening of Al-Si-coated boron steel, submitted to Journal of Materials Processing Technology, 08/ 2018. [10] L., WIDDER; A., GRAFL; A., LEBEL; C., TOMASTIK; J., BRENNER. Triboanalysis of hypoid gear components in drive trains. Tribologie und Schmierungstechnik, 58(2): 11 - 15, 2011. Aus Wissenschaft und Forschung 24 Tribologie + Schmierungstechnik · 65. Jahrgang · 6/ 2018 provement in wear protection within the wear track, since equally damaged surfaces were detected. Contrary, the application of both release agents A and C, respectively, lead to significant enhancement of the wear resistance. For these release agents inside the wear tracks no presence of severe wear manifestations could be identified. The original grinding grooves were still present and only minor asperity levelling could be observed within the wear tracks. Conclusion For concluding remarks we can summarize the results of the tribotests and surface analyses after release agent applications on a ring-on-disc system as follows: • Friction values significantly reduced after application of all release agents at both applied temperatures, whereas only release agent A could prevent the friction peak at test initiation at testing temperatures of 500 °C. For present temperatures the contained graphite maintained its beneficial tribological properties and does not react with sample surfaces. • Release agents A and C could effectively prevent wear occurrences for both applied temperatures. For release agent B severe wear and plastic deformations of the contacting surfaces was detected, similar to the dry reference samples. • Release agent B only showed significant improvements of friction behavior for low temperature application, whereas at 500 °C friction reduction and wear protection abilities were practically non-existent. 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