Aus Wissenschaft und Forschung 19 Tribologie + Schmierungstechnik · 66. Jahrgang · 4/ 5/ 2019 DOI 10.30419/ TuS-2019-0020 Structural and chemical alterations in transfer films Marco Enger, Jürgen Erlewein, Timo Ziegler, Jürgen Eder* Trockenlaufende Verbundlager sind für zahlreiche Anwendungen von großem Interesse, da sie bemerkenswerte Gleiteigenschaften über ein breitgefächertes p,v-Spektrum bieten. Das Gleitverhalten von trockenlaufenden Welle-Lager-Kontakten wird durch zahlreiche Faktoren beeinflusst. Ein wesentlicher Faktor ist sicherlich die Bildung von qualitativ hochwertigen Transferfilmen auf der Wellenoberfläche (angreifende Gegenkörperoberfläche). Der vorliegende Artikel untersucht die Übertragungsfilmbildung (Transferfilmbildung) und die daran beteiligten Prozesse in einem typischen Verbundlager-Welle-Kontakt. Verschleiß- und Reibungsexperimente wurden auf einem Stiftscheibe-Tribometer durchgeführt. Die Versuche erfolgten bei einer konstanten p,v-Kombination: p = 11,3 MPa in Kombination mit einer kontinuierlichen Gleitgeschwindigkeit von 0,035 ms -1 . Die Ergebnisse zeigten eine wiederholbare und typische Einlaufcharakteristik bevor das System in seinen Betriebszustand überging. Diese Schwankung/ Veränderung im Gleitverhalten wird durch diverse Prozesse verursacht, die während des Gleitvorgangs an den Grenzflächen stattfinden. Digitalmikroskopische Untersuchungen ergaben erste Erkenntnisse über strukturelle Veränderungen innerhalb des Transferfilms und des Lagermaterials. Komplementäre mikrostrukturelle (REM und FIB-Schnitte) und chemische Analysen (EDX und XPS) wurden ebenfalls durchgeführt, um die Natur dieser Prozesse besser zu verstehen. Die Mikrostrukturanalyse der Transferfilme zeigte deutliche strukturelle Veränderungen innerhalb der Transferfilmarchitektur mit fortschreitendem Gleitvorgang. Chemische Analyse der verschiedenen Transferfilm-Evolutionsstufen lieferte zusätzliche Informationen, dass chemische Prozesse ebenfalls von entscheidender Bedeutung sind. Unter Berücksichtigung aller Aspekte hat der vorliegende Beitrag die anfänglich aufgestellte Hypothese bestätigt, dass Reibungs- und Verschleißreaktionen des Welle-Lager- Kontakts mit strukturellen Änderungen innerhalb des Transferfilms und des Lagermaterials sowie mit Variationen der Transferfilmchemie in Zusammenhang gebracht werden können. Schlüsselwörter Verbundlager, Übertragsfilme, Reibung, PTFE Dry running plain composite bearings have been of great interest for numerous applications because they feature remarkable sliding performance over a wideranging p,v-spectrum. The sliding performance of dry running shaft bearing contacts is affected by numerous factors. One essential factor is assuredly the formation and development of high quality transfer films on the shaft surface (attacking counter surface). The present article explores the transfer film formation and accompanied processes in a common composite bearing shaft contact. Wear and friction experiments were conducted using a Pin on Disk test device. The test conditions were: 11.3 MPa combined with a continuous sliding speed of 0.035 ms -1 . The experiments revealed fluctuating and re-measurable run-in friction and wear performance until the system enters its steady state stage. This fluctuation in performance was supposed to be caused by multiple processes taking place at the interface of both contacting surfaces throughout the sliding process. General surface investigations of the attacking counter face and the worn pin surfaces discovered first insights regarding structural changes within the transfer film and bearing material. Careful micro structural (SEM and FIB sections) and chemical analyses (EDX and XPS) were performed on the stressed surfaces to discover the nature of those processes. The micro structural analysis of the transfer films revealed structural alterations within the transfer film architecture with progress in covered sliding distance. Chemical analysis of the diverse transfer film stages provided useful information that chemical processes are also of crucial importance. Taking all aspects into account the present paper confirmed the initial introduced hypotheses that observed friction and wear responses of the shaft bearing contact can be linked to structural changes within the transfer film and bearing material along with variations of the transfer film chemistry. Keywords composite bearing materials, transfer films, friction, PTFE Kurzfassung Abstract * Dr. Marco Enger Dipl.-Ing. Jürgen Erlewein Timo Ziegler, B.Sc. Dipl.-Ing. Jürgen Eder GGB Heilbronn GmbH, 74078 Heilbronn TuS_4_5_2019.qxp_T+S_2018 23.08.19 13: 15 Seite 19 ple processes are included encompassing structural and chemical alterations within the transfer film along with structural changes of the composite bearing material. 2 Materials and experimental details A frequently used composite bearing material is subject of this investigation. The structural design/ architecture of this material is shown in figure 1. The bearing material exhibits a multi-layer structure. This 3 bonded layer structure is composed of an impregnated anti-friction overlay enriched with PTFE and fillers (PTFE bearing lining), a porous sintered bronze interlayer for high wear resistance and at the same time being a mechanical interlocking system for the PTFE bearing liner. The third layer involved is a steel or bronze backing for high mechanical strength. This composite bearing material was paired with a nonhardened 42CrMo4 steel counter face with a concentrically grinded surface finish of R z ~ 2 µm and R a ~ 0.17 µm. Roughness measurements were performed perpendicular to the roughness peaks and valleys. For each test a new counter face was used. It is well-known that variations of the counter face micro geometry can dramatically affect tribological outcomes. As a consequence, efforts were made to reproduce counter faces within a precisely controlled roughness tolerance win- Aus Wissenschaft und Forschung 20 Tribologie + Schmierungstechnik · 66. Jahrgang · 4/ 5/ 2019 DOI 10.30419/ TuS-2019-0020 1 Introduction The efficiency of mechanically engineered aggregates has grown considerably in recent years. For the individual construction elements used, this means that with advances in technology, their performance potential must also be significantly improved and increased. One frequently and widespread construction element used in mechanical engineering is certainly the plain sliding bearing. Sliding bearings are used as lubricated or dry running systems. The performance of dry running PTFE based composite bearings depends on a multitude of factors. A central and essential feature is the capability of the bearing material to form a protective transfer film on the opposing surface (shaft surface). Apart from the pure formation of the transfer film, its “quality” is assuredly of crucial importance. This statement is consistent with numerous research works for polymer composites [1-6]. The findings can be summarized that a thin and coherent, robust and tenacious and welladhering transfer film is favored compared to thick, patchy or fragmentary and poor adhering transfer films. A robust and coherent transfer film shields and protects effectively the composite surface from further damage by the attacking counter face [1-6]. It is believed that the quality of transfer films is affected by diverse factors dictated by the related tribological system. As a reflection it can be postulated that the transfer film could also be understood as a response of a specific tribological system encompassing following aspects: induced collective stress, the specific property profile of the opposing surface (composition, morphology ...), the composition of the bearing material itself and most likely the conditions of the surrounding atmosphere are factors to consider. Understanding the transfer film formation and accompanied processes in bush/ shaft contacts is seen as an essential knowledge for fundamental product understanding and builds a key pillar for future investigations and materials. In this paper, we explore the evolution of transfer films and accompanied processes in a common composite bearing shaft contact. During friction and wear experiments this tribo-contact showed re-measurable and almost congruent friction and wear responses. All tests revealed a typical and clearly pronounced transition phase in friction and wear curve characteristics until the system enters its operational window. This manner is often simplified using the common expression running-in of a tribological system including all essential processes (chemical, structural, dimensional …) in the interface of two contacting surfaces in relative motion to achieve a stable operational performing window. Focused goal of this research is to create a link between friction and wear curve shapes observed in tribo-experiments to concrete events taking place in the tribo-contact interface throughout sliding. It is speculated that multi- Figure 1: Structural design composite bearing material Figure 2: Schematic illustration Pin on Disk test setup TuS_4_5_2019.qxp_T+S_2018 23.08.19 13: 15 Seite 20 dow to guarantee practically “identical” counter faces for each test run. The friction and wear experiments were conducted on a Universal-Material-Tester 3 (UMT 3) rig from Bruker. The UMT 3 test device is a versatile and modular test system with which generally all common tribological model test configurations (contact and sliding motion conditions) can be realized. For the tests a flat-on-flat test configuration was utilized and realized through a Pin on Disk test setup (pin = bearing material, disk = steel disk). A schematic illustration of this test configuration is given in figure 2. The tribological tests were operated at one constant p,vcombination - the contact pressure was 11.3 MPa combined with a continuous sliding speed of 0.035 m/ s. The tests were done with variable, however, defined test durations to evaluate and document alterations within the tribo-contact interface occurring throughout sliding. The tests were interrupted after an appreciable variation in friction characteristic was apparent on the online measurement as shown in the friction sliding distance plot (figure 3). Friction and wear responses of the system were recorded using an online data logging system. The normal and tangential forces were detected with a two axis load sensor based on a DMS measurement system. The system wear was measured via a capacitance sensor system with a linear resolution of 50 nm mounted on the pin sample holder. Prior to testing the test samples (pins + disks) were pre-conditioned with a wet chemical cleaning process → degreasing and cleaning of the surfaces. The tests were performed at standard ambient laboratory conditions: ambient medium → air, room temperature ~ 22 °C, relative humidity ~ 50 %. The tests were done with a statistic of 3 repeatable measurements. Post-test surface investigations were performed on the tribologically stressed surfaces using different techniques: • A Keyence digital microscope was used to capture first insights regarding processes that took place within tribo-contact interface during the sliding process. • Complementary surface investigations were conducted to gain deeper and more profound understanding of those events: o Micro structural analysis (SEM observations + FIBsections) were performed on the transfer films. Main goal is to achieve useful information concerning the morphology/ structure of the transfer films. o EDX (Energy Dispersive X-Ray Analysis) was applied on the counter face / transfer film to gain evidence which elements contribute to the transfer film. o Furthermore, the evolved transfer films were investigated using XPS (X-Ray Photoelectron Spectroscopy) → better understanding of compositional variations within the transfer film and further identifying the new species formed throughout sliding. 3 Results 3.1 Friction and wear results Figure 3 shows typical measured friction and wear curves of the composite bearing shaft contact observed during the friction and wear experiments. The friction and wear curves exhibit characteristic phases. The composite bearing steel contact starts with a low initial friction which slightly increases during the first sliding meters (stage 1). A declining friction characteristic is then observed and the system enters a low level friction phase Aus Wissenschaft und Forschung 21 Tribologie + Schmierungstechnik · 66. Jahrgang · 4/ 5/ 2019 DOI 10.30419/ TuS-2019-0020 Figure 3: Re-measureable friction and wear characteristic of a composite bearing material TuS_4_5_2019.qxp_T+S_2018 23.08.19 13: 15 Seite 21 up an initial polymer based transfer film on the attacking counter face. Figure 5a shows the surface observations of the worn surfaces during the first few sliding meters. The test was interrupted after the first increase in friction. The polymer overlay is slightly worn resulting in a bronze exposure. As a consequence of this the bronze structure starts to come in touch with the attacking counter face. The counter face observation suggests the presence of transferred polymer onto the steel counter face building a transfer film. Images of the tribologically stressed surfaces during the low friction phase are shown in figure 5b. The polymeric overlay is completely worn resulting in a bronze exposure across the entire pin surface. The transfer film is still present on the counter face and the imagery suggests that it is predominantly composed of the liner polymer. Thus the low friction level can be ascribed to the generated transfer film and the still high polymer content in the bearing structure. Figure 5c illustrates the microscope images of the sliding contact during stage 3. The test was interrupted shortly afterwards the noticeable rise in friction. This increase in friction is coinciding with a significant structural alteration within the bearing material as shown in image 5c. The pin observation reveals an ongoing bronze exposure. The higher content of exposed bronze structure abrades the initially built transfer film and is then transferred onto the opposing steel surface. It is assumed that the transfer film is composed of PTFE + bronze. These alterations within the contact interface result in a higher friction compared to the initial low friction. Figure 5d+e gives the microscopic observations of the stressed pins and counter surfaces during the stabilized steady state friction and wear stage (approx. 25000 and 56000 sliding cycles → stage 4 and 5). At stage 4 a high content of bronze is exposed. The ratio between polymer and bronze is still sufficient to lubricate the contact. The counter face surface observations reveal distinctive alteration within the transfer film: firstly the transfer film appears to become thinner, secondly a colour change within the transfer film is noticeable which is hypothesized to be originated by chemical processes that occurred in the contact interface (e.g. oxidation processes → oxidation is the most likely chemical process to occur). At this stage the transfer film is composed of PTFE and bronze. It is supposed that this superposition of effects is essential to facilitate an optimal steady state sliding performance. The images taken at stage 5 draw a similar picture. The polymer bronze ratio has reached a critical value; nevertheless; the friction characteristic doesn’t change at this point which would be warnings for preliminary failure. The combination of still available polymer and the evolved transfer film is sufficient to “lubricate” the contact. Aus Wissenschaft und Forschung 22 Tribologie + Schmierungstechnik · 66. Jahrgang · 4/ 5/ 2019 DOI 10.30419/ TuS-2019-0020 (stage 2). The low friction remains until a second conspicuous rise in friction is evident (stage 3). Additional friction stabilization occurs and the system transits into an optimal stable friction window (operational window → stages 4 & 5). The initial run-in wear of this tribocontact ranges between 20 and 40 µm. This transient phase is normally of short duration and the system passes into its first low level wear phase. After a certain time a second minimal rise in wear is observed and easily overlooked. This is followed by additional wear stabilization - the steady state phase is reached resulting in a linear wear characteristic. The operational friction and wear phases maintain until the system fails. The typical starting failure wear depth level is reached at approx. 60 µm. At this wear level normally a critical ratio between bronze and embedded polymer is reached which means that no sufficient quantity of polymer remains to lubricate the tribo-contact. The onset system failure starts with a slight increase and first measurable instabilities in friction. An escalating friction development is detected once the system fails completely. Failure is often accompanied with a rise in wear. System failure isn’t explicitly shown in the graphs. 3.2 General surface observations This chapter encompasses general surface observation results obtained by examining the tribologically stressed surfaces (pin and steel disk) using a digital microscope. The outcomes reveal distinctive alterations within the bearing material and the transfer film formed throughout sliding. Representative images are summarized in the subsequent image series. A general surface image of a “virgin” composite bearing pin is given in figure 4. Polymer overlay is completely intact and only a minimal bronze exposure is apparent. The visible bronze balls are located a plane deeper than the polymeric overlay. This finds its origin in the multilayer architecture of this composite bearing material. This in turn leads to an initial polymer steel contact under tribological stress providing the feature to build Figure 4: Surface image of a “virgin” composite bearing pin TuS_4_5_2019.qxp_T+S_2018 23.08.19 13: 15 Seite 22 3.3 Micro structural analysis - SEM surface observations and FIB-sections 3.3.1 SEM surface observations To determine the micro structural changes within the transfer film architecture SEM images were taken from the transfer films formed on the counter face at previously defined interrupting stages: (a) transfer film formed after 400 cycles = stage 1, b) transfer film evolution after 1100 sliding cycles = stage 2, c) transfer film stage after 6200 cycles = stage 3, d) transfer film steady state = stage 4). Important outcomes of those investigations are highlighted in this chapter. Based on these outcomes it is evident that the transfer film undergoes various structural and chemical changes during the entire sliding process. During the first sliding meters the transfer film is mainly generated by PTFE enriched wear debris which sticks on the counter face. The PTFE fragments are located in a circumferential orientation which corresponds to the sliding direction. The transfer film has a patchy and incoherent structure. Higher magnifications show that the plated Aus Wissenschaft und Forschung 23 Tribologie + Schmierungstechnik · 66. Jahrgang · 4/ 5/ 2019 DOI 10.30419/ TuS-2019-0020 sliding stage pin surface counter surface a) stage 1 b) stage 2 c) stage 3 d) stage 4 e) stage 5 Figure 5: General surface observations of the worn surfaces of a composite bearing material / 42CrMo4 tribo-contact: a) stage 1 = 400 sliding cycles, b) stage 2 = 1100 sliding rotations, c) stage 3 = 6200 rotations, d) during the steady state phase e) latest steady state stage (56000 rotations) TuS_4_5_2019.qxp_T+S_2018 23.08.19 13: 15 Seite 23 regions are located in bands in a circumferential orientation corresponding to the sliding direction. The transfer film is composed of PTFE+filler and bronze. The transfer film formed during the steady state stage still coherently masks the steel counter face. Consistent colouring of the transfer film is indicative of potential consistent transfer film chemistry or a multi-layer structure has formed. Distinctive “cracking and delamination” of the transfer film are clearly evident. The images suggest that the potential “delamination” occurs within the transfer film. The SEM observations of all transfer film stages also suggest that the transfer film thicknesses appear to vary throughout sliding. To corroborate this statement additional FIB-sections were prepared. Overall, the SEM findings already showed that the transfer film structure and chemistry strongly vary throughout sliding. Aus Wissenschaft und Forschung 24 Tribologie + Schmierungstechnik · 66. Jahrgang · 4/ 5/ 2019 DOI 10.30419/ TuS-2019-0020 PTFE wear fragments don’t adhere well on the counter surface. Thick and thin transfer film regions are also clearly apparent. This patchy and insufficient adhering transfer film ineffectively shields the composite material surface. With progress in covered sliding distance the patchy PTFE+filler debris based transfer film is most likely removed. The novel transfer film formed has an enhanced masking quality. The transfer film colour changed which is indicative that the transfer film chemistry already altered at this early stage - it isn’t longer only based on PTFE wear debris. After 6200 rotations a well-developed and coherent transfer film is formed. Only minor areas of the original steel counter face are still apparent. Pronounced colour inconsistencies are still evident indicating compositional variations within the transfer film. The diverse coloured SEM surface observation images a) stage 1 b) stage 2 c) stage 3 d) stage 4 Figure 6: SEM observations of the transfer film formed on the counter face during the sliding process: a) after 40 rotations, b) at stage 2, c) stage 3, d) during the steady state phase TuS_4_5_2019.qxp_T+S_2018 23.08.19 13: 15 Seite 24 3.3.2 FIB-sections The SEM surface investigations showed that the transfer film structure changed throughout sliding. It was also stated that the transfer film thickness appears to vary. FIB sections of the individual transfer films were prepared to substantiate those statements. Exemplary sections are summarized through the subsequent image series (figure 7). The results include the FIB sections of the transfer film formed after 400 rotations (stage 1) and the FIB sections of transfer film generated during the first steady state stage (stage 4). The transfer film thickness varies with measurable thicknesses up to several hundred nanometers. Subareas of the transfer film can have a very thin structure. At no stage the transfer film structure appears to have a consistent thickness. The thinnest overall structures were found in stage 2 and 4. In the FIB section of the steady state stage it is clearly evident that the transfer film has a 2 layer-structure. The colour difference between each layer is a hint for chemical variances. These findings provide evidence that the chemical and structural depth profile of the transfer film vary throughout sliding. The FIB section for the steady state stage also confirms the initial hypothesis that the delamination at this stage took place within the transfer film. In those areas the transfer film has a very thin structure. Further, very small and fine particles (only a few nanometres in diameter) were found in the transfer film formed at later stages. The origin of these particles is currently unclear but is subject of future investigations. 3.4 Element analysis - EDX The SEM surface images already suggest that the transfer film chemistry alter throughout sliding. EDX analyses were performed to detect which elements contribute to the transfer films at various sliding stages. Exemplary spectra are shown in figure 8 representing the element spectra of the transfer films formed after 6200 and 56000 rotations (stages 3 and 5). The EDX spectra revealed the evidence of following elements: C, O, F/ Fe, Cu, Mo/ S, Sn and Ca, respectively. As expected the element concentrations vary for both sliding distances indicating compositional changes within the transfer film with progress in sliding distance. For the longer sliding period the EDX spectrum shows more pronounced peaks of O and Cu and in addition a weaker peak of F. The weaker F peak is potentially a result of lower polymer content in the bearing composite at this sliding stage - the polymer content decreases with an increasing wear depth level. Increased oxygen detection at later stage is supposed to be caused by chemical reactions (oxidation processes seems to be the most likely to occur). For both sliding stages the Fe detection is low indicating a consistent transfer film which is coinciding with the surface observations. Aus Wissenschaft und Forschung 25 Tribologie + Schmierungstechnik · 66. Jahrgang · 4/ 5/ 2019 DOI 10.30419/ TuS-2019-0020 a) FIB-section stage 1 b) FIB-section stage 4 Fi E l FIB i f h f fil d h 42C M 4 l Figure 7: Exemplary FiB sections of the transfer films generated on the 42CrMo4 steel counter face a) EDX spectrum stage 3 b) EDX spectrum stage 5 Figure 8: EDX spectra of the transfer film formed at different stages TuS_4_5_2019.qxp_T+S_2018 23.08.19 13: 15 Seite 25 contaminations. The potential presence of C-F groups is indicative for mechanical scissoring of the PTFE chain which would suggest that PTFE is involved in chemical processes. O-C=O groups are a potential hint for carboxylate groups formed throughout sliding (e.g. Sawyer et al.). The variations in the C 1s spectrum can also be attributed to the various oxidation processes occurring in the tribo-contact. Exact clarification of the introduced hypotheses is subject of further research studies. The first clear detection of Cu is measurable at point two of the friction characteristic (corresponding sliding rotations are 1100) which confirms the SEM surface observations that suggested that the transfer film formed during the first stage is generated by PTFE based wear debris. At stage 2 Cu is present in metallic form. With progressing test time the Cu 3p 3/ 2 spectra vary and an additional satellite peak is clearly observed → the transferred Cu undergoes an oxidation process. The Sn 3d spectrum also revealed that Sn is also detectable in an oxidized form in the transfer film (Sn 3d spectrum isn’t shown). Aus Wissenschaft und Forschung 26 Tribologie + Schmierungstechnik · 66. Jahrgang · 4/ 5/ 2019 DOI 10.30419/ TuS-2019-0020 3.5 Chemical analysis - XPS XPS were performed on the transfer films using a Physical Instruments Quantera SXM with a focused monochromatic AL K-alpha X-ray. The XPS results also revealed that various chemical processes took place throughout the sliding process. Further, the XPS analysis showed that the transfer film undergoes compositional variations throughout sliding. Exemplary results are provided based on selected detail spectra. For all sliding distances the C 1s spectrum is dominated by a peak at ~ 292 eV which is linked to carbon from CF2. It is evident that the fluoropolymer transfer already occurs in the early stages of the sliding process matching well the microscopic surface observations. With progress in sliding the C 1s spectrum becomes more complex and the emergence of additional C-groups is clearly evident suggesting that the chemical complexity of the transfer film increased. Following groups can be detected: CF3 at 694 eV, O-C=O (~ 289.5 eV), C- F/ C=O (288.5 eV), C-O (286.5 eV) and C-H (285 eV), respectively. C-H peak at 285 eV can be attributed to C 1s spectrum - stage 1 C 1s spectrum - stage 3 Fi 9 C 1 f 1 d 3 Figure 9: C 1s spectra for stages 1 and 3 Cu 3p 3/ 2 - stage 1 Cu 3p 3/ 2 - stage 2 Cu 3p 3/ 2 - stage 5 Figure 10: Cu 3p3/ 2 spectra for sliding stages 1, 2 and 5 Mo 3d virgin pin Mo 3d - stage 2 Figure 11: Mo 3d spectra for sliding stages: virgin pin, transfer film sliding stage 2 TuS_4_5_2019.qxp_T+S_2018 23.08.19 13: 15 Seite 26 Mo 3d spectrum of the transfer film formed at stage 2 is shown in figure 11. This figure also represents the spectrum of a virgin pin. The spectrum peaked at ~ 229 and 232.5 eV - both peaks can be attributed to MoS 2 . Comparing this spectrum with the Mo 3d spectrum of the transfer film an obvious difference is given. The differences in the spectra, especially the additional peak located at 236 eV can be linked to oxidation processes of Mo to MoO x . 4 Conclusion A common bearing material steel contact was investigated using a common Pin on Disk test setup. The results revealed a clear pronounced and re-measurable transient running-in phase before the system entered its operational performing window. Post-test surface investigations were conducted using structural and chemical analyzing tools to better understand the nature of the alterations within the contact interface and linking these events to friction and wear response changes. Based on the main findings following general statements can be concluded: • This study confirmed the initial hypotheses that the specific friction and wear characteristics of the composite bearing shaft contact is coincident with concrete running-in processes and associated transfer film formation occurrences in the contact interface. • Transfer film characterization revealed multiple chemical processes that took place in the composite bearing 42CrMo4 tribo-contact. • Strong structural alterations occurred within the transfer film and bearing material throughout sliding. • It seems that this superposition of effects is the guarantor for optimal sliding. References [1] D. R. Haidar, J, Ye, A.C. Moore, D. L. Burris Assessing quantitative metrics of transfer film quality as indictors of polymer wear performance Wear 2017 [2] D.L. Burris, B. Boesel, G. R. Bourne, W. G. Sawyer Polymeric Nanocomposites for Tribological Applications Macromolecular Materials and Engineering 2007 [3] Q.-H. Wang, J. Xu, W. Sheen, Q. Xue The effect of nanometer SiC filler on the tribological behaviour of PEEK Wear 1997 [4] F. Li, K. Hu, J. Li, B. Zhao The friction and wear characteristic of nanometer ZnO filled polytetrafluorethylene Wear 2001 [5] E. Padenko, L. J. van Rooyen, B. Wetzel, J. Karger-Kocsis “Ultralow” Sliding Wear PTFE Nanocomposites with Functionalized Graphene, GfT Conference, Conference proceedings 2016 [6] J. Ye, D. L. Burris, T. Xie A Review of Transfer Films and Their Role in Ultra-Low- Wear Sliding Polymers Lubricants 2016 [7] X. Zhang, M. Cresswell Inorganic Controlled Release Technology: Materials and Concepts for Advanced Drug Formulations Elsevier 2016 Aus Wissenschaft und Forschung 27 Tribologie + Schmierungstechnik · 66. Jahrgang · 4/ 5/ 2019 DOI 10.30419/ TuS-2019-0020 TuS_4_5_2019.qxp_T+S_2018 23.08.19 13: 15 Seite 27