Tribologie + Schmierungstechnik 64. Jahrgang 3/ 2017 1 Introduction Mechanical face seals have been used in industry for decades due to their simple but effective structure: two rings with flat surfaces are mechanically held against each other to restrict leakage during their relative rotation. In the interface between them, a lubrication film has been formed by the sealed liquid or gas [1]. Since the film is a kind of transition unit between atmosphere and higher P-T (pressure and temperatures) region inside the pump, a gradient exists and phase change may occur when P-T condition cross the phase line. This is called “puffing”, which normally leads to serious wear and failure in mechanical seal. Lymer [2] and Orcutt [3] are two of the first researchers to study “puffing” experimentally around 1970s. During following decades, several theoretical and experimental research work has been done in this field, but uncertainties still exist: (1) unknown and inexact real contacting frictional area [4]; (2) small wear debris are trapped between frictional surfaces and aggravates wear [5]; (3) uncertainty on the ratio of vapour and liquid, which affect the synthesized viscosity and temperature field [6-8]; (4) influences from “opening force”, which is difficult to control and measure precisely [6,8-11]; (5) in many water lubricating cases, tribo-chemical processes are involved [12,13]. In some recent works, the existence of vapour has been proved radically by experiments from Wang [14], and the primary agreement can be built between calculation and experiments by Migout [8]. 11 Aus Wissenschaft und Forschung * Le Jin 1,2 Dr.rer.nat. Herbert Scheerer 1 Dr.-Ing. Georg Andersohn 1 Prof. Dr.-Ing. Matthias Oechsner 1 1 Zentrum für Konstruktionswerkstoffe (Staatliche Materialprüfungsanstalt Darmstadt (MPA) und Fachgebiet und Institut für Werkstoffkunde (IfW)) der Technischen Universität Darmstadt 2 SEC-KSB Nuclear Pumps & Valves Co., Ltd., Shanghai, China Tribological behaviour of surface contact friction test with water lubrication at elevated temperatures L. Jin, H. Scheerer, G. Andersohn, M. Oechsner* Eingereicht: 30. 10. 2016 Nach Begutachtung angenommen: 21. 1. 2017 In diesem Beitrag werden experimentelle Untersuchungen zum tribologischen Verhalten von Wasser geschmierten Gleitpaarungen bei erhöhten Temperaturen vorgestellt. Zur Simulation von Start-Stopp- Vorgängen und zur Abbildung von Grenzreibungsphänomenen, beispielsweise in Wasserpumpen, wurde ein modifiziertes Schwingreib-Verschleißsystem verwendet. Das System erlaubt die Beheizung des Grundkörpers und die Zuführung von Wasser in den Gleitspalt. Übersteigen die Temperaturen des Wassers im Gleitspalt den Siedepunkt treten zunächst auf der Atmosphärenseite und später auf der Wasserseite Bereiche mit stärkerem Verschleiß auf, was zu instabilem Reibungsverhalten führt. In dem Beitrag werden schließlich mögliche Anwendungen dieser Versuchsmethode auf das tribologische Verhalten von Dichtungen bei erhöhten Temperaturen diskutiert. Schlüsselwörter Wasserschmierung; Dampfphase; Flächenkontaktreibung; Höhere Temperatur; Oszillation This paper presents an experimental work on general tribological behaviour with water lubrication at elevated temperatures. A surface contact friction type with reciprocating movement is chosen to simulate the start-stop and accident working condition of mechanical seals in water pumps. From the results, when the heating temperature is above a critical value that is higher than saturation point, vaporization in friction gap firstly occurs near atmosphere side, and then expands into water side. Vaporization causes unstable friction behaviour and severe wear of the tribo-couple. Finally, potential applications of this friction test for evaluating the wear behaviour of seals at elevated temperatures are discussed. Keywords water as lubricant; vaporization; surface contact friction; elevated temperature; oscillation Kurzfassung Abstract T+S_3_17 03.04.17 15: 12 Seite 11 12 Tribologie + Schmierungstechnik 64. Jahrgang 3/ 2017 Most of the published experimental works are based on scaled mechanical face seal including two coaxial rings with flat surface contacting to each other, one of which rotates at uniform velocity and the other keeps static. This test is effective to study the tribological behaviour in steady running condition, but not for start-stop process or accident condition, which includes more dangerous acceleration and deceleration periods. The aim of the present paper is to perform surface contact friction tests with variable velocity to simulate the startstop process or accident condition. Heating for friction gap is exerted to seek “puffing”. 2 Materials and methods 2.1 Friction test methodology A surface contact oscillation sliding friction test has been performed on SRV III friction system, which is manufactured by Optimol Instruments Prüftechnik GmbH. Main testing mechanism is illustrated in Figure 1. Friction tests run in a test chamber of SRV, in which atmospheric temperature and humidity can be detected. Frictional surfaces come from two vertical arranged cylinder samples. Lower sample is static and upper sample performs reciprocating movement along horizontal direction. Normal load is exerted vertically from top of the upper sample. In order to keep contacting pressure as uniform as possible, the exerting point of normal load is controlled within 0.1 mm away from the axial of upper sample. Load is transmitted by a ball connection to avoid friction surface tilting and measured by sensors below the bottom of lower sample. Friction coefficient can be calculated from normal load and friction force, which is also measured by force sensors below lower sample and recorded in-situ every second. Upper sample is machined into cylinder shape with a through coaxial tunnel in centre. An outer circulation is built to pump lubricant from a tank through this tunnel into heated friction gap. Actual temperature in friction gap can be measured by a sensor below lower sample and controlled by SRV. Temperature deviation between sensor and friction gap position is calibrated smaller than 0.1 °C when heating temperature is lower than 200 °C using steel samples without upper sample. The pressure of pumped lubricant can be controlled to 4 bar maximum. During tests, normal vertical load, frequency and stroke of upper sample are set as constant values. Heating temperature is fixed in each test, but changed from 40 °C to 160 °C for different tests. Test parameters are listed in Table 1. For each heating temperature, same tests are repeated at least 3 times. Before friction start, a preheating process for 10 min is performed to get a stable temperature field, during which water is being pumped with feeding pressure. Aus Wissenschaft und Forschung Figure 1: Surface contact oscillation friction test with pumped lubricant Table 1: Parameters used in friction test Lubricant Deionized water Vertical load 50 N Mean Contacting Pressure 1.16 MPa Frequency 50 Hz Stroke 1 mm Feeding pressure (from pressure gauge) 1 bar Heating temperature from 40 °C to 160 °C Test duration 45 min Sealing performance No leakage Figure 2: Friction samples Table 2: Properties of test material Density [g/ cm 3 ] 7.8 Hardness [HV] 650-710 Elastic module [GPa] 210 Thermal conductivity [W/ mK] 4.6 Dry friction coefficient 0.92 C [%] Si [%] Mn [%] Cr [%] Fe [%] 0.93-1.05 0.15-0.35 0.25-0.45 1.35-1.60 rest T+S_3_17 03.04.17 15: 12 Seite 12 Tribologie + Schmierungstechnik 64. Jahrgang 3/ 2017 2.2 Test samples material Friction samples are manufactured from commercial used steel 1.3505 (100Cr6), Figure 2. Containment and main physical parameters are listed in Table 2. Both friction surfaces are grounded to a roughness of Ra < 0.2 µm. Upper sample is disposable but lower sample can be used four times in different friction locations without influence on each other. 2.3 Analytical methods Statistical results for friction coefficient during last 15 min of each temperature (including at least 3 same repeated tests) are calculated to evaluate the friction status. A local region of worn friction scar is chosen to evaluate wear conditions, in which friction direction is vertical with lubricant fluid path. Linear surface morphology from centre to margin is measured by Hommel-Etamic T8000 (Jenoptik AG) with a touch-type needle, of which resolution is lower than 10 nm. Mean depth of wear scar is calculated from the morphology profile by the same equation for surface roughness Ra following DIN EN ISO 4287, and the minimum point in profile is taken as the maximum wear depth for each temperature. An energy-dispersive X-ray spectroscopy equipping with scanning electron microscope (SEM/ EDX) is used to describe micro wear phenomenon and to identify element containment change in different locations of worn surface on lower sample. 3 Results 3.1 In-Situ test record Environment sensor in test chamber has detected some sudden raises of humidity soon after friction coefficient increases sharply, Figure 3. Arrows indicate humidity peaks measured by in-situ record during friction test at 160 °C. At the same moment, “puffing” voice could be heard. Since there is no visible liquid leakage during tests and no other possible vapour sources in the test chamber, these sudden raises of humidity is definitely caused by leaking vapour from friction gap. This phenomenon can prove the existence of vaporization in friction gap. 3.2 Wear condition When temperatures are lower than or equal 130 °C, mean wear depths are not larger than 0.2 µm that is similar with roughness of original surface, figure 4. When heating temperatures are above 140 °C, mean and maximum values of wear depth increase obviously and both have positive correlation with heating temperature. To study detailed wear mechanisms at elevated temperatures, original worn scars’ surface morphology profiles 13 Aus Wissenschaft und Forschung Figure 3: In-situ record from one of the friction test at 160 °C Figure 4: Mean (a) and maximum (b) wear depth of each heating temperature T+S_3_17 03.04.17 15: 12 Seite 13 14 Tribologie + Schmierungstechnik 64. Jahrgang 3/ 2017 are researched. Three typical profiles can be found at different temperatures in figure 5. In each test, wear conditions are locally different. In the regions around centre (C), similar mild wear features could be observed compared to lower temperature. Severe wear occurs more near margin (M) as shown in grey colour, and its area becomes larger when heating temperature gets higher. These wear scars appear randomly and linear parallel with movement direction, shown as narrow peaks in profiles. 3.3 SEM-EDX analysis During friction tests iron is oxidized due to high flash temperature and water provides oxygen source for this reaction. Oxygen distribution map represents the local oxidation degree and helps us comprehend partial friction and wear. In our tests SEM-EDX analysis is used to identify it. For the substrate material 100Cr6 it is difficult to measure the exact oxygen containment quantitatively by EDX because the result will be influenced by the spectrum data of chromium. Only relative comparison between different locations in one scar will make sense due to constant influence from chromium within the single measurement. Mass ratios between oxygen and iron in different local regions are compared in figure 6. From position (3) to (4), which represent from centre to margin, opposite changing tendencies of oxygen containment can be found: increasing at 100 °C but decreasing at 150 °C. Aus Wissenschaft und Forschung Figure 7: Statistical results of friction coefficient at different heating temperatures Figure 6: SEM-EDX analysis results: (a) SEM, wear scar at 150 °C and EDX detection positions; (b) SEM, wear scar at 100 °C and EDX detection positions; (c) mass ratio between oxygen and iron in different detection positions: (1) no friction but contact with water, (2) friction direction parallel with fluid path, (3) friction direction vertical with fluid path, near centre, (4) friction direction vertical with fluid path, near margin, (5) no friction or water contacted Figure 5: Surface morphology profiles when heating temperatures at 140 °C, 150 °C and 160 °C 3.4 Friction coefficient Statistical results of friction coefficient at different temperatures are illustrated in figure 7, from which general relationship can be found. When temperature is lower than 130 °C, mean friction coefficient increases together with higher heating temperature and reaches its maximum value around 130-140 °C. However, it cannot continue to increase if temperature keeps above 140 °C, and friction status is unstable due to larger standard deviations. T+S_3_17 03.04.17 15: 12 Seite 14 Tribologie + Schmierungstechnik 64. Jahrgang 3/ 2017 4 Discussion 4.1 Surface contact reciprocating friction test In this study a surface contact reciprocating friction test is developed to confirm lubricant presented in the friction gap. The test is repeatable and effective because of similar statistical results and stable in-situ records. In these tests a small temperature gradient exists in lubricant film due to constant thermal condition when no leakage is observed. According to designed structure temperature should increase slightly along the fluid path from centre to margin because of worse cooling effect of atmosphere. More severe oxidation process around margin than centre from EDX results at 100 °C (figure 6) can prove this temperature distribution. Because no leakage is observed even at lower temperature, such as 40 °C, liquid should be obstruction by solid somewhere on fluid path rather than vaporization. Liquid is totally separated from atmosphere and the lubricant pressure in friction gap is approximately considered as constant. Similar with actual products, nearly all related parameters, such as surface waviness, roughness, load distribution, velocity, chemical properties, liquid pressure, temperature field, wear debris etc., will influence the friction status in tests. Larger contact area will magnify the negative effect of morphology related parameter because it is difficult to keep all local performances uniform on entire surfaces. Even small local failure or inhomogeneous micro-structure may affect the friction state by debris. This feature brings more difficulties on mechanism analysis, but also brings more advantages on simulation for actual service condition. Compared to scaled surface seal tests with uniform rotating velocity, reciprocating movement in this test with changing velocity creates a more severe environment for friction pair. It is a kind of simulation test for startstop or accident working condition, during which more than 80 % failures occurs. From this perspective, results from this test have more significance for life-time performance of mechanical seal. Moreover, samples in this test can be machined into small dimension, by which lower cost and shorter duration can be expected. Compared to traditional friction tests for material, which normally belongs to point contact friction type, friction coefficient of surface contact test has shown more complexity, because the detected friction force is a resultant including two parts: liquid friction caused by laminar shear and solid boundary friction. However, on the other hand, surface contact type can provide a relative lower and constant mean contact pressure during tests, which cannot be got from point contact friction tests, because area will increase due to wear of the sphere surface. In this sense the described test is better for material wear test with lower load and longer test duration. Additionally it is possible to research the effects of wear debris between frictional surfaces because most of the generated debris cannot be displaced. This point is more meaningful for tribo-chemical related friction case [13,15]. 4.2 Friction behaviour and wear mechanism Lubrication and wear mechanisms will be discussed in the three temperature ranges “40 °C to 120 °C”, “120 °C to 140 °C” and “140 °C to 160 °C”. 4.2.1. Lower temperature range from 40 °C to 120 °C Besides 120 °C, most water in friction gap should keep in liquid phase because temperatures are lower than saturation point at this feeding pressure. As there is no obvious severe wear features visible by means of photos or surface morphology profiles of scars, full liquid lubrication films exist in all tests. Low wear depth at these temperatures (figure 4) can also verify that it is a fully liquid lubricated condition with slight wear. From the viewpoint of Biswas [16], main mechanism for this kind of mild wear is primarily an oxidation process of growth and spalling of oxide at the asperity level, which acts as a protection layer. High oxygen containment from EDX analysis supports this conclusion (figure 6). Similar with Hanlon’s opinion [17], soft and small pieces of debris that are detached from oxidation layer will not aggravate wear situation. Friction coefficient tends to increase when temperature rises due to changed viscosity (figure 7). When temperature increases, based on Reynolds equation, vertical loading capacity of liquid film decreases since reduced viscosity [18]. Solid asperities bear more vertical load and the ratio of solid friction increases, which is larger than liquid friction. As a result, synthesized friction coefficient becomes larger. From the point of view of classic Stribeck curve, same conclusion can be derived: friction state of this group of tests should be mixedlubrication, in which friction coefficient shows negative correlation with viscosity, therefore positive with temperature [18]. 4.2.2 Elevated temperature range from 140 °C to 160 °C In this range, wear is severe but friction coefficient is not so that high. This phenomenon can be explained by vaporization and flow patterns in friction gap. To get better understanding of lubrication condition in friction gap, a boiling model for two-phase forced flow through uniformly heated horizontal channels can be referenced. Liquid boiling starts at the onset of nucleate boiling (ONB) point due to heat flux from superheated channel wall. Along fluid path, flowing patterns are successively liquid (no boiling), bubbly (incipient nucleate 15 Aus Wissenschaft und Forschung T+S_3_17 03.04.17 15: 12 Seite 15 16 Tribologie + Schmierungstechnik 64. Jahrgang 3/ 2017 boiling dominated), plug (large and continuous bubbles), annular, mist and single-phase vapour [19, 20]. In our tests, flow patterns in latter part should be different because bubbles are destroyed by reciprocating movement and friction gap is too tight compared to traditional flow tunnel. However, these will not affect the interesting factor on the initial position of ONB, after which liquid content tends to reduce and lubrication status may deteriorate. Wear conditions in figure 5 have good agreement with this model. At higher temperatures, ONB appears closer to centre that is the start of fluid path. This vaporization process can be verified by EDX in figure 6. At 150 °C, oxygen containment in severe wear region around position 4 is lower than position 3 because there is less water near margin that provides Oxygen source. Similar model is proposed by Hughes [21] and proved by Migout [8] with uniform velocity. This work can qualitatively verify its applicability for variable velocity. Wear mechanism in this temperature range is more complicated compared to lower temperatures. Liquid lubrication near centre causes mild wear, mechanism of which is similar with lower temperature range. Discrete appeared deep scars are the typical feature of severe wear. Dominate mechanism is abrasive wear [22]. Third body from wear particles will accelerate it by abrasion [23], since most of them cannot be displaced out of friction gap in this test. Therefore, it is meaningful for us to discuss how to reduce the amount and size of wear debris to decrease the total wear. In this kind of self-mated surface contact friction, the first wear debris may be formed by accumulation of plastic strain at or near the surface, fracture of contacted asperities or pieces by fatigue crack propagation [5]. Accordingly, improving the parallelism of friction surface during installation, reducing original defects near surface, or preventing inhomogeneous micro-hardness should be possible ways. Additionally, surface roughness should be kept in a proper range, because extreme smooth surfaces may lead to larger Van der Waals force [24] that is harmful for wear. Lower friction coefficients appeared at elevated temperatures of 150 °C and 160 °C, (figure 7). Similar results could be found in published work with uniform velocities [3,8,9,11,21]. A primary consensus on the mechanism of this phenomenon is due to “opening force” caused by severe vaporization process in local and sealed region [3,11,21]. In the flash moment, volume of water increases sharply resulting in high partial pressure to counterpoise normal load. Correspondingly, vertical load on solid asperities decreases and friction force lower down. Larger deviations of friction coefficient in tests at 150 °C and 160 °C indicate that they are not steady friction process, because a stable “opening force” needs constant superheated temperature field, adequate liquid water source and changeless solid surface condition. None of these factors are possible in this type of test. These results have proved the applicability of the theory “opening force” in reciprocating changed velocity. 4.2.3 Transition temperature range from 120 °C to 140 °C Transition temperature range exists between lower and elevated temperatures, in which vaporization initially occurs. Typical boiling curve when water is heated in pool can help to understand tribological behaviour in this temperature range. Although temperatures are higher than saturation point, additional superheated temperature is needed for onset of nucleate, which is approximately 5 °C higher than saturation point. The inflection point to start fully developed nucleate boiling is around 10 -12 °C higher than saturation point [20]. Therefore, if temperature is lower than this inflection point, only isolated small bubbles can be formed locally with extreme P-T condition, such as water around asperities contact region. The reciprocating changed velocity induces irregular fluid field, which is harmful for vapour bubble’s nucleation and growth [20]. Local vapour generates mildly and cannot form enough “opening force” to counterpoise vertical load obviously. Lubrication mechanism is similar with lower temperature range, mainly full liquid lubrication. Compared to elevated temperature range, this liquid lubricated condition with mild local boiling causes higher friction coefficient but with slight wear though temperatures are higher than saturation point. Theoretically, for each particular case, a critical temperature should exist, higher than which vapour will start to generate. This critical temperature should be higher than the sum of saturation point and minimum superheat value. It is not a general constant value because kinetic energy required by nucleation is influenced by solid condition: roughness, water infiltration, surface chemical performance etc. In this group of tests, this critical temperature should be between 130 °C and 140 °C, because friction coefficients become unstable and smaller from 130 °C to 140 °C, and severe wear zones appear incipiently around margin (ONB point). This is similar with Migout’s point stated as temperature threshold [8]. 4.3 Potential application Based on results, potential applications of vapour in mechanical seal could be expected. As discussed in transition temperature range, seals can work safe with little wear below critical temperature which is higher than saturation point. Accordingly, methods to raise this critical value would make sense to expand the safety working scope of water pumps, for example, optimizing solid surface to enlarge the incipience kinetic wall for nucleates boiling. On the other hand, vaporization causes severe wear, but is benefit for reducing friction and enhancing sealing performance. Theoretically, it is possible to control va- Aus Wissenschaft und Forschung T+S_3_17 03.04.17 15: 12 Seite 16 Tribologie + Schmierungstechnik 64. Jahrgang 3/ 2017 porization only at the very margin of friction gap to play the role as preventing liquid leakage while keeping most of gap filled with liquid as lubrication film. In another word, seal temperature is controlled to set the ONB point at margin. In this case, material in margin area should be optimized with better anti-wear performance. This test also gives us potential possibilities to study the behaviour of forced boiling liquid flow with high frequency vibration and friction. The power plant industry should be more interested in such test methods, for example researches on fretting wear between U-tube and supporting plate in steam generator. 5 Conclusion In this study, a quantity of surface contact oscillation friction tests with water as lubricant has been performed. Temperature factor is studied in the range from 40 °C to 160 °C, in which general friction behaviour and wear features are discussed. The conclusions are stated as follow: (1) Vapour can be generated when heating temperature is higher or equal than 140 °C due to vaporization, which firstly starts at the outer margin of friction gap, and then expands its scope into centre when temperature increases. (2) Vaporization at elevated temperature will lead the mean friction coefficient lower but unstable because of the effect of “opening force”. Vaporization will aggravate wear condition due to liquid stagnation. (3) A critical value for heating temperature should exist as the start temperature of severe vaporization in large area. At this temperature, tribo-couple has the maximum friction coefficient but normal mild wear. This critical value is higher than saturation point due to superheat excursion in boiling curve. (4) This kind of surface contact oscillation friction test is effective and efficient. Comparing to point contact friction test, it is better for material wear test with low contacting pressure and researches on the effects of wear debris. It gives us more possibilities for friction and wear research. Acknowledgement Authors present thanks to Dr. Stephan Bross, Mr. Frank Sehr and Dr. Maike van Geldern from KSB AG for their constructional discussion and advices on this test. References [1] A.O. Lebeck, Principles and design of mechanical face seals, Wiley, New York, 1991. [2] Lymer A. 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