离合器摩擦片材料的表面特性和摩擦学性能研究.pdf

Investigation of surface characteristics and tribological behavior of clutch plate materials Qian Zou a,n, Chethan Raoa, Gary Barbera, Ben Zhoub, Yucong Wangb aDepartment of Mechanical Engineering, Oakland University, Rochester, MI 48309, USA bGM Powertrain, General Motors Corporation, Pontiac, MI 48340, USA a r t i c l e i n f o Article history: Received 1 September 2012 Accepted 13 October 2012 Available online 20 November 2012 Keywords: Clutch plate materials Friction Wear Surface characterization a b s t r a c t The surface characteristics and tribological behavior of clutch plate materials strongly affect the lifetime of the clutch and infl uence the dynamic behavior of the entire vehicle. The friction coeffi cient between the friction plate and the separator plate decreases with the number of engagement cycles. As a result, the transmission torque decreases with time. In this study, the surface characteristics of clutch plate materials have been investigated since it is closely related to friction. WYKO optical surface profi ler with TTM module and a GM created special fi xture were used to measure the surface characteristics of various clutch plate materials under different contact loads. In addition, the tribological behavior of the clutch plate materials has been studied. A UMT2 micro-tribometer was used to measure the friction characteristics of various clutch plate materials under dry and lubricated conditions. The effect of the loading and the sliding speed on the friction coeffi cient has been investigated. The correlation between the surface characteristics and the friction behavior of clutch materials was explored. The tested samples were analyzed to quantify the wear volume. The completion of the study provided a better understanding of the relationship between the material structure and the performance of clutch plates and can be used as a reference for clutch material selection and transmission design. & 2012 Elsevier B.V. All rights reserved. 1. Introduction A clutch mainly consists of two parts, i.e. friction plates and separator plates. These plates engage and disengage to transmit speed and torque. The transmitted torque is proportional to the overall friction coeffi cient of the clutch plates. The friction behavior of clutch plates is critical for overall performance of the transmission and it depends on the sliding velocity, the normal pressure, the lubrication, the temperature and the surface topography of the clutch plates. It is necessary to investigate the effect of surface topography on the friction behavior of the clutch plates. In 2007, Bezzazi and coworkers 1 reported a comparison between pin-on-disk test results with those obtained through SAE J661 standard test on clutch plates. They developed an equation for clutch facing morphology, measured the temperature on the lateral side of the disk in the pin-on-disk testing and obtained the correlation of coeffi cient of friction with the sliding velocity and the temperature. In Bezzazis work, friction and wear tests on polymeric composite clutch facing materials under no lubrication conditions were performed on a pin-on-disk tribometer to eval- uate the effect of sliding distance, sliding velocity, normal load and temperature on the tribological behavior of the composite material/steel pair. The tribometer consists of a stationary loading pin sliding against a rotating disk with its axis perpendicular to the disk. The pins ran on the disks with two degrees of freedom: a vertical one, which allows normal load application by direct contact with the surface of the disk, and a horizontal one for friction measurement. The operating conditions were chosen so that the load remains constant, and the temperature is main- tained at two different levels. All the experiments were per- formed under dry sliding conditions. Eventually, test results from the tribometer were compared with the results obtained from standard chase machine testing. The test procedure on the chase machine is based on SAE standard J661a. All the tests were conducted on both sides of the friction plate. It was observed that the friction coeffi cient becomes stable for higher sliding distances. However, during the running in phase the friction coeffi cient behavior is quite different from one test to another. Eventually the comparison is made between a standard SAE J661 test and a pin-on-disk test and there was signifi cant difference in temperature because in the standard test, drum temperature was measured and in the pin-on-disk test, the temperature was measured on the lateral side of the clutch plate. In addition, there Contents lists available at SciVerse ScienceDirect journal homepage: Wear 0043-1648/$-see front matter & 2012 Elsevier B.V. All rights reserved. /10.1016/j.wear.2012.10.024 nCorresponding author. E-mail address: (Q. Zou). Wear 302 (2013) 13781383 was also difference in materials used and the dimensions of the plates. Therefore, only a quantitative comparison could be done. Results showed that the friction coeffi cient behavior as function of temperature was well characterized by the pin-ondisk test. In 2007 Marklund and coworkers 2 investigated the friction characteristics of a wet clutch using a pin-on-disk test. They developed a mathematical expression among the friction coeffi - cient, the sliding speed and the temperature which could be used to compute coeffi cient of friction. They also used a pin-on-disk test to show that the coeffi cient of friction depends on the sliding velocity and the temperature, but not on the load applied on the clutch plates. In Marklunds work, a special holder was developed for the pin-on-disk test to enable the local friction measurement for the material combinations used in wet clutch systems. In the pin-on- disk test, a stationary pin is loaded axially while in contact with a rotating disk and a thermocouple that measures the temperature at about 0.3 mm from the contact surface is inserted in the specimen. The tests started at ambient temperature, then the sliding velocity was gradually increased and then decreased. The temperature in the test specimen increases due to the frictional heating as velocity increases, and the temperature decreases as the velocity decreases. This temperature change is recorded and it was obvious that there was a delay in temperature measurement.Friction coeffi cientobtainedfromthistesting method was compared with a traditional friction measurement method to investigate the correlation. It was observed that friction coeffi cient remained the same for both testing methods. It was observed that the load did not greatly infl uence on the friction coeffi cient in these measurements. However, the friction coeffi cient signifi cantly depended on the sliding velocity and the temperature. In 1997, Holgerson 3 conducted tests using a wet clutch test rig which can apply a drive torque during engagement and vary the sliding velocity, the force, the inertia and the lubrication. Measured data included the normal force, the brake torque, the sliding velocity and the temperature over time. The friction characteristics as well as the power and the temperature were investigated. A simple model was developed to estimate the engagement performance which gave a good approximation of the performance measured in the tests. In 2005, Nyman and coworkers 4 conducted tests on a wet clutch to determine the infl uence of the surface topography on friction characteristics. To measure the surface topography they used vertical scanning interferometry. Different parameters such as the skewness, the average slope of a surface and the arithmetic mean curvature of a surface were used for surface characteriza- tion of clutch plates. However, their results were inadequate to characterize the clutch plates. In the course of the analysis and observation of different procedures for the friction test on clutch plates, the following results were obtained: (1) The friction coeffi cient does not depend on the load, but depends on the sliding velocity and the tem- perature. It decreases with increasing temperature and sliding velocity. (2) Friction coeffi cient remains stable after a running in phase. It is stable at a level which depends on both the sliding velocity and temperature. (3) Comparison of the pin-on-disk test results with those obtained through SAE J661a standard tests shows that the typical features regarding the friction coeffi cient behaviors are identical. Comparison between the pin-on-disk test results with those obtained from a full size friction disk test rig shows a successful correlation as well. (4) Using the clutch facing material as the disk in the pin-on-disk machine eliminates the glazing effect which is frequently observed on other tests where the friction material is reduced to a small sample. The literature review shows that very little research has been done to understand the friction characteristics of the clutch plates. There was no standard test for surface characterization of the clutch plates and no study on correlating the surface characteristics with the friction performance of the clutch plates. The objectives of the present research include: (1) To develop a surface characterization method for clutch plates used in current automotive transmissions. (2) To conduct friction and wear tests to determine the friction characteristics and wear performance of the clutch plates. (3) To determine a correlation between the results obtained from the surface characterization and the friction and wear tests. (4) To provide a baseline for further research and development of clutch plates. 2. Surface characterization 2.1. Experimental setup The surface topography measurement was carried out using a WYKO optical surface profi ler. The experimental setup is shown in Fig. 1. Rectangular samples cut from a friction plate were used. Before placing the specimen into the sample holder, the glass plate was cleaned with acetone to remove dust and any foreign particles, which would otherwise lead to inaccurate measurement of the surface topography. The sample holder clamped the glass plate and the friction plate together and the load was applied by tightening the bolts on the sample holder. A load cell connected to a load transducer was used to record the force applied on the specimen. When measuring the surface profi le, the frictional plate speci- men was placed underneath a glass plate, which facilitates the application of load on the friction plate and the measurement of surface topography. Through Transmissive Media (TTM) is an additional lens necessary to make a measurement through the glass. In this study, a maximum contact pressure of 4.6 MPa often used in real transmissions was applied. The real contact area betweentheclutchplatesampleandtheglassplateis 4.8?10?5m2. Therefore, the applied load corresponding to the contact pressure of 4.6 MPa is about 220 N. In the real application, the pressure on the clutch plate varies from zero to the maximum. Therefore, a series of load changing from zero to the maximum value of 220 N, with increments of 10 N, were applied on the friction plate during the surface characterization. In this study, several clutch plate materials (marked as clutch plate-A, B, etc.) have been tested. A similar trend was observed for all the materials. Therefore, only the test data for clutch plate-A is presented here. 2.2. Surface topography measurement of clutch plate-A Fig. 2 shows the surface profi les of clutch plate-A at applied loads of 10 N, 30 N and 50 N. From the fi gures it can be seen that with the increase of the applied load, the peaks on the surface profi le were compressed, and the contact area (the red area in the fi gures) between the clutch plate and glass plate increased. Fig. 3 shows how the average roughness Ra changes with the applied load. It can be seen that the average roughness Ra decreased slowly with the increase of the applied load. Fig. 4 marks the change of Rpk/Rkwith the increase of the applied load. Although the Ra value decreases slowly with increasing applied load, the Rpk/Rkratio dropped quickly at the beginning and reached a plateau state after that. This means the deformation Sample holder Friction Plate Sample Glass plate Bolt Fig. 1. Experimental setup for surface characterization. Q. Zou et al. / Wear 302 (2013) 137813831379 mainly occurred at the peaks of the surface profi le. After the load reached a certain point, the deformation of the peaks reached its maximum value and no further deformation was produced. This also means the real contact area remains the same when the applied load is beyond a critical point. For clutch plate-A, the critical load is about 30 N. 3. Friction and wear characterization 3.1. Friction test under fully lubricated conditions Friction tests were conducted on a UMT-2 universal micro- tribometer. A UMT-2 Universal micro-tribometer has a base, which is used to hold the specimen. The base can also hold suffi cient lubricant to submerge the specimens during operation. A unique specimen holder was developed for a reciprocating friction test to enable the separator plate to be fastened onto it. The specimen holder is further connected to the upper stage and facilitates the measurement of different parameters such as the frictional force, the normal load and the coeffi cient of friction. The friction plate specimen of 40 mm?5 mm and the separator plate specimen of 22 mm?6 mm were cut from the plates. The edges of the separator plate specimen were made blunt in order to prevent plowing on the surface of the friction plate specimen. Fig. 5 shows a specimen being cut from a separator plate and a similar machining process was followed to obtain a friction plate specimen. The friction plate specimen was held rigidly on the base and the separator plate specimen was held rigidly on the upper sample holder. The sketch of the specimen holder is shown in Fig. 6. Before assembling the specimen onto the machine, acetone was used to clean the base and the upper specimen holder in order to remove debris, lubricant and dirt from the surfaces. DEXRON 5 automatic transmission fl uid was used to conduct thetestsunderfullylubricatedconditions.DEXRONisa Fig. 2. Surface profi les of clutch plate-A at different applied loads. 0 1 2 3 4 5 6 7 0102030405060708090 100 110 120 140 170 220 Average Roughness Ra (m) Applied Load (N) Fig. 3. Average roughness Ra vs. the applied load. Fig. 4. Rpk/Rkvs. the applied load. Q. Zou et al. / Wear 302 (2013) 137813831380 registered trademark of the General Motors Corporation. The used lubricant was replaced with fresh lubricant prior to running a test. The lubricated friction test was conducted under various loads. Each test was repeated three times. The following conditions were applied for the friction tests with lubricant: Test methodReciprocating test Friction plate dimensions40 mm?5 mm Separator plate dimensions14 mm?6 mm Contact area30 mm2 TemperatureRoom Temperature Test duration30 min Lubrication conditionLubricated Reciprocating speed160 rpm During the reciprocating test the temperature and the reci- procating speed were kept constant. Different loads were applied in order to investigate the effect of the load on the coeffi cient of friction. The range of the load applied for friction tests is not the same as the range of load applied for surface characterization since the contact areas are different. The range of the load applied for the friction tests was calculated based on the maximum application pressure and the contact area between the friction plate and separator plate specimen. The range of the load applied during the lubricated friction test is shown in Table 1. Fig. 7 shows the lubricated friction test results on the clutch plate-A under 6 N of load. The curves of the applied load (Fz), the friction force (Ff ) and the coeffi cient of friction (COF) vs. time were plotted. From the fi gure it can be seen that during the 30 min test, the friction force is very stable with only small variation. The curve of the average coeffi cient of friction vs. the test load was plotted and is shown in Fig. 8. The results depicted that the friction coeffi cient remained nearly constant during the test through the entire range of the load. From the results it can be seen that under fully lubricated conditions, the effect of the surface characteristics on the tribological performance of the friction materials is not signifi cant. During the friction test, the friction plate sample is completely submerged in the lubricant and this made the friction plate surface to be very dark. The wear of the specimen is minimal and cannot be measured and correlated with the friction test. 3.2. Friction test under dry conditions Wet clutches are usually oil immersed clutch plates. The fric- tional properties of the fl uid surrounding the clutch are extremely important. However, during the running in process of an engine transmission, the clutch starts operating with minimal lubricant and Fig. 5. Specimen cut from the separator plate. Separator Plate Friction Plate Fig. 6. Reciprocating test setup. Table 1 Applied load in lubricated friction test. Test no.Contact pressure (MPa)Contact area (mm2)Applied load (N) 10.130.03 20.230.06 30.430.012 41.030.030 52.630.078 64.430.0132 0200400600800100012001400160018002000 Time,sec -7 -6 -5 -4 -3 -2 -1 0 1 Fz,N 0 0.5 1.0 1.5 2.0 2.5 3.0 Ff,N 0 0.05 0.10 0.15 0.20 COF, Fig. 7. Friction test on clutch plate-A at 6 N (ATF Lubricated). Fig. 8. COF vs. load for clutch plate-A (ATF Lubricated). Q. Zou et al. / Wear 302 (2013) 137813831381 it is important to understand the friction characteristics of the clutch during this phase of operation. Therefore, tests under dry conditions were conducted. The test condition and the procedure for dry friction tests are the same as for the lubr