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Tribology and Lubrication Technology November 2012 : Page 69

Peer-reviewed Description of Wear Debris from On-Line Ferrograph Images by Their Statistical Color SE-MIN PARK 1 , TAE-YOUNG WON 1 , GI-HOON KIM 2 , DAE-SUNG KIM 2 , and YOUNG-ZE LEE 1 1 School of Mechanical Engineering Sungkyunkwan University Suwon GyeongGi-Do, Korea 2 Korea Automotive Technology Institute Chunahn, Korea Manuscript received February 10, 2010 Manuscript accepted September 13, 2010 Review led by Jim Netzel KEY WORDS Rubber Seals; Abrasive Wear; Dust Particle; SEM; Size of Particle; Particle Ingredient ABSTRACT In an automobile chassis system, several environmental factors reduce the durabil-ity of automotive components. In particular, dust particles entering through rubber seals increase the friction and wear of sliding surfaces. Increased wear causes noise, fracture, and reduced service life of components. In this study, dust particles were col-lected on the chassis while driving. The particle sizes and ingredients of the dust were analyzed using microscopy, scanning electron microscopy, and energy-dispersive X-ray. It was found that SiO 2 and Al 2 O 3 were the main ingredients of the dust particles. Based on our analysis of the data, wear tests were performed with the rubber seal specimens using a ball-on-disk type sliding wear tester. When SiO 2 and Al 2 O 3 par-ticles came into contact with the rubber seal specimen, two-body and three-body abrasive wear took place. In tests of SiO 2 particles only, the wear volumes of rubber seal specimens were increased by increasing the particle’s injection amount and the size of the dust. In a test of the mixed particles (SiO 2 and Al 2 O 3 ), four mixing ratios (8:1, 4:1, 2:1, 1:2 [SiO 2 :Al 2 O 3 ]) were used based on the analysis data. When the same size particles of SiO 2 and Al 2 O 3 were used, the wear amounts of the rubber increased with a higher percentage of Al 2 O 3 , which is harder than SiO 2 . When mixed particles with 26.6 μm of SiO 2 and 1 μm of Al 2 O 3 were used, wear increased with increased amounts of SiO 2 . Editor’s Note: For the last few decades, we have been introduced to “lifetime lubri-cated” parts on our vehicles, but this does not translate to maintenance-free. While we have far fewer components requiring relubrication, we have found new components in need of replace-ment. Probably one of the most predominant in this category is the CV joint commonly found on front-wheel drive vehicles. The failure mode for this component is usually loss of lubrication trig-gered by failure of the CV boot, which is typically made from rubber. This boot is exposed to significant contamination kicked up by the tires off the road surface. This month’s Editor’s Choice paper attempts to qualify and quantify the wear in such parts to help us understand the failure mode and perhaps find a way to design it out. Evan Zabawski, CLS Editor INTRODUCTION The durability of an automobile chassis system is directly related to the safety of passengers and property. One of the main safety issues is the wear of automo-tive components. Because the chassis system is exposed to dust, soil, and oil leaks, the components are easily damaged by the abrasive action of particles. In order to improve the reliability of the components in the chassis system, a study on wear should be performed with automobile components under real driving conditions. However, many studies of the reliability of the chassis were based only on the method of accelerated tests (Duyun and Lauwerys 1 ; Degrange, et al. 2 ; Thominea, et al. 3 ) and simulation of the seals (Lee, et al. 4 ; Shen and Salant 5 ). Environmental factors such as temperature, humidity, radiation intensity, and inflow of dust particles are associated with the reliability of the chassis system (Tuszynski, et al. 6 ; Schutz, et al. 7 ; Thomas, et al. 8 ). Abrasive wear usually occurs TRIB OL OG Y & L UBRIC A TION TE CHNOL OG Y NO VEMBER 2 012 • 69 WWW .S TLE. OR G

Peer Reviewed Paper

Jim Netzel

<br /> Description of Wear Debris from On-Line Ferrograph Images by Their Statistical Color<br /> <br /> KEY WORDS<br /> Rubber Seals; Abrasive Wear; Dust Particle; SEM; Size of Particle; Particle Ingredient<br /> <br /> ABSTRACT<br /> In an automobile chassis system, several environmental factors reduce the durability of automotive components. In particular, dust particles entering through rubber seals increase the friction and wear of sliding surfaces. Increased wear causes noise, fracture, and reduced service life of components. In this study, dust particles were collected on the chassis while driving. The particle sizes and ingredients of the dust were analyzed using microscopy, scanning electron microscopy, and energy-dispersive Xray. It was found that SiO2 and Al2O3 were the main ingredients of the dust particles. Based on our analysis of the data, wear tests were performed with the rubber seal specimens using a ball-on-disk type sliding wear tester. When SiO2 and Al2O3 particles came into contact with the rubber seal specimen, two-body and three-body abrasive wear took place. In tests of SiO2 particles only, the wear volumes of rubber seal specimens were increased by increasing the particle’s injection amount and the size of the dust. In a test of the mixed particles (SiO2 and Al2O3), four mixing ratios (8:1, 4:1, 2:1, 1:2 [SiO2:Al2O3]) were used based on the analysis data. When the same size particles of SiO2 and Al2O3 were used, the wear amounts of the rubber increased with a higher percentage of Al2O3, which is harder than SiO2. When mixed particles with 26.6 µm of SiO2 and 1 µm of Al2O3 were used, wear increased with increased amounts of SiO2.<br /> <br /> INTRODUCTION<br /> The durability of an automobile chassis system is directly related to the safety of passengers and property. One of the main safety issues is the wear of automotive components. Because the chassis system is exposed to dust, soil, and oil leaks, the components are easily damaged by the abrasive action of particles. In order to improve the reliability of the components in the chassis system, a study on wear should be performed with automobile components under real driving conditions. However, many studies of the reliability of the chassis were based only on the method of accelerated tests (Duyun and Lauwerys1; Degrange, et al.2; Thominea, et al.3) and simulation of the seals (Lee, et al.4; Shen and Salant5). Environmental factors such as temperature, humidity, radiation intensity, and inflow of dust particles are associated with the reliability of the chassis system (Tuszynski, et al.6; Schutz, et al.7; Thomas, et al.8). Abrasive wear usually occurs on rubber seals, boot rubber, and bushing rubber. Wear on the rubber parts of sealing devices causes leakage of oil and also reduces the endurance of wheel bearings and ball joints (Sefert and Westcott9; Uedelhoven, et al.10). This can result in noisy and uncomfortable driving and frequent mechanical difficulties (Ayala, et al.11; Pei, et al.12).<br /> <br /> In this study, dust particles were collected on the chassis during a drive in the northern province of China. The particles were classified into several road conditions: paved or unpaved road, rural or urban road, and highway. The samples were analyzed to verify the size of dust particles, and the ingredients of the dust particles were analyzed using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy. Based on these data, particles for wear tests were selected with real ingredients, adequate sizes, and incoming amounts. Wear tests were performed with rubber seal specimens using a ball-on-disk type sliding wear tester.<br /> <br /> EXPERIMENTAL DETAILS<br /> Collection of Dust Particles<br /> To verify the effect of dust particles on the wear of chassis components, the dust particles were collected directly on the roads during actual driving. Several filters were attached to the joint bushing in the chassis system. The average temperature in January was near -25°C. Test driving was done for more than 2,000 km. After driving for 100 km, the samples were collected and the filters were replaced. Dust particles were classified based on several road conditions such as rural road (paved or unpaved), urban road, and highway. The roads were selected to compare the size and composition of the various types of dust. The environmental conditions, such as residential area, industrial area, or rural area, might affect the size and the composition of the dust. Depending on the road conditions, deicing chemicals may have been present. Those factors could be related to different wear amounts and to mechanisms between particles and rubber seals in the chassis system. Collected dust samples were analyzed using optical microscopy, SEM, and EDX spectroscopy to obtain size distributions and composition.<br /> <br /> Analysis of Dust Particles<br /> Figures 1 and 2 show SEM images and an EDX analysis, respectively, of dust particles collected for each road condition. Various dust samples were clumped together as shown in Figure 1. The images of the dust samples were taken on the surfaces of the filters. For the rural road, the main ingredients were silicon and oxygen and possibly silica. There were only limited amounts of other metallic components. In the urban sample, various components such as Mg, Fe, and Ca were found on the dust lumps in contrast to the rural samples. Because a deicing chemical such as NaCl cannot typically be used on rural roads, the weight percentage of Na in the dust was very low. On the urban road, the main components of dust were alumina and silica. The intensity ratio of alumina to silica was 1:2. Compared to the rural samples, the components of Fe and Mg were higher. Also, there was more Na on the urban road due to the use of deicing chemicals. In the case of the highway road, the main components of dust were alumina and silica, similar to the urban road. The ratio of alumina to silica was 1:8. The difference in the ratio was probably due to various industrial structures around the highway roads. The Na component was greater on the highway road due to the deicing chemicals.<br /> <br /> Figure 3 shows the size distribution of dust particles collected for each road condition based on an image analyzing program. On the rural roads, the size of dust particles on the unpaved roads was slightly larger than those on the paved roads. On paved roads, small particles of less than 40 µm were abundant compared to those on the unpaved road. Relatively larger particles were found on the unpaved rural roads. The range of dust particle sizes on the rural roads was 40–75 µm, regardless of whether the road was paved. On the urban roads, the range of dust particle sizes was 40–75 µm, and the range ratio of 20–30 µm to 40–75 µm was around 2:5. In the case of highway roads, the ranges of dust particle sizes were 20–30 µm and 40–75 µm and the range ratio of 20–30 µm to 40–75 µm was around 3:5.<br /> <br /> Wear Tests<br /> A ball-on-disk sliding tester was used to investigate the wear characteristics of the rubber seal material as shown in Figure 4 (Cho and Lee13). A servomotor was used to rotate the shaft. The lower flat disk of rubber was mounted on the rotating shaft in a water bath. A ball was located in the upper holder, which was clamped to a fixed arm and a friction sensor. Contact was achieved by pressing the ball against the flat surface under a normal load using dead weights. AISI 52100 steel was used for the 10-mm-diameter ball. As shown in Figure 5, rubber seal material was used for the disk specimens with a diameter of 66 mm and a thickness of 3 mm. The surface roughness was 0.69 µm in Ra or 1.01 µm in Rq. All specimens were cleaned before the tests with distilled water.<br /> <br /> The wear tests were performed using the dust samples from various road conditions. From the analysis of dust on paved or unpaved rural roads, the main component of the dust was silica, as shown in Table 1. Silica was used in the tests using the singular dust in order to investigate the effects of the amount of dust and dust particle sizes on the wear of rubber. Next, silica and alumina were mixed to use as the composite dust based on the actual ratio of SiO2 and Al2O3. The hardness of silica was 800 HB and the hardness of alumina was 2,100 HB. The mixed dusts were selected based on the results of the previous road tests, as shown in Table 1.<br /> <br /> The dusts were mixed with distilled water to distribute the dust on the sliding surfaces. A slow sliding speed of 0.04 m/s (30 rpm) was used in all tests to maintain a condition of boundary lubrication. Each test was performed for 25 min and a load of 20 N. The test conditions were more severe than real contact conditions in order to produce sufficient wear tracks on the rubber surfaces.<br /> <br /> RESULTS AND DISCUSSION<br /> Because the dust particles are much harder than rubber, it was expected that the particles would cause abrasion on the sliding surfaces. Figure 6 shows wear tracks on the rubber surface, which represents behavior such as stick-slip (Fukahori and Yamazaki14,15; Lim, et al.16; Jeong, et al.17). Sliding particles were regularly embedded into the rubber after some sliding distance.<br /> <br /> The surfaces of the rubber seal were damaged by abrasive wear in the contact area between the dust and the rubber. As the abrasive particles contacted the surface of the rubber seal, many scratches were formed by three-body abrasive wear (Sin, et al.18); Wirojanupatump and Shipway19; Kim, et al.20). Scanning electron micrographs of the worn surfaces of the rubber seal are shown in Figure 7. When the ductile rubber was abraded by dust particles, the rubber was plastically deformed by a ploughing mechanism as shown in Figure 7a. Figure 7b shows silica embedded in the surface of the rubber seal. The embedded particles caused two-body abrasion on the steel surface (Figure 7c). After removing the silica from the surface of the rubber, craters were found on the surface. As a result of these deformations, the rubber seal was damaged due to abrasive wear.<br /> <br /> As increasing the amount of silica the wear tests were performed with a load of 20 N and a rotation of 30 rpm. The test time was 25 min. Dust with particle size of 26.6 µm was used in the tests. The silica was mixed with 10 mL of water. The amount of silica was increased in increments of 0.5 g from 0.0 to 2.5 g. As shown in Figure 8, the volume loss of the rubber seal increased up to 1.5 g of silica. After that, the wear volume did not show a significant change.<br /> <br /> Scanning electron micrographs of the worn surfaces of rubber seal material with increasing amounts of silica are shown in Figure 9. With 0.5 g of silica, the rubber surface showed several scratches (dotted lines). As the amount of silica increased, the number of scratches on the rubber surface increased up to 1.5 g. The ratio of 1.5 g silica/10 mL water was the critical ratio. The damage to the rubber seal no longer increased with increasing amounts of silica. Figure 10 shows an enlarged view of the rubber surface that was tested with 2.5 g of silica. Because silica was agglomerated on the entire rubber surface, the wear amount did not increase.<br /> <br /> Dust with particle sizes of 2.0, 10.9, and 26.6 µm was used in the next tests. Figure 11 shows the wear volumes of rubber seal material versus the size of silica particles.Scanning electron micrographs of the worn surfaces of rubber seals for silica particle sizes of 2.0,10.9,and 26.6 um were wider than the surface of 2.0 and 10.9 um. By increasing the size of abrasive partcles,the applied load at the contact area between the abrasive particles and the surface of the rubber seal increased. Increasing the applied load resulted in deep grooves on the surfaces of the rubber seal. By increasing the size of the dust partcles, the grooves became wider and deeper and the wear volume increased.<br /> <br /> Wear tests were performed using composite dusts with alumina and silica as shown in table 1. The total mass of the Dust Was fixed at 1.5g. the size of silica was 0.8um and the size of silica was 0.8 pm and the size of alumina was 1 µm. Figure 13 shows the wear volume of the rubber seal using mixed dust particles. By increasing the amount of silica, the wear volumes decreased. Because alumina is much harder than silica, the wear of rubber was strongly influenced by the amount of alumina. Next, tests were performed using large silica particles of 26.6 µm and alumina particles of 1 µm.<br /> <br /> Figure 14 shows the wear volume of the rubber seal for mixed dust particles. By increasing the amount of silica, wear volume increased. Scanning electron micrographs of the worn rubber surfaces in mixed dust are shown in Figure 15. By increasing the amount of silica, the scratches on the rubber surface were wider and deeper. Silica is softer than alumina, but the sizes of the silica particles were larger than the alumina particles. Therefore, the size of the soft particles is associated with the wear of the rubber to a greater degree than the hardness of the particles.<br /> <br /> CONCLUSION<br /> In this study, dust particle sizes and content were analyzed, and wear tests were conducted with silica and alumina to investigate the wear characteristics of rubber seal material in an automobile chassis system. The following conclusions were obtained:<br /> • The main component of dust on the rural road was silica. On the urban road and highway, the main components were alumina and silica. On the urban road, various components such as Mg, Fe, and Ca were found on lumps of dust compared to the rural road. On the rural roads, the sizes of dust particles were larger than those on the urban road. The range of dust size for all road conditions was 40–75 µm.<br /> • The primary damage mechanism of the rubber seal material was abrasive wear. Increasing the amount of silica flowing into the chassis system increased the wear volume of the rubber seal. The critical silica-to- water ratio was 1.5 g/10 mL. Increasing the size of dust particles resulted in increased wear volume.<br /> • In the wear test using the composite dust with alumina (1 µm) and silica (0.8 µm), by increasing the amount of silica, the wear volumes of the rubber seal decreased. Using composite dust with alumina (1 µm) and silica (26.6 µm), the wear volume of the rubber seal increased. The larger, soft dust particles resulted in more wear on the rubber seal than the smaller, hard dust particles.

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