- Email: [email protected]
Frictional Wear and Thermal Fatigue Behaviours of Biomimetic Coupling Materials for Brake Drums
Journal of Bionic Engineering Suppl. (2008) 20–27
Frictional Wear and Thermal Fatigue Behaviours of Biomimetic Coupling Materials for Brake Drums Yu Zhao1, Lu-quan Ren1, Xin Tong2, Hong Zhou2, Li Chen1 1. Key Laboratory of Terrain-Machine Bionics Engineering (Ministry of Education, China), Jilin University, Changchun 130022, P. R. China 2. The Key Lab of Automobile Material (Ministry of Education, China), Jilin University, Changchun 130022, P. R. China
Abstract In order to enhance frictional wear resistance and thermal fatigue resistance of brake drums, two kinds of biomimetic coupling materials are prepared by laser surface melting and laser coating technology respectively. These materials are all compounded by base metal and biomimetic coupling units, and have different coupling characteristics, such as the variation of the unit shape, different microstructures and hardness, different chemical compositions of units. The frictional wear resistance and thermal fatigue resistance of biomimetic coupling materials and gray cast iron used for brake drum are compared. The results indicate that frictional wear resistance and thermal fatigue resistance of biomimetic coupling sample is better than that of untreated sample, and among the biomimetic samples, laser coating treated sample has superior resistance to wear and thermal fatigue comparing with laser melting treated sample. Keywords: frictional wear, thermal fatigue, biomimetic coupling, brake drum, gray cast iron Copyright © 2008, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved.
1 Introduction Brake drum is one of the most important components in brake system of trucks. According to the principles of braking, brake drum receives powerful pressure and static or kinetic friction force from brake disk. Especially the sliding velocity and specific pressure increase with the speed of automobiles, which result in heavier wear and higher temperature. It is reported that the surface temperature of brake drum can reach as high as 900 C, and the alternating heating and cooling caused by frequent braking also lead to initiation and propagation of thermal cracks[14]. Fig. 1 shows the wear and thermal fatigue characteristics of a disabled brake drum. The above damages to brake drum do a lot of harm to the braking performance and even the safety of automobiles. Compared with frictional materials, the material for brake drum has a single category and slow development. There are mainly two kinds of braking materials, cast Corresponding author: Yu Zhao E-mail: [email protected]
iron and aluminium base composite. Gray cast iron has long been used because of its low cost and desirable properties such as low melting point, good fluidity and cast-ability, excellent machinability etc. Concretely, HT150 and HT200 are more commonly used by inboards, while outboards prefer gray cast iron grade 250 and high-carbon gray cast iron. However, these materials
Fig. 1 Thermal fatigue cracks and wear patterns of a fail brake drum.
Zhao et al.: Frictional Wear and Thermal Fatigue Behaviours of Biomimetic Coupling Materials for Brake Drums
can not satisfy applications due to their low hardness and intensity. In order to refine and increase pearlite microstructure, some alloy elements are added to cast iron. For example, the high carbon low alloyed cast irons wherein carbon content is more than 7.36 % and silicon content is less than 1.5 %, containing molybdenum, cuprum or nickel are widely applied in Britain and America; while in Germany the added alloy elements are cuprum, chromium and stannum[5,6]. Although the intensity and hardness of cast iron can be enhanced by this method, yet its disadvantages are weak resistance to thermal fatigue and high waste of alloy elements. For lightening weight and improving thermal conductivity of braking material, the ceramic reinforced aluminum base composite has become a new researching point, and the usually used hard grains are SiC, TiC, Si3N4 and Al2O3. But the high cost and difficult selection of brake disk limit their applications[711]. Therefore, it is urgent to develop new materials for brake drums. Bionics is a science that imitates the principles of bio-systems to build new technological systems of useful components. Over the past two decades, bionics has had a profound influence on engineering material research, because the unique structures, compositions and correspondingly excellent properties of biological and natural materials gave researchers many clues to improve the properties of engineering materials or increase the reliability of structural components. In fact, biology can not only adapt to the environment, but also satisfy its survival with the lowest energy cost. Therefore, the new components or materials, which are designed and manufactured by biomimetic methods, also have advantages such as high efficiency and low consumption of energy. Research on biomimetic non-smooth surface found that the bulldozer blades with such surfaces can reduce sliding resistance against soil[12]. In recent years, the technology of biomimetic non-smooth surface has been applied to brake disks for trains, brake drums for trucks and a lot of hot-work dies, aiming to solve the problem of frictional wear, abrasive wear or thermal fatigue[13,14]. The efforts are all about changing the morphologies of biomimetic non-smooth surface such as shape, size and convex-concave of non-smooth units. With further investigation, we discover that some com-
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plex problems in engineering systems are difficult to solve completely by our invented biomimetic method which only has single changing factor. In this research, two biomimetic coupling materials are prepared by laser surface melting and laser coating technology respectively. These materials are all compounded by base metal and biomimetic coupling units, with different coupling characteristics, such as varied units’ shape, dissimilar formed structure between base metal and units, different chemical compositions of units and so on. The frictional wear and thermal fatigue resistance of the coupling materials and gray cast iron are compared.
2 Experiments 2.1 Experimental materials A gray cast iron codenamed HT200, which is widely used for brake drums, is applied as the base metal material whose microstructure consists of flake graphite and pearlite shown in Fig. 2. According to different molding technology, biomimetic coupling units are divided into Laser Surface Melting Coupling Unit (LMU) and Laser Coating Coupling Unit (LCU) respectively. Among them, the LCU is made through laser coating of Fe-based self-fluxing alloy (Fe30A). The chemical compositions of the materials are given in Table 1 and Table 2.
Fig. 2 Microstructure of HT200 gray cast iron. Table 1 Chemical compositions of HT200 gray cast iron (wt%) Compositions Content
C
Si
Mn
P
S
Cu
Cr
3.250 1.570 0.920 0.060 0.059 0.500 0.270
Fe Bal.
Journal of Bionic Engineering (2008) Suppl.
22
Table 2 Chemical compositions of Fe30A alloying power (wt%) Compositions Content
C
B
Si
Cr
Ni
0.5–0.6 1.0–2.0 2.0–3.5 12–14
Mo
30–37 4.0–6.0
Fe Bal.
2.2 Sample preparation A solid state Nd-YAG laser of 1.06 μm wavelength and maximum power of 300 W is employed for laser surface melting and laser coating. The base metal is mechanically polished, using progressively finer grades of silicon carbide impregnated emery paper. By controlling the laser tracks, LMU is formed to latticed shape with spacing of 3 mm. For processing LCU, the base metal has some notches of 1 mm width and 1 mm depth, which are used for filling Fe-based self-fluxing alloy. Laser tracks are along the longitude direction of notch, and a shielding gas of argon is used to minimize contamination of the treated surface. Other laser processing parameters are listed in Table 3. Table 3 Laser parameters for processing biomimetic units Pulse duration
Frequency
Electric current
Laser spot diameter
Scanning speed
7.0 ms
10 Hz
120 A
0.8 mm
1 mm·s1
Wear samples of 14 mm length, 10 mm width and 10 mm thickness and thermal fatigue samples of 40 mm length, 20 mm width and 6 mm thickness are cut by an electric spark machine, and a 3 mm diameter round hole is drilled at one side of each thermal fatigue sample, so that they can be fixed onto the plate of the thermal fatigue experimental machine. According to different materials, samples are divided into three kinds: Untreated Sample (US), Laser Surface Melting Sample (LMS), and Laser Surface Coating Sample (LCS). 2.3 Experimental method Transverse sections of biomimetic coupling units are cut parallel to the length direction, and standard methods of metallography are followed. The microstructure is characterized by JSM-5500LV scanning electronic microscope. A XTL-2400 stereomicroscope is used to study the initiation and propagation of thermal cracks. Phases and compounds formed in the coating zone are identified by D/max-RC X-ray Diffraction
(XRD) with Cu K radiation operated at a voltage of 40 KV, a current of 40 mA, and a scanning rate of 40·min1. The alloying element’s distribution of micro-zones is analyzed by an EDAX-Falcon Energy Diffraction Spectrometer (EDS) which is equipped on a JSM-5500LV scanning electronic microscope. A Vickers Microhardness Tester (model 5104, manufactured by Buehler Co. Ltd., USA) is used for the microhardness measurement. Dry sliding tests are performed using an MM-200 block-on-ring wear tester (made by the Xuanhua Testing Machine Factory). All the friction and wear tests are carried out at room temperature. Experimental samples slide on a rotating GCr15 steel ring. The GCr15 steel has an average hardness of 61–63 HRC and the sizes of 16 mm inside diameter, 50 mm outside diameter and 10 mm thickness. A normal load of 8 kg is used for wear tests. The rotational speed of the ring is 400 r·min1 and the sliding time is 30 min. Before each test, the surface of the steel ring is polished to a roughness of about 0.1 mm, while samples were ultrasonically cleaned in anhydrous alcohol and dried before and after wear tests. The mass loss is measured by a sensitive electronic balance with an accuracy of 0.0001 g. The difference in mass of three test blocks before and after the experiment gives the average mass loss. To assist the analysis of wear mechanisms, the worn surfaces of block specimens are examined by LEXT-OLS 3000 OLYMPUS laser confocal scanning microscope. Thermal fatigue tests are carried out by a selfrestrain thermal fatigue testing machine. The samples are heated by a high temperature electric resistance furnace, and cooled by running water. This machine can record thermal cycle times automatically. The furnace temperature is controlled by thyristor control of type (KSY-12-16), and the sample temperature is monitored by a thermocouple attached to the sample at the centre of its length. A complete thermal cycle includes heating to 600±5 C in 70 seconds, and then cooling to 25±5 C in 3 seconds. The samples are free from any externally applied load, and are taken out to observe the cracks every 200 cycles, then the thermal fatigue resistance of samples is evaluated.
Zhao et al.: Frictional Wear and Thermal Fatigue Behaviours of Biomimetic Coupling Materials for Brake Drums
3 Results and Discussion 3.1 Structure characteristics of biomimetic coupling units Fig. 3 shows the cross-section morphology of biomimetic coupling unit. Observation indicates that the unit contains two areas. One is the melted zone at the outer layer which is bright in Fig. 3, because cast iron is melted completely in this zone, and subsequently recrystallized under a fast cooling rate. Due to thermal conduction, the materials around the melted zone takes solid phase transformation, thus this area is called transformed zone. Fig. 4 shows the SEM micrographs taken from the transverse section of LMU. It can be seen that the melted zone (see Fig. 4a) is composed of Fe3C, M and Ar, and the Fe3C is reticular. The microstructures of the transformed zone, as shown in Fig. 4b, consist of M, a little Ar and some un-dissolved G. The crystal grains in the whole unit zone are very fine and compact, and these results agree well with the findings of Shen and Li[15], which can be explained by the rapid cooling rate during solidification caused by laser processing method.
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Fig. 5 shows the SEM micrographs taken from the transverse section of LCU. It can be seen that the melted zone (see Fig. 5a) is composed of Ar, martensite, M7C3, M23C6, and Fe2B. Therefore, the unit is strengthened by solution strength and dispersion strength. Because the heating temperature of the transformed zone is between eutectoid reaction temperature and eutectic transformation temperature, the microstructures in this zone of LMU and LCU are similar, and they all have martensite, Ar and un-solved graphite. The X-ray diffraction results also identify the above microstructure.
Fig. 3 Cross-section morphology of biomimetic coupling unit.
Fig. 4 Microstructure of the LMU.
Fig. 5 Microstructure of the LCU.
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Journal of Bionic Engineering (2008) Suppl.
3.2 Micro-hardness of biomimetic coupling units The curves of hardness distribution in the LMU and LCU are shown in Fig. 6. The reported micro-hardness is measured along the direction of laser irradiation (from the top of unit to the bottom), which are obtained at different locations on the polished and etched cross-sections of the biomimetic unit for each sample. The average micro-hardness of the substrate is 280HV, while that of biomimetic unit is much higher. It can be seen that the micro-hardness of LMU varies from 561 HV to 603 HV, and that of LCU varies from 567 HV to 632 HV. The micro-hardness of LCU is higher than that of LMU, which can be attributed to the solution of carbon, the formation of martensite, the refinement of structure, the large proportion of cementite, especially the solution strength and the precipitation of alloying carbide. Micro-hardness is of particular importance in assessing wear resistance coatings, because it is considered as a good indication of coating integrity and wear resistance.
Fig. 6 Micro-hardness distributing of LMU and LCU.
3.3 Frictional wear resistance 3.3.1 Wear mass loss The wear resistance samples are divided into 3 groups. One group is untreated, the others are processed to biomimetic coupling samples by laser melting and laser coating technology respectively. Three samples of each group are chosen for wear tests. The average mass losses of different samples are shown in Fig. 7. It can be seen that the wear losses of the biomimetic coupling samples is 48.6% less than that of the untreated sample in the same sliding way against the steel blocks. Among
biomimetic coupling groups, the sample treated by laser coating with Fe30A has less wear mass loss.
Fig. 7 Wear mass loss of test samples.
3.3.2 Wear pattern Fig. 8 shows the worn surface of US, LMS and LCS after sliding wear tests against the wearing ring GCr15. As seen in Fig. 8a, some materials are lifted and sheared off in the wear process; the wear grooves with irregular shapes are in various depth. Besides, macroscopic observation reveals that some red-brown and black wear debris, whose chemical nature is Fe2O3 and Fe3O4[16], clearly appear on the worn surfaces and some materials adhere to the worn surfaces and other materials adhere to the steel counterparts. This phenomenon indicates that there is severe plastic deformation on the surface of US, some materials are pulled out and transferred to the counterpart surface resulting in adhesive wear. Deep grooves on the wear surface are generated as a result of cutting action, which is produced by wear debris. Some of the debris is iron oxides, which primarily originated from the contacting surface of GCr15 ring and are oxidized during the dry sliding wear process. Figs. 8b and 8c compare the morphologies of the worn surface of the biomimetic samples under the same sliding conditions. Compared with the US, the extent of the abrasive wear is lower and there is less red-brown and black wear debris on the steel counterpart observed by macrographs. The dominating wear mechanism is adhesive wear, whereas the abrasive and oxidation signs are not obvious. Moreover, the LCS displays better wear resistance than the LMS, because of its greater hardness.
Zhao et al.: Frictional Wear and Thermal Fatigue Behaviours of Biomimetic Coupling Materials for Brake Drums
(a) US
(b) LMS
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(c) LCS
Fig. 8 Worn surface of different samples after a sliding wear test.
3.4 Thermal fatigue resistance 3.4.1 The number and length of thermal cracks During thermal fatigue tests, the number of thermal cracks on each sample surface is counted after 2000 thermal cycles, and in the counting, if any crack is longer than 0.5 mm it is defined as one crack. Fig. 9 shows the number of thermal cracks, indicating that the number of cracks on US is always the most, then the LMS, and the LCS. Based on the above results, it can be deduced that biomimetic coupling units on sample’s surface have superior resistance to the initiation of thermal crack, the biomimetic coupling sample, especially formed by laser coating with Fe30A has higher resistance. Fig. 9 also shows the measured length of crack of different samples. It can be seen that after the same thermal cycles (2000), the length of crack on US surface is always the longest, then the LMS, and the LCS. In fact, the biomimetic coupling samples, especially formed by laser coating with Fe30A, have the higher resistance to crack propa-
gation. Based on the above number and length of cracks, it can be deduced that the thermal fatigue resistance of biomimetic coupling sample is better than that of un-treated sample, and among biomimetic samples, laser coating treated sample has superior resistance to thermal fatigue compared with laser melting treated sample.
Fig. 9 Crack number and length of different test samples.
Fig. 10 Microcosmic place of crack initiation.
3.4.2 Crack initiation resistance of biomimetic coupling unit It is discovered that macroscopical place where thermal crack initiated is mainly from the fixed hole and the edge of sample. After 50 thermal cycles, microcosmic place where crack initiated is also observed, and from Fig. 10 it can be seen that the place is at the sharp point of graphite. Due to nearly no intensity, the graphite is just like a hole distributing in the matrix, thus a crack initiated there first. According to the microstructure in melted zone, where graphite is not found, the source of crack initiation disappears. Therefore, processing units on sample surface, especially at the edge of sample can decrease the number of thermal cracks obviously.
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Journal of Bionic Engineering (2008) Suppl.
3.4.3 Crack propagation resistance of non-smooth unit Many micro-cracks extend along the graphite as their initiation source, and these micro-cracks connect each other to form a main crack. Observations of the propagation routes of main crack indicates that main crack propagates along the graphite and the direction of their shortest distance, showing that propagation of main crack is realized by bridge connections with micro-cracks in front of it (see Fig. 11). In fact, biomimetic coupling unit not only causes such an advantageous inhibition to crack initiation, but also resists crack propagation. It can be explained as that, in the unit area there is no graphite phase, hence there is no bridge connection between main crack and micro-crack which grows relying on graphite. That is to say, the route of crack propagation is cut off by the unit.
Fig. 11 Bridge connections between main crack and micro-cracks.
3.4.4 Effects of alloying elements Though microstructure of biomimetic unit obtained by laser melting also has characteristics of no graphite phase and refined grain sizes, which can improve thermal fatigue resistance of samples, it is still worse than that treated by laser coating of Fe30A. It is well known that crack will lanate biomimetic unit, then gets further propagation when fatigue damages are bigger than the rupture strength of the unit. In addition, the oxidation occurs badly during thermal tests, and repeated building and falling of oxide film at crack point promotes the destruction of the unit. So the higher strength and oxidation resistance of the unit, the better thermal fatigue resistance that biomimetic sample has. Due to laser coating, non-smooth unit is rich in Cr, Ni, Mo, austenite is strengthened by the solid solution of them, and eutectic carbide M7C3 has higher strength and hardness
comparing with cementite Fe3C. On the other hand, Cr is an element which can improve heat-durability of cast iron, because it forms an oxide film rich in Cr with oxygen, like Cr2O3 or FeCr2O4 which can restrain oxidation again[17]. Based on the above reasons, thermal fatigue resistance of biomimetic coupling sample can be improved further by laser coating of Fe30A powder.
4 Conclusion Biomimetic coupling unit contains melted zone and transformed zone. The melted zone of laser melting treated unit is composed of Fe3C, M and Ar; while the melted zone of laser coating treated unit consists of Ar, martensite, (Cr,Fe)7C3, (Cr,Fe)23C6, Fe2B and some austenite is strengthened by the solid solution of Cr, Ni, Mo elements. The heating temperature of transformed zone is between eutectoid reaction temperature and eutectic transformation temperature, and only solid-state phase change occurs in this zone. The average micro-hardness of substrate is 280 HV, while that of biomimetic unit is much higher. The micro-hardness of laser melting treated unit varies from 561 HV to 603 HV, and that of laser coating treated unit varies from 567 HV to 632 HV. Frictional wear and thermal fatigue resistance of biomimetic coupling sample is better than that of untreated sample, and among the biomimetic samples, laser coating treated sample has superior resistance to wear and thermal fatigue comparing with laser melting treated sample.
Acknowledgement This work is financially supported by the Project 985-Automotive Engineering of Jilin University, the National Natural Science Fund of China (Grant no. 50635030) and the Science and Development Foundation of Jilin (Grant no. 20060196).
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[6] Yamazaki T, Shibuya T, Jin C J, Kikuta T, Nakatani N. Lining of hydraulic cylinder made of cast iron with copper alloy. Journal of Materials Processing Technology, 2006, 172, [7]
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[11] Alpas A T, Zhang J. Effect of SiC particulate reinforcement on the dry sliding wear of aluminum-silicon alloys (A356). Wear, 1992, 155, 83104. [12] Ren L Q, Han Z W, Li J Q, Tong J. Effects of non-smooth characteristics on bionic bulldozer blades in resistance reduction against soil. Journal of Terramechanics, 2003, 39, 221230. [13] Chen B C. Morphology of Vehicle Running Gear and Theory
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1991, 150, 195206. [16] Habig K H. Wear Behavior and Hardness of Materials, Mechanical Industry Publisher, Beijing, 1987. (in Chinese) [17] Chen J J, Yu Z S, Xu G K. High Cr Alloyed Cast Iron and Application, Metallurgy Industry Press, Beijing, 1999.
Frictional Wear and Thermal Fatigue Behaviours of Biomimetic Coupling Materials for Brake Drums Yu Zhao1, Lu-quan Ren1, Xin Tong2, Hong Zhou2, Li Chen1 1. Key Laboratory of Terrain-Machine Bionics Engineering (Ministry of Education, China), Jilin University, Changchun 130022, P. R. China 2. The Key Lab of Automobile Material (Ministry of Education, China), Jilin University, Changchun 130022, P. R. China
Abstract In order to enhance frictional wear resistance and thermal fatigue resistance of brake drums, two kinds of biomimetic coupling materials are prepared by laser surface melting and laser coating technology respectively. These materials are all compounded by base metal and biomimetic coupling units, and have different coupling characteristics, such as the variation of the unit shape, different microstructures and hardness, different chemical compositions of units. The frictional wear resistance and thermal fatigue resistance of biomimetic coupling materials and gray cast iron used for brake drum are compared. The results indicate that frictional wear resistance and thermal fatigue resistance of biomimetic coupling sample is better than that of untreated sample, and among the biomimetic samples, laser coating treated sample has superior resistance to wear and thermal fatigue comparing with laser melting treated sample. Keywords: frictional wear, thermal fatigue, biomimetic coupling, brake drum, gray cast iron Copyright © 2008, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved.
1 Introduction Brake drum is one of the most important components in brake system of trucks. According to the principles of braking, brake drum receives powerful pressure and static or kinetic friction force from brake disk. Especially the sliding velocity and specific pressure increase with the speed of automobiles, which result in heavier wear and higher temperature. It is reported that the surface temperature of brake drum can reach as high as 900 C, and the alternating heating and cooling caused by frequent braking also lead to initiation and propagation of thermal cracks[14]. Fig. 1 shows the wear and thermal fatigue characteristics of a disabled brake drum. The above damages to brake drum do a lot of harm to the braking performance and even the safety of automobiles. Compared with frictional materials, the material for brake drum has a single category and slow development. There are mainly two kinds of braking materials, cast Corresponding author: Yu Zhao E-mail: [email protected]
iron and aluminium base composite. Gray cast iron has long been used because of its low cost and desirable properties such as low melting point, good fluidity and cast-ability, excellent machinability etc. Concretely, HT150 and HT200 are more commonly used by inboards, while outboards prefer gray cast iron grade 250 and high-carbon gray cast iron. However, these materials
Fig. 1 Thermal fatigue cracks and wear patterns of a fail brake drum.
Zhao et al.: Frictional Wear and Thermal Fatigue Behaviours of Biomimetic Coupling Materials for Brake Drums
can not satisfy applications due to their low hardness and intensity. In order to refine and increase pearlite microstructure, some alloy elements are added to cast iron. For example, the high carbon low alloyed cast irons wherein carbon content is more than 7.36 % and silicon content is less than 1.5 %, containing molybdenum, cuprum or nickel are widely applied in Britain and America; while in Germany the added alloy elements are cuprum, chromium and stannum[5,6]. Although the intensity and hardness of cast iron can be enhanced by this method, yet its disadvantages are weak resistance to thermal fatigue and high waste of alloy elements. For lightening weight and improving thermal conductivity of braking material, the ceramic reinforced aluminum base composite has become a new researching point, and the usually used hard grains are SiC, TiC, Si3N4 and Al2O3. But the high cost and difficult selection of brake disk limit their applications[711]. Therefore, it is urgent to develop new materials for brake drums. Bionics is a science that imitates the principles of bio-systems to build new technological systems of useful components. Over the past two decades, bionics has had a profound influence on engineering material research, because the unique structures, compositions and correspondingly excellent properties of biological and natural materials gave researchers many clues to improve the properties of engineering materials or increase the reliability of structural components. In fact, biology can not only adapt to the environment, but also satisfy its survival with the lowest energy cost. Therefore, the new components or materials, which are designed and manufactured by biomimetic methods, also have advantages such as high efficiency and low consumption of energy. Research on biomimetic non-smooth surface found that the bulldozer blades with such surfaces can reduce sliding resistance against soil[12]. In recent years, the technology of biomimetic non-smooth surface has been applied to brake disks for trains, brake drums for trucks and a lot of hot-work dies, aiming to solve the problem of frictional wear, abrasive wear or thermal fatigue[13,14]. The efforts are all about changing the morphologies of biomimetic non-smooth surface such as shape, size and convex-concave of non-smooth units. With further investigation, we discover that some com-
21
plex problems in engineering systems are difficult to solve completely by our invented biomimetic method which only has single changing factor. In this research, two biomimetic coupling materials are prepared by laser surface melting and laser coating technology respectively. These materials are all compounded by base metal and biomimetic coupling units, with different coupling characteristics, such as varied units’ shape, dissimilar formed structure between base metal and units, different chemical compositions of units and so on. The frictional wear and thermal fatigue resistance of the coupling materials and gray cast iron are compared.
2 Experiments 2.1 Experimental materials A gray cast iron codenamed HT200, which is widely used for brake drums, is applied as the base metal material whose microstructure consists of flake graphite and pearlite shown in Fig. 2. According to different molding technology, biomimetic coupling units are divided into Laser Surface Melting Coupling Unit (LMU) and Laser Coating Coupling Unit (LCU) respectively. Among them, the LCU is made through laser coating of Fe-based self-fluxing alloy (Fe30A). The chemical compositions of the materials are given in Table 1 and Table 2.
Fig. 2 Microstructure of HT200 gray cast iron. Table 1 Chemical compositions of HT200 gray cast iron (wt%) Compositions Content
C
Si
Mn
P
S
Cu
Cr
3.250 1.570 0.920 0.060 0.059 0.500 0.270
Fe Bal.
Journal of Bionic Engineering (2008) Suppl.
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Table 2 Chemical compositions of Fe30A alloying power (wt%) Compositions Content
C
B
Si
Cr
Ni
0.5–0.6 1.0–2.0 2.0–3.5 12–14
Mo
30–37 4.0–6.0
Fe Bal.
2.2 Sample preparation A solid state Nd-YAG laser of 1.06 μm wavelength and maximum power of 300 W is employed for laser surface melting and laser coating. The base metal is mechanically polished, using progressively finer grades of silicon carbide impregnated emery paper. By controlling the laser tracks, LMU is formed to latticed shape with spacing of 3 mm. For processing LCU, the base metal has some notches of 1 mm width and 1 mm depth, which are used for filling Fe-based self-fluxing alloy. Laser tracks are along the longitude direction of notch, and a shielding gas of argon is used to minimize contamination of the treated surface. Other laser processing parameters are listed in Table 3. Table 3 Laser parameters for processing biomimetic units Pulse duration
Frequency
Electric current
Laser spot diameter
Scanning speed
7.0 ms
10 Hz
120 A
0.8 mm
1 mm·s1
Wear samples of 14 mm length, 10 mm width and 10 mm thickness and thermal fatigue samples of 40 mm length, 20 mm width and 6 mm thickness are cut by an electric spark machine, and a 3 mm diameter round hole is drilled at one side of each thermal fatigue sample, so that they can be fixed onto the plate of the thermal fatigue experimental machine. According to different materials, samples are divided into three kinds: Untreated Sample (US), Laser Surface Melting Sample (LMS), and Laser Surface Coating Sample (LCS). 2.3 Experimental method Transverse sections of biomimetic coupling units are cut parallel to the length direction, and standard methods of metallography are followed. The microstructure is characterized by JSM-5500LV scanning electronic microscope. A XTL-2400 stereomicroscope is used to study the initiation and propagation of thermal cracks. Phases and compounds formed in the coating zone are identified by D/max-RC X-ray Diffraction
(XRD) with Cu K radiation operated at a voltage of 40 KV, a current of 40 mA, and a scanning rate of 40·min1. The alloying element’s distribution of micro-zones is analyzed by an EDAX-Falcon Energy Diffraction Spectrometer (EDS) which is equipped on a JSM-5500LV scanning electronic microscope. A Vickers Microhardness Tester (model 5104, manufactured by Buehler Co. Ltd., USA) is used for the microhardness measurement. Dry sliding tests are performed using an MM-200 block-on-ring wear tester (made by the Xuanhua Testing Machine Factory). All the friction and wear tests are carried out at room temperature. Experimental samples slide on a rotating GCr15 steel ring. The GCr15 steel has an average hardness of 61–63 HRC and the sizes of 16 mm inside diameter, 50 mm outside diameter and 10 mm thickness. A normal load of 8 kg is used for wear tests. The rotational speed of the ring is 400 r·min1 and the sliding time is 30 min. Before each test, the surface of the steel ring is polished to a roughness of about 0.1 mm, while samples were ultrasonically cleaned in anhydrous alcohol and dried before and after wear tests. The mass loss is measured by a sensitive electronic balance with an accuracy of 0.0001 g. The difference in mass of three test blocks before and after the experiment gives the average mass loss. To assist the analysis of wear mechanisms, the worn surfaces of block specimens are examined by LEXT-OLS 3000 OLYMPUS laser confocal scanning microscope. Thermal fatigue tests are carried out by a selfrestrain thermal fatigue testing machine. The samples are heated by a high temperature electric resistance furnace, and cooled by running water. This machine can record thermal cycle times automatically. The furnace temperature is controlled by thyristor control of type (KSY-12-16), and the sample temperature is monitored by a thermocouple attached to the sample at the centre of its length. A complete thermal cycle includes heating to 600±5 C in 70 seconds, and then cooling to 25±5 C in 3 seconds. The samples are free from any externally applied load, and are taken out to observe the cracks every 200 cycles, then the thermal fatigue resistance of samples is evaluated.
Zhao et al.: Frictional Wear and Thermal Fatigue Behaviours of Biomimetic Coupling Materials for Brake Drums
3 Results and Discussion 3.1 Structure characteristics of biomimetic coupling units Fig. 3 shows the cross-section morphology of biomimetic coupling unit. Observation indicates that the unit contains two areas. One is the melted zone at the outer layer which is bright in Fig. 3, because cast iron is melted completely in this zone, and subsequently recrystallized under a fast cooling rate. Due to thermal conduction, the materials around the melted zone takes solid phase transformation, thus this area is called transformed zone. Fig. 4 shows the SEM micrographs taken from the transverse section of LMU. It can be seen that the melted zone (see Fig. 4a) is composed of Fe3C, M and Ar, and the Fe3C is reticular. The microstructures of the transformed zone, as shown in Fig. 4b, consist of M, a little Ar and some un-dissolved G. The crystal grains in the whole unit zone are very fine and compact, and these results agree well with the findings of Shen and Li[15], which can be explained by the rapid cooling rate during solidification caused by laser processing method.
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Fig. 5 shows the SEM micrographs taken from the transverse section of LCU. It can be seen that the melted zone (see Fig. 5a) is composed of Ar, martensite, M7C3, M23C6, and Fe2B. Therefore, the unit is strengthened by solution strength and dispersion strength. Because the heating temperature of the transformed zone is between eutectoid reaction temperature and eutectic transformation temperature, the microstructures in this zone of LMU and LCU are similar, and they all have martensite, Ar and un-solved graphite. The X-ray diffraction results also identify the above microstructure.
Fig. 3 Cross-section morphology of biomimetic coupling unit.
Fig. 4 Microstructure of the LMU.
Fig. 5 Microstructure of the LCU.
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3.2 Micro-hardness of biomimetic coupling units The curves of hardness distribution in the LMU and LCU are shown in Fig. 6. The reported micro-hardness is measured along the direction of laser irradiation (from the top of unit to the bottom), which are obtained at different locations on the polished and etched cross-sections of the biomimetic unit for each sample. The average micro-hardness of the substrate is 280HV, while that of biomimetic unit is much higher. It can be seen that the micro-hardness of LMU varies from 561 HV to 603 HV, and that of LCU varies from 567 HV to 632 HV. The micro-hardness of LCU is higher than that of LMU, which can be attributed to the solution of carbon, the formation of martensite, the refinement of structure, the large proportion of cementite, especially the solution strength and the precipitation of alloying carbide. Micro-hardness is of particular importance in assessing wear resistance coatings, because it is considered as a good indication of coating integrity and wear resistance.
Fig. 6 Micro-hardness distributing of LMU and LCU.
3.3 Frictional wear resistance 3.3.1 Wear mass loss The wear resistance samples are divided into 3 groups. One group is untreated, the others are processed to biomimetic coupling samples by laser melting and laser coating technology respectively. Three samples of each group are chosen for wear tests. The average mass losses of different samples are shown in Fig. 7. It can be seen that the wear losses of the biomimetic coupling samples is 48.6% less than that of the untreated sample in the same sliding way against the steel blocks. Among
biomimetic coupling groups, the sample treated by laser coating with Fe30A has less wear mass loss.
Fig. 7 Wear mass loss of test samples.
3.3.2 Wear pattern Fig. 8 shows the worn surface of US, LMS and LCS after sliding wear tests against the wearing ring GCr15. As seen in Fig. 8a, some materials are lifted and sheared off in the wear process; the wear grooves with irregular shapes are in various depth. Besides, macroscopic observation reveals that some red-brown and black wear debris, whose chemical nature is Fe2O3 and Fe3O4[16], clearly appear on the worn surfaces and some materials adhere to the worn surfaces and other materials adhere to the steel counterparts. This phenomenon indicates that there is severe plastic deformation on the surface of US, some materials are pulled out and transferred to the counterpart surface resulting in adhesive wear. Deep grooves on the wear surface are generated as a result of cutting action, which is produced by wear debris. Some of the debris is iron oxides, which primarily originated from the contacting surface of GCr15 ring and are oxidized during the dry sliding wear process. Figs. 8b and 8c compare the morphologies of the worn surface of the biomimetic samples under the same sliding conditions. Compared with the US, the extent of the abrasive wear is lower and there is less red-brown and black wear debris on the steel counterpart observed by macrographs. The dominating wear mechanism is adhesive wear, whereas the abrasive and oxidation signs are not obvious. Moreover, the LCS displays better wear resistance than the LMS, because of its greater hardness.
Zhao et al.: Frictional Wear and Thermal Fatigue Behaviours of Biomimetic Coupling Materials for Brake Drums
(a) US
(b) LMS
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(c) LCS
Fig. 8 Worn surface of different samples after a sliding wear test.
3.4 Thermal fatigue resistance 3.4.1 The number and length of thermal cracks During thermal fatigue tests, the number of thermal cracks on each sample surface is counted after 2000 thermal cycles, and in the counting, if any crack is longer than 0.5 mm it is defined as one crack. Fig. 9 shows the number of thermal cracks, indicating that the number of cracks on US is always the most, then the LMS, and the LCS. Based on the above results, it can be deduced that biomimetic coupling units on sample’s surface have superior resistance to the initiation of thermal crack, the biomimetic coupling sample, especially formed by laser coating with Fe30A has higher resistance. Fig. 9 also shows the measured length of crack of different samples. It can be seen that after the same thermal cycles (2000), the length of crack on US surface is always the longest, then the LMS, and the LCS. In fact, the biomimetic coupling samples, especially formed by laser coating with Fe30A, have the higher resistance to crack propa-
gation. Based on the above number and length of cracks, it can be deduced that the thermal fatigue resistance of biomimetic coupling sample is better than that of un-treated sample, and among biomimetic samples, laser coating treated sample has superior resistance to thermal fatigue compared with laser melting treated sample.
Fig. 9 Crack number and length of different test samples.
Fig. 10 Microcosmic place of crack initiation.
3.4.2 Crack initiation resistance of biomimetic coupling unit It is discovered that macroscopical place where thermal crack initiated is mainly from the fixed hole and the edge of sample. After 50 thermal cycles, microcosmic place where crack initiated is also observed, and from Fig. 10 it can be seen that the place is at the sharp point of graphite. Due to nearly no intensity, the graphite is just like a hole distributing in the matrix, thus a crack initiated there first. According to the microstructure in melted zone, where graphite is not found, the source of crack initiation disappears. Therefore, processing units on sample surface, especially at the edge of sample can decrease the number of thermal cracks obviously.
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Journal of Bionic Engineering (2008) Suppl.
3.4.3 Crack propagation resistance of non-smooth unit Many micro-cracks extend along the graphite as their initiation source, and these micro-cracks connect each other to form a main crack. Observations of the propagation routes of main crack indicates that main crack propagates along the graphite and the direction of their shortest distance, showing that propagation of main crack is realized by bridge connections with micro-cracks in front of it (see Fig. 11). In fact, biomimetic coupling unit not only causes such an advantageous inhibition to crack initiation, but also resists crack propagation. It can be explained as that, in the unit area there is no graphite phase, hence there is no bridge connection between main crack and micro-crack which grows relying on graphite. That is to say, the route of crack propagation is cut off by the unit.
Fig. 11 Bridge connections between main crack and micro-cracks.
3.4.4 Effects of alloying elements Though microstructure of biomimetic unit obtained by laser melting also has characteristics of no graphite phase and refined grain sizes, which can improve thermal fatigue resistance of samples, it is still worse than that treated by laser coating of Fe30A. It is well known that crack will lanate biomimetic unit, then gets further propagation when fatigue damages are bigger than the rupture strength of the unit. In addition, the oxidation occurs badly during thermal tests, and repeated building and falling of oxide film at crack point promotes the destruction of the unit. So the higher strength and oxidation resistance of the unit, the better thermal fatigue resistance that biomimetic sample has. Due to laser coating, non-smooth unit is rich in Cr, Ni, Mo, austenite is strengthened by the solid solution of them, and eutectic carbide M7C3 has higher strength and hardness
comparing with cementite Fe3C. On the other hand, Cr is an element which can improve heat-durability of cast iron, because it forms an oxide film rich in Cr with oxygen, like Cr2O3 or FeCr2O4 which can restrain oxidation again[17]. Based on the above reasons, thermal fatigue resistance of biomimetic coupling sample can be improved further by laser coating of Fe30A powder.
4 Conclusion Biomimetic coupling unit contains melted zone and transformed zone. The melted zone of laser melting treated unit is composed of Fe3C, M and Ar; while the melted zone of laser coating treated unit consists of Ar, martensite, (Cr,Fe)7C3, (Cr,Fe)23C6, Fe2B and some austenite is strengthened by the solid solution of Cr, Ni, Mo elements. The heating temperature of transformed zone is between eutectoid reaction temperature and eutectic transformation temperature, and only solid-state phase change occurs in this zone. The average micro-hardness of substrate is 280 HV, while that of biomimetic unit is much higher. The micro-hardness of laser melting treated unit varies from 561 HV to 603 HV, and that of laser coating treated unit varies from 567 HV to 632 HV. Frictional wear and thermal fatigue resistance of biomimetic coupling sample is better than that of untreated sample, and among the biomimetic samples, laser coating treated sample has superior resistance to wear and thermal fatigue comparing with laser melting treated sample.
Acknowledgement This work is financially supported by the Project 985-Automotive Engineering of Jilin University, the National Natural Science Fund of China (Grant no. 50635030) and the Science and Development Foundation of Jilin (Grant no. 20060196).
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