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Spherical dendritic particles formed in cavitation erosion
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Materials Letters 62 (2008) 2707 – 2709 www.elsevier.com/locate/matlet
Spherical dendritic particles formed in cavitation erosion Chen Haosheng ⁎, Liu Shihan, Wang Jiadao, Chen Darong State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China Received 19 December 2007; accepted 9 January 2008 Available online 16 January 2008
Abstract Micro spherical particles were found in a vibrating cavitation erosion experiment. Examination of the spherical particles reveals the dendritic pattern on the surface and the hollow structure of the interior. The surface and interior structures of the particle are also related to the particle’s size. For smaller particle, the dendritic structure is replaced by the fine cell and the interior becomes solid. Such special structures are considered to be the result of particle’s solidification from molten state at a rapid cooling rate, which happens in a special transient environment with transient high temperature and high pressure provided by cavity collapsing. The specific area and surface tension force are the main reasons for the different structures of the particles in different size. © 2008 Elsevier B.V. All rights reserved. Keywords: Cavitation erosion; Thermal effect; Spherical particle; Dendritic
1. Introduction In early 1970s, Scott and Mills [1,2] reported the finding of spherical particles in rolling contact fatigue. The spherical particle was thought to be formed from tongues of metal removed by cavitation erosion due to the high-pressure lubricant entrapped in the surface propagating crack. It was reported that similar spherical particles were formed under conditions of fretting, sliding and rolling contact fatigue [3–5], and the deformation of the wear debris on fatigue fracture surface is thought to be the main reason. On the other hand, in cavitation erosion, where the metal–metal contact condition does not exist, the spherical particles were also found, and the formation of spherical particles is more easily envisaged from the molten state [6–8]. Here we show the clear surface crystallization patterns and interior structure of the spherical particles, which are the direct evidence on the formation of the spherical particles from the molten state. It indicates the importance of the thermal effect in cavitation erosion, and it also helps to explore
⁎ Corresponding author. Tel.: +57 086 10 62797646; fax: +57 086 10 62781379. E-mail address: [email protected] (C. Haosheng). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.018
the growth of the crystal in a special environment with transient high temperature and high pressures. 2. Experiment and results A controlled experiment was conducted to explore the formation mechanism using a standard vibrating cavitation erosion apparatus [9]. The specimens were made of 40Cr mild steel, and their surfaces were polished. Experiments were conducted in de-ionized water for 15 min, and the temperature was kept at 20 °C during the test. After the experiment, particles was collected and observed under a scanning electron microscope (SEM). Spherical particles were found in the experiments ranging in diameter from 1 μm up to 30 μm. Energy dispersive X-ray analysis (EDS) of the spheres showed that they have the same chemical composition as the matrix material. A significant result of the particle was the dendritic patterns on the particle's surface as shown in Fig. 1(a) and (b). The dendritic arm is much thinner than ones from conventional solidification, and such fine dendrite is the production of very rapid cooling from the molten state. Breakages are found on the spherical surfaces. Seeing from the cracks, the interior of the particle is not solid. The hollow structure of the interior and the dendritic patterns on the surface prove that the particle is solidified from molten state at a high cooling rate, and they also indicate the existence of a transient environment with extremely high temperature and pressure on the solid
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C. Haosheng et al. / Materials Letters 62 (2008) 2707–2709
Fig. 2. An undeveloped particle in diameter of 5.2 μm. Its surface is covered by the fine cells.
attributed to the effect of the surface tension. As discussed above, the particle is originally in molten state. In such state, the surface tension force has an inverse relation to the particle's radius. Smaller particles will have larger surface tension force. The surface tension force causes the spherical shape of the melted particle and provides a pull force pointing to the center. Thus, the smaller particle will be solid under the larger surface tension force.
3. Conclusions
Fig. 1. Spherical particles formed in the cavitation erosion. (a) Particle in diameter of 16.3 μm, and its surface is covered by fine dendrite. (b) Particle in diameter of 11.0 μm, with clear breach on the surface.
surface. As researchers have pointed out [10,11], such environment can be generated at the moment of cavity collapsing. The temperature can reach 5075 K and the pressure is higher than 109 Pa. On the other hand, experimental and numerical results prove that such environment exists only in a very short duration [12–15], so the molten spherical particles will experience a rapid cooling process in the surrounding water. Thus, the special surface and interior structures of the spherical particles are the production of the special environment in cavitation erosion. The dendritic pattern is related to the particle size. As shown in Figs. 1–3, the number of the dendritic arms reduces as the radius of the particle decreases. The reason is that a smaller particle will have larger specific area and smaller heat capacity, which will result in a more rapid cooling process. Under this condition, the dendritic arms are difficult to grow and then their number reduces. An undeveloped particle shown in Fig. 2 confirms this viewpoint. On its surface, the fine dendrite was replaced by the fine cells, which are the production of crystallization under faster cooling speed. Additionally, the melted root and the blunt nose of the cell tip prove that the particles are from the molten state. The thickness of the particle's wall is also under the size effect. The wall thickness increases as the particle’s diameter decreases according to the particles shown in Fig. 1. The particle would be even a solid ball as shown in Fig. 3 when it is small enough. Such phenomenon can be
In summary, the spherical particles found in cavitation erosion were formed from molten state and experienced a rapid cooling process. The surface and the interior structures of the particle are under the effect of size related surface tension force and specific area. Additionally, the spherical particles are the results of the high temperature if the cavitation erosion does happen in the rolling or sliding contact.
Fig. 3. A particle in diameter of 1.9 μm. It has a smooth surface and only few grain boundaries can be seen on the surface.
C. Haosheng et al. / Materials Letters 62 (2008) 2707–2709
Acknowledgments Project (No. 50505020) was supported by NSFC, and the Project (2007CB707702) was supported by National Basic Research Program of China. Also, the authors would like to thank Yang Wenyan (Tsinghua University) for her help in preparing the SEM pictures. References [1] D. Scott, H. Mills, Nature 241 (1973) 115–116. [2] D. Scott, H. Mills, Wear 16 (1970) 234–237. [3] L.F. Stowers, E. Rabinowiez, J. App. Phys. 43 (1972) 2485–2487.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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J. Lankford, D.L. Davidson, Metall. Trans. A 14A (1983) 1227–1230. J.W. Tichler, D. Scott, Wear 16 (1970) 229–233. A.P. Thiruvengadam, ASLE Trans. 21 (1977) 344–350. J.E. Davies, Wear 107 (1986) 189. Y.S. Jin, C.B. Wang, Wear 131 (1989) 315–328. H.S. Chen, S.H. Liu, D.R. Chen, J.D. Wang, J. App. Phys. 101 (2007) 103510–1~5. R.T. Knapp, J.W. Daily, F.G. Hammit, Cavitation, McGraw – Hill, New York, 1970. E.B. Flint, K.S. Suslick, Science 253 (1991) 1397–1399. M.S. Plesset, R.B. Chapman, J. Fluid Mech. 47 (1971) 283–290. H.C. Yeh, W.J. Yang, J. Appl. Phys. 19 (1968) 3156–3165. C.L. Kling, Ph.D. thesis, University of Michigan, 1970. N.K. Bourne, J.E. Field, J. Appl. Phys. 78 (1995) 4423–4428.
Materials Letters 62 (2008) 2707 – 2709 www.elsevier.com/locate/matlet
Spherical dendritic particles formed in cavitation erosion Chen Haosheng ⁎, Liu Shihan, Wang Jiadao, Chen Darong State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China Received 19 December 2007; accepted 9 January 2008 Available online 16 January 2008
Abstract Micro spherical particles were found in a vibrating cavitation erosion experiment. Examination of the spherical particles reveals the dendritic pattern on the surface and the hollow structure of the interior. The surface and interior structures of the particle are also related to the particle’s size. For smaller particle, the dendritic structure is replaced by the fine cell and the interior becomes solid. Such special structures are considered to be the result of particle’s solidification from molten state at a rapid cooling rate, which happens in a special transient environment with transient high temperature and high pressure provided by cavity collapsing. The specific area and surface tension force are the main reasons for the different structures of the particles in different size. © 2008 Elsevier B.V. All rights reserved. Keywords: Cavitation erosion; Thermal effect; Spherical particle; Dendritic
1. Introduction In early 1970s, Scott and Mills [1,2] reported the finding of spherical particles in rolling contact fatigue. The spherical particle was thought to be formed from tongues of metal removed by cavitation erosion due to the high-pressure lubricant entrapped in the surface propagating crack. It was reported that similar spherical particles were formed under conditions of fretting, sliding and rolling contact fatigue [3–5], and the deformation of the wear debris on fatigue fracture surface is thought to be the main reason. On the other hand, in cavitation erosion, where the metal–metal contact condition does not exist, the spherical particles were also found, and the formation of spherical particles is more easily envisaged from the molten state [6–8]. Here we show the clear surface crystallization patterns and interior structure of the spherical particles, which are the direct evidence on the formation of the spherical particles from the molten state. It indicates the importance of the thermal effect in cavitation erosion, and it also helps to explore
⁎ Corresponding author. Tel.: +57 086 10 62797646; fax: +57 086 10 62781379. E-mail address: [email protected] (C. Haosheng). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.018
the growth of the crystal in a special environment with transient high temperature and high pressures. 2. Experiment and results A controlled experiment was conducted to explore the formation mechanism using a standard vibrating cavitation erosion apparatus [9]. The specimens were made of 40Cr mild steel, and their surfaces were polished. Experiments were conducted in de-ionized water for 15 min, and the temperature was kept at 20 °C during the test. After the experiment, particles was collected and observed under a scanning electron microscope (SEM). Spherical particles were found in the experiments ranging in diameter from 1 μm up to 30 μm. Energy dispersive X-ray analysis (EDS) of the spheres showed that they have the same chemical composition as the matrix material. A significant result of the particle was the dendritic patterns on the particle's surface as shown in Fig. 1(a) and (b). The dendritic arm is much thinner than ones from conventional solidification, and such fine dendrite is the production of very rapid cooling from the molten state. Breakages are found on the spherical surfaces. Seeing from the cracks, the interior of the particle is not solid. The hollow structure of the interior and the dendritic patterns on the surface prove that the particle is solidified from molten state at a high cooling rate, and they also indicate the existence of a transient environment with extremely high temperature and pressure on the solid
2708
C. Haosheng et al. / Materials Letters 62 (2008) 2707–2709
Fig. 2. An undeveloped particle in diameter of 5.2 μm. Its surface is covered by the fine cells.
attributed to the effect of the surface tension. As discussed above, the particle is originally in molten state. In such state, the surface tension force has an inverse relation to the particle's radius. Smaller particles will have larger surface tension force. The surface tension force causes the spherical shape of the melted particle and provides a pull force pointing to the center. Thus, the smaller particle will be solid under the larger surface tension force.
3. Conclusions
Fig. 1. Spherical particles formed in the cavitation erosion. (a) Particle in diameter of 16.3 μm, and its surface is covered by fine dendrite. (b) Particle in diameter of 11.0 μm, with clear breach on the surface.
surface. As researchers have pointed out [10,11], such environment can be generated at the moment of cavity collapsing. The temperature can reach 5075 K and the pressure is higher than 109 Pa. On the other hand, experimental and numerical results prove that such environment exists only in a very short duration [12–15], so the molten spherical particles will experience a rapid cooling process in the surrounding water. Thus, the special surface and interior structures of the spherical particles are the production of the special environment in cavitation erosion. The dendritic pattern is related to the particle size. As shown in Figs. 1–3, the number of the dendritic arms reduces as the radius of the particle decreases. The reason is that a smaller particle will have larger specific area and smaller heat capacity, which will result in a more rapid cooling process. Under this condition, the dendritic arms are difficult to grow and then their number reduces. An undeveloped particle shown in Fig. 2 confirms this viewpoint. On its surface, the fine dendrite was replaced by the fine cells, which are the production of crystallization under faster cooling speed. Additionally, the melted root and the blunt nose of the cell tip prove that the particles are from the molten state. The thickness of the particle's wall is also under the size effect. The wall thickness increases as the particle’s diameter decreases according to the particles shown in Fig. 1. The particle would be even a solid ball as shown in Fig. 3 when it is small enough. Such phenomenon can be
In summary, the spherical particles found in cavitation erosion were formed from molten state and experienced a rapid cooling process. The surface and the interior structures of the particle are under the effect of size related surface tension force and specific area. Additionally, the spherical particles are the results of the high temperature if the cavitation erosion does happen in the rolling or sliding contact.
Fig. 3. A particle in diameter of 1.9 μm. It has a smooth surface and only few grain boundaries can be seen on the surface.
C. Haosheng et al. / Materials Letters 62 (2008) 2707–2709
Acknowledgments Project (No. 50505020) was supported by NSFC, and the Project (2007CB707702) was supported by National Basic Research Program of China. Also, the authors would like to thank Yang Wenyan (Tsinghua University) for her help in preparing the SEM pictures. References [1] D. Scott, H. Mills, Nature 241 (1973) 115–116. [2] D. Scott, H. Mills, Wear 16 (1970) 234–237. [3] L.F. Stowers, E. Rabinowiez, J. App. Phys. 43 (1972) 2485–2487.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
2709
J. Lankford, D.L. Davidson, Metall. Trans. A 14A (1983) 1227–1230. J.W. Tichler, D. Scott, Wear 16 (1970) 229–233. A.P. Thiruvengadam, ASLE Trans. 21 (1977) 344–350. J.E. Davies, Wear 107 (1986) 189. Y.S. Jin, C.B. Wang, Wear 131 (1989) 315–328. H.S. Chen, S.H. Liu, D.R. Chen, J.D. Wang, J. App. Phys. 101 (2007) 103510–1~5. R.T. Knapp, J.W. Daily, F.G. Hammit, Cavitation, McGraw – Hill, New York, 1970. E.B. Flint, K.S. Suslick, Science 253 (1991) 1397–1399. M.S. Plesset, R.B. Chapman, J. Fluid Mech. 47 (1971) 283–290. H.C. Yeh, W.J. Yang, J. Appl. Phys. 19 (1968) 3156–3165. C.L. Kling, Ph.D. thesis, University of Michigan, 1970. N.K. Bourne, J.E. Field, J. Appl. Phys. 78 (1995) 4423–4428.