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The effect of laser surface texturing on frictional performance of face seal
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 7 ( 2 0 0 8 ) 96–100
journal homepage: www.elsevier.com/locate/jmatprotec
The effect of laser surface texturing on frictional performance of face seal Wan Yi, Xiong Dang-Sheng ∗ Department of Materials Science & Engineering, Nanjing University of Science and Technology, Xiaolingwei 200#, Nanjing, Jiangsu 210094, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Laser surface texturing technology has some advantages in reducing friction and wear of
Received 15 September 2006
mechanical parts in industry. In this paper, a Nd:YAG laser was used to generate micropores
Received in revised form
on T8 steel surface and the structure and morphology features of surface micropores were
11 April 2007
observed. Tribological experiments were conducted with a ring-on-disc tester under various
Accepted 2 June 2007
loads and speeds. It is shown that the maximum PV value of face seal can be increased by hydrodynamic effect of micropores. © 2007 Elsevier B.V. All rights reserved.
Keywords: Laser texturing Micropore Frictional performance PV value
1.
Introduction
The increasing demand for high pressure and speed in modern seal system calls for more ideas for better performing seal. High pressure mechanical seals rely on hydrostatic effects to generate fluid film stiffness that is essential for safe and reliable operation (Wong et al., 2001; Etsion and Halperin, 2003; Wang et al., 2003). Elastic hydrodynamic lubricating theory indicates the surface smoothness may affect hydrodynamic lubrication and load carrying capacity. The most significant pressure build-up in hydrodynamic lubrication is achieved by separating surfaces in relative motion to reduce friction and wear (Pettersson and Jacobson, 2003; Sahlin and Glavatskin, 2005). Laser surface texturing may be an ideal technology for applications in mechanical face seal, as well as in various components in engine such as piston ring and cylinder and thrust bearings, involving creation of an array of microdimples or channels artificially distributed on the mating surface with a
∗
Corresponding author. Tel.: +86 25 84315325. E-mail address: [email protected] (D.-S. Xiong). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.06.019
pulsed laser beam (Du et al., 2005; Schreck and Zum Gahr, 2005; Etsion, 2004). In this study, a Nd:YAG laser was used to generate micropores on T8 steel surfaces. The friction characteristics of mechanical seal under different loads and speeds were investigated and analyzed with a ring-on-disc tribometer.
2.
Experimental details
2.1.
Processing of laser surface-micropored
The T8 steel discs were 48 mm in diameter with 62HRC hardness, and the end surfaces were polished to the roughness less than 0.05 m Ra . The 316L stainless steel ring had an internal diameter of 32 mm, and an external diameter of 46 mm, and its surface roughness was about 0.02 m Ra . In the present work, a Nd:YAG laser by with a wavelength of 1064 nm, a pulse width of 450 ns was used to induce pores on
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 7 ( 2 0 0 8 ) 96–100
3.
Results
3.1.
Morphology of micropores
97
Fig. 2 shows the optical images of disc surface at different magnifications. It can be seen from in Fig. 2(b) that the diameter of pore (d) is about 200 m, the transverse distance (l) is 400 m and the radial interval (D) is 800 m as details in Fig. 2(a). The pore area ratio was obtained from the following equation: r=
Fig. 1 – Ring-on-disc friction testing.
d2 4lD
(1)
The calculated pore area ratio was about 10%. During laser texturing process, metallic materials absorbing laser energy were ejected out of the surface as a result of plastic deformation. For the high energy density, ablation area formed rapidly and melt material around the pores was the typical feature of laser-induced microstructure. The energy and interaction time between laser beam and target material engendered an orbicular area surrounding the pore, called “heat-affected zone” as shown in Fig. 2(b). These raised ridges
the surface of T8 steel discs with a pulse energy of 30 J. Micropores were distributed with an annular array. All the samples were cleaned by ultrasonic bath in acetone for 3 min. The morphology of laser texturing pores was observed using an optical microscope.
2.2.
Friction testing method and test conditions
Friction tests were carried out using a ring-on-disc tribological tester MG-2000 which is shown in Fig. 1. The upper ring was fixed into a metallic holder, which was loaded vertically to a laser textured bottom disc. The disc was driven by a motor to a certain rotational speeds. Friction coefficients were calculated from the frictional torque automatically recorded. The average diameter of the frictional track on the samples was about 40 mm. Before tests, all the samples were cleaned in an acetone bath ultrasonically for 3 min. The T8 steel disc was smeared with a thin base oil at the beginning, then mounted onto a centrifuge with a rotational speed of 400 rpm for 1 min to spin the excessive oil, keeping a thin oil film on the surface. The testing conditions are listed in Table 1. When the load and speed exceeded a critical value, friction coefficient suddenly increased. The maximal PV values of face seal pairs were calculated for both polished and micropored discs.
Table 1 – Testing condition Load (N) Sliding speed (m/s) Lubricant Temperature (◦ C) Humidity (%)
50–600 0.4–3 Smeared oil 18–20 40–60
Fig. 2 – Optical images of laser texturing surface at different magnifications.
98
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 7 ( 2 0 0 8 ) 96–100
Fig. 4 – Friction coefficient of polished and micropored disc with different speeds (a) and loads (b). Fig. 3 – Profiles of the micropore on laser-texturing disc at (a) the center and (b) the heat-affected zone.
were easily removed by polishing for 3 min using a diamond paste with a size of 500 nm. The profiles through the center of pores and heat-affected zone are shown in Fig. 3. It is clear that the depth in the center area (Fig. 3(a)) is in the range of 8–10 m, while the depth of heat-affected zone (Fig. 3(b)) decreased gradually with the increasing distance from the center. There was a ring-like area around the pores and remains melt trace after deburring process.
3.2.
Friction curves
The PV value is the estimation of the stability in hydrodynamic film and the antiwearability of seal material. P is the pressure of medium in seal cavity, and V is the velocity between the contacted surfaces. Fig. 4 shows the friction coefficients of micropored and polished discs against stainless steel with different speeds (a) and loads (b). Almost all the samples exhibited similar trends. In the present experiments, the load and speed increased with a stepwise manner and it can be seen that the friction coefficients of all rubbing pairs decreased at the initial stage, but increased gradually with load and speed. The friction coefficient of polished disc increased suddenly when load and
speed exceeded 200 N and 1.4 m/s, while the laser-micropored disc increased the critical point of high friction coefficient to the operating condition of 500 N and 2 m/s. The micropored seal surface was demonstrated to improve the maximum PV value to 2.5 times compared with the polished one, and the friction coefficients decreased slightly with PV value increasing.
3.3.
Worn surface
The worn surfaces for two discs after friction tests are shown in Fig. 5. A weak strip left to the pore can be seen in Fig. 5(a), and the wear in other areas remained abrasive trace induced by polishing process. While the polished disc in Fig. 5(b) had a rougher worn surface with quite a few parallel ploughs. Appropriately distributed pores considered in the present study played a role in affording the retention lubricating film to the contact surface along the sliding direction and acted to trap worn debris, which helped to reduce the undesirable ploughs. The experimental results clarified the potential of micropores for reduction in friction. In fact, the tribological properties of laser surface texturing are connected with the parameters of micropores (depth, diameter and density), which can be optimized for operation conditions from a theoretical analysis (Gyk and Etsion, 2006).
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 7 ( 2 0 0 8 ) 96–100
99
Fig. 6 – Lubrication mechanism of micropores for oil supply effect.
oil-reserviored structure fails easily in case of oil film broken. It is confirmed the pores play an important role of complementary and retention of lubricating film to contact surfaces as to prolong the hydrodynamic lubrication regime. When the oil reaches merely to the valley (shown by A or less), the lubricating film is unable to supply the oil-broken regions as to engender dry friction finally. Still much work is needed before we design the surface texturing parameter can be optimized for different condition.
4.
Conclusions
Frictional properties of laser-micropored surface were assessed through ring-on-disc tests, simulating a face seal contact interface with different loads and speeds. The findings are concluded as follows:
Fig. 5 – Optical microphoto of worn surface on discs (a) micropored surface and (b) polished surface.
3.4.
Discussion
In this paper, a proposed mechanism explanation has been established to investigate the oil supply effect of micropores on the friction and lubricating process. It was assumed that the dimension of micro-asperity on the texturing surfaces small and neglectable, compared with pore sizes. The lubricating effect is highly dependent on the oil content during friction process. At first, the oil adsorption layer is formed on contact surfaces to minimize surface energy. The thickness of oil film increases steadily as to fill the pores, as result of oil mass increasing. The friction occurs primarily in the peak contacted area (C in Fig. 6) irrespective of oil mass. In this test, a thin oil film was smeared on the surfaces and therefore a certain amount of oil was prestored in each pore. When the lubricating film is broken in the peak region, oil in the pore as schematically shown in position B can be supplied to the peak region, spontaneously by virtue of the free energy reduction. Lubricating film on the polished surface without such regular
(1) A Nd:YAG laser was used to produce micropores on T8 steel. The laser-textured micropore had a dimension of 200 m diameter and 10 m depth, 10% dimple-density. (2) All the surfaces had similar trends with the friction coefficients decreased at the initial stage and increased gradually with load and speed. Compared with the polished surface, the laser-micropored seal surface can improve the maximum PV value to 2.5 times. (3) Micropore can decrease contact area to reduce stiction, trap debris to decrease plough and act as lubricant reservoirs to provide oil to contacted surface.
Acknowledgements This work was supported by High Technology Research Project of Jiangsu Province No: BG2007046, and Productive Translation Item of College Scientific Research of Jiangsu Province Educational Office No: JHB06-04.
references
Du, D., He, Y.F., Sui, B., Xiong, L.J., Zhang, H., 2005. Laser texturing of rollers by pulsed Nd:YAG laser. J. Mater. Process. Technol. 161, 456–461.
100
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 7 ( 2 0 0 8 ) 96–100
Etsion, I., 2004. Improving tribological performance of mechanical components by laser surface texturing. Tribol. Lett. 17 (4), 733–737. Etsion, I., Halperin, G., 2003. A laser surface terxtured hydrostatic mechanical seal. Sealing Technol. 3, 6–10. Gyk, G., Etsion, I., 2006. Testing piston rings with partial laser surface texturing for friction reduction. Wear 216, 792–796. Pettersson, U., Jacobson, S., 2003. Influence of surface texture on boundary lubricated sliding contacts. Tribol. Int. 36, 857–864.
Sahlin, F., Glavatskin, S.B., 2005. Two-dimensional CFD-analysis of micro-patterned surfaces in hydrodynamic lubrication. J. Tribol. 127, 96–102. Schreck, S., Zum Gahr, K.-H., 2005. Laser-assisted structuring of ceramic and steel surfaces for improving tribological properties. Appl. Surf. Sci. 247, 616–622. Wang, X., Kato, K., Adachi, K., Aizawa, K., 2003. Loads carrying capacity map for the surface texture design of SiC thrust bearing sliding in water. Tribol. Int. 36, 189–197. Wong, H.C., Umehara, N., Kato, K., 2001. The effect of surface roughness on friction of ceramics sliding in water. Wear 218, 237–243.
journal homepage: www.elsevier.com/locate/jmatprotec
The effect of laser surface texturing on frictional performance of face seal Wan Yi, Xiong Dang-Sheng ∗ Department of Materials Science & Engineering, Nanjing University of Science and Technology, Xiaolingwei 200#, Nanjing, Jiangsu 210094, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Laser surface texturing technology has some advantages in reducing friction and wear of
Received 15 September 2006
mechanical parts in industry. In this paper, a Nd:YAG laser was used to generate micropores
Received in revised form
on T8 steel surface and the structure and morphology features of surface micropores were
11 April 2007
observed. Tribological experiments were conducted with a ring-on-disc tester under various
Accepted 2 June 2007
loads and speeds. It is shown that the maximum PV value of face seal can be increased by hydrodynamic effect of micropores. © 2007 Elsevier B.V. All rights reserved.
Keywords: Laser texturing Micropore Frictional performance PV value
1.
Introduction
The increasing demand for high pressure and speed in modern seal system calls for more ideas for better performing seal. High pressure mechanical seals rely on hydrostatic effects to generate fluid film stiffness that is essential for safe and reliable operation (Wong et al., 2001; Etsion and Halperin, 2003; Wang et al., 2003). Elastic hydrodynamic lubricating theory indicates the surface smoothness may affect hydrodynamic lubrication and load carrying capacity. The most significant pressure build-up in hydrodynamic lubrication is achieved by separating surfaces in relative motion to reduce friction and wear (Pettersson and Jacobson, 2003; Sahlin and Glavatskin, 2005). Laser surface texturing may be an ideal technology for applications in mechanical face seal, as well as in various components in engine such as piston ring and cylinder and thrust bearings, involving creation of an array of microdimples or channels artificially distributed on the mating surface with a
∗
Corresponding author. Tel.: +86 25 84315325. E-mail address: [email protected] (D.-S. Xiong). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.06.019
pulsed laser beam (Du et al., 2005; Schreck and Zum Gahr, 2005; Etsion, 2004). In this study, a Nd:YAG laser was used to generate micropores on T8 steel surfaces. The friction characteristics of mechanical seal under different loads and speeds were investigated and analyzed with a ring-on-disc tribometer.
2.
Experimental details
2.1.
Processing of laser surface-micropored
The T8 steel discs were 48 mm in diameter with 62HRC hardness, and the end surfaces were polished to the roughness less than 0.05 m Ra . The 316L stainless steel ring had an internal diameter of 32 mm, and an external diameter of 46 mm, and its surface roughness was about 0.02 m Ra . In the present work, a Nd:YAG laser by with a wavelength of 1064 nm, a pulse width of 450 ns was used to induce pores on
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 7 ( 2 0 0 8 ) 96–100
3.
Results
3.1.
Morphology of micropores
97
Fig. 2 shows the optical images of disc surface at different magnifications. It can be seen from in Fig. 2(b) that the diameter of pore (d) is about 200 m, the transverse distance (l) is 400 m and the radial interval (D) is 800 m as details in Fig. 2(a). The pore area ratio was obtained from the following equation: r=
Fig. 1 – Ring-on-disc friction testing.
d2 4lD
(1)
The calculated pore area ratio was about 10%. During laser texturing process, metallic materials absorbing laser energy were ejected out of the surface as a result of plastic deformation. For the high energy density, ablation area formed rapidly and melt material around the pores was the typical feature of laser-induced microstructure. The energy and interaction time between laser beam and target material engendered an orbicular area surrounding the pore, called “heat-affected zone” as shown in Fig. 2(b). These raised ridges
the surface of T8 steel discs with a pulse energy of 30 J. Micropores were distributed with an annular array. All the samples were cleaned by ultrasonic bath in acetone for 3 min. The morphology of laser texturing pores was observed using an optical microscope.
2.2.
Friction testing method and test conditions
Friction tests were carried out using a ring-on-disc tribological tester MG-2000 which is shown in Fig. 1. The upper ring was fixed into a metallic holder, which was loaded vertically to a laser textured bottom disc. The disc was driven by a motor to a certain rotational speeds. Friction coefficients were calculated from the frictional torque automatically recorded. The average diameter of the frictional track on the samples was about 40 mm. Before tests, all the samples were cleaned in an acetone bath ultrasonically for 3 min. The T8 steel disc was smeared with a thin base oil at the beginning, then mounted onto a centrifuge with a rotational speed of 400 rpm for 1 min to spin the excessive oil, keeping a thin oil film on the surface. The testing conditions are listed in Table 1. When the load and speed exceeded a critical value, friction coefficient suddenly increased. The maximal PV values of face seal pairs were calculated for both polished and micropored discs.
Table 1 – Testing condition Load (N) Sliding speed (m/s) Lubricant Temperature (◦ C) Humidity (%)
50–600 0.4–3 Smeared oil 18–20 40–60
Fig. 2 – Optical images of laser texturing surface at different magnifications.
98
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 7 ( 2 0 0 8 ) 96–100
Fig. 4 – Friction coefficient of polished and micropored disc with different speeds (a) and loads (b). Fig. 3 – Profiles of the micropore on laser-texturing disc at (a) the center and (b) the heat-affected zone.
were easily removed by polishing for 3 min using a diamond paste with a size of 500 nm. The profiles through the center of pores and heat-affected zone are shown in Fig. 3. It is clear that the depth in the center area (Fig. 3(a)) is in the range of 8–10 m, while the depth of heat-affected zone (Fig. 3(b)) decreased gradually with the increasing distance from the center. There was a ring-like area around the pores and remains melt trace after deburring process.
3.2.
Friction curves
The PV value is the estimation of the stability in hydrodynamic film and the antiwearability of seal material. P is the pressure of medium in seal cavity, and V is the velocity between the contacted surfaces. Fig. 4 shows the friction coefficients of micropored and polished discs against stainless steel with different speeds (a) and loads (b). Almost all the samples exhibited similar trends. In the present experiments, the load and speed increased with a stepwise manner and it can be seen that the friction coefficients of all rubbing pairs decreased at the initial stage, but increased gradually with load and speed. The friction coefficient of polished disc increased suddenly when load and
speed exceeded 200 N and 1.4 m/s, while the laser-micropored disc increased the critical point of high friction coefficient to the operating condition of 500 N and 2 m/s. The micropored seal surface was demonstrated to improve the maximum PV value to 2.5 times compared with the polished one, and the friction coefficients decreased slightly with PV value increasing.
3.3.
Worn surface
The worn surfaces for two discs after friction tests are shown in Fig. 5. A weak strip left to the pore can be seen in Fig. 5(a), and the wear in other areas remained abrasive trace induced by polishing process. While the polished disc in Fig. 5(b) had a rougher worn surface with quite a few parallel ploughs. Appropriately distributed pores considered in the present study played a role in affording the retention lubricating film to the contact surface along the sliding direction and acted to trap worn debris, which helped to reduce the undesirable ploughs. The experimental results clarified the potential of micropores for reduction in friction. In fact, the tribological properties of laser surface texturing are connected with the parameters of micropores (depth, diameter and density), which can be optimized for operation conditions from a theoretical analysis (Gyk and Etsion, 2006).
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 7 ( 2 0 0 8 ) 96–100
99
Fig. 6 – Lubrication mechanism of micropores for oil supply effect.
oil-reserviored structure fails easily in case of oil film broken. It is confirmed the pores play an important role of complementary and retention of lubricating film to contact surfaces as to prolong the hydrodynamic lubrication regime. When the oil reaches merely to the valley (shown by A or less), the lubricating film is unable to supply the oil-broken regions as to engender dry friction finally. Still much work is needed before we design the surface texturing parameter can be optimized for different condition.
4.
Conclusions
Frictional properties of laser-micropored surface were assessed through ring-on-disc tests, simulating a face seal contact interface with different loads and speeds. The findings are concluded as follows:
Fig. 5 – Optical microphoto of worn surface on discs (a) micropored surface and (b) polished surface.
3.4.
Discussion
In this paper, a proposed mechanism explanation has been established to investigate the oil supply effect of micropores on the friction and lubricating process. It was assumed that the dimension of micro-asperity on the texturing surfaces small and neglectable, compared with pore sizes. The lubricating effect is highly dependent on the oil content during friction process. At first, the oil adsorption layer is formed on contact surfaces to minimize surface energy. The thickness of oil film increases steadily as to fill the pores, as result of oil mass increasing. The friction occurs primarily in the peak contacted area (C in Fig. 6) irrespective of oil mass. In this test, a thin oil film was smeared on the surfaces and therefore a certain amount of oil was prestored in each pore. When the lubricating film is broken in the peak region, oil in the pore as schematically shown in position B can be supplied to the peak region, spontaneously by virtue of the free energy reduction. Lubricating film on the polished surface without such regular
(1) A Nd:YAG laser was used to produce micropores on T8 steel. The laser-textured micropore had a dimension of 200 m diameter and 10 m depth, 10% dimple-density. (2) All the surfaces had similar trends with the friction coefficients decreased at the initial stage and increased gradually with load and speed. Compared with the polished surface, the laser-micropored seal surface can improve the maximum PV value to 2.5 times. (3) Micropore can decrease contact area to reduce stiction, trap debris to decrease plough and act as lubricant reservoirs to provide oil to contacted surface.
Acknowledgements This work was supported by High Technology Research Project of Jiangsu Province No: BG2007046, and Productive Translation Item of College Scientific Research of Jiangsu Province Educational Office No: JHB06-04.
references
Du, D., He, Y.F., Sui, B., Xiong, L.J., Zhang, H., 2005. Laser texturing of rollers by pulsed Nd:YAG laser. J. Mater. Process. Technol. 161, 456–461.
100
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 7 ( 2 0 0 8 ) 96–100
Etsion, I., 2004. Improving tribological performance of mechanical components by laser surface texturing. Tribol. Lett. 17 (4), 733–737. Etsion, I., Halperin, G., 2003. A laser surface terxtured hydrostatic mechanical seal. Sealing Technol. 3, 6–10. Gyk, G., Etsion, I., 2006. Testing piston rings with partial laser surface texturing for friction reduction. Wear 216, 792–796. Pettersson, U., Jacobson, S., 2003. Influence of surface texture on boundary lubricated sliding contacts. Tribol. Int. 36, 857–864.
Sahlin, F., Glavatskin, S.B., 2005. Two-dimensional CFD-analysis of micro-patterned surfaces in hydrodynamic lubrication. J. Tribol. 127, 96–102. Schreck, S., Zum Gahr, K.-H., 2005. Laser-assisted structuring of ceramic and steel surfaces for improving tribological properties. Appl. Surf. Sci. 247, 616–622. Wang, X., Kato, K., Adachi, K., Aizawa, K., 2003. Loads carrying capacity map for the surface texture design of SiC thrust bearing sliding in water. Tribol. Int. 36, 189–197. Wong, H.C., Umehara, N., Kato, K., 2001. The effect of surface roughness on friction of ceramics sliding in water. Wear 218, 237–243.