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Grinding texture as a useful surface to reduce the friction of carbide-derived carbon coatings
NEW CARBON MATERIALS Volume 28, Issue 1, Feb 2013 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2013, 28(1):71–75.
RESEARCH PAPER
Grinding texture as a useful surface to reduce the friction of carbide-derived carbon coatings SUI Jian1,2, LU Jin-jun1,* 1
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China;
2
The College of Chemistry and Chemical Engineering, Yibin University, Yibin 644000，China
Abstract: A 150 μm thick SiC coating was ground to form V-shaped and U-shaped grooves with a 10 μm depth, which was followed by partial chlorination to convert the coating to a carbide-derived carbon (CDC) coating. The effect of the grooves in the CDC coating on its tribological properties was investigated. The grooves were removed by further grinding for comparison. It was found that the grooved CDC coating exhibited improved tribological properties in sliding against Si3N4 in both dry and moist conditions compared with the coating without grooves. The friction coefficient of the grooved CDC coating (0.05) against Si3N4 was lower than that of the ungrooved coating under moist condition,s which can be accounted for by a hydrodynamic lubrication mechanism in the vicinity of the grooves. The grooves were also a useful surface texture to reduce the friction coefficient of the CDC coating in sliding against Si3N4 under dry conditions. Key Words: Carbide derived carbon; SiC; Grinding marks; Water; Surface texture
1
Introduction
Silicon carbide (SiC) has already been used as sliding components lubricated with aqueous media, e.g. piston pumps, journal bearings, and mechanical seals, owing to its superior mechanical and chemical properties, including high hardness and good wear resistance[1,2]. The friction and wear of SiC under water lubrication are very attractive. It was reported that the friction coefficient of self-mated SiC in water was as low as 0.01 after the running-in process [3]. Wong et al.[4] observed a very low friction coefficient (0.003 8) for SiC in water lubricated condition with a cylinder-on-disk apparatus. The excellent tribological properties of SiC in water were elucidated from the view point of tribochemistry, i.e. tribochemical product SiO2 and its hydride in water acting as lubricants, and the smoothened contact surface by tribochemical wear [4-6]. However, the sliding SiC pairs in water could fail to maintain low friction coefficient and even lead to seizure or degradation of SiC surface under some conditions, such as temporary loss of lubricants and occasional overloading [7,8]. Moreover, drawbacks were found for self-mated SiC [9], including a thick tribochemical film as lubricant and a long running-in process with large friction coefficient. To eliminate these problems, surface modification on SiC coating is necessary. Diamond like carbon (DLC) coating was used to improve the tribological performance in various environments for sliding parts, including advanced ceramics. For carbide
ceramic, an alternative approach to improve the tribological behavior of SiC was to convert SiC to carbide-derived carbon (CDC) [10,12] by chlorination [13]. The CDC coating was formed by transforming carbide to carbon layer in chlorine or chlorine-containing atmosphere at elevated temperature. The CDC coating had some superior advantages to DLC coatings, e.g. high growth rate, good coating adherence, unlimited thickness, and low residual stress [14]. The presence of graphitic carbon, carbon onion, and nano-crystalline diamond in CDC coating leads to excellent friction and wear properties in various conditions. The friction coefficient of the CDC coating in humid air was comparable to that of graphite. Moreover, the friction coefficient of the CDC coating tended to decrease with decreasing humidity, which is superior to graphite and glassy carbon [15]., The CDC coating in inert environment (e.g. dry N2) had a friction coefficient less than 0.05, better than that in humid air [16,17]. The tribological performance of the CDC coating in water proved to be very good [18]. These results suggested that the tribological performance of carbide substrates could be greatly improved by formation of CDC coating in both dry and wet conditions. SiC disk is always polished before chlorination. However, a ground SiC disk with grinding marks was incidentally treated in a chlorine-containing atmosphere. It was found that the grinding marks on SiC were held after chlorination. It is well known that surface texture on the sliding surface can enhance the tribological performance, especially in fluids [19]. Therefore, the purpose of this paper is to investigate the effect of grinding
Received date: 09 July 2012; Revised date: 10 October 2012 *Corresponding author. E-mail: [email protected] Copyright©2013, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(13)60067-0
SUI Jian et al. / New Carbon Materials, 2013, 28(1): 71–75
marks on the friction and wear of CDC coating in water.
2
Experimental
The chlorination apparatus to convert carbide to CDC is composed of a gas supply system and a heat treatment system as described in details elsewhere[11]. The hot pressed SiC disk was provided by Shanghai Institute of Ceramic, Chinese Academy of Sciences. Before chlorination, the Φ25 mm × 8 mm SiC disk was ground by a diamond wheel to form marks in the disk. In chlorination experiment, SiC was placed in a silica tube furnace with a diameter of 32 mm and a length of 1 000 mm. The tube was purged with a 5% Cl2/Ar gas mixture with a flow rate of 100 mL/min before heated to the desired reaction temperature. The chlorination was performed at 1 000 o C in the above gas mixture for 5 h. After chlorination, the sample was cooled down to room temperature under Ar flow. The grooves on the CDC coating, left by marks were removed by a further grinding as a comparison. Scanning electron microscopy (SEM) was used to investigate surface texture of the CDC coating. Friction and wear tests were performed on a THT tribometer (CSM Ltd., Swizertland) with a ball-on-disk configuration (a stationary ball sliding on a rotating disk). The stationary ball was made of Si3N4 with a diameter of 3 mm. The load and the speed were 5 N and 0.20 m/s, respectively. The total sliding distance was 300 m. The rubbing surfaces were submerged in distilled water at room temperature. In
order to eliminate the effect of flowing water on the frictional force, the volume of filled water was kept constant and the forces derived from flowing water were measured with no contact pressure for calibration. Consequently, based on the friction coefficient recorded by tribometer and the ratio of flowing water derived forces to the given load, the friction coefficient for the CDC coating sliding against Si3N4 in water was calculated.
3
Results and discussion
SEM observation revealed that original grinding marks on SiC surface were preserved as seen in Fig. 1a. The grinding marks were V-shaped and U-shaped grooves approximately paralleling to each other as shown in the inlet of Fig. 1a. Most of them had a width of 10 to 30 µm, a depth of 4 to 10 µm and a length from 10 µm to several millimeters. Typical features of the CDC coating were clearly observed under a high magnification in Fig. 1b. Because the maximum depth (10 µm) of the grinding marks was less than 10% of the thickness (150 µm) of the CDC coating as shown in Fig. 1c, it is possible to remove these grooves with minimum damage to the CDC coating by the further grinding. Friction coefficient of the CDC coating against Si3N4 was as low as 0.05, lower than that of the ground CDC coating against Si3N4. This indicated that grooves in the CDC coating were favorable to reduce friction coefficient of the coating. In addition, the frictional trace of the former was smooth while
Fig. 1 (a, b) SEM images of the as-received CDC coating and (c) the cross-sectional micrograph of CDC coating and SiC substrate.
SUI Jian et al. / New Carbon Materials, 2013, 28(1): 71–75
In a sliding cycle, the angle of sliding direction to the grinding marks kept varying. The role of the grooves for friction reduction can be best explained when the angle was 90° as shown in Fig. 3a and 3b. For the CDC coating, the grooves were held during sliding and a hydrodynamic lubrication was built up in the vicinity of the grooves. However, for the ground CDC coating without grooves, there was no such mechanism and the ground CDC coating exhibited a higher friction coefficient than that of the CDC coating. According to the above results, it is reasonable to conclude that the grinding marks in CDC coating are a cost effective surface texture to lower friction coefficient of the CDC coating. Fig. 2 Frictional traces of as-received CDC coating and the ground CDC coating in sliding against Si3N4 in water.
that of the latter was not smooth and had many fluctuations as shown in Fig. 2. A short running-in period before the steady friction coefficient was found for the CDC coating against Si3N4 while no such running-in period was found for the ground CDC coating against Si3N4. These mean that the grinding marks were useful for friction reduction.
The excellent tribological performance of the marked CDC coating against Si3N4 in both dry sliding (µ=0.13)[18] and water lubricated condition (µ=0.05) is useful for application where a sudden lack of water at the tribo-interface happens. Wear particles generated by dry wear and invading hard particles can be trapped in the grooves to reduce or eliminate a three-body abrasive wear for the CDC coating. Therefore, the presence of grooves is beneficial to reduce abrasive friction in sliding contact.
Fig. 3 (a) SEM micrograph of wear track of CDC coating in water and (b) Corresponding 3D profile of wear region in water.
4
[4]
Conclusions
ceramics under water-lubricated conditions[J]. Tribol Lett, 1998, 5(4): 303-308.
Grinding marks as a cost effective surface texture are useful to reduce friction of carbide-derived carbon coating in water and occasional dry condition.
[5]
References
[6]
[1]
speed sliding[J]. Wear, 1988, 121: 107-116.
lubrication sliding[J]. Tribol Int, 1993, 26: 83-92. P,
Litula
P.
Load-carrying
capability
of
Si3N4 and SiC under water lubrication[J]. Tribol Int, 2002, 35: 129-135. [7]
[3]
Sasaki S. The effects of the surrounding atmosphere on the friction and wear of alumina, zirconia, silicon nitride and silicon carbide[J]. Wear, 1989,134: 185-200.
Takadoum J, Zsiga Z, Ben Rhouma M, et al. Correlation between friction coefficient and wear mechanism of SiC/SiC
water-lubricated ceramic journal bearings[J]. Tribol Int, 1997, 27: 315-321.
Chen M, Kato K, Adachi K. The comparisons of sliding speed and normal load effect on friction coefficients of self-mated
Erikson L C, Blomberg A, Hogmark S, et al. Tribological
Andersson
Ishigaki H, Nagata R, Iwasa M. Effect of absorbed water on friction of hot-pressed silicon nitride and silicon carbide at low
characterization of alumina and silicon carbide under [2]
Wong H C, Umehara N, Kato K. Frictional characteristics of
system[J]. J Mater Sci Lett, 1994, 13: 474-476. [8]
Zum Gahr K H, Blattner R, Hwang D-H, et al. Micro-and macro-tribological properties of SiC ceramics in sliding contact[J]. Wear, 2001, 250: 299-310.
SUI Jian et al. / New Carbon Materials, 2013, 28(1): 71–75
[9]
humidity on the tribological properties of carbide-derived
Chen M, Kato K, Adachi K. Friction and wear of self-mated
carbon (CDC) films on silicon carbide. Tribol Lett[J], 2003,
SiC and Si3N4 sliding in water[J]. Wear, 2001, 250: 246-255.
15(1): 51-55.
[10] [ Gogotsi Y G, Yoshimura M. Formation of carbon films on carbides under hydrothermal conditions[J]. Nature, 1994, 367:
[16]
[11]
Technol, 2006, 3(3): 236-244.
Gogotsi Y G, Jeon I-D, McNallan M J. Carbon coatings on silicon carbide by reaction with chlorine-containing gases[J]. J
Erdemir A, Kovalchenko A, White C, et al. Synthesis and tribology of carbide-derived carbon films[J]. Int J Appl Ceram
628-630. [17]
Carrol B, Gogotsi Y G, Kovalchenko A, et al. Tribological characterization of carbide-derived carbon (CDC) films in dry
Mater Chem, 1997, 7: 1841-1848. [12] [ Ersoy D A, McNallan M J, Gogotsi Y G, et al. Tribological
and humid environments[J]. Nanostructured Materials and
properties of carbon coatings produced by high temperature
Coatings for Biomedical and Sensor Applications, 2003, 102: 119-130.
chlorination of silicon carbide[J]. STLE Tribol Trans, 2000, 43: 809-815. [13]
[18]
porous carbide-derived carbons by chlorination of mesoporous titanium carbides[J]. New Carbon Materials, 2009, 24(3): 243-250. [14]
Ersoy D A, McNallan M J, Gogotsi Y G. Carbon coatings produced by high temperature chlorination of silicon carbide ceramics[J]. Mater Res Innov, 2001, 5: 55-62.
[15]
Carrolla B, Gogotsi Y G, Kovalchenko A, et al. Effect of
Sui J, Lu J J. Friction and wear of carbide derived carbon coating sliding against Si3N4 under the lubrication of water[J].
Cheng G, Long D H, Liu X J, et al. Fabrication of hierarchical
Tribology (in Chinese), 2011, 31: 498-503. [19]
Hamilton D B, Walowit J A, Allen C M. A theory of lubrication by micro-irregularities[J]. J Basic Eng, 1966, 88: 177-185.
RESEARCH PAPER
Grinding texture as a useful surface to reduce the friction of carbide-derived carbon coatings SUI Jian1,2, LU Jin-jun1,* 1
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China;
2
The College of Chemistry and Chemical Engineering, Yibin University, Yibin 644000，China
Abstract: A 150 μm thick SiC coating was ground to form V-shaped and U-shaped grooves with a 10 μm depth, which was followed by partial chlorination to convert the coating to a carbide-derived carbon (CDC) coating. The effect of the grooves in the CDC coating on its tribological properties was investigated. The grooves were removed by further grinding for comparison. It was found that the grooved CDC coating exhibited improved tribological properties in sliding against Si3N4 in both dry and moist conditions compared with the coating without grooves. The friction coefficient of the grooved CDC coating (0.05) against Si3N4 was lower than that of the ungrooved coating under moist condition,s which can be accounted for by a hydrodynamic lubrication mechanism in the vicinity of the grooves. The grooves were also a useful surface texture to reduce the friction coefficient of the CDC coating in sliding against Si3N4 under dry conditions. Key Words: Carbide derived carbon; SiC; Grinding marks; Water; Surface texture
1
Introduction
Silicon carbide (SiC) has already been used as sliding components lubricated with aqueous media, e.g. piston pumps, journal bearings, and mechanical seals, owing to its superior mechanical and chemical properties, including high hardness and good wear resistance[1,2]. The friction and wear of SiC under water lubrication are very attractive. It was reported that the friction coefficient of self-mated SiC in water was as low as 0.01 after the running-in process [3]. Wong et al.[4] observed a very low friction coefficient (0.003 8) for SiC in water lubricated condition with a cylinder-on-disk apparatus. The excellent tribological properties of SiC in water were elucidated from the view point of tribochemistry, i.e. tribochemical product SiO2 and its hydride in water acting as lubricants, and the smoothened contact surface by tribochemical wear [4-6]. However, the sliding SiC pairs in water could fail to maintain low friction coefficient and even lead to seizure or degradation of SiC surface under some conditions, such as temporary loss of lubricants and occasional overloading [7,8]. Moreover, drawbacks were found for self-mated SiC [9], including a thick tribochemical film as lubricant and a long running-in process with large friction coefficient. To eliminate these problems, surface modification on SiC coating is necessary. Diamond like carbon (DLC) coating was used to improve the tribological performance in various environments for sliding parts, including advanced ceramics. For carbide
ceramic, an alternative approach to improve the tribological behavior of SiC was to convert SiC to carbide-derived carbon (CDC) [10,12] by chlorination [13]. The CDC coating was formed by transforming carbide to carbon layer in chlorine or chlorine-containing atmosphere at elevated temperature. The CDC coating had some superior advantages to DLC coatings, e.g. high growth rate, good coating adherence, unlimited thickness, and low residual stress [14]. The presence of graphitic carbon, carbon onion, and nano-crystalline diamond in CDC coating leads to excellent friction and wear properties in various conditions. The friction coefficient of the CDC coating in humid air was comparable to that of graphite. Moreover, the friction coefficient of the CDC coating tended to decrease with decreasing humidity, which is superior to graphite and glassy carbon [15]., The CDC coating in inert environment (e.g. dry N2) had a friction coefficient less than 0.05, better than that in humid air [16,17]. The tribological performance of the CDC coating in water proved to be very good [18]. These results suggested that the tribological performance of carbide substrates could be greatly improved by formation of CDC coating in both dry and wet conditions. SiC disk is always polished before chlorination. However, a ground SiC disk with grinding marks was incidentally treated in a chlorine-containing atmosphere. It was found that the grinding marks on SiC were held after chlorination. It is well known that surface texture on the sliding surface can enhance the tribological performance, especially in fluids [19]. Therefore, the purpose of this paper is to investigate the effect of grinding
Received date: 09 July 2012; Revised date: 10 October 2012 *Corresponding author. E-mail: [email protected] Copyright©2013, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(13)60067-0
SUI Jian et al. / New Carbon Materials, 2013, 28(1): 71–75
marks on the friction and wear of CDC coating in water.
2
Experimental
The chlorination apparatus to convert carbide to CDC is composed of a gas supply system and a heat treatment system as described in details elsewhere[11]. The hot pressed SiC disk was provided by Shanghai Institute of Ceramic, Chinese Academy of Sciences. Before chlorination, the Φ25 mm × 8 mm SiC disk was ground by a diamond wheel to form marks in the disk. In chlorination experiment, SiC was placed in a silica tube furnace with a diameter of 32 mm and a length of 1 000 mm. The tube was purged with a 5% Cl2/Ar gas mixture with a flow rate of 100 mL/min before heated to the desired reaction temperature. The chlorination was performed at 1 000 o C in the above gas mixture for 5 h. After chlorination, the sample was cooled down to room temperature under Ar flow. The grooves on the CDC coating, left by marks were removed by a further grinding as a comparison. Scanning electron microscopy (SEM) was used to investigate surface texture of the CDC coating. Friction and wear tests were performed on a THT tribometer (CSM Ltd., Swizertland) with a ball-on-disk configuration (a stationary ball sliding on a rotating disk). The stationary ball was made of Si3N4 with a diameter of 3 mm. The load and the speed were 5 N and 0.20 m/s, respectively. The total sliding distance was 300 m. The rubbing surfaces were submerged in distilled water at room temperature. In
order to eliminate the effect of flowing water on the frictional force, the volume of filled water was kept constant and the forces derived from flowing water were measured with no contact pressure for calibration. Consequently, based on the friction coefficient recorded by tribometer and the ratio of flowing water derived forces to the given load, the friction coefficient for the CDC coating sliding against Si3N4 in water was calculated.
3
Results and discussion
SEM observation revealed that original grinding marks on SiC surface were preserved as seen in Fig. 1a. The grinding marks were V-shaped and U-shaped grooves approximately paralleling to each other as shown in the inlet of Fig. 1a. Most of them had a width of 10 to 30 µm, a depth of 4 to 10 µm and a length from 10 µm to several millimeters. Typical features of the CDC coating were clearly observed under a high magnification in Fig. 1b. Because the maximum depth (10 µm) of the grinding marks was less than 10% of the thickness (150 µm) of the CDC coating as shown in Fig. 1c, it is possible to remove these grooves with minimum damage to the CDC coating by the further grinding. Friction coefficient of the CDC coating against Si3N4 was as low as 0.05, lower than that of the ground CDC coating against Si3N4. This indicated that grooves in the CDC coating were favorable to reduce friction coefficient of the coating. In addition, the frictional trace of the former was smooth while
Fig. 1 (a, b) SEM images of the as-received CDC coating and (c) the cross-sectional micrograph of CDC coating and SiC substrate.
SUI Jian et al. / New Carbon Materials, 2013, 28(1): 71–75
In a sliding cycle, the angle of sliding direction to the grinding marks kept varying. The role of the grooves for friction reduction can be best explained when the angle was 90° as shown in Fig. 3a and 3b. For the CDC coating, the grooves were held during sliding and a hydrodynamic lubrication was built up in the vicinity of the grooves. However, for the ground CDC coating without grooves, there was no such mechanism and the ground CDC coating exhibited a higher friction coefficient than that of the CDC coating. According to the above results, it is reasonable to conclude that the grinding marks in CDC coating are a cost effective surface texture to lower friction coefficient of the CDC coating. Fig. 2 Frictional traces of as-received CDC coating and the ground CDC coating in sliding against Si3N4 in water.
that of the latter was not smooth and had many fluctuations as shown in Fig. 2. A short running-in period before the steady friction coefficient was found for the CDC coating against Si3N4 while no such running-in period was found for the ground CDC coating against Si3N4. These mean that the grinding marks were useful for friction reduction.
The excellent tribological performance of the marked CDC coating against Si3N4 in both dry sliding (µ=0.13)[18] and water lubricated condition (µ=0.05) is useful for application where a sudden lack of water at the tribo-interface happens. Wear particles generated by dry wear and invading hard particles can be trapped in the grooves to reduce or eliminate a three-body abrasive wear for the CDC coating. Therefore, the presence of grooves is beneficial to reduce abrasive friction in sliding contact.
Fig. 3 (a) SEM micrograph of wear track of CDC coating in water and (b) Corresponding 3D profile of wear region in water.
4
[4]
Conclusions
ceramics under water-lubricated conditions[J]. Tribol Lett, 1998, 5(4): 303-308.
Grinding marks as a cost effective surface texture are useful to reduce friction of carbide-derived carbon coating in water and occasional dry condition.
[5]
References
[6]
[1]
speed sliding[J]. Wear, 1988, 121: 107-116.
lubrication sliding[J]. Tribol Int, 1993, 26: 83-92. P,
Litula
P.
Load-carrying
capability
of
Si3N4 and SiC under water lubrication[J]. Tribol Int, 2002, 35: 129-135. [7]
[3]
Sasaki S. The effects of the surrounding atmosphere on the friction and wear of alumina, zirconia, silicon nitride and silicon carbide[J]. Wear, 1989,134: 185-200.
Takadoum J, Zsiga Z, Ben Rhouma M, et al. Correlation between friction coefficient and wear mechanism of SiC/SiC
water-lubricated ceramic journal bearings[J]. Tribol Int, 1997, 27: 315-321.
Chen M, Kato K, Adachi K. The comparisons of sliding speed and normal load effect on friction coefficients of self-mated
Erikson L C, Blomberg A, Hogmark S, et al. Tribological
Andersson
Ishigaki H, Nagata R, Iwasa M. Effect of absorbed water on friction of hot-pressed silicon nitride and silicon carbide at low
characterization of alumina and silicon carbide under [2]
Wong H C, Umehara N, Kato K. Frictional characteristics of
system[J]. J Mater Sci Lett, 1994, 13: 474-476. [8]
Zum Gahr K H, Blattner R, Hwang D-H, et al. Micro-and macro-tribological properties of SiC ceramics in sliding contact[J]. Wear, 2001, 250: 299-310.
SUI Jian et al. / New Carbon Materials, 2013, 28(1): 71–75
[9]
humidity on the tribological properties of carbide-derived
Chen M, Kato K, Adachi K. Friction and wear of self-mated
carbon (CDC) films on silicon carbide. Tribol Lett[J], 2003,
SiC and Si3N4 sliding in water[J]. Wear, 2001, 250: 246-255.
15(1): 51-55.
[10] [ Gogotsi Y G, Yoshimura M. Formation of carbon films on carbides under hydrothermal conditions[J]. Nature, 1994, 367:
[16]
[11]
Technol, 2006, 3(3): 236-244.
Gogotsi Y G, Jeon I-D, McNallan M J. Carbon coatings on silicon carbide by reaction with chlorine-containing gases[J]. J
Erdemir A, Kovalchenko A, White C, et al. Synthesis and tribology of carbide-derived carbon films[J]. Int J Appl Ceram
628-630. [17]
Carrol B, Gogotsi Y G, Kovalchenko A, et al. Tribological characterization of carbide-derived carbon (CDC) films in dry
Mater Chem, 1997, 7: 1841-1848. [12] [ Ersoy D A, McNallan M J, Gogotsi Y G, et al. Tribological
and humid environments[J]. Nanostructured Materials and
properties of carbon coatings produced by high temperature
Coatings for Biomedical and Sensor Applications, 2003, 102: 119-130.
chlorination of silicon carbide[J]. STLE Tribol Trans, 2000, 43: 809-815. [13]
[18]
porous carbide-derived carbons by chlorination of mesoporous titanium carbides[J]. New Carbon Materials, 2009, 24(3): 243-250. [14]
Ersoy D A, McNallan M J, Gogotsi Y G. Carbon coatings produced by high temperature chlorination of silicon carbide ceramics[J]. Mater Res Innov, 2001, 5: 55-62.
[15]
Carrolla B, Gogotsi Y G, Kovalchenko A, et al. Effect of
Sui J, Lu J J. Friction and wear of carbide derived carbon coating sliding against Si3N4 under the lubrication of water[J].
Cheng G, Long D H, Liu X J, et al. Fabrication of hierarchical
Tribology (in Chinese), 2011, 31: 498-503. [19]
Hamilton D B, Walowit J A, Allen C M. A theory of lubrication by micro-irregularities[J]. J Basic Eng, 1966, 88: 177-185.