The effect of surface texturing on reducing the friction and wear of steel under lubricated sliding contact

The effect of surface texturing on reducing the friction and wear of steel under lubricated sliding contact

Applied Surface Science 273 (2013) 199–204 Contents lists available at SciVerse ScienceDirect Applied Surface Science journal homepage: www.elsevier...

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Applied Surface Science 273 (2013) 199–204

Contents lists available at SciVerse ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

The effect of surface texturing on reducing the friction and wear of steel under lubricated sliding contact Wei Tang a,b , Yuankai Zhou a , Hua Zhu a,∗ , Haifeng Yang a a b

School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China

a r t i c l e

i n f o

Article history: Received 7 September 2012 Received in revised form 2 February 2013 Accepted 2 February 2013 Available online 11 February 2013 Keywords: Surface texturing Friction reduction Wear reduction Load carrying capacity

a b s t r a c t Surface texturing is a widely used approach to improve the load capacity, the wear resistance, and the friction coefficient of tribological mechanical components. This study experimentally investigates the effect of surface texturing on reducing friction and wear. A numerical model of the load carrying capacity of multi-dimples is developed to analyze the relevant mechanism, and the effect of surface texturing on different dimple area fractions is evaluated to determine the optimal dimple pattern. The results show that surface texturing is important for reducing friction and wear. Changes in dimple area fraction can dramatically reduce friction and wear. The results indicate a 5% optimal dimple area fraction can generate the greatest hydrodynamic pressure compared with other fractions and can reduce friction and wear up to 38% and 72%, respectively. The theoretical model and the experimental results are found to be closely correlated. The generation of hydrodynamic pressure, the function of micro-trap for wear debris and the micro-reservoirs for lubricant retention are the main causes for the reduction in the friction and wear of the surface texturing. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Because approximately 40% of total energy losses result from engine friction and wear loss, friction and wear must be reduced to improve fuel consumption [1]. Surface texturing is an emerging effective method for improving the tribological performance of the mechanical components [2,3]. Since 1960s, Hamilton et al. [4] proposed the idea that surface texturing in the form of micro asperities acted as micro hydrodynamic bearings between two parallel surfaces, subsequent studies have investigated the hydrodynamic effect of surface texture. It has been shown that the selective micro-texture is effective for generating additional load capacity under hydrodynamic lubrication conditions and the cavitation is the main reason of this phenomenon [5–8]. Generally, the pressure increases in the converging film regions, while it decreases in the diverging film regions. The cavity generated in diverging film regions truncates the negative pressure and additional load capacity or positive pressure is generated by texture between contact surfaces [3,9,10], the schematic is shown in Fig. 1 [4,11]. Today, various forms and geometric features of surface texturing for tribological applications are carried out widely and various texturing techniques are also employed in these studies

∗ Corresponding author. Tel.: +86 13952207512. E-mail address: [email protected] (H. Zhu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.02.013

including machining, photoetching, etching techniques, ion beam texturing, and laser texturing [12,13]. According to the literature, the introduction of texturing may trap wear debris, thus reducing the ploughing and deformation components of friction [9,14], and may also act as a micro-reservoir for lubricants, thus reducing the friction and increasing the lifetime of the sliding contacts [15–17]. The effect of surface texturing on reducing friction and wear was depended considerably on the shape, size, density, and pattern of dimples [18,12,19,20]. For example, the experimental work by Etsion and Sher [19] indicated that partial LST piston rings could consume up to 4% less fuel compared with non-textured conventional barrel-shaped rings. Wang’s study suggested that pattern mixed with the large dimples (350 ␮m) and small dimples (40 ␮m) could result in higher critical load over that with small or large dimples only, and three times greater than that of untextured surface [20]. The previous studies also show that it existed an optimum texturing parameter for maximum load carrying capacity and minimum friction coefficient for different shape surface texturing, and the ranges of the optimum texturing parameter were varied in deferent operation conditions [8,10]. Most studies of surface texturing have focused on the experimental aspect, and recently, theoretical research has received more attention. This study experimentally investigates the effect of surface texturing on reducing the friction and wear of a steel surface. A numerical model of the load carrying capacity of multi-dimples is developed to analyze the mechanism

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Fig. 2. Schematic of the friction and wear test.

machine. Each dimple cell was an axisymmetric cylindrical segment with a diameter d (d = 300 ␮m, 500 ␮m, and 700 ␮m), and depth h1 (h1 = 50 ␮m) and was located in the center of an imaginary square cell with length l (l = 2 mm) (as shown in Fig. 3). Following previous studies [21,22], three dimple area fractions (2%, 5%, and 10%) that are varied by changing the diameter of each dimple were chosen and were defined as the following: Sp =

d2 (%) 4l2

(1)

After being fabricated with the texture, the samples were deburred, cleaned by ultrasonication in acetone and deionized water for 2 h, and dried with flowing air to keep the surface conditions as constant as possible. A non-textured sample was also measured for comparison. Four sets of samples were used in the tests, and each set was tested once. Fig. 1. Schematic of positive pressure sourced from cavitation.

2.2. Friction and wear measurements

responsible for reducing friction and wear with surface texturing, and the effect of surface texturing with several dimple area fractions is evaluated to determine the optimal dimple pattern. 2. Experimental details 2.1. Sample preparation In this study, two different hardness materials which are commonly used in the tribological mechanical components were employed in the test and the chemical composition is shown in Table 1. The high alloy steel with a hardness of HRC 62 was chosen as the upper specimen for better wear resistance and the medium carbon steel with a hardness of HRC 20 was chosen as the lower textured specimen to study the friction and wear reduction effect of the surface texturing. The two flat steel specimens were fixed and slid against each other; this process is shown schematically in Fig. 2. All lower steel samples were firstly lapped to achieve a surface roughness of 0.04 ␮m RMS and were then textured to the regular arrays of cylindrical dimples using a miniature engraving

Friction and wear tests were performed using a multi-functional tribometer (UMT, CETR, Campbell, CA, USA) under linear reciprocating motion and full lubrication. The tests were conducted at a normal load of 150 N (corresponding to a nominal contact pressures of 8.3 MPa), a frequency of 6 Hz and a stroke length of 20 mm (resulting in an average sliding speed of 0.24 m/s) in a laboratory atmosphere (25 ◦ C, RH 55%). A lubricant with a dynamic viscosity of 0.04678 Pa s and a density of 850 kg/m3 was used. To ensure full contact with the mating surface, running-in was performed for all samples before tests [23]. In the running-in process, the normal load was gradually increased in increments of approximately 50 N, at a velocity of 120 mm/s. When the friction force stabilized, it indicated that the running-in had finished. Then, the friction and wear tests were performed. The normal load and the friction force were measured with a stress sensor and the data were digitized and were collected on a personal computer. The average data of the friction signal were calculated, and the friction coefficient was obtained. To study the effect of the surface texturing on reducing wear, the wear debris in lubricant should be separated and collected after the tests. The separation and collection of the wear debris proceeded as follows: after a 7 h test, the wear debris and the lubricant were

Table 1 Chemical composition of the samples (wt%).

Upper sample (high alloy steel) Lower sample (medium carbon steel)

C

Si

Mn

Cr

S

P

2.16 0.45

0.27 0.23

0.35 0.66

11.62 0.15

0.021 0.025

0.012 0.008

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Fig. 3. Schematic of (a) two contacting surfaces with relative sliding velocity  and a fluid film thickness of h0 or h0 + h1 and (b) a dimple cell. An axisymmetric cylindrical segment with diameter d and depth h1 is located in the center of an imaginary square cell with the length l. The bottom of the dimple is has a 120◦ taper.

separated by centrifugation at 1500 rpm for 10 min. The deposit was placed and was dried at 25 ◦ C in a vacuum oven for 24 h. Then, the deposit was cleaned by ultrasonication in acetone for 2 h. The above procedure was repeated until the wear debris was collected. The size of the obtained wear debris was investigated using a scanning electron microscope (SEM). The amount of the wear debris was investigated using a weighing method. Before and after the test, the samples were cleaned by ultrasonication in acetone and deionized water for 2 h, dried with flowing air, and weighted with an electronic balance (accuracy is ±0.001 g). Before and after the test, the sample’s surface morphology was examined using a three-dimensional profilometer (MicroXam, Phase-Shift, Tucson, AZ, USA) to determine the change in surface roughness. 3. Analytical model To elucidate the mechanism responsible for reducing the friction and wear with surface texturing, a numerical model of multidimples based on the Reynolds equation was developed. The effect of different dimple area fraction on hydrodynamic lubrication under sliding contact was evaluated. The schematic of the two contacting surfaces and the dimple cell is presented in Fig. 3. The upper and lower contact surfaces are considered to constitute the non-textured surface and textured surface, respectively, and are separated by a fluid with a uniform thickness (h0 ). The textured surface shows regular arrays of cylindrical dimples. Each dimple cell is an axisymmetric cylindrical segment with a diameter d, and depth h1 and is located in the center of an imaginary square cell with length l. All dimple cell parameters in the numerical simulation are identical in the experiment. The following assumptions apply: • The fluid is a Newtonian liquid with a constant viscosity of 0.04678 Pa s and a constant density of 850 kg/m3 . The volume force and the inertia force are neglected.

• The flow’s rate equals to the surface velocity. • The contact surfaces are fully lubricated, and the fluid’s film thickness h0 remains constant. • Because the film thickness h0 is extremely thin, the change in the pressure distribution along each dimple’s z-axis is neglected. • No external force is applied to the fluid film.

Based on these assumptions, the hydrodynamic pressure distribution between two contact surfaces for a Newtonian liquid is obtained from the steady-state Reynolds equations: ∂ ∂x

 h3

∂p ∂x

 +

∂ ∂y

 h3

∂p ∂y

 = 6

∂h ∂x

(2)

where x and y are the Cartesian coordinates as shown in Fig. 3; P is fluid’s film pressure; h is the film thickness, which is defined in Eq. (3);  is fluid’s dynamic viscosity; and  is the sliding velocity.

 h(x, y) =

h0

(x, y) ∈ /˚

h1 + h0

(x, y) ∈ ˚

(3)

where Ф is the dimple area. Considering that the interaction between the adjacent dimples is significant [24,25], a grid of 3 × 3 dimple cells is chosen to obtain the film pressure between the two contact surfaces. Because the cavitation phenomenon generates the fluid film’s positive pressure, the film’s load carrying capacity is calculated using Half-Sommerfeld boundary condition; specifically, the pressure in the negative pressure area will be considered as zero, and only the positive pressure will be used to calculate the average pressure [26,27]. According to the Half-Sommerfeld boundary condition, the following boundary conditions are assumed as shown in Fig. 4 and Eq. (4): the pressure of the boundaries at the fluid’s inlet and outlet

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Table 2 Average coefficient of friction of different dimple area fraction samples. Dimple area fraction (%)

Coefficient of friction

0

2

5

10

0.136 ± 0.034

0.123 ± 0.021

0.08 ±0.02

0.106 ±0.03

Table 3 Wear amount and surface roughness of different dimple area fraction samples after the 7 h test. Dimple area fraction (%)

Wear amount (mg) RMS (nm)

0

2

5

10

18 ± 1.8 865 ± 55

9 ± 1.5 705 ± 67

5 ± 1.6 521 ± 48

6 ± 1.0 670 ± 62

Fig. 5. The typical coefficient of friction as a function of time for the textured and non-textured surface.

is considered to be zero, and the environmental pressure is not less than zero.

⎧ ⎨ P(x = 0), y = (P x3), l = y ⎩ Fig. 4. A grid of 3 × 3 grid of dimple cells.

P(x, y = 0) = (Px, y3), = l

(4)

Pk (x, y) ≥ 0

Based on the above boundary conditions and on the combination of Eqs. (2) and (3), the film pressure between the two contact

Fig. 6. SEM images of the typical wear debris produced by different dimple area fraction samples after the 7 h test.

W. Tang et al. / Applied Surface Science 273 (2013) 199–204

surfaces can be obtained and will be used to explain the mechanism responsible for reducing the friction and wear with surface texturing in the next section. 4. Results and discussion Fig. 5 displays the typical coefficient of friction for testing time plots of different dimple area fraction samples (0%, 2%, 5%, and 10%), and Table 2 presents the average coefficient of friction of different dimple area fraction samples. The results show that all textured samples have a lower coefficient of friction than the non-textured sample, and that the coefficient of friction of the 5% dimple area fraction sample stabilizes at the lowest value, approximately 0.08; this value is approximately38% lower than that of the non-textured sample, which indicates that the surface texturing substantially affects the friction reduction and that the 5% dimple area fraction tends to yield the minimum friction force. Table 3 and Fig. 6 presents the wear amount, the surface roughness and the SEM of the typical wear debris produced by different

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dimple area fraction samples after a 7 h test. They shows that all textured samples have a lower wear amount and surface roughness, and a smaller wear debris than the non-textured sample after the 7 h test. The wear amount of the 5% dimple area fraction sample is approximately 72% less than that of the non-textured sample. All results indicate that the surface texturing substantially affects the wear reduction and that the 5% dimple area fraction sample tends to yield the minimum wear as well as better wear topography. The surface texturing of the sliding surface is important in reducing friction and wear. To explain the mechanism responsible for these reductions of the surface texturing, the effect of the different dimple area fraction on the hydrodynamic lubrication under sliding contact is evaluated, and according to Refs. [2,18,12] the average pressure distribution of the fluid film is chosen as an index to evaluate the effect of the hydrodynamic pressure generation on the surface texturing; the results are shown in Fig. 7. Fig. 7(a) shows that normal to the sliding directions, the hydrodynamic pressure is strongly influenced by the adjacent dimples

Fig. 7. (a) Average pressure distribution over 3 × 3 dimples of different dimple area fraction samples and (b) average pressure versus different dimple area fraction samples.

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and the interaction between the adjacent dimples become significant with increasing dimple area fraction, the similar results are also shown in Ref. [24]. Because of this interaction, the pressure does not decrease to zero along the width boundary, thereby indicating that the interaction between the dimples significantly affects the hydrodynamic pressure distribution. Fig. 7(b) shows that all textured samples can generate greater hydrodynamic pressure compared with the non-textured sample and will thus increase the load carrying capacity. During the friction, hydrodynamic pressure is generated in the narrow gap between the mating surfaces. The load carrying capacity can be provided by each dimple because of an asymmetric hydrodynamic pressure distribution over the dimples that results from local cavitation in the diverging clearance of the dimples [11]. Fig. 7(b) also shows that the 5% dimple area fraction sample generates the greatest hydrodynamic pressure of all the dimple area fractions and thus yields maximum load carrying capacity. According to the numerical analysis and the experimental results, the generation of the surface texturing’s hydrodynamic pressure is crucial to reducing friction and wear, and 5% is the optimal dimple area fraction value for achieving minimum friction and wear, which is consistent with the previous study results in Ref. [12]. Additionally, the dimples on the surface also serve as traps for the wear debris and as micro-reservoirs for lubricant retention, thus reducing the ploughing and deformation components of friction and wear. 5. Conclusion The effect of surface texturing on reducing the friction and wear of a steel surface is experimentally investigated. The theoretical and experimental results of a model of multi-dimples based on the Reynolds equation are shown to be closely correlated. The surface texturing is important in reducing friction and wear. The generation of hydrodynamic pressure, the function of micro-trap for wear debris, and the micro-reservoirs for lubricant retention are the main mechanisms responsible for reducing the friction and wear with surface texturing. The change in the dimple area fraction can dramatically reduce friction and wear. A 5% optimal dimple area fraction, which can generate the greatest hydrodynamic pressure of all tested dimple area fractions and can reduce friction and wear up to 38% and 72%, respectively, is identified. Acknowledgments The authors acknowledge financial support from the National Natural Science Foundation of China 51205394 and 50975276, the China Postdoctoral Science Foundation funded project 2011M500966, the Fundamental Research Funds for the Central Universities 2010QNA24, Specialized Research Fund for the Doctoral Program of Higher Education 20120095120014, and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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