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TiC ceramic sliding against steel under dry friction
Materials and Design 52 (2013) 234–245
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Effect of laser surface texturing on Si3N4/TiC ceramic sliding against steel under dry friction Xing Youqiang, Deng Jianxin ⇑, Feng Xiuting, Yu Sheng Department of Mechanical Engineering, Shandong University, Jinan 250061, PR China
a r t i c l e
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Article history: Received 6 April 2013 Accepted 24 May 2013 Available online 5 June 2013 Keywords: Laser surface texturing Friction Wear Ceramic matrix composite
a b s t r a c t In efforts to investigate the influence of the surface texturing on the Si3N4/TiC ceramic, laser surface texturing (LST) was performed on the Si3N4/TiC ceramic by an Nd:YAG laser and different geometrical characteristics of regular-arranged micro-grooved textures were fabricated on the surfaces. The tribological properties of the textured and smooth samples were investigated by carrying out sliding wear tests against steel balls under dry condition using a ball-on-disk tribometer. Effect of surface texturing on the stress distribution was studied by finite element method (FEM). Results show that the textured surfaces exhibited lower friction coefficient and excellent anti-wear properties compared with smooth surfaces. The tribological characteristics depended greatly on the size and density of the micro-grooves, and the geometrical characteristics of the surface textures have a significant effect on the tribological behavior. Among the patterns investigated, the wavy-grooved samples exhibit the lowest friction coefficient and wear rate; and a large texture density may be the best for reduction of friction and wear of textured samples. While, the wear rate of balls sliding against textured surfaces is larger than that of balls sliding against smooth surfaces. FEM results show that surface texturing can improve the stress distribution of contact interfaces and reduce stress concentration. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction During the last few decades, Si3N4-based ceramic has been increasingly used as wear resistant materials: cutting tools [1,2], engine components [3,4], and bearings [5,6], due to its excellent intrinsic properties, such as low density, low thermal expansion, high hardness, good chemical inertness and high corrosion resistance. Generally speaking, Si3N4-based ceramic can be used under lubricated and dry conditions; however, under dry condition ceramic shows serious friction and wear between the contact surfaces of the sliding components. As Si3N4-based ceramics are increasingly used for various engineering applications, which must possess the ability to withstand the severe operating condition, and the surface friction and wear are important to the life of components. Thus, a large number of studies have been made on Si3N4-based ceramic as frictional materials in literatures. Akdogan and Stolarski [7] studied that when Si3N4 ceramic sliding against stainless steel without lubricants, severity of wear between stainless steel and Si3N4 ceramic might be attributed to a composite form of adhesion and three-body abrasive wear. Zhao et al. [8] ⇑ Corresponding author. Address: Department of Mechanical Engineering, Shandong University, No. 17923 Jingshi Road, Jinan 250061, Shandong Province, PR China. Tel.: +86 531 88399769. E-mail address: [email protected] (J. Deng). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.05.077
studied the tribological characteristics of Si3N4 ceramic sliding against stainless steel under unlubricated sliding conditions; the results showed that the wear of ceramic is mainly caused by the adhesion-peeling off process. When transferred stainless steel flats are peeled off the ceramic surface, some ceramic fragments or grains are also pulled out and taken away. Gautier and Kato [9] studied the wear mechanism of silicon nitride in unlubricated sliding against steel, it is suggested that the wear of ceramics is mainly related to transgranular or intergranular fracture. Steel discs presented much more intricate wear patterns, including abrasion, oxidation and delamination. It is known that the surface topography has a significant influence on the friction behavior and the wear resistance. Surface texturing is an effective way for improving the tribological properties for many years. Its main effects are to trap wear debris, store lubricants, and increase the load carrying capacity [10–15]. In the past decades, the surface texturing has been used in many fields to improve the tribological performances of interfaces, including bearings [16,17], engine cylinder liners [18,19], cutting tools [20–22], and seal rings [23,24]. The dimples and grooves may be the effective patterns on the tribo-surfaces. Efforts have been made to compare the effect of dimples and grooves on tribological performances, and it also has been used on the surface of ceramics [25–29]. Despite the above researches, different researchers studied in different experimental settings showed the different results. Suh
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Y. Xing et al. / Materials and Design 52 (2013) 234–245 Table 1 Properties of Si3N4/TiC ceramic, AISI 440C steel. Material (wt%)
Density (g cm3)
Hardness (HV)
Young’s modulus (GPa)
Poisson’s ratio
Fracture toughness (MPa m1/2)
Bending strength (MPa)
Thermal expansion (106 mm1 °C1)
Thermal conductivity (Wm1 °C1)
Si3N4/TiC35% AISI 440C
3.51 7.62
2191 746
340 197
0.29 0.29
6.2 18.3
830 1900
3.0 10.2
23 24.2
Table 2 Geometric parameters of textured samples. Width (lm)
Distance (lm)
Darea of wave (%)
Darea of line (%)
50 50 50 50
100 200 300 400
70.7 35.36 23.57 17.68
50 25 16.67 12.5
et al. [30] reported that friction control can be achieved by fabricating the micro-grooved crosshatch pattern on a contact surface, and the crosshatch angle of groove texture seems to be an important parameter to design surface texture as well as groove aspect ratio. Yuan et al. [31] studied the orientation effects of micro-grooves on sliding surfaces, the results showed that the merits of perpendicular and parallel orientation may swap under different contact conditions. Zum Gahr et al. [32] investigated the friction coefficient and film thickness of 100Cr6/sapphire pairs, which showed that textures perpendicular to the sliding direction generated greater film thickness and lower friction coefficient than that of parallel orientation, which is contradicted to the experimental results obtained by Hata et al. [33]. Therefore, the friction performance may be quite sensitive to the geometric characteristics of the surface textures and the materials, and it needs to be further studied. This paper studies the effect of different grooves fabricated by laser surface texturing (LST) on a silicon nitride ceramic disc plate
mated with 440C stainless steel. The effect of textures on the tribological properties was investigated by carrying out dry sliding tests on a ball-on disk tribometer. Although the past results contribute a lot for us to understand the effect of textures, there are still many uncertainties during the design of textures patterns. The aim of this investigation was to study the friction and wear behavior of textured surfaces of Si3N4 ceramic sliding against the steel under dry condition, and most important, the effects of the geometrical characteristics of the micro-grooved textures. 2. Experimental details 2.1. Specimens and surface texturing The materials used for the tests were hot-pressed Si3N4/TiC ceramic and AISI440C stainless steel ball, the main component and mechanical properties of the samples were listed in Table 1. The Si3N4/TiC discs were 16 mm 16 mm 8 mm, and the surfaces were lapped and polished to the roughness less than Ra 0.02 lm. The AISI440C stainless steel balls have a diameter of 9.5 mm, and their surface roughness was about Ra 0.01 lm. The LST (laser surface texturing) was applied on the Si3N4/TiC discs with a wavelength of 1064 nm Nd:YAG laser, pulse of 10 ns duration, voltage of 19.5 V, frequency of 6000 Hz and scanning speed of 5 mm/s. The wavy and linear grooves were fabricated on the samples. Textures were produced with fixed distance rang-
Fig. 1. Optical micrographs of textured surfaces: (a) wavy groove, (b) linear groove, (c) a single groove, (d) schematic diagram of ST-WG sample, and (e) schematic diagram of ST-WG sample.
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Fig. 2. Surface topographies of textured samples detected by a white light interferometer: (a) ST-WG and (b) ST-LG.
Fig. 3. SEM microphotograph of a groove (a), surrounding the groove area and EDS (b).
ing from 100 lm to 400 lm, the width of the grooves was about 50 lm and the depth was about 50 lm. The geometric parameters of textures were listed in Table 2, in which the density (Darea) is the area ratio of textured surfaces with total surfaces.
In order to assess the effect of the textures, the smooth samples were used to test for comparison. The samples with smooth surfaces, wavy grooves and linear grooves are named SS, ST-WG and ST-LG, respectively. The morphology of textured samples was ob-
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1000
-Si3N4 -Si3N4 TiC TiO2 TiO
Lin (Cps)
800
600
400
200
0 10
30
50
70
2-Theta-Scale
Fig. 6. Comparison of average friction coefficient of smooth samples and textured samples with different distance.
Fig. 4. X-ray diffraction (XRD) analysis of the textured Si3N4/TiC surface.
Fig. 5. Comparison of friction coefficient of the smooth samples and textured samples with different distance.
served using an optical microscope, scanning electron microscopy (SEM) and white light interferometer. 2.2. Friction and wear test Reciprocating sliding friction tests according to ASTM G133-05 (2010) standard were carried out using a ball-on-disk tribometer (UMT-2, USA). The upper specimen was a stainless steel ball (AISI
440C), and the lower specimen was a Si3N4/TiC ceramic sample. The lower specimen was fixed, and the ball was reciprocating sliding against the lower specimen. Two parameters were varied to investigate the influence of the textures on the friction conditions: texture distance (texture density): 100, 200, 300 and 400 lm, and texture type: wavy groove and linear groove. All tests were carried out with a load of 45 N, sliding speed of 5 mm/s and sliding time of 30 min. Each parameter combination is repeated three times.
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Before the tests, all the samples were cleaned in ultrasonic bath by alcohol for 30 min and then dried. The worn surfaces of the samples were observed by an optical microscope, scanning electron microscopy (SEM) and white light interferometer (Wyko NT9300, USA). The friction coefficient can be obtained directly by the software on the UMT-2 tribometer. The worn volumes of the discs were calculated by surface worn profiles measured by a white light interferometer and the worn volumes of the balls were calculated by the spherical crown measured by an optical microscope. The wear rate is calculated as follows: Fig. 7. Variations of the wear rates of smooth samples and textured samples with different distance.
K¼
V FL
ð1Þ
where V is the worn volume (mm3), F is the normal load (N), and L is the sliding distance (m). 3. Results 3.1. Morphology of textured surface
Fig. 8. Variations of the wear rates of the balls sliding against smooth samples and textured samples with different distance.
Fig. 1 shows the surface topography and schematic diagram of textured samples and the stereoscopic profile of a single microgroove, which is observed by an optical microscope. It can be seen that two different texturing patterns are arranged on the surfaces evenly, the width of the micro-groove is about 50 lm and the depth is about 50 lm; the angle of the wavy pattern is 90° and the cross-sectional profile of the micro-groove is triangle. Fig. 2 shows topography of textured samples detected by a white light interferometer. SEM microphotograph of a single groove and surrounding the micro-groove is illustrated in Fig. 3. It can be seen that the heat-affected zone is formed around the groove due to the laser with high energy density, vapor blast ejections of melted ceramic and rapid condensation. The bulges around grooves are a little higher than
Fig. 9. Optical micrographs of worn surfaces of smooth and textured samples with distance of 200 lm after 30 min dry sliding friction: (a) SS, (b) ST-WG, and (c) ST-LG.
Fig. 10. SEM micrographs of worn surfaces of SS samples after 30 min dry sliding friction and EDX analysis of point A.
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Fig. 11. SEM micrographs of worn surfaces of ST-WG samples with the distance of 200 lm after 30 min dry sliding friction and EDX analysis of point A and point B.
Fig. 12. SEM micrographs of worn surfaces of ST-LG samples with the distance of 200 lm after 30 min dry sliding friction and EDX analysis of point A and point B.
the plan area, which increases the surface roughness of textured samples. The samples are consisted of three parties: substrate, interlayer and oxide layer. The outermost layer contains a higher content of O, and based on the results of EDS (Fig. 3) and XRD (Fig. 4), the TiO2 and TiO are formed on the surface. Thus, the effect of oxidation may be taken into account in the friction process.
3.2. Friction and wear properties The friction coefficient of smooth surfaces and two kinds of textured surfaces with different distance is shown in Fig. 5. It can be seen that the friction coefficient of smooth samples (SS) in all experimental conditions is initially relatively lower than that of
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Fig. 13. Three-dimensional surface topographies of the wear tracks on the (a) SS, (b) ST-WG, and (c) ST-LG samples after 30 min dry sliding friction detected by a white light interferometer.
textured surfaces (ST-WG and ST-LG) with the same distance. The friction coefficient of all samples increases and then tends to be stable waves with the time under all experimental conditions. Against smooth surfaces, the friction coefficient stabilizes at a value 0.6–0.7; while against textured surfaces, it stabilizes at 0.4– 0.6, and ST-WG samples have the lowest friction coefficient. It also can be seen that the textured samples reach a steady state with less time. The average friction coefficient in the stage of stable waves is calculated for comparison in Fig. 6. It is shown that both the textured samples (ST-WG and ST-LG) show lower values of average friction coefficient than smooth samples (SS), the average friction coefficient increases with the increase of texture distance. The average friction coefficient of ST-WG and ST-LG samples is reduced by 22.1% and 12.4% at the distance of 100 lm, respectively; while at the distance of 400 lm, the reduction of the average friction coefficient of the textured samples (ST-WG and ST-LG) is not obvious compared with smooth samples (SS), and it is 5.1% and 3.9% for ST-WG and ST-LG samples, respectively. The texture distance of 100 lm may be the best for friction reduction on all tests. The specific wear rates of the Si3N4/TiC ceramic samples are shown in Fig. 7. It can be seen that the wear rates of all textured samples (ST-WG and ST-LG) are lower than that of smooth samples (SS) under all conditions. At the same distance, ST-WG samples have the lowest wear rates. The wear rates of textured samples increase with increasing texture distance from 100 lm to 400 lm. The wear rates of balls sliding against smooth and textured samples with different distance are calculated and shown in Fig. 8. Apparently, the wear rates of all balls sliding against textured samples (ST-WG and ST-LG) are larger than that sliding against smooth samples (SS); the balls sliding against ST-WG samples show the highest wear rates at all tests. In addition, the wear
rates show a declining trend with increasing texture distance for textured samples compared with smooth samples. Fig. 9 shows the optical micrographs of worn surfaces of smooth and textured samples with distance of 200 lm after 30 min dry friction. From the optical micrographs, it is observed that wear scars formed by mechanical rubbing can be seen clearly, and in some case, the wear scars are different. After tests the microgrooves are still present, and the grooves are filled with a lot of wear debris. SEM micrographs of worn surfaces of three different samples (SS, ST-WG and ST-LG) after 30 min sliding friction are presented in the Figs. 10–12. Obviously, severe surface damage can be seen of SS sample in the form of many ploughs and adhesions on the worn surface (Fig. 10a and b). The EDX analysis of point A confirms the presence of many adhesions of Fe, Fig. 10c. However, the worn on the textured surfaces is milder compared with that of SS samples surfaces, and few ploughs and adhesions can be seen on the surfaces among textures, see Figs. 11 and 12. It also can be seen that SEM micrographs of worn surfaces and results of EDX of point A (inside the grooves) of all the textured surfaces exposed that the grooves are filled with wear particles. Comparison of EDX analysis of point A and point B, the amount of Fe of point B is lower than that of point A, see Figs. 11 and 12. The three-dimensional topographies of the wear tracks on three kinds of samples after 30 min dry friction are shown in Fig. 13. It is shown that the adhered materials for smooth samples (Fig. 13a) are larger than that of textured samples (Fig. 13b and c); the textures (Fig. 13b and c) still exist after the friction process. To quantify the wear of the samples, two-dimensional topographies and wear profiles fitted by B-spline curves of the wear tracks of three kinds of samples after cleaned in an ultrasonic bath by alcohol are detected by a white light interferometer (see Fig. 14).
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The worn surfaces of the balls sliding against three kinds of samples after 30 min dry sliding friction are shown in Fig. 15. Several conclusions can be obtained from the worn surfaces: the wear scars on balls sliding against textured samples (Fig. 15a and b) are larger than that of the ball sliding against smooth surfaces (Fig. 15c); the enlarged SEM micrographs of the ball-worn surfaces (Fig. 15a0 –c0 ) indicated that wear occurred in all cases by an abrasive mechanism. However, pronounced abrasive grooves visible as scratches were obvious in balls sliding against the smooth samples (Fig. 15c0 ) than that of the balls sliding against textured samples (Fig. 15a0 and b0 ). 3.3. Effect of surface texturing on the stress distribution In the process of sliding friction, the stress is an important reason to cause wear of the surface, and surface texturing will influ-
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ence the stress and its distribution. The finite element method (FEM) is an effective way to estimate the stress and characterize its distribution. Fig. 16 shows the FEM gridding model of three different samples. To obtain more accurate simulation results, the upper surfaces of the textured samples are refined mesh. The mechanical properties of the sample materials are listed in Table 1. In order to reduce the computing time in the analysis and simplify the FEM model, the upper ball specimen surface is simplified to a cylindrical pin and the tests were studied by the finite element analysis after a cycle of reciprocating motion. A pressure of 572.9 MPa is applied on the upper surfaces of pins according to calculation of the Hertz contact theory. The results of the von Mises stress distribution in the SS, ST-WG, and ST-LG samples are shown in Fig. 17. It can be seen that the large stress concentration occurs on SS sample surfaces
Fig. 14. Two-dimensional surface topographies and profiles of the wear tracks on the samples after 30 min dry sliding friction detected by a white light interferometer: (a) and (a0 ) SS, (b) and (b0 ) ST-WG, and (c) and (c0 ) ST-LG.
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Fig. 15. SEM micrographs of ball-worn surfaces sliding against different samples: (a) ball-worn surface sliding against ST-WG with the distance of 200 lm, (a0 ) enlarged SEM micrograph corresponding to (a), (b) ball-worn surface sliding against ST-LG with the distance of 200 lm, (b0 ) enlarged SEM micrograph corresponding to (b), (c) ball-worn surface sliding against SS, and (c0 ) enlarged SEM micrograph corresponding to (c).
Fig. 16. FEM gridding model of samples: (a) SS, (b) ST-WG with the distance of 200 lm, and (c) ST-LG with the distance of 200 lm.
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Fig. 17. The von Mises stress distribution of the samples surfaces: (a) SS, (b) ST-WG, (c) ST-LG.
Groove
(a)
(b)
(c)
440 C steel ball
Wear debris
Si3 N 4 / TiC ceramic
Fig. 18. Friction and wear reduction mechanism of textured surfaces.
(Fig. 17a), while large stress only occurs at the edges of the textures of ST-WG and ST-LG samples surfaces (Fig. 17b and c). In comparison with SS sample (Fig. 17a), the stress distribution of ST-WG and ST-LG samples is more uniform, but the stress magnitude is a litter larger. For SS sample, the maximum von Mises stress is 887.2 MPa, while it is 1263 MPa and 957.4 MPa for ST-WG and ST-LG samples. 4. Discussion In this paper, the effect of the grooved textures on the Si3N4/TiC ceramic under dry condition is studied. The results show that the textured surface can improve the tribological performance of the Si3N4/TiC ceramic disc under dry friction. The reduction of the contact areas between the balls and the textured samples and the effect of capturing debris of the grooves may be responsible for the reduction of the friction coefficient and wear rate of the textured samples [34]. While the initial friction coefficient of the textured surface is higher than that of the smooth surface, which is due to the large surface roughness of the textured surface increased by the bulges formed by melting and vaporization, and the textured surface can reduce the run-in time, see Fig. 5. The distance and pat-
tern of the textures have an important influence on tribological behavior of the Si3N4/TiC samples and the 440C steel balls (Figs. 5–8). A smaller distance between the grooves should facilitate both the friction coefficient and wear rate of the Si3N4/TiC samples. It is due to the larger texture density corresponding to the smaller texture distance (see Table 2) enable to reduce the larger contact area, collect more debris and thus has a lower friction coefficient and wear rate. The schematic diagram of texture effect mechanism is shown in Fig. 18. It can be explained that when the movement begins between the couples, cracks occurs on the surfaces of the disc and ball (Fig. 18a); proceed with the friction, materials are peeled off the couples (Fig. 18b), then the grooves can capture the debris effectively (Fig. 18c), which can be seen from the SEM of the worn textured sample surfaces (see Figs. 11 and 12). Thus, when wear particles escape from the contact zone into the grooves, the friction and wear are decreased, which is in line with the literatures [35– 37]. The results of the friction coefficient show a high dependence on the geometry of the textured surfaces. Lowest values of the friction coefficient are obtained by ST-WG samples and the highest values are obtained by the SS samples under all experimental con-
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Fig. 19. Schematic of sliding area, length and force with different textured samples.
ditions. Friction property is strongly influenced by sliding length, area of the ball in the textures and the retarding force of the bulges of the textures [30,38,39]. The sliding area (A) and length (l) moving on the textured surface parallel to the sliding direction were shown in Fig. 19. In the view point of geometrical parameters in grooved pattern (Fig. 19a and b), it was already confirmed that friction coefficient is associated with the width (w) and angle (h), so the groove sliding area ‘‘A’’ can be calculated as it is shown in Fig. 19a0 and b0 by red color, which shows low friction performance according to the sliding area. It can be assumed that friction coefficient is a function of the parameter A [38]. For wavy textures, h is 45°, while it is 90° for linear textures. The sliding area (A) moving on the textured surface for the wavy and linear textures can be recorded as A1 and A2. It can be calculated that A1 is larger than A2. Finally, the increasing of the sliding area and the length moving on the textured surface enhanced the effect of lubricating and capturing debris (Fig. 19a0 and b0 ). The main retarding force FR, which is perpendicular to direction of the grooved edges, is shown in Fig. 19a00 and b00 , it is obvious that FR1 is smaller than that of FR2. Thus, the ST-WG had the least friction coefficient and the wear rate (Figs. 5–7). As we all know that the excessive stress is an important reason to cause wear, The results of the simulation (Fig. 17a–c) show that the textures can prevent excessive stress concentration in the contact zone and the stress distribution is uniform after a cycle movement, which reduces the friction and wear of the samples. On the other hand, EDS and XRD of the textured surface indicated the oxidation after laser surface texturing, and TiO2 produced on the textured samples may act as a lubricant, Figs. 3 and 4, which contributes to the improvement of tribological performance [40]. SEM micrographs of the worn surfaces for SS sample exhibit a severe surface damage, the wear mechanism is mainly ploughs and adhesions (Fig. 10). It can be seen that a lot of wear debris filled in the micro-grooves, and few ploughs and adhesions can be seen on the surface among textures (Figs. 11 and 12), which is
the effect of capturing debris of textures. Thus, the wear of the textured surface is reduced effectively. Looking to the worn topographies of smooth surface detected by a white light interferometer we note that a spread of debris covers the whole wear track (Fig. 13a). However, it shows few wear debris on surfaces among grooves of textured samples (Fig. 13b and c), which is consistent with that shown in Figs. 10–12. This indicates that wear debris move from the contact region into grooves. The surface worn topographies and the wear profiles of samples after being cleaned in ultrasonic bath by alcohol and then dried are detected by a white light interferometer (Fig. 14); the wear depths of textured samples (ST-WG and ST-LG) are less than that of smooth samples (SS), which confirms the reduction of the wear of the textured samples. Note that the wear scars on balls sliding against textured discs are larger than those sliding against smooth surfaces, Fig. 15, which is consistent with the literature [41], the wear of balls is dependent on the roughness of the contact samples and the textured samples have a larger roughness (rougher samples produced more wear on the ball); on the other hand, the sharp edges around the grooved have a micro-cutting effect, which would cut the contact ball surface and lead to a higher wear of the balls [12]. Thus, surface texturing could be beneficial if accelerated wear on the counter-face is acceptable, such as cutting tools [42]. 5. Conclusions Laser surface texturing (LST) is a new and interesting method for the surface finishing of ceramics, its possibilities complement the variety of conventional finishing methods. Two different patterned micro-grooves were made on the surface of the Si3N4/TiC ceramic, reciprocating dry sliding wear tests were carried out with the samples (SS, ST-WG and ST-LG) sliding against 440C steel ball under dry condition in this research. The main conclusions from this study are summarized in the following:
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(1) Surface texturing has a strong effect on the friction and wear performance of sliding surfaces and the friction coefficient of the textured ceramics was quite sensitive to the geometrical characteristics of the surface texturing. In this research, the ST-WG and ST-LG samples have lower friction coefficient and wear rates than that of SS samples, and the ST-WG samples have the best effect on friction and wear reduction. The large texture density (small texture distance) may be the best for reduction of friction and wear of textured samples. The wear form of Si3N4 ceramics in sliding pairs is mainly caused by the adhesions and ploughs between the rubbing surfaces. The adhesions and ploughs on the worn tracks of the ST-WG and ST-LG samples are milder compared with that of the SS samples. (2) The wear scars on balls sliding against textured samples are larger than those sliding against smooth surfaces. SEM micrographs of the ball-worn surfaces indicate that wear occurs in all cases by an abrasive mechanism, while, pronounced abrasive grooves visible as scratches are obvious in balls sliding against the smooth samples than that of the balls sliding against textured samples, the effect of capturing debris by textures may be responsible for it. Based on the studies, LST could be beneficial in terms of friction reduction if accelerated wear on the counter-face is acceptable. (3) Finite element analysis results show that surface texturing can improve the stress distribution of contact interfaces. In comparison with the SS sample, ST-WG and ST-LG samples reduce the stress concentration of the surface, and the stress distribution of ST-WG and ST-LG samples is more uniform. Thus, less stress concentration reduces the wear on the surface of textured samples.
Acknowledgments This work is supported by ‘‘The National Natural Science Foundation of China (51075237)’’, ‘‘The Taishan Scholar Program of Shandong Province’’, ‘‘The Natural Science Foundation of Shandong Province (ZR2010EZ002)’’, ‘‘The Independent Innovation Foundation of Shandong University (2011JC001)’’, and ‘‘The Specialized Research Fund for Doctoral Program of Higher Education (20110131130002)’’. References [1] Zhao XZ, Liu JJ, Zhu BL, Miao HZ, Luo ZB. Wear simulation of Si3N4 cutting tool material on a Pin-On-Disc tester. Ceram Int 1997;23:483–8. [2] Souzaa JVC, Nonoa MCA, Ribeirob MV, Machadoa JPB, Silvac OMM. Cutting forces in turning of gray cast iron using silicon nitride based cutting tool. Mater Des 2009;30:2715–20. [3] Kitamura K, Takebayashi H, Ikeda M, Percoulis H, Development of Ceramic Cam Roller Follower for Engine Application. SAE Tech. Paper No. 972774; 1997. [4] Walzer P, Heinrich H, Langer M. Ceramic components in passenger-car diesel engines. Mater Des 1986;7:75–80. [5] Katz RN. Ceramics for high performance rolling element bearings: a review and assessment. Mater Des 1987;8:108–12. [6] Wang L, Snidle RW, Gu L. Rolling contact silicon nitride bearing technology: a review of recent research. Wear 2000;246:159–73. [7] Akdogan G, Stolarski TA. Wear in metal/silicon nitride sliding pairs. Ceram Int 2003;29:435–46. [8] Zhao XZ, Liu JJ, Zhu BL, Miao HZ, Luo BZ. Wear behavior of Si3N4 ceramic cutting tool material against stainless steel in dry and water-lubricated conditions. Ceram Int 1999;25:309–15. [9] Gautier P, Kato K. Wear mechanisms of silicon nitride, partially stabilized zirconia and alumina in unlubricated sliding against steel. Wear 1993;162– 164:305–13.
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[23] Pawlus P, Galda L, Dzierwa A, Koszela W. Abrasive wear resistance of textured steel rings. Wear 2009;267:1873–82. [24] Etsion I, Sher E. Improving fuel efficiency with laser surface textured piston rings. Tribol Int 2009;42:542–7. [25] Wakuda M, Yamauchi Y, Kanzaki S, Yasuda Y. Effect of surface texturing on friction reduction between ceramic and steel materials under lubricated sliding contact. Wear 2003;254:356–63. [26] Yamakiri H, Sasaki S, Kurita T, Kasashima N. Effects of laser surface texturing on friction behavior of silicon nitride under lubrication with water. Tribol Int 2011;44:579–84. [27] Schreck S. Zum Gahr KH. Laser-assisted structuring of ceramic and steel surfaces for improving tribological properties. Appl Surf Sci 2005;247:616–22. [28] Xing YQ, Deng JX, Tan YQ, Zhou HM. Effect of surface textures on friction properties of Al2O3/TiC ceramics. Surf Eng 2012;28:605–11. [29] Wang XL, Kato K, Adachi K, Aizawa K. Load carrying capacity map for the surface texture design of SiC thrust bearing sliding in water. Tribol Int 2003;36:189–97. [30] Suh MS, Chae YH, Kim SS, Hinoki T, Kohyama A. Effect of geometrical parameters in micro-grooved crosshatch pattern under lubricated sliding friction. Tribol Int 2010;43:1508–17. [31] Yuan SH, Huang W, Wang XL. Orientation effects of micro-grooves on sliding surfaces. Tribol Int 2011;44:1047–54. [32] Zum Gahr KH, Wahl R, Wauthier K. Experimental study of the effect of microtexturing on oil lubricated ceramic/steel friction pairs. Wear 2009;267:1241–51. [33] Hata H, Nakahara T, Aoki H. Measurement of friction in lightly load hydrodynamic sliders with striated roughness. In: Proceedings of the winter annual meeting of the American Society of Mechanical Engineers. Chicago, Illinois; 1980; p. 75–92. [34] Kim SS, Lee HG, Lee DG. Tribological behaviors of carbon composite grooved surfaces. Compos Struct 2005;71:238–45. [35] Fu Y, Wei J, Batchelor AW. Some considerations on the mitigation of fretting damage by the application of surface modification technologies. J Mater Process Technol 2000;99:231–45. [36] Volchok A, Halperin G, Etsion I. The effect of surface regular microtopography on fretting fatigue life. Wear 2002;253:509–15. [37] Varenberg M, Halperin G, Etsion I. Different aspects of the role of wear debris in fretting wear. Wear 2002;252:902–10. [38] Costa HL, Hutchings IM. Hydrodynamic lubrication of textured steel surfaces under reciprocating sliding conditions. Tribol Int 2007;40:1227–38. [39] Franzen V, Witulski J, Brosius A, Trompeter M, Tekkaya AE. Textured surfaces for deep drawing tools by rolling. Int J Mach Tool Manu 2010;50:969–76. [40] Deng JX, Ai X, Li ZQ. Friction and wear behavior of A12O3/TiB2 composite against cemented carbide in various atmospheres at elevated temperature. Wear 1996;195:128–32. [41] Kovalchenko A, Ajayi O, Erdemir A, Fenske G. 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Effect of laser surface texturing on Si3N4/TiC ceramic sliding against steel under dry friction Xing Youqiang, Deng Jianxin ⇑, Feng Xiuting, Yu Sheng Department of Mechanical Engineering, Shandong University, Jinan 250061, PR China
a r t i c l e
i n f o
Article history: Received 6 April 2013 Accepted 24 May 2013 Available online 5 June 2013 Keywords: Laser surface texturing Friction Wear Ceramic matrix composite
a b s t r a c t In efforts to investigate the influence of the surface texturing on the Si3N4/TiC ceramic, laser surface texturing (LST) was performed on the Si3N4/TiC ceramic by an Nd:YAG laser and different geometrical characteristics of regular-arranged micro-grooved textures were fabricated on the surfaces. The tribological properties of the textured and smooth samples were investigated by carrying out sliding wear tests against steel balls under dry condition using a ball-on-disk tribometer. Effect of surface texturing on the stress distribution was studied by finite element method (FEM). Results show that the textured surfaces exhibited lower friction coefficient and excellent anti-wear properties compared with smooth surfaces. The tribological characteristics depended greatly on the size and density of the micro-grooves, and the geometrical characteristics of the surface textures have a significant effect on the tribological behavior. Among the patterns investigated, the wavy-grooved samples exhibit the lowest friction coefficient and wear rate; and a large texture density may be the best for reduction of friction and wear of textured samples. While, the wear rate of balls sliding against textured surfaces is larger than that of balls sliding against smooth surfaces. FEM results show that surface texturing can improve the stress distribution of contact interfaces and reduce stress concentration. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction During the last few decades, Si3N4-based ceramic has been increasingly used as wear resistant materials: cutting tools [1,2], engine components [3,4], and bearings [5,6], due to its excellent intrinsic properties, such as low density, low thermal expansion, high hardness, good chemical inertness and high corrosion resistance. Generally speaking, Si3N4-based ceramic can be used under lubricated and dry conditions; however, under dry condition ceramic shows serious friction and wear between the contact surfaces of the sliding components. As Si3N4-based ceramics are increasingly used for various engineering applications, which must possess the ability to withstand the severe operating condition, and the surface friction and wear are important to the life of components. Thus, a large number of studies have been made on Si3N4-based ceramic as frictional materials in literatures. Akdogan and Stolarski [7] studied that when Si3N4 ceramic sliding against stainless steel without lubricants, severity of wear between stainless steel and Si3N4 ceramic might be attributed to a composite form of adhesion and three-body abrasive wear. Zhao et al. [8] ⇑ Corresponding author. Address: Department of Mechanical Engineering, Shandong University, No. 17923 Jingshi Road, Jinan 250061, Shandong Province, PR China. Tel.: +86 531 88399769. E-mail address: [email protected] (J. Deng). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.05.077
studied the tribological characteristics of Si3N4 ceramic sliding against stainless steel under unlubricated sliding conditions; the results showed that the wear of ceramic is mainly caused by the adhesion-peeling off process. When transferred stainless steel flats are peeled off the ceramic surface, some ceramic fragments or grains are also pulled out and taken away. Gautier and Kato [9] studied the wear mechanism of silicon nitride in unlubricated sliding against steel, it is suggested that the wear of ceramics is mainly related to transgranular or intergranular fracture. Steel discs presented much more intricate wear patterns, including abrasion, oxidation and delamination. It is known that the surface topography has a significant influence on the friction behavior and the wear resistance. Surface texturing is an effective way for improving the tribological properties for many years. Its main effects are to trap wear debris, store lubricants, and increase the load carrying capacity [10–15]. In the past decades, the surface texturing has been used in many fields to improve the tribological performances of interfaces, including bearings [16,17], engine cylinder liners [18,19], cutting tools [20–22], and seal rings [23,24]. The dimples and grooves may be the effective patterns on the tribo-surfaces. Efforts have been made to compare the effect of dimples and grooves on tribological performances, and it also has been used on the surface of ceramics [25–29]. Despite the above researches, different researchers studied in different experimental settings showed the different results. Suh
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Y. Xing et al. / Materials and Design 52 (2013) 234–245 Table 1 Properties of Si3N4/TiC ceramic, AISI 440C steel. Material (wt%)
Density (g cm3)
Hardness (HV)
Young’s modulus (GPa)
Poisson’s ratio
Fracture toughness (MPa m1/2)
Bending strength (MPa)
Thermal expansion (106 mm1 °C1)
Thermal conductivity (Wm1 °C1)
Si3N4/TiC35% AISI 440C
3.51 7.62
2191 746
340 197
0.29 0.29
6.2 18.3
830 1900
3.0 10.2
23 24.2
Table 2 Geometric parameters of textured samples. Width (lm)
Distance (lm)
Darea of wave (%)
Darea of line (%)
50 50 50 50
100 200 300 400
70.7 35.36 23.57 17.68
50 25 16.67 12.5
et al. [30] reported that friction control can be achieved by fabricating the micro-grooved crosshatch pattern on a contact surface, and the crosshatch angle of groove texture seems to be an important parameter to design surface texture as well as groove aspect ratio. Yuan et al. [31] studied the orientation effects of micro-grooves on sliding surfaces, the results showed that the merits of perpendicular and parallel orientation may swap under different contact conditions. Zum Gahr et al. [32] investigated the friction coefficient and film thickness of 100Cr6/sapphire pairs, which showed that textures perpendicular to the sliding direction generated greater film thickness and lower friction coefficient than that of parallel orientation, which is contradicted to the experimental results obtained by Hata et al. [33]. Therefore, the friction performance may be quite sensitive to the geometric characteristics of the surface textures and the materials, and it needs to be further studied. This paper studies the effect of different grooves fabricated by laser surface texturing (LST) on a silicon nitride ceramic disc plate
mated with 440C stainless steel. The effect of textures on the tribological properties was investigated by carrying out dry sliding tests on a ball-on disk tribometer. Although the past results contribute a lot for us to understand the effect of textures, there are still many uncertainties during the design of textures patterns. The aim of this investigation was to study the friction and wear behavior of textured surfaces of Si3N4 ceramic sliding against the steel under dry condition, and most important, the effects of the geometrical characteristics of the micro-grooved textures. 2. Experimental details 2.1. Specimens and surface texturing The materials used for the tests were hot-pressed Si3N4/TiC ceramic and AISI440C stainless steel ball, the main component and mechanical properties of the samples were listed in Table 1. The Si3N4/TiC discs were 16 mm 16 mm 8 mm, and the surfaces were lapped and polished to the roughness less than Ra 0.02 lm. The AISI440C stainless steel balls have a diameter of 9.5 mm, and their surface roughness was about Ra 0.01 lm. The LST (laser surface texturing) was applied on the Si3N4/TiC discs with a wavelength of 1064 nm Nd:YAG laser, pulse of 10 ns duration, voltage of 19.5 V, frequency of 6000 Hz and scanning speed of 5 mm/s. The wavy and linear grooves were fabricated on the samples. Textures were produced with fixed distance rang-
Fig. 1. Optical micrographs of textured surfaces: (a) wavy groove, (b) linear groove, (c) a single groove, (d) schematic diagram of ST-WG sample, and (e) schematic diagram of ST-WG sample.
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Fig. 2. Surface topographies of textured samples detected by a white light interferometer: (a) ST-WG and (b) ST-LG.
Fig. 3. SEM microphotograph of a groove (a), surrounding the groove area and EDS (b).
ing from 100 lm to 400 lm, the width of the grooves was about 50 lm and the depth was about 50 lm. The geometric parameters of textures were listed in Table 2, in which the density (Darea) is the area ratio of textured surfaces with total surfaces.
In order to assess the effect of the textures, the smooth samples were used to test for comparison. The samples with smooth surfaces, wavy grooves and linear grooves are named SS, ST-WG and ST-LG, respectively. The morphology of textured samples was ob-
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1000
-Si3N4 -Si3N4 TiC TiO2 TiO
Lin (Cps)
800
600
400
200
0 10
30
50
70
2-Theta-Scale
Fig. 6. Comparison of average friction coefficient of smooth samples and textured samples with different distance.
Fig. 4. X-ray diffraction (XRD) analysis of the textured Si3N4/TiC surface.
Fig. 5. Comparison of friction coefficient of the smooth samples and textured samples with different distance.
served using an optical microscope, scanning electron microscopy (SEM) and white light interferometer. 2.2. Friction and wear test Reciprocating sliding friction tests according to ASTM G133-05 (2010) standard were carried out using a ball-on-disk tribometer (UMT-2, USA). The upper specimen was a stainless steel ball (AISI
440C), and the lower specimen was a Si3N4/TiC ceramic sample. The lower specimen was fixed, and the ball was reciprocating sliding against the lower specimen. Two parameters were varied to investigate the influence of the textures on the friction conditions: texture distance (texture density): 100, 200, 300 and 400 lm, and texture type: wavy groove and linear groove. All tests were carried out with a load of 45 N, sliding speed of 5 mm/s and sliding time of 30 min. Each parameter combination is repeated three times.
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Before the tests, all the samples were cleaned in ultrasonic bath by alcohol for 30 min and then dried. The worn surfaces of the samples were observed by an optical microscope, scanning electron microscopy (SEM) and white light interferometer (Wyko NT9300, USA). The friction coefficient can be obtained directly by the software on the UMT-2 tribometer. The worn volumes of the discs were calculated by surface worn profiles measured by a white light interferometer and the worn volumes of the balls were calculated by the spherical crown measured by an optical microscope. The wear rate is calculated as follows: Fig. 7. Variations of the wear rates of smooth samples and textured samples with different distance.
K¼
V FL
ð1Þ
where V is the worn volume (mm3), F is the normal load (N), and L is the sliding distance (m). 3. Results 3.1. Morphology of textured surface
Fig. 8. Variations of the wear rates of the balls sliding against smooth samples and textured samples with different distance.
Fig. 1 shows the surface topography and schematic diagram of textured samples and the stereoscopic profile of a single microgroove, which is observed by an optical microscope. It can be seen that two different texturing patterns are arranged on the surfaces evenly, the width of the micro-groove is about 50 lm and the depth is about 50 lm; the angle of the wavy pattern is 90° and the cross-sectional profile of the micro-groove is triangle. Fig. 2 shows topography of textured samples detected by a white light interferometer. SEM microphotograph of a single groove and surrounding the micro-groove is illustrated in Fig. 3. It can be seen that the heat-affected zone is formed around the groove due to the laser with high energy density, vapor blast ejections of melted ceramic and rapid condensation. The bulges around grooves are a little higher than
Fig. 9. Optical micrographs of worn surfaces of smooth and textured samples with distance of 200 lm after 30 min dry sliding friction: (a) SS, (b) ST-WG, and (c) ST-LG.
Fig. 10. SEM micrographs of worn surfaces of SS samples after 30 min dry sliding friction and EDX analysis of point A.
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Fig. 11. SEM micrographs of worn surfaces of ST-WG samples with the distance of 200 lm after 30 min dry sliding friction and EDX analysis of point A and point B.
Fig. 12. SEM micrographs of worn surfaces of ST-LG samples with the distance of 200 lm after 30 min dry sliding friction and EDX analysis of point A and point B.
the plan area, which increases the surface roughness of textured samples. The samples are consisted of three parties: substrate, interlayer and oxide layer. The outermost layer contains a higher content of O, and based on the results of EDS (Fig. 3) and XRD (Fig. 4), the TiO2 and TiO are formed on the surface. Thus, the effect of oxidation may be taken into account in the friction process.
3.2. Friction and wear properties The friction coefficient of smooth surfaces and two kinds of textured surfaces with different distance is shown in Fig. 5. It can be seen that the friction coefficient of smooth samples (SS) in all experimental conditions is initially relatively lower than that of
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Fig. 13. Three-dimensional surface topographies of the wear tracks on the (a) SS, (b) ST-WG, and (c) ST-LG samples after 30 min dry sliding friction detected by a white light interferometer.
textured surfaces (ST-WG and ST-LG) with the same distance. The friction coefficient of all samples increases and then tends to be stable waves with the time under all experimental conditions. Against smooth surfaces, the friction coefficient stabilizes at a value 0.6–0.7; while against textured surfaces, it stabilizes at 0.4– 0.6, and ST-WG samples have the lowest friction coefficient. It also can be seen that the textured samples reach a steady state with less time. The average friction coefficient in the stage of stable waves is calculated for comparison in Fig. 6. It is shown that both the textured samples (ST-WG and ST-LG) show lower values of average friction coefficient than smooth samples (SS), the average friction coefficient increases with the increase of texture distance. The average friction coefficient of ST-WG and ST-LG samples is reduced by 22.1% and 12.4% at the distance of 100 lm, respectively; while at the distance of 400 lm, the reduction of the average friction coefficient of the textured samples (ST-WG and ST-LG) is not obvious compared with smooth samples (SS), and it is 5.1% and 3.9% for ST-WG and ST-LG samples, respectively. The texture distance of 100 lm may be the best for friction reduction on all tests. The specific wear rates of the Si3N4/TiC ceramic samples are shown in Fig. 7. It can be seen that the wear rates of all textured samples (ST-WG and ST-LG) are lower than that of smooth samples (SS) under all conditions. At the same distance, ST-WG samples have the lowest wear rates. The wear rates of textured samples increase with increasing texture distance from 100 lm to 400 lm. The wear rates of balls sliding against smooth and textured samples with different distance are calculated and shown in Fig. 8. Apparently, the wear rates of all balls sliding against textured samples (ST-WG and ST-LG) are larger than that sliding against smooth samples (SS); the balls sliding against ST-WG samples show the highest wear rates at all tests. In addition, the wear
rates show a declining trend with increasing texture distance for textured samples compared with smooth samples. Fig. 9 shows the optical micrographs of worn surfaces of smooth and textured samples with distance of 200 lm after 30 min dry friction. From the optical micrographs, it is observed that wear scars formed by mechanical rubbing can be seen clearly, and in some case, the wear scars are different. After tests the microgrooves are still present, and the grooves are filled with a lot of wear debris. SEM micrographs of worn surfaces of three different samples (SS, ST-WG and ST-LG) after 30 min sliding friction are presented in the Figs. 10–12. Obviously, severe surface damage can be seen of SS sample in the form of many ploughs and adhesions on the worn surface (Fig. 10a and b). The EDX analysis of point A confirms the presence of many adhesions of Fe, Fig. 10c. However, the worn on the textured surfaces is milder compared with that of SS samples surfaces, and few ploughs and adhesions can be seen on the surfaces among textures, see Figs. 11 and 12. It also can be seen that SEM micrographs of worn surfaces and results of EDX of point A (inside the grooves) of all the textured surfaces exposed that the grooves are filled with wear particles. Comparison of EDX analysis of point A and point B, the amount of Fe of point B is lower than that of point A, see Figs. 11 and 12. The three-dimensional topographies of the wear tracks on three kinds of samples after 30 min dry friction are shown in Fig. 13. It is shown that the adhered materials for smooth samples (Fig. 13a) are larger than that of textured samples (Fig. 13b and c); the textures (Fig. 13b and c) still exist after the friction process. To quantify the wear of the samples, two-dimensional topographies and wear profiles fitted by B-spline curves of the wear tracks of three kinds of samples after cleaned in an ultrasonic bath by alcohol are detected by a white light interferometer (see Fig. 14).
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The worn surfaces of the balls sliding against three kinds of samples after 30 min dry sliding friction are shown in Fig. 15. Several conclusions can be obtained from the worn surfaces: the wear scars on balls sliding against textured samples (Fig. 15a and b) are larger than that of the ball sliding against smooth surfaces (Fig. 15c); the enlarged SEM micrographs of the ball-worn surfaces (Fig. 15a0 –c0 ) indicated that wear occurred in all cases by an abrasive mechanism. However, pronounced abrasive grooves visible as scratches were obvious in balls sliding against the smooth samples (Fig. 15c0 ) than that of the balls sliding against textured samples (Fig. 15a0 and b0 ). 3.3. Effect of surface texturing on the stress distribution In the process of sliding friction, the stress is an important reason to cause wear of the surface, and surface texturing will influ-
241
ence the stress and its distribution. The finite element method (FEM) is an effective way to estimate the stress and characterize its distribution. Fig. 16 shows the FEM gridding model of three different samples. To obtain more accurate simulation results, the upper surfaces of the textured samples are refined mesh. The mechanical properties of the sample materials are listed in Table 1. In order to reduce the computing time in the analysis and simplify the FEM model, the upper ball specimen surface is simplified to a cylindrical pin and the tests were studied by the finite element analysis after a cycle of reciprocating motion. A pressure of 572.9 MPa is applied on the upper surfaces of pins according to calculation of the Hertz contact theory. The results of the von Mises stress distribution in the SS, ST-WG, and ST-LG samples are shown in Fig. 17. It can be seen that the large stress concentration occurs on SS sample surfaces
Fig. 14. Two-dimensional surface topographies and profiles of the wear tracks on the samples after 30 min dry sliding friction detected by a white light interferometer: (a) and (a0 ) SS, (b) and (b0 ) ST-WG, and (c) and (c0 ) ST-LG.
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Fig. 15. SEM micrographs of ball-worn surfaces sliding against different samples: (a) ball-worn surface sliding against ST-WG with the distance of 200 lm, (a0 ) enlarged SEM micrograph corresponding to (a), (b) ball-worn surface sliding against ST-LG with the distance of 200 lm, (b0 ) enlarged SEM micrograph corresponding to (b), (c) ball-worn surface sliding against SS, and (c0 ) enlarged SEM micrograph corresponding to (c).
Fig. 16. FEM gridding model of samples: (a) SS, (b) ST-WG with the distance of 200 lm, and (c) ST-LG with the distance of 200 lm.
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Fig. 17. The von Mises stress distribution of the samples surfaces: (a) SS, (b) ST-WG, (c) ST-LG.
Groove
(a)
(b)
(c)
440 C steel ball
Wear debris
Si3 N 4 / TiC ceramic
Fig. 18. Friction and wear reduction mechanism of textured surfaces.
(Fig. 17a), while large stress only occurs at the edges of the textures of ST-WG and ST-LG samples surfaces (Fig. 17b and c). In comparison with SS sample (Fig. 17a), the stress distribution of ST-WG and ST-LG samples is more uniform, but the stress magnitude is a litter larger. For SS sample, the maximum von Mises stress is 887.2 MPa, while it is 1263 MPa and 957.4 MPa for ST-WG and ST-LG samples. 4. Discussion In this paper, the effect of the grooved textures on the Si3N4/TiC ceramic under dry condition is studied. The results show that the textured surface can improve the tribological performance of the Si3N4/TiC ceramic disc under dry friction. The reduction of the contact areas between the balls and the textured samples and the effect of capturing debris of the grooves may be responsible for the reduction of the friction coefficient and wear rate of the textured samples [34]. While the initial friction coefficient of the textured surface is higher than that of the smooth surface, which is due to the large surface roughness of the textured surface increased by the bulges formed by melting and vaporization, and the textured surface can reduce the run-in time, see Fig. 5. The distance and pat-
tern of the textures have an important influence on tribological behavior of the Si3N4/TiC samples and the 440C steel balls (Figs. 5–8). A smaller distance between the grooves should facilitate both the friction coefficient and wear rate of the Si3N4/TiC samples. It is due to the larger texture density corresponding to the smaller texture distance (see Table 2) enable to reduce the larger contact area, collect more debris and thus has a lower friction coefficient and wear rate. The schematic diagram of texture effect mechanism is shown in Fig. 18. It can be explained that when the movement begins between the couples, cracks occurs on the surfaces of the disc and ball (Fig. 18a); proceed with the friction, materials are peeled off the couples (Fig. 18b), then the grooves can capture the debris effectively (Fig. 18c), which can be seen from the SEM of the worn textured sample surfaces (see Figs. 11 and 12). Thus, when wear particles escape from the contact zone into the grooves, the friction and wear are decreased, which is in line with the literatures [35– 37]. The results of the friction coefficient show a high dependence on the geometry of the textured surfaces. Lowest values of the friction coefficient are obtained by ST-WG samples and the highest values are obtained by the SS samples under all experimental con-
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Fig. 19. Schematic of sliding area, length and force with different textured samples.
ditions. Friction property is strongly influenced by sliding length, area of the ball in the textures and the retarding force of the bulges of the textures [30,38,39]. The sliding area (A) and length (l) moving on the textured surface parallel to the sliding direction were shown in Fig. 19. In the view point of geometrical parameters in grooved pattern (Fig. 19a and b), it was already confirmed that friction coefficient is associated with the width (w) and angle (h), so the groove sliding area ‘‘A’’ can be calculated as it is shown in Fig. 19a0 and b0 by red color, which shows low friction performance according to the sliding area. It can be assumed that friction coefficient is a function of the parameter A [38]. For wavy textures, h is 45°, while it is 90° for linear textures. The sliding area (A) moving on the textured surface for the wavy and linear textures can be recorded as A1 and A2. It can be calculated that A1 is larger than A2. Finally, the increasing of the sliding area and the length moving on the textured surface enhanced the effect of lubricating and capturing debris (Fig. 19a0 and b0 ). The main retarding force FR, which is perpendicular to direction of the grooved edges, is shown in Fig. 19a00 and b00 , it is obvious that FR1 is smaller than that of FR2. Thus, the ST-WG had the least friction coefficient and the wear rate (Figs. 5–7). As we all know that the excessive stress is an important reason to cause wear, The results of the simulation (Fig. 17a–c) show that the textures can prevent excessive stress concentration in the contact zone and the stress distribution is uniform after a cycle movement, which reduces the friction and wear of the samples. On the other hand, EDS and XRD of the textured surface indicated the oxidation after laser surface texturing, and TiO2 produced on the textured samples may act as a lubricant, Figs. 3 and 4, which contributes to the improvement of tribological performance [40]. SEM micrographs of the worn surfaces for SS sample exhibit a severe surface damage, the wear mechanism is mainly ploughs and adhesions (Fig. 10). It can be seen that a lot of wear debris filled in the micro-grooves, and few ploughs and adhesions can be seen on the surface among textures (Figs. 11 and 12), which is
the effect of capturing debris of textures. Thus, the wear of the textured surface is reduced effectively. Looking to the worn topographies of smooth surface detected by a white light interferometer we note that a spread of debris covers the whole wear track (Fig. 13a). However, it shows few wear debris on surfaces among grooves of textured samples (Fig. 13b and c), which is consistent with that shown in Figs. 10–12. This indicates that wear debris move from the contact region into grooves. The surface worn topographies and the wear profiles of samples after being cleaned in ultrasonic bath by alcohol and then dried are detected by a white light interferometer (Fig. 14); the wear depths of textured samples (ST-WG and ST-LG) are less than that of smooth samples (SS), which confirms the reduction of the wear of the textured samples. Note that the wear scars on balls sliding against textured discs are larger than those sliding against smooth surfaces, Fig. 15, which is consistent with the literature [41], the wear of balls is dependent on the roughness of the contact samples and the textured samples have a larger roughness (rougher samples produced more wear on the ball); on the other hand, the sharp edges around the grooved have a micro-cutting effect, which would cut the contact ball surface and lead to a higher wear of the balls [12]. Thus, surface texturing could be beneficial if accelerated wear on the counter-face is acceptable, such as cutting tools [42]. 5. Conclusions Laser surface texturing (LST) is a new and interesting method for the surface finishing of ceramics, its possibilities complement the variety of conventional finishing methods. Two different patterned micro-grooves were made on the surface of the Si3N4/TiC ceramic, reciprocating dry sliding wear tests were carried out with the samples (SS, ST-WG and ST-LG) sliding against 440C steel ball under dry condition in this research. The main conclusions from this study are summarized in the following:
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(1) Surface texturing has a strong effect on the friction and wear performance of sliding surfaces and the friction coefficient of the textured ceramics was quite sensitive to the geometrical characteristics of the surface texturing. In this research, the ST-WG and ST-LG samples have lower friction coefficient and wear rates than that of SS samples, and the ST-WG samples have the best effect on friction and wear reduction. The large texture density (small texture distance) may be the best for reduction of friction and wear of textured samples. The wear form of Si3N4 ceramics in sliding pairs is mainly caused by the adhesions and ploughs between the rubbing surfaces. The adhesions and ploughs on the worn tracks of the ST-WG and ST-LG samples are milder compared with that of the SS samples. (2) The wear scars on balls sliding against textured samples are larger than those sliding against smooth surfaces. SEM micrographs of the ball-worn surfaces indicate that wear occurs in all cases by an abrasive mechanism, while, pronounced abrasive grooves visible as scratches are obvious in balls sliding against the smooth samples than that of the balls sliding against textured samples, the effect of capturing debris by textures may be responsible for it. Based on the studies, LST could be beneficial in terms of friction reduction if accelerated wear on the counter-face is acceptable. (3) Finite element analysis results show that surface texturing can improve the stress distribution of contact interfaces. In comparison with the SS sample, ST-WG and ST-LG samples reduce the stress concentration of the surface, and the stress distribution of ST-WG and ST-LG samples is more uniform. Thus, less stress concentration reduces the wear on the surface of textured samples.
Acknowledgments This work is supported by ‘‘The National Natural Science Foundation of China (51075237)’’, ‘‘The Taishan Scholar Program of Shandong Province’’, ‘‘The Natural Science Foundation of Shandong Province (ZR2010EZ002)’’, ‘‘The Independent Innovation Foundation of Shandong University (2011JC001)’’, and ‘‘The Specialized Research Fund for Doctoral Program of Higher Education (20110131130002)’’. References [1] Zhao XZ, Liu JJ, Zhu BL, Miao HZ, Luo ZB. Wear simulation of Si3N4 cutting tool material on a Pin-On-Disc tester. Ceram Int 1997;23:483–8. [2] Souzaa JVC, Nonoa MCA, Ribeirob MV, Machadoa JPB, Silvac OMM. Cutting forces in turning of gray cast iron using silicon nitride based cutting tool. Mater Des 2009;30:2715–20. [3] Kitamura K, Takebayashi H, Ikeda M, Percoulis H, Development of Ceramic Cam Roller Follower for Engine Application. SAE Tech. Paper No. 972774; 1997. [4] Walzer P, Heinrich H, Langer M. Ceramic components in passenger-car diesel engines. Mater Des 1986;7:75–80. [5] Katz RN. Ceramics for high performance rolling element bearings: a review and assessment. Mater Des 1987;8:108–12. [6] Wang L, Snidle RW, Gu L. Rolling contact silicon nitride bearing technology: a review of recent research. Wear 2000;246:159–73. [7] Akdogan G, Stolarski TA. Wear in metal/silicon nitride sliding pairs. Ceram Int 2003;29:435–46. [8] Zhao XZ, Liu JJ, Zhu BL, Miao HZ, Luo BZ. Wear behavior of Si3N4 ceramic cutting tool material against stainless steel in dry and water-lubricated conditions. Ceram Int 1999;25:309–15. [9] Gautier P, Kato K. Wear mechanisms of silicon nitride, partially stabilized zirconia and alumina in unlubricated sliding against steel. Wear 1993;162– 164:305–13.
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