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Ultra-low friction coefficient in alumina–silicon nitride pair lubricated with water
Wear 296 (2012) 656–659
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Ultra-low friction coefficient in alumina–silicon nitride pair lubricated with water Vanderlei Ferreira a, Humberto Naoyuki Yoshimura b, Amilton Sinatora a,n a b
~ Paulo, Av. Prof. Mello Moraes, 2231, Sa~ o Paulo, SP 05508-970 , Brazil Surface Phenomena Laboratory, Department of Mechanical Engineering, Polytechnic School, University of Sao ´, SP 09210-170, Brazil Center for Engineering, Modeling and Applied Social Sciences, Universidade Federal do ABC, Rua Santa Ade´lia, 166, Santo Andre
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 February 2012 Received in revised form 6 July 2012 Accepted 25 July 2012 Available online 2 August 2012
In a ball-on-disc wear test, an alumina ceramic body sliding against a silicon nitride ceramic body in water achieved an ultra-low friction coefficient (ULFC) of 0.004. The profilometer and EDX measurements indicated that the ULFC regime in this unmated Al2O3–Si3N4 pair was achieved because of the formation of a flat and smooth interface of nanometric roughness, which favored the hydrodynamic lubrication. The triboreactions formed silicon and aluminum hydroxides which contributed to decrease roughness and shear stress at the contact interface. This behavior enables the development of low energy loss water-based tribological systems using oxide ceramics. & 2012 Elsevier B.V. All rights reserved.
Keywords: Sliding friction Surface topography Engineering ceramics Profilometry Surface analysis
1. Introduction Environmental pollution and shortage of resources have raised the number of government regulations requiring more fuel-efficient vehicles. These vehicles will demand better motor thermal efficiency and low-friction tribological components. Ceramic waterlubricated tribological systems are being improved due to accelerated technological development and are candidate to replace metal oil-lubricated ones. Friction coefficient values in the range of 0.010–0.008 and wear rates of 1 10 11 (mm3 N 1 mm 1) have been referred as ultra-low friction coefficient and ultra-low wear [1–3]. Sliding tests performed with self-mated ceramic pairs of SiC or Si3N4 in water have led to friction values in this range [4,5]. Self-mated SiC or Si3N4 sliding pairs have been studied to elucidate the mechanism responsible for the ultra-low friction coefficient (ULFC) in water. The mechanisms commonly associated with this behavior include: hydrodynamic lubrication by a thin film of water at the interface [4]; boundary lubrication by a silica film formed on the surfaces [6]; and mixed lubrication involving the first two mechanisms could represent a contribution to the triboreactions [5,7]. In order to deepen understanding of the mechanisms responsible for the ULFC values, tests were performed to evaluate the effects of: water temperature [5],
n
Corresponding author. Tel./fax: þ 55 11 3091 9855. E-mail address: [email protected] (A. Sinatora).
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.07.030
nature of the mated pair (SiC–SiC or Si3N4–Si3N4) [8], initial roughness [8,9], testing load [10] and speed [11,12]. As opposed to the ULFC regime observed in silicon-containing covalent ceramics, friction studies on oxide ceramics in water show distinct tribological behavior. In the case of alumina, steady-state friction values of about 0.25 have been reported [6,12,13]. These values are one order of magnitude greater than the friction coefficient values for SiC–SiC or Si3N4–Si3N4 pairs referred above. Adopting some approach in order to induce the occurrence of ultra-low friction coefficient regime in oxide ceramics can have great technological impact, since these ceramics are usually less expensive and more generally applied than covalent ceramics. In this work, for the first time it is reported that an alumina ceramic sliding against a silicon nitride ceramic in water can achieve the ULFC regime. The possible mechanisms related to the achievement of friction coefficient as low as 0.004 in this unmatched Al2O3–Si3N4 pair are discussed. 2. Material and methods A high purity alumina powder doped with 750 ppm MgO (AKS-3030A, Sumitomo Chemical Co., Japan) was uniaxially pressed at 45 MPa to form discs using a cylindrical steel die (diameter of 64 mm). The discs were then pressed at 200 MPa in a cold isostatic press and afterward sintered at 1650 1C for 1 h in air using an electric furnace.
V. Ferreira et al. / Wear 296 (2012) 656–659
The flat surfaces of the sintered discs were machined and then a hole was drilled in the center of each disc in order to fix it in the wear test equipment. Before the wear test, the flat surfaces were lapped using copper lapping discs and a 15 mm diamond suspension, resulting in Al2O3 discs of 51 mm of diameter and 6.5 mm of thickness. A contact profilometer (Kosaka, Surfcorder—1700a) was used to determine the RMS roughness, resulting in a value of 350 nm. Lubricated sliding wear tests were conducted using a ball-ondisc configuration in the Plint TE67 equipment. Commercial Si3N4 bearing balls of 11.11 mm of diameter were used in the tests. During the test, the discs were rotated at a wear track tangential speed of 1.0 m/s (562 rpm). The Si3N4 ball was pressed against the Al2O3 disc using a dead weight system at a normal load of 53.17 70.01 N, which resulted in a mean Hertzian pressure of 900 MPa. The tests were performed at room temperature (2371 1C) with a flux of distilled and deionized water (18.2 MO cm, Millipore Milli-Q System) covering the ball and disc contact interface. In order to ensure that the contact interface was continuously submersed, the water was poured on the central area of the rotating disc, so that the centrifugal force would spread the water to the area of contact. One liter of water was recirculated in a closed circuit with a peristaltic pump. The friction force was monitored with a cell load of 0.10 N of accuracy at a frequency of 1 Hz. When the measurements indicated the beginning of the UFLC regime, the mean friction force and friction coefficient were monitored and measured for 10 min to ensure the reliability of the values. Five tests were carried out to confirm the reproducibility of the results. The specimens were cleaned in an ultrasound bath during 20 min in acetone before and after the wear tests. This procedure was followed after the tests in order to remove from the surface loosely adherent wear debris which could have altered the roughness profiles. The roughness profiles were measured before and after the tests without filtering. The worn surfaces of tested ball and disc were analyzed by optical microscopy and X-ray fluorescence spectrometry, EDX (Shimadzu, EDX-720) with a Na–Sc detector. The measured properties of Al2O3 disc and Si3N4 ball were, respectively: 3.96 and 3.26 Mg m 3 for density; 16.3 and 14.6 GPa for Vickers hardness (HVN10-98N); 403 and 316 GPa for Young’s modulus; 3.5 and 5.9 MPa m1/2 for fracture toughness,
657
KIc The microstructure of Al2O3 ceramic had few residual pores of size smaller than 1 mm usually at the grain-boundaries, a-Al2O3 grains of near-isometric shape and average grain size of 4 mm. The microstructure of Si3N4 ceramic had elongated b-Si3N4 grains with hexagonal cross section and bimodal size distribution (small grains of about 0.5 mm and fewer and larger grains of up to 3 mm of length) and an intergranular phase typical of liquid-phase sintered materials.
3. Results Fig. 1 shows a typical curve of measured friction coefficient. The curves of all five experiments exhibited the same behavior. In the first period, the friction coefficient increased rapidly up to 0.4–0.5. Then, it was followed by a transient period during which there was high friction coefficient variation: both the amplitude of peaks and the mean value of friction coefficient decreased with the increase in testing time. These two high friction regimes are called running-in period. After approximately 5373 min, the friction coefficient reached 0.00470.002, the UFLC regime value, and then remained stable for at least 10 min, after which the test was interrupted. Two ULFC results achieved 0.004, but most of them achieved 0.002, which is the resolution limit of the test. During the test, a wear track was produced on the alumina disc (Fig. 2a) and a regular circular plateau was formed on the silicon nitride ball (Fig. 2b). Considering that the wear in ULFC regime is negligible due to its very low friction coefficient, it can be inferred that both wear track and plateau were formed during the running-in period. The diameter of this plateau was used to calculate the average apparent pressure, resulting in 10 MPa. The determined values for volumetric loss were of 0.4270.08 mm3 in the sphere and approximately 0.08770.002 mm3 in the wear track on alumina disc. The roughness profile obtained from the cross-section of the worn track on alumina disc is shown in Fig. 3a. Outside the wear track the RMS roughness value was 350 nm and the topography displayed valleys of more than 1.0 mm in depth. The wear track in turn exhibited a quite different feature: the roughness summits were worn away, and the valleys seemed to have been fulfilled, probably by the wear debris, as shown in Fig. 3b. As a consequence,
Fig. 1. Typical behavior of the friction coefficient during the Si3N4 sphere on Al2O3 disc sliding test. In detail the last minutes of the test during which the friction coefficient of 0.002 was reached.
658
V. Ferreira et al. / Wear 296 (2012) 656–659
Fig. 2. Wear track on alumina disc (a), and wear plateau on silicon nitride ball (b).
Fig. 4. Energy dispersive X-ray fluorescence spectra from the alumina surface: (a) outside; and (b) inside the wear track (notice the presence of the silicon peak).
4. Discussion
Fig. 3. Topographies of: (a) the alumina disc profile outside and across the wear track (notice the disappearance of valleys under the wear track); (b) wear track on alumina disc at higher magnification; (c) plateau on silicon nitride ball.
the RMS roughness decreased to values in the range of 20–46 nm, one order of magnitude lower than the initial value. The silicon nitride plateau showed a very regular surface with RMS roughness values in the range of 2–8 nm (Fig. 3c). The EDX spectra obtained from the alumina surfaces before and after the wear test are shown in Fig. 4. A silicon peak is observed in the wear track (Fig. 4b), indicating that the material from silicon nitride ball was transferred to the alumina worn surface.
The results of the wear test showed that the ultra-low friction coefficient (ULFC) regime in the unmatched Al2O3–Si3N4 pair in water is achieved after a given sliding time when surpassing the running-in period. The tests also indicated that it is possible to reach a friction coefficient as low as 0.004 (Fig. 1). Apparently, there are no works in the literature which investigated the friction coefficient behavior of silicon nitride sliding against alumina. Likewise, there are no works reporting the ULFC values for oxide ceramics. Friction coefficients of this magnitude during under water ceramic sliding have only been reported for matched sliding pairs of silicon nitride [4,5,9] and silicon carbide [11,12]. Moreover, the Al2O3–Si3N4 pair exhibited a very different friction behavior from the alumina–alumina pair sliding in water [6,12,13]. The Al2O3–Al2O3 pairs do not display great oscillations in the friction coefficient magnitude during the experiment, and throughout the tests the friction coefficient does not decrease below 0.2, a value far from the ULFC of 0.004 reported in this work. Among the sliding tests performed with self-mated ceramic pairs of Al2O3–Al2O3 and Si3N4–Si3N4 in the same conditions of load, velocity and water lubrication of this study only the Si3N4– Si3N4 pairs achieved the ULFC regime [12]. Previous works [5,8,9,12] reported that the ULFC in Si3N4– Si3N4 or SiC–SiC sliding pairs in water is preceded by the formation of a hydrated silica layer on the surface of the ceramics. Sugita and co-workers [14] studied the relationship between
V. Ferreira et al. / Wear 296 (2012) 656–659
[28SiHþ]/[28Si þ] and [28SiOþ ]/[28Siþ] in the silica layer on silicon nitride surface. They measured an increase in the concentration of [28SiHþ ] and [28SiO þ] and suggested that at first Si is oxidized and afterwards hydrated, which could indicate that some degree of wear would be necessary to reach the ULFC regime. It was later suggested that the formation of silicon oxide [5] and its hydration can occur by the following reactions: Si3N4 þ6H2O23SiO2 þ 4NH3
(1)
SiO2 þ2H2O2Si(OH)4
(2)
The wear events yielded a flat surface on the Si3N4 ball (Fig. 2b), resulting in an apparent contact pressure of 10 MPa and a flat surface roughness ranging from 2 to 8 nm RMS (Fig. 3c), both similar to the measured values in Si3N4–Si3N4 sliding pairs [5,9]. The formation of a smooth flat surface on the sphere seems to be necessary so that the water lubricant film can work properly. The flat surface reduces the mean contact pressure whereas the low roughness reduces the water film thickness to the necessary minimum to activate the hydrodynamic lubrication. In the wear track on the counter-body (alumina disc, Fig. 2a), the decrease of the roughness value from 350 to 30 nm (Fig. 3b) also contributed to the decrease of the friction coefficient. According to the H2O–Al2O3 phase diagram from Wefers [15], an aluminum hydroxide (Al(OH)3) layer may be formed on the wear track under 10 MPa pressure. Since the aluminum hydroxide has lubricant characteristics [16], the in situ formation of this compound may have reduced the shear stresses at the contact interface. A similar argument was considered by Presser and co-workers [17] to explain the hydration of silicon carbide under water lubrication. The EDX spectrum (Fig. 4b) showed that the wear track on the alumina disc had been partly covered by a silicon-based compound. Considering Sugita and co-workers’ findings [14] and reactions (1) and (2), it is possible to infer that hydrated silicon oxide was formed on the alumina wear track. Both silicon and aluminum hydroxides debris have possibly filled the starting roughness valleys on the surface of alumina disc reducing its depth and resulting, along with the wear of alumina asperity summits, in a flat wear track as shown in Fig. 3a. This investigation showed that the friction coefficient of the Al2O3–Si3N4 pair was reduced from 0.4–0.5 at the beginning of the test to 0.004 at the steady-state regime. This ultra-low friction coefficient was achieved possibly because a flat smooth interface was formed, covering the alumina disc with a soft layer of aluminum hydroxide, and both body (Si3N4 ball) and counterbody (Al2O3 disc) with a hydrated silicon oxide layer. The measured ULFC value is in the same range as the one obtained in Si3N4–Si3N4 or SiC–SiC pairs which have been studied since the 1980s. Further investigation is needed to improve the friction mechanisms in aqueous medium in order to design new tribological systems with oxide ceramics.
659
5. Conclusions Ultra-low friction coefficient (ULFC) was achieved in an unmated Al2O3–Si3N4 sliding pair under water. This result showed that the ULFC is not an exclusive phenomenon of Si3N4–Si3N4 and SiC–SiC mated pairs. Since the friction coefficient between Al2O3 and Si3N4 is in the same range of the mated Si3N4 and SiC pairs, the silicon nitride is considered to have a key role in the development of ULFC. The tribolayer of aluminum hydroxide and hydrated silicon oxide formed in situ contributes to reach the ULFC. The results pave the way to the development of very low friction loss mechanical systems running under water with oxide ceramics. References [1] J.M. Martin, C. Donnet, T. Le Mogne, T. Epicier, Superlubricity of molybdenum disulphide, Physical Review B 48 (14) (1993) 10583–10586. [2] S. Myhra, J.W.B. Summers, E.H. Kisi, Ti3SiC2—a layered ceramic exhibiting ultra-low friction, Materials Letters 39 (1) (1999) 6–11. [3] M. Chhowalla, G.A.J. Amaratunga, Thin films of fullerene-like MoS2 nanoparticles with ultra-low friction and wear, Nature 407 (6801) (2000) 164–167. [4] H. Tomizawa, T.E. Fischer, Friction and wear of silicon nitride and silicon carbide in water: hydrodynamic lubrication at low sliding speed obtained by tribochemical wear, Tribology Transactions 30 (1) (1987) 41–46. [5] J. Xu, K. Kato, Formation of tribochemical layer of ceramics sliding in water and its role for low friction, Wear 245 (1–2) (2000) 61–75. [6] P. Andersson, Water-lubricated pin-on-disc tests with ceramics, Wear 154 (1) (1992) 37–47. [7] R.S. Gates, S.M. Hsu, Tribochemistry between water and Si3N4 and SiC: induction time analysis, Tribology Letters 17 (3) (2004) 399–407. [8] M. Chen, K. Kato, K. Adachi, Friction and wear of self-mated SiC and Si3N4 sliding in water, Wear 250 (1–12) (2001) 246–255. [9] S. Jahanmir, Y. Ozmen, L.K. Ives, Water lubrication of silicon nitride in sliding, Tribology Letters 17 (3) (2004) 409–417. [10] L. Jordi, C. Iliev, T.E. Fischer, Lubrication of silicon nitride and silicon carbide by water: running in, wear and operation of sliding bearings, Tribology Letters 17 (3) (2004) 367–376. [11] M. Chen, K. Kato, K. Adachi, The comparisons of sliding speed and normal load effect on friction coefficients of self-mated Si3N4 and SiC under water lubrication, Tribology International 35 (3) (2002) 129–135. [12] D.A. Rani, Y. Yoshizawa, H. Hyuga, K. Hirao, Y. Yamauchi, Tribological behavior of ceramic materials (Si3N4, SiC and Al2O3) in aqueous medium, Journal of the European Ceramic Society 24 (10–11) (2004) 3279–3284. [13] M. Kalin, S. Jahanmir, G. Drazˇicˇ, Wear mechanisms of glass-infiltrated alumina sliding against alumina in water, Journal of the American Ceramic Society 88 (2) (2005) 346–352. [14] T. Sugita, K. Ueda, Y. Kanemura, Material removal mechanism of silicon nitride during rubbing in water, Wear 97 (1) (1984) 1–8. [15] K. Wefers, Nomenclature, preparation, and properties of aluminum oxides, oxide hydroxides, and trihydroxides, in: L.D. Hart (Ed.), Alumina Chemicals: Science and Technology Handbook, American Ceramic Society, Westerville, OH, 1990, pp. 13–22. [16] T. Nagaoka, C. Duran, T. Isobe, Y. Hotta, K. Watari, Hydraulic alumina binder for extrusion of alumina ceramics, Journal of the American Ceramic Society 90 (12) (2007) 3998–4001. [17] V. Presser, O. Krummhauer, K.G. Nickel, A. Kailer, C. Berthold, C. Raisch, Tribological and hydrothermal behaviour of silicon carbide under water lubrication, Wear 266 (7–8) (2009) 771–781.
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Ultra-low friction coefficient in alumina–silicon nitride pair lubricated with water Vanderlei Ferreira a, Humberto Naoyuki Yoshimura b, Amilton Sinatora a,n a b
~ Paulo, Av. Prof. Mello Moraes, 2231, Sa~ o Paulo, SP 05508-970 , Brazil Surface Phenomena Laboratory, Department of Mechanical Engineering, Polytechnic School, University of Sao ´, SP 09210-170, Brazil Center for Engineering, Modeling and Applied Social Sciences, Universidade Federal do ABC, Rua Santa Ade´lia, 166, Santo Andre
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 February 2012 Received in revised form 6 July 2012 Accepted 25 July 2012 Available online 2 August 2012
In a ball-on-disc wear test, an alumina ceramic body sliding against a silicon nitride ceramic body in water achieved an ultra-low friction coefficient (ULFC) of 0.004. The profilometer and EDX measurements indicated that the ULFC regime in this unmated Al2O3–Si3N4 pair was achieved because of the formation of a flat and smooth interface of nanometric roughness, which favored the hydrodynamic lubrication. The triboreactions formed silicon and aluminum hydroxides which contributed to decrease roughness and shear stress at the contact interface. This behavior enables the development of low energy loss water-based tribological systems using oxide ceramics. & 2012 Elsevier B.V. All rights reserved.
Keywords: Sliding friction Surface topography Engineering ceramics Profilometry Surface analysis
1. Introduction Environmental pollution and shortage of resources have raised the number of government regulations requiring more fuel-efficient vehicles. These vehicles will demand better motor thermal efficiency and low-friction tribological components. Ceramic waterlubricated tribological systems are being improved due to accelerated technological development and are candidate to replace metal oil-lubricated ones. Friction coefficient values in the range of 0.010–0.008 and wear rates of 1 10 11 (mm3 N 1 mm 1) have been referred as ultra-low friction coefficient and ultra-low wear [1–3]. Sliding tests performed with self-mated ceramic pairs of SiC or Si3N4 in water have led to friction values in this range [4,5]. Self-mated SiC or Si3N4 sliding pairs have been studied to elucidate the mechanism responsible for the ultra-low friction coefficient (ULFC) in water. The mechanisms commonly associated with this behavior include: hydrodynamic lubrication by a thin film of water at the interface [4]; boundary lubrication by a silica film formed on the surfaces [6]; and mixed lubrication involving the first two mechanisms could represent a contribution to the triboreactions [5,7]. In order to deepen understanding of the mechanisms responsible for the ULFC values, tests were performed to evaluate the effects of: water temperature [5],
n
Corresponding author. Tel./fax: þ 55 11 3091 9855. E-mail address: [email protected] (A. Sinatora).
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.07.030
nature of the mated pair (SiC–SiC or Si3N4–Si3N4) [8], initial roughness [8,9], testing load [10] and speed [11,12]. As opposed to the ULFC regime observed in silicon-containing covalent ceramics, friction studies on oxide ceramics in water show distinct tribological behavior. In the case of alumina, steady-state friction values of about 0.25 have been reported [6,12,13]. These values are one order of magnitude greater than the friction coefficient values for SiC–SiC or Si3N4–Si3N4 pairs referred above. Adopting some approach in order to induce the occurrence of ultra-low friction coefficient regime in oxide ceramics can have great technological impact, since these ceramics are usually less expensive and more generally applied than covalent ceramics. In this work, for the first time it is reported that an alumina ceramic sliding against a silicon nitride ceramic in water can achieve the ULFC regime. The possible mechanisms related to the achievement of friction coefficient as low as 0.004 in this unmatched Al2O3–Si3N4 pair are discussed. 2. Material and methods A high purity alumina powder doped with 750 ppm MgO (AKS-3030A, Sumitomo Chemical Co., Japan) was uniaxially pressed at 45 MPa to form discs using a cylindrical steel die (diameter of 64 mm). The discs were then pressed at 200 MPa in a cold isostatic press and afterward sintered at 1650 1C for 1 h in air using an electric furnace.
V. Ferreira et al. / Wear 296 (2012) 656–659
The flat surfaces of the sintered discs were machined and then a hole was drilled in the center of each disc in order to fix it in the wear test equipment. Before the wear test, the flat surfaces were lapped using copper lapping discs and a 15 mm diamond suspension, resulting in Al2O3 discs of 51 mm of diameter and 6.5 mm of thickness. A contact profilometer (Kosaka, Surfcorder—1700a) was used to determine the RMS roughness, resulting in a value of 350 nm. Lubricated sliding wear tests were conducted using a ball-ondisc configuration in the Plint TE67 equipment. Commercial Si3N4 bearing balls of 11.11 mm of diameter were used in the tests. During the test, the discs were rotated at a wear track tangential speed of 1.0 m/s (562 rpm). The Si3N4 ball was pressed against the Al2O3 disc using a dead weight system at a normal load of 53.17 70.01 N, which resulted in a mean Hertzian pressure of 900 MPa. The tests were performed at room temperature (2371 1C) with a flux of distilled and deionized water (18.2 MO cm, Millipore Milli-Q System) covering the ball and disc contact interface. In order to ensure that the contact interface was continuously submersed, the water was poured on the central area of the rotating disc, so that the centrifugal force would spread the water to the area of contact. One liter of water was recirculated in a closed circuit with a peristaltic pump. The friction force was monitored with a cell load of 0.10 N of accuracy at a frequency of 1 Hz. When the measurements indicated the beginning of the UFLC regime, the mean friction force and friction coefficient were monitored and measured for 10 min to ensure the reliability of the values. Five tests were carried out to confirm the reproducibility of the results. The specimens were cleaned in an ultrasound bath during 20 min in acetone before and after the wear tests. This procedure was followed after the tests in order to remove from the surface loosely adherent wear debris which could have altered the roughness profiles. The roughness profiles were measured before and after the tests without filtering. The worn surfaces of tested ball and disc were analyzed by optical microscopy and X-ray fluorescence spectrometry, EDX (Shimadzu, EDX-720) with a Na–Sc detector. The measured properties of Al2O3 disc and Si3N4 ball were, respectively: 3.96 and 3.26 Mg m 3 for density; 16.3 and 14.6 GPa for Vickers hardness (HVN10-98N); 403 and 316 GPa for Young’s modulus; 3.5 and 5.9 MPa m1/2 for fracture toughness,
657
KIc The microstructure of Al2O3 ceramic had few residual pores of size smaller than 1 mm usually at the grain-boundaries, a-Al2O3 grains of near-isometric shape and average grain size of 4 mm. The microstructure of Si3N4 ceramic had elongated b-Si3N4 grains with hexagonal cross section and bimodal size distribution (small grains of about 0.5 mm and fewer and larger grains of up to 3 mm of length) and an intergranular phase typical of liquid-phase sintered materials.
3. Results Fig. 1 shows a typical curve of measured friction coefficient. The curves of all five experiments exhibited the same behavior. In the first period, the friction coefficient increased rapidly up to 0.4–0.5. Then, it was followed by a transient period during which there was high friction coefficient variation: both the amplitude of peaks and the mean value of friction coefficient decreased with the increase in testing time. These two high friction regimes are called running-in period. After approximately 5373 min, the friction coefficient reached 0.00470.002, the UFLC regime value, and then remained stable for at least 10 min, after which the test was interrupted. Two ULFC results achieved 0.004, but most of them achieved 0.002, which is the resolution limit of the test. During the test, a wear track was produced on the alumina disc (Fig. 2a) and a regular circular plateau was formed on the silicon nitride ball (Fig. 2b). Considering that the wear in ULFC regime is negligible due to its very low friction coefficient, it can be inferred that both wear track and plateau were formed during the running-in period. The diameter of this plateau was used to calculate the average apparent pressure, resulting in 10 MPa. The determined values for volumetric loss were of 0.4270.08 mm3 in the sphere and approximately 0.08770.002 mm3 in the wear track on alumina disc. The roughness profile obtained from the cross-section of the worn track on alumina disc is shown in Fig. 3a. Outside the wear track the RMS roughness value was 350 nm and the topography displayed valleys of more than 1.0 mm in depth. The wear track in turn exhibited a quite different feature: the roughness summits were worn away, and the valleys seemed to have been fulfilled, probably by the wear debris, as shown in Fig. 3b. As a consequence,
Fig. 1. Typical behavior of the friction coefficient during the Si3N4 sphere on Al2O3 disc sliding test. In detail the last minutes of the test during which the friction coefficient of 0.002 was reached.
658
V. Ferreira et al. / Wear 296 (2012) 656–659
Fig. 2. Wear track on alumina disc (a), and wear plateau on silicon nitride ball (b).
Fig. 4. Energy dispersive X-ray fluorescence spectra from the alumina surface: (a) outside; and (b) inside the wear track (notice the presence of the silicon peak).
4. Discussion
Fig. 3. Topographies of: (a) the alumina disc profile outside and across the wear track (notice the disappearance of valleys under the wear track); (b) wear track on alumina disc at higher magnification; (c) plateau on silicon nitride ball.
the RMS roughness decreased to values in the range of 20–46 nm, one order of magnitude lower than the initial value. The silicon nitride plateau showed a very regular surface with RMS roughness values in the range of 2–8 nm (Fig. 3c). The EDX spectra obtained from the alumina surfaces before and after the wear test are shown in Fig. 4. A silicon peak is observed in the wear track (Fig. 4b), indicating that the material from silicon nitride ball was transferred to the alumina worn surface.
The results of the wear test showed that the ultra-low friction coefficient (ULFC) regime in the unmatched Al2O3–Si3N4 pair in water is achieved after a given sliding time when surpassing the running-in period. The tests also indicated that it is possible to reach a friction coefficient as low as 0.004 (Fig. 1). Apparently, there are no works in the literature which investigated the friction coefficient behavior of silicon nitride sliding against alumina. Likewise, there are no works reporting the ULFC values for oxide ceramics. Friction coefficients of this magnitude during under water ceramic sliding have only been reported for matched sliding pairs of silicon nitride [4,5,9] and silicon carbide [11,12]. Moreover, the Al2O3–Si3N4 pair exhibited a very different friction behavior from the alumina–alumina pair sliding in water [6,12,13]. The Al2O3–Al2O3 pairs do not display great oscillations in the friction coefficient magnitude during the experiment, and throughout the tests the friction coefficient does not decrease below 0.2, a value far from the ULFC of 0.004 reported in this work. Among the sliding tests performed with self-mated ceramic pairs of Al2O3–Al2O3 and Si3N4–Si3N4 in the same conditions of load, velocity and water lubrication of this study only the Si3N4– Si3N4 pairs achieved the ULFC regime [12]. Previous works [5,8,9,12] reported that the ULFC in Si3N4– Si3N4 or SiC–SiC sliding pairs in water is preceded by the formation of a hydrated silica layer on the surface of the ceramics. Sugita and co-workers [14] studied the relationship between
V. Ferreira et al. / Wear 296 (2012) 656–659
[28SiHþ]/[28Si þ] and [28SiOþ ]/[28Siþ] in the silica layer on silicon nitride surface. They measured an increase in the concentration of [28SiHþ ] and [28SiO þ] and suggested that at first Si is oxidized and afterwards hydrated, which could indicate that some degree of wear would be necessary to reach the ULFC regime. It was later suggested that the formation of silicon oxide [5] and its hydration can occur by the following reactions: Si3N4 þ6H2O23SiO2 þ 4NH3
(1)
SiO2 þ2H2O2Si(OH)4
(2)
The wear events yielded a flat surface on the Si3N4 ball (Fig. 2b), resulting in an apparent contact pressure of 10 MPa and a flat surface roughness ranging from 2 to 8 nm RMS (Fig. 3c), both similar to the measured values in Si3N4–Si3N4 sliding pairs [5,9]. The formation of a smooth flat surface on the sphere seems to be necessary so that the water lubricant film can work properly. The flat surface reduces the mean contact pressure whereas the low roughness reduces the water film thickness to the necessary minimum to activate the hydrodynamic lubrication. In the wear track on the counter-body (alumina disc, Fig. 2a), the decrease of the roughness value from 350 to 30 nm (Fig. 3b) also contributed to the decrease of the friction coefficient. According to the H2O–Al2O3 phase diagram from Wefers [15], an aluminum hydroxide (Al(OH)3) layer may be formed on the wear track under 10 MPa pressure. Since the aluminum hydroxide has lubricant characteristics [16], the in situ formation of this compound may have reduced the shear stresses at the contact interface. A similar argument was considered by Presser and co-workers [17] to explain the hydration of silicon carbide under water lubrication. The EDX spectrum (Fig. 4b) showed that the wear track on the alumina disc had been partly covered by a silicon-based compound. Considering Sugita and co-workers’ findings [14] and reactions (1) and (2), it is possible to infer that hydrated silicon oxide was formed on the alumina wear track. Both silicon and aluminum hydroxides debris have possibly filled the starting roughness valleys on the surface of alumina disc reducing its depth and resulting, along with the wear of alumina asperity summits, in a flat wear track as shown in Fig. 3a. This investigation showed that the friction coefficient of the Al2O3–Si3N4 pair was reduced from 0.4–0.5 at the beginning of the test to 0.004 at the steady-state regime. This ultra-low friction coefficient was achieved possibly because a flat smooth interface was formed, covering the alumina disc with a soft layer of aluminum hydroxide, and both body (Si3N4 ball) and counterbody (Al2O3 disc) with a hydrated silicon oxide layer. The measured ULFC value is in the same range as the one obtained in Si3N4–Si3N4 or SiC–SiC pairs which have been studied since the 1980s. Further investigation is needed to improve the friction mechanisms in aqueous medium in order to design new tribological systems with oxide ceramics.
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5. Conclusions Ultra-low friction coefficient (ULFC) was achieved in an unmated Al2O3–Si3N4 sliding pair under water. This result showed that the ULFC is not an exclusive phenomenon of Si3N4–Si3N4 and SiC–SiC mated pairs. Since the friction coefficient between Al2O3 and Si3N4 is in the same range of the mated Si3N4 and SiC pairs, the silicon nitride is considered to have a key role in the development of ULFC. The tribolayer of aluminum hydroxide and hydrated silicon oxide formed in situ contributes to reach the ULFC. The results pave the way to the development of very low friction loss mechanical systems running under water with oxide ceramics. References [1] J.M. Martin, C. Donnet, T. Le Mogne, T. Epicier, Superlubricity of molybdenum disulphide, Physical Review B 48 (14) (1993) 10583–10586. [2] S. Myhra, J.W.B. Summers, E.H. Kisi, Ti3SiC2—a layered ceramic exhibiting ultra-low friction, Materials Letters 39 (1) (1999) 6–11. [3] M. Chhowalla, G.A.J. Amaratunga, Thin films of fullerene-like MoS2 nanoparticles with ultra-low friction and wear, Nature 407 (6801) (2000) 164–167. [4] H. Tomizawa, T.E. Fischer, Friction and wear of silicon nitride and silicon carbide in water: hydrodynamic lubrication at low sliding speed obtained by tribochemical wear, Tribology Transactions 30 (1) (1987) 41–46. [5] J. Xu, K. Kato, Formation of tribochemical layer of ceramics sliding in water and its role for low friction, Wear 245 (1–2) (2000) 61–75. [6] P. Andersson, Water-lubricated pin-on-disc tests with ceramics, Wear 154 (1) (1992) 37–47. [7] R.S. Gates, S.M. Hsu, Tribochemistry between water and Si3N4 and SiC: induction time analysis, Tribology Letters 17 (3) (2004) 399–407. [8] M. Chen, K. Kato, K. Adachi, Friction and wear of self-mated SiC and Si3N4 sliding in water, Wear 250 (1–12) (2001) 246–255. [9] S. Jahanmir, Y. Ozmen, L.K. Ives, Water lubrication of silicon nitride in sliding, Tribology Letters 17 (3) (2004) 409–417. [10] L. Jordi, C. Iliev, T.E. Fischer, Lubrication of silicon nitride and silicon carbide by water: running in, wear and operation of sliding bearings, Tribology Letters 17 (3) (2004) 367–376. [11] M. Chen, K. Kato, K. Adachi, The comparisons of sliding speed and normal load effect on friction coefficients of self-mated Si3N4 and SiC under water lubrication, Tribology International 35 (3) (2002) 129–135. [12] D.A. Rani, Y. Yoshizawa, H. Hyuga, K. Hirao, Y. Yamauchi, Tribological behavior of ceramic materials (Si3N4, SiC and Al2O3) in aqueous medium, Journal of the European Ceramic Society 24 (10–11) (2004) 3279–3284. [13] M. Kalin, S. Jahanmir, G. Drazˇicˇ, Wear mechanisms of glass-infiltrated alumina sliding against alumina in water, Journal of the American Ceramic Society 88 (2) (2005) 346–352. [14] T. Sugita, K. Ueda, Y. Kanemura, Material removal mechanism of silicon nitride during rubbing in water, Wear 97 (1) (1984) 1–8. [15] K. Wefers, Nomenclature, preparation, and properties of aluminum oxides, oxide hydroxides, and trihydroxides, in: L.D. Hart (Ed.), Alumina Chemicals: Science and Technology Handbook, American Ceramic Society, Westerville, OH, 1990, pp. 13–22. [16] T. Nagaoka, C. Duran, T. Isobe, Y. Hotta, K. Watari, Hydraulic alumina binder for extrusion of alumina ceramics, Journal of the American Ceramic Society 90 (12) (2007) 3998–4001. [17] V. Presser, O. Krummhauer, K.G. Nickel, A. Kailer, C. Berthold, C. Raisch, Tribological and hydrothermal behaviour of silicon carbide under water lubrication, Wear 266 (7–8) (2009) 771–781.