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Oxide-film-dependent tribological behaviors of Ti3SiC2
Wear 262 (2007) 1079–1085
Oxide-film-dependent tribological behaviors of Ti3SiC2 Zhenying Huang, Hongxiang Zhai ∗ , Minglin Guan, Xin Liu, Mingxing Ai, Yang Zhou Center of Materials Engineering, Beijing Jiaotong University, Beijing 100044, China Received 5 June 2005; received in revised form 17 October 2006; accepted 13 November 2006 Available online 13 December 2006
Abstract The tribological behaviors of high pure bulk Ti3 SiC2 dry-sliding against a low carbon steel disk were investigated on a block-on-disk type tester under several sliding speeds from 5 m/s to 60 m/s and normal pressures from 0.1 MPa to 0.8 MPa. It was found that both the coefficient of friction and the wear rate of Ti3 SiC2 were dependent on the presence of a frictional film consisting of oxides of Ti, Si and Fe in the Ti3 SiC2 friction surface. The oxide film was formed and the percentage of coverage was increased with increasing the normal pressure for the medium sliding speeds of 20 m/s and 40 m/s. The oxide film could be formed but was maintained difficultly with increasing the normal pressure when the sliding speed was up to 60 m/s. Few of the oxides was generated and hence almost no oxide film was formed in the friction surface when the sliding speed was 5 m/s. The presence of the oxide film induced the coefficient of friction to reduce, but made the wear rate of Ti3 SiC2 increase. The coefficient of friction and the wear rate (×10−6 mm3 /Nm) of Ti3 SiC2 were of 0.53 and 0.91, 0.26 and 1.35, 0.16 and 2.05, and 0.29 and 3.75 under the normal pressure of 0.8 MPa and the sliding speed of 5 m/s, 20 m/s, 40 m/s and 60 m/s, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Ti3 SiC2 ; Tribological behavior; Oxide film; Antifriction effect
1. Introduction There were many literatures [1–9] showing that the layered ternary carbide Ti3 SiC2 had an unusual combination of properties in electrical and thermal conductivity, thermal shock resistance, damage tolerance, machinability and other useful properties. In fact, it also has a good tribological property [10–14]. However, for the complexity of the tribological problem, a further study is necessary to understand some tribological behaviors and the relevant mechanisms of Ti3 SiC2 . In early, Myhra et al. [15] had measured the friction coefficient of the basal plane of Ti3 SiC2 , using a lateral force microscopy, to be as low as 2 × 10−3 to 5 × 10−3 , suggesting that the layered Ti3 SiC2 might be a good self-antifriction material. However, the later test results reported by El-Raghy et al. [16] showed that the friction coefficient of polycrystalline bulk Ti3 SiC2 sliding dryly against a 440C steel pin on a pinon-disk type tester was as high as 0.83. In addition, the wear rates of Ti3 SiC2 were as large as 4.25 × 10−3 mm3 /Nm and 1.34 × 10−3 mm3 /Nm for the fine grained one (3–5 m) and the
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coarse grained one (100–150 m), respectively. Such large coefficient of friction and wear rate may be unacceptable, if using Ti3 SiC2 as a tribological material with self-antifriction function. In fact, the result may not be so disappointing. In the previous work [10–14], we have investigated some friction and wear behaviors of a nearly pure bulk Ti3 SiC2 sliding dryly against low carbon steel on a block-on-disk type tester. The results showed that the coefficient of friction was 0.27, and the wear rate of Ti3 SiC2 was as low as 2.2 × 10−6 mm3 /Nm for the sliding speed of 20 m/s and normal pressure of 0.8 MPa. It is an interesting question that why the tribological property of Ti3 SiC2 exhibited so large difference between the testing results. In general, for any a friction-pair making up of real materials, the friction and wear behavior would be influenced strongly by the lubricating state of the friction surfaces [17–19]. The results of the previous work by Lim et al. [17] have shown that, the sliding friction and wear behaviors between dry metal surfaces are determined, at low sliding speeds (v = 1 m/s for steel) by surface roughness and by the plastic (and perhaps elastic) properties of the surfaces. At higher speeds (v = 1 m/s for steel), the surface condition is modified by local heating (which can cause oxidation or even melting); then the coefficient of friction depends in a reproducible way on sliding velocity and bearing pressure. It is conceivable that, if either of the friction surfaces is able to
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generate a film which has antifriction effect during friction, the coefficient of friction and the wear rate will be reduced. Otherwise, a larger coefficient of friction and wear rate may appear due to severe interferences of asperities between friction surfaces. This may also be a dominant factor to determine whether or not the Ti3 SiC2 exhibiting a good tribological performance. The work reported herein is one of a series of investigations devoted to the synthesis and tribological applications of the ternary carbides including Ti3 SiC2 and Ti3 AlC2 [10–14,20–22]. The purpose of this paper is to exhibit some effects of a selfgenerating oxide film in the friction surface of Ti3 SiC2 on the tribology behaviors. 2. Experimental procedure The Ti3 SiC2 sample used in this study was fabricated by a hot-pressing process, which had been described in detail elsewhere [10]. The sample has a high purity; the content of Ti3 SiC2 phase in the sample is estimated to be larger than 98% in volume. Fig. 1 is a typical SEM micrograph of the Ti3 SiC2 sample, showing that most of Ti3 SiC2 grains have a plate-like or lenticular shape, and the average size of the grains is about 20 m in the elongated direction. The specific weight of the sample is 4.31 g/cm3 , measured by the Archimedes’ method, and the relative volume density is about 95% for the theoretical density of 4.53 g/cm3 [2]. The Ti3 SiC2 sample was machined into blocks with dimensions of 10 mm × 10 mm × 12 mm for friction and wear tests on a block-on-disk type friction tester, which had been introduced elsewhere [23]. A low carbon steel (containing 0.2% carbon) disk with diameter of 300 mm and thickness of 10 mm was used as the friction counterpart. Tests were performed at room temperature of 20–22 ◦ C and relative humidity of 23–25%. The sliding speed and normal pressure were set at several levels, from 5 m/s to 60 m/s, and from 0.1 MPa to 0.8 MPa, respectively. The sliding distance for one continuous friction process was determined as 24,000 m. Such a long sliding distance is necessary to accurately measure the mass loss of the Ti3 SiC2 block
Fig. 1. Typical SEM micrograph showing microstructures of the Ti3 SiC2 sample, where the observed surface has been etched in a mixed acid of HNO3 and HF.
in one continuous friction process, since the Ti3 SiC2 is very wear-resistant. The mass loss of the Ti3 SiC2 block was measured using an electronic balance with ±10−4 g accuracy after every continuous friction process. The wear rate of the Ti3 SiC2 was calculated from the mass loss per sliding distance and per normal load (normal pressure × area of friction surface). The coefficient of friction was measured automatically by the computer system. Tests were repeated three times for every given test condition, and the average was used as the evaluated data. In addition, whenever the sliding speed and/or the normal pressure were changed, a pre-abrasion was made to eliminate any possible influence of the loading history on the friction surface. The friction surfaces of the Ti3 SiC2 as well as the steel disk were observed and analyzed by a scanning electron microscopy (SEM: Hitachi, S-3500N) equipped with an energy dispersion spectroscopy (EDS: Oxford, Inca) associating with the measured data of the friction coefficient and the wear rate, in order to understand some friction and wear behaviors and the relevant mechanisms. 3. Results 3.1. Friction behaviors A typical data curve of the measured coefficient of friction is shown in Fig. 2. The transition behavior that the coefficient of friction increases from an initial value to a relatively steady value and the fluctuation behavior that the coefficient of friction randomly fluctuates throughout the entire friction process, as general behaviors of the dry-sliding friction, have been characterized in our previous work [23]. The transition behavior is mainly related with the thermal instabilities of friction surfaces, and the fluctuation behavior is governed essentially by the interference mechanisms of asperities between friction surfaces. The transition and steady stages can be divided by a characteristic time, at which the coefficient of friction has reached 97% of the steady value. The randomly fluctuating behavior in the steady stage has been confirmed as yielding to a normal distribution
Fig. 2. A typical data curve of the measured coefficient of friction; the iconography exhibits the normal distribution characteristics of the coefficient of friction in the steady stage.
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Fig. 3. The coefficients of friction as functions of the normal pressure for different sliding speeds.
Fig. 4. The wear rates of the Ti3 SiC2 as functions of the normal pressure for different sliding speeds.
[23]. The insert in the upper right-hand corner of Fig. 2 is a statistical result for the measured data, showing typical characteristics of normal distribution. In fact, all of the measured data curves are identical in the statistical characteristics. Thus, the expectation of the normal distribution has been used as an average to evaluate the coefficient of friction in magnitude for every testing condition. Fig. 3 shows the friction coefficients as functions of normal pressure for different sliding speeds. Notably, the friction coefficients are speed- and pressure-dependent. For the low sliding speed of 5 m/s, the coefficient of friction monotonously increases from 0.42 to 0.53 with increasing the normal pressure from 0.1 MPa to 0.8 MPa. However, for the medium sliding speed of 20 m/s, only when the normal pressure increases from 0.1 MPa to 0.2 MPa the coefficient of friction increases from 0.25 to 0.36, then it gradually reduces to 0.27 with increasing the normal pressure to 0.8 MPa. A similar pressure-dependent change is exhibited at the sliding speed of 40 m/s, but the maximum value and the corresponding pressure are changed. The coefficient of friction increases from a quite small value of 0.08 to 0.21 with increasing the normal pressure from 0.1 MPa to 0.4 MPa, and then gradually reduces to 0.18 with increasing the normal pressure to 0.8 MPa. An inverse change is found at the higher sliding speed of 60 m/s. The coefficient of friction slowly increases from 0.12 to 0.2 with increasing the normal pressure from 0.1 MPa to 0.6 MPa, and then shows a faster increasing tendency, but only increases to 0.29 when the normal pressure increases to 0.8 MPa. Clearly, the sliding speed is an assignable factor. The coefficient of friction notably reduces with increase in the sliding speed, unless a larger normal pressure is applied when the sliding speed is high. These evidences suggest that the friction behavior could be governed by a changing antifriction mechanism, which is dependent upon both of the sliding speed and the normal pressure.
wear rates increase with increase in the sliding speed unless the normal pressure applied is small at the sliding speed of 60 m/s. This speed-dependent change is reversed to that of the coefficient of friction in magnitude, i.e., a smaller wear rate of Ti3 SiC2 corresponds to a larger coefficient of friction, and vice versa. For the low speed of 5 m/s, the wear rate increases slightly from 0.64 × 10−6 mm3 /Nm to 0.90 × 10−6 mm3 /Nm with increasing the normal pressure from 0.1 MPa to 0.8 MPa. However, for the medium speeds, the wear rates decrease slightly from 1.8 × 10−6 mm3 /Nm to 1.37 × 10−6 mm3 /Nm and from 2.5 × 10−6 mm3 /Nm to 2.0 × 10−6 mm3 /Nm, with the normal pressure increasing from 0.1 MPa to 0.8 MPa, for sliding speeds of 20 m/s and 40 m/s, respectively. An inverse change similar to the behavior in the coefficient of friction is found at high speed of 60 m/s. The wear rate increases from a relatively low value of 1.0 × 10−6 mm3 /Nm to 2.11 × 10−6 mm3 /Nm with increasing the normal pressure from 0.1 MPa to 0.5 MPa, then increased at a larger rate to 3.62 × 10−6 mm3 /Nm with further increasing the normal pressure to 0.8 MPa. These behaviors suggest that the wear rate of Ti3 SiC2 could be governed by a complex mechanism, relating to the state of friction surfaces.
3.2. Wear behaviors The wear rates of the Ti3 SiC2 are shown in Fig. 4 as functions of the normal pressure for different sliding speeds. Clearly, the
3.3. Friction surface behaviors The friction surfaces were observed by SEM associating with several typical behaviors of the coefficient of friction and the wear rate. Fig. 5 is the observed micrographs for the Ti3 SiC2 friction surfaces. It can be seen that the friction surfaces are different in image with the sliding speed and normal pressure changing. Clearly, there is a frictional film covering the Ti3 SiC2 friction surface for the medium speed of 20 m/s, and the percentage of coverage is increasing with the increase of the normal pressure. As shown in Fig. 5(c and d), the film only partially covers the friction surface in the case of 0.2 MPa, while it completely covers the friction surface in the case of 0.8 MPa and the film surface is very smooth accordingly. In fact, the similar statuses are also presented at the sliding speed of 40 m/s. However, when the sliding speed increases to 60 m/s, the statuses are
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Fig. 5. SEM micrographs exhibiting the Ti3 SiC2 friction surfaces for the different sliding speeds and the normal pressures: (a) 5 m/s and 0.2 MPa, (b) 5 m/s and 0.8 MPa, (c) 20 m/s and 0.2 MPa, (d) 20 m/s and 0.8 MPa, (e) 60 m/s and 0.2 MPa, and (f) 60 m/s and 0.8 MPa.
changed. There is yet a completely covered and smoother film existing in the friction surface for the low normal pressure of 0.2 MPa, but the film becomes poor and non-uniform when the normal pressure increases to 0.8 MPa, as shown in Fig. 5(e and f). It is worth to note that there are only few porphyritic films existing in the Ti3 SiC2 friction surface when the sliding speed is 5 m/s, irrespective of the normal pressure of 0.2 MPa or 0.8 MPa. The thickness of the film was observed from the cross-section of the film. Fig. 6 is a micrograph showing the cross-section state of the film formed under the sliding speed of 20 m/s and the normal pressure of 0.8 MPa. The thickness of the film is estimated to be about 1.0 m. This is a typical thickness for the film, when it completely covered the Ti3 SiC2 friction surface. Fig. 7 are typical SEM micrographs showing the friction surfaces of the steel disk for the lower sliding speed of 5 m/s and
the medium sliding speed of 20 m/s, corresponding to the filmlacking in Ti3 SiC2 friction surface (Fig. 5(b)) and the film-full in Ti3 SiC2 friction surface (Fig. 5(d)), respectively. The friction surface for the sliding speed of 5 m/s is quite rough and there are obvious scratches on the surface, while the friction surface for the speed of 20 m/s is smoother, and obviously there is a film covering the friction surface. 3.4. Compositions of the film The compositions of the films covering the friction surfaces of the Ti3 SiC2 and the steel disk were identified by the EDS. Fig. 8(a and b) are the analyzed results for the Ti3 SiC2 friction surface and the steel disk friction surface, which have been shown in Figs 5(d) and 7(b), respectively. There are only
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Fig. 6. A typical SEM micrograph showing the cross-section of the film, which was formed under the sliding speed of 20 m/s and the normal pressure of 0.8 MPa.
elements of oxygen, titanium, silicon and a little amount of ferrum, but no carbon existing in the Ti3 SiC2 friction surface, indicating that the film consists of the oxides of titanium, silicon and ferrum. There are the same elements existing in the friction surfaces of the steel disk, indicating that the film covering the friction surface of the steel disk consists of the same oxides as the Ti3 SiC2 friction surface. The only difference between the two surfaces is the content of the elements. It is conceivable that the oxides of titanium and silicon existing in the steel disk surface are transferred from the Ti3 SiC2 friction surface, and the ferric oxide in the Ti3 SiC2 friction surface is transferred from the steel disk surface. Fig. 9 is a typical SEM micrograph showing the wear debris, which is collected under the sliding speed of 20 m/s and normal pressure of 0.8 MPa. Most of the wear debris shows a lamella shape, and the chemical composition is identified to be the same to the Ti3 SiC2 friction surface, indicating that the wear debris is mainly the product of the oxide films. This means that the Ti3 SiC2 is not directly contacted with the steel disk during the friction process, but is insulated by the oxide film. Thus, the friction and wear behaviors could be governed, to a great extent, by the oxide film when it has been formed.
Fig. 8. EDS analysis results for the friction surface of (a) the Ti3 SiC2 and (b) the steel disk.
To further identify the phase behavior of the oxide film, the XRD analysis was used. In fact, it is quite difficult to obtain an XRD pattern of the frictional film directly in the friction surface, because the X-ray could readily penetrate the film and reach into the surface of the matrix. So, we used the wear debris to be analyzed. Fig. 10 is the XRD pattern of the wear debris shown in Fig. 9. The diffraction peaks belong to iron titanate (Fe2.25 Ti0.75 O4 ), iron silicate (FeSiO3 ) and a very small amount of Ti3 SiC2 , indicating that the frictional film consisted of cocrystalline or partially cocrystalline iron titanate and iron silicate. Obviously, the Fe2.25 Ti0.75 O4 and FeSiO3 are the crystallization of the oxides of ferrum, titanium and silicon. The forming of them could be related to a mechanical alloying reaction during the impact of asperities between the friction surfaces of Ti3 SiC2 and the steel disk.
Fig. 7. SEM micrographs exhibiting the friction surfaces of the low carbon steel disk for the sliding speed and the normal pressure: (a) 5 m/s and 0.8 MPa, and (b) 20 m/s and 0.8 MPa.
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Fig. 9. SEM micrograph showing the worn debris collected under the sliding speed of 20 m/s and normal pressure of 0.8 MPa.
Fig. 10. XRD pattern of the wear debris shown in Fig. 9.
4. Discussions It has been shown that the oxide film is soft and flowable during the sliding friction [10–12]. Such a film would play an antifriction part in the friction surfaces, while the speed and pressure-dependent behaviors of the friction coefficient could be related to the change in covering percentage of the oxide film in the friction surface. It is conceivable that a good oxide film with a higher percentage of coverage results in a smaller coefficient of friction, and a poor oxide film with a lower percentage of coverage results in a larger friction coefficient. Indeed, there was such a corresponding relation between the coefficient of friction and the percentage of coverage. In the cases of the medium speeds of 20 m/s and 40 m/s, the increased percentages of coverage resulted in the reducing of the coefficients of friction with increasing the normal pressure. In the case of the higher speed of 60 m/s, the reducing percentage of coverage resulted in the increasing of the coefficient of friction with increasing the normal pressure. As the same causal relation, in the case of the lower sliding speed of 5 m/s, the larger coefficient of friction was due to the lack of the oxide film. It is worth to indi-
cate that the coefficient of friction as high as 0.83 measured by El-Raghy et al. [16] could be attributed to the poor friction surfaces of the Ti3 SiC2 as well as the friction counterpart. In fact, there were almost no observable films existing in the friction surfaces. The changing behaviors of the wear rates of Ti3 SiC2 are also related closely with the existing of the oxide film. For the lack of the oxide film, the Ti3 SiC2 friction surface for the low sliding speed of 5 m/s would be harder than the steel disk, and consequently the friction surface of the steel disk was scratched remarkably as shown in Fig. 6(a). Clearly, in this case, the lower wear rate of Ti3 SiC2 was at the price of the severe wear of the steel disk. As the soft oxide film is formed and completed with increasing the normal pressure, the hardness relation between the Ti3 SiC2 and the steel disk was changed for the medium speeds of 20 m/s and 40 m/s. Since the softer oxide film would be readily worn, the wear rate of Ti3 SiC2 was increased accordingly. There would be more oxides generated at the higher sliding speed of 60 m/s due to the higher frictional temperature, but they were difficult to be maintained in the friction surface when the normal pressure was larger, due to the crowding out effect of the friction surfaces during the sliding friction process. Consequently, the wear rate of Ti3 SiC2 increased fast. In addition, the change in the viscosity of the oxide film could also be an influencing factor. The viscosity would be reduced with increasing the sliding speed and/or the normal pressure, since the frictional temperature in the friction surface rises. Then the oxide film is difficult to be maintained for the larger sliding speed and normal pressure could be related with the change of the viscosity as well. Some literatures [24–26] have shown that Ti3 SiC2 can be oxidized at a certain temperature in air, and forms a dense, adhesive and layered scale consisting of SiO2 and TiO2 on the surface of Ti3 SiC2 . Essentially, when two surfaces slide together, most of the work done against friction is turned into heat [17–19]. The resulting rise in temperature may modify the mechanical and metallurgical properties of the sliding surfaces, and it may make them oxidize or even melt; all these things influence the friction of coefficient and the rate of wear. Lim et al. [17] have developed wear mechanism and temperature maps for different materials in dry-sliding contact, where bulk surface (Tb ) and flash (Tf ) temperature are plotted as isothermal contours on normalized sliding velocity and contact pressure axes. The oxidation occurring on the Ti3 SiC2 friction surface may also due to the generating of frictional heat, which is proportional to the friction work μLV, where L is the normal load, V the sliding speed, and μ is the coefficient of friction [27]. Thus, the rate of generating oxides is an increasing function of the L, V and μ, and hence the larger L and/or V as well as μ the larger generating rate of the oxides in unit time. On the other hand, simultaneously with the generating, the oxides would be consumed continuously from the friction surface as wear, and the consuming rate in unit time would be also an increasing function of the L and V. Hence, the larger L and/or V the larger consuming rate. The oxide film can be formed on the friction surface, only when the generating rate is larger than the consuming rate. However, the thickness of the oxide film is limited by the viscosity of the oxides during fric-
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tion as well as the roughness of friction surface, cannot exceed a characteristic thickness, and the redundant oxides would be removed from the friction surface as the wear debris. Therefore, the oxide film can be maintained at the friction surface, if only the generating rate is equal to the consuming rate when a steady oxide film has been formed. This means that a larger generating rate could result in a larger wear rate. The oxide film cannot be formed, or a formed oxide film will be damaged, if the generating rate is smaller than the consuming rate. These states can be characterized as: ⎧ ⎨ ωg > ωc , the oxide film can be formed; ωg = ωc , the oxide film can be maintained; (1) ⎩ ωg < ωc ,
the oxide film cannot be formed or will be damaged.
where ωg and ωc are the generating rate and the consuming rate, respectively. The state could be changed with the sliding speed and/or the normal pressure, since the ωg and ωc are different function of the L and V. The states for the low sliding speed of 5 m/s and the normal pressures applied from 0.1 MPa to 0.8 MPa can be classified to ωg < ωc . However, the states for the medium speeds of 20 m/s and 40 m/s could be changed from ωg < ωc to ωg = ωc with increasing the normal pressure. Particularly, the states for the higher sliding speed of 60 m/s could be changed from ωg > ωc to ωg = ωc and further to ωg < ωc with the increase in normal pressure. These changes will be further characterized in other paper. 5. Conclusions (1) The friction and wear behaviors of Ti3 SiC2 sliding dryly against low carbon steel are dependent strongly upon the presence of a frictional oxide film on the Ti3 SiC2 friction surface. (2) The frictional film consists of the oxides of Ti, Si and Fe, and its percentage of coverage at the Ti3 SiC2 friction surface is increased with increasing the sliding speed and/or normal pressure in a limited range. (3) A larger generating rate of the oxides increases the covering percentage of the oxide film. In results, the coefficient of friction is reduced due to the antifriction effect of the oxide film, but the wear rate of Ti3 SiC2 is increased due to the consuming of the oxides. (4) The oxide film can be formed and continuously maintained for the medium sliding speeds, but maintained with difficulty when the sliding speed is high. Almost no oxide film can be formed when the sliding speed is low. Acknowledgements This work was supported by the National Science Foundation of China (NSFC) under Grant No. 50472045, the National High Technology Program (863 Program) of China under Grant No. 2003AA332080 and the Excellent Doctor Fund of Beijing Jiaotong University under Grant No. 48014.
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Oxide-film-dependent tribological behaviors of Ti3SiC2 Zhenying Huang, Hongxiang Zhai ∗ , Minglin Guan, Xin Liu, Mingxing Ai, Yang Zhou Center of Materials Engineering, Beijing Jiaotong University, Beijing 100044, China Received 5 June 2005; received in revised form 17 October 2006; accepted 13 November 2006 Available online 13 December 2006
Abstract The tribological behaviors of high pure bulk Ti3 SiC2 dry-sliding against a low carbon steel disk were investigated on a block-on-disk type tester under several sliding speeds from 5 m/s to 60 m/s and normal pressures from 0.1 MPa to 0.8 MPa. It was found that both the coefficient of friction and the wear rate of Ti3 SiC2 were dependent on the presence of a frictional film consisting of oxides of Ti, Si and Fe in the Ti3 SiC2 friction surface. The oxide film was formed and the percentage of coverage was increased with increasing the normal pressure for the medium sliding speeds of 20 m/s and 40 m/s. The oxide film could be formed but was maintained difficultly with increasing the normal pressure when the sliding speed was up to 60 m/s. Few of the oxides was generated and hence almost no oxide film was formed in the friction surface when the sliding speed was 5 m/s. The presence of the oxide film induced the coefficient of friction to reduce, but made the wear rate of Ti3 SiC2 increase. The coefficient of friction and the wear rate (×10−6 mm3 /Nm) of Ti3 SiC2 were of 0.53 and 0.91, 0.26 and 1.35, 0.16 and 2.05, and 0.29 and 3.75 under the normal pressure of 0.8 MPa and the sliding speed of 5 m/s, 20 m/s, 40 m/s and 60 m/s, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Ti3 SiC2 ; Tribological behavior; Oxide film; Antifriction effect
1. Introduction There were many literatures [1–9] showing that the layered ternary carbide Ti3 SiC2 had an unusual combination of properties in electrical and thermal conductivity, thermal shock resistance, damage tolerance, machinability and other useful properties. In fact, it also has a good tribological property [10–14]. However, for the complexity of the tribological problem, a further study is necessary to understand some tribological behaviors and the relevant mechanisms of Ti3 SiC2 . In early, Myhra et al. [15] had measured the friction coefficient of the basal plane of Ti3 SiC2 , using a lateral force microscopy, to be as low as 2 × 10−3 to 5 × 10−3 , suggesting that the layered Ti3 SiC2 might be a good self-antifriction material. However, the later test results reported by El-Raghy et al. [16] showed that the friction coefficient of polycrystalline bulk Ti3 SiC2 sliding dryly against a 440C steel pin on a pinon-disk type tester was as high as 0.83. In addition, the wear rates of Ti3 SiC2 were as large as 4.25 × 10−3 mm3 /Nm and 1.34 × 10−3 mm3 /Nm for the fine grained one (3–5 m) and the
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coarse grained one (100–150 m), respectively. Such large coefficient of friction and wear rate may be unacceptable, if using Ti3 SiC2 as a tribological material with self-antifriction function. In fact, the result may not be so disappointing. In the previous work [10–14], we have investigated some friction and wear behaviors of a nearly pure bulk Ti3 SiC2 sliding dryly against low carbon steel on a block-on-disk type tester. The results showed that the coefficient of friction was 0.27, and the wear rate of Ti3 SiC2 was as low as 2.2 × 10−6 mm3 /Nm for the sliding speed of 20 m/s and normal pressure of 0.8 MPa. It is an interesting question that why the tribological property of Ti3 SiC2 exhibited so large difference between the testing results. In general, for any a friction-pair making up of real materials, the friction and wear behavior would be influenced strongly by the lubricating state of the friction surfaces [17–19]. The results of the previous work by Lim et al. [17] have shown that, the sliding friction and wear behaviors between dry metal surfaces are determined, at low sliding speeds (v = 1 m/s for steel) by surface roughness and by the plastic (and perhaps elastic) properties of the surfaces. At higher speeds (v = 1 m/s for steel), the surface condition is modified by local heating (which can cause oxidation or even melting); then the coefficient of friction depends in a reproducible way on sliding velocity and bearing pressure. It is conceivable that, if either of the friction surfaces is able to
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generate a film which has antifriction effect during friction, the coefficient of friction and the wear rate will be reduced. Otherwise, a larger coefficient of friction and wear rate may appear due to severe interferences of asperities between friction surfaces. This may also be a dominant factor to determine whether or not the Ti3 SiC2 exhibiting a good tribological performance. The work reported herein is one of a series of investigations devoted to the synthesis and tribological applications of the ternary carbides including Ti3 SiC2 and Ti3 AlC2 [10–14,20–22]. The purpose of this paper is to exhibit some effects of a selfgenerating oxide film in the friction surface of Ti3 SiC2 on the tribology behaviors. 2. Experimental procedure The Ti3 SiC2 sample used in this study was fabricated by a hot-pressing process, which had been described in detail elsewhere [10]. The sample has a high purity; the content of Ti3 SiC2 phase in the sample is estimated to be larger than 98% in volume. Fig. 1 is a typical SEM micrograph of the Ti3 SiC2 sample, showing that most of Ti3 SiC2 grains have a plate-like or lenticular shape, and the average size of the grains is about 20 m in the elongated direction. The specific weight of the sample is 4.31 g/cm3 , measured by the Archimedes’ method, and the relative volume density is about 95% for the theoretical density of 4.53 g/cm3 [2]. The Ti3 SiC2 sample was machined into blocks with dimensions of 10 mm × 10 mm × 12 mm for friction and wear tests on a block-on-disk type friction tester, which had been introduced elsewhere [23]. A low carbon steel (containing 0.2% carbon) disk with diameter of 300 mm and thickness of 10 mm was used as the friction counterpart. Tests were performed at room temperature of 20–22 ◦ C and relative humidity of 23–25%. The sliding speed and normal pressure were set at several levels, from 5 m/s to 60 m/s, and from 0.1 MPa to 0.8 MPa, respectively. The sliding distance for one continuous friction process was determined as 24,000 m. Such a long sliding distance is necessary to accurately measure the mass loss of the Ti3 SiC2 block
Fig. 1. Typical SEM micrograph showing microstructures of the Ti3 SiC2 sample, where the observed surface has been etched in a mixed acid of HNO3 and HF.
in one continuous friction process, since the Ti3 SiC2 is very wear-resistant. The mass loss of the Ti3 SiC2 block was measured using an electronic balance with ±10−4 g accuracy after every continuous friction process. The wear rate of the Ti3 SiC2 was calculated from the mass loss per sliding distance and per normal load (normal pressure × area of friction surface). The coefficient of friction was measured automatically by the computer system. Tests were repeated three times for every given test condition, and the average was used as the evaluated data. In addition, whenever the sliding speed and/or the normal pressure were changed, a pre-abrasion was made to eliminate any possible influence of the loading history on the friction surface. The friction surfaces of the Ti3 SiC2 as well as the steel disk were observed and analyzed by a scanning electron microscopy (SEM: Hitachi, S-3500N) equipped with an energy dispersion spectroscopy (EDS: Oxford, Inca) associating with the measured data of the friction coefficient and the wear rate, in order to understand some friction and wear behaviors and the relevant mechanisms. 3. Results 3.1. Friction behaviors A typical data curve of the measured coefficient of friction is shown in Fig. 2. The transition behavior that the coefficient of friction increases from an initial value to a relatively steady value and the fluctuation behavior that the coefficient of friction randomly fluctuates throughout the entire friction process, as general behaviors of the dry-sliding friction, have been characterized in our previous work [23]. The transition behavior is mainly related with the thermal instabilities of friction surfaces, and the fluctuation behavior is governed essentially by the interference mechanisms of asperities between friction surfaces. The transition and steady stages can be divided by a characteristic time, at which the coefficient of friction has reached 97% of the steady value. The randomly fluctuating behavior in the steady stage has been confirmed as yielding to a normal distribution
Fig. 2. A typical data curve of the measured coefficient of friction; the iconography exhibits the normal distribution characteristics of the coefficient of friction in the steady stage.
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Fig. 3. The coefficients of friction as functions of the normal pressure for different sliding speeds.
Fig. 4. The wear rates of the Ti3 SiC2 as functions of the normal pressure for different sliding speeds.
[23]. The insert in the upper right-hand corner of Fig. 2 is a statistical result for the measured data, showing typical characteristics of normal distribution. In fact, all of the measured data curves are identical in the statistical characteristics. Thus, the expectation of the normal distribution has been used as an average to evaluate the coefficient of friction in magnitude for every testing condition. Fig. 3 shows the friction coefficients as functions of normal pressure for different sliding speeds. Notably, the friction coefficients are speed- and pressure-dependent. For the low sliding speed of 5 m/s, the coefficient of friction monotonously increases from 0.42 to 0.53 with increasing the normal pressure from 0.1 MPa to 0.8 MPa. However, for the medium sliding speed of 20 m/s, only when the normal pressure increases from 0.1 MPa to 0.2 MPa the coefficient of friction increases from 0.25 to 0.36, then it gradually reduces to 0.27 with increasing the normal pressure to 0.8 MPa. A similar pressure-dependent change is exhibited at the sliding speed of 40 m/s, but the maximum value and the corresponding pressure are changed. The coefficient of friction increases from a quite small value of 0.08 to 0.21 with increasing the normal pressure from 0.1 MPa to 0.4 MPa, and then gradually reduces to 0.18 with increasing the normal pressure to 0.8 MPa. An inverse change is found at the higher sliding speed of 60 m/s. The coefficient of friction slowly increases from 0.12 to 0.2 with increasing the normal pressure from 0.1 MPa to 0.6 MPa, and then shows a faster increasing tendency, but only increases to 0.29 when the normal pressure increases to 0.8 MPa. Clearly, the sliding speed is an assignable factor. The coefficient of friction notably reduces with increase in the sliding speed, unless a larger normal pressure is applied when the sliding speed is high. These evidences suggest that the friction behavior could be governed by a changing antifriction mechanism, which is dependent upon both of the sliding speed and the normal pressure.
wear rates increase with increase in the sliding speed unless the normal pressure applied is small at the sliding speed of 60 m/s. This speed-dependent change is reversed to that of the coefficient of friction in magnitude, i.e., a smaller wear rate of Ti3 SiC2 corresponds to a larger coefficient of friction, and vice versa. For the low speed of 5 m/s, the wear rate increases slightly from 0.64 × 10−6 mm3 /Nm to 0.90 × 10−6 mm3 /Nm with increasing the normal pressure from 0.1 MPa to 0.8 MPa. However, for the medium speeds, the wear rates decrease slightly from 1.8 × 10−6 mm3 /Nm to 1.37 × 10−6 mm3 /Nm and from 2.5 × 10−6 mm3 /Nm to 2.0 × 10−6 mm3 /Nm, with the normal pressure increasing from 0.1 MPa to 0.8 MPa, for sliding speeds of 20 m/s and 40 m/s, respectively. An inverse change similar to the behavior in the coefficient of friction is found at high speed of 60 m/s. The wear rate increases from a relatively low value of 1.0 × 10−6 mm3 /Nm to 2.11 × 10−6 mm3 /Nm with increasing the normal pressure from 0.1 MPa to 0.5 MPa, then increased at a larger rate to 3.62 × 10−6 mm3 /Nm with further increasing the normal pressure to 0.8 MPa. These behaviors suggest that the wear rate of Ti3 SiC2 could be governed by a complex mechanism, relating to the state of friction surfaces.
3.2. Wear behaviors The wear rates of the Ti3 SiC2 are shown in Fig. 4 as functions of the normal pressure for different sliding speeds. Clearly, the
3.3. Friction surface behaviors The friction surfaces were observed by SEM associating with several typical behaviors of the coefficient of friction and the wear rate. Fig. 5 is the observed micrographs for the Ti3 SiC2 friction surfaces. It can be seen that the friction surfaces are different in image with the sliding speed and normal pressure changing. Clearly, there is a frictional film covering the Ti3 SiC2 friction surface for the medium speed of 20 m/s, and the percentage of coverage is increasing with the increase of the normal pressure. As shown in Fig. 5(c and d), the film only partially covers the friction surface in the case of 0.2 MPa, while it completely covers the friction surface in the case of 0.8 MPa and the film surface is very smooth accordingly. In fact, the similar statuses are also presented at the sliding speed of 40 m/s. However, when the sliding speed increases to 60 m/s, the statuses are
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Fig. 5. SEM micrographs exhibiting the Ti3 SiC2 friction surfaces for the different sliding speeds and the normal pressures: (a) 5 m/s and 0.2 MPa, (b) 5 m/s and 0.8 MPa, (c) 20 m/s and 0.2 MPa, (d) 20 m/s and 0.8 MPa, (e) 60 m/s and 0.2 MPa, and (f) 60 m/s and 0.8 MPa.
changed. There is yet a completely covered and smoother film existing in the friction surface for the low normal pressure of 0.2 MPa, but the film becomes poor and non-uniform when the normal pressure increases to 0.8 MPa, as shown in Fig. 5(e and f). It is worth to note that there are only few porphyritic films existing in the Ti3 SiC2 friction surface when the sliding speed is 5 m/s, irrespective of the normal pressure of 0.2 MPa or 0.8 MPa. The thickness of the film was observed from the cross-section of the film. Fig. 6 is a micrograph showing the cross-section state of the film formed under the sliding speed of 20 m/s and the normal pressure of 0.8 MPa. The thickness of the film is estimated to be about 1.0 m. This is a typical thickness for the film, when it completely covered the Ti3 SiC2 friction surface. Fig. 7 are typical SEM micrographs showing the friction surfaces of the steel disk for the lower sliding speed of 5 m/s and
the medium sliding speed of 20 m/s, corresponding to the filmlacking in Ti3 SiC2 friction surface (Fig. 5(b)) and the film-full in Ti3 SiC2 friction surface (Fig. 5(d)), respectively. The friction surface for the sliding speed of 5 m/s is quite rough and there are obvious scratches on the surface, while the friction surface for the speed of 20 m/s is smoother, and obviously there is a film covering the friction surface. 3.4. Compositions of the film The compositions of the films covering the friction surfaces of the Ti3 SiC2 and the steel disk were identified by the EDS. Fig. 8(a and b) are the analyzed results for the Ti3 SiC2 friction surface and the steel disk friction surface, which have been shown in Figs 5(d) and 7(b), respectively. There are only
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Fig. 6. A typical SEM micrograph showing the cross-section of the film, which was formed under the sliding speed of 20 m/s and the normal pressure of 0.8 MPa.
elements of oxygen, titanium, silicon and a little amount of ferrum, but no carbon existing in the Ti3 SiC2 friction surface, indicating that the film consists of the oxides of titanium, silicon and ferrum. There are the same elements existing in the friction surfaces of the steel disk, indicating that the film covering the friction surface of the steel disk consists of the same oxides as the Ti3 SiC2 friction surface. The only difference between the two surfaces is the content of the elements. It is conceivable that the oxides of titanium and silicon existing in the steel disk surface are transferred from the Ti3 SiC2 friction surface, and the ferric oxide in the Ti3 SiC2 friction surface is transferred from the steel disk surface. Fig. 9 is a typical SEM micrograph showing the wear debris, which is collected under the sliding speed of 20 m/s and normal pressure of 0.8 MPa. Most of the wear debris shows a lamella shape, and the chemical composition is identified to be the same to the Ti3 SiC2 friction surface, indicating that the wear debris is mainly the product of the oxide films. This means that the Ti3 SiC2 is not directly contacted with the steel disk during the friction process, but is insulated by the oxide film. Thus, the friction and wear behaviors could be governed, to a great extent, by the oxide film when it has been formed.
Fig. 8. EDS analysis results for the friction surface of (a) the Ti3 SiC2 and (b) the steel disk.
To further identify the phase behavior of the oxide film, the XRD analysis was used. In fact, it is quite difficult to obtain an XRD pattern of the frictional film directly in the friction surface, because the X-ray could readily penetrate the film and reach into the surface of the matrix. So, we used the wear debris to be analyzed. Fig. 10 is the XRD pattern of the wear debris shown in Fig. 9. The diffraction peaks belong to iron titanate (Fe2.25 Ti0.75 O4 ), iron silicate (FeSiO3 ) and a very small amount of Ti3 SiC2 , indicating that the frictional film consisted of cocrystalline or partially cocrystalline iron titanate and iron silicate. Obviously, the Fe2.25 Ti0.75 O4 and FeSiO3 are the crystallization of the oxides of ferrum, titanium and silicon. The forming of them could be related to a mechanical alloying reaction during the impact of asperities between the friction surfaces of Ti3 SiC2 and the steel disk.
Fig. 7. SEM micrographs exhibiting the friction surfaces of the low carbon steel disk for the sliding speed and the normal pressure: (a) 5 m/s and 0.8 MPa, and (b) 20 m/s and 0.8 MPa.
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Fig. 9. SEM micrograph showing the worn debris collected under the sliding speed of 20 m/s and normal pressure of 0.8 MPa.
Fig. 10. XRD pattern of the wear debris shown in Fig. 9.
4. Discussions It has been shown that the oxide film is soft and flowable during the sliding friction [10–12]. Such a film would play an antifriction part in the friction surfaces, while the speed and pressure-dependent behaviors of the friction coefficient could be related to the change in covering percentage of the oxide film in the friction surface. It is conceivable that a good oxide film with a higher percentage of coverage results in a smaller coefficient of friction, and a poor oxide film with a lower percentage of coverage results in a larger friction coefficient. Indeed, there was such a corresponding relation between the coefficient of friction and the percentage of coverage. In the cases of the medium speeds of 20 m/s and 40 m/s, the increased percentages of coverage resulted in the reducing of the coefficients of friction with increasing the normal pressure. In the case of the higher speed of 60 m/s, the reducing percentage of coverage resulted in the increasing of the coefficient of friction with increasing the normal pressure. As the same causal relation, in the case of the lower sliding speed of 5 m/s, the larger coefficient of friction was due to the lack of the oxide film. It is worth to indi-
cate that the coefficient of friction as high as 0.83 measured by El-Raghy et al. [16] could be attributed to the poor friction surfaces of the Ti3 SiC2 as well as the friction counterpart. In fact, there were almost no observable films existing in the friction surfaces. The changing behaviors of the wear rates of Ti3 SiC2 are also related closely with the existing of the oxide film. For the lack of the oxide film, the Ti3 SiC2 friction surface for the low sliding speed of 5 m/s would be harder than the steel disk, and consequently the friction surface of the steel disk was scratched remarkably as shown in Fig. 6(a). Clearly, in this case, the lower wear rate of Ti3 SiC2 was at the price of the severe wear of the steel disk. As the soft oxide film is formed and completed with increasing the normal pressure, the hardness relation between the Ti3 SiC2 and the steel disk was changed for the medium speeds of 20 m/s and 40 m/s. Since the softer oxide film would be readily worn, the wear rate of Ti3 SiC2 was increased accordingly. There would be more oxides generated at the higher sliding speed of 60 m/s due to the higher frictional temperature, but they were difficult to be maintained in the friction surface when the normal pressure was larger, due to the crowding out effect of the friction surfaces during the sliding friction process. Consequently, the wear rate of Ti3 SiC2 increased fast. In addition, the change in the viscosity of the oxide film could also be an influencing factor. The viscosity would be reduced with increasing the sliding speed and/or the normal pressure, since the frictional temperature in the friction surface rises. Then the oxide film is difficult to be maintained for the larger sliding speed and normal pressure could be related with the change of the viscosity as well. Some literatures [24–26] have shown that Ti3 SiC2 can be oxidized at a certain temperature in air, and forms a dense, adhesive and layered scale consisting of SiO2 and TiO2 on the surface of Ti3 SiC2 . Essentially, when two surfaces slide together, most of the work done against friction is turned into heat [17–19]. The resulting rise in temperature may modify the mechanical and metallurgical properties of the sliding surfaces, and it may make them oxidize or even melt; all these things influence the friction of coefficient and the rate of wear. Lim et al. [17] have developed wear mechanism and temperature maps for different materials in dry-sliding contact, where bulk surface (Tb ) and flash (Tf ) temperature are plotted as isothermal contours on normalized sliding velocity and contact pressure axes. The oxidation occurring on the Ti3 SiC2 friction surface may also due to the generating of frictional heat, which is proportional to the friction work μLV, where L is the normal load, V the sliding speed, and μ is the coefficient of friction [27]. Thus, the rate of generating oxides is an increasing function of the L, V and μ, and hence the larger L and/or V as well as μ the larger generating rate of the oxides in unit time. On the other hand, simultaneously with the generating, the oxides would be consumed continuously from the friction surface as wear, and the consuming rate in unit time would be also an increasing function of the L and V. Hence, the larger L and/or V the larger consuming rate. The oxide film can be formed on the friction surface, only when the generating rate is larger than the consuming rate. However, the thickness of the oxide film is limited by the viscosity of the oxides during fric-
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tion as well as the roughness of friction surface, cannot exceed a characteristic thickness, and the redundant oxides would be removed from the friction surface as the wear debris. Therefore, the oxide film can be maintained at the friction surface, if only the generating rate is equal to the consuming rate when a steady oxide film has been formed. This means that a larger generating rate could result in a larger wear rate. The oxide film cannot be formed, or a formed oxide film will be damaged, if the generating rate is smaller than the consuming rate. These states can be characterized as: ⎧ ⎨ ωg > ωc , the oxide film can be formed; ωg = ωc , the oxide film can be maintained; (1) ⎩ ωg < ωc ,
the oxide film cannot be formed or will be damaged.
where ωg and ωc are the generating rate and the consuming rate, respectively. The state could be changed with the sliding speed and/or the normal pressure, since the ωg and ωc are different function of the L and V. The states for the low sliding speed of 5 m/s and the normal pressures applied from 0.1 MPa to 0.8 MPa can be classified to ωg < ωc . However, the states for the medium speeds of 20 m/s and 40 m/s could be changed from ωg < ωc to ωg = ωc with increasing the normal pressure. Particularly, the states for the higher sliding speed of 60 m/s could be changed from ωg > ωc to ωg = ωc and further to ωg < ωc with the increase in normal pressure. These changes will be further characterized in other paper. 5. Conclusions (1) The friction and wear behaviors of Ti3 SiC2 sliding dryly against low carbon steel are dependent strongly upon the presence of a frictional oxide film on the Ti3 SiC2 friction surface. (2) The frictional film consists of the oxides of Ti, Si and Fe, and its percentage of coverage at the Ti3 SiC2 friction surface is increased with increasing the sliding speed and/or normal pressure in a limited range. (3) A larger generating rate of the oxides increases the covering percentage of the oxide film. In results, the coefficient of friction is reduced due to the antifriction effect of the oxide film, but the wear rate of Ti3 SiC2 is increased due to the consuming of the oxides. (4) The oxide film can be formed and continuously maintained for the medium sliding speeds, but maintained with difficulty when the sliding speed is high. Almost no oxide film can be formed when the sliding speed is low. Acknowledgements This work was supported by the National Science Foundation of China (NSFC) under Grant No. 50472045, the National High Technology Program (863 Program) of China under Grant No. 2003AA332080 and the Excellent Doctor Fund of Beijing Jiaotong University under Grant No. 48014.
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