Microstructure and dry sliding wear behavior of hot-extruded AlSiCuPb bearing alloys

Microstructure and dry sliding wear behavior of hot-extruded AlSiCuPb bearing alloys

Materials Characterization 48 (2002) 347 – 357 Microstructure and dry sliding wear behavior of hot-extruded AlSiCuPb bearing alloys J. Ana,b,*, Y.B. ...

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Materials Characterization 48 (2002) 347 – 357

Microstructure and dry sliding wear behavior of hot-extruded AlSiCuPb bearing alloys J. Ana,b,*, Y.B. Liub, Q.Y. Zhanga, C. Donga a

State Key Laboratory for Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116023, People’s Republic of China b Department of Materials Science and Engineering, Nanling Campus of Jilin University, Changchun 130025, People’s Republic of China Received 20 May 2002; accepted 20 July 2002

Abstract The microstructures, mechanical properties and dry wear behavior of hot-extruded AlSiCuPb bearing alloys have been studied. It showed that the hot-extruded AlSiCuPb alloys possessed microstructures with uniformly distributed lead particles and fine broken grains of silicon, and exhibited further improved mechanical properties in comparison to the stir cast ones. With increasing the lead content, the wear rate decreased greatly and wear rateload curves took different form for extruded bearing alloys containing 20% and 25% lead. Optical observation revealed the reason was formation of a black compact film of lubricant covering almost the entire worn surface of specimens at highly applied load level. This film is a mixture of different constituents containing the elements Al, Si, O, Fe and Pb. D 2002 Elsevier Science Inc. All rights reserved. Keywords: Hot extrusion; ASiCuPb bearing alloys; Wear behavior

1. Introduction Hypermonotectic Al – Pb alloys suffer from inherent metallurgical problems due to the segregation that arises from the large miscibility gap in the liquid state and the wide difference in densities of aluminum and lead. These problems can be overcome by employing unconventional fabrication techniques such as stircast, spray deposition, powder metallurgy, Maran*

Corresponding author. Department of Materials Science and Engineering, Nanling Campus of Jilin University, Renmin Street 142, 130025 Changchun, People’s Republic of China. Tel.: +86-431-5705874; fax: +86-4315705876. E-mail address: [email protected] (J. An).

goni-convection casting and mechanical alloying, thereby rendering these alloys viable as advanced bearing materials for automotive applications [1 – 7]. Using these techniques, a large amount of lead (over 20 wt.%) can be incorporated in an aluminum matrix, with the subsequent solidified Al – Pb alloys exhibiting a microstructure of fine soft lead particles homogeneously dispersed in the mechanically strong aluminum matrix. Instead of the standard bearing materials like Cu – Sn – Pb or Al – Sn alloys, today’s higher engine temperatures require the use of materials whose soft phase has a higher melting temperature and which have an appreciably higher volume content of dispersed soft phase. Generally, this can only be achieved using Al – Pb alloys of hypermonotectic composition [8]. Most of the work to date on these

1044-5803/02/$ – see front matter D 2002 Elsevier Science Inc. All rights reserved. PII: S 1 0 4 4 - 5 8 0 3 ( 0 2 ) 0 0 2 8 8 - 7

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Fig. 1. XRD patterns for (a) as-cast and (b) hot extruded Al – 4Si – 1Cu – 20Pb alloys.

alloys has been focused on the microstructure and characteristics of fraction and wear of as-cast alloys. However, in these alloys, especially those formed by

stircast and rheocast, a large number of casting defects exist in the ingots produced by powerful stirring during fabrication in atmosphere [2,3,9,10].

Fig. 2. Light optical micrographs of AlSiCuPb alloys: (a) as-cast Al – 4Si – 1Cu – 15Pb alloy; (b) as-cast Al – 4Si – 1Cu – 25Pb alloy; (c) as-cast Al – 4Si – 1Cu – 25Pb alloy, showing silicon particles; (d) hot-extruded Al – 4Si – 1Cu – 25Pb alloy.

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Fig. 3. Microstructure of hot-extruded Al – 4Si – 1Cu – 25Pb. SEM micrograph (a) and the corresponding dot-map images for Al (b), Si (c) and Pb (d).

Porosity as high as over 10% and the presence of acicular eutectic silicon particles inevitably have significant negative effects on the mechanical properties of AlSiPb alloys, and obscure the real excellent friction and wear properties. Of more significance is the fact that the aluminum-based bearing alloys usually are hot-extruded into strips, then bonded to steel sheet, forming a bimetallic component before the bearing surface is machined [11,12]. Therefore,

there is much interest in the mechanical properties of these hot-extruded Al – Pb alloys, especially their wear and friction characteristics, in their states close to those under working condition. Reports on investigations into the microstructure and friction and wear characteristics of hot-extruded AlSiCuPb alloys are rare and more work is required to gain a better understanding of the wear mechanisms involved. The main purpose of the present

Table 1 Mechanical properties, density and porosity of the as-cast and the hot-extruded Al – Si – Cu base alloy and Al – Si – Cu – Pb alloys Alloy composition

Hardness (HRB)

Ultimate tensile strength (MN m  2)

Elongation to fracture (%)

Density (g cm  3)

Porosity (%)

As-cast Al – 4Si – 1Cu As-cast Al – 4Si – 1Cu – 10Pb As-cast Al – 4Si – 1Cu – 15Pb As-cast Al – 4Si – 1Cu – 20Pb As-cast Al – 4Si – 1Cu – 25Pb Hot-extruded Al – 4Si – 1Cu Hot-extruded Al – 4Si – 1Cu – 10Pb Hot-extruded Al – 4Si – 1Cu – 15Pb Hot-extruded Al – 4Si – 1Cu – 20Pb Hot-extruded Al – 4Si – 1Cu – 25Pb

45 42 39 37 34 67 65 59 54 40

160 124 105 82 79 185 174 169 162 156

4.3 2.7 2.1 1.3 0.5 21.4 18.1 15.0 14.2 12.3

2.61 2.58 2.47 2.57 2.51 2.67 2.62 2.91 3.08 3.11

4.3 12.5 19.7 20.1 25.2 2.14 4.3 5.4 4.1 7.6

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Fig. 4. The variation of wear rate with load for the as-cast and the hot-extruded versions of the base Al – 4Si – 1Cu alloy and the Al – 4Si – 1Cu – 20Pb alloy.

paper is to study changes in the microstructure and mechanical properties of AlSiCuPb alloys before and after hot extrusion, and the effects of microstructure and a lubricant film on the wear behavior against hard surface of steel under dry sliding condition.

2. Experimental details 2.1. Stir casting and hot extrusion A 2.5-kg amount of base alloy, Al – 4%Si – 1.0% Cu – 0.5%Mg – 0.4%Mn – 1.0%Sn, was charged into a crucible kept in a resistance-heated vertical muffle furnace. When the molten melt reached 700 C, the furnace was switched off and preheated baffles were

pushed into the crucible. In the meantime, the desired amount of lead shots was added into the base alloy melt at an appropriate rate, and the melt agitated at 40 rps with a nine-bladed flat stirrer. After stirring for 300 s, the crucible was taken out of the furnace and the turbulent melt poured into a steel mold. The complete details of the procedure are given in two earlier papers [11,12]. The processing yielded 70-mm diameter cylindrical ingots containing 10%, 15%, 20% and 25% lead (by weight). These were then machined into specimens of 20-mm diameter and 25-mm-thick. Finally, they were hot-extruded at a rate of 4:1 at 400 C into bars 10 mm in diameter. Specimens were prepared from the ingots and the hot-extruded bars for microstructural investigation using standard grinding and polishing procedures,

Fig. 5. The variation of wear rate with load for the hot-extruded AlSiCuPb alloys.

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Fig. 6. The variation of the coefficient of friction with load for the hot-extruded base Al – 4Si – 1Cu alloy and the AlSiCuPb alloys.

and the grain and lead particle sizes subsequently estimated using the LOM and SEM. The ingots and hot-extruded bars were also machined into standard tensometer specimens, and tension tests were performed using an Instron tension-testing machine. 2.2. Wear testing

Pin specimens were weighed both before and after testing on a METTLER-TOLEDO MXD-1000 single pan electrical balance that gave readings to 0.1 mg. From these measurements, it was possible to calculate the average volume of material lost due to wear over a travel distance of 376.8 m.

Friction and wear testing was conducted with a pin-on-disc type machine. All friction and wear tests were carried out under dry sliding conditions at a relative humidity of about 60% and an ambient temperature of 22 C. Pin wear test specimens of f6  12 mm were machined from both ingot and hot-extruded bar material, and included various lead contents. The disc was 70 mm in diameter and made of high carbon chromium steel hardened to a hardness of 57 HRC. The disc rotational speed was kept constant at 78.5  10  2 ms  1 throughout the investigation. The flat surfaces of both the pin specimens and the steel disc were ground to a constant surface finish of about 0.4 mm. Specimens were thoroughly degreased by acetone and dried before the commencement of each wear test. During the tests, the load was increased generally in 20 N increments until the maximum possible load could be applied or until seizure took place (indicated by abnormal noise and vibration in the pin-disc assembly). The coefficient of friction, m, was then calculated from the friction moment to an accuracy of ± 0.01 J (recorded from the signal from strain gauges mounted on the torque tube in the testing machine) using the following formula: m ¼ M =RN

ð1Þ

where M is the friction moment, R the radius of wear track (0.03 m) and N the normal load.

Fig. 7. Light optical micrographs of the morphology of the black lubricating films on worn surfaces of Al – 4Si – 1Cu – 20Pb at different loads: (a) 120 N and (b) 200 N.

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The worn surfaces of the wear pins were examined with a Nikon light optical microscope (LOM), a JEOL 8600 scanning electron microscope (SEM) with an attached energy dispersive X-ray analyser (EDX) and a VG ESCALAB Mk II X-ray photoelectron spectroscope (XPS). A Rigaku X-ray diffractometer (XRD) was used to analyze the phase constituents of the test materials and wear debris. 3. Results and discussion 3.1. Effect of hot extrusion on microstructures and mechanical properties Fig. 1 shows the XRD analyses of the as-cast and the hot-extruded AlSiCuPb alloys. These indicate that

the phase constituents in both alloys are an aluminum-rich phase (a-Al), a lead-rich phase and a silicon phase. No new phases were produced during the hot extrusion processing. Fig. 2 consists of LOM photographs of as-cast and hot-extruded AlSiCuPb alloys containing various lead contents. In the case of the ascast material, the aluminum-rich phase appears in the form of white dendrite, while the lead-rich phase was in the form of dark spherical or nearly spherical particles, uniformly distributed in the matrix of the primary aluminum phase. The average size of the lead particles increased with increasing lead content, from 50 mm at 15 wt.% Pb to 80 mm at 25 wt.% and Pb. In Fig. 2(c), a higher magnification view of the specimen shown in Fig. 2(b), coarse acicular eutectic silicon particles, of average length of about 10 mm,

Fig. 8. XPS spectra of a worn surface of Al – 4Si – 1Cu – 20Pb at a load of 200 N: (a) Si 2p and (b) Pb 4f7/2.

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can be observed continuously distributed along the aaluminum boundaries. The hot extrusion process changed the microstructure considerably, as can be seen in Fig. 2(d). The structure has become more compact, and the grain size of the a-Al greatly reduced. The average sizes of the lead particles decreased by an order of magnitude from those observed in the as-cast material, ranging from 6 to 8.5 mm as the lead content increased from 15 to 25 wt.%. To better understand the changes in silicon and lead particles, aluminum, silicon and lead mapping of the hot-extruded AlSiCuPb alloy is shown in Fig. 3. It can be seen that acicular eutectic silicon particles were broken into small grains with an average size of 5 mm and distributed on the original border of a-Al. The size of lead particles decreased considerably, some of them similar to those of the broken silicon grains. The great difference in microstructures between the as-cast and the hot-extruded conditions suggests a corresponding difference in mechanical properties. These can be seen in Table 1, which lists the mechanical properties of the as-cast and the hot-extruded AlSiCuPb alloys as well as those of the base AlSiCu alloys. All the properties of alloys, especially those of AlSiCuPb alloys, were improved significantly by the hot extrusion processing. This can be attributed to combination of two main factors: (1) Reduction of porosity. Vigorous stirring of the melt at a high rotating speed induced a large amount of porosity. The porosity in the as-cast AlSiCuPb alloys varied from 12% to 25% and their properties rapidly deteriorated. However,

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hot extrusion reduced the porosity to within a range of 4 – 7%. (2) Refinement of the microstructure. This is reflected in decreases in the size of both the a-Al grains and the lead particles, the breaking up of the needle-shaped eutectic silicon into small grains and an even distribution throughout. 3.2. Wear behavior In order to understand the effect of hot extrusion on the tribological behavior of AlSiCuPb alloys, a comparison was made between the Al – 4Si – 1Cu – 20Pb alloy and the base AlSiCu alloy. Fig. 4 illustrates the changes in wear rate for both the base alloy and the Al – 4Si – 1Cu – 20Pb alloy before and after hot extrusion. The resistance to wear for both alloys improved remarkably following hot extrusion. This is especially notable in the resistance to seizure for the Al – 4Si – 1Cu – 20Pb alloy, where the seizure load increased from 180 to 300 N after hot extrusion. Again, the reason for the superior performance of the hot-extruded material is due to the combination of significant changes in the microstructure, including porosity, the grain size and morphology of a-Al, lead-rich phase and silicon phase after hot extrusion. Fig. 5 shows the variation with load of the wear rate for hot-extruded alloys. It is clearly revealed that the wear rate decreased significantly with increasing lead content. There is also a difference in the forms of the wear curves, depending on the lead content. The wear rate curves of the base alloy and the AlSiCuPb alloys containing 10 and 15 wt.% lead maintain an essentially linear upward slope until just prior to

Fig. 9. XRD analysis of the wear debris of Al – 4Si – 1Cu – 20Pb at 200 N.

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Table 2 The composition (at.%) of worn and unworn surfaces of Al – 4Si – 1Cu – 20Pb at 200 N Element

Unworn surface

Worn surface

O Al Si Cu Pb Fe

46.40 35.26 15.67 0.18 2.37 0.11

54.68 23.53 15.96 0.28 5.33 0.22

seizure taking place. According to Archard [13], the wear W is directly proportional to the load P under dry sliding conditions as given by the following equation: W ¼

KPs pm

ð2Þ

where s is the sliding distance, pm is the hardness of the materials under test and K is a constant, which has a larger value for severe wear than for mild wear. However, in contrast, the wear curves for the AlSiCuPb alloys containing 20 and 25 wt.% lead exhibit linearity only up to a certain point (around 200 N for the former and 120 N for the latter). At higher loads, the curves deviate from linearity until the final onset of seizure. This phenomenon illustrates that different wear mechanisms are in place for these higher lead content alloys than those present with the base alloy and the lower lead content alloys. The evident change in wear mechanism is also presented in curves of the coefficient of friction with load shown in Fig. 6. Increases in the lead content result in decreases in the friction coefficient under the same load, with the alloys containing 20% and 25% lead exhibiting the lowest coefficients of friction These curves are indicative of the existence of some kind of lubricating effect. The ‘‘plateau’’ characteristic in the wear rate curve at high loads is very similar to that observed in Al-based particle-reinforced composites [14 – 16]. The SiC or Al2O3 particles in these composites helps to form a mechanical mixed layer (MML) on the worn surface during the sliding process, and the wear rate subsequently increases very slowly with the applied load, displaying an almost plateau-like effect. The MML plays an important role in the wear of SiC particle-reinforced aluminum composites. Venkataraman and Sundararajan [17] performed hardness measurements on the MML. These showed that the MML formed on the worn surface of SiC-reinforced aluminum composite was substantially harder than the bulk material

because it contained a fine mixture of Fe, Al and SiC phases. The absence of a MML at the worn subsurface of the matrix alloy was considered to be the reason that the wear resistance of aluminum matrix alloy was worse than that of composite. In the present investigation, a black lubricating film was observed on almost the entire worn surface of the AlSiCuPb bearing alloys and was directly related to the occurrence of the plateau in the wear rate curves at the higher loads. Macroscopic observation revealed that the morphology of the worn surface varied with load. The black film was readily apparent on the worn surfaces of specimens containing 20% and 25% lead tested at the highest applied loads. The film played a large role in reducing the friction coefficient and improving the resistance to wear and seizure. In the case of Al – 4Si – 1Cu – 20Pb alloy, when the load was less than 80 N, black powder adhered to worn surface. At loads higher than 100 N, the powder disappeared but a black film began to be seen in some local area on the worn surface. With the increasing load the area covered by black film increased gradually. When the load increased to a higher level (more than 220 N), the black film covered almost the entire worn surface. The results of macroscopic observations corresponded perfectly to the plateau in the wear rate curve and the lowest stage of friction coefficient, and a similar corresponding relationship occurred in the case of the Al – 4Si – 1Cu – 25Pb alloy. LOM microstructural analysis gave further proof of the existence of the black lubricating film, and revealed that at low loads the film appeared light and thin, and only partially covered the substrate. This was in contrast to the observations at high loads where a dark, thick film covered almost the entire substrate. The variation with load of the extent of coverage by the lubricating film is illustrated in Fig. 7. The chemical nature of the lubricating film was investigated using XPS analysis of the worn surface of an Al – 4Si – 1Cu – 20Pb specimen. The analysis revealed that the aluminum, silicon and lead existed mainly in compound states. The aluminum was found to be present as Al2O3 and, in addition, there appeared to be a small amount of an iron component containing Fe2O3. However, the exact silicon and lead compounds could not be determined. According to Fig. 8(a), the Si (2p) binding energy was 101.2 eV. This is probably indicative of the presence of a silicate, based on available literature data (e.g., Refs. [18,19]). Fig. 8(b) shows the binding energy of the lead component to be 139.2 eV, midway between the 136.9 eV of Pb (4f7/2) and the 141.7 eV of Pb (4f5/2),

Fig. 10. Worn surfaces of the Al – 4Si – 1Cu – 20Pb at different applied loads: (a) 60 N, (b) 140 N, (c) 200 N, (d) 240 N, (e) 280 N, (f) deformation in the subsurface, 280 N, (g) crack in the subsurface, 280 N and (h) 300 N.

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at approximately the binding energy associated with PbO. Thus, a possible silicate may be a-lead silicate (Pb4SiO6). XRD analysis of the wear debris (which contains constituents of the lubricating film) (Fig. 9) shows that the debris contains Pb4SiO6, thereby supporting this theory. Others have also found this compound in the wear debris of Al – Pb alloys at the similar load conditions [20]. Thus, based on the analysis results, it is postulated that the black lubricating film is a mixture of Al2O3, Fe2O3 and Pb4SiO6. These results also indicate that the film of lubricant actually is the product of several reactions: the lead, aluminum and silicon in the subsurface reacted with oxygen in atmosphere and the lead, silicon and oxygen reacted each other under high contact pressure and surface temperature. This film is different from the pure lead film that has been considered by many workers [2 – 5,7,10]. Table 2 shows the surface composition obtained by the XPS analysis of the worn and unworn surfaces of an Al – 4Si – 1Cu – 20Pb specimen. This also reflects the increase in Pb and O concentrations following the formation of the lubricant film. The results in this paper revealed that no pure lead was found in the film, and that the lead existed in the form of a compound and became an effective constitute of mixed lubricating film. This may be attributed to increased friction heating at loads higher than those applied to the as-cast Al – Pb alloys in by other studies [2 – 4,7,10]. This film at the interface of mating surfaces restricts metal – metal contact, and hence resistance to wear and friction is improved. Further investigations into the constituents of this film are needed. The beneficial effect of the mixed film of lubricant on the wear of the AlSiCuPb alloys is reflected in the change in the morphology of the worn surfaces as observed in SEM studies. At a low load of 60 N, the worn surface appeared smooth and consisted of small grooves (Fig. 10(a)), and some black powder was present at the bottom. X-ray examination of the wear debris from this stage clearly showed the presence of lead oxide and alumina, indicating that oxidative wear was the main mechanism. In the load range of 80 – 200 N, the characteristics of worn surface were similar, the worn surface consisting of grooves and large shallow dimples (Fig. 10(b)). The surface and cross-section SEM images of the worn samples show that, within the range of 200 – 280 N, the wear surface appeared smooth, due to the formation of the compact black lubricating film that covered almost the entire worn surface (Fig. 10(c) and (d)). A few craters were also found in some regions in the surface. With increasing load, the rough area of crater region increased (Fig. 10(e)) and apparent plastic deformation occurred in the subsurface region. This led to the formation of cracking (Fig. 10(f) and (g)), indicating

that the crack initiation and growth was mainly in the subsurface. Thus, at this point, delamination wear is the main mechanism. The presence of the lubricant film determines the delamination wear mechanism in AlSiCuPb alloys, because when the film covers almost the entire wear surface it effectively restricts metal-to-metal contact between pin and disc. This prevents the occurrence of adhesive wear as early as with the base alloy. The wear rate in this region increased very slowly with the applied load, almost reaching a plateau. A probable reason is that the propagation of subsurface cracking requires the attainment of a certain stress condition, and the crack can be kept relatively stable in a certain load range, resulting in a relatively constant wear rate. This phenomenon was also observed in the SiC particlereinforced A356 and spray-deposited AlCuMn alloy [16]. Fig. 10(h) presents the worn surface at the load of 300 N, and shows many cracks in the rough region where a great mount of material transfer from the pin to the disc occurred. This is typical of adhesive wear.

4. Conclusions 1. Hot extrusion considerably improved the microstructures and mechanical properties of stircast AlSiCuPb alloys and greatly decreased the porosity. 2. The hot-extruded AlSiCuPb alloys demonstrated better wear resistance than the base alloy. The wear rate decreased with lead content, with the effect being most prominent with the alloys containing 20% and 25% lead. 3. The presence of a plateau phenomenon in the wear rate curves of the higher lead content alloys is attributed to a film of lubricant covering almost the entire worn surface. The film was determined to be a mixture of different constituents containing Al, Si, O, Fe and Pb.

Acknowledgements The authors thank the Research Fund for the National Science Foundation of China.

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