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Comparative research on wear characteristics of spheroidal graphite cast iron and carbon steel
Wear 274–275 (2012) 84–93
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Wear journal homepage: www.elsevier.com/locate/wear
Comparative research on wear characteristics of spheroidal graphite cast iron and carbon steel M.X. Wei b , S.Q. Wang a,b,∗ , X.H. Cui b,∗∗ a b
School of Environment and Materials Engineering, Yantai University, 264005 Yantai, China School of Materials Science and Engineering, Jiangsu University, 212013 Zhenjiang, China
a r t i c l e
i n f o
Article history: Received 27 December 2010 Received in revised form 2 August 2011 Accepted 8 August 2011 Available online 25 August 2011 Keywords: Sliding wear Steel Cast iron Tribochemistry Electron microscopy
a b s t r a c t Wear characteristics of a spheroidal graphite cast iron and a carbon steel were studied under atmospheric conditions at 25–400 ◦ C. The spheroidal graphite cast iron presented obviously different wear behaviors from the carbon steel, which may be attributed to the presence of graphite. With an increase of ambient temperature, tribo-oxides of carbon steel substantially increased and its substrate softened, thus severe wear, oxidative mild wear, oxidative wear and extrusive wear took turns to prevail. However, compared with carbon steel in the same case, tribo-oxides were markedly reduced in the spheroidal graphite cast iron, thus oxidative mild wear and oxidative wear did not appear due to the lack of oxides. It is suggested that less tribo-oxides in the spheroidal graphite cast iron may be attributed to the reduction of graphite to tribo-oxides during sliding. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fe–C alloys are oxidized due to their thermodynamically instability in air. During sliding, especially at a higher sliding velocity, normal load and/or ambient temperature, oxidation is intensified and tribo-oxides form on worn surfaces [1,2]. The development of a tribo-oxide layer would markedly affect wear behaviors and mechanisms of Fe–C alloys [1–4]. Hence, oxidative wear plays an important role in the wear process, and the research on oxidative wear requires more attention. Steels and graphite cast iron as important Fe–C alloys are popularly adopted in various engineering applications. Extensive research on oxidative wear of steels has been carried out and the wear behavior and mechanism of steels are clearly understood. However, there has been less study of graphite cast iron and hence the wear behavior and mechanism of graphite cast iron are less clear. Quinn et al. studied the oxidative wear of ferroalloys under mild wear conditions at room- and slightly elevated temperatures and proposed a mild oxidative wear model [1]. Wilson et al. studied the influence of wear oxide debris particles on reducing wear and
∗ Corresponding author at: School of Environment and Materials Engineering, Yantai University, 264005 Yantai, China. Tel.: +86 5356706039; fax: +86 5356706039. ∗∗ Corresponding author. Tel.: +86 51188797618; fax: +86 511 8791919. E-mail addresses: shuqi [email protected] (S.Q. Wang), [email protected] (X.H. Cui). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.08.015
proposed another important oxidative wear model [2]. The wear oxide debris particles have been shown to form nano-structured glaze layers at a higher temperature to enhance wear resistance [3]. They shared the same viewpoint that the tribo-oxides play an important role in oxidative wear; tribo-oxides can form on worn surfaces and prevent the metal–metal contact and adhesion, thus reducing wear rate. Most subsequent studies on oxidative wear originated from Quinn et al. s work, and tribo-oxides were found to reduce wear by forming a protective layer [4–6]. In most cases, oxidative wear means a mild wear. However, when the matrix does not have enough strength to support tribo-oxide layer, tribo-oxides may not reduce wear and tribo-oxides were reported not to prevent wear in some studies [7,8]. Inman et al. pointed out that when no glaze formed due to worse sinterablity of oxides, severe wear prevailed [9]. Wang et al. further studied oxidative wear of steels in severe testing conditions and pointed out that tribo-oxides did not always prevent wear, especially when the mild-severe wear transition occurred in oxidative wear [10,11]. Additionally, we suggested that the two types of oxidative wear should be distinguished and termed oxidative mild wear and oxidative wear [12]. Inman et al. also suggested a division of oxidative wear/mild wear into abrasive mild wear and protective mild wear as well as severe wear into standard severe wear and abrasion-assisted-severe-wear [9,13]. Abrasionassisted-severe-wear meant that in some cases tribo-oxides did not prevent wear, on the contrary accelerated wear. Riahi and Alpas [14] produced the first wear map for grey cast iron, where wear mechanisms were classified into ultra-mild, mild
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Table 1 Chemical compositions of the steels and spheroidal graphite cast iron used in this study. Materials
Element (wt.%) C
800-2 1045 D2
3.40–3.80 0.42–0.50 1.45–1.70
Cr
Mo
V
Si
Mn
S
P
– <0.25 11.00–12.50
– 0.46 0.40–0.60
– 0.25 0.15–0.30
2.40–3.00 0.17–0.37 ≤0.40
0.50–0.80 0.50–0.80 ≤0.40
≤0.05 <0.04 ≤0.030
≤0.05 <0.04 ≤0.030
and severe wear. In ultra-mild wear, a compacted oxide film covered contacting surfaces; also mild wear was defined as oxidative wear. Severe wear appeared when oxide layer did not stably form due to delamination of tribo-oxides. They also pointed out that the excellent wear resistance of grey cast iron during dry sliding under low loading conditions was attributed to graphite flakes and formation of a graphite film. Liu [15] studied the influence of surface oxidation on the wear resistance of cast irons and found that oxidation increased wear rate when cast iron was of high strength and hardness (high resistance to adhesion and microcutting); but when cast iron was of low strength and hardness (low resistance to adhesion and micro-cutting), oxidation reduced wear rate. Whether oxidation increased or reduced wear depended on graphite morphology, microstructure and hardness of matrix; they were considered to be key factors deciding on whether wear rate increased or decreased due to oxidation. Hirasata and Hayashi [16] studied friction and wear of several kinds of cast irons under severe sliding conditions and found that the wear rates were strongly dependent on hardness variation. No tribo-oxides were noted to exist, only a plastically deformed layer. Prasad [17] studied the effect of microstructural features and test conditions on the wear of cast iron. He pointed out that the governing phenomena of wear in the alloyed cast iron might be due to adhesive wear which generated oxide particulates that further led to abrasive wear. When abrasive wear mode was prominent, the wear took place by plowing or microcutting of the hard debris particulates. The main operating materials removal mechanism was spalling associated with substantial microcracking and with a limited extent of adhesion and abrasion during sliding. It is clear that the formation of tribo-oxides and their role in the wear of graphite iron have been not clarified until now. In order to obtain a better understanding of these, the dry sliding wear behavior of spheroid graphite iron was studied in comparison with that of carbon steel. They possessed the same microstructure and hardness, the only significant difference being the presence of spheroid graphite. The amount and morphology of tribo-oxides were examined from worn surfaces and subsurfaces; the tribooxides and their role in wear were explored, focused on the influences of graphite on the formation of tribo-oxides and wear behavior.
2. Experimental procedure 2.1. Materials and heat-treatment Commercial 1045 carbon steel and 800-2 spheroidal graphite cast iron were selected as pins of 6 mm diameter and 12 mm length, while D2 steel was choosed as disc ‘counter-surfaces’ of 70 mm diameter and 8 mm thickness. Their chemical compositions are listed in Table 1. 1045 steel was austenitized at 840 ◦ C for 20 min, and then cooled in water, finally tempered at 400 ◦ C for 2 h to achieve tempered martensite. The spheroidal graphite cast iron was austenitized at 900 ◦ C for 20 min, cooled in oil, then tempered at 500 ◦ C for 2 h, attaining tempered martensite and spheroidal graphite. The carbon steel and spheroidal graphite cast iron presented the same hardness (40 HRC), and their microstructures are shown in Fig. 1. Commercial D2 steel was austenitized at 1150 ◦ C for 20 min, oil quenched and tempered three times at 550 ◦ C for 2 h to achieve a hardness of 60 HRC. 2.2. Wear test An MG-200 type pin-on-disc high temperature wear testing machine was employed to perform wear tests for a distance of 1.2 × 103 m at a velocity of 1 m/s within an ambient temperature range of 25–400 ◦ C and a load range of 50–200 N. Wear was determined by measuring the mass loss of the pin specimen using an electronic balance (0.00001 g). Mass loss was converted into volume loss using the density of the materials (7.85 and 7.30 g/cm3 for the carbon steel and spheroidal graphite cast iron, respectively). The wear rate was defined as volume loss by unit distance (mm3 /mm). Data for each wear value was calculated from the average of three tests. Moment of friction force between the pin and disk during wear was measured by a sensor and automatically recorded by a personal computer, then transformed to friction coefficient. 2.3. Evaluation techniques Phases on worn surfaces were identified by a D/Max-2500/pc type X-ray diffractometer (XRD). Morphologies of worn surface and subsurface were examined by a JSM-7001F type scanning electron
Fig. 1. Heat-treated microstructures of the carbon steel (a) and spheroidal graphite cast iron (b).
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Fig. 2. Wear rate vs. load curves of the carbon steel (a) and spheroidal graphite cast iron (b) sliding at various ambient temperatures.
microscope (SEM). Hardness of pin specimen was measured by an HR-150A type Rockwell apparatus. 3. Results and analysis 3.1. Wear and friction behaviors Fig. 2 shows wear rate vs. load curves of the carbon steel and spheroidal graphite cast iron sliding at various ambient temperatures. The wear rate of the carbon steel at 25 ◦ C increased approximately linearly with increasing load. At 200 ◦ C, the wear rate abruptly decreased as the load was raised from 50 to 100 N, then increased again slightly with further increasing load. The wear rate at 400 ◦ C increased gradually at first with increasing load, then started to rise rapidly once the load surpassed 125 N, exceeding the detection limit of the testing machine (Fig. 2a). At 25–200 ◦ C, the spheroidal graphite cast iron presented almost the same wear rate vs. load curves, only the wear rate at 200 ◦ C was slightly higher than that at 25 ◦ C. However, the wear rate at 400 ◦ C was markedly higher than those at 25–200 ◦ C, and increased sharply once the load surpassed 125 N (Fig. 2b). Comparisons of wear rates between the carbon steel and spheroidal graphite cast iron sliding at various ambient temperatures are shown in Fig. 3. The wear rate of the spheroidal graphite cast iron at 25 ◦ C was lower than that of the carbon steel at loads below 150 N, but higher once over 150 N (Fig. 3a).
At 200 ◦ C, the same phenomenon happened again, only the transition load was 100 N (Fig. 3b). At 400 ◦ C, the wear rate of the spheroidal graphite cast iron was marginally higher than that of the carbon steel at loads below 100 N, and their difference gradually became greater with further increasing load (Fig. 3c). The variation of friction coefficient with sliding distance for the carbon steel and spheroidal graphite cast iron sliding under different conditions is shown in Fig. 4. In most cases, the spheroidal graphite cast iron presented a lower friction coefficient than the carbon steel. An increase in load resulted in a decrease in friction coefficient of the materials at every ambient temperature. The fluctuation of friction coefficient of the carbon steel was slight at 25 ◦ C and under 150 N at 400 ◦ C, enlarged under 100–200 N at 200 ◦ C and under 50–100 N at 400 ◦ C. The spheroidal graphite cast iron was of small fluctuation of friction coefficient in most cases and presented a large fluctuation of friction coefficient merely under 50 N at 400 ◦ C. 3.2. XRD analysis for worn surfaces X-ray diffraction patterns of worn surfaces of the carbon steel and spheroidal graphite cast iron sliding under various conditions are shown in Fig. 5. At 25 ◦ C, only trace tribo-oxides were identified on worn surfaces of the carbon steel. The amount of tribo-oxides gradually increased with increasing ambient temperature from 25 ◦ C to 200 ◦ C; a small amount of tribo-oxides appeared
Fig. 3. Comparisons of wear rate between the carbon steel and spheroidal graphite cast iron sliding at 25 (a), 200 (b) and 400 ◦ C (c).
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Fig. 4. Friction coefficient vs. sliding distance curves of the carbon steel (a)–(c) and the spheroidal graphite cast iron (d)–(f) sliding under various conditions.
at 200 ◦ C (Fig. 5a and b). However, at 400 ◦ C, a large amount of tribo-oxides were generated; the peaks corresponding to oxides significantly surpassed those of iron. But once the load reached 150 N, the amount of tribo-oxides markedly decreased (Fig. 5c). It was observed in the spheroidal graphite cast iron that merely trace tribo-oxides formed at 25–200 ◦ C and a small amount of tribooxides appeared at 400 ◦ C (Fig. 5d–f). Compared with the carbon steel, the amount of tribo-oxides generated on worn surfaces of the spheroidal graphite cast iron was obviously reduced, even at 400 ◦ C.
3.3. SEM morphology of worn surfaces SEM morphologies of worn surfaces of the carbon steel and spheroidal graphite cast iron sliding under various conditions are shown in Figs. 6 and 7, respectively. At 25 ◦ C, obvious adhesive and a trace of abrasive mark were identified in the carbon steel (Fig. 6a and b). In addition, tribo-oxides sparsely scattered on the worn surface under high load, which can be confirmed by XRD results and section morphology of worn surfaces. The amount and area of tribo-oxides increased as the load surpassed 100 N at 200 ◦ C; tribo-oxide layer formed with some delaminated craters and a trace of abrasive mark (Fig. 6c and d). At 400 ◦ C, a smooth tribo-oxide layer with delaminated craters was found across the whole worn surface (Fig. 6e and f). However, at a load of 150 N, any trace of smooth tribo-oxide layers and delaminated craters disappeared, instead only a small amount of fine grooves parallel to the sliding direction and many tiny oxide particles appeared (Fig. 6g). For the spheroidal graphite cast iron at 25–200 ◦ C, adhesive trace appeared without tribo-oxides on worn surfaces (Fig. 7a–d). At 400 ◦ C, although a small, sparse amount of tribo-oxides appeared on the partial worn surface, adhesive was still predominant worn characteristic (Fig. 7e). As the load reached 150 N, the worn surface also became smooth just as happened in the carbon steel, but there were no oxide particles (Fig. 7f).
3.4. Section morphology of worn surfaces and subsurfaces In order to ascertain the formation of tribo-oxides on worn surfaces, section morphologies of worn surfaces and subsurfaces of the carbon steel and spheroidal graphite cast iron sliding under various conditions were investigated, as shown in Figs. 8 and 9, respectively. For the carbon steel at 25 ◦ C, tribo-oxides did not form under a load of 50 N (Fig. 8a), but appeared under a load of 100 N and higher. Such oxides were distributed sparsely on worn surfaces and reached a thickness of less than 3 m (Fig. 8b). At 200 ◦ C, no tribooxide layer appeared was observed under a load of 50 N (Fig. 8c), however, once the load reached 100 N or above, a tribo-oxide layer with a thickness of about 8–10 m existed (Fig. 8d). At 400 ◦ C, a large amount of tribo-oxides formed and the thickness increased to 20–25 m (Fig. 8e). However, as the load reached 150 N, the thickness of tribo-oxide layer substantially decreased to less than 0.5 m (Fig. 8f). This may explain why the peaks corresponding to iron surpassed those of oxides (Fig. 4c). For the spheroidal graphite cast iron, no tribo-oxide layer appeared on worn surfaces at 25 ◦ C (Fig. 9a and b). At 200 ◦ C, there was reduced tribo-oxides formation (compared to carbon steel) with a reduced thickness of less than 3 m (Fig. 9c and d). Due to the presence of graphite in the spheroidal graphite cast iron, rupture seemed to appear readily on the worn surface, as shown in the inserted image of Fig. 9b. At 400 ◦ C, the amount of tribooxides correspondingly increased, and tribo-oxide layer was still thin (less than 5 m) and discontinuous (Fig. 9e and f). At 150 N, the elongated graphite appeared companied by massive plastic deformation in the sub-surface region.
4. Discussion At 25 ◦ C, almost no or trace tribo-oxides appeared on worn surfaces of the carbon steel and spheroidal graphite cast iron, thus metal–metal contact was not avoided. Shear stress resulted from friction force would cause deformation of contacting asperities, leading eventually rupture inside the softer pin resulting in mass
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Fig. 5. X-ray diffraction patterns for worn surfaces of the carbon steel (a)–(c) and spheroidal graphite cast iron (d)–(f) sliding under various conditions.
loss [18,19], thus a severe wear prevailed. Archard [19] proposed an equation to evaluate wear resistance in this regime, where material wear loss is directly proportional to normal load and sliding distance, but inversely proportional to the softer material’s real hardness (the pin specimen in this study). Although the carbon steel and spheroidal graphite cast iron possessed the same original hardness (40 HRC), they presented different wear behaviors, which might be due to the graphite in the spheroidal graphite cast iron. At loads below 150 N, the wear rate of the spheroidal graphite cast iron was slightly lower than that of the carbon steel due to graphite lubrication. However, on application of a 200 N
load, the lubrication effect lessened and even disappeared. In this case, the presence of graphite led to easy fracture of worn surfaces and the wear rate of the spheroidal graphite cast iron thus rapidly increased. For carbon steel, as ambient temperature was elevated to 200 ◦ C, severe wear still operated under 50 N load. With increasing load, more tribo-oxides started to form on worn surfaces due to increased frictional heating. These tribo-oxides did not cover the whole worn surface, and preferentially formed at contacting asperities [10]. When the tribo-oxide layer reaches a thickness of about 10 m, they could prevent metal–metal contact and adhesion,
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Fig. 6. Morphologies of worn surfaces of the carbon steel sliding under various conditions: (a) 50 N, 25 ◦ C; (b) 200 N, 25 ◦ C; (c) 50 N, 200 ◦ C; (d) 200 N, 200 ◦ C; (e) 50 N, 400 ◦ C; (f) 100 N, 400 ◦ C; (g) 150 N, 400 ◦ C.
and protect the underlying material against mechanical damage [10,11]. In this case, the oxidative mild wear regime proposed by Quinn et al. [1] and Wilson et al. [2] prevailed. The wear rate of the carbon steel therefore dropped abruptly on a raising load to 100 N and above this, the wear rate increased slowly and steadily with increasing load. However, tribo-oxides generated on worn surfaces of the spheroidal graphite cast iron were too little and to thin to avoid
metal–metal contact and adhesion, thus the severe wear still dominated. During sliding, the increasing friction heat and high ambient temperature would result in the decrease in the hardness of worn surface and subsurface, thus the wear rate of the spheroidal graphite cast iron at 200 ◦ C further increased compared with that at 25 ◦ C. Therefore, the original material hardness in Archard’s equation [19] should be replaced by the real surface hardness. In severe wear of the spheroidal graphite cast iron at 200 ◦ C, graphite
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Fig. 7. Morphologies of worn surfaces of the spheroidal graphite cast iron sliding under various conditions: (a) 50 N, 25 ◦ C; (b) 200 N, 25 ◦ C; (c) 50 N, 200 ◦ C; (d) 200 N, 200 ◦ C; (e) 50 N, 400 ◦ C; (f) 150 N, 400 ◦ C.
lubrication still functioned and affected wear behavior. Thus, the wear rate of the spheroidal graphite cast iron was lower than that of the carbon steel at loads below 100 N. At 400 ◦ C, a dense glaze tribo-oxide layer with a thickness of about 20–25 m formed over the whole worn surface of the carbon steel, thus the oxidative mild wear prevailed at low load. In such a case, tribo-oxides are considered to play a decisive role in determining wear behavior, irrespective of matrix microstructures. With increasing load of 100–125 N, the matrix was thermally softened probably due to dynamic recovery and recrystallization [10,11]. Not enough mechanical support for the tribo-oxide layer could be provided due to a large scale plastic deformation of the softened matrix. Both tribo-oxide and matrix affect wear behavior, and the oxidative wear, rather than oxidative mild wear began to prevail [10–12]. At 150 N, high compressive and shear stresses caused a massive plastically deformed region due to severe thermal softening of 1045 steel, the uppermost material was liable to extrusion out from the contact surface, resulting in a high wear rate. Although tribo-oxides were inevitably produced at 400 ◦ C, the formed tribolayer of less than 0.5 m was too thin to offer wear protection.
This wear could be classified as extrusive wear as reported by So et al. [20]. The level of tribo-oxides observed on the worn surface of the spheroidal graphite cast iron also increased on raising the temperature to 400 ◦ C, however, there were still not present to levels seen under equivalent conditions with carbon steel. Only a small amount of tribo-oxides partially covered the worn surface and their amount and thickness (less than 5 m) was insufficient to eliminate metal–metal contact completely. It is possible that the tribo-oxides might have reduced wear to some extent, but severe wear was still the dominant wear mechanism in this case. It is worth noting that the lubrication effect of graphite seemed to disappear and so the wear rate of the spheroidal graphite cast iron was slightly higher than that of the carbon steel at loads below 100 N. On application of a 150 N load, the extrusive wear also prevailed for the spheroidal graphite cast iron. Unlike the extrusive wear regime observed with carbon steel, fracture of worn surface at the interface between graphite and matrix was noted. As a result, there was a greater mass loss at high load compared with that of carbon steel, due to lower strength and plasticity of the spheroidal
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Fig. 8. Section morphologies of worn surfaces and subsurfaces of the carbon steel sliding under various conditions: (a) 50 N, 25 ◦ C; (b) 200 N, 25 ◦ C; (c) 50 N, 200 ◦ C; (d) 200 N, 200 ◦ C; (e) 50 N, 400 ◦ C; (f) 150 N, 400 ◦ C.
graphite cast iron. It appears that the graphite cast iron did not facilitate any advantage during elevated-temperature wear due to loss of graphite lubrication. The spheroidal graphite cast iron possessed the same microstructure and hardness as the carbon steel, except for graphite. These different wear behaviors and wear regimes of the spheroidal graphite cast iron should be attributed to the presence of said graphite. During sliding, it is suggested that the graphite in the spheroidal graphite cast iron was gradually exposed, then ground down to carbon powder to cover worn surface. Graphite as a strong reducer would cause the following reactions during elevated-temperature wear: 2C + O2 → 2CO
(1)
4CO + Fe3 O4 → 3Fe + 4CO2
(2)
3CO + Fe2 O3 → 2Fe + 3CO2
(3)
These reactions could occur during sliding, i.e. graphite powder reacts first with oxygen to produce carbon monoxide, which then
reduces the tribo-oxides generated on the worn surface. The levels of tribo-oxide on the worn surface of the spheroidal graphite cast iron may therefore be substantially reduced by these reactions, resulting in the differently observed wear behaviors and wear regimes. Except for its indirect function (on tribo-oxides), graphite would directly affect wear behavior of the spheroidal graphite cast iron. The role of graphite in the spheroidal graphite cast iron can be summarized as follows. Graphite reduced wear as a solid lubricant under the low-load conditions (at 25–200 ◦ C), however, accelerated wear under a high load due to this high load causing fracture of the worn surface at the interface between graphite and matrix. At 400 ◦ C, it is suggested that tribo-oxides were chemically reduced by the described chemical reactions, hence explaining the markedly reduced levels of tribo-oxide and graphite on worn surfaces. Hence, the ability of both graphite and tribo-oxides to reduce wear were substantially impaired. It must be noted that graphite chemically reduced tribo-oxide during wear is a suggested mechanism. This is because graphite would physically affect oxide sintering and layer formation
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Fig. 9. Section morphologies of worn surfaces and subsurfaces of the spheroidal graphite cast iron sliding under various conditions: (a) 50 N, 25 ◦ C; (b) 200 N, 25 ◦ C; (c) 50 N, 200 ◦ C; (d) 200 N, 200 ◦ C; (e) 50 N, 400 ◦ C; (f) 150 N, 400 ◦ C.
during sliding. If graphite worsen oxide sintering, tribo-oxides would be reduced. Thus, the role of graphite during wear needs further investigation.
5. Conclusions (1) With increasing ambient temperature, the wear rate of the carbon steel first decreased (with a minimum at 200 ◦ C), then increased, while the wear rate of the spheroidal graphite cast iron continuously increased. At 400 ◦ C, once a higher load of 150 N was applied, the wear rates of both materials rapidly increased. (2) For the carbon steel, severe wear prevailed at 25 ◦ C and 50 N, at 200 ◦ C. Oxidative mild wear operated under a load ranging from 100 to 200 N at 200 ◦ C and also at 50 N only at 400 ◦ C. Oxidative wear occurred within a load range of 100–125 N at 400 ◦ C. Extrusive wear appeared with further increasing load. (3) For the spheroidal graphite cast iron, tribo-oxides were substantially reduced due to the suggested reduction of tribooxides by graphite. Thus severe wear was always dominant
regime at 25–400 ◦ C, except for a load of 150 N at 400 ◦ C, where extrusive wear with fracture prevailed. (4) Graphite reduced wear under the conditions of lower load (at 25–200 ◦ C), however, accelerated wear under a high load because of fracture at the interface between graphite and matrix. At 400 ◦ C, it is suggested that tribo-oxides were chemically reduced by the described chemical reactions. Hence, the ability of both graphite and tribo-oxides to reduce wear was substantially impaired. Acknowledgements Financial supports of our work by National Natural Science Foundation of China (No. 51071078) and Natural Science Fund of Jiangsu Province (No. BK2009221) are gratefully acknowledged. References [1] T.F.J. Quinn, J.L. Sullivan, D.M. Rowson, Developments in the oxidational theory of mild wear, Tribol. Int. 13 (1980) 153–158.
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Wear journal homepage: www.elsevier.com/locate/wear
Comparative research on wear characteristics of spheroidal graphite cast iron and carbon steel M.X. Wei b , S.Q. Wang a,b,∗ , X.H. Cui b,∗∗ a b
School of Environment and Materials Engineering, Yantai University, 264005 Yantai, China School of Materials Science and Engineering, Jiangsu University, 212013 Zhenjiang, China
a r t i c l e
i n f o
Article history: Received 27 December 2010 Received in revised form 2 August 2011 Accepted 8 August 2011 Available online 25 August 2011 Keywords: Sliding wear Steel Cast iron Tribochemistry Electron microscopy
a b s t r a c t Wear characteristics of a spheroidal graphite cast iron and a carbon steel were studied under atmospheric conditions at 25–400 ◦ C. The spheroidal graphite cast iron presented obviously different wear behaviors from the carbon steel, which may be attributed to the presence of graphite. With an increase of ambient temperature, tribo-oxides of carbon steel substantially increased and its substrate softened, thus severe wear, oxidative mild wear, oxidative wear and extrusive wear took turns to prevail. However, compared with carbon steel in the same case, tribo-oxides were markedly reduced in the spheroidal graphite cast iron, thus oxidative mild wear and oxidative wear did not appear due to the lack of oxides. It is suggested that less tribo-oxides in the spheroidal graphite cast iron may be attributed to the reduction of graphite to tribo-oxides during sliding. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fe–C alloys are oxidized due to their thermodynamically instability in air. During sliding, especially at a higher sliding velocity, normal load and/or ambient temperature, oxidation is intensified and tribo-oxides form on worn surfaces [1,2]. The development of a tribo-oxide layer would markedly affect wear behaviors and mechanisms of Fe–C alloys [1–4]. Hence, oxidative wear plays an important role in the wear process, and the research on oxidative wear requires more attention. Steels and graphite cast iron as important Fe–C alloys are popularly adopted in various engineering applications. Extensive research on oxidative wear of steels has been carried out and the wear behavior and mechanism of steels are clearly understood. However, there has been less study of graphite cast iron and hence the wear behavior and mechanism of graphite cast iron are less clear. Quinn et al. studied the oxidative wear of ferroalloys under mild wear conditions at room- and slightly elevated temperatures and proposed a mild oxidative wear model [1]. Wilson et al. studied the influence of wear oxide debris particles on reducing wear and
∗ Corresponding author at: School of Environment and Materials Engineering, Yantai University, 264005 Yantai, China. Tel.: +86 5356706039; fax: +86 5356706039. ∗∗ Corresponding author. Tel.: +86 51188797618; fax: +86 511 8791919. E-mail addresses: shuqi [email protected] (S.Q. Wang), [email protected] (X.H. Cui). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.08.015
proposed another important oxidative wear model [2]. The wear oxide debris particles have been shown to form nano-structured glaze layers at a higher temperature to enhance wear resistance [3]. They shared the same viewpoint that the tribo-oxides play an important role in oxidative wear; tribo-oxides can form on worn surfaces and prevent the metal–metal contact and adhesion, thus reducing wear rate. Most subsequent studies on oxidative wear originated from Quinn et al. s work, and tribo-oxides were found to reduce wear by forming a protective layer [4–6]. In most cases, oxidative wear means a mild wear. However, when the matrix does not have enough strength to support tribo-oxide layer, tribo-oxides may not reduce wear and tribo-oxides were reported not to prevent wear in some studies [7,8]. Inman et al. pointed out that when no glaze formed due to worse sinterablity of oxides, severe wear prevailed [9]. Wang et al. further studied oxidative wear of steels in severe testing conditions and pointed out that tribo-oxides did not always prevent wear, especially when the mild-severe wear transition occurred in oxidative wear [10,11]. Additionally, we suggested that the two types of oxidative wear should be distinguished and termed oxidative mild wear and oxidative wear [12]. Inman et al. also suggested a division of oxidative wear/mild wear into abrasive mild wear and protective mild wear as well as severe wear into standard severe wear and abrasion-assisted-severe-wear [9,13]. Abrasionassisted-severe-wear meant that in some cases tribo-oxides did not prevent wear, on the contrary accelerated wear. Riahi and Alpas [14] produced the first wear map for grey cast iron, where wear mechanisms were classified into ultra-mild, mild
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Table 1 Chemical compositions of the steels and spheroidal graphite cast iron used in this study. Materials
Element (wt.%) C
800-2 1045 D2
3.40–3.80 0.42–0.50 1.45–1.70
Cr
Mo
V
Si
Mn
S
P
– <0.25 11.00–12.50
– 0.46 0.40–0.60
– 0.25 0.15–0.30
2.40–3.00 0.17–0.37 ≤0.40
0.50–0.80 0.50–0.80 ≤0.40
≤0.05 <0.04 ≤0.030
≤0.05 <0.04 ≤0.030
and severe wear. In ultra-mild wear, a compacted oxide film covered contacting surfaces; also mild wear was defined as oxidative wear. Severe wear appeared when oxide layer did not stably form due to delamination of tribo-oxides. They also pointed out that the excellent wear resistance of grey cast iron during dry sliding under low loading conditions was attributed to graphite flakes and formation of a graphite film. Liu [15] studied the influence of surface oxidation on the wear resistance of cast irons and found that oxidation increased wear rate when cast iron was of high strength and hardness (high resistance to adhesion and microcutting); but when cast iron was of low strength and hardness (low resistance to adhesion and micro-cutting), oxidation reduced wear rate. Whether oxidation increased or reduced wear depended on graphite morphology, microstructure and hardness of matrix; they were considered to be key factors deciding on whether wear rate increased or decreased due to oxidation. Hirasata and Hayashi [16] studied friction and wear of several kinds of cast irons under severe sliding conditions and found that the wear rates were strongly dependent on hardness variation. No tribo-oxides were noted to exist, only a plastically deformed layer. Prasad [17] studied the effect of microstructural features and test conditions on the wear of cast iron. He pointed out that the governing phenomena of wear in the alloyed cast iron might be due to adhesive wear which generated oxide particulates that further led to abrasive wear. When abrasive wear mode was prominent, the wear took place by plowing or microcutting of the hard debris particulates. The main operating materials removal mechanism was spalling associated with substantial microcracking and with a limited extent of adhesion and abrasion during sliding. It is clear that the formation of tribo-oxides and their role in the wear of graphite iron have been not clarified until now. In order to obtain a better understanding of these, the dry sliding wear behavior of spheroid graphite iron was studied in comparison with that of carbon steel. They possessed the same microstructure and hardness, the only significant difference being the presence of spheroid graphite. The amount and morphology of tribo-oxides were examined from worn surfaces and subsurfaces; the tribooxides and their role in wear were explored, focused on the influences of graphite on the formation of tribo-oxides and wear behavior.
2. Experimental procedure 2.1. Materials and heat-treatment Commercial 1045 carbon steel and 800-2 spheroidal graphite cast iron were selected as pins of 6 mm diameter and 12 mm length, while D2 steel was choosed as disc ‘counter-surfaces’ of 70 mm diameter and 8 mm thickness. Their chemical compositions are listed in Table 1. 1045 steel was austenitized at 840 ◦ C for 20 min, and then cooled in water, finally tempered at 400 ◦ C for 2 h to achieve tempered martensite. The spheroidal graphite cast iron was austenitized at 900 ◦ C for 20 min, cooled in oil, then tempered at 500 ◦ C for 2 h, attaining tempered martensite and spheroidal graphite. The carbon steel and spheroidal graphite cast iron presented the same hardness (40 HRC), and their microstructures are shown in Fig. 1. Commercial D2 steel was austenitized at 1150 ◦ C for 20 min, oil quenched and tempered three times at 550 ◦ C for 2 h to achieve a hardness of 60 HRC. 2.2. Wear test An MG-200 type pin-on-disc high temperature wear testing machine was employed to perform wear tests for a distance of 1.2 × 103 m at a velocity of 1 m/s within an ambient temperature range of 25–400 ◦ C and a load range of 50–200 N. Wear was determined by measuring the mass loss of the pin specimen using an electronic balance (0.00001 g). Mass loss was converted into volume loss using the density of the materials (7.85 and 7.30 g/cm3 for the carbon steel and spheroidal graphite cast iron, respectively). The wear rate was defined as volume loss by unit distance (mm3 /mm). Data for each wear value was calculated from the average of three tests. Moment of friction force between the pin and disk during wear was measured by a sensor and automatically recorded by a personal computer, then transformed to friction coefficient. 2.3. Evaluation techniques Phases on worn surfaces were identified by a D/Max-2500/pc type X-ray diffractometer (XRD). Morphologies of worn surface and subsurface were examined by a JSM-7001F type scanning electron
Fig. 1. Heat-treated microstructures of the carbon steel (a) and spheroidal graphite cast iron (b).
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Fig. 2. Wear rate vs. load curves of the carbon steel (a) and spheroidal graphite cast iron (b) sliding at various ambient temperatures.
microscope (SEM). Hardness of pin specimen was measured by an HR-150A type Rockwell apparatus. 3. Results and analysis 3.1. Wear and friction behaviors Fig. 2 shows wear rate vs. load curves of the carbon steel and spheroidal graphite cast iron sliding at various ambient temperatures. The wear rate of the carbon steel at 25 ◦ C increased approximately linearly with increasing load. At 200 ◦ C, the wear rate abruptly decreased as the load was raised from 50 to 100 N, then increased again slightly with further increasing load. The wear rate at 400 ◦ C increased gradually at first with increasing load, then started to rise rapidly once the load surpassed 125 N, exceeding the detection limit of the testing machine (Fig. 2a). At 25–200 ◦ C, the spheroidal graphite cast iron presented almost the same wear rate vs. load curves, only the wear rate at 200 ◦ C was slightly higher than that at 25 ◦ C. However, the wear rate at 400 ◦ C was markedly higher than those at 25–200 ◦ C, and increased sharply once the load surpassed 125 N (Fig. 2b). Comparisons of wear rates between the carbon steel and spheroidal graphite cast iron sliding at various ambient temperatures are shown in Fig. 3. The wear rate of the spheroidal graphite cast iron at 25 ◦ C was lower than that of the carbon steel at loads below 150 N, but higher once over 150 N (Fig. 3a).
At 200 ◦ C, the same phenomenon happened again, only the transition load was 100 N (Fig. 3b). At 400 ◦ C, the wear rate of the spheroidal graphite cast iron was marginally higher than that of the carbon steel at loads below 100 N, and their difference gradually became greater with further increasing load (Fig. 3c). The variation of friction coefficient with sliding distance for the carbon steel and spheroidal graphite cast iron sliding under different conditions is shown in Fig. 4. In most cases, the spheroidal graphite cast iron presented a lower friction coefficient than the carbon steel. An increase in load resulted in a decrease in friction coefficient of the materials at every ambient temperature. The fluctuation of friction coefficient of the carbon steel was slight at 25 ◦ C and under 150 N at 400 ◦ C, enlarged under 100–200 N at 200 ◦ C and under 50–100 N at 400 ◦ C. The spheroidal graphite cast iron was of small fluctuation of friction coefficient in most cases and presented a large fluctuation of friction coefficient merely under 50 N at 400 ◦ C. 3.2. XRD analysis for worn surfaces X-ray diffraction patterns of worn surfaces of the carbon steel and spheroidal graphite cast iron sliding under various conditions are shown in Fig. 5. At 25 ◦ C, only trace tribo-oxides were identified on worn surfaces of the carbon steel. The amount of tribo-oxides gradually increased with increasing ambient temperature from 25 ◦ C to 200 ◦ C; a small amount of tribo-oxides appeared
Fig. 3. Comparisons of wear rate between the carbon steel and spheroidal graphite cast iron sliding at 25 (a), 200 (b) and 400 ◦ C (c).
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Fig. 4. Friction coefficient vs. sliding distance curves of the carbon steel (a)–(c) and the spheroidal graphite cast iron (d)–(f) sliding under various conditions.
at 200 ◦ C (Fig. 5a and b). However, at 400 ◦ C, a large amount of tribo-oxides were generated; the peaks corresponding to oxides significantly surpassed those of iron. But once the load reached 150 N, the amount of tribo-oxides markedly decreased (Fig. 5c). It was observed in the spheroidal graphite cast iron that merely trace tribo-oxides formed at 25–200 ◦ C and a small amount of tribooxides appeared at 400 ◦ C (Fig. 5d–f). Compared with the carbon steel, the amount of tribo-oxides generated on worn surfaces of the spheroidal graphite cast iron was obviously reduced, even at 400 ◦ C.
3.3. SEM morphology of worn surfaces SEM morphologies of worn surfaces of the carbon steel and spheroidal graphite cast iron sliding under various conditions are shown in Figs. 6 and 7, respectively. At 25 ◦ C, obvious adhesive and a trace of abrasive mark were identified in the carbon steel (Fig. 6a and b). In addition, tribo-oxides sparsely scattered on the worn surface under high load, which can be confirmed by XRD results and section morphology of worn surfaces. The amount and area of tribo-oxides increased as the load surpassed 100 N at 200 ◦ C; tribo-oxide layer formed with some delaminated craters and a trace of abrasive mark (Fig. 6c and d). At 400 ◦ C, a smooth tribo-oxide layer with delaminated craters was found across the whole worn surface (Fig. 6e and f). However, at a load of 150 N, any trace of smooth tribo-oxide layers and delaminated craters disappeared, instead only a small amount of fine grooves parallel to the sliding direction and many tiny oxide particles appeared (Fig. 6g). For the spheroidal graphite cast iron at 25–200 ◦ C, adhesive trace appeared without tribo-oxides on worn surfaces (Fig. 7a–d). At 400 ◦ C, although a small, sparse amount of tribo-oxides appeared on the partial worn surface, adhesive was still predominant worn characteristic (Fig. 7e). As the load reached 150 N, the worn surface also became smooth just as happened in the carbon steel, but there were no oxide particles (Fig. 7f).
3.4. Section morphology of worn surfaces and subsurfaces In order to ascertain the formation of tribo-oxides on worn surfaces, section morphologies of worn surfaces and subsurfaces of the carbon steel and spheroidal graphite cast iron sliding under various conditions were investigated, as shown in Figs. 8 and 9, respectively. For the carbon steel at 25 ◦ C, tribo-oxides did not form under a load of 50 N (Fig. 8a), but appeared under a load of 100 N and higher. Such oxides were distributed sparsely on worn surfaces and reached a thickness of less than 3 m (Fig. 8b). At 200 ◦ C, no tribooxide layer appeared was observed under a load of 50 N (Fig. 8c), however, once the load reached 100 N or above, a tribo-oxide layer with a thickness of about 8–10 m existed (Fig. 8d). At 400 ◦ C, a large amount of tribo-oxides formed and the thickness increased to 20–25 m (Fig. 8e). However, as the load reached 150 N, the thickness of tribo-oxide layer substantially decreased to less than 0.5 m (Fig. 8f). This may explain why the peaks corresponding to iron surpassed those of oxides (Fig. 4c). For the spheroidal graphite cast iron, no tribo-oxide layer appeared on worn surfaces at 25 ◦ C (Fig. 9a and b). At 200 ◦ C, there was reduced tribo-oxides formation (compared to carbon steel) with a reduced thickness of less than 3 m (Fig. 9c and d). Due to the presence of graphite in the spheroidal graphite cast iron, rupture seemed to appear readily on the worn surface, as shown in the inserted image of Fig. 9b. At 400 ◦ C, the amount of tribooxides correspondingly increased, and tribo-oxide layer was still thin (less than 5 m) and discontinuous (Fig. 9e and f). At 150 N, the elongated graphite appeared companied by massive plastic deformation in the sub-surface region.
4. Discussion At 25 ◦ C, almost no or trace tribo-oxides appeared on worn surfaces of the carbon steel and spheroidal graphite cast iron, thus metal–metal contact was not avoided. Shear stress resulted from friction force would cause deformation of contacting asperities, leading eventually rupture inside the softer pin resulting in mass
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Fig. 5. X-ray diffraction patterns for worn surfaces of the carbon steel (a)–(c) and spheroidal graphite cast iron (d)–(f) sliding under various conditions.
loss [18,19], thus a severe wear prevailed. Archard [19] proposed an equation to evaluate wear resistance in this regime, where material wear loss is directly proportional to normal load and sliding distance, but inversely proportional to the softer material’s real hardness (the pin specimen in this study). Although the carbon steel and spheroidal graphite cast iron possessed the same original hardness (40 HRC), they presented different wear behaviors, which might be due to the graphite in the spheroidal graphite cast iron. At loads below 150 N, the wear rate of the spheroidal graphite cast iron was slightly lower than that of the carbon steel due to graphite lubrication. However, on application of a 200 N
load, the lubrication effect lessened and even disappeared. In this case, the presence of graphite led to easy fracture of worn surfaces and the wear rate of the spheroidal graphite cast iron thus rapidly increased. For carbon steel, as ambient temperature was elevated to 200 ◦ C, severe wear still operated under 50 N load. With increasing load, more tribo-oxides started to form on worn surfaces due to increased frictional heating. These tribo-oxides did not cover the whole worn surface, and preferentially formed at contacting asperities [10]. When the tribo-oxide layer reaches a thickness of about 10 m, they could prevent metal–metal contact and adhesion,
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Fig. 6. Morphologies of worn surfaces of the carbon steel sliding under various conditions: (a) 50 N, 25 ◦ C; (b) 200 N, 25 ◦ C; (c) 50 N, 200 ◦ C; (d) 200 N, 200 ◦ C; (e) 50 N, 400 ◦ C; (f) 100 N, 400 ◦ C; (g) 150 N, 400 ◦ C.
and protect the underlying material against mechanical damage [10,11]. In this case, the oxidative mild wear regime proposed by Quinn et al. [1] and Wilson et al. [2] prevailed. The wear rate of the carbon steel therefore dropped abruptly on a raising load to 100 N and above this, the wear rate increased slowly and steadily with increasing load. However, tribo-oxides generated on worn surfaces of the spheroidal graphite cast iron were too little and to thin to avoid
metal–metal contact and adhesion, thus the severe wear still dominated. During sliding, the increasing friction heat and high ambient temperature would result in the decrease in the hardness of worn surface and subsurface, thus the wear rate of the spheroidal graphite cast iron at 200 ◦ C further increased compared with that at 25 ◦ C. Therefore, the original material hardness in Archard’s equation [19] should be replaced by the real surface hardness. In severe wear of the spheroidal graphite cast iron at 200 ◦ C, graphite
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Fig. 7. Morphologies of worn surfaces of the spheroidal graphite cast iron sliding under various conditions: (a) 50 N, 25 ◦ C; (b) 200 N, 25 ◦ C; (c) 50 N, 200 ◦ C; (d) 200 N, 200 ◦ C; (e) 50 N, 400 ◦ C; (f) 150 N, 400 ◦ C.
lubrication still functioned and affected wear behavior. Thus, the wear rate of the spheroidal graphite cast iron was lower than that of the carbon steel at loads below 100 N. At 400 ◦ C, a dense glaze tribo-oxide layer with a thickness of about 20–25 m formed over the whole worn surface of the carbon steel, thus the oxidative mild wear prevailed at low load. In such a case, tribo-oxides are considered to play a decisive role in determining wear behavior, irrespective of matrix microstructures. With increasing load of 100–125 N, the matrix was thermally softened probably due to dynamic recovery and recrystallization [10,11]. Not enough mechanical support for the tribo-oxide layer could be provided due to a large scale plastic deformation of the softened matrix. Both tribo-oxide and matrix affect wear behavior, and the oxidative wear, rather than oxidative mild wear began to prevail [10–12]. At 150 N, high compressive and shear stresses caused a massive plastically deformed region due to severe thermal softening of 1045 steel, the uppermost material was liable to extrusion out from the contact surface, resulting in a high wear rate. Although tribo-oxides were inevitably produced at 400 ◦ C, the formed tribolayer of less than 0.5 m was too thin to offer wear protection.
This wear could be classified as extrusive wear as reported by So et al. [20]. The level of tribo-oxides observed on the worn surface of the spheroidal graphite cast iron also increased on raising the temperature to 400 ◦ C, however, there were still not present to levels seen under equivalent conditions with carbon steel. Only a small amount of tribo-oxides partially covered the worn surface and their amount and thickness (less than 5 m) was insufficient to eliminate metal–metal contact completely. It is possible that the tribo-oxides might have reduced wear to some extent, but severe wear was still the dominant wear mechanism in this case. It is worth noting that the lubrication effect of graphite seemed to disappear and so the wear rate of the spheroidal graphite cast iron was slightly higher than that of the carbon steel at loads below 100 N. On application of a 150 N load, the extrusive wear also prevailed for the spheroidal graphite cast iron. Unlike the extrusive wear regime observed with carbon steel, fracture of worn surface at the interface between graphite and matrix was noted. As a result, there was a greater mass loss at high load compared with that of carbon steel, due to lower strength and plasticity of the spheroidal
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Fig. 8. Section morphologies of worn surfaces and subsurfaces of the carbon steel sliding under various conditions: (a) 50 N, 25 ◦ C; (b) 200 N, 25 ◦ C; (c) 50 N, 200 ◦ C; (d) 200 N, 200 ◦ C; (e) 50 N, 400 ◦ C; (f) 150 N, 400 ◦ C.
graphite cast iron. It appears that the graphite cast iron did not facilitate any advantage during elevated-temperature wear due to loss of graphite lubrication. The spheroidal graphite cast iron possessed the same microstructure and hardness as the carbon steel, except for graphite. These different wear behaviors and wear regimes of the spheroidal graphite cast iron should be attributed to the presence of said graphite. During sliding, it is suggested that the graphite in the spheroidal graphite cast iron was gradually exposed, then ground down to carbon powder to cover worn surface. Graphite as a strong reducer would cause the following reactions during elevated-temperature wear: 2C + O2 → 2CO
(1)
4CO + Fe3 O4 → 3Fe + 4CO2
(2)
3CO + Fe2 O3 → 2Fe + 3CO2
(3)
These reactions could occur during sliding, i.e. graphite powder reacts first with oxygen to produce carbon monoxide, which then
reduces the tribo-oxides generated on the worn surface. The levels of tribo-oxide on the worn surface of the spheroidal graphite cast iron may therefore be substantially reduced by these reactions, resulting in the differently observed wear behaviors and wear regimes. Except for its indirect function (on tribo-oxides), graphite would directly affect wear behavior of the spheroidal graphite cast iron. The role of graphite in the spheroidal graphite cast iron can be summarized as follows. Graphite reduced wear as a solid lubricant under the low-load conditions (at 25–200 ◦ C), however, accelerated wear under a high load due to this high load causing fracture of the worn surface at the interface between graphite and matrix. At 400 ◦ C, it is suggested that tribo-oxides were chemically reduced by the described chemical reactions, hence explaining the markedly reduced levels of tribo-oxide and graphite on worn surfaces. Hence, the ability of both graphite and tribo-oxides to reduce wear were substantially impaired. It must be noted that graphite chemically reduced tribo-oxide during wear is a suggested mechanism. This is because graphite would physically affect oxide sintering and layer formation
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Fig. 9. Section morphologies of worn surfaces and subsurfaces of the spheroidal graphite cast iron sliding under various conditions: (a) 50 N, 25 ◦ C; (b) 200 N, 25 ◦ C; (c) 50 N, 200 ◦ C; (d) 200 N, 200 ◦ C; (e) 50 N, 400 ◦ C; (f) 150 N, 400 ◦ C.
during sliding. If graphite worsen oxide sintering, tribo-oxides would be reduced. Thus, the role of graphite during wear needs further investigation.
5. Conclusions (1) With increasing ambient temperature, the wear rate of the carbon steel first decreased (with a minimum at 200 ◦ C), then increased, while the wear rate of the spheroidal graphite cast iron continuously increased. At 400 ◦ C, once a higher load of 150 N was applied, the wear rates of both materials rapidly increased. (2) For the carbon steel, severe wear prevailed at 25 ◦ C and 50 N, at 200 ◦ C. Oxidative mild wear operated under a load ranging from 100 to 200 N at 200 ◦ C and also at 50 N only at 400 ◦ C. Oxidative wear occurred within a load range of 100–125 N at 400 ◦ C. Extrusive wear appeared with further increasing load. (3) For the spheroidal graphite cast iron, tribo-oxides were substantially reduced due to the suggested reduction of tribooxides by graphite. Thus severe wear was always dominant
regime at 25–400 ◦ C, except for a load of 150 N at 400 ◦ C, where extrusive wear with fracture prevailed. (4) Graphite reduced wear under the conditions of lower load (at 25–200 ◦ C), however, accelerated wear under a high load because of fracture at the interface between graphite and matrix. At 400 ◦ C, it is suggested that tribo-oxides were chemically reduced by the described chemical reactions. Hence, the ability of both graphite and tribo-oxides to reduce wear was substantially impaired. Acknowledgements Financial supports of our work by National Natural Science Foundation of China (No. 51071078) and Natural Science Fund of Jiangsu Province (No. BK2009221) are gratefully acknowledged. References [1] T.F.J. Quinn, J.L. Sullivan, D.M. Rowson, Developments in the oxidational theory of mild wear, Tribol. Int. 13 (1980) 153–158.
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