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Subsurface characteristics of an abraded Fe-0.4 wt%C pearlitic steel: A nanoindentation study
Wear 263 (2007) 1575–1578
Short communication
Subsurface characteristics of an abraded Fe-0.4 wt%C pearlitic steel: A nanoindentation study Futoshi Katsuki ∗ , Mitsuharu Yonemura Corporate Research and Development Laboratories, Sumitomo Metal Industries Limited, 1-8, Fuso-cho, Amagasaki 660-0891, Japan Received 1 September 2006; received in revised form 11 December 2006; accepted 7 January 2007 Available online 23 May 2007
Abstract The present investigation of a unidirectionally abraded pearlitic (0.4 wt%C) steel and the steel with vanadium addition (0.1 wt%V) elucidates the work hardening and the reorientation near the surface layer caused by abrasion, particularly its relation to the wear behavior. The abraded surfaces were examined with a nanoindentation apparatus to evaluate the variation of nanohardness and elastic modulus with depth below sliding contact on a nanometer scale. It has been found that the wear rate of each specimen was almost equal, although the nanohardness of the abraded surface of the vanadium addition steel was lower than that of the vanadium free one. The elastic modulus of the vanadium free steel increased with decreasing the indentation depth below the abraded surface to about 1.3 times the original value (∼200 GPa), while the vanadium addition steel was constant. The varying nature of influence of interlamellar spacing and the vanadium addition on the wear response of the pearlitic steel has been discussed in terms of the ratio between hardness and elastic modulus of the abraded surface which corresponds to a plasticity factor. © 2007 Elsevier B.V. All rights reserved. Keywords: Abrasive wear; Pearlite steel; Nanoindentation; Work hardening; Surface texturing
1. Introduction Considerable plastic deformation, which is produced by frictional forces in a surface layers of ductile materials, causes the microstructural changes localized within a small volume of material adjacent to contact surfaces. The homogeneity of the surface may decrease to some extent, which could be reflected by changes in surface mechanical properties and therefore an alternation of the friction and wear behavior of the materials. Studies have been made of the relationship between typical microstructures of steels (such as pearlite, martensite, bainite and ferrite) and wear behavior of materials during wear [1,2]. Abrasion-induced microstructure changes were also studied via metallographic examinations of longitudinal sections (parallel to the sliding direction) and the surface hardness using a microhardness tester [3]. The ratio between hardness and elastic modulus of the abraded surface which corresponds to a plasticity factor, is commonly used to establish the abrasive wear severity of materials [4]. However, accurate evaluation of the hardness cannot be accomplished with conventional testing techniques because the
severe plastic deformation and work hardening occurred in the region very close to the abraded surface. Furthermore, despite the importance of the influence of the elastic properties upon the deflection and the stress in metal surfaces under load, there appears to have been no conscious effort to measure the elastic modulus of the abraded surface to control wear [5]. The purpose of the present investigation of unidirectionally abraded surface of a pearlitic (0.4 wt%C) steel is to examine the work hardening and the change of the elastic properties near the surface layer, particularly its relation to the wear behavior. An attempt has also been made to examine the role of the interlamellar spacing and microalloying with vanadium addition (0.1 wt%V) in controlling wear behavior [6]. We use a depth sensing nanoindentation technique, which is a powerful tool for nanomechanical testing at a very small scale and very high resolution [7]. The relationship between the nanomechanical properties of the surface layer and the wear behavior are considered to clarify the micromechanism of the abrasive wear of the pearlite steel. 2. Experimental
∗
Corresponding author. Tel.: +81 6 6489 5736; fax: +81 6 6489 5960. E-mail address: [email protected] (F. Katsuki).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.01.092
The pearlite steel utilized in this study was prepared by an induction furnace. The chemical composition is shown in
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F. Katsuki, M. Yonemura / Wear 263 (2007) 1575–1578
Table 1 The chemical composition (wt%), cooling rate (◦ /min) from 950 to 300 ◦ C and bulk hardness by the conventional vickers indenter of specimens Number
C
Si
V
Cr
CR
Hv
1 2 3 4 5 6
0.38 0.38 0.38 0.37 0.37 0.37
<0.01 <0.01 <0.01 <0.01 <0.01 <0.01
<0.001 <0.001 <0.001 0.1 0.1 0.1
1.5 1.5 1.5 1.5 1.5 1.5
0.5 2 7 0.5 2 7
165 193 206 174 182 208
Table 1. Hot rolled bars of 20 mm diameter, forged above 1100 ◦ C from a 90 mm diameter, were cooled in an ambient air. The bars were austenitized at 950 ◦ C for 3600 s and then continuously cooled in a vacuum of about 10−5 Pa. Subsequently, the bars were cooled different rate to change its interlamellar spacing. A spacing range of 165–208 nm was achieved by carrying out continuous cooling from the austenitized temperature to 300 ◦ C on two medium carbon steels, one a carbon steel with chromium and the other containing small additions of vanadium, together with chromium. The details of the cooling rate and corresponding interlamellar spacing, bulk hardness by the conventional vickers indenter are given in Table 1. The volume fraction of a primary ferrite phase of each specimen was less than 10%. Testing surface (2 mm × 3.7 mm) of the specimen was mirror-polished by emery paper and buffing, then finished by an electrolytic polishing with the Pt cathode in a solution of 5% percholic acid, 95% acetic acid at 8 ◦ C under the potential of 40–50 V to remove the mechanically damaged surface layer. Two-body abrasive wear test was performed on a small square tip (2 mm × 3.7 mm × 1.3 mm) of the specimen. The tests were conducted using a pin-on-disc machine. The specimen is ground on an abrasive paper at an applied load of 2.1 N, a sliding speed of 0.66 m/s and traversal distance of 1980 m. Crushed silica particles (size: 15–67 m) were used as the abrasive medium. The specimens were thoroughly cleaned, dried and weighed prior to and after each wear test. The wear rate was calculated from weight loss measurement. Weight loss data were converted to volume loss using steel density of 7760 kg/m3 . Nanoindentation tests were conducted on abraded surfaces worn under the same condition using a commercially available apparatus (Triboscope, Hysitron Inc., Minneapolis, MN). A Berkovich indenter was employed, and the tip truncation was calibrated by using a reference specimen of fused silica. Analyses for the tip calibration and the calculation of nanohardness and the elastic modulus were conducted by the method used by Oliver and Pharr presented detailed discussion about the calculation methodology [8]. The testing load values were 100, 200, 500 and 1000 N. The loading rate was constant of 200 N/s. After abrasion tests, specimens were sectioned perpendicular to the wear surfaces. Longitudinal sections (parallel to the sliding direction) were examined in scanning electron microscope (SEM) to clarify the wear-induced microstructural changes in the subsurface regions.
Fig. 1. The influence of the inverse of the square root of the interlamellar spacing (S−1/2 ) on vickers hardness (Hv) of the steel.
3. Results Fig. 1 shows the bulk hardness by vickers indenter (Hv) of the steel plotted as a function of the inverse of the square root of the interlamellar spacing (S−1/2 ). Hv hardness followed Hall-Petch relationship, with the interlamellar spacing; finer the lamellae, higher the Hv hardness [6]. Independence of the interlamellar spacing, it is also shown that vanadium additions enhance the bulk hardness. Fig. 2 reveals the abrasive wear rate (wear volume/sliding distance) as a function of the Hv hardness of the steel. It is clear that the minimum wear rate occurred at about same Hv hardness in both the vanadium addition and the vanadium free steel. It may be noted that higher hardness does not necessarily mean better wear resistance. Metallographic examination of longitudinal sections reveals shear deformation under the worn surface, as shown in Fig. 3. Below the worn surface, lamellae of the specimen No. 1, which are initially perpendicular to the abrasive surface, are displaced towards the sliding direction. The fine pearlitic structure is observed with plastic deformation approximately extending a micrometer from the surface. The extent of deformation is so severe very close to the surface that it is not possible to resolve the pearlitic structure by SEM. The surface nanohardness of the specimens are plotted as a function of the indentation depth in Fig. 4(a–c). On each specimen, nanoindentation was performed at four different loads of 100, 200, 500 and 1000 N and average values at each load are
Fig. 2. Wear rate of vanadium free and addition steels vs. vickers hardness (Hv).
F. Katsuki, M. Yonemura / Wear 263 (2007) 1575–1578
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Fig. 5. Elastic modulus as a function of indentation depth.
4. Discussions Fig. 3. Scanning electron microscope image of a longitudinal section of specimen No. 1.
plotted in Fig. 4. Substantial abrasion induced work hardening is observed. The presence of the fine pearlitic structure and flow lines in the layers close to the abraded surface (Fig. 3) agreed well with the higher hardness of the regions. There was a considerable hardening in the case of coarser pearlitic structure, as shown in Fig. 4(a). On the other hand, as shown in Fig. 4(c), the extent of hardening was less with decreasing interlamellar spacing, while the bulk Hv hardness is larger than that in the case of coarser spacing (Fig. 1). It was also clear that the extent of hardening was more in the case of the vanadium free steel than that of the vanadium addition steel. It may be noted that the nanohardness of the vanadium free steel (Nos. 1–3) is larger than that of the vanadium addition steel (Nos. 4–6), while the Hv hardness of the vanadium free steel is smaller than that of the vanadium addition one (Fig. 1). The calculated elastic modulus of the steel in various conditions as a function of the indentation depth in Fig. 5. The elastic modulus of the vanadium free steel increased with decreasing the depth below the abraded surface to about 1.2–1.3 times the original value (∼200 GPa), while the vanadium addition steel was almost constant. The original value is corresponds to the elastic modulus of a polycrystalline iron with a random orientation [5].
As shown in Fig. 3, the lamellae structure were bent and reoriented in the sliding direction. Fine pearlitic structure was observed just beneath the abraded surface. It is reviewed that the formation of the fine substructure under the worn surface is due to the heavy shear deformation introduced by wear, and such a sliding contact creates a near-surface layer having different structure and mechanical properties than the base material [1–3,9]. The plastic deformation zone beneath an indenter expands with increasing penetration depth of the indenter. Bhattachaya and Nix suggests that indentations to about 20% of the surface layer thickness sample the surface layer alone [10]. As shown in Fig. 4(a–c), the maximum hardness appeared to occur at a contact depth smaller than 15 nm. When the contact depth is 15 nm, the corresponding plastically deformed zone beneath the indenter would expand to about 75 nm in diameter on the hemispherical approximation, which is in good agreements with the depth of severe deformed layer for lamellar pearlitic structures, as shown in Fig. 3. Fig. 6 shows abrasive wear rate versus average surface nanohardness corresponding to the contact depth less than 15 nm. The wear rate of each specimen is found to be a linear function of their surface nanohardness. These results indicate that the reduced wear rate with decrease in interlamellar spacing during surface deformation may be attributed to increased surface nanohardness of the pearlite steel which control the cutting efficiency of the abrasive wear [11].
Fig. 4. Surface nanohardness of Nos. 1–6 after abrasive wear tests. (a) Coarse lamella, (b) medium lamella and (c) fine lamella.
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equal. These results imply that the deformation of the subsurface region and the dynamic change of the plasticity factor is an important factor for controlling the abrasive wear resistance of the pearlite steel. The combination of low elastic modulus, which spreads out the load, with high hardness, to resist the applied load, should result in maximum possible resistance to abrasive wear. 5. Conclusion
Fig. 6. Wear rate as a function of average surface nanohardness corresponding to the tip contact depth less than 15 nm.
It may be further noted, although the surface nanohardness of the steel with the vanadium addition was lower than that of the vanadium free one, the wear rate of each specimen is almost equal. Wear is characterized as a process of surface damage due to mechanical contact of matter. Thus the type of contact is very important for all wear losses. Greenwood and Williamson proposed a plasticity factor, the ratio of hardness to elastic modulus of the abraded surface which is used to establish the abrasive wear severity of materials [12]. This factor gives an indication of the depth of penetration that a metal can tolerate without exceeding its elastic limit. As shown in Fig. 5, the elastic modulus of the vanadium free steel increased with decreasing the contact depth below the abraded surface to about 1.3 times the original value (∼200 GPa), while the vanadium addition steel was constant. These results indicate that in the case of vanadium free steel, high elastic modulus which concentrates the load on the steel surface should result in lower resistance to abrasive wear. Polycrystalline metals, under unidirectionally abrasion, will give up their random crystalline arrangement and assume a preferred orientation. Metals with a preferred orientation exhibit many peculiarities associated with a single crystal. The elastic modulus of a single crystal of iron is 132 GPa in a direction parallel to 0 0 1. As the direction of load is tilted away from these major axes, the modulus increases until a maximum value of 284 GPa is obtained when the load acts in a 1 1 1 direction [5]. If texturing of the initially random oriented surface layers has occurred during abrasion, the higher elastic modulus of the vanadium free steel may be attributed to the surface texture exhibiting high elastic modulus, such as (1 1 1) texture [13,14]. The results clearly show vanadium additions can enhance the bulk hardness and strength of medium carbon pearlite steels, as shown in Fig. 1. This enhancement is achieved at the expense of some reduction in ductility [15]. Therefore, the extent of the surface hardening and subjected to wear induced deformation may decrease with decrease in ductility due to difficulty in decreasing the interlamellar spacing. As a result, below a critical value of ductility, the surface layer gets removed because of a low work hardening exponent. It may be further noted this removal would prevent the subsurface region of the vanadium addition steel from texturing, which leads to the increase of the elastic modulus by abrasion. Thus the wear rate of each specimen is almost
The effects of the abrasion-induced subsurface changes on the wear behavior of a pearlite steel have been examined by using a nanoindentation technique. It has been found, although the surface nanohardness of the steel with the vanadium addition was lower than that of the vanadium free one, the wear rate of each specimen is almost equal. The contact modulus of the vanadium free steel increased with decreasing the indentation depth below the abraded surface to about 1.3 times the original value (∼200 GPa), while the vanadium addition steel was almost constant. The simple wear mechanism extracted from the surface hardness and elastic modulus by a nanoindentation technique may be useful for controlling the wear characteristics, an important requirement in the qualitative understanding of wear behavior. References [1] P.L. Hurricks, Some metallurgical factors controlling the adhesive and abrasive wear resistance of steels. A review, Wear 26 (1973) 285–304. [2] J. Larsen-Basse, Role of microstructure and mechanical properties in abrasion, Scripta Metall. Mater. 24 (1990) 821–826. [3] B.K. Prasad, S.V. Prasad, Abrasion-induced microstructural changes during low stress abrasion of a plain carbon (0.5% C) steel, Wear 151 (1991) 1–12. [4] K.H. Zum Gahr, Microstructure and Wear of Materials, Elsevier, New York, 1987, pp. 52–53. [5] T.L. Oberle, Properties influencing wear of metals, J. Metals 3 (1951) 438–439. [6] S. Bhattacharyya, Wear and friction in steel, aluminum and magnesium alloys. I. Pearlitic and spheroidized steels, Wear 61 (1980) 133–141. [7] B. Bhushan, Handbook of Micro/Nano-Tribology, CRC Press, New York, 1995, pp. 321–348. [8] W.C. Oliver, G.M. Pharr, Improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564–1583. [9] D.A. Rigney, J.E. Hammerberg, Unlubricated sliding behavior or metals, MRS Bulletin 23 (6) (1998) 32–36. [10] A.K. Bhattacharya, W.D. Nix, Analysis of elastic and plastic deformation associated with indentation testing of thin films on substrates, Int. J. Solids Struct. 24 (1988) 1287–1298. [11] O.P. Modi, D.P. Mondal, B.K. Prasad, M. Singh, H.K. Khaira, Abrasive wear behavior of a high carbon steel: effects of microstructure and experimental parameters and correlation with mechanical properties, Mater. Sci. Eng. A 343 (2003) 235–242. [12] J. Greenwood, J. Williamson, Contact of nominally flat surfaces, Proc. R. Soc. A 295 (1966) 300–319. [13] D.R. Wheeler, D.H. Buckley, Texturing in metals as a result of sliding, Wear 33 (1975) 65–74. [14] V.D. Scott, H. Wilman, Surface re-orientation caused on metals by abrasion—its nature, origin and relation to friction and wear, Proc. R. Soc. A 247 (1958) 353–368. [15] G.L. Dunlop, C.-J. Carlsson, G. Frimodig, Precipitation of VC in ferrite and pearlite during direct transformation of a medium carbon microalloyed steel, Metall. Trans. 9A (1978) 261–266.
Short communication
Subsurface characteristics of an abraded Fe-0.4 wt%C pearlitic steel: A nanoindentation study Futoshi Katsuki ∗ , Mitsuharu Yonemura Corporate Research and Development Laboratories, Sumitomo Metal Industries Limited, 1-8, Fuso-cho, Amagasaki 660-0891, Japan Received 1 September 2006; received in revised form 11 December 2006; accepted 7 January 2007 Available online 23 May 2007
Abstract The present investigation of a unidirectionally abraded pearlitic (0.4 wt%C) steel and the steel with vanadium addition (0.1 wt%V) elucidates the work hardening and the reorientation near the surface layer caused by abrasion, particularly its relation to the wear behavior. The abraded surfaces were examined with a nanoindentation apparatus to evaluate the variation of nanohardness and elastic modulus with depth below sliding contact on a nanometer scale. It has been found that the wear rate of each specimen was almost equal, although the nanohardness of the abraded surface of the vanadium addition steel was lower than that of the vanadium free one. The elastic modulus of the vanadium free steel increased with decreasing the indentation depth below the abraded surface to about 1.3 times the original value (∼200 GPa), while the vanadium addition steel was constant. The varying nature of influence of interlamellar spacing and the vanadium addition on the wear response of the pearlitic steel has been discussed in terms of the ratio between hardness and elastic modulus of the abraded surface which corresponds to a plasticity factor. © 2007 Elsevier B.V. All rights reserved. Keywords: Abrasive wear; Pearlite steel; Nanoindentation; Work hardening; Surface texturing
1. Introduction Considerable plastic deformation, which is produced by frictional forces in a surface layers of ductile materials, causes the microstructural changes localized within a small volume of material adjacent to contact surfaces. The homogeneity of the surface may decrease to some extent, which could be reflected by changes in surface mechanical properties and therefore an alternation of the friction and wear behavior of the materials. Studies have been made of the relationship between typical microstructures of steels (such as pearlite, martensite, bainite and ferrite) and wear behavior of materials during wear [1,2]. Abrasion-induced microstructure changes were also studied via metallographic examinations of longitudinal sections (parallel to the sliding direction) and the surface hardness using a microhardness tester [3]. The ratio between hardness and elastic modulus of the abraded surface which corresponds to a plasticity factor, is commonly used to establish the abrasive wear severity of materials [4]. However, accurate evaluation of the hardness cannot be accomplished with conventional testing techniques because the
severe plastic deformation and work hardening occurred in the region very close to the abraded surface. Furthermore, despite the importance of the influence of the elastic properties upon the deflection and the stress in metal surfaces under load, there appears to have been no conscious effort to measure the elastic modulus of the abraded surface to control wear [5]. The purpose of the present investigation of unidirectionally abraded surface of a pearlitic (0.4 wt%C) steel is to examine the work hardening and the change of the elastic properties near the surface layer, particularly its relation to the wear behavior. An attempt has also been made to examine the role of the interlamellar spacing and microalloying with vanadium addition (0.1 wt%V) in controlling wear behavior [6]. We use a depth sensing nanoindentation technique, which is a powerful tool for nanomechanical testing at a very small scale and very high resolution [7]. The relationship between the nanomechanical properties of the surface layer and the wear behavior are considered to clarify the micromechanism of the abrasive wear of the pearlite steel. 2. Experimental
∗
Corresponding author. Tel.: +81 6 6489 5736; fax: +81 6 6489 5960. E-mail address: [email protected] (F. Katsuki).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.01.092
The pearlite steel utilized in this study was prepared by an induction furnace. The chemical composition is shown in
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F. Katsuki, M. Yonemura / Wear 263 (2007) 1575–1578
Table 1 The chemical composition (wt%), cooling rate (◦ /min) from 950 to 300 ◦ C and bulk hardness by the conventional vickers indenter of specimens Number
C
Si
V
Cr
CR
Hv
1 2 3 4 5 6
0.38 0.38 0.38 0.37 0.37 0.37
<0.01 <0.01 <0.01 <0.01 <0.01 <0.01
<0.001 <0.001 <0.001 0.1 0.1 0.1
1.5 1.5 1.5 1.5 1.5 1.5
0.5 2 7 0.5 2 7
165 193 206 174 182 208
Table 1. Hot rolled bars of 20 mm diameter, forged above 1100 ◦ C from a 90 mm diameter, were cooled in an ambient air. The bars were austenitized at 950 ◦ C for 3600 s and then continuously cooled in a vacuum of about 10−5 Pa. Subsequently, the bars were cooled different rate to change its interlamellar spacing. A spacing range of 165–208 nm was achieved by carrying out continuous cooling from the austenitized temperature to 300 ◦ C on two medium carbon steels, one a carbon steel with chromium and the other containing small additions of vanadium, together with chromium. The details of the cooling rate and corresponding interlamellar spacing, bulk hardness by the conventional vickers indenter are given in Table 1. The volume fraction of a primary ferrite phase of each specimen was less than 10%. Testing surface (2 mm × 3.7 mm) of the specimen was mirror-polished by emery paper and buffing, then finished by an electrolytic polishing with the Pt cathode in a solution of 5% percholic acid, 95% acetic acid at 8 ◦ C under the potential of 40–50 V to remove the mechanically damaged surface layer. Two-body abrasive wear test was performed on a small square tip (2 mm × 3.7 mm × 1.3 mm) of the specimen. The tests were conducted using a pin-on-disc machine. The specimen is ground on an abrasive paper at an applied load of 2.1 N, a sliding speed of 0.66 m/s and traversal distance of 1980 m. Crushed silica particles (size: 15–67 m) were used as the abrasive medium. The specimens were thoroughly cleaned, dried and weighed prior to and after each wear test. The wear rate was calculated from weight loss measurement. Weight loss data were converted to volume loss using steel density of 7760 kg/m3 . Nanoindentation tests were conducted on abraded surfaces worn under the same condition using a commercially available apparatus (Triboscope, Hysitron Inc., Minneapolis, MN). A Berkovich indenter was employed, and the tip truncation was calibrated by using a reference specimen of fused silica. Analyses for the tip calibration and the calculation of nanohardness and the elastic modulus were conducted by the method used by Oliver and Pharr presented detailed discussion about the calculation methodology [8]. The testing load values were 100, 200, 500 and 1000 N. The loading rate was constant of 200 N/s. After abrasion tests, specimens were sectioned perpendicular to the wear surfaces. Longitudinal sections (parallel to the sliding direction) were examined in scanning electron microscope (SEM) to clarify the wear-induced microstructural changes in the subsurface regions.
Fig. 1. The influence of the inverse of the square root of the interlamellar spacing (S−1/2 ) on vickers hardness (Hv) of the steel.
3. Results Fig. 1 shows the bulk hardness by vickers indenter (Hv) of the steel plotted as a function of the inverse of the square root of the interlamellar spacing (S−1/2 ). Hv hardness followed Hall-Petch relationship, with the interlamellar spacing; finer the lamellae, higher the Hv hardness [6]. Independence of the interlamellar spacing, it is also shown that vanadium additions enhance the bulk hardness. Fig. 2 reveals the abrasive wear rate (wear volume/sliding distance) as a function of the Hv hardness of the steel. It is clear that the minimum wear rate occurred at about same Hv hardness in both the vanadium addition and the vanadium free steel. It may be noted that higher hardness does not necessarily mean better wear resistance. Metallographic examination of longitudinal sections reveals shear deformation under the worn surface, as shown in Fig. 3. Below the worn surface, lamellae of the specimen No. 1, which are initially perpendicular to the abrasive surface, are displaced towards the sliding direction. The fine pearlitic structure is observed with plastic deformation approximately extending a micrometer from the surface. The extent of deformation is so severe very close to the surface that it is not possible to resolve the pearlitic structure by SEM. The surface nanohardness of the specimens are plotted as a function of the indentation depth in Fig. 4(a–c). On each specimen, nanoindentation was performed at four different loads of 100, 200, 500 and 1000 N and average values at each load are
Fig. 2. Wear rate of vanadium free and addition steels vs. vickers hardness (Hv).
F. Katsuki, M. Yonemura / Wear 263 (2007) 1575–1578
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Fig. 5. Elastic modulus as a function of indentation depth.
4. Discussions Fig. 3. Scanning electron microscope image of a longitudinal section of specimen No. 1.
plotted in Fig. 4. Substantial abrasion induced work hardening is observed. The presence of the fine pearlitic structure and flow lines in the layers close to the abraded surface (Fig. 3) agreed well with the higher hardness of the regions. There was a considerable hardening in the case of coarser pearlitic structure, as shown in Fig. 4(a). On the other hand, as shown in Fig. 4(c), the extent of hardening was less with decreasing interlamellar spacing, while the bulk Hv hardness is larger than that in the case of coarser spacing (Fig. 1). It was also clear that the extent of hardening was more in the case of the vanadium free steel than that of the vanadium addition steel. It may be noted that the nanohardness of the vanadium free steel (Nos. 1–3) is larger than that of the vanadium addition steel (Nos. 4–6), while the Hv hardness of the vanadium free steel is smaller than that of the vanadium addition one (Fig. 1). The calculated elastic modulus of the steel in various conditions as a function of the indentation depth in Fig. 5. The elastic modulus of the vanadium free steel increased with decreasing the depth below the abraded surface to about 1.2–1.3 times the original value (∼200 GPa), while the vanadium addition steel was almost constant. The original value is corresponds to the elastic modulus of a polycrystalline iron with a random orientation [5].
As shown in Fig. 3, the lamellae structure were bent and reoriented in the sliding direction. Fine pearlitic structure was observed just beneath the abraded surface. It is reviewed that the formation of the fine substructure under the worn surface is due to the heavy shear deformation introduced by wear, and such a sliding contact creates a near-surface layer having different structure and mechanical properties than the base material [1–3,9]. The plastic deformation zone beneath an indenter expands with increasing penetration depth of the indenter. Bhattachaya and Nix suggests that indentations to about 20% of the surface layer thickness sample the surface layer alone [10]. As shown in Fig. 4(a–c), the maximum hardness appeared to occur at a contact depth smaller than 15 nm. When the contact depth is 15 nm, the corresponding plastically deformed zone beneath the indenter would expand to about 75 nm in diameter on the hemispherical approximation, which is in good agreements with the depth of severe deformed layer for lamellar pearlitic structures, as shown in Fig. 3. Fig. 6 shows abrasive wear rate versus average surface nanohardness corresponding to the contact depth less than 15 nm. The wear rate of each specimen is found to be a linear function of their surface nanohardness. These results indicate that the reduced wear rate with decrease in interlamellar spacing during surface deformation may be attributed to increased surface nanohardness of the pearlite steel which control the cutting efficiency of the abrasive wear [11].
Fig. 4. Surface nanohardness of Nos. 1–6 after abrasive wear tests. (a) Coarse lamella, (b) medium lamella and (c) fine lamella.
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F. Katsuki, M. Yonemura / Wear 263 (2007) 1575–1578
equal. These results imply that the deformation of the subsurface region and the dynamic change of the plasticity factor is an important factor for controlling the abrasive wear resistance of the pearlite steel. The combination of low elastic modulus, which spreads out the load, with high hardness, to resist the applied load, should result in maximum possible resistance to abrasive wear. 5. Conclusion
Fig. 6. Wear rate as a function of average surface nanohardness corresponding to the tip contact depth less than 15 nm.
It may be further noted, although the surface nanohardness of the steel with the vanadium addition was lower than that of the vanadium free one, the wear rate of each specimen is almost equal. Wear is characterized as a process of surface damage due to mechanical contact of matter. Thus the type of contact is very important for all wear losses. Greenwood and Williamson proposed a plasticity factor, the ratio of hardness to elastic modulus of the abraded surface which is used to establish the abrasive wear severity of materials [12]. This factor gives an indication of the depth of penetration that a metal can tolerate without exceeding its elastic limit. As shown in Fig. 5, the elastic modulus of the vanadium free steel increased with decreasing the contact depth below the abraded surface to about 1.3 times the original value (∼200 GPa), while the vanadium addition steel was constant. These results indicate that in the case of vanadium free steel, high elastic modulus which concentrates the load on the steel surface should result in lower resistance to abrasive wear. Polycrystalline metals, under unidirectionally abrasion, will give up their random crystalline arrangement and assume a preferred orientation. Metals with a preferred orientation exhibit many peculiarities associated with a single crystal. The elastic modulus of a single crystal of iron is 132 GPa in a direction parallel to 0 0 1. As the direction of load is tilted away from these major axes, the modulus increases until a maximum value of 284 GPa is obtained when the load acts in a 1 1 1 direction [5]. If texturing of the initially random oriented surface layers has occurred during abrasion, the higher elastic modulus of the vanadium free steel may be attributed to the surface texture exhibiting high elastic modulus, such as (1 1 1) texture [13,14]. The results clearly show vanadium additions can enhance the bulk hardness and strength of medium carbon pearlite steels, as shown in Fig. 1. This enhancement is achieved at the expense of some reduction in ductility [15]. Therefore, the extent of the surface hardening and subjected to wear induced deformation may decrease with decrease in ductility due to difficulty in decreasing the interlamellar spacing. As a result, below a critical value of ductility, the surface layer gets removed because of a low work hardening exponent. It may be further noted this removal would prevent the subsurface region of the vanadium addition steel from texturing, which leads to the increase of the elastic modulus by abrasion. Thus the wear rate of each specimen is almost
The effects of the abrasion-induced subsurface changes on the wear behavior of a pearlite steel have been examined by using a nanoindentation technique. It has been found, although the surface nanohardness of the steel with the vanadium addition was lower than that of the vanadium free one, the wear rate of each specimen is almost equal. The contact modulus of the vanadium free steel increased with decreasing the indentation depth below the abraded surface to about 1.3 times the original value (∼200 GPa), while the vanadium addition steel was almost constant. The simple wear mechanism extracted from the surface hardness and elastic modulus by a nanoindentation technique may be useful for controlling the wear characteristics, an important requirement in the qualitative understanding of wear behavior. References [1] P.L. Hurricks, Some metallurgical factors controlling the adhesive and abrasive wear resistance of steels. A review, Wear 26 (1973) 285–304. [2] J. Larsen-Basse, Role of microstructure and mechanical properties in abrasion, Scripta Metall. Mater. 24 (1990) 821–826. [3] B.K. Prasad, S.V. Prasad, Abrasion-induced microstructural changes during low stress abrasion of a plain carbon (0.5% C) steel, Wear 151 (1991) 1–12. [4] K.H. Zum Gahr, Microstructure and Wear of Materials, Elsevier, New York, 1987, pp. 52–53. [5] T.L. Oberle, Properties influencing wear of metals, J. Metals 3 (1951) 438–439. [6] S. Bhattacharyya, Wear and friction in steel, aluminum and magnesium alloys. I. Pearlitic and spheroidized steels, Wear 61 (1980) 133–141. [7] B. Bhushan, Handbook of Micro/Nano-Tribology, CRC Press, New York, 1995, pp. 321–348. [8] W.C. Oliver, G.M. Pharr, Improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564–1583. [9] D.A. Rigney, J.E. Hammerberg, Unlubricated sliding behavior or metals, MRS Bulletin 23 (6) (1998) 32–36. [10] A.K. Bhattacharya, W.D. Nix, Analysis of elastic and plastic deformation associated with indentation testing of thin films on substrates, Int. J. Solids Struct. 24 (1988) 1287–1298. [11] O.P. Modi, D.P. Mondal, B.K. Prasad, M. Singh, H.K. Khaira, Abrasive wear behavior of a high carbon steel: effects of microstructure and experimental parameters and correlation with mechanical properties, Mater. Sci. Eng. A 343 (2003) 235–242. [12] J. Greenwood, J. Williamson, Contact of nominally flat surfaces, Proc. R. Soc. A 295 (1966) 300–319. [13] D.R. Wheeler, D.H. Buckley, Texturing in metals as a result of sliding, Wear 33 (1975) 65–74. [14] V.D. Scott, H. Wilman, Surface re-orientation caused on metals by abrasion—its nature, origin and relation to friction and wear, Proc. R. Soc. A 247 (1958) 353–368. [15] G.L. Dunlop, C.-J. Carlsson, G. Frimodig, Precipitation of VC in ferrite and pearlite during direct transformation of a medium carbon microalloyed steel, Metall. Trans. 9A (1978) 261–266.