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Two-body abrasive wear characteristics of Nylon clay nanocomposites—effect of grit size, load, and sliding velocity
Materials Science and Engineering A 435–436 (2006) 181–186
Two-body abrasive wear characteristics of Nylon clay nanocomposites—effect of grit size, load, and sliding velocity G. Srinath, R. Gnanamoorthy ∗ Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India Received 26 February 2006; received in revised form 10 June 2006; accepted 21 July 2006
Abstract Nylon nanocomposites show a good performance under dry sliding condition but exhibits poor abrasive wear resistance. The effect of grit size, normal load, and sliding velocity on the abrasive wear characteristics of Nylon nanocomposites are reported. Ploughing was the dominant mode of material removal in pristine Nylon while cutting wear mechanism was dominant in Nylon nanocomposites. An empirical relation between the wear volume and the parameters such as normal load sliding speed, sliding distance, and material properties is developed. © 2006 Elsevier B.V. All rights reserved. Keywords: Nylon nanocomposites; Abrasive wear; Ratner–Lancaster factor; Empirical relation
1. Introduction Abrasive wear situations are encountered in applications such as vanes and gears; pumps handling industrial fluids; bearings in steel mills subjected to heat; chute liners abraded by coke, coal, and mineral ores; food processors, etc. [1]. Abrasive wear resistance is a system property rather than a material property [2]. It depends on the abrasive particle size, applied stress, sliding velocity, chemical composition, and the wear mechanism involved. Polymers, although soft, have a high degree of abrasive wear resistance compared with metals of same hardness. The superior performance of polymers is attributed to its inability to fracture the grits and to produce fresh sharp edges [3]. Various fillers and fibres are used as reinforcements to improve the virgin polymers’ mechanical properties as polymers have low strength compared to metals. Incorporation of fibres and fillers in polymers affects the tribo performance, but it is found to be beneficial under some wear conditions while detrimental in some other wear situations [3–5]. Incorporation of fillers and fibres in most polymer composites are found to affect the abrasive wear resistance severely [4,5]. Amount of filler added, filler matrix interaction,
∗
Corresponding author. Tel.: +91 44 2257 4691; fax: +91 44 2257 4652. E-mail address: [email protected] (R. Gnanamoorthy).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.117
and type of the matrix used affect the abrasive wear resistance [3]. Inclusion of the nano sized fillers or reinforcements in polymer matrix evolve a new class of materials called polymer nanocomposites. Polymer nanocomposites exhibit superior mechanical properties, barrier properties, resistance to solvent intake, thermal stability, and flame retardancy than pristine polymer [6]. From tribological point of view, under dry sliding conditions, polymer nanocomposites performed much better than the pristine materials [7–9]. This is due to the lesser material removal, as the size of nano additives are more or less same as the segments of the surrounding polymer chain and also the nano fillers aid in the formation of a tenacious transfer layer [9]. Nylon clay nanocomposites, showed an improved sliding friction and wear performance under dry sliding conditions when slide against a ground stainless steel counterface [8]. However, the Nylon clay nanocomposites performed poorly under abrasive wear mode [10]. The reason for the deterioration of the wear resistance of the Nylon clay nanocomposites was attributed to the decrease in the ductility or strain at break due to the addition of clay. Many models explain the abrasive wear behaviour of polymers, which correlate the wear loss with the material property. The model proposed by Ratner–Lancaster suggests that the abrasive wear loss of the material is inversely proportional to the product of the tensile stress at break and tensile strain at break [10]. The model proposes that the energy required to remove a
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unit volume of material from the surface is proportional to the bulk fracture energy of the material [11]. The operating parameters such as load, velocity, and grit size of the abrasive medium influence the abrasive wear performance of polymer composites. For most of the materials the abrasive wear behaviour is affected with the load, sliding speed, and grit size [1–5,12]. With increase in load, speed, and grit size the abrasive wear of polyaryletherketone composites increases [1]. The abrasive wear rate of PMMA increases rapidly with abrasive grit size until a critical grit size is reached beyond which it becomes independent of grit diameter [12]. A detail study on the effect of system variables that influence the abrasive wear performance of Nylon clay nanocomposites will aid in the selection of material composition for certain applications. This paper describes the effect of load, speed, clay loading, and grit size on the abrasive wear performance of Nylon clay nanocomposites. An empirical relation between these variables and abrasive wear volume is developed.
Fig. 1. Schematic of the pin-on-disk tribometer used in abrasive wear studies.
2. Materials and experimental procedure Layered silicate clays like montmorillonite, hectorite, and saponite are the few potential precursors used for producing nanolevel dispersion [6]. These clays have silicate layers held together by weak bonds. The organically modified clays or organoclays help in better intercalation. Commercial grade Nylon 6 is used in preparing the nanocomposites. The organoclay used was montmorillonite modified by 2-methyl, 2hydrogenated tallow quaternary ammonium chloride (supplied by Elementis Specialties, USA). Nylon nanocomposites with 1, 3, and 5% organoclay (by weight) was prepared by melt intercalation technique. X-ray diffraction carried on the injectionmoulded specimens showed the good exfoliation of the clay in the matrix [8,10]. The detailed methodology of processing and characterisation are discussed elsewhere [8]. Mechanical properties of Nylon nanocomposites are listed in Table 1. Abrasive wear tests were conducted on a pin-on-disk type machine modified to test under abrasive mode. The schematic of the test rig is shown in Fig. 1. The SiC waterproof emery sheets of grades #80, #100, #180, and #320 were used. The abrasive paper was rigidly affixed on a steel disk using ethyl cyanoacrylate-based adhesive. Cylindrical specimens of dimensions 8 mm in diameter and 20 mm in length were injection moulded. The friction force was measured using a force transducer fixed on the loading arm. A non-contact laser displacement transducer was used to measure the linear wear of pins during tests. Friction force and change in dimensions of the pin were measured continuously and data were stored using a personal
Fig. 2. Effect of grit size on the abrasive wear loss of Nylon nanocomposites at a normal load of 5 N and sliding velocity of 0.2 m/s.
computer based data acquisition system. Friction and wear tests were conducted at normal loads 5 and 10 N and sliding velocities 0.2, 0.4, and 1 m/s. Tests were conducted under laboratory conditions (30 ± 3 ◦ C, RH 57 ± 5%). Initial mass of the pins was measured using an electronic balance of 0.1 mg accuracy. The surface roughness was measured before and after tests using a perthometer. Tests were run up to a sliding distance of 180 m. After the test, the pin was cleaned and the specimen mass and surface roughness were measured. Three tests were conducted under each test condition and the average values of measured friction force, wear, and mass loss were used for further analysis. The wear loss is quantified by both mass loss and dimension changes. The wear quantified by dimension change is measured on-line using a laser displacement transducer, which directly gives the change in length of specimen during wear process. The dimensional changes that occur on the disc surface are negligible. 3. Results and discussion The effect of grit size on the abrasive wear loss of Nylon nanocomposites is shown in Fig. 2. Tests conducted on coarse
Table 1 Tensile properties of Nylon nanocomposites Clay content in nanocomposite (wt.%)
Tensile strength (MPa)
0 1 3 5
45 44 47 51
± ± ± ±
5 4 4 3
Strain at break εb
Ratner–Lancaster factor 1/σ b εb (×10−2 MPa−1 )
3.84 2.66 2.03 0.95
0.59 0.90 1.10 2.10
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grits #80 and #100 sheet, which has an average particle size of about 266 and 173 m, respectively, resulted in high wear loss in all the compositions tested. The abrasive wear resistance of the materials on grit #80 are poor due to the large sized abrasive particle, which removes more material during the wear process. Moreover as the abrasive particles are large in size, the asperity contact area, i.e., the real contact area between the abrasive grits and the asperities on pin surface is less. Hence the stresses at these contacts are high for the applied load leading to an early crack initiation and high material removal. The material
Fig. 4. Effect of normal load on the abrasive wear loss of Nylon nanocomposites (grit no. #180, velocity 0.2 m/s).
removal rate is high owing to the ploughing and cutting action of the grits. As the particle size decreases, the material removal rate decreases [3,13]. Grits of grade #180 and #320, having an average particle size of 71 and 31 m, respectively, exhibits almost same wear loss under given test conditions. With increasing the grit numbers, i.e., medium and finer grade grits, the abrasive wear loss of Nylon and nanocomposites becomes independent of the grit size. Among the materials tested, Nylon nanocomposites with 5% clay showed the maximum wear loss when slide under all grit sizes used, due to its least Ratner–Lancaster factor [10]. It is also to be noted that, when the grit size is high, the variation in the wear volume loss among Nylon nanocomposites is also high. Scanning electron micrographs of the worn surfaces of Nylon 6 + 5% clay abraded on abrasive sheets of grades #80, #180, and #320 are shown in Fig. 3. On the worn surface of nanocomposite abraded against the abrasive sheet #80, the abrasive grooves are deep and wide (Fig. 3a). However the worn surfaces of the same material slide against abrasive sheets of grade #180 and #320 indicate shallow and narrow grooves due to the smaller size of the abrasive particles (Fig. 3b and c). Effect of the applied normal load on the abrasive wear loss is shown in Fig. 4. Increase in normal load increases the contact stresses, depth of penetration of the grit on the polymer surface, and hence the wear loss. Under both 5 and 10 N normal loads, pristine Nylon showed a lower abrasive wear loss than the Nylon nanocomposites. Fig. 5 shows the effect of sliding veloc-
Fig. 3. Scanning electron micrographs of worn surfaces of Nylon 6 + 5% clay pin at normal load of 5 N on abrasive papers of: (a) grit no. #80; (b) grit no. #180; and (c) grit no. #320.
Fig. 5. Effect of sliding velocity on the abrasive wear of Nylon nanocomposites (grit no. #180, normal load 5 N).
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Fig. 6. Effect of sliding distance on the abrasive wear of Nylon nanocomposites (grit no. #180, normal load 5 N, velocity 0.2 m/s).
ity on the abrasive wear behaviour of Nylon nanocomposites. Increase in sliding velocity increases the abrasive wear loss of Nylon nanocomposites due to excessive plastic deformation on the polymer surface [3]. The wear loss quantified based on the dimension changes of the pin that occur during the sliding wear tests is shown in Fig. 6. In multi pass testing (repeated passage of pin over the same wear track), the amount of material removed per unit distance, i.e., the wear rate, gradually decreases with continuous sliding due to the entrapment of wear debris between the abrasive grits. Due to clogging of the abrasive grits by wear debris, the effectiveness of abrasive grit to penetrate or cut the polymer surface is reduced and hence the wear rate [3]. Clogging of abrasive grits depends upon the wear debris size and also the abrasive grit size. Although the wear rate decreases with increase in sliding distance, the abrasive wear loss increases with sliding distance for all the materials tested. The clogging of abrasive grits has a severe effect on the coefficient of friction of Nylon nanocomposites. The effect of clay content, sliding distance, and grit size on the coefficient of friction is shown in Fig. 7. The coefficient of friction of Nylon nanocomposites with 1, 3, and 5% clay, when slide against #320 grit sheet, is less compared to that in tests conducted at other grit sizes. Small size of abrasive particles and effective clogging
of grits by wear debris contributes to the variation in coefficient of friction. Pristine Nylon exhibited a least coefficient of friction on grit #180 paper. Except pristine Nylon, coefficient of friction of all other materials exhibited a decreasing trend with sliding distance and also Nylon nanocomposites showed a lesser coefficient of friction than pristine Nylon. The efficiency of clogging plays a dominant role on the coefficient of friction [10]. Addition of clay affects the abrasive wear mechanism. Scanning electron micrographs of worn surfaces of pins are shown in Fig. 8. Deep grooves indicating severe ploughing action was observed in pristine Nylon (Fig. 8a). As Nylon 6 is ductile, the material removal by ploughing mechanism is dominant [14]. As the clay content increases, nanocomposites become brittle and cutting action dominates. Worn surfaces of Nylon nanocomposites with 1, 3, and 5% clay (Fig. 8b–d) show the presence of grooves and particles indicating cutting action. The damage area in Nylon nanocomposites with 5% clay is large indicating severe cutting action (Fig. 8d). The surface roughness measured perpendicular to the sliding direction of the pins worn against abrasive paper with grit #80, #100, #180, and #320 are shown in Fig. 9. Pins worn against #80 grit paper are rough compared with others. Larger size of the abrasive grains in #80 grit paper causes greater damage to pin surface. Nylon nanocomposite pins with 5% clay shows an excessive surface damage due to its brittle nature and the dominant cutting wear mechanism. This is also evident from the micrograph in Fig. 8. 4. Relation between the abrasive wear loss and operating parameters Many attempts have been made to correlate the wear volume to the operating parameters and bulk properties of materials [15–17]. Empirical relation had been arrived by principles of dimensional analysis by Kar and Bahadur [15] for polyoxymethyelene (POM) and PTFE filled POM. The equation proposed by Kar and Bahadur [15] was modified by Viswanath and Bello [16]. As the operating parameters such as load, velocity,
Fig. 7. Effect of abrasive grit size and sliding distance on the coefficient of friction of Nylon nanocomposites at normal load of 5 N and velocity of 0.2 m/s.
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Fig. 8. Scanning electron micrographs of the worn pins tested on the grit no. #180 at a load of 5 N and sliding velocity 0.2 m/s: (a) Nylon 6; (b) Nylon + 1% clay; (c) Nylon + 3% clay; and (d) Nylon + 5% clay.
and sliding distance affect the wear volume, an empirical relation between these variables and the wear volume was attempted using the principles of dimensional analysis. For the sake of simplicity, the effect of grit size was neglected as the wear volume at higher grit grades are more or less the same. In other words, when the abrasive particle size is smaller, the grit size does not have an appreciable effect on wear. The abrasive wear volume loss of polymer/composite depends on the load, velocity, sliding distance, and the material parameter Ratner–Lancaster factor, R (1/σ b εb ). Hence W = f (L, v, S, R, ρ)
(1)
where W is the wear volume (m3 ), L is the normal load (N), v is the sliding velocity (m/s), R is the Ratner–Lancaster factor (m2 /N), S is the sliding distance (m), and ρ is the density of the material (kg/mm3 ).
According to Buckingham pi theorem, three dimensionless groups are formed LR L W (2) =f S3 S 2 ρ(Sv)2 Hence b W L LR a =K S3 S2 ρ(Sv)2
(3)
i.e., W = KLa+b v−2b S 3−2a−2b Ra ρb
(4)
The wear rate is W = KLa+b v−2b S 2−2a−2b Ra ρb S
(5)
where K is the wear constant. The values of a, b and K are determined from the experimental data obtained during sliding tests conducted on Nylon nanocomposites slide against grit #180 abrasive paper. Here the wear is represented as wear rate as the experimental data is available for a sliding distance of 180 m. As the density is nearly same for the test materials, it is combined with the constant K. Then Eq. (5) becomes
Fig. 9. Effect of grit size on the final surface roughness of pins.
W = 1.18 × 10−05 L0.77 v0.07 R0.84 S 0.46 S
(6)
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1. Abrasive wear resistance in nanocomposites depends on the size of abrasive medium. Increase in the normal load and sliding velocity increases the material loss. 2. Wear rate of Nylon nanocomposites decreases with the sliding distance due to repeated passage of the pin over same wear track, which causes clogging of abrasive grit by wear debris. 3. Clogging decreases the coefficient of friction of nanocomposites. Nanocomposites with 1, 3, and 5% clay exhibited low coefficient of friction when abraded against #320 grit sheet. 4. Ploughing is dominant wear mechanism in pristine Nylon, while cutting mechanism is predominant in Nylon nanocomposites. Surface damage of pin is more when abraded by coarse grit. 5. Wear rate prediction based on the empirical relation agrees well with experimental data. References
Fig. 10. Actual and predicted wear rates for Nylon and Nylon nanocomposites abraded on grit no. #180.
The wear rate predicted from the empirical equation agrees with the experimental data from test conducted on grit #80 (Fig. 10). As the effect of temperature rise that occur during sliding was not considered, there is a difference in the actual and predicted wear loss in tests conducted at high sliding velocity, 1 m/s, is observed. The temperature at interface plays a major role and alters the wear drastically. 5. Conclusions Based on the abrasive wear tests conducted on Nylon nanocomposites following conclusions are drawn.
[1] A.P. Harsha, U.S. Tewari, Polym. Test. 21 (2002) 697–709. [2] R.G. Bayer, Mechanical Wear Prediction and Prevention, Marcel Dekker Inc., New York, 1994. [3] G.W. Stachowaik, A.W. Batchelor, Engineering Tribology, Butterworth HeinMann, Boston, 2000. [4] J. Bijwe, J. Indumathi, J. John Rajesh, M. Fahim, Wear 249 (2001) 715– 726. [5] J. Bijwe, J. John Rajesh, A. Jeyakumar, A. Gosh, U.S. Tewari, Tribol. Int. 33 (2000) 697–706. [6] M. Alexandre, P. Dubois, Mater. Sci. Eng. R 28 (2000) 1–63. [7] Q.-H. Wang, J. Xu, W. Shen, Q. Xue, Wear 209 (1997) 316–321. [8] G. Srinath, R. Gnanamoorthy, J. Mater. Sci. 40 (2005) 2897– 2901. [9] A. Dasari, Z.-Z. Yu, Y.-W. Mai, G.-H. Hu, J. Varlet, Compos. Sci. Technol. 65 (2005) 2314–2328. [10] G. Srinath, R. Gnanamoorthy, J. Mater. Sci., in press. [11] J.O. Bello, R.J.K. Wood, Wear 258 (2005) 294–302. [12] H. Sin, N. Saka, P. Suh, Wear 55 (1979) 163–190. [13] A.A. Torrance, Wear 253 (2002) 813–819. [14] K.G. Bundisnki, Wear 203–204 (1997) 302–309. [15] M.K. Kar, S. Bahadur, Wear 30 (1974) 337–348. [16] N. Viswanath, D.G. Bello, Wear 181–183 (1995) 42–49. [17] J.A. Williams, Tribol. Int. 38 (2005) 863–870.
Two-body abrasive wear characteristics of Nylon clay nanocomposites—effect of grit size, load, and sliding velocity G. Srinath, R. Gnanamoorthy ∗ Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India Received 26 February 2006; received in revised form 10 June 2006; accepted 21 July 2006
Abstract Nylon nanocomposites show a good performance under dry sliding condition but exhibits poor abrasive wear resistance. The effect of grit size, normal load, and sliding velocity on the abrasive wear characteristics of Nylon nanocomposites are reported. Ploughing was the dominant mode of material removal in pristine Nylon while cutting wear mechanism was dominant in Nylon nanocomposites. An empirical relation between the wear volume and the parameters such as normal load sliding speed, sliding distance, and material properties is developed. © 2006 Elsevier B.V. All rights reserved. Keywords: Nylon nanocomposites; Abrasive wear; Ratner–Lancaster factor; Empirical relation
1. Introduction Abrasive wear situations are encountered in applications such as vanes and gears; pumps handling industrial fluids; bearings in steel mills subjected to heat; chute liners abraded by coke, coal, and mineral ores; food processors, etc. [1]. Abrasive wear resistance is a system property rather than a material property [2]. It depends on the abrasive particle size, applied stress, sliding velocity, chemical composition, and the wear mechanism involved. Polymers, although soft, have a high degree of abrasive wear resistance compared with metals of same hardness. The superior performance of polymers is attributed to its inability to fracture the grits and to produce fresh sharp edges [3]. Various fillers and fibres are used as reinforcements to improve the virgin polymers’ mechanical properties as polymers have low strength compared to metals. Incorporation of fibres and fillers in polymers affects the tribo performance, but it is found to be beneficial under some wear conditions while detrimental in some other wear situations [3–5]. Incorporation of fillers and fibres in most polymer composites are found to affect the abrasive wear resistance severely [4,5]. Amount of filler added, filler matrix interaction,
∗
Corresponding author. Tel.: +91 44 2257 4691; fax: +91 44 2257 4652. E-mail address: [email protected] (R. Gnanamoorthy).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.117
and type of the matrix used affect the abrasive wear resistance [3]. Inclusion of the nano sized fillers or reinforcements in polymer matrix evolve a new class of materials called polymer nanocomposites. Polymer nanocomposites exhibit superior mechanical properties, barrier properties, resistance to solvent intake, thermal stability, and flame retardancy than pristine polymer [6]. From tribological point of view, under dry sliding conditions, polymer nanocomposites performed much better than the pristine materials [7–9]. This is due to the lesser material removal, as the size of nano additives are more or less same as the segments of the surrounding polymer chain and also the nano fillers aid in the formation of a tenacious transfer layer [9]. Nylon clay nanocomposites, showed an improved sliding friction and wear performance under dry sliding conditions when slide against a ground stainless steel counterface [8]. However, the Nylon clay nanocomposites performed poorly under abrasive wear mode [10]. The reason for the deterioration of the wear resistance of the Nylon clay nanocomposites was attributed to the decrease in the ductility or strain at break due to the addition of clay. Many models explain the abrasive wear behaviour of polymers, which correlate the wear loss with the material property. The model proposed by Ratner–Lancaster suggests that the abrasive wear loss of the material is inversely proportional to the product of the tensile stress at break and tensile strain at break [10]. The model proposes that the energy required to remove a
182
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unit volume of material from the surface is proportional to the bulk fracture energy of the material [11]. The operating parameters such as load, velocity, and grit size of the abrasive medium influence the abrasive wear performance of polymer composites. For most of the materials the abrasive wear behaviour is affected with the load, sliding speed, and grit size [1–5,12]. With increase in load, speed, and grit size the abrasive wear of polyaryletherketone composites increases [1]. The abrasive wear rate of PMMA increases rapidly with abrasive grit size until a critical grit size is reached beyond which it becomes independent of grit diameter [12]. A detail study on the effect of system variables that influence the abrasive wear performance of Nylon clay nanocomposites will aid in the selection of material composition for certain applications. This paper describes the effect of load, speed, clay loading, and grit size on the abrasive wear performance of Nylon clay nanocomposites. An empirical relation between these variables and abrasive wear volume is developed.
Fig. 1. Schematic of the pin-on-disk tribometer used in abrasive wear studies.
2. Materials and experimental procedure Layered silicate clays like montmorillonite, hectorite, and saponite are the few potential precursors used for producing nanolevel dispersion [6]. These clays have silicate layers held together by weak bonds. The organically modified clays or organoclays help in better intercalation. Commercial grade Nylon 6 is used in preparing the nanocomposites. The organoclay used was montmorillonite modified by 2-methyl, 2hydrogenated tallow quaternary ammonium chloride (supplied by Elementis Specialties, USA). Nylon nanocomposites with 1, 3, and 5% organoclay (by weight) was prepared by melt intercalation technique. X-ray diffraction carried on the injectionmoulded specimens showed the good exfoliation of the clay in the matrix [8,10]. The detailed methodology of processing and characterisation are discussed elsewhere [8]. Mechanical properties of Nylon nanocomposites are listed in Table 1. Abrasive wear tests were conducted on a pin-on-disk type machine modified to test under abrasive mode. The schematic of the test rig is shown in Fig. 1. The SiC waterproof emery sheets of grades #80, #100, #180, and #320 were used. The abrasive paper was rigidly affixed on a steel disk using ethyl cyanoacrylate-based adhesive. Cylindrical specimens of dimensions 8 mm in diameter and 20 mm in length were injection moulded. The friction force was measured using a force transducer fixed on the loading arm. A non-contact laser displacement transducer was used to measure the linear wear of pins during tests. Friction force and change in dimensions of the pin were measured continuously and data were stored using a personal
Fig. 2. Effect of grit size on the abrasive wear loss of Nylon nanocomposites at a normal load of 5 N and sliding velocity of 0.2 m/s.
computer based data acquisition system. Friction and wear tests were conducted at normal loads 5 and 10 N and sliding velocities 0.2, 0.4, and 1 m/s. Tests were conducted under laboratory conditions (30 ± 3 ◦ C, RH 57 ± 5%). Initial mass of the pins was measured using an electronic balance of 0.1 mg accuracy. The surface roughness was measured before and after tests using a perthometer. Tests were run up to a sliding distance of 180 m. After the test, the pin was cleaned and the specimen mass and surface roughness were measured. Three tests were conducted under each test condition and the average values of measured friction force, wear, and mass loss were used for further analysis. The wear loss is quantified by both mass loss and dimension changes. The wear quantified by dimension change is measured on-line using a laser displacement transducer, which directly gives the change in length of specimen during wear process. The dimensional changes that occur on the disc surface are negligible. 3. Results and discussion The effect of grit size on the abrasive wear loss of Nylon nanocomposites is shown in Fig. 2. Tests conducted on coarse
Table 1 Tensile properties of Nylon nanocomposites Clay content in nanocomposite (wt.%)
Tensile strength (MPa)
0 1 3 5
45 44 47 51
± ± ± ±
5 4 4 3
Strain at break εb
Ratner–Lancaster factor 1/σ b εb (×10−2 MPa−1 )
3.84 2.66 2.03 0.95
0.59 0.90 1.10 2.10
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grits #80 and #100 sheet, which has an average particle size of about 266 and 173 m, respectively, resulted in high wear loss in all the compositions tested. The abrasive wear resistance of the materials on grit #80 are poor due to the large sized abrasive particle, which removes more material during the wear process. Moreover as the abrasive particles are large in size, the asperity contact area, i.e., the real contact area between the abrasive grits and the asperities on pin surface is less. Hence the stresses at these contacts are high for the applied load leading to an early crack initiation and high material removal. The material
Fig. 4. Effect of normal load on the abrasive wear loss of Nylon nanocomposites (grit no. #180, velocity 0.2 m/s).
removal rate is high owing to the ploughing and cutting action of the grits. As the particle size decreases, the material removal rate decreases [3,13]. Grits of grade #180 and #320, having an average particle size of 71 and 31 m, respectively, exhibits almost same wear loss under given test conditions. With increasing the grit numbers, i.e., medium and finer grade grits, the abrasive wear loss of Nylon and nanocomposites becomes independent of the grit size. Among the materials tested, Nylon nanocomposites with 5% clay showed the maximum wear loss when slide under all grit sizes used, due to its least Ratner–Lancaster factor [10]. It is also to be noted that, when the grit size is high, the variation in the wear volume loss among Nylon nanocomposites is also high. Scanning electron micrographs of the worn surfaces of Nylon 6 + 5% clay abraded on abrasive sheets of grades #80, #180, and #320 are shown in Fig. 3. On the worn surface of nanocomposite abraded against the abrasive sheet #80, the abrasive grooves are deep and wide (Fig. 3a). However the worn surfaces of the same material slide against abrasive sheets of grade #180 and #320 indicate shallow and narrow grooves due to the smaller size of the abrasive particles (Fig. 3b and c). Effect of the applied normal load on the abrasive wear loss is shown in Fig. 4. Increase in normal load increases the contact stresses, depth of penetration of the grit on the polymer surface, and hence the wear loss. Under both 5 and 10 N normal loads, pristine Nylon showed a lower abrasive wear loss than the Nylon nanocomposites. Fig. 5 shows the effect of sliding veloc-
Fig. 3. Scanning electron micrographs of worn surfaces of Nylon 6 + 5% clay pin at normal load of 5 N on abrasive papers of: (a) grit no. #80; (b) grit no. #180; and (c) grit no. #320.
Fig. 5. Effect of sliding velocity on the abrasive wear of Nylon nanocomposites (grit no. #180, normal load 5 N).
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Fig. 6. Effect of sliding distance on the abrasive wear of Nylon nanocomposites (grit no. #180, normal load 5 N, velocity 0.2 m/s).
ity on the abrasive wear behaviour of Nylon nanocomposites. Increase in sliding velocity increases the abrasive wear loss of Nylon nanocomposites due to excessive plastic deformation on the polymer surface [3]. The wear loss quantified based on the dimension changes of the pin that occur during the sliding wear tests is shown in Fig. 6. In multi pass testing (repeated passage of pin over the same wear track), the amount of material removed per unit distance, i.e., the wear rate, gradually decreases with continuous sliding due to the entrapment of wear debris between the abrasive grits. Due to clogging of the abrasive grits by wear debris, the effectiveness of abrasive grit to penetrate or cut the polymer surface is reduced and hence the wear rate [3]. Clogging of abrasive grits depends upon the wear debris size and also the abrasive grit size. Although the wear rate decreases with increase in sliding distance, the abrasive wear loss increases with sliding distance for all the materials tested. The clogging of abrasive grits has a severe effect on the coefficient of friction of Nylon nanocomposites. The effect of clay content, sliding distance, and grit size on the coefficient of friction is shown in Fig. 7. The coefficient of friction of Nylon nanocomposites with 1, 3, and 5% clay, when slide against #320 grit sheet, is less compared to that in tests conducted at other grit sizes. Small size of abrasive particles and effective clogging
of grits by wear debris contributes to the variation in coefficient of friction. Pristine Nylon exhibited a least coefficient of friction on grit #180 paper. Except pristine Nylon, coefficient of friction of all other materials exhibited a decreasing trend with sliding distance and also Nylon nanocomposites showed a lesser coefficient of friction than pristine Nylon. The efficiency of clogging plays a dominant role on the coefficient of friction [10]. Addition of clay affects the abrasive wear mechanism. Scanning electron micrographs of worn surfaces of pins are shown in Fig. 8. Deep grooves indicating severe ploughing action was observed in pristine Nylon (Fig. 8a). As Nylon 6 is ductile, the material removal by ploughing mechanism is dominant [14]. As the clay content increases, nanocomposites become brittle and cutting action dominates. Worn surfaces of Nylon nanocomposites with 1, 3, and 5% clay (Fig. 8b–d) show the presence of grooves and particles indicating cutting action. The damage area in Nylon nanocomposites with 5% clay is large indicating severe cutting action (Fig. 8d). The surface roughness measured perpendicular to the sliding direction of the pins worn against abrasive paper with grit #80, #100, #180, and #320 are shown in Fig. 9. Pins worn against #80 grit paper are rough compared with others. Larger size of the abrasive grains in #80 grit paper causes greater damage to pin surface. Nylon nanocomposite pins with 5% clay shows an excessive surface damage due to its brittle nature and the dominant cutting wear mechanism. This is also evident from the micrograph in Fig. 8. 4. Relation between the abrasive wear loss and operating parameters Many attempts have been made to correlate the wear volume to the operating parameters and bulk properties of materials [15–17]. Empirical relation had been arrived by principles of dimensional analysis by Kar and Bahadur [15] for polyoxymethyelene (POM) and PTFE filled POM. The equation proposed by Kar and Bahadur [15] was modified by Viswanath and Bello [16]. As the operating parameters such as load, velocity,
Fig. 7. Effect of abrasive grit size and sliding distance on the coefficient of friction of Nylon nanocomposites at normal load of 5 N and velocity of 0.2 m/s.
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Fig. 8. Scanning electron micrographs of the worn pins tested on the grit no. #180 at a load of 5 N and sliding velocity 0.2 m/s: (a) Nylon 6; (b) Nylon + 1% clay; (c) Nylon + 3% clay; and (d) Nylon + 5% clay.
and sliding distance affect the wear volume, an empirical relation between these variables and the wear volume was attempted using the principles of dimensional analysis. For the sake of simplicity, the effect of grit size was neglected as the wear volume at higher grit grades are more or less the same. In other words, when the abrasive particle size is smaller, the grit size does not have an appreciable effect on wear. The abrasive wear volume loss of polymer/composite depends on the load, velocity, sliding distance, and the material parameter Ratner–Lancaster factor, R (1/σ b εb ). Hence W = f (L, v, S, R, ρ)
(1)
where W is the wear volume (m3 ), L is the normal load (N), v is the sliding velocity (m/s), R is the Ratner–Lancaster factor (m2 /N), S is the sliding distance (m), and ρ is the density of the material (kg/mm3 ).
According to Buckingham pi theorem, three dimensionless groups are formed LR L W (2) =f S3 S 2 ρ(Sv)2 Hence b W L LR a =K S3 S2 ρ(Sv)2
(3)
i.e., W = KLa+b v−2b S 3−2a−2b Ra ρb
(4)
The wear rate is W = KLa+b v−2b S 2−2a−2b Ra ρb S
(5)
where K is the wear constant. The values of a, b and K are determined from the experimental data obtained during sliding tests conducted on Nylon nanocomposites slide against grit #180 abrasive paper. Here the wear is represented as wear rate as the experimental data is available for a sliding distance of 180 m. As the density is nearly same for the test materials, it is combined with the constant K. Then Eq. (5) becomes
Fig. 9. Effect of grit size on the final surface roughness of pins.
W = 1.18 × 10−05 L0.77 v0.07 R0.84 S 0.46 S
(6)
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1. Abrasive wear resistance in nanocomposites depends on the size of abrasive medium. Increase in the normal load and sliding velocity increases the material loss. 2. Wear rate of Nylon nanocomposites decreases with the sliding distance due to repeated passage of the pin over same wear track, which causes clogging of abrasive grit by wear debris. 3. Clogging decreases the coefficient of friction of nanocomposites. Nanocomposites with 1, 3, and 5% clay exhibited low coefficient of friction when abraded against #320 grit sheet. 4. Ploughing is dominant wear mechanism in pristine Nylon, while cutting mechanism is predominant in Nylon nanocomposites. Surface damage of pin is more when abraded by coarse grit. 5. Wear rate prediction based on the empirical relation agrees well with experimental data. References
Fig. 10. Actual and predicted wear rates for Nylon and Nylon nanocomposites abraded on grit no. #180.
The wear rate predicted from the empirical equation agrees with the experimental data from test conducted on grit #80 (Fig. 10). As the effect of temperature rise that occur during sliding was not considered, there is a difference in the actual and predicted wear loss in tests conducted at high sliding velocity, 1 m/s, is observed. The temperature at interface plays a major role and alters the wear drastically. 5. Conclusions Based on the abrasive wear tests conducted on Nylon nanocomposites following conclusions are drawn.
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