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Scratch behavior of the aged hydrogenated nitrile butadiene rubber
Wear 352-353 (2016) 155–159
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Scratch behavior of the aged hydrogenated nitrile butadiene rubber Zhongmeng Zhu, Qian Cheng, Chengkai Jiang, Jianwei Zhang, Han Jiang n Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu 610031, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 November 2015 Received in revised form 13 February 2016 Accepted 17 February 2016 Available online 23 February 2016
The objective of this work was to study the scratch behavior of the aged hydrogenated nitrile butadiene rubber (HNBR) including its scratch damage modes and their corresponding mechanism. A home-made device was used to perform the scratch testing on the virgin and thermal-aged HNBR substrates with a progressively increasing depth. The scratch resistance of HNBR declined dramatically with the increase of the aging time. The surface topography and damage modes of the scratched HNBR samples were investigated. Two types of scratch damage mode, i.e. the flake peeling and the helical-form damage were observed and both exhibited the characteristics of tearing. Specimens of two different geometries were used to measure the effects of aging time on tear strength of HNBR. It was found that there was a strong correlation between the strength data from two types of tear tests with the scratch damage modes. This finding provides a valuable guidance for improving the scratch resistance of the rubber materials and prolonging their service life. & 2016 Elsevier B.V. All rights reserved.
Keywords: Scratch testing Polymers Fracture Surface morphology Tear strength Thermal aging
1. Introduction During the daily use of rubber components over a wide range of applications such as tires, seals and vibration absorbers etc. [1], the surface scratch due to the relative movement with other counterparts not only seriously decreases the esthetics, but also degrades the material performance and eventually shortens the service life of the rubber products. Moreover, the scratch performance of rubber materials can be tremendously worsened by the inevitable aging caused by the harsh service environment and complex loading conditions [2–4]. Therefore, the study of the scratch behavior of the aged rubber is of great importance for engineering applications. Different from the investigation on the rubber wear, which focuses on the wear volume loss during a repeated abrasion [5–9], the research on rubber scratch, a single pass with a point contact, discusses the evolution and mechanism of the scratch damage [10]. The scratch behavior has been intensively studied on a wide range of plastic polymers such as polycarbonate [10,11], polymethyl methacrylate [12], polypropylene [13], thermoplastic polyolefin [10,14] and polymer-based composites [15–17], but not much attention has been given to rubbers. Schallamach investigated the single pass friction behavior of natural rubber using a stainless steel needle and found that the tensile stress in the substrate behind the needle produced lateral tears of the rubber across the sliding trace [18–20]. Briscoe n
Corresponding author. Tel.: þ 86 28 87601442; fax: þ 86 28 87600797. E-mail address: [email protected] (H. Jiang).
http://dx.doi.org/10.1016/j.wear.2016.02.010 0043-1648/& 2016 Elsevier B.V. All rights reserved.
found that the decreased cone angle of the scratch tip led to a more severe scratch damage of the rubber substrate and the bulk material tearing was accounted for the damage [21]. Low indicated that the scratch velocity could aggravated the scratch damage of styrene butadiene rubber, neoprene and ethylene propylene diene monomer rubber while the carbon black reinforcement could improve their scratch resistance [22]. The correlation between the material properties and possible scratch damage modes of rubbers was not fully investigated. Moreover, little work has been conducted to study the scratch behavior of the aged rubbers. In this work, the scratch tests with a progressive increasing scratch depth were carried out on the virgin and thermal aged HNBR using a home-made scratch device. The surface morphology of the scratched HNBR was investigated to identify the scratch damage modes. Then two types of tear strength were measured for the samples with different aging time. The correlation between different scratch damage modes and tear strengths was established. This study could help the material researchers to better understand the scratch behavior of rubbers and thus to improve its overall scratch resistance.
2. Experiment 2.1. Material and thermal aging experiments The HNBR plates (Kaidi Northwest Rubber Co. Ltd.) were 90% saturated, containing 50% acrylonitrile content and reinforced
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Fig. 1. Shape and size of the tear specimens of HNBR. (a) Type T (b) Type B.
with 30 phr hydrated silica. All the samples were thermally aged at a moderate temperature, i.e. 90 °C (about 120 °C higher than the Tg of HNBR), in an environment chamber (WG03, INBORN) referencing the literatures [23,24]. After reaching the prescribed aging time, the samples were removed from the chamber and air cooled to the ambient temperature. Then, they were stabilized in air for more than 16 h. 2.2. Tear tests An electronic universal testing machine (AGS-J, SHIMADZU) was utilized to measure the tear strengths of the HNBR at room temperature (about 25 °C). Two types of tear specimen, type T (Fig. 1(a)) and type B (Fig. 1(b)), were selected following ASTM D624-00 (2012) since they represented two different fracture modes in nature, i.e. type T tear for mode III fracture and type B tear for mode I fracture respectively. The tear tests were carried out under a loading rate of 500mm/min for type T and 50 mm/min for type B tearing respectively according to ASTM D624-00 (2012). Then the values of the tear strengths were calculated and the average value from five repeated tests was recorded. 2.3. Scratch tests and post-scratch analysis The scratch tests were performed under the ambient temperature using a home-made scratch device, which is schematically illustrated in Fig. 2. The scratch test unit comprises of a servo motor that drives the scratch tip moving tangentially with a prescribed speed. The increasing scratch depth is controlled with the help of another servo motor. The rigidity of the apparatus is enough to prevent the possible vibrations during scratch. To avoid its arching in front of the tip during scratching, the rubber sample was glued on an epoxy plate and then the plate was well clamped on the test stage. A conical stainless steel indenter (half cone angle: 22.5°) with a spherical tip (diameter (d): E 0.1 mm) was used in this work. A progressive increasing scratch depth (D) from 0 to 1.5 mm was adopted. The scratch length was 100 mm and the scratch speed was kept constant at 25 mm/s. The formation of the
Fig. 2. Schematic illustration of the home-made scratch device.
scratch damages has been in-situ observed with a high-speed video (Photron FASTCAM Mini UX50). Three duplicated scratch tests were carried out under each aging condition. The surface topography of the scratched sample was characterized using a super depth digital microscope (VHX-1000, Keyence). The good resolution (600 600 dpi) images of the scratches were obtained using an optical scanner (LiDE 210, Canon).
3. Results and discussion 3.1. Aging effect on the scratch behavior The scratch damage images of the HNBRs at different aging time under a linearly increased scratch depth are exhibited in Fig. 3. It is clear that the longer the aging time is, the more severe the scratch damage is. Only a minor scratch is visible for the virgin specimen even at the latest stage of the scratch process. However,
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Fig. 3. Scanned images of HNBR with different aging times scratched with linearly increasing scratch depth of 0–1.5 mm. (△: Onset of flake peeling ▲: Onset of helical-form damage).
Fig. 4. Flake peeling and helical-form scratch damages (a) optical scan image of scratched specimen with 30 days aging, (b) micro image of the flake peeling, (c) schematic illustration of the flake peeling, (d) micro image of the helical-form damage and (e) Schematic illustration of the helical-form damage.
an obvious groove, whose length is larger than one third of the whole scratch path, can be found for the sample after 90 days aging. This indicates that the scratch resistance of the material dramatically declines with the increase of the aging time. The microscopy observation was carried out on the scratched samples at different aging time. The sample with 30 days aging is shown in Fig. 4(a) as a good representation. The evolution of the scratch damage can be described in three stages. Firstly, the relatively shallow scratch depth and low stress level only induce some minor ridges (known as Schallamach ridges) on the surface of the specimen. These parallel ridges between which the rubber buckles inward lie perpendicular to the sliding direction and have been called as “abrasion patterns” [18–20]. During the second stage of the scratch process, the flake peeling, shown in Fig. 4(b), occurs on the surface of HNBR with the increasing scratch depth. Its formation is schematically illustrated in Fig. 4(c). It is believed that this type of damage is initiated from a stress concentration point, due to the minor surface defect or the filler close to the surface. When the endured stress of the rubber substrate behind the indenter goes beyond the material strength, the surface material, stuck to the forward-moving indenter, peels up gradually from the initial point of damage. The peeling process
is similar to the type T tearing to some extent. After the dissipation of the energy, the peeling stops and leaves a rubber flake in an ellipse shape contour. When this process occurred periodically, the repeated flake peeling is well observed in the range of median scratch depth. It should be mentioned that, during this stage, the scratch tip still does not pierce into the rubber substrate, in spite of the serious damage already occurred. As the scratch depth further increases, the scratch tip finally pierces into the substrate and then the repeated helical-form damage (Fig. 4(d)) appears. Fig. 4(e) schematically demonstrated its formation. A crack initiates from the defect point where the scratch tip pierced into the substrate. As the indenter moves forward, the crack gradually extends to both sides of the groove. Then dragged by the moving indenter, the material accumulates in front of the tip and curls forward. This induces the crack propagated along the sliding direction and leaves a helical-form damage on the scratched surface eventually. This process is somewhat similar to the propagation of the mode I crack which is corresponded to the type B tearing to some extent. It should be noted that the surface properties, degrading with aging process, influence the scratch behavior to a certain extent. However, considering the size scale of the scratch damage whose
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Z. Zhu et al. / Wear 352-353 (2016) 155–159
Fig. 5. Correlation between the onset of flake peeling and the type T tear strength (a) evolution of the onset depth of flake peeling and the type T tear strength with aging time (b) linear correlation between the onset of flake peeling and the type T tear strength of the aged HNBR.
onset depth exceeds 0.5 mm, the bulk properties of the aged HNBR should be the dominant factors for the studied scratch performance. 3.2. Correlation between the onsets of scratch damages and the tear strengths As discussed above, the form of type T tearing occurs in the stage two of HNBR scratch process and the type B tearing appears corresponding to the stage three with the helical-form damage. The correlation between the scratch damage modes and the tear strengths of HNBR is discussed below. Fig. 5(a) shows the evolution of the onset depth of flake peeling and the strength of type T tearing with the increase of the aging time. The scission of the molecular chains during the thermal aging degrades the material strengths of the HBNR specimens. The type T tear strength drops sharply at the first 15 days and then gradually decreases to a stable value. The onset depth of flake peeling decreases in the same tendency with the aging time. After aging for 90 days, both onset depth of flake peeling and type T tear strength decline approximately 60%. It is well known that the surface flaws such as minor cracks and edge nicks will develops with the aging process. Combined with the decline of the type T tear strength, aging process results in the severe degradation of the HNBR’s resistance to the flake peeling damage. A Pearson correlation analysis was carried out, which is shown in Fig. 5(b). A strong linear correlation can be found between the onset depth of
Fig. 6. Correlation between the onset of helical-form damage and the type B tear strength (a) evolution of the onset depth of helical-form damage and the type B tear strength with aging time (b) linear correlation between the onset of helicalform damage and the type B tear strength of the aged HNBR.
flake peeling and the type T tear strength with R2 ¼0.919. This denotes that the type T tear strength could be a good index of the scratch resistance corresponding to the flake peeling damage of HNBR. As shown in Fig. 6(a), the onset depth of helical-form damage and the type B tear strength decrease in the same trend with the development of aging process. The result of the Pearson correlation analysis is shown in Fig. 6(b). Similarly, a strong linear correlation exists between the onset depth of the helical-form damage and the type B tear strength with R2 ¼0.923. This implies that the scratch resistance corresponding to the helicalform damage of HNBR could be evaluated using the type B tear strength. Although in the studied range, the linear correlation is enough to describe the relationship between the onset scratch depths and tear strengths, the physical mechanism behind it is not fully understood yet. This should be explored in the future work. In addition to the tear strengths of HNBR, we also measured the tensile strength and the fracture strain as well. However, there is only very weak correlation, if exists, between the onsets of the scratch damage and the tensile properties. It is clear that the scratch resistance of rubber materials can be improved by enhancing these two types of tear properties. For instance, reinforcing the rubber compounds with carbon black and increasing the crosslink density [25,26] are good options to
Z. Zhu et al. / Wear 352-353 (2016) 155–159
improve the tear properties of rubber materials thus to enhance their overall scratch performance as well.
4. Conclusions This work investigated the scratch behavior of the aged HNBR under a linearly increasing scratch depth with a home-made scratch device. The following conclusions are found. (1) Thermal aging had a significant effect on the scratch behavior of HNBR. The scratch resistance of HNBR dramatically declines with the development of the aging process. (2) With the increase of the scratch depth, two scratch damage modes, i.e. the flake peeling and the helical-form damage, occur in sequence on the surface of the HNBR. (3) The scratch damage mechanism of the flake peeling is found to be correlated with the type T tearing while the type B tearing is accounted for the helical-form damage during scratching. This finding provides a good guidance to improve the scratch resistance of the rubbery materials.
Acknowledgments The work is supported by National Natural Science Foundation of China (11172249, 11272269). The authors would also like to thank the partial financial support from Ministry of Education of China (NCET-12-0938), Science and Technology Department of Sichuan Province (2013JQ0010)
References [1] M. Morton, Rubber Technology, 3rd ed., Springer Science & Business Media, Dordrecht, 1999. [2] C. Dong, C. Yuan, X. Bai, X. Yan, Z. Peng, Tribological properties of aged nitrile butadiene rubber under dry sliding conditions, Wear 322 (2015) 226–237. [3] F. Guo, X. Jia, L. Huang, R.F. Salant, Y. Wang, The effect of aging during storage on the performance of a radial lip seal, Polym. Degrad. Stab. 98 (2013) 2193–2200. [4] Z. Zhu, C. Jiang, Q. Cheng, J. Zhang, S. Guo, Y. Xiong, B. Fu, W. Yang, H. Jiang, Accelerated aging test of hydrogenated nitrile butadiene rubber using the time–temperature–strain superposition principle, RSC Adv. 5 (2015) 90178–90183.
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[5] J. Lancaster, Abrasive wear of polymers, Wear 14 (1969) 223–239. [6] L. Wang, X. Guan, G. Zhang, Friction and wear behaviors of carbon-based multilayer coatings sliding against different rubbers in water environment, Tribol. Int. 64 (2013) 69–77. [7] A. Shaaban, S.M. Se, M. Dimin, J.M. Juoi, M.H.M. Husin, N.M.M. Mitan, Influence of heating temperature and holding time on biochars derived from rubber wood sawdust via slow pyrolysis, J. Anal. Appl. Pyrolysis 107 (2014) 31–39. [8] J. Han, Y. Zhang, C. Wu, L. Xie, Y. Ma, Wet sliding abrasion of natural rubber composites filled with carbon black at different applied loads, J. Macromol. Sci. Part B 54 (2015) 401–410. [9] S. Nahvi, P. Shipway, D. McCartney, Particle motion and modes of wear in the dry sand–rubber wheel abrasion test, Wear 267 (2009) 2083–2091. [10] H. Jiang, R. Browning, H.J. Sue, Understanding of scratch-induced damage mechanisms in polymers, Polymer 50 (2009) 4056–4065. [11] J. Zhang, H. Jiang, C. Jiang, Q. Cheng, G. Kang, In-situ observation of temperature rise during scratch testing of poly (methylmethacrylate) and polycarbonate, Tribol. Int. 95 (2016) 1–4. [12] C. Gauthier, S. Lafaye, R. Schirrer, Elastic recovery of a scratch in a polymeric surface: experiments and analysis, Tribol. Int. 34 (2001) 469–479. [13] H. Jiang, Q. Cheng, C. Jiang, J. Zhang, L. Yonghua, Effect of stick-slip on the scratch performance of polypropylene, Tribol. Int. 91 (2015) 1–5. [14] R.L. Browning, H. Jiang, A. Moyse, et al., Scratch behavior of soft thermoplastic olefins: effects of ethylene content and testing rate, J. Mater. Sci. 43 (2008) 1357–1365. [15] R.D.K. Misra, R. Hadal, S.J. Duncan, Surface damage behavior during scratch deformation of mineral reinforced polymer composites, Acta Mater. 52 (2004) 4363–4376. [16] W. Brostow, V. Kovačevic, D. Vrsaljko, J. Whitworth, Tribology of polymers and polymer-based composites, J. Mater. Educ. 32 (2010) 273. [17] L.F. Giraldo, W. Brostow, E. Devaux, B.L. López, L.D. Pérez, Scratch and wear resistance of polyamide 6 reinforced with multiwall carbon nanotubes, J. Nanosci. Nanotechnol. 8 (2008) 3176–3183. [18] A. Schallamach, Abrasion of rubber by a needle, J. Polym. Sci. 9 (1952) 385–404. [19] A. Schallamach, Friction and abrasion of rubber, Rubber Chem. Technol. 31 (1958) 982–1014. [20] A. Schallamach, How does rubber slide? Wear 17 (1971) 301–312. [21] K. Maeda, A. Bismarck, B. Briscoe, Mechanisms of scratching frictions and damage maps for rubber compounds, Wear 259 (2005) 651–660. [22] K. Low, W. Teo, Characteristics of SBR, neoprene and EPDM compounds in a single-pass pendulum scratch, Tribol. Int. 54 (2012) 9–16. [23] N. Roch, P. Heuillet, C. Janin, et al., Mechanical and tribological behavior of HNBR modified by ion implantation, influence of aging, Surf. Coat. Technol. 209 (2012) 58–63. [24] Y. Xiong, B.Q. Fu, S.Y. Guo, et al., Study of stress accelerated aging behavior of sulfured nitrile-butadiene rubber, Equip. Environ. Eng. 9 (2012) 52–55. [25] G. Hamed, Tearing of vulcanized rubber, Rubber Chem. Technol. 78 (2005) 548–553. [26] K. Tsunoda, J. Busfield, C. Davies, A. Thomas, Effect of materials variables on the tear behaviour of a non-crystallising elastomer, J. Mater. Sci. 35 (2000) 5187–5198.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Scratch behavior of the aged hydrogenated nitrile butadiene rubber Zhongmeng Zhu, Qian Cheng, Chengkai Jiang, Jianwei Zhang, Han Jiang n Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu 610031, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 November 2015 Received in revised form 13 February 2016 Accepted 17 February 2016 Available online 23 February 2016
The objective of this work was to study the scratch behavior of the aged hydrogenated nitrile butadiene rubber (HNBR) including its scratch damage modes and their corresponding mechanism. A home-made device was used to perform the scratch testing on the virgin and thermal-aged HNBR substrates with a progressively increasing depth. The scratch resistance of HNBR declined dramatically with the increase of the aging time. The surface topography and damage modes of the scratched HNBR samples were investigated. Two types of scratch damage mode, i.e. the flake peeling and the helical-form damage were observed and both exhibited the characteristics of tearing. Specimens of two different geometries were used to measure the effects of aging time on tear strength of HNBR. It was found that there was a strong correlation between the strength data from two types of tear tests with the scratch damage modes. This finding provides a valuable guidance for improving the scratch resistance of the rubber materials and prolonging their service life. & 2016 Elsevier B.V. All rights reserved.
Keywords: Scratch testing Polymers Fracture Surface morphology Tear strength Thermal aging
1. Introduction During the daily use of rubber components over a wide range of applications such as tires, seals and vibration absorbers etc. [1], the surface scratch due to the relative movement with other counterparts not only seriously decreases the esthetics, but also degrades the material performance and eventually shortens the service life of the rubber products. Moreover, the scratch performance of rubber materials can be tremendously worsened by the inevitable aging caused by the harsh service environment and complex loading conditions [2–4]. Therefore, the study of the scratch behavior of the aged rubber is of great importance for engineering applications. Different from the investigation on the rubber wear, which focuses on the wear volume loss during a repeated abrasion [5–9], the research on rubber scratch, a single pass with a point contact, discusses the evolution and mechanism of the scratch damage [10]. The scratch behavior has been intensively studied on a wide range of plastic polymers such as polycarbonate [10,11], polymethyl methacrylate [12], polypropylene [13], thermoplastic polyolefin [10,14] and polymer-based composites [15–17], but not much attention has been given to rubbers. Schallamach investigated the single pass friction behavior of natural rubber using a stainless steel needle and found that the tensile stress in the substrate behind the needle produced lateral tears of the rubber across the sliding trace [18–20]. Briscoe n
Corresponding author. Tel.: þ 86 28 87601442; fax: þ 86 28 87600797. E-mail address: [email protected] (H. Jiang).
http://dx.doi.org/10.1016/j.wear.2016.02.010 0043-1648/& 2016 Elsevier B.V. All rights reserved.
found that the decreased cone angle of the scratch tip led to a more severe scratch damage of the rubber substrate and the bulk material tearing was accounted for the damage [21]. Low indicated that the scratch velocity could aggravated the scratch damage of styrene butadiene rubber, neoprene and ethylene propylene diene monomer rubber while the carbon black reinforcement could improve their scratch resistance [22]. The correlation between the material properties and possible scratch damage modes of rubbers was not fully investigated. Moreover, little work has been conducted to study the scratch behavior of the aged rubbers. In this work, the scratch tests with a progressive increasing scratch depth were carried out on the virgin and thermal aged HNBR using a home-made scratch device. The surface morphology of the scratched HNBR was investigated to identify the scratch damage modes. Then two types of tear strength were measured for the samples with different aging time. The correlation between different scratch damage modes and tear strengths was established. This study could help the material researchers to better understand the scratch behavior of rubbers and thus to improve its overall scratch resistance.
2. Experiment 2.1. Material and thermal aging experiments The HNBR plates (Kaidi Northwest Rubber Co. Ltd.) were 90% saturated, containing 50% acrylonitrile content and reinforced
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Z. Zhu et al. / Wear 352-353 (2016) 155–159
Fig. 1. Shape and size of the tear specimens of HNBR. (a) Type T (b) Type B.
with 30 phr hydrated silica. All the samples were thermally aged at a moderate temperature, i.e. 90 °C (about 120 °C higher than the Tg of HNBR), in an environment chamber (WG03, INBORN) referencing the literatures [23,24]. After reaching the prescribed aging time, the samples were removed from the chamber and air cooled to the ambient temperature. Then, they were stabilized in air for more than 16 h. 2.2. Tear tests An electronic universal testing machine (AGS-J, SHIMADZU) was utilized to measure the tear strengths of the HNBR at room temperature (about 25 °C). Two types of tear specimen, type T (Fig. 1(a)) and type B (Fig. 1(b)), were selected following ASTM D624-00 (2012) since they represented two different fracture modes in nature, i.e. type T tear for mode III fracture and type B tear for mode I fracture respectively. The tear tests were carried out under a loading rate of 500mm/min for type T and 50 mm/min for type B tearing respectively according to ASTM D624-00 (2012). Then the values of the tear strengths were calculated and the average value from five repeated tests was recorded. 2.3. Scratch tests and post-scratch analysis The scratch tests were performed under the ambient temperature using a home-made scratch device, which is schematically illustrated in Fig. 2. The scratch test unit comprises of a servo motor that drives the scratch tip moving tangentially with a prescribed speed. The increasing scratch depth is controlled with the help of another servo motor. The rigidity of the apparatus is enough to prevent the possible vibrations during scratch. To avoid its arching in front of the tip during scratching, the rubber sample was glued on an epoxy plate and then the plate was well clamped on the test stage. A conical stainless steel indenter (half cone angle: 22.5°) with a spherical tip (diameter (d): E 0.1 mm) was used in this work. A progressive increasing scratch depth (D) from 0 to 1.5 mm was adopted. The scratch length was 100 mm and the scratch speed was kept constant at 25 mm/s. The formation of the
Fig. 2. Schematic illustration of the home-made scratch device.
scratch damages has been in-situ observed with a high-speed video (Photron FASTCAM Mini UX50). Three duplicated scratch tests were carried out under each aging condition. The surface topography of the scratched sample was characterized using a super depth digital microscope (VHX-1000, Keyence). The good resolution (600 600 dpi) images of the scratches were obtained using an optical scanner (LiDE 210, Canon).
3. Results and discussion 3.1. Aging effect on the scratch behavior The scratch damage images of the HNBRs at different aging time under a linearly increased scratch depth are exhibited in Fig. 3. It is clear that the longer the aging time is, the more severe the scratch damage is. Only a minor scratch is visible for the virgin specimen even at the latest stage of the scratch process. However,
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Fig. 3. Scanned images of HNBR with different aging times scratched with linearly increasing scratch depth of 0–1.5 mm. (△: Onset of flake peeling ▲: Onset of helical-form damage).
Fig. 4. Flake peeling and helical-form scratch damages (a) optical scan image of scratched specimen with 30 days aging, (b) micro image of the flake peeling, (c) schematic illustration of the flake peeling, (d) micro image of the helical-form damage and (e) Schematic illustration of the helical-form damage.
an obvious groove, whose length is larger than one third of the whole scratch path, can be found for the sample after 90 days aging. This indicates that the scratch resistance of the material dramatically declines with the increase of the aging time. The microscopy observation was carried out on the scratched samples at different aging time. The sample with 30 days aging is shown in Fig. 4(a) as a good representation. The evolution of the scratch damage can be described in three stages. Firstly, the relatively shallow scratch depth and low stress level only induce some minor ridges (known as Schallamach ridges) on the surface of the specimen. These parallel ridges between which the rubber buckles inward lie perpendicular to the sliding direction and have been called as “abrasion patterns” [18–20]. During the second stage of the scratch process, the flake peeling, shown in Fig. 4(b), occurs on the surface of HNBR with the increasing scratch depth. Its formation is schematically illustrated in Fig. 4(c). It is believed that this type of damage is initiated from a stress concentration point, due to the minor surface defect or the filler close to the surface. When the endured stress of the rubber substrate behind the indenter goes beyond the material strength, the surface material, stuck to the forward-moving indenter, peels up gradually from the initial point of damage. The peeling process
is similar to the type T tearing to some extent. After the dissipation of the energy, the peeling stops and leaves a rubber flake in an ellipse shape contour. When this process occurred periodically, the repeated flake peeling is well observed in the range of median scratch depth. It should be mentioned that, during this stage, the scratch tip still does not pierce into the rubber substrate, in spite of the serious damage already occurred. As the scratch depth further increases, the scratch tip finally pierces into the substrate and then the repeated helical-form damage (Fig. 4(d)) appears. Fig. 4(e) schematically demonstrated its formation. A crack initiates from the defect point where the scratch tip pierced into the substrate. As the indenter moves forward, the crack gradually extends to both sides of the groove. Then dragged by the moving indenter, the material accumulates in front of the tip and curls forward. This induces the crack propagated along the sliding direction and leaves a helical-form damage on the scratched surface eventually. This process is somewhat similar to the propagation of the mode I crack which is corresponded to the type B tearing to some extent. It should be noted that the surface properties, degrading with aging process, influence the scratch behavior to a certain extent. However, considering the size scale of the scratch damage whose
158
Z. Zhu et al. / Wear 352-353 (2016) 155–159
Fig. 5. Correlation between the onset of flake peeling and the type T tear strength (a) evolution of the onset depth of flake peeling and the type T tear strength with aging time (b) linear correlation between the onset of flake peeling and the type T tear strength of the aged HNBR.
onset depth exceeds 0.5 mm, the bulk properties of the aged HNBR should be the dominant factors for the studied scratch performance. 3.2. Correlation between the onsets of scratch damages and the tear strengths As discussed above, the form of type T tearing occurs in the stage two of HNBR scratch process and the type B tearing appears corresponding to the stage three with the helical-form damage. The correlation between the scratch damage modes and the tear strengths of HNBR is discussed below. Fig. 5(a) shows the evolution of the onset depth of flake peeling and the strength of type T tearing with the increase of the aging time. The scission of the molecular chains during the thermal aging degrades the material strengths of the HBNR specimens. The type T tear strength drops sharply at the first 15 days and then gradually decreases to a stable value. The onset depth of flake peeling decreases in the same tendency with the aging time. After aging for 90 days, both onset depth of flake peeling and type T tear strength decline approximately 60%. It is well known that the surface flaws such as minor cracks and edge nicks will develops with the aging process. Combined with the decline of the type T tear strength, aging process results in the severe degradation of the HNBR’s resistance to the flake peeling damage. A Pearson correlation analysis was carried out, which is shown in Fig. 5(b). A strong linear correlation can be found between the onset depth of
Fig. 6. Correlation between the onset of helical-form damage and the type B tear strength (a) evolution of the onset depth of helical-form damage and the type B tear strength with aging time (b) linear correlation between the onset of helicalform damage and the type B tear strength of the aged HNBR.
flake peeling and the type T tear strength with R2 ¼0.919. This denotes that the type T tear strength could be a good index of the scratch resistance corresponding to the flake peeling damage of HNBR. As shown in Fig. 6(a), the onset depth of helical-form damage and the type B tear strength decrease in the same trend with the development of aging process. The result of the Pearson correlation analysis is shown in Fig. 6(b). Similarly, a strong linear correlation exists between the onset depth of the helical-form damage and the type B tear strength with R2 ¼0.923. This implies that the scratch resistance corresponding to the helicalform damage of HNBR could be evaluated using the type B tear strength. Although in the studied range, the linear correlation is enough to describe the relationship between the onset scratch depths and tear strengths, the physical mechanism behind it is not fully understood yet. This should be explored in the future work. In addition to the tear strengths of HNBR, we also measured the tensile strength and the fracture strain as well. However, there is only very weak correlation, if exists, between the onsets of the scratch damage and the tensile properties. It is clear that the scratch resistance of rubber materials can be improved by enhancing these two types of tear properties. For instance, reinforcing the rubber compounds with carbon black and increasing the crosslink density [25,26] are good options to
Z. Zhu et al. / Wear 352-353 (2016) 155–159
improve the tear properties of rubber materials thus to enhance their overall scratch performance as well.
4. Conclusions This work investigated the scratch behavior of the aged HNBR under a linearly increasing scratch depth with a home-made scratch device. The following conclusions are found. (1) Thermal aging had a significant effect on the scratch behavior of HNBR. The scratch resistance of HNBR dramatically declines with the development of the aging process. (2) With the increase of the scratch depth, two scratch damage modes, i.e. the flake peeling and the helical-form damage, occur in sequence on the surface of the HNBR. (3) The scratch damage mechanism of the flake peeling is found to be correlated with the type T tearing while the type B tearing is accounted for the helical-form damage during scratching. This finding provides a good guidance to improve the scratch resistance of the rubbery materials.
Acknowledgments The work is supported by National Natural Science Foundation of China (11172249, 11272269). The authors would also like to thank the partial financial support from Ministry of Education of China (NCET-12-0938), Science and Technology Department of Sichuan Province (2013JQ0010)
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