- Email: [email protected]
Effect of stick-slip on the scratch performance of polypropylene
Tribology International 91 (2015) 1–5
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Effect of stick-slip on the scratch performance of polypropylene Han Jiang a,n, Qian Cheng a, Chengkai Jiang a, Jianwei Zhang a, Yonghua Lib a Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China b Kingfa Science & Technology Co., LTD., Guangzhou 510520, China
art ic l e i nf o
a b s t r a c t
Article history: Received 31 March 2015 Received in revised form 3 June 2015 Accepted 19 June 2015 Available online 30 June 2015
For the scratch process of polypropylene (PP), the stick-slip phenomenon always exists and contributes to the observed periodic surface damage patterns. The stick-slip, one main cause for scratch visibility, alters the surface characteristics of substrate and eventually induces scratch visibility. Both ASTM/ISO and Erichsen scratch test methods are employed to study the stick-slip phenomenon and its effects on scratch behavior of PP. Image analysis shows that the stick-slip phenomenon was responsible for material removal and severe surface damage. Possible solutions to improve the scratch performance of polymeric materials are also discussed in this paper. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Scratch Stick-slip Polypropylene Periodic damage pattern
1. Introduction Widely utilized in automotive industry, consumer electronics, mechanical engineering and package industry, polypropylene (PP) has shown certain limitations, such as poor thermal barrier performance, low modulus, weak impact strength, and poor scratch resistance. The visible scratch damage on surface, one crucial concern for PP's applications, induced by other relatively harder objects can diminish esthetic appeal, thus causing the loss of attractiveness or durability, but the intended functionality of components may still serve. The injection molding auto parts including control panel, door, and glove box will inevitably experience scratches during the process of manufacturing, transporting and daily using. To overcome this obstruction, it is necessary to understand the fundamental mechanism(s) behind the visibility induced by scratch. Recently, many research works have been performed on the scratch behavior of polymeric materials [1–12]. It has been found that many factors, such as the crystallinity, additives, slip agent, toughening agent and surface texture have influences on the scratch resistance of PP based material. The recently established ASTM/ISO scratch test method [13,14] makes it possible to evaluate the polymer scratch resistance systematically and quantitatively. Investigations have been done on the scratch damage modes for different types of polymers including PP, PE, Epoxy, PS, PC, Nylon, SAN [1–12,15,16] and their micro/nano-composites
n
Corresponding author. Tel.: þ 86 28 87601442. E-mail address: [email protected] (H.-n. Jiang).
http://dx.doi.org/10.1016/j.triboint.2015.06.024 0301-679X/& 2015 Elsevier Ltd. All rights reserved.
[17–28]. Various scratch patterns, such as mar, fish-scale, parabolic crack, and material plow have been observed [15,29–31]. The shape of those surface scratch patterns, as well as the possible damage mechanisms, can be quite different [15,16]. What has been observed is that all of them are in a similar periodic fashion. The recurring scratch pattern can be observed not only on polymer materials [32,33] but also on non-polymeric materials such as ceramic and glass [34,35], as well as in nano-scale polishing process [36], thus proving that the recurrent scratch damage pattern is a common phenomenon for most materials. The periodic change of stress/strain status induced by the stick-slip is found to be an important mechanism of polymer scratch damage behavior [15,29]. Physically, the stick-slip between scratch tip and substrate is the instability from the couple of scratch head and underlay substrate. It is regarded as the physical cause of the repeated pattern of polymer scratch. Generally, the onset of scratch visibility of PP based materials always occurs close to the beginning of periodic fish-scale pattern caused by stick-slip [6,15,29]. To improve PP's scratch resistance and postpone the onset of scratch visibility, it is crucial to explain the physical nature of such a periodic feature. An analytical model has been proposed to explain the mechanistic process of the oscillatory stick-slip movement of the scratch tip [37]. In this paper, two types of scratch loading approaches, linearly increasing normal load per the ASTM/ISO method and constant normal load per the Erichsen delta-L method, are performed to analyze the stick-slip feature. The experimental results certify that the stickslip phenomenon is well correlated with the scratch visibility. The effectiveness of the analytical model to describe the stick-slip phenomenon is also presented.
2
H. Jiang et al. / Tribology International 91 (2015) 1–5
Fig. 1. Typical scratch periodical fish-scale feature of PP: (a) system 1 (scanner), (b) system 2 @6N (SEM) and (c) system 3 @14N (SEM).
system 3 which is composed of copolymer (HHP6, SINAOPEC) and POE (ENGAGE8150, Dow's Chemical) without the additives. Both systems were scratched with different levels of constant normal loads: 6, 10 and 14 N respectively. Scanning Electron Microscope (HITACHI S3000N) was then employed to analyze the characteristics of scratched surface. The brightness increase of the cross hatch scratched area was measured with a Spectrophotometer (ColorQuest XE, HunterLab). The brightness difference from a scratched and non-scratch surface was obtained as the so-called Delta-L to give an expression of the brightness deviation in numeric values. Fig. 2. Cross section SEM photograph of the system 3 (Delta-L ¼ 5.1).
3. Results and discussion 2. Experiment 3.1. The evaluation of scratch resistance: Delta-L vs. scratch width A scratch machine (SMS V4) following the ASTM/ISO polymer scratch standard was utilized to perform the linearly increasing normal load (1–30 N) scratch tests on the injected PP panels (system 1, with 70% polypropylene and 30% ethylene–propylene rubber (EPR), Advanced Composites). The diameter of scratch tip is 1 mm. The scratch test is conducted at the speed of 100 mm/s. High resolution optical scanner (Epson 4870) was used to investigate the periodic fish-scale scratch feature. Utilizing the scratcher Erichsen 430P, other two injected PP sample panels (system 2 and 3) were tested following the plastic interior components testing of scratch resistance (PV3952, Volkswagen Automotive) to create cross-hatch scratch patterns. The diameter of scratch tip is 1 mm. The scratch test speed is performed at the speed of 1000 mm/min. The system 2, modified with Talc 3000 (GuiGuang Talc Development CO., LTD.) and commercial available anti-scratch additives, antioxidant (B225, CIBA), was supposed to exhibit better scratch resistance than the
Many parameters can be adopted to evaluate the scratch performance of polymeric materials. While the critical load level at the onset of scratch visibility is reported in the ASTM/ISO method, the difference of the brightness of sample surface (L) before and after the scratch test, named as Delta-L, is employed by Erichsen method to evaluate the material scratch performance. The worse the scratch resistance the material performs, the larger the value of Delta-L is. Although there is no straightforward correlation between these two parameters, the value of Delta-L could be a good qualitative index to evaluate the scratch resistance under certain load level. The generally agreed upon view is that the scratch width, the residual scratch path left by the tip movement, is the characteristic of scratch deformation with great significance [37], which can be conveniently measured by the top-view or cross-section images, either from optical scanner (Fig. 1a) or SEM (Fig. 1b and c, Fig. 2).
H. Jiang et al. / Tribology International 91 (2015) 1–5
3
0.75
Scratch Width (mm)
0.70
Stick
Slip
0.65 0.60 0.55 0.50 0.45 0.40 0
1
2
3
4
5
ΔL
Fig. 4. SEM photograph of scratch damage of system 3 (100 ).
Fig. 3. Delta-L vs. scratch width of PP composite.
The worse the scratch resistance of material is, the wider the residual scratch path is. The correlation between the Delta-L and the scratch width of PP composites can be easily identified from Fig. 3. Generally, under the same level of scratch normal load, the Delta-L increases with the broadening of scratch width. When the scratch tip travels on the material with poor scratch resistance, the tangential force between tip and substrate produce severe deformation/damage. Thus, both scratch width and brightness change are chosen as the index for the evaluation of scratch resistance in this paper.
3.2. Stick-slip phenomenon Fig. 4 shows the constant normal load scratch of system 3 (Delta-L ¼5.1) in which the serious whitening can be observed. Although the scratch tip is designed to move at a constant speed, its actual velocity oscillates due to the repeated formation (stick) and breakage (slip) of the adhesion between tip and substrate. The area marked as “stick” is where the scratch tip slows down, sticking to the substrate. The accumulated material in front of the tip obstructs the horizontal movement of the tip, as well as introduces additional resistance force. The friction and resistance from the material has to overcome to allow the scratch tip moving forward again. Then, with less friction and resistance force, the tip will slip, i.e. move relatively faster and smoother, and following by another cycle of stick-slip. The distance between each cycle is roughly constant at 0.35–0.40 mm. The scratch process involves not only the stick-slip tangential movement but also the periodically vertical impact of tip onto the PP substrate. The resultant force could be much larger than the prescribed load condition. This will induced pre-mature sever damage on the substrate material. The void illustrated in Fig. 5 is believed to be the damage induced by the stick-slip during the scratch process of PP based composites. Fig. 6 is the SEM images of the system 2 (Delta-L¼ 0.8). With better scratch resistance, the scratch path is relatively smooth showing little stick-slip phenomenon. No sever material removal can be found. For the material with better scratch resistance, the scratch tip slips more smoothly than the one with poor scratch resistance. Then the possible scratch damage is minimized. The recurring of fish-scale patterns, other than fish-scale itself, taken as the phenomenological outcome from stick-slip behavior, can be identified in the typical scratch images for all three systems.
Fig. 5. SEM photograph of scratch damage (5000 ).
The distance between two adjunct fish-scales can be obtained from these images. Fig. 7 shows the measured distance between two adjunct fishscales for three systems under different load levels. While it statically keeps unchanged for the same constant load of crosshatch test, in the case of increasing normal load, the higher the normal load, the larger the distance between two adjunct fishscales is found. However the scratch normal load is applied, constant or linearly increasing, the distance of periodic fish-scale pattern is always proportional to the normal load level for all investigated systems. This experimental finding agrees well with the analytical model [37]. Furthermore, the system 3 with poor scratch resistance shows a larger stick-slip periodic distance than that of the system 2 under the same normal load level. It suggests that when the scratch tip moves on the material with worse scratch resistance, a lower load is required to overcome the “stick” and for “slip” to occur. There is no straightforward correlation between the value of critical load and Delta-L, from the ASTM/ISO and Erichsen methods respectively. However, the similar scratch periodic feature is the fundamental mechanisms behind the investigated thermoplastic polyolefins (TPOs) scratch, thus the periodic stick-slip phenomenon may be adopted as a useful approach for the correlation. Under the same normal load, the periodic distance between two
4
H. Jiang et al. / Tribology International 91 (2015) 1–5
Fig. 6. SEM photograph of scratch damage (Delta-L ¼0.8): (a) 100 and (b) 5000 .
resistance of PP. To prepare PP with good scratch resistance, the stick-slip phenomenon during the scratch should be minimized.
Acknowledgments This paper is supported by National Natural Science Foundation of China (11172249, 11472231), Ministry of Education of China (NCET-12–0938) and Science and Technology Department of Sichuan Province (2013JQ0010). The authors also would like to express sincere gratitude to the valuable help on the experiment from the research group of Professor H.-J. Sue in Texas A&M University. References Fig. 7. Distance between two adjacent fish-scale patterns of three systems at different normal loads.
adjacent fish-scale features is negatively correlated to the scratch resistance. To sum up, obviously, to improve the polymer scratch resistance by delaying the onset of scratch visibility, the stick-slip phenomenon should be delayed or minimized.
4. Summary No matter which scratch test method is adopted, the ISO/ASTM method or the Erichsen method, the stick-slip phenomenon during the scratch process is the fundamental mechanism causing scratch visibility in most TPOs systems. The wider the residual scratch path and the larger the Delta-L, the worse the scratch performance is. Validated by the experimental results, the stickslip phenomenon can be correlated with the material performance and the load level as described by the analytical model. It should be noted that there exist certain cases that scratch resistance in visibility can be improved with longer stick-slip distance and longer fish-scale patterns. For the catalog of PP material studied in this work, we found that a poor scratch resistance material exhibits a larger distance between two adjacent fish-scale patterns. This distance may be used as a parameter to quantify scratch
[1] Briscoe BJ, Delfino A, Pelillo E. Single-pass pendulum scratching of polystyrene and polymethylmethacrylate. Wear 1999;225–229:319–28. [2] Brostow W, Chonkaew W, Mirshams R, Srivastava A. Characterization of grooves in scratch resistance testing. Polym Eng Sci 2008;48:2060–5. [3] Pelletier H, Gauthier C, Schirrer R. Relationship between contact geometry and average plastic strain during scratch tests on amorphous polymers. Tribol Int 2010;43:796–809. [4] Kathalewar M, Sabnis A, Waghoo G. Effect of incorporation of surface treated zinc oxide on non-isocyanate polyurethane based nano-composite coatings. Prog Organ Coat 2013;76:1215–29. [5] Kim B, Kim H, Choi B, Lee H. An experimental study of the scratch properties of poly(methyl methacrylate) as a function of the concentration of added slip agent. Tribol Int 2011;44:2035–41. [6] Jiang H, Browning RL, Hossaina MM, Sue H-J, Fujiwara M. Quantitative evaluation of scratch visibility resistance of polymers. Appl Surf Sci 2010;256:6324–9. [7] Chivatanasoontorn V, Tsukise S, Kotaki M. Surface texture effect on scratch behavior of injection molded plastics. Polym Eng Sci 2012;52:1862–7. [8] Friedrich K, Sue HJ, Liu P, Almajid A. Scratch resistance of high performance polymers. Tribol Int 2011;44:1032–46. [9] Hadal RS, Misra RDK. Scratch deformation behavior of thermoplastic materials with significant differences in ductility. Mater Sci Eng A 2005;398:252–61. [10] Myshkin NK, Grigoryev AY, Chizhik SA, Choi KY. Surface roughness and texture analysis in microscale. Wear 2003;254:1001–9. [11] Brostow W, Deborde J-L, Jaklewicz M, Olszynski P. Tribology with emphasis on polymers: friction, scratch resistance and wear. J Mater Ed 2003;25:119–32. [12] Myshkin NK, Petrokovets MI, Kovalev AV. Tribology of polymers: friction, wear and mass transfer. Tribol Int 2005;38:910–21. [13] ISO 19252:2008, Annual Book of ISO Standards, International Organization for Standardization (2008). [14] ASTM D7027-05, Annual Book of ASTM Standards, ASTM International (2005). [15] Jiang H, Browning R, Sue H-J. Understanding of scratch-induced damage mechanisms in polymers. Polymer 2009;50:4056–65.
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[16] Hadal RS, Misra RDK. Scratch deformation behavior of thermoplastic materials with significant differences in ductility. Mater Sci Eng A 2005;398:252–61. [17] Suryapudi NL, Yuan Q, Pesacreta TC, Misra RDK. The determining role of scratch indenter radius on surface deformation of high density polyethylene and calcium carbonate reinforced composite. Mater Sci Eng A 2007;456:218–29. [18] Suryapudi NL, Ramisetti N, Misra RDK. On scratch deformation of glass fiber reinforced nylon 66. Mater Sci Eng A 2007;456:230–5. [19] Mudaliar A, Yuan Q, Misra RDK. On surface deformation of melt-intercalated polyethylene-clay nanocomposites during scratching. Polym Eng Sci 2006;46:1625–34. [20] Tanniru M, Misra RDK. Reduced susceptibility to stress whitening during tensile deformation of calcium carbonate-reinforced high density polyethylene. Mater Sci Eng A 2006;424:53–70. [21] Thridandapani RR, Mudaliar A, Yuan Q, Misra RDK. Near surface deformation associated with scratch in polypropylene-clay nanocomposites: a microscopic study. Mater Sci Eng A 2006;418:292–302. [22] Hadal RS, Yuan Q, Jog J, Misra RDK. On stress whitening during surface deformation in clay-containing polymer nanocomposites: a microstructural approach. Mater Sci Eng A 2006;418:268–81. [23] Tanniru M, Misra RDK, Bertrand K, Murphy D. The determining role of calcium carbonate on surface deformation during scratching of calcium carbonatereinforced polyethylene composites. Mater Sci Eng A 2005;404:208–20. [24] Misra RDK, Hadal R, Duncan SJ. Surface damage behavior during scratch deformation of mineral reinforced polymer composites. Acta Mater 2004;52:4363–76. [25] Misra RDK, Nathani H, Dasari A, Wanjale SD, Jog JP. The determining role of clay particles on mechanically induced surface damage and associated stress whitening in polybutene clay nanocomposites. Mater Sci Eng A 2004;386:175–85.
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[26] Dasari A, Rohrmann J, Misra RDK. Scratch deformation characteristics of micrometric wollastonite-reinforced ethylene propylene copolymer composites. Polym Eng Sci 2004;44:1738–48. [27] Hadal R, Dasari A, Rohrmann J, Misra RDK. Susceptibility to scratch surface damage of wollastonite- and talc-containing polypropylene micrometric composites. Mater Sci Eng A 2004;380:326–39. [28] Dasari A, Rohrmann J, Misra RDK. On the scratch deformation of micrometric wollastonite reinforced polypropylene composites. Mater Sci Eng A 2004;364:357–69. [29] Li K, Ni B, Li JCM. Stick-slip in the scratching of styrene-acrylonitrile copolymer. J Mater Res 1996;11:1574–80. [30] Zhang SL, Li JCM. Slip process of stick slip motion in the scratching of a polymer. Mater Sci Eng 2003;A344:182–9. [31] Brostow W, Kovacevic V, Vrsaljko D, Whitworth J. Tribology of polymers and polymer based composites. J Mater Educ 2010;32:273–90. [32] Venkatsurya PKC, Yuan Q, Misra RDK. Micromechanism of surface and subsurface deformation behavior of high density polyethylene containing dispersion of nanoparticles: an electron microscopy study and indenter-substrate interaction. Mech Mater 2011;43:254–68. [33] Yuan Q, Ramisetti N, Misra RDK. Nanoscale near surface deformation in polymer nanocomposites. Acta Mater 2008;56:2089–100. [34] Sivakumar R, Jones M, Hirao K, Kanematsu W. Scratch behavior of SiAlON ceramics. J. Eur. Ceram. Soc. 2006;26:351–9. [35] Lawn BR, Wiederhorn SM, Roberts DE. Effect of sliding friction forces on the strength of brittle materials. J Mater Sci 1984;19:2561–9. [36] Kim HJ, Yang JC, Yoon BU, Lee HD, Kim T. Nano-scale stick-slip friction model for the chatter scratch generated by chemical mechanical polishing process. J Nanosci Nanotechnol 2012;12:5683–6. [37] Jiang H., Whitcomb J.D., Sue H.-J., Mechanical modeling of scratch induced periodic damage. In: Proceedings of the 11th-annual SPE TPO conference, MI (US); 2009.
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Effect of stick-slip on the scratch performance of polypropylene Han Jiang a,n, Qian Cheng a, Chengkai Jiang a, Jianwei Zhang a, Yonghua Lib a Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China b Kingfa Science & Technology Co., LTD., Guangzhou 510520, China
art ic l e i nf o
a b s t r a c t
Article history: Received 31 March 2015 Received in revised form 3 June 2015 Accepted 19 June 2015 Available online 30 June 2015
For the scratch process of polypropylene (PP), the stick-slip phenomenon always exists and contributes to the observed periodic surface damage patterns. The stick-slip, one main cause for scratch visibility, alters the surface characteristics of substrate and eventually induces scratch visibility. Both ASTM/ISO and Erichsen scratch test methods are employed to study the stick-slip phenomenon and its effects on scratch behavior of PP. Image analysis shows that the stick-slip phenomenon was responsible for material removal and severe surface damage. Possible solutions to improve the scratch performance of polymeric materials are also discussed in this paper. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Scratch Stick-slip Polypropylene Periodic damage pattern
1. Introduction Widely utilized in automotive industry, consumer electronics, mechanical engineering and package industry, polypropylene (PP) has shown certain limitations, such as poor thermal barrier performance, low modulus, weak impact strength, and poor scratch resistance. The visible scratch damage on surface, one crucial concern for PP's applications, induced by other relatively harder objects can diminish esthetic appeal, thus causing the loss of attractiveness or durability, but the intended functionality of components may still serve. The injection molding auto parts including control panel, door, and glove box will inevitably experience scratches during the process of manufacturing, transporting and daily using. To overcome this obstruction, it is necessary to understand the fundamental mechanism(s) behind the visibility induced by scratch. Recently, many research works have been performed on the scratch behavior of polymeric materials [1–12]. It has been found that many factors, such as the crystallinity, additives, slip agent, toughening agent and surface texture have influences on the scratch resistance of PP based material. The recently established ASTM/ISO scratch test method [13,14] makes it possible to evaluate the polymer scratch resistance systematically and quantitatively. Investigations have been done on the scratch damage modes for different types of polymers including PP, PE, Epoxy, PS, PC, Nylon, SAN [1–12,15,16] and their micro/nano-composites
n
Corresponding author. Tel.: þ 86 28 87601442. E-mail address: [email protected] (H.-n. Jiang).
http://dx.doi.org/10.1016/j.triboint.2015.06.024 0301-679X/& 2015 Elsevier Ltd. All rights reserved.
[17–28]. Various scratch patterns, such as mar, fish-scale, parabolic crack, and material plow have been observed [15,29–31]. The shape of those surface scratch patterns, as well as the possible damage mechanisms, can be quite different [15,16]. What has been observed is that all of them are in a similar periodic fashion. The recurring scratch pattern can be observed not only on polymer materials [32,33] but also on non-polymeric materials such as ceramic and glass [34,35], as well as in nano-scale polishing process [36], thus proving that the recurrent scratch damage pattern is a common phenomenon for most materials. The periodic change of stress/strain status induced by the stick-slip is found to be an important mechanism of polymer scratch damage behavior [15,29]. Physically, the stick-slip between scratch tip and substrate is the instability from the couple of scratch head and underlay substrate. It is regarded as the physical cause of the repeated pattern of polymer scratch. Generally, the onset of scratch visibility of PP based materials always occurs close to the beginning of periodic fish-scale pattern caused by stick-slip [6,15,29]. To improve PP's scratch resistance and postpone the onset of scratch visibility, it is crucial to explain the physical nature of such a periodic feature. An analytical model has been proposed to explain the mechanistic process of the oscillatory stick-slip movement of the scratch tip [37]. In this paper, two types of scratch loading approaches, linearly increasing normal load per the ASTM/ISO method and constant normal load per the Erichsen delta-L method, are performed to analyze the stick-slip feature. The experimental results certify that the stickslip phenomenon is well correlated with the scratch visibility. The effectiveness of the analytical model to describe the stick-slip phenomenon is also presented.
2
H. Jiang et al. / Tribology International 91 (2015) 1–5
Fig. 1. Typical scratch periodical fish-scale feature of PP: (a) system 1 (scanner), (b) system 2 @6N (SEM) and (c) system 3 @14N (SEM).
system 3 which is composed of copolymer (HHP6, SINAOPEC) and POE (ENGAGE8150, Dow's Chemical) without the additives. Both systems were scratched with different levels of constant normal loads: 6, 10 and 14 N respectively. Scanning Electron Microscope (HITACHI S3000N) was then employed to analyze the characteristics of scratched surface. The brightness increase of the cross hatch scratched area was measured with a Spectrophotometer (ColorQuest XE, HunterLab). The brightness difference from a scratched and non-scratch surface was obtained as the so-called Delta-L to give an expression of the brightness deviation in numeric values. Fig. 2. Cross section SEM photograph of the system 3 (Delta-L ¼ 5.1).
3. Results and discussion 2. Experiment 3.1. The evaluation of scratch resistance: Delta-L vs. scratch width A scratch machine (SMS V4) following the ASTM/ISO polymer scratch standard was utilized to perform the linearly increasing normal load (1–30 N) scratch tests on the injected PP panels (system 1, with 70% polypropylene and 30% ethylene–propylene rubber (EPR), Advanced Composites). The diameter of scratch tip is 1 mm. The scratch test is conducted at the speed of 100 mm/s. High resolution optical scanner (Epson 4870) was used to investigate the periodic fish-scale scratch feature. Utilizing the scratcher Erichsen 430P, other two injected PP sample panels (system 2 and 3) were tested following the plastic interior components testing of scratch resistance (PV3952, Volkswagen Automotive) to create cross-hatch scratch patterns. The diameter of scratch tip is 1 mm. The scratch test speed is performed at the speed of 1000 mm/min. The system 2, modified with Talc 3000 (GuiGuang Talc Development CO., LTD.) and commercial available anti-scratch additives, antioxidant (B225, CIBA), was supposed to exhibit better scratch resistance than the
Many parameters can be adopted to evaluate the scratch performance of polymeric materials. While the critical load level at the onset of scratch visibility is reported in the ASTM/ISO method, the difference of the brightness of sample surface (L) before and after the scratch test, named as Delta-L, is employed by Erichsen method to evaluate the material scratch performance. The worse the scratch resistance the material performs, the larger the value of Delta-L is. Although there is no straightforward correlation between these two parameters, the value of Delta-L could be a good qualitative index to evaluate the scratch resistance under certain load level. The generally agreed upon view is that the scratch width, the residual scratch path left by the tip movement, is the characteristic of scratch deformation with great significance [37], which can be conveniently measured by the top-view or cross-section images, either from optical scanner (Fig. 1a) or SEM (Fig. 1b and c, Fig. 2).
H. Jiang et al. / Tribology International 91 (2015) 1–5
3
0.75
Scratch Width (mm)
0.70
Stick
Slip
0.65 0.60 0.55 0.50 0.45 0.40 0
1
2
3
4
5
ΔL
Fig. 4. SEM photograph of scratch damage of system 3 (100 ).
Fig. 3. Delta-L vs. scratch width of PP composite.
The worse the scratch resistance of material is, the wider the residual scratch path is. The correlation between the Delta-L and the scratch width of PP composites can be easily identified from Fig. 3. Generally, under the same level of scratch normal load, the Delta-L increases with the broadening of scratch width. When the scratch tip travels on the material with poor scratch resistance, the tangential force between tip and substrate produce severe deformation/damage. Thus, both scratch width and brightness change are chosen as the index for the evaluation of scratch resistance in this paper.
3.2. Stick-slip phenomenon Fig. 4 shows the constant normal load scratch of system 3 (Delta-L ¼5.1) in which the serious whitening can be observed. Although the scratch tip is designed to move at a constant speed, its actual velocity oscillates due to the repeated formation (stick) and breakage (slip) of the adhesion between tip and substrate. The area marked as “stick” is where the scratch tip slows down, sticking to the substrate. The accumulated material in front of the tip obstructs the horizontal movement of the tip, as well as introduces additional resistance force. The friction and resistance from the material has to overcome to allow the scratch tip moving forward again. Then, with less friction and resistance force, the tip will slip, i.e. move relatively faster and smoother, and following by another cycle of stick-slip. The distance between each cycle is roughly constant at 0.35–0.40 mm. The scratch process involves not only the stick-slip tangential movement but also the periodically vertical impact of tip onto the PP substrate. The resultant force could be much larger than the prescribed load condition. This will induced pre-mature sever damage on the substrate material. The void illustrated in Fig. 5 is believed to be the damage induced by the stick-slip during the scratch process of PP based composites. Fig. 6 is the SEM images of the system 2 (Delta-L¼ 0.8). With better scratch resistance, the scratch path is relatively smooth showing little stick-slip phenomenon. No sever material removal can be found. For the material with better scratch resistance, the scratch tip slips more smoothly than the one with poor scratch resistance. Then the possible scratch damage is minimized. The recurring of fish-scale patterns, other than fish-scale itself, taken as the phenomenological outcome from stick-slip behavior, can be identified in the typical scratch images for all three systems.
Fig. 5. SEM photograph of scratch damage (5000 ).
The distance between two adjunct fish-scales can be obtained from these images. Fig. 7 shows the measured distance between two adjunct fishscales for three systems under different load levels. While it statically keeps unchanged for the same constant load of crosshatch test, in the case of increasing normal load, the higher the normal load, the larger the distance between two adjunct fishscales is found. However the scratch normal load is applied, constant or linearly increasing, the distance of periodic fish-scale pattern is always proportional to the normal load level for all investigated systems. This experimental finding agrees well with the analytical model [37]. Furthermore, the system 3 with poor scratch resistance shows a larger stick-slip periodic distance than that of the system 2 under the same normal load level. It suggests that when the scratch tip moves on the material with worse scratch resistance, a lower load is required to overcome the “stick” and for “slip” to occur. There is no straightforward correlation between the value of critical load and Delta-L, from the ASTM/ISO and Erichsen methods respectively. However, the similar scratch periodic feature is the fundamental mechanisms behind the investigated thermoplastic polyolefins (TPOs) scratch, thus the periodic stick-slip phenomenon may be adopted as a useful approach for the correlation. Under the same normal load, the periodic distance between two
4
H. Jiang et al. / Tribology International 91 (2015) 1–5
Fig. 6. SEM photograph of scratch damage (Delta-L ¼0.8): (a) 100 and (b) 5000 .
resistance of PP. To prepare PP with good scratch resistance, the stick-slip phenomenon during the scratch should be minimized.
Acknowledgments This paper is supported by National Natural Science Foundation of China (11172249, 11472231), Ministry of Education of China (NCET-12–0938) and Science and Technology Department of Sichuan Province (2013JQ0010). The authors also would like to express sincere gratitude to the valuable help on the experiment from the research group of Professor H.-J. Sue in Texas A&M University. References Fig. 7. Distance between two adjacent fish-scale patterns of three systems at different normal loads.
adjacent fish-scale features is negatively correlated to the scratch resistance. To sum up, obviously, to improve the polymer scratch resistance by delaying the onset of scratch visibility, the stick-slip phenomenon should be delayed or minimized.
4. Summary No matter which scratch test method is adopted, the ISO/ASTM method or the Erichsen method, the stick-slip phenomenon during the scratch process is the fundamental mechanism causing scratch visibility in most TPOs systems. The wider the residual scratch path and the larger the Delta-L, the worse the scratch performance is. Validated by the experimental results, the stickslip phenomenon can be correlated with the material performance and the load level as described by the analytical model. It should be noted that there exist certain cases that scratch resistance in visibility can be improved with longer stick-slip distance and longer fish-scale patterns. For the catalog of PP material studied in this work, we found that a poor scratch resistance material exhibits a larger distance between two adjacent fish-scale patterns. This distance may be used as a parameter to quantify scratch
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