- Email: [email protected]
Mechanical properties modification of a thin film phenolic resin filled with nano silica particles
Computational Materials Science xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci
Mechanical properties modification of a thin film phenolic resin filled with nano silica particles F. Taheri-Behrooz, B. Memar Maher, M.M. Shokrieh ⇑ Composites Research Laboratory, Center of Excellence in Experimental Solid Mechanics and Dynamics, School of Mechanical Engineering, Iran University of Science and Technology, Tehran 16846-13114, Iran
a r t i c l e
i n f o
Article history: Received 1 March 2014 Received in revised form 24 August 2014 Accepted 29 August 2014 Available online xxxx Keywords: Nano silica Phenolic resin Nano indentation Scratch Hardness Elastic modulus Penetration depth
a b s t r a c t The current manuscript experimentally investigates the mechanical and wears properties of three different phenolic polymer nanocomposites. Samples are made of 1, 2 and 3 wt.% of well-dispersed silica nanoparticles in a phenolic resin. Firstly, it was found that for the phenolic polymer, the Young’s modulus steadily increased as the weight percent of silica nanoparticles was increased except for 3 wt.% nanoparticle filled samples. For the phenolic polymer with 1 and 2 wt.% nano silica, Young’s modulus was increased 7% and 12.5%, respectively in comparison with the neat phenolic resin. However, for samples with 3% nanosilica this value showed a reduction about 13.5% and 2.7% in comparison to 2% nanoparticle filled samples and pure samples, respectively. Secondly, the presence of 2 wt.% nano silica particles increased the hardness of the phenolic polymer by 27.3% compared with the pure phenolic resin. Also, it was shown by the results of scratch tests that nano silica has almost no effect on the coefficient of the friction and wear rates. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Polymeric nano-reinforced coatings have drawn considerable attention, in recent years, caused by improvements in various properties comprising scratch resistance, abrasion resistance, heat stability as well as other mechanical properties [1–4]. Traditionally, the scratch resistance of an organic coating can be improved by the addition of high content of inorganic filler. As inorganic fillers a range of different metal–oxide particles can be dispersed in the polymeric matrix [1,5–8]. The use of inorganic particles in the nano scale range is particularly attractive since it allows improving the properties of the polymers by controlling the degree of interaction between the polymer and the nano fillers [9] via a top–down approach. Phenol resins (PF), notwithstanding the century-long history, are still attracting a great deal of research interests. They are important technical materials and irreplaceable in many fields, especially in thermal insulation, coating, aeronautic utilities, electro-optical devices and composite materials due to their thermal stability, high char yield, structural integrity and solvent resistance. Phenol resin-based friction materials usually contain a large number of reinforcing and filling constituents such as reinforcing
fibers, abrasives, binders, fillers, and friction modifiers (solid lubricants). This accounts for the great dependence of their properties on the interactions and synergetic effects among the multiphase ingredients. In this sense, it is very important to correctly select and properly combine the different components so as to satisfy a number of requirements for the properties of the friction materials, such as good wear resistance, stable friction coefficient and reliable strength at a wide range of rigorous conditions. In recent years, one of the widely used techniques for the evaluation of the mechanical and tribological properties of metals, ceramics, polymers and films at ultra-microscopic level is nano indentation and nano scratch tests using depth-sensing method [10,11]. However, up to now, few papers concerning the tribological behaviors at the micro/nano-scale of the phenolic-based nano composites have been presented [12,13]. The present paper characterizes the mechanical and tribological behaviors of phenolic films and SiO2/phenolic nano composite coatings by using nano mechanics testing.
2. Materials and methods 2.1. Materials
⇑ Corresponding author. E-mail address: [email protected] (M.M. Shokrieh).
Resol-type phenolic resin (IL800) and SiO2 particles were used in the present research. The morphology of SiO2 nanoparticle is
http://dx.doi.org/10.1016/j.commatsci.2014.08.042 0927-0256/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: F. Taheri-Behrooz et al., Comput. Mater. Sci. (2014), http://dx.doi.org/10.1016/j.commatsci.2014.08.042
2
F. Taheri-Behrooz et al. / Computational Materials Science xxx (2014) xxx–xxx
Table 1 Impurities of Silicon nano-oxide [14]. Component
Cu
Zn
Fe
Mg
Na
Al
As
Sb
Pb
Hg
Content (%)
0.0
0.0005884
0.0
0.0000042
0.0
0.0
0.0
0.0000031
0.0
0.0
spherical with an average particle size of 10–15 nm, and purity of Silicon nano-oxide is 99.999% as shown in Table 1. 2.2. Sample preparation
To investigate the mechanical and tribological performances of composite coating, nano-scale indentation and scratch tests were applied by using HysitronTribo Indenter. The indentation tests were carried out underforce-control method, where the applied load is controlled according to a programmed loading function, and the displacement was continuously monitored. The loading function of the indentation in this research consists of five steps linear loading and five step sun loading segment together. The maximum applied load for all samples was 800 lN. The final values of hardness and modulus were taken as the average of five indentations carried out in different spots on the same material. To calculate hardness (H) and modulus (E) the Olive and Pharr method [15] were used:
8.2
8
Elastic modulus (GPa)
2.3. Characterization method
Fig. 2. Load–displacement curve for the specimens.
7.8 7.6 7.4 7.2
7 6.8 6.6 6.4
Pure
1 wt%
2 wt%
3 wt%
Fig. 3. Elastic modulus of the specimens.
0.6 0.5
Hardness (GPa)
Phenolic resin was mixed with 1 wt.%, 2 wt.% and 3 wt.% of SiO2 for 20 min by a mixer at 600 rpm by using a two-propelled mechanical stirrer. Then, it was mixed for extra 5 min at 100 rpm. In order to obtain a full dispersion of the nanoparticles, probe sonicator of 14 mm in diameter (Hielscher UP400S) was utilized. The sonication process was performed for 30 min with 50 s cycle using ultrasonic waves set to 50 kHz. To avoid temperature rise during the ultrasonic processing, the Pyrex beaker containing resin and nano silica were cooled by an ice-bath. During the stirring, air bubbles created due to mixture also could inversely affect the quality and the properties of final product. To avoid this effect, the prepared mixture was placed in a vacuum oven for 15 min to be completely degassed. The mixture was prepared as a thin film on a metal substrate and kept for 18 h at 40 °C temperature to complete the curing process of the specimens. It is worthy to note that to have a tendency solvent used in resin; resins were stored at room temperature to evaporate the solvent for a week which leads to smooth sample surface without any pores.
0.4 0.3 0.2 0.1 0
Pure
1 wt%
2 wt%
3 wt%
Fig. 4. Hardness of the specimens.
Table 2 Results of the indentation test.
Fig. 1. A schematic representation of the load–displacement curve [15].
Pure 1% 2% 3%
Young’s modules (GPa)
Hardness (GPa)
Max force (lN)
7.2 7.7 8.1 7.0
0.33 0.40 0.42 0.48
748.7 751.6 752.8 754.8
Please cite this article in press as: F. Taheri-Behrooz et al., Comput. Mater. Sci. (2014), http://dx.doi.org/10.1016/j.commatsci.2014.08.042
3
F. Taheri-Behrooz et al. / Computational Materials Science xxx (2014) xxx–xxx
F max Ac
1 1 #2 1 #2i ¼ þ Er E Ei
S¼
df ¼ 2Er dh
96
ð1Þ
rffiffiffiffiffi Ac
p
ð2Þ
ð3Þ
In this method, stiffness (S) is given with respect to the slope of unloading cure as depicted in Fig. 1[13]. Where h is displacement, Ac is contact area, Er is reduced modulus E and m are Young’s modulus and Poisson’s ratio of the sample, respectively, Ei and mi are the Young’s modulus and Poisson’s ratio of diamond indenter, respectively.
Scratch depth analyses (nm)
H¼
92
88
84
80 Pure
1 wt%
2 wt%
Fig. 7. Scratch depth analyses for specimens.
Fig. 5. Scratch depth analysis for 2% wt. SiO2/phenolic specimen.
Fig. 6. Scratch depth analysis for 3% wt. SiO2/phenolic specimen.
Please cite this article in press as: F. Taheri-Behrooz et al., Comput. Mater. Sci. (2014), http://dx.doi.org/10.1016/j.commatsci.2014.08.042
3 wt%
4
F. Taheri-Behrooz et al. / Computational Materials Science xxx (2014) xxx–xxx
The elastic modulus using this approach is accurate, but for polymers because of their time- and rate-dependent behavior under load, this generally leads to an incompatible evaluation of properties. But, the indentation test was repeated 5 times and the repeatability of test results was observed.
3. Results and discussion 3.1. Indentation test
Fig. 8. Friction coefficient of the samples.
Friction Coefficient
0.357
0.328
0.299
0.27 Pure Phenol
1 wt%
2 wt%
3 wt%
Fig. 9. Average of the friction coefficient of samples.
Fig. 2, shows the load–displacement curve of the specimens. By increasing the load, displacement for specimens that contain nano particle was decreased compared to the pure resin. Also, 3 wt.% SiO2/phenol, 2 wt.% SiO2/phenol and 1 wt.% SiO2/phenol composite specimens possess the less displacement, respectively. Reduced elastic modulus and hardness values for different samples are shown in Figs. 3 and 4, respectively. It can be seen from the results that the 2 wt.% SiO2/phenol specimens showed best performance compared to the others. According to the results, modulus of elasticity of 2 wt.% SiO2/phenol samples is 5.2% more than that of 1 wt.% SiO2/phenol samples and this value is 13.5% compared to the 3 wt.% SiO2 samples. This reduction is due to the high volume fraction of nano particles which was occurred in this percent as explained by Kouzeli et al. [16]. Modulus of elasticity of the samples that contains 2% SiO2 nano particles was 12.5% more than the pure sample, while the corresponding value was about 7% for 1 wt.% SiO2 sample. According to the results of hardness test, the hardness the samples increases with increasing the weight percent of the nanoparticles. The results of the indentation tests are summarized in Table 2.
Fig. 10. SEM micrographs from (a) neat phenolic film and phenol films containing (b) 1 wt.% of SiO2 (c) 2 wt.% of nano SiO2 (d)3 wt.% of nano SiO2.
Please cite this article in press as: F. Taheri-Behrooz et al., Comput. Mater. Sci. (2014), http://dx.doi.org/10.1016/j.commatsci.2014.08.042
F. Taheri-Behrooz et al. / Computational Materials Science xxx (2014) xxx–xxx
5
3.2. Scratch test
4. Conclusions
The scratch process performed at three steps. First, a pre-scan under a very small load (1 lN) was performed. Then, the indenter touched the sample surface under a certain force and scratch test was carried out. The load used in this test was 800 lN. The length of the scratch created was 10 lm. Finally, a post-scan under the same load as the pre-scan was conducted to get an image of sample after scratch. An estimate of the magnitude of the residual scratch ditch and the extent of immediate recovery could be obtained by comparing the pre-scan and post-scan images. Atomic force microscopy (AFM) image was taken from the scratched samples. Examples of these images along with the depth analysis are depicted in Figs 5 and 6. Results for four types of specimens are compared in Fig. 7. As shown in the Fig. 7, scratch depth of 2 wt.% SiO2/phenol samples is about 10.3% less than that of the pure samples. As well as, this value is 5.6% less than that of 1 wt.% SiO2/phenol samples. The friction coefficient is defined as the ratio of the tangential force to the normal force. The variation of this parameter with time is shown in Fig. 8 for different samples. According to studies of Bautista et al. [17] concerning the relation between scratch test and wear resistance, the mechanical properties extracted from the scratch test can be utilized for investigating of the wear resistance of the coating. During the scratch, the coefficient of friction changes from 0 to 0.4 for each sample, then it remains almost constant for about 30 s, and finally there is a sudden change in the graph due to isolation of the probe from the sample surface on the unloading area. As shown in the Fig. 9 the friction coefficient of the all samples are very close to each other. Therefore, this result confirmed that nano silica particles have a minor effect on the coefficient of friction and wear rate of phenolic resin. This phenomenon is in agreement with the results observed by Omrani et al. [18] that alumina nanoparticles have no effect on the friction coefficient of the epoxy specimens. In contrast to SiO2 and alumina nanoparticles, Chang and Zhang [19] showed that the TiO2 nanoparticles have a great effect on the friction coefficient of epoxy specimens.
In this paper, the effects of adding different percentages of nano-silica in phenolic resin were investigated in terms of hardness, wear and elastic modulus. The performance of fabricated nanocomposite specimens were precisely studied by scratch and indentation tests using Hysitron TriboIndenter. Results of these experiments are as follows:
3.3. SEM images A scanning electron microscope (SEM) was used to investigate the quality of particles dispersion in the resin. The SEM micrographs were obtained using a Tescan Vega II after coating the samples with a thin layer of gold. As shown in Fig. 10, the dispersion of the nano silica in 1 wt.% and 2 wt.% samples are uniform, however for 3 wt.% samples it looks non-uniform. As presented by the Kang et al. [20], increasing the filler content caused agglomeration of the nano particles during mixing and sonication of the mixture.
(a) By nano indentation tests, it was showed that adding any of 1, 2 and 3 percentages of nano SiO2 increased the elastic modulus and hardness of the specimens compared with the pure sample. The increase in elastic modulus and hardness were 12.5% and 27.3% for 2 wt.% and 7% and 21.21% for 1 wt.% nanoparticle filled samples, respectively. However, for 3 wt.% nanoparticle filled samples, the elastic modules decreased about 13.5% and 2.7% compared to 2 wt.% nanoparticle filled and pure samples, respectively. (b) By inspection of the AFM images, the maximum decrease in the depth penetration was belonged to 3 wt.% nanoparticle filled sample, then 2 wt.% nanoparticle filled samples in comparison with the pure samples. Depth penetration equals to 81.881 nm, 84.464 nm and 93 nm for 3 wt.%, 2 wt.% and pure samples, respectively. (c) According to the friction coefficient versus time graph, it was revealed that the coefficient of friction were almost identical for all three samples. This observation confirmed that nanosilica particles have a minor effect on the wear properties of the phenolic resin under investigation.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
G. Schottner et al., J. Sol–Gel Sci. Technol. 27 (1) (2003) 71–79. U. Schulz et al., Prog. Org. Coat. 42 (1) (2001) 38–48. C. Flosbach, W. Schubert, Prog. Org. Coat. 43 (1) (2001) 123–130. T. Mahrholz et al., Compos. Part A: Appl. Sci. Manuf. 40 (3) (2009) 235–243. H. Zhang et al., Polymer 49 (17) (2008) 3816–3825. H. Zhang et al., Acta Mater. 54 (7) (2006) 1833–1842. Y. Dong et al., Compos. Part A: Appl. Sci. Manuf. 42 (10) (2011) 1483–1492. C.B. Ng et al., Nanostruct. Mater. 12 (1999) 507–510. T. Adachi et al., Acta Mater. 56 (9) (2008) 2101–2109. W.C. Oliver, G.M. Pharr, J. Mater. Res. 19 (01) (2004) 3–20. L. Lin et al., Prog. Org. Coat. 40 (1) (2000) 85–91. C.L. Chiang et al., J. Polym. Sci.: Part A: Polym. Chem. 4 (2003) 905–913. A. Mirzapour et al., Compos. Part A: Appl. Sci. Manuf. 63 (2014) 159–167. Teconan Company, Nano-Silicon Oxide Datasheet, 2010. W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (06) (1992) 1564–1583. M. Kouzeli et al., Acta Mater. 49 (3) (2001) 497–505. Y. Bautista et al., Prog. Org. Coat. 70 (4) (2011) 178–185. A. Omrani et al., Mater. Chem. Phys. 114 (1) (2009) 145–150. L. Chang, Z. Zhang, Wear 260 (7) (2006) 869–878. Y. Kang et al., Appl. Surf. Sci. 258 (17) (2012) 6384–6390.
Please cite this article in press as: F. Taheri-Behrooz et al., Comput. Mater. Sci. (2014), http://dx.doi.org/10.1016/j.commatsci.2014.08.042
Contents lists available at ScienceDirect
Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci
Mechanical properties modification of a thin film phenolic resin filled with nano silica particles F. Taheri-Behrooz, B. Memar Maher, M.M. Shokrieh ⇑ Composites Research Laboratory, Center of Excellence in Experimental Solid Mechanics and Dynamics, School of Mechanical Engineering, Iran University of Science and Technology, Tehran 16846-13114, Iran
a r t i c l e
i n f o
Article history: Received 1 March 2014 Received in revised form 24 August 2014 Accepted 29 August 2014 Available online xxxx Keywords: Nano silica Phenolic resin Nano indentation Scratch Hardness Elastic modulus Penetration depth
a b s t r a c t The current manuscript experimentally investigates the mechanical and wears properties of three different phenolic polymer nanocomposites. Samples are made of 1, 2 and 3 wt.% of well-dispersed silica nanoparticles in a phenolic resin. Firstly, it was found that for the phenolic polymer, the Young’s modulus steadily increased as the weight percent of silica nanoparticles was increased except for 3 wt.% nanoparticle filled samples. For the phenolic polymer with 1 and 2 wt.% nano silica, Young’s modulus was increased 7% and 12.5%, respectively in comparison with the neat phenolic resin. However, for samples with 3% nanosilica this value showed a reduction about 13.5% and 2.7% in comparison to 2% nanoparticle filled samples and pure samples, respectively. Secondly, the presence of 2 wt.% nano silica particles increased the hardness of the phenolic polymer by 27.3% compared with the pure phenolic resin. Also, it was shown by the results of scratch tests that nano silica has almost no effect on the coefficient of the friction and wear rates. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Polymeric nano-reinforced coatings have drawn considerable attention, in recent years, caused by improvements in various properties comprising scratch resistance, abrasion resistance, heat stability as well as other mechanical properties [1–4]. Traditionally, the scratch resistance of an organic coating can be improved by the addition of high content of inorganic filler. As inorganic fillers a range of different metal–oxide particles can be dispersed in the polymeric matrix [1,5–8]. The use of inorganic particles in the nano scale range is particularly attractive since it allows improving the properties of the polymers by controlling the degree of interaction between the polymer and the nano fillers [9] via a top–down approach. Phenol resins (PF), notwithstanding the century-long history, are still attracting a great deal of research interests. They are important technical materials and irreplaceable in many fields, especially in thermal insulation, coating, aeronautic utilities, electro-optical devices and composite materials due to their thermal stability, high char yield, structural integrity and solvent resistance. Phenol resin-based friction materials usually contain a large number of reinforcing and filling constituents such as reinforcing
fibers, abrasives, binders, fillers, and friction modifiers (solid lubricants). This accounts for the great dependence of their properties on the interactions and synergetic effects among the multiphase ingredients. In this sense, it is very important to correctly select and properly combine the different components so as to satisfy a number of requirements for the properties of the friction materials, such as good wear resistance, stable friction coefficient and reliable strength at a wide range of rigorous conditions. In recent years, one of the widely used techniques for the evaluation of the mechanical and tribological properties of metals, ceramics, polymers and films at ultra-microscopic level is nano indentation and nano scratch tests using depth-sensing method [10,11]. However, up to now, few papers concerning the tribological behaviors at the micro/nano-scale of the phenolic-based nano composites have been presented [12,13]. The present paper characterizes the mechanical and tribological behaviors of phenolic films and SiO2/phenolic nano composite coatings by using nano mechanics testing.
2. Materials and methods 2.1. Materials
⇑ Corresponding author. E-mail address: [email protected] (M.M. Shokrieh).
Resol-type phenolic resin (IL800) and SiO2 particles were used in the present research. The morphology of SiO2 nanoparticle is
http://dx.doi.org/10.1016/j.commatsci.2014.08.042 0927-0256/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: F. Taheri-Behrooz et al., Comput. Mater. Sci. (2014), http://dx.doi.org/10.1016/j.commatsci.2014.08.042
2
F. Taheri-Behrooz et al. / Computational Materials Science xxx (2014) xxx–xxx
Table 1 Impurities of Silicon nano-oxide [14]. Component
Cu
Zn
Fe
Mg
Na
Al
As
Sb
Pb
Hg
Content (%)
0.0
0.0005884
0.0
0.0000042
0.0
0.0
0.0
0.0000031
0.0
0.0
spherical with an average particle size of 10–15 nm, and purity of Silicon nano-oxide is 99.999% as shown in Table 1. 2.2. Sample preparation
To investigate the mechanical and tribological performances of composite coating, nano-scale indentation and scratch tests were applied by using HysitronTribo Indenter. The indentation tests were carried out underforce-control method, where the applied load is controlled according to a programmed loading function, and the displacement was continuously monitored. The loading function of the indentation in this research consists of five steps linear loading and five step sun loading segment together. The maximum applied load for all samples was 800 lN. The final values of hardness and modulus were taken as the average of five indentations carried out in different spots on the same material. To calculate hardness (H) and modulus (E) the Olive and Pharr method [15] were used:
8.2
8
Elastic modulus (GPa)
2.3. Characterization method
Fig. 2. Load–displacement curve for the specimens.
7.8 7.6 7.4 7.2
7 6.8 6.6 6.4
Pure
1 wt%
2 wt%
3 wt%
Fig. 3. Elastic modulus of the specimens.
0.6 0.5
Hardness (GPa)
Phenolic resin was mixed with 1 wt.%, 2 wt.% and 3 wt.% of SiO2 for 20 min by a mixer at 600 rpm by using a two-propelled mechanical stirrer. Then, it was mixed for extra 5 min at 100 rpm. In order to obtain a full dispersion of the nanoparticles, probe sonicator of 14 mm in diameter (Hielscher UP400S) was utilized. The sonication process was performed for 30 min with 50 s cycle using ultrasonic waves set to 50 kHz. To avoid temperature rise during the ultrasonic processing, the Pyrex beaker containing resin and nano silica were cooled by an ice-bath. During the stirring, air bubbles created due to mixture also could inversely affect the quality and the properties of final product. To avoid this effect, the prepared mixture was placed in a vacuum oven for 15 min to be completely degassed. The mixture was prepared as a thin film on a metal substrate and kept for 18 h at 40 °C temperature to complete the curing process of the specimens. It is worthy to note that to have a tendency solvent used in resin; resins were stored at room temperature to evaporate the solvent for a week which leads to smooth sample surface without any pores.
0.4 0.3 0.2 0.1 0
Pure
1 wt%
2 wt%
3 wt%
Fig. 4. Hardness of the specimens.
Table 2 Results of the indentation test.
Fig. 1. A schematic representation of the load–displacement curve [15].
Pure 1% 2% 3%
Young’s modules (GPa)
Hardness (GPa)
Max force (lN)
7.2 7.7 8.1 7.0
0.33 0.40 0.42 0.48
748.7 751.6 752.8 754.8
Please cite this article in press as: F. Taheri-Behrooz et al., Comput. Mater. Sci. (2014), http://dx.doi.org/10.1016/j.commatsci.2014.08.042
3
F. Taheri-Behrooz et al. / Computational Materials Science xxx (2014) xxx–xxx
F max Ac
1 1 #2 1 #2i ¼ þ Er E Ei
S¼
df ¼ 2Er dh
96
ð1Þ
rffiffiffiffiffi Ac
p
ð2Þ
ð3Þ
In this method, stiffness (S) is given with respect to the slope of unloading cure as depicted in Fig. 1[13]. Where h is displacement, Ac is contact area, Er is reduced modulus E and m are Young’s modulus and Poisson’s ratio of the sample, respectively, Ei and mi are the Young’s modulus and Poisson’s ratio of diamond indenter, respectively.
Scratch depth analyses (nm)
H¼
92
88
84
80 Pure
1 wt%
2 wt%
Fig. 7. Scratch depth analyses for specimens.
Fig. 5. Scratch depth analysis for 2% wt. SiO2/phenolic specimen.
Fig. 6. Scratch depth analysis for 3% wt. SiO2/phenolic specimen.
Please cite this article in press as: F. Taheri-Behrooz et al., Comput. Mater. Sci. (2014), http://dx.doi.org/10.1016/j.commatsci.2014.08.042
3 wt%
4
F. Taheri-Behrooz et al. / Computational Materials Science xxx (2014) xxx–xxx
The elastic modulus using this approach is accurate, but for polymers because of their time- and rate-dependent behavior under load, this generally leads to an incompatible evaluation of properties. But, the indentation test was repeated 5 times and the repeatability of test results was observed.
3. Results and discussion 3.1. Indentation test
Fig. 8. Friction coefficient of the samples.
Friction Coefficient
0.357
0.328
0.299
0.27 Pure Phenol
1 wt%
2 wt%
3 wt%
Fig. 9. Average of the friction coefficient of samples.
Fig. 2, shows the load–displacement curve of the specimens. By increasing the load, displacement for specimens that contain nano particle was decreased compared to the pure resin. Also, 3 wt.% SiO2/phenol, 2 wt.% SiO2/phenol and 1 wt.% SiO2/phenol composite specimens possess the less displacement, respectively. Reduced elastic modulus and hardness values for different samples are shown in Figs. 3 and 4, respectively. It can be seen from the results that the 2 wt.% SiO2/phenol specimens showed best performance compared to the others. According to the results, modulus of elasticity of 2 wt.% SiO2/phenol samples is 5.2% more than that of 1 wt.% SiO2/phenol samples and this value is 13.5% compared to the 3 wt.% SiO2 samples. This reduction is due to the high volume fraction of nano particles which was occurred in this percent as explained by Kouzeli et al. [16]. Modulus of elasticity of the samples that contains 2% SiO2 nano particles was 12.5% more than the pure sample, while the corresponding value was about 7% for 1 wt.% SiO2 sample. According to the results of hardness test, the hardness the samples increases with increasing the weight percent of the nanoparticles. The results of the indentation tests are summarized in Table 2.
Fig. 10. SEM micrographs from (a) neat phenolic film and phenol films containing (b) 1 wt.% of SiO2 (c) 2 wt.% of nano SiO2 (d)3 wt.% of nano SiO2.
Please cite this article in press as: F. Taheri-Behrooz et al., Comput. Mater. Sci. (2014), http://dx.doi.org/10.1016/j.commatsci.2014.08.042
F. Taheri-Behrooz et al. / Computational Materials Science xxx (2014) xxx–xxx
5
3.2. Scratch test
4. Conclusions
The scratch process performed at three steps. First, a pre-scan under a very small load (1 lN) was performed. Then, the indenter touched the sample surface under a certain force and scratch test was carried out. The load used in this test was 800 lN. The length of the scratch created was 10 lm. Finally, a post-scan under the same load as the pre-scan was conducted to get an image of sample after scratch. An estimate of the magnitude of the residual scratch ditch and the extent of immediate recovery could be obtained by comparing the pre-scan and post-scan images. Atomic force microscopy (AFM) image was taken from the scratched samples. Examples of these images along with the depth analysis are depicted in Figs 5 and 6. Results for four types of specimens are compared in Fig. 7. As shown in the Fig. 7, scratch depth of 2 wt.% SiO2/phenol samples is about 10.3% less than that of the pure samples. As well as, this value is 5.6% less than that of 1 wt.% SiO2/phenol samples. The friction coefficient is defined as the ratio of the tangential force to the normal force. The variation of this parameter with time is shown in Fig. 8 for different samples. According to studies of Bautista et al. [17] concerning the relation between scratch test and wear resistance, the mechanical properties extracted from the scratch test can be utilized for investigating of the wear resistance of the coating. During the scratch, the coefficient of friction changes from 0 to 0.4 for each sample, then it remains almost constant for about 30 s, and finally there is a sudden change in the graph due to isolation of the probe from the sample surface on the unloading area. As shown in the Fig. 9 the friction coefficient of the all samples are very close to each other. Therefore, this result confirmed that nano silica particles have a minor effect on the coefficient of friction and wear rate of phenolic resin. This phenomenon is in agreement with the results observed by Omrani et al. [18] that alumina nanoparticles have no effect on the friction coefficient of the epoxy specimens. In contrast to SiO2 and alumina nanoparticles, Chang and Zhang [19] showed that the TiO2 nanoparticles have a great effect on the friction coefficient of epoxy specimens.
In this paper, the effects of adding different percentages of nano-silica in phenolic resin were investigated in terms of hardness, wear and elastic modulus. The performance of fabricated nanocomposite specimens were precisely studied by scratch and indentation tests using Hysitron TriboIndenter. Results of these experiments are as follows:
3.3. SEM images A scanning electron microscope (SEM) was used to investigate the quality of particles dispersion in the resin. The SEM micrographs were obtained using a Tescan Vega II after coating the samples with a thin layer of gold. As shown in Fig. 10, the dispersion of the nano silica in 1 wt.% and 2 wt.% samples are uniform, however for 3 wt.% samples it looks non-uniform. As presented by the Kang et al. [20], increasing the filler content caused agglomeration of the nano particles during mixing and sonication of the mixture.
(a) By nano indentation tests, it was showed that adding any of 1, 2 and 3 percentages of nano SiO2 increased the elastic modulus and hardness of the specimens compared with the pure sample. The increase in elastic modulus and hardness were 12.5% and 27.3% for 2 wt.% and 7% and 21.21% for 1 wt.% nanoparticle filled samples, respectively. However, for 3 wt.% nanoparticle filled samples, the elastic modules decreased about 13.5% and 2.7% compared to 2 wt.% nanoparticle filled and pure samples, respectively. (b) By inspection of the AFM images, the maximum decrease in the depth penetration was belonged to 3 wt.% nanoparticle filled sample, then 2 wt.% nanoparticle filled samples in comparison with the pure samples. Depth penetration equals to 81.881 nm, 84.464 nm and 93 nm for 3 wt.%, 2 wt.% and pure samples, respectively. (c) According to the friction coefficient versus time graph, it was revealed that the coefficient of friction were almost identical for all three samples. This observation confirmed that nanosilica particles have a minor effect on the wear properties of the phenolic resin under investigation.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
G. Schottner et al., J. Sol–Gel Sci. Technol. 27 (1) (2003) 71–79. U. Schulz et al., Prog. Org. Coat. 42 (1) (2001) 38–48. C. Flosbach, W. Schubert, Prog. Org. Coat. 43 (1) (2001) 123–130. T. Mahrholz et al., Compos. Part A: Appl. Sci. Manuf. 40 (3) (2009) 235–243. H. Zhang et al., Polymer 49 (17) (2008) 3816–3825. H. Zhang et al., Acta Mater. 54 (7) (2006) 1833–1842. Y. Dong et al., Compos. Part A: Appl. Sci. Manuf. 42 (10) (2011) 1483–1492. C.B. Ng et al., Nanostruct. Mater. 12 (1999) 507–510. T. Adachi et al., Acta Mater. 56 (9) (2008) 2101–2109. W.C. Oliver, G.M. Pharr, J. Mater. Res. 19 (01) (2004) 3–20. L. Lin et al., Prog. Org. Coat. 40 (1) (2000) 85–91. C.L. Chiang et al., J. Polym. Sci.: Part A: Polym. Chem. 4 (2003) 905–913. A. Mirzapour et al., Compos. Part A: Appl. Sci. Manuf. 63 (2014) 159–167. Teconan Company, Nano-Silicon Oxide Datasheet, 2010. W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (06) (1992) 1564–1583. M. Kouzeli et al., Acta Mater. 49 (3) (2001) 497–505. Y. Bautista et al., Prog. Org. Coat. 70 (4) (2011) 178–185. A. Omrani et al., Mater. Chem. Phys. 114 (1) (2009) 145–150. L. Chang, Z. Zhang, Wear 260 (7) (2006) 869–878. Y. Kang et al., Appl. Surf. Sci. 258 (17) (2012) 6384–6390.
Please cite this article in press as: F. Taheri-Behrooz et al., Comput. Mater. Sci. (2014), http://dx.doi.org/10.1016/j.commatsci.2014.08.042