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silica hybrids by sol–gel processing
Wear 262 (2007) 1048–1055
The effect of silica size on the friction and wear behaviors of polyimide/silica hybrids by sol–gel processing Shi-Quan Lai a,b , Tong-Sheng Li a,∗ , Fan-Dong Wang a , Xu-Jun Li a , Li Yue b a
Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai 200433, PR China b Laboratory of Functional Materials, School of Chemical Engineering, Anshan University of Science and Technology, Anshan, Liaoning 114004, PR China Received 29 May 2006; received in revised form 13 September 2006; accepted 17 October 2006 Available online 28 November 2006
Abstract In order to investigate the effect of silica size on the friction and wear behaviors of PI hybrids, polyimide/silica (PI/SiO2 ) hybrids with different size of silica were successfully synthesized through an in situ sol–gel reaction from pyromellitic dianhydride-4,4 -oxydianiline (PMDA—ODA) and tetraethoxysilane (TEOS). The size of silica in the hybrids was 100–800 nm. The friction and wear test for pure PI and its hybrids was carried out on a ball-on-disc wear tester under dry sliding conditions. Tensile tests on the PI/SiO2 hybrids showed that the strength and toughness of PI/SiO2 hybrids were improved simultaneously when the size of silica was less than 300 nm. The friction coefficient and wear rate of the PI hybrids firstly decreased and then increased with increasing the size of silica. The friction coefficient of the hybrid with 100 nm SiO2 was the lowest and ca.20% lower in contrast with that of pure PI. However, the lowest wear rate was recorded for the hybrid with 300 nm SiO2 , ca.20% lower than that of neat PI. These behaviors were attributed to the size effect of silica in PI matrix. Scanning electron microscopy (SEM) revealed that an appropriate size of silica in PI matrix could effectively reduce adhesive wear of PI and restrain the formation of bigger debris. © 2006 Elsevier B.V. All rights reserved. Keywords: Size; Friction; Wear; Polyimide; Silica; Hybrid
1. Introduction The predominant feature of nanometer particles lies in their small size effect [1]. If nanometer particles can be well dispersed in the surrounding polymer matrix, low content of nanometer particles can provide an enormous amount of interfacial area through which the bulk properties of the polymer can be altered. Accordingly, many attempts were made to develop nanometer particles filled polymeric composites to improve the tribological performance of the matrices in the recent years [2,3]. Wang et al. [4–7] incorporated nano-Si3 N4 , nano-SiO2 , nanoSiC and nano-ZrO2 into polyetheretherketone (PEEK), the testing results indicated that these nanometer particles could effectively reduce the wear rate of PEEK, which was attributed to a thin, uniform and tenacious transfer film formed on the surface of the counterpart. Bahadur and coworkers [8,9] inves-
∗
Corresponding author. Tel.: +86 21 65642199; fax: +86 21 5080117. E-mail address: [email protected] (T.-S. Li).
0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.10.007
tigated tribological properties of polyphenylene sulfide (PPS) filled with nanometer Al2 O3 , TiO2 , ZnO, CuO and SiC. An increase in wear resistance and a decrease of the coefficient of friction were observed under specific conditions. Moreover they also discussed wear mechanism of PPS from transfer films. Li et al. [10] reported that nanometer ZnO filled to PTFE could greatly reduce the wear of this polymer. The anti-wear mechanisms was that nanometer ZnO prevented the destruction of PTFE banded structure during the friction process. Sawyer et al. [11] explored that the friction and wear behavior of PTFE composites filled with 40 nm Al2 O3 , they found that the friction coefficient of the composite increased slightly over unfilled sample and the wear resistance increased monotonically with increasing filler concentration. Lai et al. [12–14] used nanometer attapulite and ultrafine-fine diamond to improve the tribological performance of PTFE and analyzed the mechanism of filler action in reducing the wear of PTFE polymer from material structure through differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Yu et al. [15] compared the friction and wear behaviors of polyoxymethylene (POM) filled with micrometer
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and submicron copper particles, the submicron copper particles was more effective in contrast with micrometer copper particles for lowering the wear and coefficient of friction of the composites. Ng et al. [16] dispersed TiO2 nanoparticles in epoxy, the resultant composites appeared not only to be tougher than the traditional microparticle filled epoxy but also possessed a higher scratch resistance. Cai et al. [17–19] researched tribological properties of polyimide filled with nanometer SiO2 , Al2 O3 and carbon nanotube (CNT), they reported that nanometer SiO2 , Al2 O3 and CNT could effectively enhance the friction-reduction and antiwear capacity of the composite. Lai et al. [20] showed that nano-attapulgite particles could obviously improve the tribological behaviors of polyimide. Nano-attapulgite particles restrained the formation of bigger debris. In addition, many researchers still studied the friction and wear behaviors of ultrahigh molecular weight polyethylene (UHMWPE) composites containing kaolin [21], montmorillonite (MMT) [22], nano-SiO2 [23] and nano-Al2 O3 [24], the results indicated that these materials were of outstanding friction performance. It is well known that the smaller the size of filler particles is, the larger their specific surface area becomes, and the more likely the agglomeration of the particles. However, by analyzing the literature dealing with nanoparticle filled polymeric composites for tribological applications which are prepared by mechanic dispersive mixing, it can be estimated that it is difficult to disperse nanometer particles into indeed significant nanometer particles in polymer matrix as a result of agglomeration of nanoparticles and high viscosity of polymer. Thereby, it is very important to select a right preparation method which can well disperse nanometer particles into polymer matrix in order to research small size effect of nanoparticles in tribomaterials. With this perspective in mind, a sol–gel method was utilized to prepare polyimide-silica hybrid materials in the present work. Moreover, nanoparticles in polymer matrix were controlled through altering experimental conditions. Although many researchers [25–27] had reported coupling agent could availably reduce the size of silica particles in PI/SiO2 hybrids, we do not intend to add coupling agent into our research system in this work because coupling agent can strengthen interface interaction between SiO2 particles and PI matrix, and consequently, affect the friction and wear behaviors of PI hybrids. The purpose of this work is to study the effect of the size of SiO2 particles on the friction and wear behaviors of the PI/SiO2 hybrids synthesized through a sol–gel method. Moreover, the wear mechanisms of the hybrids under dry sliding conditions are also discussed. 2. Experimental details 2.1. Materials Pyromellitic dianhydride (PMDA, chemical reagent grade), 4,4 -oxydianiline (ODA, chemical reagent grade), N,N-dimethylacetamide (DMAc, analytic reagent grade), tetraethoxysilane (TEOS) and hydrochloric acid (HCl, chemical reagent grade) were all purchased from Shanghai chemistry
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agent Co., Ltd. (China). These reagents were used without further purification. 2.2. Synthesis of poly(amic acid) Poly(amic acid) (PAA) was synthesized by putting 0.12 mol of 4,4 -oxydianiline (ODA) into a three-neck flask containing 500 g of N,N-dimethylacetamide (DMAc) under nitrogen purge in ice water. After ODA was completely dissolved in DMAc, 0.12 mol of PMDA, which was divided into three batches, was added to the flask batch-by-batch with a time interval of 30 min between batches. After all the PMDA was dissolved, the mixtures in the flask were stirred for 6 h at room temperature, and a viscous poly(amic acid) solution was obtained. The PAA solution was kept in a freezer until use. 2.3. Preparation of PI/silica hybrids The PI/SiO2 hybrids were prepared by blending the required ratios of TEOS, deionized water and HCl with 50 g PAA solution. The recipe for the PI/silica hybrids was given Table 1, SiO2 content in the hybrids was obtained according as a hypothesis that TEOS translated absolutely into SiO2 . The solutions were stirred for 6 h at room temperature until they became homogeneous and viscous. The PI/Silica hybrids for tensile test and scanning electron microscope (SEM) observation were prepared by casting the PAA mixed solution on a glass substrate. However, the samples for friction and wear test were obtained by spraying the PAA mixed solution on a steel ring with 40 mm diameter, the steel ring surface was polished with metallographic abrasive paper before spraying the PAA mixed solution. Finally, these samples were put in an air convection oven at 70 ◦ C for 2 h before the imidization step. Imidization of PAA/SiO2 was carried out by putting the samples in an air-circulation oven at 100 ◦ C, 200 ◦ C and 300 ◦ C for 1 h, respectively. SEM micrographs of the broken surface of the PI hybrids are shown in Fig. 1. It can be seen from Fig. 1 that SiO2 particles in the PI hybrids become round or spherical shapes. Moreover, these particles are evenly embedded in PI matrix. Our early Table 1 The recipe for the PI/silica hybrids Samplea
TEOS (g)
Silica contentb (wt%)
H2 O/TEOS (molar ratio)
Silica sizec (nm)
PI 0/0 PI 5/4 PI 10/2 PI 10/4 PI 10/8 PI 10/12 PI 15/4 PI 20/4
0 1.0 2.0 2.0 2.0 2.0 3.1 4.3
0 5 10 10 10 10 15 20
0 4 2 4 8 12 4 4
0 150 100 200 500 800 450 650
a The number before the slash denotes the silica weight content in the hybrid films ant the number after denotes molar ratio of H2 O and TEOS. b Silica content was calculated from a hypothesis that TEOS translated absolutely into silica. c Silica size was obtained from SEM observation of the hybrid films (seen in Fig. 1).
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Fig. 1. SEM micrographs of fracture surfaces of the PI/silica hybrids: (a) PI 5/4, (b) PI 10/2, (c) PI 10/4, (d) PI 10/8, (e) PI 10/12, (f) PI 15/4.
research reported that PI is a kind of compact structural materials [24], although from Fig. 1 it is noticed that there is a weak interaction between SiO2 particles and PI matrix. The size of silica increases with increasing the addition content of TEOS and deionized water, respectively. SiO2 size in the hybrid materials ranges between 100 nm and 800 nm. The results are listed in Table 1. Therefore, it could be inferred from Fig. 1 that the size of silica particles in PI matrix may be controlled through altering experimental conditions such as the additive content of TEOS and deionized water. 2.4. Characterization In order to study the morphology and size of SiO2 particles in PI matrix, the morphologies of broken surfaces of the PI hybrids were observed using a JSM-560LV scanning electron microscope (SEM). The samples for SEM observation were fractured in liquid nitrogen, and then were sputter coated with a thin layer of gold palladium alloy prior to SEM examina-
tion. SEM micrographs are given in Fig. 1. Tensile properties of the solution-cast films were tested with an Instron Mechanical Tester (model 5567). The crosshead speed was set as 0.5 mm/min in all experiments. The specimens were prepared by being cut into strips 4 mm × 30 mm. An average of at least five individual determinations was used. 2.5. Friction and wear test Dry sliding wear tests were conducted on a ball-on-disc friction and wear tester to evaluate the friction and wear properties of the polyimide hybrid films with different size of silica. Schematic diagram of wear tester was described in Fig. 2. A stainless steel ball was used as the counterpart, and its diameter was 3 mm. Sliding was performed under ambient conditions (temperature: 20 ± 3 ◦ C, humidity: 50 ± 10%). Each friction and wear test was carried out for 30 min. The frictional force transferred to a sensor was recorded throughout the tests. The sensor produced an electric output that depended on the frictional force.
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Fig. 2. Schematic diagram of wear tester.
The electric signal amplified by a multi-meter was stored in a computer via a general-purpose interface bus (GPIB). The wear rate K (g N−1 m−1 ) was calculated from the following relationship: M K= Pvt
(1)
where M the mass loss (g), P the load (N), v the sliding speed of the steel ring (m s−1 ), t the duration of friction (min). Three replicate friction and wear tests were carried out so as to minimize data scattering, and the average of the three replicate test results was reported in this article. After the wear tests, the worn surfaces of the hybrids and worn debris were examined by SEM in order to elucidate the friction and wear mechanisms. In order to increase the resolution for the SEM observation, the tested composite specimens were plated with gold coating to render then electric conductivity. 3. Results and discussion 3.1. Friction and wear properties
Fig. 3. Friction coefficient as a function of time of the PI hybrids with different size of silica (load: 2 N): A: PI 0/0, B: PI 5/4, C: PI 10/4, D: PI 20/4, E: PI 10/12.
coefficient of the hybrid is almost equal to that of pure PI. Comparing pure PI and the polyimide hybrids, it can be concluded that the addition of silica is favorable for lowering the friction coefficient, especially, when the size of silica in the hybrids is below 150 nm. At the same time, in Fig. 5 the calculated wear rate K (g N−1 m−1 ) versus SiO2 size is plotted. The mass loss at the end of the tests was used to calculate the wear rate of the PI hybrids through Eq. (1). From Fig. 5 it can be seen that the wear rate of the PI hybrid films decreases firstly and then increases with increasing the size of SiO2 particle in the hybrids. When SiO2 size is 300 nm, the wear rate of the PI hybrid is the lowest (2.67 × 10−8 g N−1 m−1 ), this is ca.20% lower than that of pure PI (3.35 × 10−8 g N−1 m−1 ). The average friction coefficient for this sample is μ ¯ = 0.35. These behaviors can be attributed to the effect of silica particles. Comparing Figs. 4 and 5, it can be seen that the lowest friction coefficient is not corresponding to the least wear rate. The friction and wear mechanisms which silica particles reduce the coefficient of friction and wear rate of pure
Figs. 3 and 4 show the recorded friction coefficients as a function of time and the average values as a function of the size of SiO2 in the hybrids, respectively. The average friction coefficient (μ) ¯ for each test is calculated using Eq. (2) and the variance (σ 2 ) is calculated using Eq. (3). In both the equations t is the total duration of the test in seconds 1 μ ¯ = μ dt (2) T 2 μ dt 1 2 σ = μ dt − (3) T T From Figs. 3 and 4 it is clear that first the friction coefficient decreases obviously, and then increases as the size of SiO2 particle in the hybrids increases. The friction coefficient of the hybrid with 100 nm SiO2 particle is the lowest (0.32), this is ca.20% lower in contrast with that of pure PI (0.41). However, when the size of SiO2 particles is up to 800 nm, the friction
Fig. 4. Average friction coefficient for the samples as a function of the size of silica in the hybrid films (load: 2 N).
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Fig. 5. A plot of the calculated wear rate versus the size of silica in the hybrids (2 N).
PI will be discussed through mechanical analysis of materials and SEM observations of worn surfaces and debris. The friction coefficient and wear rate of pure PI and PI/SiO2 hybrids under various sliding speeds as functions of SiO2 size at a load of 2 N are given in Figs. 6 and 7, respectively. From Fig. 6, it can be seen that the friction coefficient first decreases, and then increases as SiO2 size increases under all sliding speeds, the hybrid with 100 nm SiO2 shows the lowest friction coefficient in all samples. Furthermore, the friction coefficient of pure PI and its hybrids with different size of silica decrease with increasing sliding speed. This behavior can be put down to an increase in the orientation degree of molecular chains of polyimide in frictional interfaces with increasing sliding speed. Fig. 7 shows that the wear rate of pure PI and its hybrids with different size of silica increases with increasing sliding speed. It could be rational to infer that the sliding surface temperature of PI/SiO2 hybrids increased with sliding speed, which resulted in micromelting of the hybrid surface. Such a kind of micro-melting could be speeded at a relatively larger sliding speed, owing to the extended sliding distance.
Fig. 6. The effect of sliding speed on the friction coefficient of PI hybrids.
Fig. 7. Variation of the wear rate of PI hybrids with the size of silica at various sliding speed.
Figs. 8 and 9 show, respectively, the variations of the friction coefficient and wear rate of pure PI and PI/SiO2 hybrids with load at a sliding speed of 0.84 m/s. From Fig. 8, it can be seen that the PI hybrids with SiO2 size below 150 nm give obviously decreased friction coefficients at both low and high load compared to the bare PI, while they register larger friction coefficients at a SiO2 size above 150 nm, which is similar to what was observed in Fig. 4. In addition, a relatively small friction coefficient is registered for the hybrids at a high load. Thus, the PI/SiO2 hybrids are more suitable to the working condition of relatively high load. Fig. 9 shows the wear rate of PI/SiO2 hybrids first decreases and then increases with increasing SiO2 size in contrast with that of pure PI at both low and high load. Moreover, the PI hybrid materials give a better wear-resistance at lower load than at higher load. Simultaneously, as can be seen by the naked eye, it is interesting to notice that the hybrid with 300 nm SiO2 exhibits the best wear-resistance under the both load conditions.
Fig. 8. Variation of the friction coefficient of PI hybrids with the size of silica at various applied load.
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of PI when the size of silica is less than 800 nm. This suggests that there is a reinforcement effect as a result of the existence of SiO2 particles in the PI hybrids. This effect could be due to the small size effect of SiO2 particles in the hybrids. However, on the contrary, when the size of silica exceeds 800 nm, SiO2 destroys the mechanical properties of PI. This is attributed to agglomeration of SiO2 in the PI composites; the small size effect of SiO2 disappears, as can be seen in Fig. 1(e). Comparing tribological properties and mechanical performances of the PI hybrids, it appears that the hybrids with the best mechanical property has the best wear-resistance property, but does not correspond to the best friction behavior. This illuminates the wear behavior of materials is directly related to the mechanical properties of hybrids. Moreover, we further investigate the wear mechanisms of the PI/silica hybrids from debris and worn surfaces. Fig. 9. Variation of the wear rate of PI hybrids with the size of silica at various applied load.
3.2. Mechanical properties In order to study the wear mechanisms of the PI hybrid materials, we first analyzed mechanical properties of the PI hybrids. Fig. 10 shows that the tensile strength and elongation at break of the PI/silica hybrid materials as a function of the size of silica in the hybrids. It can be seen from Fig. 10 that the tensile strength increases with increasing SiO2 size and reaches the highest value (125.1 MPa) at a size of 300 nm, this is ca. 50% higher than that of the original PI (82.4 MPa). When the SiO2 size is above 300 nm, the tensile strength decreases obviously, although which is still higher than that of pure PI until the size of silica in the hybrid films reaches 800 nm.The elongation at break of the hybrids shows the same variation tendency with increasing SiO2 size, when the size of silica in PI matrix is 300 nm, the elongation at break (17.8) of the hybrid is ca.200% higher compared to that of pure PI (6.5). Moreover, the elongation at break of the hybrids with 800 nm SiO2 is a little less than that of pure PI. From the above analysis it may be seen that SiO2 particles in PI matrix can improve both strength and toughness
Fig. 10. Variation of the mechanic properties of the hybrid films with the size of silica.
3.3. SEM examinations Debris is a product of frictional experiments, and therefore its analysis is helpful for comprehending friction and wear mechanisms. In this work, SEM is first used for the analysis of debris. SEM micrographs of debris of pure PI and its hybrid materials are given in Fig. 11. Fig. 11 shows that both debris of pure PI and its hybrid materials consist of flakes. However, debris of the PI composites is considerably smaller than that of pure PI in size. When the size of silica in the hybrid films is less than 450 nm, the wear debris of the PI hybrids decreases with increasing the size of silica, the wear rate of the corresponding materials is also lower than that of pristine PI, as can be seen in Fig. 5. Whereas, the size of silica exceeds 450 nm, the debris size of the hybrid materials increases, so the wear rate of these materials also increases. These behaviors could be explained that the less SiO2 particles in PI matrix has bigger surface area and leads to strengthen interactive forces between PI polymer and SiO2 particles. Accordingly, the wear rate of the hybrids decreases greatly in contrast with that of pure PI. However, a bigger SiO2 particles results in the stress concentration, so this decreases the interactive forces between the PI polymer and SiO2 particles, the wear rate of the hybrids obviously increases. The SEM micrographs of the worn surfaces of pure PI and its hybrid materials under experimental conditions are shown in Fig. 12. It can be seen from Fig. 12(a) that the worn surface of pure PI under dry sliding is characterized by severe plastic deformation and micro cracking, while a large amount of rubbed PI debris is observed on the stainless steel ball counterpart surface during the friction experiment. It indicates the type of wear for pure PI is most adhesive wear. Compared to pure PI, It is interesting to notice the great difference between the morphologies of the wear traces on the pure PI and PI hybrids. The worn surfaces PI hybrids are smoother compared to that of pure PI when the size of silica in the hybrids is less than 450 nm. In particular, for PI hybrid with 300 nm SiO2 particles, the worn surface is quite smooth and no ploughed marks could be observed, this is in good agreement with the comments mentioned above. That is, the optical size of SiO2 for the best wear resistance should be recommended as 300 nm. Thus, is can also be inferred that the morphologies of the wear traces are relevant to the wear rates of
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Fig. 11. SEM morphologies of the wear debris of: (a) PI 0/0 (SiO2 size: 0 nm); (b) PI 5/4, (SiO2 size: 150 nm); (c) PI 10/4, (SiO2 size: 450 nm); (d) PI 20/4 (SiO2 size: 650 nm).
SiO2 filled PI. However, SiO2 size is above 450 nm, the worn surfaces of the hybrids are rougher, almost same as that of pure PI. All the hybrids are characterized by plastic deformation, micro cracking, and spalling under drying conditions. This is reason
that the interfacial interaction between the PI matrix and the SiO2 inorganic phase would be worsened at an extended size of SiO2 particles, which accounted for the poorer wear-resistance of the PI/SiO2 hybrids in this case as well.
Fig. 12. SEM morphologies of worn surface of: (a) PI 0/0 (SiO2 size: 0 nm); (b) PI 5/4 (SiO2 size: 150 nm); (c) PI 10/4 (SiO2 size: 450 nm); (d) PI 20/4 (SiO2 size: 650 nm).
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4. Conclusions (1) PI/SiO2 hybrid films with different size of silica are synthesized through a sol–gel reaction. (2) The strength and toughness of PI/SiO2 hybrids are enhanced, respectively, ca. 50% and 200% compared to that of pure PI when the size of silica is less than 300 nm. (3) The size of silica in the hybrid could be controlled through altering experimental conditions such as the additive content of TEOS and deionized water, and its scale is 100– 800 nm. (4) The friction coefficient and wear rate of PI hybrids with different sizes of silica firstly decrease and then increase with increasing the size of silica in the hybrids. The friction coefficient of the hybrid with 100 nm SiO2 is the lowest and ca.20% lower in contrast with that of pure PI. However, the lowest wear rate is recorded for the hybrid with 300 nm SiO2 , and ca.20% lower than that of neat PI. (5) An appropriate size of silica in PI matrix could effectively reduce adhesive wear of pure PI and prevent the formation of bigger debris. References [1] E.P. Giannelis, Adv. Mater. 8 (1996) 29. [2] E. Reynaud, C. Gauthier, J. Perez, Rev. Metall. 96 (1999) 169.
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[3] Q. Ji, M.Q. Zhang, M.Z. Rong, K. Friedrich, Chin. Plast. 15 (2001) 1 (in Chinese). [4] Q.H. Wang, Q.J. Xue, W.C. Shen, W.M. Liu, Wear 196 (1996) 82. [5] Q.H. Wang, Q.J. Xue, W.C. Shen, Tribol. Int. 30 (1997) 193. [6] Q.H. Wang, Q.J. Xue, W.M. Liu, J.M. Chen, Wear 209 (1997) 316. [7] Q.H. Wang, Q.J. Xue, W.M. Liu, W.C. Shen, J. Appl. Polym. Sci. 69 (1998) 135. [8] C.J. Schwartz, S. Bahadur, Wear 237 (2000) 261. [9] S. Bahadur, C. Sunkara, Wear 258 (2005) 1411. [10] F. Li, K. Hu, J. Li, et al., Wear 249 (2002) 877. [11] W.G. Sawyer, K.D. Freudenberg, P. Bhimaraj, et al., Wear 254 (2003) 573. [12] S.Q. Lai, T.S. Li, X.J. Liu, et al., Macromol. Mater. Eng. 289 (2004) 916. [13] S.Q. Lai, T.S. Li, X.J. Liu, et al., Tribol. Int. 39 (2006) 541. [14] S.Q. Lai, L. Yue, T.S. Li, et al., Wear 260 (2006) 462. [15] L.G. Yu, S.R. Yang, H.T. Wang, et al., J. Appl. Polym. Sci. 77 (2000) 2404. [16] C.B. Ng, L.S. Schadler, R.W. Siegel, Nanostruct. Mater. 12 (1999) 507. [17] H. Cai, F.Y. Yan, Q.J. Xue, et al., Mater. Sci. Eng. A 364 (2004) 94. [18] H. Cai, F.Y. Yan, Q.J. Xue, W.M. Liu, Polym. Test 22 (2003) 875. [19] H. Cai, F.Y. Yan, Q.J. Xue, J. Chin. Electr. Micro. Soc. 22 (2003) 420 (in Chinese). [20] S.Q. Lai, L. Yue, T.S. Li, et al., Macromol. Mater. Eng. 290 (2005) 195. [21] G.F. Gong, Y.H. Yang, X. Fu, Wear 1/2 (256) (2004) 88. [22] L. Huang, R.B. Zhang, Z.G. Jiang, et al., J. Beijing Univ. Chem. Tech. 32 (2005) 38 (in Chinese). [23] M.Q. Xue, L.Y. Zhu, Plast. Sci. Tech. 3 (2005) 22 (in Chinese). [24] S.S. Yu, Q.Z. Wang, Eng. Plast. Appl. 32 (2004) 35 (in Chinese). [25] L. Mascia, A. Kioul, J. Mater. Sci. Lett. 13 (1994) 641. [26] S. Goizet, J.C. Schrotter, M. Smaihi, A. Deratani, New J. Chem. 21 (1997) 461. [27] C. Joly, M. Smaihi, L. Porcar, R.D. Noble, Chem. Mater. 11 (1999) 2331.
The effect of silica size on the friction and wear behaviors of polyimide/silica hybrids by sol–gel processing Shi-Quan Lai a,b , Tong-Sheng Li a,∗ , Fan-Dong Wang a , Xu-Jun Li a , Li Yue b a
Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai 200433, PR China b Laboratory of Functional Materials, School of Chemical Engineering, Anshan University of Science and Technology, Anshan, Liaoning 114004, PR China Received 29 May 2006; received in revised form 13 September 2006; accepted 17 October 2006 Available online 28 November 2006
Abstract In order to investigate the effect of silica size on the friction and wear behaviors of PI hybrids, polyimide/silica (PI/SiO2 ) hybrids with different size of silica were successfully synthesized through an in situ sol–gel reaction from pyromellitic dianhydride-4,4 -oxydianiline (PMDA—ODA) and tetraethoxysilane (TEOS). The size of silica in the hybrids was 100–800 nm. The friction and wear test for pure PI and its hybrids was carried out on a ball-on-disc wear tester under dry sliding conditions. Tensile tests on the PI/SiO2 hybrids showed that the strength and toughness of PI/SiO2 hybrids were improved simultaneously when the size of silica was less than 300 nm. The friction coefficient and wear rate of the PI hybrids firstly decreased and then increased with increasing the size of silica. The friction coefficient of the hybrid with 100 nm SiO2 was the lowest and ca.20% lower in contrast with that of pure PI. However, the lowest wear rate was recorded for the hybrid with 300 nm SiO2 , ca.20% lower than that of neat PI. These behaviors were attributed to the size effect of silica in PI matrix. Scanning electron microscopy (SEM) revealed that an appropriate size of silica in PI matrix could effectively reduce adhesive wear of PI and restrain the formation of bigger debris. © 2006 Elsevier B.V. All rights reserved. Keywords: Size; Friction; Wear; Polyimide; Silica; Hybrid
1. Introduction The predominant feature of nanometer particles lies in their small size effect [1]. If nanometer particles can be well dispersed in the surrounding polymer matrix, low content of nanometer particles can provide an enormous amount of interfacial area through which the bulk properties of the polymer can be altered. Accordingly, many attempts were made to develop nanometer particles filled polymeric composites to improve the tribological performance of the matrices in the recent years [2,3]. Wang et al. [4–7] incorporated nano-Si3 N4 , nano-SiO2 , nanoSiC and nano-ZrO2 into polyetheretherketone (PEEK), the testing results indicated that these nanometer particles could effectively reduce the wear rate of PEEK, which was attributed to a thin, uniform and tenacious transfer film formed on the surface of the counterpart. Bahadur and coworkers [8,9] inves-
∗
Corresponding author. Tel.: +86 21 65642199; fax: +86 21 5080117. E-mail address: [email protected] (T.-S. Li).
0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.10.007
tigated tribological properties of polyphenylene sulfide (PPS) filled with nanometer Al2 O3 , TiO2 , ZnO, CuO and SiC. An increase in wear resistance and a decrease of the coefficient of friction were observed under specific conditions. Moreover they also discussed wear mechanism of PPS from transfer films. Li et al. [10] reported that nanometer ZnO filled to PTFE could greatly reduce the wear of this polymer. The anti-wear mechanisms was that nanometer ZnO prevented the destruction of PTFE banded structure during the friction process. Sawyer et al. [11] explored that the friction and wear behavior of PTFE composites filled with 40 nm Al2 O3 , they found that the friction coefficient of the composite increased slightly over unfilled sample and the wear resistance increased monotonically with increasing filler concentration. Lai et al. [12–14] used nanometer attapulite and ultrafine-fine diamond to improve the tribological performance of PTFE and analyzed the mechanism of filler action in reducing the wear of PTFE polymer from material structure through differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Yu et al. [15] compared the friction and wear behaviors of polyoxymethylene (POM) filled with micrometer
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and submicron copper particles, the submicron copper particles was more effective in contrast with micrometer copper particles for lowering the wear and coefficient of friction of the composites. Ng et al. [16] dispersed TiO2 nanoparticles in epoxy, the resultant composites appeared not only to be tougher than the traditional microparticle filled epoxy but also possessed a higher scratch resistance. Cai et al. [17–19] researched tribological properties of polyimide filled with nanometer SiO2 , Al2 O3 and carbon nanotube (CNT), they reported that nanometer SiO2 , Al2 O3 and CNT could effectively enhance the friction-reduction and antiwear capacity of the composite. Lai et al. [20] showed that nano-attapulgite particles could obviously improve the tribological behaviors of polyimide. Nano-attapulgite particles restrained the formation of bigger debris. In addition, many researchers still studied the friction and wear behaviors of ultrahigh molecular weight polyethylene (UHMWPE) composites containing kaolin [21], montmorillonite (MMT) [22], nano-SiO2 [23] and nano-Al2 O3 [24], the results indicated that these materials were of outstanding friction performance. It is well known that the smaller the size of filler particles is, the larger their specific surface area becomes, and the more likely the agglomeration of the particles. However, by analyzing the literature dealing with nanoparticle filled polymeric composites for tribological applications which are prepared by mechanic dispersive mixing, it can be estimated that it is difficult to disperse nanometer particles into indeed significant nanometer particles in polymer matrix as a result of agglomeration of nanoparticles and high viscosity of polymer. Thereby, it is very important to select a right preparation method which can well disperse nanometer particles into polymer matrix in order to research small size effect of nanoparticles in tribomaterials. With this perspective in mind, a sol–gel method was utilized to prepare polyimide-silica hybrid materials in the present work. Moreover, nanoparticles in polymer matrix were controlled through altering experimental conditions. Although many researchers [25–27] had reported coupling agent could availably reduce the size of silica particles in PI/SiO2 hybrids, we do not intend to add coupling agent into our research system in this work because coupling agent can strengthen interface interaction between SiO2 particles and PI matrix, and consequently, affect the friction and wear behaviors of PI hybrids. The purpose of this work is to study the effect of the size of SiO2 particles on the friction and wear behaviors of the PI/SiO2 hybrids synthesized through a sol–gel method. Moreover, the wear mechanisms of the hybrids under dry sliding conditions are also discussed. 2. Experimental details 2.1. Materials Pyromellitic dianhydride (PMDA, chemical reagent grade), 4,4 -oxydianiline (ODA, chemical reagent grade), N,N-dimethylacetamide (DMAc, analytic reagent grade), tetraethoxysilane (TEOS) and hydrochloric acid (HCl, chemical reagent grade) were all purchased from Shanghai chemistry
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agent Co., Ltd. (China). These reagents were used without further purification. 2.2. Synthesis of poly(amic acid) Poly(amic acid) (PAA) was synthesized by putting 0.12 mol of 4,4 -oxydianiline (ODA) into a three-neck flask containing 500 g of N,N-dimethylacetamide (DMAc) under nitrogen purge in ice water. After ODA was completely dissolved in DMAc, 0.12 mol of PMDA, which was divided into three batches, was added to the flask batch-by-batch with a time interval of 30 min between batches. After all the PMDA was dissolved, the mixtures in the flask were stirred for 6 h at room temperature, and a viscous poly(amic acid) solution was obtained. The PAA solution was kept in a freezer until use. 2.3. Preparation of PI/silica hybrids The PI/SiO2 hybrids were prepared by blending the required ratios of TEOS, deionized water and HCl with 50 g PAA solution. The recipe for the PI/silica hybrids was given Table 1, SiO2 content in the hybrids was obtained according as a hypothesis that TEOS translated absolutely into SiO2 . The solutions were stirred for 6 h at room temperature until they became homogeneous and viscous. The PI/Silica hybrids for tensile test and scanning electron microscope (SEM) observation were prepared by casting the PAA mixed solution on a glass substrate. However, the samples for friction and wear test were obtained by spraying the PAA mixed solution on a steel ring with 40 mm diameter, the steel ring surface was polished with metallographic abrasive paper before spraying the PAA mixed solution. Finally, these samples were put in an air convection oven at 70 ◦ C for 2 h before the imidization step. Imidization of PAA/SiO2 was carried out by putting the samples in an air-circulation oven at 100 ◦ C, 200 ◦ C and 300 ◦ C for 1 h, respectively. SEM micrographs of the broken surface of the PI hybrids are shown in Fig. 1. It can be seen from Fig. 1 that SiO2 particles in the PI hybrids become round or spherical shapes. Moreover, these particles are evenly embedded in PI matrix. Our early Table 1 The recipe for the PI/silica hybrids Samplea
TEOS (g)
Silica contentb (wt%)
H2 O/TEOS (molar ratio)
Silica sizec (nm)
PI 0/0 PI 5/4 PI 10/2 PI 10/4 PI 10/8 PI 10/12 PI 15/4 PI 20/4
0 1.0 2.0 2.0 2.0 2.0 3.1 4.3
0 5 10 10 10 10 15 20
0 4 2 4 8 12 4 4
0 150 100 200 500 800 450 650
a The number before the slash denotes the silica weight content in the hybrid films ant the number after denotes molar ratio of H2 O and TEOS. b Silica content was calculated from a hypothesis that TEOS translated absolutely into silica. c Silica size was obtained from SEM observation of the hybrid films (seen in Fig. 1).
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Fig. 1. SEM micrographs of fracture surfaces of the PI/silica hybrids: (a) PI 5/4, (b) PI 10/2, (c) PI 10/4, (d) PI 10/8, (e) PI 10/12, (f) PI 15/4.
research reported that PI is a kind of compact structural materials [24], although from Fig. 1 it is noticed that there is a weak interaction between SiO2 particles and PI matrix. The size of silica increases with increasing the addition content of TEOS and deionized water, respectively. SiO2 size in the hybrid materials ranges between 100 nm and 800 nm. The results are listed in Table 1. Therefore, it could be inferred from Fig. 1 that the size of silica particles in PI matrix may be controlled through altering experimental conditions such as the additive content of TEOS and deionized water. 2.4. Characterization In order to study the morphology and size of SiO2 particles in PI matrix, the morphologies of broken surfaces of the PI hybrids were observed using a JSM-560LV scanning electron microscope (SEM). The samples for SEM observation were fractured in liquid nitrogen, and then were sputter coated with a thin layer of gold palladium alloy prior to SEM examina-
tion. SEM micrographs are given in Fig. 1. Tensile properties of the solution-cast films were tested with an Instron Mechanical Tester (model 5567). The crosshead speed was set as 0.5 mm/min in all experiments. The specimens were prepared by being cut into strips 4 mm × 30 mm. An average of at least five individual determinations was used. 2.5. Friction and wear test Dry sliding wear tests were conducted on a ball-on-disc friction and wear tester to evaluate the friction and wear properties of the polyimide hybrid films with different size of silica. Schematic diagram of wear tester was described in Fig. 2. A stainless steel ball was used as the counterpart, and its diameter was 3 mm. Sliding was performed under ambient conditions (temperature: 20 ± 3 ◦ C, humidity: 50 ± 10%). Each friction and wear test was carried out for 30 min. The frictional force transferred to a sensor was recorded throughout the tests. The sensor produced an electric output that depended on the frictional force.
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Fig. 2. Schematic diagram of wear tester.
The electric signal amplified by a multi-meter was stored in a computer via a general-purpose interface bus (GPIB). The wear rate K (g N−1 m−1 ) was calculated from the following relationship: M K= Pvt
(1)
where M the mass loss (g), P the load (N), v the sliding speed of the steel ring (m s−1 ), t the duration of friction (min). Three replicate friction and wear tests were carried out so as to minimize data scattering, and the average of the three replicate test results was reported in this article. After the wear tests, the worn surfaces of the hybrids and worn debris were examined by SEM in order to elucidate the friction and wear mechanisms. In order to increase the resolution for the SEM observation, the tested composite specimens were plated with gold coating to render then electric conductivity. 3. Results and discussion 3.1. Friction and wear properties
Fig. 3. Friction coefficient as a function of time of the PI hybrids with different size of silica (load: 2 N): A: PI 0/0, B: PI 5/4, C: PI 10/4, D: PI 20/4, E: PI 10/12.
coefficient of the hybrid is almost equal to that of pure PI. Comparing pure PI and the polyimide hybrids, it can be concluded that the addition of silica is favorable for lowering the friction coefficient, especially, when the size of silica in the hybrids is below 150 nm. At the same time, in Fig. 5 the calculated wear rate K (g N−1 m−1 ) versus SiO2 size is plotted. The mass loss at the end of the tests was used to calculate the wear rate of the PI hybrids through Eq. (1). From Fig. 5 it can be seen that the wear rate of the PI hybrid films decreases firstly and then increases with increasing the size of SiO2 particle in the hybrids. When SiO2 size is 300 nm, the wear rate of the PI hybrid is the lowest (2.67 × 10−8 g N−1 m−1 ), this is ca.20% lower than that of pure PI (3.35 × 10−8 g N−1 m−1 ). The average friction coefficient for this sample is μ ¯ = 0.35. These behaviors can be attributed to the effect of silica particles. Comparing Figs. 4 and 5, it can be seen that the lowest friction coefficient is not corresponding to the least wear rate. The friction and wear mechanisms which silica particles reduce the coefficient of friction and wear rate of pure
Figs. 3 and 4 show the recorded friction coefficients as a function of time and the average values as a function of the size of SiO2 in the hybrids, respectively. The average friction coefficient (μ) ¯ for each test is calculated using Eq. (2) and the variance (σ 2 ) is calculated using Eq. (3). In both the equations t is the total duration of the test in seconds 1 μ ¯ = μ dt (2) T 2 μ dt 1 2 σ = μ dt − (3) T T From Figs. 3 and 4 it is clear that first the friction coefficient decreases obviously, and then increases as the size of SiO2 particle in the hybrids increases. The friction coefficient of the hybrid with 100 nm SiO2 particle is the lowest (0.32), this is ca.20% lower in contrast with that of pure PI (0.41). However, when the size of SiO2 particles is up to 800 nm, the friction
Fig. 4. Average friction coefficient for the samples as a function of the size of silica in the hybrid films (load: 2 N).
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Fig. 5. A plot of the calculated wear rate versus the size of silica in the hybrids (2 N).
PI will be discussed through mechanical analysis of materials and SEM observations of worn surfaces and debris. The friction coefficient and wear rate of pure PI and PI/SiO2 hybrids under various sliding speeds as functions of SiO2 size at a load of 2 N are given in Figs. 6 and 7, respectively. From Fig. 6, it can be seen that the friction coefficient first decreases, and then increases as SiO2 size increases under all sliding speeds, the hybrid with 100 nm SiO2 shows the lowest friction coefficient in all samples. Furthermore, the friction coefficient of pure PI and its hybrids with different size of silica decrease with increasing sliding speed. This behavior can be put down to an increase in the orientation degree of molecular chains of polyimide in frictional interfaces with increasing sliding speed. Fig. 7 shows that the wear rate of pure PI and its hybrids with different size of silica increases with increasing sliding speed. It could be rational to infer that the sliding surface temperature of PI/SiO2 hybrids increased with sliding speed, which resulted in micromelting of the hybrid surface. Such a kind of micro-melting could be speeded at a relatively larger sliding speed, owing to the extended sliding distance.
Fig. 6. The effect of sliding speed on the friction coefficient of PI hybrids.
Fig. 7. Variation of the wear rate of PI hybrids with the size of silica at various sliding speed.
Figs. 8 and 9 show, respectively, the variations of the friction coefficient and wear rate of pure PI and PI/SiO2 hybrids with load at a sliding speed of 0.84 m/s. From Fig. 8, it can be seen that the PI hybrids with SiO2 size below 150 nm give obviously decreased friction coefficients at both low and high load compared to the bare PI, while they register larger friction coefficients at a SiO2 size above 150 nm, which is similar to what was observed in Fig. 4. In addition, a relatively small friction coefficient is registered for the hybrids at a high load. Thus, the PI/SiO2 hybrids are more suitable to the working condition of relatively high load. Fig. 9 shows the wear rate of PI/SiO2 hybrids first decreases and then increases with increasing SiO2 size in contrast with that of pure PI at both low and high load. Moreover, the PI hybrid materials give a better wear-resistance at lower load than at higher load. Simultaneously, as can be seen by the naked eye, it is interesting to notice that the hybrid with 300 nm SiO2 exhibits the best wear-resistance under the both load conditions.
Fig. 8. Variation of the friction coefficient of PI hybrids with the size of silica at various applied load.
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of PI when the size of silica is less than 800 nm. This suggests that there is a reinforcement effect as a result of the existence of SiO2 particles in the PI hybrids. This effect could be due to the small size effect of SiO2 particles in the hybrids. However, on the contrary, when the size of silica exceeds 800 nm, SiO2 destroys the mechanical properties of PI. This is attributed to agglomeration of SiO2 in the PI composites; the small size effect of SiO2 disappears, as can be seen in Fig. 1(e). Comparing tribological properties and mechanical performances of the PI hybrids, it appears that the hybrids with the best mechanical property has the best wear-resistance property, but does not correspond to the best friction behavior. This illuminates the wear behavior of materials is directly related to the mechanical properties of hybrids. Moreover, we further investigate the wear mechanisms of the PI/silica hybrids from debris and worn surfaces. Fig. 9. Variation of the wear rate of PI hybrids with the size of silica at various applied load.
3.2. Mechanical properties In order to study the wear mechanisms of the PI hybrid materials, we first analyzed mechanical properties of the PI hybrids. Fig. 10 shows that the tensile strength and elongation at break of the PI/silica hybrid materials as a function of the size of silica in the hybrids. It can be seen from Fig. 10 that the tensile strength increases with increasing SiO2 size and reaches the highest value (125.1 MPa) at a size of 300 nm, this is ca. 50% higher than that of the original PI (82.4 MPa). When the SiO2 size is above 300 nm, the tensile strength decreases obviously, although which is still higher than that of pure PI until the size of silica in the hybrid films reaches 800 nm.The elongation at break of the hybrids shows the same variation tendency with increasing SiO2 size, when the size of silica in PI matrix is 300 nm, the elongation at break (17.8) of the hybrid is ca.200% higher compared to that of pure PI (6.5). Moreover, the elongation at break of the hybrids with 800 nm SiO2 is a little less than that of pure PI. From the above analysis it may be seen that SiO2 particles in PI matrix can improve both strength and toughness
Fig. 10. Variation of the mechanic properties of the hybrid films with the size of silica.
3.3. SEM examinations Debris is a product of frictional experiments, and therefore its analysis is helpful for comprehending friction and wear mechanisms. In this work, SEM is first used for the analysis of debris. SEM micrographs of debris of pure PI and its hybrid materials are given in Fig. 11. Fig. 11 shows that both debris of pure PI and its hybrid materials consist of flakes. However, debris of the PI composites is considerably smaller than that of pure PI in size. When the size of silica in the hybrid films is less than 450 nm, the wear debris of the PI hybrids decreases with increasing the size of silica, the wear rate of the corresponding materials is also lower than that of pristine PI, as can be seen in Fig. 5. Whereas, the size of silica exceeds 450 nm, the debris size of the hybrid materials increases, so the wear rate of these materials also increases. These behaviors could be explained that the less SiO2 particles in PI matrix has bigger surface area and leads to strengthen interactive forces between PI polymer and SiO2 particles. Accordingly, the wear rate of the hybrids decreases greatly in contrast with that of pure PI. However, a bigger SiO2 particles results in the stress concentration, so this decreases the interactive forces between the PI polymer and SiO2 particles, the wear rate of the hybrids obviously increases. The SEM micrographs of the worn surfaces of pure PI and its hybrid materials under experimental conditions are shown in Fig. 12. It can be seen from Fig. 12(a) that the worn surface of pure PI under dry sliding is characterized by severe plastic deformation and micro cracking, while a large amount of rubbed PI debris is observed on the stainless steel ball counterpart surface during the friction experiment. It indicates the type of wear for pure PI is most adhesive wear. Compared to pure PI, It is interesting to notice the great difference between the morphologies of the wear traces on the pure PI and PI hybrids. The worn surfaces PI hybrids are smoother compared to that of pure PI when the size of silica in the hybrids is less than 450 nm. In particular, for PI hybrid with 300 nm SiO2 particles, the worn surface is quite smooth and no ploughed marks could be observed, this is in good agreement with the comments mentioned above. That is, the optical size of SiO2 for the best wear resistance should be recommended as 300 nm. Thus, is can also be inferred that the morphologies of the wear traces are relevant to the wear rates of
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Fig. 11. SEM morphologies of the wear debris of: (a) PI 0/0 (SiO2 size: 0 nm); (b) PI 5/4, (SiO2 size: 150 nm); (c) PI 10/4, (SiO2 size: 450 nm); (d) PI 20/4 (SiO2 size: 650 nm).
SiO2 filled PI. However, SiO2 size is above 450 nm, the worn surfaces of the hybrids are rougher, almost same as that of pure PI. All the hybrids are characterized by plastic deformation, micro cracking, and spalling under drying conditions. This is reason
that the interfacial interaction between the PI matrix and the SiO2 inorganic phase would be worsened at an extended size of SiO2 particles, which accounted for the poorer wear-resistance of the PI/SiO2 hybrids in this case as well.
Fig. 12. SEM morphologies of worn surface of: (a) PI 0/0 (SiO2 size: 0 nm); (b) PI 5/4 (SiO2 size: 150 nm); (c) PI 10/4 (SiO2 size: 450 nm); (d) PI 20/4 (SiO2 size: 650 nm).
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4. Conclusions (1) PI/SiO2 hybrid films with different size of silica are synthesized through a sol–gel reaction. (2) The strength and toughness of PI/SiO2 hybrids are enhanced, respectively, ca. 50% and 200% compared to that of pure PI when the size of silica is less than 300 nm. (3) The size of silica in the hybrid could be controlled through altering experimental conditions such as the additive content of TEOS and deionized water, and its scale is 100– 800 nm. (4) The friction coefficient and wear rate of PI hybrids with different sizes of silica firstly decrease and then increase with increasing the size of silica in the hybrids. The friction coefficient of the hybrid with 100 nm SiO2 is the lowest and ca.20% lower in contrast with that of pure PI. However, the lowest wear rate is recorded for the hybrid with 300 nm SiO2 , and ca.20% lower than that of neat PI. (5) An appropriate size of silica in PI matrix could effectively reduce adhesive wear of pure PI and prevent the formation of bigger debris. References [1] E.P. Giannelis, Adv. Mater. 8 (1996) 29. [2] E. Reynaud, C. Gauthier, J. Perez, Rev. Metall. 96 (1999) 169.
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