- Email: [email protected]
Mechanical behaviors of Al2O3 nanoparticles reinforced polyetheretherketone
Materials Science and Engineering A 492 (2008) 383–391
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical behaviors of Al2 O3 nanoparticles reinforced polyetheretherketone Pan Guoliang, Guo Qiang ∗ , Tian Aiguo, He Zhiqiang School of Materials Science and Engineering, Shanghai University, Shanghai 201800, PR China
a r t i c l e
i n f o
Article history: Received 23 November 2007 Received in revised form 25 March 2008 Accepted 7 May 2008 Keywords: PEEK Alumina Mechanical behavior Fracture Composite
a b s t r a c t In order to improve the mechanical properties of Polyetheretherketone (PEEK), nanometer Al2 O3 particles were used as fillers. The effect of size and content of Al2 O3 particles, various coupling agents and dispersing methods on the mechanical properties were all studied. The reinforced PEEK filled with 15 nm and 5 wt.% Al2 O3 particles possessed higher tensile, flexural and impact performance than that filled with 90 nm and 10 wt.% Al2 O3 particles possessed. Especially, the impact strength of the PEEK filled with titanate treated nanometer Al2 O3 particles was about eight times that of neat PEEK. Furthermore, the higher tensile, flexural, compression and impact strength of PEEK filled with nanometer Al2 O3 particles than that of PEEK filled with nanometer SiO2 particles were presented. In addition, through the analysis of fractographs for tensile specimens by scanning electrical microscope (SEM), the relativity between mechanical characteristics and fracture mechanism were discussed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction PEEK is a kind of high performance engineering plastic with outstanding performance: high mechanical properties, high temperature resistance and self-lubrication etc. However, higher mechanical properties are required under some special severe conditions such as used as gears, piston rings and slide bearings [1]. Inorganic nanometer particles of which diameter is less than 100 nm possess the properties of great specific surface area, high surface energy and large number of atom surface defects in comparison with normal size inorganic particles. The filling of inorganic nanometer particles into PEEK matrix has been done in many literatures to improve the strength, rigidity and tenacity of PEEK [2–6]. However it is difficult to keep the scale and dispersion homogenization of inorganic nanometer particles in PEEK due to the easy reunion of nanometer particle, the insolubility of PEEK and its high melting process temperature of above 400 ◦ C [7]. It has appeared in the literatures that PEEK matrix was filled with nanometer particles: Al2 O3 , SiO2 , SiC, Si3 N4 , AlN and ZrO2 etc. those all emphasized on the improvement in friction and wear properties [8–17], but less report emphasized on the effect of the diameter and mass fraction of filled nanometer Al2 O3 particles on mechanical properties of PEEK. In addition, the effect of different coupling agents and various
∗ Correspondence author. Tel.: +86 21 69982791; fax: +86 21 69982840. E-mail addresses: [email protected], pangl [email protected] (G. Qiang). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.05.026
dispersing methods on mechanical behaviors are all investigated in this paper.
2. Experimental 2.1. Preparation of polymer specimens PEEK powder of 250 m in diameter is supplied by Jilin Univ. and that of 50 m in diameter is from Victrex Co. Ltd. The Al2 O3 particles of 15 nm in diameter are prepared by Jiangsu Univ. and that of 90 nm in diameter are purchased from Bona Technology Co. Ltd. The SiO2 particles of 12 nm in diameter are supplied by Degussa-huls Pacific Co. Ltd. Titanate, Stearic acid, Sodium stearate and Silane which selected as coupling agents are all purchased from SINOPHARM Group Corp. Inorganic nanometer fillers were firstly mixed with coupling agents in absolute ethanol, and then blended with PEEK powder, finally prepared by heat compression moulding. Four kinds of dispersing methods were used to blend the mixture with PEEK powder. The first one is dry powder direct mechanical mixing method (DMM), which is to dry the mixed solution of fillers by heating at 110 ◦ C for 3 h, and mixed it with dry PEEK powder by mechanical method. The second one is liquid-solid mechanical dispersing method (LSMD), which is to blend the mixed solution with PEEK powder by mechanical dispersion directly, then to filtrate and dry it at 110 ◦ C for 8 h. The third one is ultrasonic dispersing method (UD), which is to disperse PEEK powder in mixed solution with
95 5 – – UD Titanate 90 – – – 10 LSMD Silane 95 – – – 5 LSMD Silane
95 5 – – LSMD Titanate
U141 L434 L424
L141
95 5 – – BMD Titanate
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
B141
384
95 – 5 – – LSMD Sodium stearate 95 – 5 – – LSMD Stearic acid
90 – 10 – – LSMD Titanate
L123 L122
L131
Fig. 1. DSC curves of specimens D000, L120 and L121.
ultrasound for 1.5 h. Absolute ethanol was selected as liquid medium, and the mixture was stirred by a glass stick continuously during the ultrasonic dispersion. The last one is ball milling dispersing method (BMD), which is to blend the mixed solution with PEEK powder in a ball mill for 1.5 h. Some Si3 N4 balls of 2–6 mm in diameter was set in the machine to milling the mixture and absolute ethanol was also used as liquid medium. During the molding process, the material was heated to 340 ◦ C under the pressure of 20 MPa and kept for 30 min, then heated to 365 ◦ C, followed by cooling to 100 ◦ C in the mould while kept the pressure, finally opened the mould and cooled to room temperature. The diameter and content of inorganic nanometer particles, coupling agents and various dispersing methods are all listed in Table 1.
95 – 5 – – LSMD Titanate 95 – 5 – – LSMD – 95 – – 5 – DMM Titanate
L121 L120 D221
A PerkinElmer differential scanning calorimeter (DSC) was used to obtain the thermograms of neat PEEK and inorganic nanometer particles reinforced PEEK. The temperature used was 50–450 ◦ C with a helium atmosphere and the samples were heated at 10 ◦ C min−1 . The glass transition temperature (Tg ), melting temperature (Tm ) and enthalpy of melting (Hm ) of samples were all calculated. Crystallinity () is an important factor which would be positive with mechanical properties of PEEK and could be calculated as Eq. (1).
D121
95 – 5 – – DMM Titanate
D000
100 – – – – – –
=
Hf Hf∗
× 100%
(1)
where the melting enthalpy (Hf ) can be obtained from the area that is enclosed by the peak of DSC melting curves and the baseline. And Hf∗ is the melting enthalpy while the crystallinity is 100%.
PEEK (˚ 250 m, wt.%) PEEK (˚ 50 m, wt.%) ˚ 15 nm Al2 O3 ˚ 90 nm Al2 O3 ˚ 12 nm SiO2 Dispersing method Surface treating agent
2.3. Atomic force microscope analysis
Prescription code
Table 1 Composition of inorganic nanometer particles/PEEK composites and dispersing methods
2.2. Differential scanning calorimeter analysis
Dispersion of nanometer particles in PEEK matrix was examined by atomic force microscopy (AFM) using a Nanoscope E atomic force microscope (Digital instrument, USA) at room temperature. Specimens of D121, L121 and L131 were chosen to be inspected, which was about 10 mm × 10 mm × 3 mm in size. Transect of specimen was wheted by metallographic abrasive paper and polished by pure water before examination.
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
Fig. 2. AFM micrograph of dispersion of nanometer Al2 O3 particles in the PEEK composites. (a) D121; (b) L121; (c) L131.
385
386
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
surfaces were cleaned with pure alcohol to eliminate impurities and coated with a thin evaporated layer of gold to improve conductivity before examination. 3. Results and discussion 3.1. Thermal behaviors of PEEK filled with Al2 O3 nanoparticles Thermal and crystallization behaviors of composites were studied by DSC experiments. The results showed that the glass transition temperature (Tg ) of neat PEEK is 118 ◦ C however that of L120 and L121 both filled with 5 wt.% and 15 nm Al2 O3 particles are all about 138 ◦ C. The melting temperature (Tm ) of neat PEEK is 343 ◦ C, however that of L120 and L121 are 340 ◦ C and 341 ◦ C, respectively. As is shown in Fig. 1, the Hf of specimens D000, L120 and L121 are 42.42, 24.37 and 28.18 J/g, respectively, which demonstrates that the crystallinity has decreased for the filling of inorganic nanometer particles. It is possible that the filling of nanometer particles would result in the improvement of rigidity of PEEK chain segment, and obstruct the ordered arrangement of PEEK chain segment. 3.2. Dispersion of nanometer Al2 O3 particles in PEEK
Fig. 3. Effect of diameter of nanometer Al2 O3 particles on the mechanical properties. (a) tensile, compressive and flexural strength; (b) impact strength and ball indentation hardness.
2.4. Mechanical properties examination Tensile and compressive tests were carried out on a universal testing machine (CSS-441000) under ambient condition at a nominal strain rate of 1.5 mm min−1 and 1 mm min−1 , respectively. The dimension of tensile specimens meets the requirement of National Standard Testing Methods GB/T1041-92. Impact strength of the samples was measured with XCJ-4 Impact Tester at room temperature according to GB/T16420-1996. Ball indentation hardness of the samples was measured with PHBI-625 Plastic Ball Indentation Hardness Tester according to GB2298-82. Flexural tests were carried out on a universal material testing machine (INSTRON 1195, INSTRON Corporation, U.K.) according to GB/T 16419-1996, and the specimens were tested with a crosshead speed of 1.0 mm/min. The flexural strength was calculated as Eq. (2). f = 1.5L0 · P · (B · H 2 )
−1
As is shown in Fig. 2(a), which is the AFM micrograph of PEEK filled with 5 wt.% and 15 nm Al2 O3 particles and dispersed by dry powder direct mechanical mixing method. The white spots in figure are nanometer Al2 O3 particles which almost keep uniform dispersion without any visible aggregating, and the maximal scale of it is about 90 nm. Fig. 2(b) shows the micrograph of specimen L121 which is of the same components with D121 but dispersed by LSMD. The maximal scale of Al2 O3 particles is about 40 nm which is smaller than that of D121. Composition and dispersing method of specimen L131 is the same as L121 except that the content of Al2 O3 particles is 10%. As is shown in Fig. 1(c), the maximal scale of Al2 O3 particles in it approaches 100 nm, which indicates that the nanometer structure composites could be achieved by these preparation methods. According to Fig. 2(b) and (c), as the dispersing homogenization of specimen is concerned, it can be found that filling of 5 wt.% Al2 O3 is superior to that of 10 wt.% Al2 O3 and liquid-solid mechanical dispersing method is better than dry powder direct mechanical mixing method.
(2)
where f is flexural strength, L0 is the span between two acting points on specimen, P is the force loaded on specimen according to a given flexibility, B is the width of specimen and H is the thickness of specimen. 2.5. Fractograph analysis A JXA-840A scanning electron microscope (SEM) was used to evaluate the fractograph of tensile testing specimens. The fractured
Fig. 4. Effect of diameter and mass fraction of Al2 O3 particles on tensile and flexural modulus.
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
387
Fig. 5. SEM micrographs with different magnification of tensile fracture surface for neat PEEK and composites filled with various diameters Al2 O3 particles. (a), (b) and (c) D000; (d), (e) and (f) D121; (g) and (h) D221.
3.3. Effect of filler nanoparticle size on mechanical properties and fracture mechanism As is shown in Fig. 3(a) and (b), specimens filled with 15 nm Al2 O3 particles and dispersed by DMM possess maximal tensile and
impact strength, which increased by 5% and 5.6 times than neat PEEK, respectively. However, compressive and flexural strength would increase as the diameter of Al2 O3 particles rises. Compressive and flexural strength of specimen filled with 90 nm Al2 O3 increased by 21% and 17% than that of neat PEEK, respectively. Fig. 4
Fig. 6. Effect of content of various nanometer particles on the mechanical properties. (a) tensile, compressive and flexural strength; (b) impact strength and ball indentation hardness.
388
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
Fig. 7. SEM micrographs with different magnification of tensile fracture surface for PEEK composites filled with various contents Al2 O3 or SiO2 . (a), (b) and (c) L121; (d), (e) and (f) L131; (g) L424.
shows the variation of tensile and flexural modulus with diameter of Al2 O3 particles. It can be seen that the flexural modulus monotonically increases with the increase in the diameter of Al2 O3 particles, while the variation of tensile modulus with diameter has no similar trend.
Nanometer particles possess large specific surface area, high surface activity and better interactivity with the polymer chain segment in comparison with normal size particles, so the filling of it could improve the toughness, rigidity and strength of composites [18]. Meanwhile the rigid inorganic particles in polymer
Fig. 8. Comparison of mechanical properties of composites filled with various coupling agents treated Al2 O3 particles: (a) tensile and flexural strength; (b) impact strength and ball indentation hardness.
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
389
Fig. 9. Variation of mechanical properties of PEEK composites with different dispersing methods. (a) tensile, compressive and flexural strength; (b) impact strength and ball indentation hardness.
would lead to the concentration of stress, then easily resulting in more microcracks and more absorption to impact energy. In addition, inorganic particles could interrupt and delay the spread of microcrack or stop its transformation to the fracture crack. With the increase of diameter, the specific surface area of inorganic particles would decrease, then lead to the weakening of interaction between inorganic particles and polymer, finally would result in
the decrease of tensile and impact strength. However, compressive and flexural strength would increase with the improvement of material rigidity. Representative SEM micrographs of the tensile fracture surface of D000, D121 and D221 at various magnifications are all shown in Fig. 5. Dimples are distinctly visible in the micrograph of D000 (see Fig. 5(a)). The high magnification micrograph of one of the
Fig. 10. SEM micrographs with different magnification of tensile fracture surface for PEEK composites prepared by different dispersing methods. (a), (b) and (c) L141; (d), (e) and (f) U141; (g), (h) and (i) B141.
390
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
dimples in Fig. 5(a) is shown in Fig. 5(b). The bottom of the dimple appears smooth but some distinctly visible radial striae can be found on the edge (see Fig. 5(b) and (c)). The fractograph of D121 appears rough multilayer structure with distinct edges like mica (see Fig. 5(d) and (e)). The multilayer structure could increase the area on which the tensile force acts, which could account for the raise of tensile strength. And one of the layers is composed of abundant of smooth grains in high magnification micrograph as shown in Fig. 5(f). The micrograph of D221 also appears multilayer structure analogous to that of D121, but the number of layers is fewer (see Fig. 5(g) and (h)), which indicates that tensile strength would not increase as the diameter of Al2 O3 particles rises. 3.4. Effect of filler content on mechanical properties and fracture mechanism Tensile, impact, compressive, flexural strength and ball indentation hardness as the function of content of inorganic nanometer particles are all shown in Fig. 6(a) and (b). All mechanical properties of specimens L121 and L131 which dispersed by LSMD are superior to that of D000, except that tensile strength of L131 is lower than that of D000. Furthermore, tensile, compressive and impact strength of L121 are optimal of all and the impact strength of L121 is even eight times that of D000. But the higher flexural strength and ball indentation hardness of L131 are also presented compared with L121. In addition, all the mechanical properties of nanometer SiO2 reinforced PEEK are superior to that of D000 except for the tensile strength of L424 and L434, but the improvement of it is worse than that of nanometer Al2 O3 reinforced PEEK. Analogous with specimens filled with nanometer Al2 O3 particles, tensile, compressive and impact strength of L424 are superior to that of L434, except that other two mechanical properties of L434 are better than that of L424, even the ball indentation hardness of specimen L434 is the highest of all. As is shown in above Fig. 4, the flexural modulus also monotonically increases with the increase in mass fraction of filled Al2 O3 particles. But for tensile modulus, when the content adds to 10% tensile modulus decreases. It can be found that the variation of modulus is almost in correspondence with that of strength in this test. Representative SEM micrographs of the tensile fracture surface of L121, L131 and L424 at various magnifications are all shown in Fig. 7, respectively. The micrograph of L121 also appears multilayer structure but the drop in level is greater than that of D121, which would account for the higher strength in tensile test (see Fig. 7(a)). The edge of one of the layer shown in Fig. 7(a) is composed of smooth grains, which are less than 100 nm in diameter (see Fig. 7(b) and (c). However, there is a big smooth area in the middle of the micrograph of L131 as shown in Fig. 7(d), which indicates more brittle fracture characteristic than other specimens. Clear plastic deformation in the edge of the smooth area could be found at higher magnification as shown in Fig. 7(e) and (f). And lots of holes could be found in the micrograph of L424 as shown in Fig. 7(g), which indicates that the compatibility of PEEK matrix and SiO2 is not good and account for its lowest tensile strength of all specimens. 3.5. Effect of coupling agents for nanoparticle treatment on mechanical properties As is shown in Fig. 8(a) and (b), tensile, the flexural and impact strength of specimens which filled with coupling agents treated nanometer Al2 O3 particles are higher than that of neat PEEK, which implies the well interaction between the nanometer Al2 O3 particles and polymer segment chain. Furthermore, the impact strength of specimens filled with coupling agents treated Al2 O3 is higher than that of L120 filled with untreated Al2 O3 . It is possible that the gen-
eration of ductile interface layer between the surface of particles and coupling agents improves the absorption to impact energy. But there is no analogous trend in the others mechanical properties, which is possible due to the weakening of strength and rigidity of composites for the generation of ductile interface. 3.6. Effect of dispersing methods of nanoparticles in PEEK on mechanical properties Four kind of dispersing methods: DMM, LSMD, UD and BMD were used to disperse fillers in PEEK matrix. Fig. 9(a) and (b) give the variation of mechanical properties with these different dispersing methods. It is seen that the compressive, flexural and impact strength of L121 have improved by 14.8%, 10.4% and 15.7%, respectively, compared with that of D121. But the tensile strength and ball indentation hardness of L121 are slightly greater than that of D121. As far as methods of LSMD, UD and BMD are concerned, the variation of mechanical properties with various dispersing methods is slightly. Among the three kinds of specimens, the tensile and compressive strength of B141 are superior, while the flexural strength and ball indentation hardness of U141 are best. Fig. 10 shows the SEM micrographs with different magnification of tensile fracture surface for PEEK composites by different dispersing methods. From Fig. 10(a), it can be seen that there are abundant of layers on the fractograph of L141, and Fig. 10(b) also displays the obviously tough fracture characteristic. But Fig. 10(c) shows that there are large numbers of grains about 100 nm in diameter, which would be Al2 O3 conglomeration or fragments of PEEK resin. According to Fig. 10(d), the fractograph also displays abundant of layers. But it is seen that there are some great conglomeration with diameter above 2 m from Fig. 10(e) and (f), which reveal that method of ultrasonic cannot smash the larger grains of Al2 O3 . Fig. 10(g) and (h) show that the surface of fracture is smoother than above two and there are little layers, but there are more homogeneous dispersed grains with more little diameter than others, which prove that ball milling dispersing method can better comminute the conglomeration and disperse it. 4. Conclusion (1) The filling of Al2 O3 nanometer particles results in a decrease of crystallinity compared with neat PEEK. This is possibly due to the increase of rigidity of PEEK chain for the filling, which could encumber the rearrange of PEEK chain in space. In addition, the glass transition temperature of nanometer Al2 O3 particles reinforced PEEK has increased about 20 ◦ C, however there is a slight reduction in melting point compared with neat PEEK. (2) AFM micrographs of specimen D121, L121 and L131 could confirm that nanometer Al2 O3 particles are dispersed in PEEK with nanometer size. And dispersing homogenization of 5 wt.% inorganic particles in specimen is superior to that of 10 wt.% particles. In addition, liquid-solid mechanical dispersing method is better than dry powder direct mechanical mixing method. (3) Tensile strength, ball indentation hardness and impact strength of PEEK filled with 15 nm Al2 O3 particles are better than those of PEEK filled with 90 nm Al2 O3 particles, however, the latter exhibits better performance in flexural and compressive strength but has lowest value of ball indentation hardness of all. (4) PEEK filled with 5 wt.% Al2 O3 represents better performance than that filled with 10 wt.% Al2 O3 in tensile, compressive and impact strength, but the latter exhibits better performance in flexural strength and ball indentation hardness. In addition,
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
all mechanical properties of PEEK filled with nanometer SiO2 particles are worse than that of PEEK filled with Al2 O3 particles except for the ball indentation hardness. Generally, the variation of modulus is almost in correspondence with that of strength. (5) The treatment of coupling agent results in an increase in toughness, but decrease in hardness. And impact strength of PEEK filled with titanate treated nanometer Al2 O3 particles is about eight times that of neat PEEK or 2 times that of PEEK filled with untreated Al2 O3 . (6) As mechanical properties are concerned, liquid–solid mechanical dispersing method is superior to dry powder direct mechanical mixing method. But the difference among liquid–solid mechanical dispersing method, ultrasonic dispersing method and ball milling dispersing method are slightly. It should be noted that the ball milling dispersing method could better comminute the conglomeration and disperse inorganic particles in PEEK resin homogeneously.
391
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
P.J. Rae, E.N. Brown, E.B. Orler, Polymer 48 (2007) 598–615. M.C. Kuo, J.C. Huang, Mater. Chem. Phys. 99 (2006) 258–268. Y.H. Lai, M.C. Kuo, J.C. Huang, et al., Mater. Sci. Eng. A 458 (2007) 158–169. M.C. Kuo, C.M. Tsai, J.C. Huang, et al., Mater. Chem. Phys. 90 (2005) 185–195. Q. Guo, A.G. Tian, Eng. Plast. Appl. 29 (2001) 19–21. M.H. Jen, Y.C. Tseng, C.H. Wu, Compos. Sci. Technol. 65 (2005) 775–779. X.D. Peng, H.Y. Ma, Q.F. Zeng, et al., Mocaxue Xuebao/Tribol. (China) 24 (2004) 240–243. Q.J. Xue, Q.H. Wang, Wear 213 (1997) 54–58. P. Werner, V. Altstadt, R. Jaskulka, et al., Wear 257 (2004) 1006–1014. H.B. Qiao, Q. Guo, A.G. Tian, et al., Tribol. Int. 40 (2007) 105–110. X.D. Peng, Q.F. Zeng, H.Y. Ma, et al., Run Hua Yu Mi Feng/Lubric. Eng. (China) 4 (2005) 56–58. X.D. Peng, H.Y. Ma, Y. Lei, Shiyou Daxue Xuebao/J. Univ. Petrol. (China) 28 (2004) 68–70. R.K. Goyal, A.N. Tiwari, U.P. Mulik, Appl. Sci. Manuf. 38 (2007) 512–524. Q.H. Wang, Q.J. Xue, W.C. Shen, Tribol. Int. 30 (1997) 193–197. Q.H. Wang, Q.J. Xue, H.W. Liu, et al., Wear 198 (1996) 216–219. Q.H. Wang, Q.J. Xue, W.M. Liu, et al., Wear 243 (2000) 140–146. Q.H. Wang, Q.J. Xue, W.M. Liu, et al., J. Appl. Polym. Sci. 79 (2001) 1394–1400. Q.H. Wang, W. Shen, J. Xu, et al., Wear 209 (1997) 316–321.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical behaviors of Al2 O3 nanoparticles reinforced polyetheretherketone Pan Guoliang, Guo Qiang ∗ , Tian Aiguo, He Zhiqiang School of Materials Science and Engineering, Shanghai University, Shanghai 201800, PR China
a r t i c l e
i n f o
Article history: Received 23 November 2007 Received in revised form 25 March 2008 Accepted 7 May 2008 Keywords: PEEK Alumina Mechanical behavior Fracture Composite
a b s t r a c t In order to improve the mechanical properties of Polyetheretherketone (PEEK), nanometer Al2 O3 particles were used as fillers. The effect of size and content of Al2 O3 particles, various coupling agents and dispersing methods on the mechanical properties were all studied. The reinforced PEEK filled with 15 nm and 5 wt.% Al2 O3 particles possessed higher tensile, flexural and impact performance than that filled with 90 nm and 10 wt.% Al2 O3 particles possessed. Especially, the impact strength of the PEEK filled with titanate treated nanometer Al2 O3 particles was about eight times that of neat PEEK. Furthermore, the higher tensile, flexural, compression and impact strength of PEEK filled with nanometer Al2 O3 particles than that of PEEK filled with nanometer SiO2 particles were presented. In addition, through the analysis of fractographs for tensile specimens by scanning electrical microscope (SEM), the relativity between mechanical characteristics and fracture mechanism were discussed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction PEEK is a kind of high performance engineering plastic with outstanding performance: high mechanical properties, high temperature resistance and self-lubrication etc. However, higher mechanical properties are required under some special severe conditions such as used as gears, piston rings and slide bearings [1]. Inorganic nanometer particles of which diameter is less than 100 nm possess the properties of great specific surface area, high surface energy and large number of atom surface defects in comparison with normal size inorganic particles. The filling of inorganic nanometer particles into PEEK matrix has been done in many literatures to improve the strength, rigidity and tenacity of PEEK [2–6]. However it is difficult to keep the scale and dispersion homogenization of inorganic nanometer particles in PEEK due to the easy reunion of nanometer particle, the insolubility of PEEK and its high melting process temperature of above 400 ◦ C [7]. It has appeared in the literatures that PEEK matrix was filled with nanometer particles: Al2 O3 , SiO2 , SiC, Si3 N4 , AlN and ZrO2 etc. those all emphasized on the improvement in friction and wear properties [8–17], but less report emphasized on the effect of the diameter and mass fraction of filled nanometer Al2 O3 particles on mechanical properties of PEEK. In addition, the effect of different coupling agents and various
∗ Correspondence author. Tel.: +86 21 69982791; fax: +86 21 69982840. E-mail addresses: [email protected], pangl [email protected] (G. Qiang). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.05.026
dispersing methods on mechanical behaviors are all investigated in this paper.
2. Experimental 2.1. Preparation of polymer specimens PEEK powder of 250 m in diameter is supplied by Jilin Univ. and that of 50 m in diameter is from Victrex Co. Ltd. The Al2 O3 particles of 15 nm in diameter are prepared by Jiangsu Univ. and that of 90 nm in diameter are purchased from Bona Technology Co. Ltd. The SiO2 particles of 12 nm in diameter are supplied by Degussa-huls Pacific Co. Ltd. Titanate, Stearic acid, Sodium stearate and Silane which selected as coupling agents are all purchased from SINOPHARM Group Corp. Inorganic nanometer fillers were firstly mixed with coupling agents in absolute ethanol, and then blended with PEEK powder, finally prepared by heat compression moulding. Four kinds of dispersing methods were used to blend the mixture with PEEK powder. The first one is dry powder direct mechanical mixing method (DMM), which is to dry the mixed solution of fillers by heating at 110 ◦ C for 3 h, and mixed it with dry PEEK powder by mechanical method. The second one is liquid-solid mechanical dispersing method (LSMD), which is to blend the mixed solution with PEEK powder by mechanical dispersion directly, then to filtrate and dry it at 110 ◦ C for 8 h. The third one is ultrasonic dispersing method (UD), which is to disperse PEEK powder in mixed solution with
95 5 – – UD Titanate 90 – – – 10 LSMD Silane 95 – – – 5 LSMD Silane
95 5 – – LSMD Titanate
U141 L434 L424
L141
95 5 – – BMD Titanate
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
B141
384
95 – 5 – – LSMD Sodium stearate 95 – 5 – – LSMD Stearic acid
90 – 10 – – LSMD Titanate
L123 L122
L131
Fig. 1. DSC curves of specimens D000, L120 and L121.
ultrasound for 1.5 h. Absolute ethanol was selected as liquid medium, and the mixture was stirred by a glass stick continuously during the ultrasonic dispersion. The last one is ball milling dispersing method (BMD), which is to blend the mixed solution with PEEK powder in a ball mill for 1.5 h. Some Si3 N4 balls of 2–6 mm in diameter was set in the machine to milling the mixture and absolute ethanol was also used as liquid medium. During the molding process, the material was heated to 340 ◦ C under the pressure of 20 MPa and kept for 30 min, then heated to 365 ◦ C, followed by cooling to 100 ◦ C in the mould while kept the pressure, finally opened the mould and cooled to room temperature. The diameter and content of inorganic nanometer particles, coupling agents and various dispersing methods are all listed in Table 1.
95 – 5 – – LSMD Titanate 95 – 5 – – LSMD – 95 – – 5 – DMM Titanate
L121 L120 D221
A PerkinElmer differential scanning calorimeter (DSC) was used to obtain the thermograms of neat PEEK and inorganic nanometer particles reinforced PEEK. The temperature used was 50–450 ◦ C with a helium atmosphere and the samples were heated at 10 ◦ C min−1 . The glass transition temperature (Tg ), melting temperature (Tm ) and enthalpy of melting (Hm ) of samples were all calculated. Crystallinity () is an important factor which would be positive with mechanical properties of PEEK and could be calculated as Eq. (1).
D121
95 – 5 – – DMM Titanate
D000
100 – – – – – –
=
Hf Hf∗
× 100%
(1)
where the melting enthalpy (Hf ) can be obtained from the area that is enclosed by the peak of DSC melting curves and the baseline. And Hf∗ is the melting enthalpy while the crystallinity is 100%.
PEEK (˚ 250 m, wt.%) PEEK (˚ 50 m, wt.%) ˚ 15 nm Al2 O3 ˚ 90 nm Al2 O3 ˚ 12 nm SiO2 Dispersing method Surface treating agent
2.3. Atomic force microscope analysis
Prescription code
Table 1 Composition of inorganic nanometer particles/PEEK composites and dispersing methods
2.2. Differential scanning calorimeter analysis
Dispersion of nanometer particles in PEEK matrix was examined by atomic force microscopy (AFM) using a Nanoscope E atomic force microscope (Digital instrument, USA) at room temperature. Specimens of D121, L121 and L131 were chosen to be inspected, which was about 10 mm × 10 mm × 3 mm in size. Transect of specimen was wheted by metallographic abrasive paper and polished by pure water before examination.
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
Fig. 2. AFM micrograph of dispersion of nanometer Al2 O3 particles in the PEEK composites. (a) D121; (b) L121; (c) L131.
385
386
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
surfaces were cleaned with pure alcohol to eliminate impurities and coated with a thin evaporated layer of gold to improve conductivity before examination. 3. Results and discussion 3.1. Thermal behaviors of PEEK filled with Al2 O3 nanoparticles Thermal and crystallization behaviors of composites were studied by DSC experiments. The results showed that the glass transition temperature (Tg ) of neat PEEK is 118 ◦ C however that of L120 and L121 both filled with 5 wt.% and 15 nm Al2 O3 particles are all about 138 ◦ C. The melting temperature (Tm ) of neat PEEK is 343 ◦ C, however that of L120 and L121 are 340 ◦ C and 341 ◦ C, respectively. As is shown in Fig. 1, the Hf of specimens D000, L120 and L121 are 42.42, 24.37 and 28.18 J/g, respectively, which demonstrates that the crystallinity has decreased for the filling of inorganic nanometer particles. It is possible that the filling of nanometer particles would result in the improvement of rigidity of PEEK chain segment, and obstruct the ordered arrangement of PEEK chain segment. 3.2. Dispersion of nanometer Al2 O3 particles in PEEK
Fig. 3. Effect of diameter of nanometer Al2 O3 particles on the mechanical properties. (a) tensile, compressive and flexural strength; (b) impact strength and ball indentation hardness.
2.4. Mechanical properties examination Tensile and compressive tests were carried out on a universal testing machine (CSS-441000) under ambient condition at a nominal strain rate of 1.5 mm min−1 and 1 mm min−1 , respectively. The dimension of tensile specimens meets the requirement of National Standard Testing Methods GB/T1041-92. Impact strength of the samples was measured with XCJ-4 Impact Tester at room temperature according to GB/T16420-1996. Ball indentation hardness of the samples was measured with PHBI-625 Plastic Ball Indentation Hardness Tester according to GB2298-82. Flexural tests were carried out on a universal material testing machine (INSTRON 1195, INSTRON Corporation, U.K.) according to GB/T 16419-1996, and the specimens were tested with a crosshead speed of 1.0 mm/min. The flexural strength was calculated as Eq. (2). f = 1.5L0 · P · (B · H 2 )
−1
As is shown in Fig. 2(a), which is the AFM micrograph of PEEK filled with 5 wt.% and 15 nm Al2 O3 particles and dispersed by dry powder direct mechanical mixing method. The white spots in figure are nanometer Al2 O3 particles which almost keep uniform dispersion without any visible aggregating, and the maximal scale of it is about 90 nm. Fig. 2(b) shows the micrograph of specimen L121 which is of the same components with D121 but dispersed by LSMD. The maximal scale of Al2 O3 particles is about 40 nm which is smaller than that of D121. Composition and dispersing method of specimen L131 is the same as L121 except that the content of Al2 O3 particles is 10%. As is shown in Fig. 1(c), the maximal scale of Al2 O3 particles in it approaches 100 nm, which indicates that the nanometer structure composites could be achieved by these preparation methods. According to Fig. 2(b) and (c), as the dispersing homogenization of specimen is concerned, it can be found that filling of 5 wt.% Al2 O3 is superior to that of 10 wt.% Al2 O3 and liquid-solid mechanical dispersing method is better than dry powder direct mechanical mixing method.
(2)
where f is flexural strength, L0 is the span between two acting points on specimen, P is the force loaded on specimen according to a given flexibility, B is the width of specimen and H is the thickness of specimen. 2.5. Fractograph analysis A JXA-840A scanning electron microscope (SEM) was used to evaluate the fractograph of tensile testing specimens. The fractured
Fig. 4. Effect of diameter and mass fraction of Al2 O3 particles on tensile and flexural modulus.
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
387
Fig. 5. SEM micrographs with different magnification of tensile fracture surface for neat PEEK and composites filled with various diameters Al2 O3 particles. (a), (b) and (c) D000; (d), (e) and (f) D121; (g) and (h) D221.
3.3. Effect of filler nanoparticle size on mechanical properties and fracture mechanism As is shown in Fig. 3(a) and (b), specimens filled with 15 nm Al2 O3 particles and dispersed by DMM possess maximal tensile and
impact strength, which increased by 5% and 5.6 times than neat PEEK, respectively. However, compressive and flexural strength would increase as the diameter of Al2 O3 particles rises. Compressive and flexural strength of specimen filled with 90 nm Al2 O3 increased by 21% and 17% than that of neat PEEK, respectively. Fig. 4
Fig. 6. Effect of content of various nanometer particles on the mechanical properties. (a) tensile, compressive and flexural strength; (b) impact strength and ball indentation hardness.
388
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
Fig. 7. SEM micrographs with different magnification of tensile fracture surface for PEEK composites filled with various contents Al2 O3 or SiO2 . (a), (b) and (c) L121; (d), (e) and (f) L131; (g) L424.
shows the variation of tensile and flexural modulus with diameter of Al2 O3 particles. It can be seen that the flexural modulus monotonically increases with the increase in the diameter of Al2 O3 particles, while the variation of tensile modulus with diameter has no similar trend.
Nanometer particles possess large specific surface area, high surface activity and better interactivity with the polymer chain segment in comparison with normal size particles, so the filling of it could improve the toughness, rigidity and strength of composites [18]. Meanwhile the rigid inorganic particles in polymer
Fig. 8. Comparison of mechanical properties of composites filled with various coupling agents treated Al2 O3 particles: (a) tensile and flexural strength; (b) impact strength and ball indentation hardness.
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
389
Fig. 9. Variation of mechanical properties of PEEK composites with different dispersing methods. (a) tensile, compressive and flexural strength; (b) impact strength and ball indentation hardness.
would lead to the concentration of stress, then easily resulting in more microcracks and more absorption to impact energy. In addition, inorganic particles could interrupt and delay the spread of microcrack or stop its transformation to the fracture crack. With the increase of diameter, the specific surface area of inorganic particles would decrease, then lead to the weakening of interaction between inorganic particles and polymer, finally would result in
the decrease of tensile and impact strength. However, compressive and flexural strength would increase with the improvement of material rigidity. Representative SEM micrographs of the tensile fracture surface of D000, D121 and D221 at various magnifications are all shown in Fig. 5. Dimples are distinctly visible in the micrograph of D000 (see Fig. 5(a)). The high magnification micrograph of one of the
Fig. 10. SEM micrographs with different magnification of tensile fracture surface for PEEK composites prepared by different dispersing methods. (a), (b) and (c) L141; (d), (e) and (f) U141; (g), (h) and (i) B141.
390
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
dimples in Fig. 5(a) is shown in Fig. 5(b). The bottom of the dimple appears smooth but some distinctly visible radial striae can be found on the edge (see Fig. 5(b) and (c)). The fractograph of D121 appears rough multilayer structure with distinct edges like mica (see Fig. 5(d) and (e)). The multilayer structure could increase the area on which the tensile force acts, which could account for the raise of tensile strength. And one of the layers is composed of abundant of smooth grains in high magnification micrograph as shown in Fig. 5(f). The micrograph of D221 also appears multilayer structure analogous to that of D121, but the number of layers is fewer (see Fig. 5(g) and (h)), which indicates that tensile strength would not increase as the diameter of Al2 O3 particles rises. 3.4. Effect of filler content on mechanical properties and fracture mechanism Tensile, impact, compressive, flexural strength and ball indentation hardness as the function of content of inorganic nanometer particles are all shown in Fig. 6(a) and (b). All mechanical properties of specimens L121 and L131 which dispersed by LSMD are superior to that of D000, except that tensile strength of L131 is lower than that of D000. Furthermore, tensile, compressive and impact strength of L121 are optimal of all and the impact strength of L121 is even eight times that of D000. But the higher flexural strength and ball indentation hardness of L131 are also presented compared with L121. In addition, all the mechanical properties of nanometer SiO2 reinforced PEEK are superior to that of D000 except for the tensile strength of L424 and L434, but the improvement of it is worse than that of nanometer Al2 O3 reinforced PEEK. Analogous with specimens filled with nanometer Al2 O3 particles, tensile, compressive and impact strength of L424 are superior to that of L434, except that other two mechanical properties of L434 are better than that of L424, even the ball indentation hardness of specimen L434 is the highest of all. As is shown in above Fig. 4, the flexural modulus also monotonically increases with the increase in mass fraction of filled Al2 O3 particles. But for tensile modulus, when the content adds to 10% tensile modulus decreases. It can be found that the variation of modulus is almost in correspondence with that of strength in this test. Representative SEM micrographs of the tensile fracture surface of L121, L131 and L424 at various magnifications are all shown in Fig. 7, respectively. The micrograph of L121 also appears multilayer structure but the drop in level is greater than that of D121, which would account for the higher strength in tensile test (see Fig. 7(a)). The edge of one of the layer shown in Fig. 7(a) is composed of smooth grains, which are less than 100 nm in diameter (see Fig. 7(b) and (c). However, there is a big smooth area in the middle of the micrograph of L131 as shown in Fig. 7(d), which indicates more brittle fracture characteristic than other specimens. Clear plastic deformation in the edge of the smooth area could be found at higher magnification as shown in Fig. 7(e) and (f). And lots of holes could be found in the micrograph of L424 as shown in Fig. 7(g), which indicates that the compatibility of PEEK matrix and SiO2 is not good and account for its lowest tensile strength of all specimens. 3.5. Effect of coupling agents for nanoparticle treatment on mechanical properties As is shown in Fig. 8(a) and (b), tensile, the flexural and impact strength of specimens which filled with coupling agents treated nanometer Al2 O3 particles are higher than that of neat PEEK, which implies the well interaction between the nanometer Al2 O3 particles and polymer segment chain. Furthermore, the impact strength of specimens filled with coupling agents treated Al2 O3 is higher than that of L120 filled with untreated Al2 O3 . It is possible that the gen-
eration of ductile interface layer between the surface of particles and coupling agents improves the absorption to impact energy. But there is no analogous trend in the others mechanical properties, which is possible due to the weakening of strength and rigidity of composites for the generation of ductile interface. 3.6. Effect of dispersing methods of nanoparticles in PEEK on mechanical properties Four kind of dispersing methods: DMM, LSMD, UD and BMD were used to disperse fillers in PEEK matrix. Fig. 9(a) and (b) give the variation of mechanical properties with these different dispersing methods. It is seen that the compressive, flexural and impact strength of L121 have improved by 14.8%, 10.4% and 15.7%, respectively, compared with that of D121. But the tensile strength and ball indentation hardness of L121 are slightly greater than that of D121. As far as methods of LSMD, UD and BMD are concerned, the variation of mechanical properties with various dispersing methods is slightly. Among the three kinds of specimens, the tensile and compressive strength of B141 are superior, while the flexural strength and ball indentation hardness of U141 are best. Fig. 10 shows the SEM micrographs with different magnification of tensile fracture surface for PEEK composites by different dispersing methods. From Fig. 10(a), it can be seen that there are abundant of layers on the fractograph of L141, and Fig. 10(b) also displays the obviously tough fracture characteristic. But Fig. 10(c) shows that there are large numbers of grains about 100 nm in diameter, which would be Al2 O3 conglomeration or fragments of PEEK resin. According to Fig. 10(d), the fractograph also displays abundant of layers. But it is seen that there are some great conglomeration with diameter above 2 m from Fig. 10(e) and (f), which reveal that method of ultrasonic cannot smash the larger grains of Al2 O3 . Fig. 10(g) and (h) show that the surface of fracture is smoother than above two and there are little layers, but there are more homogeneous dispersed grains with more little diameter than others, which prove that ball milling dispersing method can better comminute the conglomeration and disperse it. 4. Conclusion (1) The filling of Al2 O3 nanometer particles results in a decrease of crystallinity compared with neat PEEK. This is possibly due to the increase of rigidity of PEEK chain for the filling, which could encumber the rearrange of PEEK chain in space. In addition, the glass transition temperature of nanometer Al2 O3 particles reinforced PEEK has increased about 20 ◦ C, however there is a slight reduction in melting point compared with neat PEEK. (2) AFM micrographs of specimen D121, L121 and L131 could confirm that nanometer Al2 O3 particles are dispersed in PEEK with nanometer size. And dispersing homogenization of 5 wt.% inorganic particles in specimen is superior to that of 10 wt.% particles. In addition, liquid-solid mechanical dispersing method is better than dry powder direct mechanical mixing method. (3) Tensile strength, ball indentation hardness and impact strength of PEEK filled with 15 nm Al2 O3 particles are better than those of PEEK filled with 90 nm Al2 O3 particles, however, the latter exhibits better performance in flexural and compressive strength but has lowest value of ball indentation hardness of all. (4) PEEK filled with 5 wt.% Al2 O3 represents better performance than that filled with 10 wt.% Al2 O3 in tensile, compressive and impact strength, but the latter exhibits better performance in flexural strength and ball indentation hardness. In addition,
P. Guoliang et al. / Materials Science and Engineering A 492 (2008) 383–391
all mechanical properties of PEEK filled with nanometer SiO2 particles are worse than that of PEEK filled with Al2 O3 particles except for the ball indentation hardness. Generally, the variation of modulus is almost in correspondence with that of strength. (5) The treatment of coupling agent results in an increase in toughness, but decrease in hardness. And impact strength of PEEK filled with titanate treated nanometer Al2 O3 particles is about eight times that of neat PEEK or 2 times that of PEEK filled with untreated Al2 O3 . (6) As mechanical properties are concerned, liquid–solid mechanical dispersing method is superior to dry powder direct mechanical mixing method. But the difference among liquid–solid mechanical dispersing method, ultrasonic dispersing method and ball milling dispersing method are slightly. It should be noted that the ball milling dispersing method could better comminute the conglomeration and disperse inorganic particles in PEEK resin homogeneously.
391
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
P.J. Rae, E.N. Brown, E.B. Orler, Polymer 48 (2007) 598–615. M.C. Kuo, J.C. Huang, Mater. Chem. Phys. 99 (2006) 258–268. Y.H. Lai, M.C. Kuo, J.C. Huang, et al., Mater. Sci. Eng. A 458 (2007) 158–169. M.C. Kuo, C.M. Tsai, J.C. Huang, et al., Mater. Chem. Phys. 90 (2005) 185–195. Q. Guo, A.G. Tian, Eng. Plast. Appl. 29 (2001) 19–21. M.H. Jen, Y.C. Tseng, C.H. Wu, Compos. Sci. Technol. 65 (2005) 775–779. X.D. Peng, H.Y. Ma, Q.F. Zeng, et al., Mocaxue Xuebao/Tribol. (China) 24 (2004) 240–243. Q.J. Xue, Q.H. Wang, Wear 213 (1997) 54–58. P. Werner, V. Altstadt, R. Jaskulka, et al., Wear 257 (2004) 1006–1014. H.B. Qiao, Q. Guo, A.G. Tian, et al., Tribol. Int. 40 (2007) 105–110. X.D. Peng, Q.F. Zeng, H.Y. Ma, et al., Run Hua Yu Mi Feng/Lubric. Eng. (China) 4 (2005) 56–58. X.D. Peng, H.Y. Ma, Y. Lei, Shiyou Daxue Xuebao/J. Univ. Petrol. (China) 28 (2004) 68–70. R.K. Goyal, A.N. Tiwari, U.P. Mulik, Appl. Sci. Manuf. 38 (2007) 512–524. Q.H. Wang, Q.J. Xue, W.C. Shen, Tribol. Int. 30 (1997) 193–197. Q.H. Wang, Q.J. Xue, H.W. Liu, et al., Wear 198 (1996) 216–219. Q.H. Wang, Q.J. Xue, W.M. Liu, et al., Wear 243 (2000) 140–146. Q.H. Wang, Q.J. Xue, W.M. Liu, et al., J. Appl. Polym. Sci. 79 (2001) 1394–1400. Q.H. Wang, W. Shen, J. Xu, et al., Wear 209 (1997) 316–321.