- Email: [email protected]
Flat end and Berkovich instrumented indentation of N and Si irradiated polyethylene – viscoelastic behavior, hardness and elastic modulus
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 257 (2007) 510–514 www.elsevier.com/locate/nimb
Flat end and Berkovich instrumented indentation of N and Si irradiated polyethylene – viscoelastic behavior, hardness and elastic modulus C.E. Foerster
a,*
, J.H. Stankievicz b, F.C. Serbena a, C.M. Lepienski b, F.C. Zawislak
a
c
Depto. de Fı´sica-UEPG, Av. Carlos Cavalcanti 4748, 84030-900 Ponta Grossa, PR, Brazil b Depto. de Fı´sica-UFPR, CP 19044, 81531-990 Curitiba, PR, Brazil c Instituto de Fı´sica-UFRGS, CP 15051, 91501-970 Porto Alegre, RS, Brazil Available online 4 February 2007
Abstract Viscoelasticity, hardness and elastic modulus of N (30 keV) and Si (1.5 MeV) irradiations on polyethylene were investigated. The ion fluences were between 1 · 1013 and 1 · 1016 ions cm2. A flat end indenter was employed to measure the viscoelastic behavior by holding a constant load. Hardness and elastic modulus was obtained by using a Berkovich indenter. Pristine hardness (20 MPa) reaches about 5 GPa after Si irradiation. Polymer viscosity at constant load was calculated from the derivative of the flat end indenter penetration. Viscosity increases about 10 times for Si irradiation compared to pristine. The effect of ion irradiation is less pronounced for the viscosity parameter than for hardness or elastic modulus. These results are discussed considering that the indenter penetration during constant load is affected by displacement of the entire region under the irradiated layer and not only from the hard irradiated layer as in the case of the hardness. 2007 Elsevier B.V. All rights reserved. PACS: 62.20.Qp; 61.80.X; 61.82.Pv; 62.20.Hg; 81.40.Wx Keywords: Ion irradiation; Mechanical properties; Viscosity; Polyethylene; Instrumented indentation
1. Introduction Irradiation by energetic ions is known to cause strong modifications in polymeric structures. The ion bombardment promotes the formation of unsaturated bonds, cross-linking, cleavage of polymeric chains and emission of atomic and molecular fragments. Chain scission leads to decreasing molecular weight while cross-linking cause an increase. These effects can change the polymer phase, the chemical structure, the crystallinity and also the molecular weight. The modifications in the physical and chemical properties can be systematically studied by controlling parameters such as ion energy, ion fluence and ion species. *
Corresponding author. Tel.: +55 42 3220 3044; fax: +55 42 3220 3042. E-mail address: [email protected] (C.E. Foerster).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.221
The improvements in the polymer properties are related to electronic energy transfer (excitation and ionization) and the degradation in properties (as a result of chain scission – displacement reactions) to nuclear energy transfer [1–3]. Very hard surface layers in polymers have been obtained by high-energy ion-beam processing. This ion process allows obtaining a new class of materials that are lightweight and have the flexibility of polymers, combined with high surface hardness and wear resistance higher than for some usual metallic alloys. Several studies about the ion irradiation effects on mechanical properties of ion irradiated polymer can be found in the literature: (a) Metallic ion implantations (Cr, Ti, Si and Pt) in polyethylene, polycarbonate and polyetherimide [4]; (b) Kapton, Teflon, Tefzel and Mylar irradiated with B, N, C, Si and Fe, singly or simultaneously by dual or triple beams [5]; (c) C60 films He,
C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 510–514
Ar, N and Xe irradiated [6], photoresist films [7] and silsesquioxane films [8]. Instrumented indentation is a convenient method for determining the mechanical properties of solids. Analysis of the unloading load–displacement response, which is assumed to be elastic, allows obtaining hardness and elastic modulus [9]. Polymers present a time-dependent behavior when placed under load. In this case, it is necessary a careful approach to obtain the adequate estimation of mechanical properties from conventional instrumented indentation tests. Application of a step load and subsequent measurement of depth as a function of time can be used to calculate time depended viscoelastic properties of the material. Materials that deform elastically but exhibit time-dependent behavior are called viscoelastic and materials in which time-dependent plastic deformation occurs are viscoplastic [10]. Polymeric structures indented with a flat end indenter will have a viscoelastic behavior at low loads and a viscoplastic behavior if the load is high enough. In present work we investigated the effect of N and Si irradiation at different energies and fluences on the viscoelastic properties, hardness and elastic modulus of polyethylene (PE). The viscoelastic properties were measured through time variation of the flat ended tip penetration at constant loads. 2. Experimental procedure and modeling considerations 2.1. Sample preparation and methods Samples of low density polyethylene PB 681/59 were obtained from OPP Polietilenos do Brasil S.A. Sheets forms with dimensions of 11 · 11 · 0.1 cm were made from pellets using aluminum molds and polyester foils in a Schulz press model PHS 15. The thermo pressing was made at 130 C at cycles of 5 min with no load, 5 min with 20 kN and 5 min with 40 kN. The sheets were cooled at ambient temperature. The irradiated samples were cut from the sheets. The Si 1.5 MeV irradiations (Tandreton – 3 MeV implanter) and N 30 keV (500 keV – HVEE implanter), were made in the range from 1013 to 1016 ions cm2. In order to avoid local heating and damage the ion beam current densities were lower than 50 nA cm2. The ion irradiation parameters were estimated by using the TRIM code ˚ [11]. For N the ion beam parameters were: Se = 1.3 eV/A ˚ with Rp = 0.12 lm. For Si were: and Sn = 1.2 eV/A ˚ e Sn = 0.4 eV/A ˚ with Rp = 2.24 lm. Se = 10 eV/A The irradiated samples were also characterized by Raman spectroscopy. The spectra were obtained by using a He–Ne laser of 30 mW for excitations with k = 632.9 nm. The laser spot was about 2 lm and the samples were moved quickly in a random pattern to avoid local heating and photodamage of sample surface. Hardness (H) and elastic modulus (E) measurements were performed using instrumented supplied by the Nanoindenter XPTM from MTS Inc. following the Oliver and
511
Pharr method [9]. For each sample were performed 16–25 indentations at eight different loads, from 4 to 200 mN, with a pyramidal Berkovich tip (apex angle of 65). The viscoelastic properties were measured by instrumented indentation with a truncated 60 conical indenter of circular diameter equal to 13 lm. The indentations were made with a 3 s step loading process followed by 400 s at a constant applied load. The lowest load was 125 lN. The following steps were made at loads twice the previous loads. The penetration depth was recorded during the constant load holding. The derivative dh/dt was measured in all steps in the last 100 s at constant load. 2.2. Theoretical considerations about viscoelastic and viscoplastic measurements by instrumented penetrations According to Sakai [12] for a loading history P(t 0 ) considering a flat-ended cylindrical indenter with diameter d, the penetration depth h(t) is expressed by Z 4ð1 m2 Þ t dP ðt0 Þ 0 hðtÞ ¼ Dðt t0 Þ ð1Þ dt ; k f pd dt0 0 where D(t t 0 ) is the creep compliance function [13], kf is a constant depending on the indenter geometry and m is the Poisson coefficient. If a step load is applied the above equation is reduced to hðtÞ ¼
4ð1 m2 Þ DðtÞ: k f pd
ð2Þ
At low loads a viscoelastic steady state behavior h(t) = vst can occur (vs is the deformation rate). In this case [12], the penetration depth h(t) is given by hðtÞ ¼
2ð1 tÞP 0 t : g k f pd
ð3Þ
Considering that kf = 4/p for a flat end cylindrical indenter we have g¼
ð1 tÞ P 0 dh 2d dt
ð4Þ
Viscosity can be then calculated from Eq. (4). It was considered that m = 0.4 for polyethylene. 3. Results and discussion In Fig. 1 are shown the Raman spectra of selected Si and N irradiated PE samples. For the pristine sample are indicated the typical Ag, B2g and B3g active rotational Raman peaks in the region from 1000 to 1500 cm1 [14]. Not indexed peaks (with low intensity) are also present in the spectrum and they can be associated to impurities since we are working with a commercial PE. The increase in ion fluences modifies the Raman spectra. The effect of the ion irradiation is summarized in Fig. 1 through the highest ion fluences (1 · 1016 ions cm2). The spectra are very different in comparison to pristine sample.
C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 510–514
G
D 16
+
-2
Intensity (a.u.)
1x10 Si .cm
16
+
-2
1x10 N .cm
B2g Ag B2g
Ag B2g
B3g
Ag
Pristine
600
900
1200
1500
1800
Energy (cm-1) Fig. 1. Raman spectra for pristine and N and Si irradiated samples at highest fluences. In the figure are indicated the Raman actives modes for pristine and the hydrogenated amorphous carbon D and G peaks.
Irradiations with N at fluences lower than 1 · 1015 N+ cm2 (not shown here) do not produce significant modifications in the Raman spectra. This indicates that the original polymer structure remains almost the same. Fluences higher than 3 · 1015 N+ cm2 generate structural modifications, starting an amorphization process. For Si irradiation, amorphization process starts at lower fluences (1 · 1014 Si+ cm2) than for N irradiations. The polymer amorphization is identified by the presence of the D and G peaks in the broad energy branch between 1000 and 1600 cm1. These peaks can be attributed to hydrogenated amorphous carbon structure [15]. Comparing the spectra at the highest fluences, it is verified that N irradiation is less efficient to amorphize the PE structure. At 1 · 1016 Si+ cm2 the PE Raman peaks are suppressed, meanwhile for nitrogen at this fluence pristine Raman active modes are still present. The literature indicates that energy transference by electronic processes is more efficient to destroy polymer structure than by nuclear process (atomic displacementsballistic effects) [1]. The pristine structure is modified by hydrogen and C–H radicals losses and formation of double bonds that results in an increase in cross-link and surface oxidation. Present results also indicate that the electronic component is the major factor to promote structural modifications in PE. Depending on ions species and fluences, the electronic component creates excitations and ionizations in the polymer structure that result in a hydrogenated amorphous carbon structure. The electronic stopping power of Si irradiation is much higher than for N. Then, for the same density of deposited energy (/(Se + Sn)), the electronic component due to Si irradiation is much more significant than for N. The N fluences used in this work were not high enough to produce a complete destruction of the polymer structure since some typical PE Raman peaks remains even after irradiation at the maximum fluence (Fig. 1).
In Fig. 2 the hardness profiles at highest Si and N fluences are compared to pristine value ( 20 MPa) until a depth of 1400 nm. At shallow tip penetrations the hardness reaches 5 GPa after Si irradiation at 1 · 1016 Si+ cm2. This is a typical value for amorphous hydrogenated carbon structures reported in the literature [16,17]. Despite that Si Rp position is much higher than for N, the Si irradiation hardening effect at near surface is much intense than at deeper regions. The hardness increase is due to structure modifications induced by the ion travel (excitations and ionizations processes) and not by the presence of it as a foreigner atom. This corroborates the assumption that the ballistic effect is not significant for structural changes in PE. In Fig. 3 are shown the hardness and the elastic modulus at tip penetration depths of about 100 nm for all irradiated samples. This depth was choosing to compare similar near surface irradiated regions since the Rp are very different for the used ions energies. N irradiated samples have hardness around 0.2 GPa. This is about 10 times the pristine hardness (20 MPa). The elastic modulus does not increase significantly (from 0.3 GPa (pristine) to 0.5 GPa) as hardness. This occurs since the elastic modulus, measured by instrumented indentation, corresponds to the response from the large elastic field around the tip in opposition of the small plastic deformation field that is preponderant for the hardness measurements. For Si irradiations H and E increases more significantly with the deposited energy density than for N. Hardness at 100 nm in depth for Si irradiation can increase until to 2 GPa (100 times higher than pristine value), meanwhile E increases to 8 GPa (25 times). These differences in the relative increase are due to the small range influence of the plastic field compared to the long range of the elastic field. In addition there is an influence of the effect of a thin hard layer on the soft substrate.
10 Pristine 16 + -2 10 N .cm 16
+
10 Si .cm
Hardness (GPa)
512
-2
1
0.1
0.01 0
200
400
600
800
1000
1200
1400
Contact depth (nm) Fig. 2. Hardness profiles for pristine and irradiated samples at highest N and Si fluences.
C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 510–514
10
1
1
* 0.1
0.1
E - Pristine H - Pristine
*
0.01 1E-3
Elastic Modulus
0.01
a
Si
0.1 1 10 o 3 φ(Sn + Se) (eV/A )
N
Si
0.01
Fig. 3. Hardness and elastic modulus values for all irradiated samples, at 100 nm in depth, as a function of the deposited density of energy. It is also indicated the respective pristine H and E values.
Considering the deposited energy density (Fig. 3), it is ˚ 3 there is a tranimportant to observe that around 1.0 eV/A sition between unmodified to modified structure and the mechanical properties. Below this energy the mechanical properties were not significantly changed. This statement was also previously observed in C60, AZ-1350TM and POSS films submitted to ion irradiations [6–8]. However, we note in this work that N does not produce the same effect as Si at the same deposited density energy. We choose a small N energy in order to have a strong difference in electronic stopping powers and understand the effect of this component in the microstructure change. Probably if N irradiation would be made at higher energies, the value of ˚ 3 to produce structural changes could be verified. 1.0 eV/A This rule of thumb for significant changes in mechanical properties is then probably more related to the electronic component and not to the ballistic component of the deposited density energy. Typical displacements versus time curves are shown in Fig. 4(a) for N and in Fig. 4(b) for Si irradiations at fluences of 1 · 1013 and 1 · 1016 ions cm2. The viscosity of the samples is related to the derivative of the displacement at constant load as indicated by Eq. (4). High viscosity corresponds to a small value for displacement derivative during constant load segment. The hardness effect is observed by initial displacement during loading. Small displacements correspond to harder samples. Clearly it is observed in Fig. 4 for the initial displacement and each time when the load is increased to a new higher step value (the load value at each step is indicated in Fig. 4). The viscosity values were calculated from data obtained at each applied load. At loads of 1000 lN and higher viscoplastic effects are observed since the displacement derivative does not varies linearly with load as for the first three steps. The displacement derivative was calculated in the last 100 s of each step.
2000 13
+
-2
1x10 N .cm 16 + -2 1x10 N .cm
1600
Viscoplastic deformation
1200 Viscosity calculation
2000μN
800 1000μΝ
400 125μΝ
0
100
0
250μΝ
500
500μN
1000
1500
2000
2500
Time (s)
b Displacement into Surface (nm)
N
Elastic Modulus (GPa)
Hardness (GPa)
Hardness
Displacement into Surface (nm)
10
513
2000 13
Si 10 16 Si 10
1600
Viscoplastic deformation
1200 Viscosity calculation
800 400
2000μN
125μΝ 250μΝ
0 0
500
500μN
1000
1000μN
1500
2000
2500
Time (s) Fig. 4. Displacements versus time curves at different loads for flat end tip at lowest and highest ion fluences: (a) Nitrogen and (b) silicon irradiation. The steps for viscosity calculation and the viscoplastic regime are indicated.
In Fig. 5 it is shown the calculated viscosity for all loads for the lowest and highest N and Si fluences. Irradiation does not affect the viscosity as hardness and even for elastic modulus. The viscosity in this case is a measure of the surface creep with time. The bulk contributions to creep surpass that from near surface hardening. Consequently, an increase in hardness does not correspond to a similar increase in the measured viscosity. The viscoelastic behavior comes from molecular movement since the molecules are submitted to a long distance elastic field. This is important in the case of irradiated samples submitted to constant loads, where the surface is hard but the viscoelastic behavior will be not so affect by the irradiation. The increase in hardness is more pronounced that the increase in viscosity parameter. The difference is related to the range of elastic and plastic fields that influences the measured properties by instrumented indentation. With this technique the plastic field and elastic or viscoelastic
514
C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 510–514
The PE viscosity, after ion irradiation, is not significantly modified. The elastic field under the indenter is of long range in comparison to the plastic deformation field (hardness). Consequently the response from the unmodified volume is predominant compared to the ion modified near surface region volume. This certify that ion irradiation assure a good contact resistance (high hardness and low wear) and also a good compliance for polymers when it is necessary.
Viscosity ( x10
12
Pa.s)
10
1 Pristine 13 + -2 1x10 Si .cm 13 + -2 1x10 N .cm 16 + -2 1x10 Si .cm 16 + -2 1x10 N .cm
0.1 100
1000
10000
Load on Sample (µN) Fig. 5. Viscosity as a function of the applied loads for samples irradiated at the lowest and highest fluences.
fields have different magnitudes. Then, the different volume field regions under the indenter affect the measured properties. Hardness and tribological effects are restricted to the near surface affected region while the elastic and viscoelastic behavior involves the modified surface layer and the bulk region under the modified layer. The sample has then high surface strength but the elastic and viscoelastic properties remain less affected, which is important to assure a good contact resistance and a very good compliance when it is necessary. 4. Conclusions Our results showed that high energy Si irradiation (1.5 MeV) on PE increases surface hardness from 20 MPa until 2 GPa at high fluence (1 · 1016 Si+ cm2). For low N energy (30 keV) even at the same high fluence the hardness increases only to 200 MPa. The electronic stopping power for Si is much higher than for N. This confirms that the electronic component of the transferred energy is the most important mechanism to have high hardness in ion irradiated PE.
Acknowledgement We would like to acknowledge the Brazilian agencies CNPq and CAPES (Procad) for the financial support and special thanks to Dr. Naira Balzaretti (IF-UFRGS) for Raman data acquisitions. References [1] J.C. Pivin, Thin Solid Films 263 (1995) 185. [2] G.R. Rao, E.H. Lee, J. Mat. Res. 11 (10) (1996) 2661. [3] E.H. Lee, G.R. Rao, L.K. Mansur, Trends Polym. Sci. 4 (7) (1996) 229. [4] G.R. Rao, R.K. Monar, E.H. Lee, J.R. Treglio, Surf. Coat. Technol. 64 (2) (1994) 69. [5] E.H. Lee, M.B. Lewis, P.J. Blau, L.K. Mansur, J. Mat. Res. 6 (3) (1991) 610. [6] C.E. Foerster, F.C. Serbena, C.M. Lepienski, D.L. Baptista, F.C. Zawislak, Nucl. Instr. and Meth. B 148 (1999) 634. [7] C.M. Lepienski, I.T.S. Garcia, C.E. Foerster, F.C. Serbena, F.C. Zawislak, Nucl. Instr. and Meth. B 175 (2001) 668. [8] C.E. Foerster, F.C. Serbena, I.T.S. Garcia, C.M. Lepienski, L.S. Roman, J.R. Galva˜o, F.C. Zawislak, Nucl. Instr. and Meth. B 218 (2004) 375. [9] W. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [10] A.C. Fischer-Cripps, Mater. Sci. Eng. A 385 (2004) 74. [11] J.F. Ziegler, SRIM, the program is downloaded from htttp:// www.research.ibm.com/ionbeam/home. [12] M. Sakai, S. Shimizu, J. Non-Cryst. Solids 282 (2001) 236. [13] I.H. Shames, F.A. Cozzarelli, Elastic and Inelastic Stress Analysis, Prentice-Hall, Englewood Cliffs, NJ, 1992 (Chapter 6). [14] M. Tasumi, T. Shimanouchi, J. Chem. Phys 43 (1965) 1245. [15] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (20) (2000) 14905. [16] D.L. Baptista, C.E. Foerster, C.M. Lepienski, F.C. Zawislak, Nucl. Instr. and Meth. B 218 (2004) 61. [17] E.C. Rangel, N.C. Cruz, C.M. Lepienski, Nucl. Instr. and Meth. B 191 (2002) 704.
Nuclear Instruments and Methods in Physics Research B 257 (2007) 510–514 www.elsevier.com/locate/nimb
Flat end and Berkovich instrumented indentation of N and Si irradiated polyethylene – viscoelastic behavior, hardness and elastic modulus C.E. Foerster
a,*
, J.H. Stankievicz b, F.C. Serbena a, C.M. Lepienski b, F.C. Zawislak
a
c
Depto. de Fı´sica-UEPG, Av. Carlos Cavalcanti 4748, 84030-900 Ponta Grossa, PR, Brazil b Depto. de Fı´sica-UFPR, CP 19044, 81531-990 Curitiba, PR, Brazil c Instituto de Fı´sica-UFRGS, CP 15051, 91501-970 Porto Alegre, RS, Brazil Available online 4 February 2007
Abstract Viscoelasticity, hardness and elastic modulus of N (30 keV) and Si (1.5 MeV) irradiations on polyethylene were investigated. The ion fluences were between 1 · 1013 and 1 · 1016 ions cm2. A flat end indenter was employed to measure the viscoelastic behavior by holding a constant load. Hardness and elastic modulus was obtained by using a Berkovich indenter. Pristine hardness (20 MPa) reaches about 5 GPa after Si irradiation. Polymer viscosity at constant load was calculated from the derivative of the flat end indenter penetration. Viscosity increases about 10 times for Si irradiation compared to pristine. The effect of ion irradiation is less pronounced for the viscosity parameter than for hardness or elastic modulus. These results are discussed considering that the indenter penetration during constant load is affected by displacement of the entire region under the irradiated layer and not only from the hard irradiated layer as in the case of the hardness. 2007 Elsevier B.V. All rights reserved. PACS: 62.20.Qp; 61.80.X; 61.82.Pv; 62.20.Hg; 81.40.Wx Keywords: Ion irradiation; Mechanical properties; Viscosity; Polyethylene; Instrumented indentation
1. Introduction Irradiation by energetic ions is known to cause strong modifications in polymeric structures. The ion bombardment promotes the formation of unsaturated bonds, cross-linking, cleavage of polymeric chains and emission of atomic and molecular fragments. Chain scission leads to decreasing molecular weight while cross-linking cause an increase. These effects can change the polymer phase, the chemical structure, the crystallinity and also the molecular weight. The modifications in the physical and chemical properties can be systematically studied by controlling parameters such as ion energy, ion fluence and ion species. *
Corresponding author. Tel.: +55 42 3220 3044; fax: +55 42 3220 3042. E-mail address: [email protected] (C.E. Foerster).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.221
The improvements in the polymer properties are related to electronic energy transfer (excitation and ionization) and the degradation in properties (as a result of chain scission – displacement reactions) to nuclear energy transfer [1–3]. Very hard surface layers in polymers have been obtained by high-energy ion-beam processing. This ion process allows obtaining a new class of materials that are lightweight and have the flexibility of polymers, combined with high surface hardness and wear resistance higher than for some usual metallic alloys. Several studies about the ion irradiation effects on mechanical properties of ion irradiated polymer can be found in the literature: (a) Metallic ion implantations (Cr, Ti, Si and Pt) in polyethylene, polycarbonate and polyetherimide [4]; (b) Kapton, Teflon, Tefzel and Mylar irradiated with B, N, C, Si and Fe, singly or simultaneously by dual or triple beams [5]; (c) C60 films He,
C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 510–514
Ar, N and Xe irradiated [6], photoresist films [7] and silsesquioxane films [8]. Instrumented indentation is a convenient method for determining the mechanical properties of solids. Analysis of the unloading load–displacement response, which is assumed to be elastic, allows obtaining hardness and elastic modulus [9]. Polymers present a time-dependent behavior when placed under load. In this case, it is necessary a careful approach to obtain the adequate estimation of mechanical properties from conventional instrumented indentation tests. Application of a step load and subsequent measurement of depth as a function of time can be used to calculate time depended viscoelastic properties of the material. Materials that deform elastically but exhibit time-dependent behavior are called viscoelastic and materials in which time-dependent plastic deformation occurs are viscoplastic [10]. Polymeric structures indented with a flat end indenter will have a viscoelastic behavior at low loads and a viscoplastic behavior if the load is high enough. In present work we investigated the effect of N and Si irradiation at different energies and fluences on the viscoelastic properties, hardness and elastic modulus of polyethylene (PE). The viscoelastic properties were measured through time variation of the flat ended tip penetration at constant loads. 2. Experimental procedure and modeling considerations 2.1. Sample preparation and methods Samples of low density polyethylene PB 681/59 were obtained from OPP Polietilenos do Brasil S.A. Sheets forms with dimensions of 11 · 11 · 0.1 cm were made from pellets using aluminum molds and polyester foils in a Schulz press model PHS 15. The thermo pressing was made at 130 C at cycles of 5 min with no load, 5 min with 20 kN and 5 min with 40 kN. The sheets were cooled at ambient temperature. The irradiated samples were cut from the sheets. The Si 1.5 MeV irradiations (Tandreton – 3 MeV implanter) and N 30 keV (500 keV – HVEE implanter), were made in the range from 1013 to 1016 ions cm2. In order to avoid local heating and damage the ion beam current densities were lower than 50 nA cm2. The ion irradiation parameters were estimated by using the TRIM code ˚ [11]. For N the ion beam parameters were: Se = 1.3 eV/A ˚ with Rp = 0.12 lm. For Si were: and Sn = 1.2 eV/A ˚ e Sn = 0.4 eV/A ˚ with Rp = 2.24 lm. Se = 10 eV/A The irradiated samples were also characterized by Raman spectroscopy. The spectra were obtained by using a He–Ne laser of 30 mW for excitations with k = 632.9 nm. The laser spot was about 2 lm and the samples were moved quickly in a random pattern to avoid local heating and photodamage of sample surface. Hardness (H) and elastic modulus (E) measurements were performed using instrumented supplied by the Nanoindenter XPTM from MTS Inc. following the Oliver and
511
Pharr method [9]. For each sample were performed 16–25 indentations at eight different loads, from 4 to 200 mN, with a pyramidal Berkovich tip (apex angle of 65). The viscoelastic properties were measured by instrumented indentation with a truncated 60 conical indenter of circular diameter equal to 13 lm. The indentations were made with a 3 s step loading process followed by 400 s at a constant applied load. The lowest load was 125 lN. The following steps were made at loads twice the previous loads. The penetration depth was recorded during the constant load holding. The derivative dh/dt was measured in all steps in the last 100 s at constant load. 2.2. Theoretical considerations about viscoelastic and viscoplastic measurements by instrumented penetrations According to Sakai [12] for a loading history P(t 0 ) considering a flat-ended cylindrical indenter with diameter d, the penetration depth h(t) is expressed by Z 4ð1 m2 Þ t dP ðt0 Þ 0 hðtÞ ¼ Dðt t0 Þ ð1Þ dt ; k f pd dt0 0 where D(t t 0 ) is the creep compliance function [13], kf is a constant depending on the indenter geometry and m is the Poisson coefficient. If a step load is applied the above equation is reduced to hðtÞ ¼
4ð1 m2 Þ DðtÞ: k f pd
ð2Þ
At low loads a viscoelastic steady state behavior h(t) = vst can occur (vs is the deformation rate). In this case [12], the penetration depth h(t) is given by hðtÞ ¼
2ð1 tÞP 0 t : g k f pd
ð3Þ
Considering that kf = 4/p for a flat end cylindrical indenter we have g¼
ð1 tÞ P 0 dh 2d dt
ð4Þ
Viscosity can be then calculated from Eq. (4). It was considered that m = 0.4 for polyethylene. 3. Results and discussion In Fig. 1 are shown the Raman spectra of selected Si and N irradiated PE samples. For the pristine sample are indicated the typical Ag, B2g and B3g active rotational Raman peaks in the region from 1000 to 1500 cm1 [14]. Not indexed peaks (with low intensity) are also present in the spectrum and they can be associated to impurities since we are working with a commercial PE. The increase in ion fluences modifies the Raman spectra. The effect of the ion irradiation is summarized in Fig. 1 through the highest ion fluences (1 · 1016 ions cm2). The spectra are very different in comparison to pristine sample.
C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 510–514
G
D 16
+
-2
Intensity (a.u.)
1x10 Si .cm
16
+
-2
1x10 N .cm
B2g Ag B2g
Ag B2g
B3g
Ag
Pristine
600
900
1200
1500
1800
Energy (cm-1) Fig. 1. Raman spectra for pristine and N and Si irradiated samples at highest fluences. In the figure are indicated the Raman actives modes for pristine and the hydrogenated amorphous carbon D and G peaks.
Irradiations with N at fluences lower than 1 · 1015 N+ cm2 (not shown here) do not produce significant modifications in the Raman spectra. This indicates that the original polymer structure remains almost the same. Fluences higher than 3 · 1015 N+ cm2 generate structural modifications, starting an amorphization process. For Si irradiation, amorphization process starts at lower fluences (1 · 1014 Si+ cm2) than for N irradiations. The polymer amorphization is identified by the presence of the D and G peaks in the broad energy branch between 1000 and 1600 cm1. These peaks can be attributed to hydrogenated amorphous carbon structure [15]. Comparing the spectra at the highest fluences, it is verified that N irradiation is less efficient to amorphize the PE structure. At 1 · 1016 Si+ cm2 the PE Raman peaks are suppressed, meanwhile for nitrogen at this fluence pristine Raman active modes are still present. The literature indicates that energy transference by electronic processes is more efficient to destroy polymer structure than by nuclear process (atomic displacementsballistic effects) [1]. The pristine structure is modified by hydrogen and C–H radicals losses and formation of double bonds that results in an increase in cross-link and surface oxidation. Present results also indicate that the electronic component is the major factor to promote structural modifications in PE. Depending on ions species and fluences, the electronic component creates excitations and ionizations in the polymer structure that result in a hydrogenated amorphous carbon structure. The electronic stopping power of Si irradiation is much higher than for N. Then, for the same density of deposited energy (/(Se + Sn)), the electronic component due to Si irradiation is much more significant than for N. The N fluences used in this work were not high enough to produce a complete destruction of the polymer structure since some typical PE Raman peaks remains even after irradiation at the maximum fluence (Fig. 1).
In Fig. 2 the hardness profiles at highest Si and N fluences are compared to pristine value ( 20 MPa) until a depth of 1400 nm. At shallow tip penetrations the hardness reaches 5 GPa after Si irradiation at 1 · 1016 Si+ cm2. This is a typical value for amorphous hydrogenated carbon structures reported in the literature [16,17]. Despite that Si Rp position is much higher than for N, the Si irradiation hardening effect at near surface is much intense than at deeper regions. The hardness increase is due to structure modifications induced by the ion travel (excitations and ionizations processes) and not by the presence of it as a foreigner atom. This corroborates the assumption that the ballistic effect is not significant for structural changes in PE. In Fig. 3 are shown the hardness and the elastic modulus at tip penetration depths of about 100 nm for all irradiated samples. This depth was choosing to compare similar near surface irradiated regions since the Rp are very different for the used ions energies. N irradiated samples have hardness around 0.2 GPa. This is about 10 times the pristine hardness (20 MPa). The elastic modulus does not increase significantly (from 0.3 GPa (pristine) to 0.5 GPa) as hardness. This occurs since the elastic modulus, measured by instrumented indentation, corresponds to the response from the large elastic field around the tip in opposition of the small plastic deformation field that is preponderant for the hardness measurements. For Si irradiations H and E increases more significantly with the deposited energy density than for N. Hardness at 100 nm in depth for Si irradiation can increase until to 2 GPa (100 times higher than pristine value), meanwhile E increases to 8 GPa (25 times). These differences in the relative increase are due to the small range influence of the plastic field compared to the long range of the elastic field. In addition there is an influence of the effect of a thin hard layer on the soft substrate.
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Contact depth (nm) Fig. 2. Hardness profiles for pristine and irradiated samples at highest N and Si fluences.
C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 510–514
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Fig. 3. Hardness and elastic modulus values for all irradiated samples, at 100 nm in depth, as a function of the deposited density of energy. It is also indicated the respective pristine H and E values.
Considering the deposited energy density (Fig. 3), it is ˚ 3 there is a tranimportant to observe that around 1.0 eV/A sition between unmodified to modified structure and the mechanical properties. Below this energy the mechanical properties were not significantly changed. This statement was also previously observed in C60, AZ-1350TM and POSS films submitted to ion irradiations [6–8]. However, we note in this work that N does not produce the same effect as Si at the same deposited density energy. We choose a small N energy in order to have a strong difference in electronic stopping powers and understand the effect of this component in the microstructure change. Probably if N irradiation would be made at higher energies, the value of ˚ 3 to produce structural changes could be verified. 1.0 eV/A This rule of thumb for significant changes in mechanical properties is then probably more related to the electronic component and not to the ballistic component of the deposited density energy. Typical displacements versus time curves are shown in Fig. 4(a) for N and in Fig. 4(b) for Si irradiations at fluences of 1 · 1013 and 1 · 1016 ions cm2. The viscosity of the samples is related to the derivative of the displacement at constant load as indicated by Eq. (4). High viscosity corresponds to a small value for displacement derivative during constant load segment. The hardness effect is observed by initial displacement during loading. Small displacements correspond to harder samples. Clearly it is observed in Fig. 4 for the initial displacement and each time when the load is increased to a new higher step value (the load value at each step is indicated in Fig. 4). The viscosity values were calculated from data obtained at each applied load. At loads of 1000 lN and higher viscoplastic effects are observed since the displacement derivative does not varies linearly with load as for the first three steps. The displacement derivative was calculated in the last 100 s of each step.
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Time (s) Fig. 4. Displacements versus time curves at different loads for flat end tip at lowest and highest ion fluences: (a) Nitrogen and (b) silicon irradiation. The steps for viscosity calculation and the viscoplastic regime are indicated.
In Fig. 5 it is shown the calculated viscosity for all loads for the lowest and highest N and Si fluences. Irradiation does not affect the viscosity as hardness and even for elastic modulus. The viscosity in this case is a measure of the surface creep with time. The bulk contributions to creep surpass that from near surface hardening. Consequently, an increase in hardness does not correspond to a similar increase in the measured viscosity. The viscoelastic behavior comes from molecular movement since the molecules are submitted to a long distance elastic field. This is important in the case of irradiated samples submitted to constant loads, where the surface is hard but the viscoelastic behavior will be not so affect by the irradiation. The increase in hardness is more pronounced that the increase in viscosity parameter. The difference is related to the range of elastic and plastic fields that influences the measured properties by instrumented indentation. With this technique the plastic field and elastic or viscoelastic
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The PE viscosity, after ion irradiation, is not significantly modified. The elastic field under the indenter is of long range in comparison to the plastic deformation field (hardness). Consequently the response from the unmodified volume is predominant compared to the ion modified near surface region volume. This certify that ion irradiation assure a good contact resistance (high hardness and low wear) and also a good compliance for polymers when it is necessary.
Viscosity ( x10
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Pa.s)
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Load on Sample (µN) Fig. 5. Viscosity as a function of the applied loads for samples irradiated at the lowest and highest fluences.
fields have different magnitudes. Then, the different volume field regions under the indenter affect the measured properties. Hardness and tribological effects are restricted to the near surface affected region while the elastic and viscoelastic behavior involves the modified surface layer and the bulk region under the modified layer. The sample has then high surface strength but the elastic and viscoelastic properties remain less affected, which is important to assure a good contact resistance and a very good compliance when it is necessary. 4. Conclusions Our results showed that high energy Si irradiation (1.5 MeV) on PE increases surface hardness from 20 MPa until 2 GPa at high fluence (1 · 1016 Si+ cm2). For low N energy (30 keV) even at the same high fluence the hardness increases only to 200 MPa. The electronic stopping power for Si is much higher than for N. This confirms that the electronic component of the transferred energy is the most important mechanism to have high hardness in ion irradiated PE.
Acknowledgement We would like to acknowledge the Brazilian agencies CNPq and CAPES (Procad) for the financial support and special thanks to Dr. Naira Balzaretti (IF-UFRGS) for Raman data acquisitions. References [1] J.C. Pivin, Thin Solid Films 263 (1995) 185. [2] G.R. Rao, E.H. Lee, J. Mat. Res. 11 (10) (1996) 2661. [3] E.H. Lee, G.R. Rao, L.K. Mansur, Trends Polym. Sci. 4 (7) (1996) 229. [4] G.R. Rao, R.K. Monar, E.H. Lee, J.R. Treglio, Surf. Coat. Technol. 64 (2) (1994) 69. [5] E.H. Lee, M.B. Lewis, P.J. Blau, L.K. Mansur, J. Mat. Res. 6 (3) (1991) 610. [6] C.E. Foerster, F.C. Serbena, C.M. Lepienski, D.L. Baptista, F.C. Zawislak, Nucl. Instr. and Meth. B 148 (1999) 634. [7] C.M. Lepienski, I.T.S. Garcia, C.E. Foerster, F.C. Serbena, F.C. Zawislak, Nucl. Instr. and Meth. B 175 (2001) 668. [8] C.E. Foerster, F.C. Serbena, I.T.S. Garcia, C.M. Lepienski, L.S. Roman, J.R. Galva˜o, F.C. Zawislak, Nucl. Instr. and Meth. B 218 (2004) 375. [9] W. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [10] A.C. Fischer-Cripps, Mater. Sci. Eng. A 385 (2004) 74. [11] J.F. Ziegler, SRIM, the program is downloaded from htttp:// www.research.ibm.com/ionbeam/home. [12] M. Sakai, S. Shimizu, J. Non-Cryst. Solids 282 (2001) 236. [13] I.H. Shames, F.A. Cozzarelli, Elastic and Inelastic Stress Analysis, Prentice-Hall, Englewood Cliffs, NJ, 1992 (Chapter 6). [14] M. Tasumi, T. Shimanouchi, J. Chem. Phys 43 (1965) 1245. [15] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (20) (2000) 14905. [16] D.L. Baptista, C.E. Foerster, C.M. Lepienski, F.C. Zawislak, Nucl. Instr. and Meth. B 218 (2004) 61. [17] E.C. Rangel, N.C. Cruz, C.M. Lepienski, Nucl. Instr. and Meth. B 191 (2002) 704.