The test involves penetrating a sample material using an indenter, while the penetration depth and load are recorded so that the stiffness and hardness of the ...
Proceedings of International Conference on Nanotechnology for the Forest Products Industry, Marriott Marquis, Atlanta, GA, April 26-28, 2006.
Nanoindentation as a Tool for Understanding Nano-mechanical Properties of Wood Cell Wall and Biocomposites Siqun Wang1), Seung-Hwan Lee1), William T.Y. Tze2), Tim Rials1), George M. Pharr3) 1) Tennessee Forest Products Center, The University of Tennessee, Knoxville, Tennessee 2) Currently Department of Bio-based Products, University of Minnesota, St. Paul, Minnesota 3) Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee and Metals and Ceramic Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee
ABSTRACT Cellulose fibers, cell wall fragments and cellulose microfibrils can be used as reinforced materials for biocomposite manufacture. Understanding nano-mechanical properties of those materials will be very useful to show their full theoretical potential. Nanoindentation testing is a technique that determines the mechanical properties of a material in the sub-micron/nano scale. The test involves penetrating a sample material using an indenter, while the penetration depth and load are recorded so that the stiffness and hardness of the indented location can be subsequently calculated. The objective of this study was to understand the mechanical properties of wood cell wall, regenerated cellulose fiber and cellulose fiberreinforced polypropylene composite using nanoindentation. The effect of microfibril angle on the hardness and stiffness values of wood cell wall and the mechanical properties of interphase between regenerated cellulose fiber and polypropylene were investigated. Time dependent mechanical properties of regenerated cellulose fiber in longitudinal and transverse direction were also investigated.
1. INTRODUCTION Many researchers in composite science are critically considering about the effective use of natural fibers for composite structure and suggesting those as viable alternative fibers with conventional fibers, such as glass, carbon or aramid fibers and talc [1-4]. Natural fibers over conventional fillers has numerous advantages, for examples, low cost, low density, high toughness, acceptable specific strength properties, ease of processing and separation, biodegradability. Furthermore, the utilization of natural resources has important meaning in the environmental view of the fixation of carbon dioxide and the save of the petroleum stock. With a green concept in recent years, natural fiber-reinforced polymer composites (NFRPC) have also attracted attention as a method of recycling natural fibers and plastic waste, and for preparing waterresistant natural fiber based materials without using formaldehyde-based adhesives [5-10]. The greatest growth potential for NFRPCs is in building industries. Products include decking, fencing, industrial flooring, landscape timber, railings, and molding [11]. In particular, the decking market is the largest and fastest growing NFRPC market. The mechanical properties of natural fibers as the reinforcing materials of polymers will be very important to design the final composite. Especially, understanding nano-mechanical properties of those materials will be very useful to show their full theoretical potential. Thus, this study aimed at the investigation of nanoscale mechanical properties of wood cell wall and the interphase in regenerated cellulose fiber-reinforced PP composite by using continuous nanoindentation. Time-dependant mechanical properties of regenerated cellulose fiber were also investigated. Nanoindentation technique is very useful to measure the mechanical properties of wood cell wall, regenerated cellulose fiber, and the interphase of composite with nano or submicron scale. For example, the size of indent is in the same order of magnitude of the wood cell-wall thickness, which is reported to be 4.6-5.5 micrometer and 9.2-13.4 micrometer in thickness, respectively, for earlywood and latewood of loblolly pine. Furthermore, the interphase has a thickness less than 5 μm, so its properties were rarely reported in literature, because experimental means of unequivocally establishing the interphase properties have not been yet fully available. Nanoindentation testing can facilitate the research on nano-mechanical properties of these materials.
2. NANOINDETATION INSTRUMENT AND INDENTATION PROCEDURE Nanoindentation testing is a technique that determines the mechanical properties of a material in the submicron/nano scale. The test involves penetrating a sample material using an indenter, while the penetration depth and load are recorded so that the stiffness and hardness of the indented location can be subsequently calculated. The indenter head can be 100 nm in radius (in the case of Berkovic indenter), and the penetration can be up to one or two micrometers in depth, with the resulting indent having a linear dimension in the order of micrometers. In particular, the continuous nanoindentation technique is one of the significant improvements in nanoindentation testing. The continuous measurement technique offers a tool to probe stiffness as a function of indentation depth in one single experiment. That is, cycles of indentation, each of which consists of incremental loading and partial unloading, are performed until a final desired depth is attained. Each loading-and-partial unloading cycle provides a value of hardness and stiffness; hence as the penetration progresses, various hardness or stiffness values were determined as a function of the indentation depth. Continuous nanoindentation was performed as following six steps: approaching to surface as accurately as possible, loading to peak load, holding the indenter at peak load, unloading 90% of peak load for 50 seconds, holding the indenter after 90% unloading for 100 seconds, and finally, unloading completely. An arrayed line of indents was made from fiber and matrix with different spacing, depending on indentation depth. The load-displacement data from the nanoindentation tests can be used to calculate hardness and elastic modulus. The hardness ( H ) of the samples for an indentation depth (h) can be calculated from the following equation: P (1) H = max A
where Pmax refers to the load measured at a maximum depth of penetration (h) in an indentation cycle, while A refers to the projected area of contact between the indenter and sample at Pmax . The combined modulus of the system, or reduced indentation modulus ( Er ) can be determined from the following expression [12]: dP 1 π (2) Er = dh 2 A where dP is the slope of the line in tangent to the initial unloading curve in the load-displacement plot. dh
The sample modulus ( E s ) can then be calculated as follows:
(
Es = 1 −ν where
νs
and
νi
2 s
)
⎛ 1 1 − ν i2 ⎞ ⎜ ⎟ ⎜E − E ⎟ i ⎝ r ⎠
−1
(3)
are the Poisson’s ratios of the specimen and indenter, respectively, while Ei is the
modulus of the indenter.
3. APPLICATION 3.1. Wood Cell Wall Five different annual rings of a loblolly pine, with microfibril angles (MFA) between 15 and 36 degrees, were examined. The continuous measurement technique employed in this study yielded series of hardness and modulus values as a function of the indentation depth. To express the hardness or moduli of a sample, various approaches (Figure 1) were used – taking the values corresponding to 100 nm (H100 or E100) and 200 nm (H200 or E200) indentation depth, or averaging the values for a depth of more than 200 nm (Have, >200 or Eave, >200). The hardness and modulus values (Hu or Eu) were also determined from the ultimate unloading curve. Furthermore, the hardness ( H o ) and modulus ( E o ) values that are independent of penetration depth were also determined and found highly consistent between adjacent tracheids. The H o and E o
values are 19% and 4%, respectively, higher than the values determined using the conventional, unloading approach (Figure 2). Analyses of E o values confirm previous findings that the cell-wall longitudinal moduli at low MFA are underestimated, while the values at higher MFA are considerably reasonable. The present study also ingeniously relates the cell-wall longitudinal hardness and stiffness to the moduli of elasticity and rupture (MOE and MOR) of wood. The measured MOE (MFA ≥ 28o) and MOR closely correlate with the values predicted from E o and H o . Hence E o (at high MFA) and H o could at least be considered relative quantities for modeling and comparisons aimed at probing cell-wall mechanical changes associated with certain growth and utilization processes.
Longitudinal modulus (GPa)
16 14 12
Eave, >200
10 8 6
E100
4 0
50
E200
100
150
200
250
300
350
Displacement (nm)
Figure 1. The depth-dependent longitudinal stiffness. 0.60 y = 1.191x 2
R = 0.95
Ho (GPa)
0.50
0.40
0.30 0.30
0.35
0.40 Hu (GPa)
0.45
0.50
Figure 2. Relationship between the hardness values determined from the “intrinsic” (Ho) and “unloading” approaches (Hu). 3.2. Lyocell Fiber and Polypropylene (PP) Figure 2 shows a series of hardness (H) and elastic modulus (E) values of cellulose fiber and PP matrix obtained by continuous nanoindentation with 100 nm depth. It was found that these values became constant from approximately 30 nm indentation depth. The intrinsic hardness and elastic modulus values, which is independent on indentation depth, were 0.69 and 18.65 GPa in the cellulose fiber and 0.16 and 4.67 GPa in the PP matrix, respectively.
0.16 0.14
0.8
15
0.6 10
0.4
5
0.2 0
Hardness (GPa) aaa
Hardness (GPa) aaaa
20
Elastic modulus (GPa)aaa
(A) Cellulose fiber 1
0.12
20
40
60
80
6
0.1 0.08
4
0.06 0.04
2
0.02 0
0 0
8 (B) PP matrix Elastic modulus (GPa)aaa
25
1.2
0 0
100
20
40
60
80
100
Displacement, h (nm)
Displacement, h (nm)
Figure 2. The dependency of hardness (●) and elastic modulus (■) on indentation depth. 3.3. Time-dependent Mechanical Properties Time-dependent mechanical properties of two regenerated cellulose fibers with different tensile modulus were investigated. The hardness and elastic modulus of all samples decreased as the applied load was increased (Figure 3). Figure 4 shows that decreasing the loading rate also decreased the hardness and elastic modulus. Creep exponents were also evaluated from the linear relationship between contact stress and the contact strain rate (Figure 5). The value of creep exponent for the regenerated cellulose fiber with higher tensile modulus was 0.54, which is almost the same with all samples.
● ■ ○ □
0.3
14
Lyo950 (longitudinal) Lyo500 (longitudinal) Lyo950 (transverse) Lyo500 (transverse)
Elastic modulus (GPa) aaa
Hardness (GPa) aaa
0.4
0.2
0.1
● ■ ○ □
12
Lyo950 (longitudinal) Lyo500 (longitudinal) Lyo950 (transverse) Lyo500 (transverse)
10 8 6 4
0.0 0
500
1000
1500
Load (μN)
2000
2500
0
500
1000
1500
2000
2500
Load (μN)
Figure 3. Effect of load on hardness and elastic modulus for 200 sec of holding time at each load. Sample Code: Lyo950 (longitudinal) with 15 GPa of tensile modulus, Lyo500 (longitudinal) with 8 GPa of tensile modulus.
0.5
16
8 0.2 4
-1
0.3
Log (έ) (sec )
12
Hardness (GPa) aaa
Elastic modulus (GPa)aaa
-3.375
0.4
-3.415 -0.65
0 0
20
40
60
80
100
-3.395
-3.405
0.1
0
-3.385
120
-0.6
-0.55
-0.5
Log (H) (GPa)
Loading rate (μN/s)
Figure 4. Effect of loading rate on elastic modulus and hardness under the load of 2000 μN for 200 sec of holding time. Sample code: Lyo500 (longitudinal).
Figure 5. Plots of Logσ (H) and Logε. Sample code: Lyo950 (longitudinal). Load: 0.5 μN.
3.4. Interphace Properties The nanoscale mechanical properties of the interphase in a cellulose fiber-reinforced polypropylene composite were investigated. The interphase was modified by maleic anhydride PP and γ-APS sizing on fiber surface. To evaluate the interphase, a line of indents was produced from the fiber to the matrix. A gradient profile in the hardness and modulus across the intephase region was observed as shown in Figure 6. The distinct properties of the transition zone were revealed by 4 indents with 30 nm depth and 260 nm spacing. 0.8
20 ● Test 1 ■ Test 2
● Test 1 ■ Test 2
■
Elastic modulus (GPa)aaaa
Hardness (GPa) aaa
0.6 ■ ●
0.4
0.2
(A)
16
■
■ ●
12
8
4
(B)
0
0
0
0.8
1.6
2.4
3.2
Spacing of indentation (μm)
4
4.8
0
0.8
1.6
2.4
3.2
4
4.8
Spacing of indentation (μm)
Figure 6. Variation of hardness (A) and elastic modulus (B) across the interphase region between the fiber and PP matrix obtained by nanoindentation with 30 nm depth and 260 nm spacing.
4. SUMMARY The objective of this study was to understand the mechanical properties of wood cell wall, regenerated cellulose fiber and cellulose fiber-reinforced polypropylene composite using nanoindentation. The effect of
microfibril angle on the hardness and stiffness values of wood cell wall and the mechanical properties of interphase between regenerated cellulose fiber and polypropylene were investigated. Intrinsic hardness and modulus values that are independent of penetration depth were 19% and 4%, respectively, higher than the values determined using the conventional, unloading approach. Analyses of elastic modulus values confirm previous findings that the cell-wall longitudinal moduli at low MFA are underestimated, while the values at high MFA are considerably reasonable. The present study also ingeniously relates the cell-wall longitudinal hardness and stiffness to the moduli of elasticity and rupture (MOE and MOR) of wood. The intrinsic hardness and elastic modulus values of regenerated cellulose fiber and polypropylene were 0.69 and 18.65 GPa, and 0.16 and 4.67 GPa, respectively. In the creep test, the hardness and elastic modulus of regenerated cellulose fiber decreased as the applied load was increased. Decreasing the loading rate also decreased the hardness and elastic modulus. Creep exponent for regenerated cellulose fiber with higher tensile modulus was 0.54, which shows almost same value with all samples. The distinct properties of the transition zone in regenerated cellulose fiber-reinforced PP composite were revealed by 4 indents with 30 nm depth and 260 nm spacing.
ACKNOWLEDGEMENTS The project was supported by the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, grant number # 2005-02645 and the USDA Wood Utilization Research Grant. Instrumentation for the nanoindentation work was provided through the SHaRE Program at the Oak Ridge National Laboratory, which was sponsored by the Division of Materials Science and Engineering, U.S. Department of Energy, under contract DE-AC05-000R22725 with UT-Battelle, LLC.
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