Transcript Nano-Mechanical Properties of Cellulose Fibers by Nanoindentation

Nano-Mechanical Properties of Cellulose Fibers
by Nanoindentation
2007 International Conference on
Nanotechnology for the Forest Products Industry
Knoxville Convention Center, June 13-15, 2007
Siqun Wang, Seung-Hwan Lee, Cheng Xing,
George M. Pharr
University of Tennessee
Introductions
Siqun Wang
• Wood-plastic composite is a very promising material
to achieve durability without using toxic chemicals.
Deck and fence
Pool and docks
Door panel (Audi A2)
Source from Nexwood website
Introductions
Siqun Wang
To design fiber reinforced polymer composites, we
need to know
• Matrix
• Fiber
• Interphase
Research Goals and Materials
Siqun Wang
Objectives:
• To investigate nano mechanical properties of cellulose fibers by nanoindentation;
• To compare data between nanoindentation and conventional tensile test;
• To understand what could happen if fiber diameter is too small or fiber cell wall is too
narrow.
Materials:
• Two types of Lyocell fiber
• Refined wood fibers under different
refining steam pressure
The image of refining fiber a) juvenile wood at 2 MPa, b) mature wood
at 2 MPa, c) juvenile wood at 18 MPa, and d) mature at 18 MPa.
Research Goals and Materials
Siqun Wang
Materials:
•Refined wood fibers under different refining steam pressure
2 Bars
12 Bars
18 Bars
Experimental Method
Siqun Wang
• Lyocell fiber:
• Tensile test (Tensile modulus)
• Nanoindentation (hardness, elastic modulus,
creep)
• X-ray diffraction (crystallinity)
• AFM
• Refined wood fibers under different refining
steam pressure:
• Nanoindentation (hardness, elastic modulus,
creep)
• AFM
• Nanoindentation influenced by neighboring
materials via finite element analysis
Single fiber nanomechanical testing system
Nanoindentation Instrument and Indentation
Procedure
Lf
Lf
Le
77
63.5
h
Le
30
s
S = 2 × Le/tan30 º
H = Lf + Le
Lf = h (tan77 º )
Le = h(tan65.3 º)
H = h (tan77 º + tan 65.3 º)
S = 2h (tan65.3 º )/(tan30 º)
Geometry of nano-indenter
(Berkovich diamond tip)
Indent
marks
Schematic of the NANO II
Indenter
Nanoindentation Instrument and Indentation
Procedure
Indentation force, P
Hardness (H):
Pmax
P
H 

2
A
24.5hc
Displacement, h
(Oliver and Pharr)
Typical load-displacement curve
dP 1 
Er 
dh 2 A

Es  1 

dP/dh
Unloading
hc
Elastic modulus (Es):
2
s
Loading
 1 1 


Ei
 Er
Er is reduced elastic modulus, which accounts
for the fact that elastic deformation occurs in
both the sample and the indenter.
2
i



1
Vs and Vi (0.07) are the Poisson’s ratios of
the specimen and indenter, respectively.
Ei is the modulus of the indenter (1141 GPa).
Nanoindentation Instrument and Indentation
Procedure
Continuous stiffness measurement : One of the significant
improvements in nanoindentation test
Load, P
With single experiment, cycles of
indentation, each of which consists
of incremental loading and partial
unloading, are performed until a
final desired depth is attained.
Displacement, h
Each loading-and-partial unloading
cycle provides a series of values of
hardness and elastic modulus.
Results – Lyocell fiber
Siqun Wang
Tensile modulus of Lyocell fibers by single fiber tensile test and
sample codes for specimens
Sample Code
Fiber direction
Lyo13 (L)
Longitudinal
Lyo13 (T)
Transverse
Lyo10 (L)
Longitudinal
Lyo10 (T)
Transverse
Tensile modulus
(GPa)
Index of crystallinity
(%)
12.64 (2.94)
67.5
-
-
10.36 (1.88)
65.4
-
-
The value in parenthesis is the standard deviation (SD)
Results – Lyocell fiber
Siqun Wang
Hardness and elastic modulus of Lyocell fibers measured by
continuous nanoindentation
Sample
Mean value from 150
to 300 nm depth (GPa)
Unloading value at final
indentation depth (GPa)
H mean
E mean
Hu
Eu
Lyo13 (L)
0.44
(0.06)
13.19
(0.10)
0.43
(0.05)
13.10
(0.10)
Lyo13 (T)
0.32
(0.02)
6.77
(0.28)
0.33
(0.02)
6.69
(0.25)
Lyo10 (L)
0.33
(0.05)
11.51
(1.27)
0.32
(0.06)
11.42
(1.25)
Lyo10 (T)
0. 30
(0.01)
6.09
(0.14)
0. 30
(0.01)
6.01
(0.13)
Each is the average value from 5 indents. The value
in parenthesis is the standard deviation (SD).
Results – Lyocell fiber
Siqun Wang
Hardness and elastic modulus of Lyocell fibers measured by continuous
nanoindentation
Sample
Mean value from
150 to 300 nm depth
(GPa)
Unloading value at final
indentation depth (GPa)
H mean
E mean
Hu
Eu
Lyo13 (L)
0.44
(0.06)
13.19
(0.10)
0.43
(0.05)
13.10
(0.10)
Lyo13 (T)
0.32
(0.02)
6.77
(0.28)
0.33
(0.02)
6.69
(0.25)
Lyo10 (L)
0.33
(0.05)
11.51
(1.27)
0.32
(0.06)
11.42
(1.25)
Lyo10 (T)
0. 30
(0.01)
6.09
(0.14)
0. 30
(0.01)
6.01
(0.13)
Each is the average value from 5 indents. The
value in parenthesis is the standard deviation
(SD).
Sample
Code
Fiber direction
Lyo13 (L)
Longitudinal
Lyo13 (T)
Transverse
Lyo10 (L)
Longitudinal
Lyo10 (T)
Transverse
Tensile
modulus
(GPa)
12.64
(2.94)
Index of
crystallinity
(%)
67.5
10.36
(1.88)
-
65.4
-
Results – Lyocell fiber
Siqun Wang
Creep behaviors of Lyocell fibers
1.2
-3.15
200s
Lyo10
(T)
Lyo8 (T)
0.8
-1
Log (έ) (sec )
Load (mN) aaa
1
Hold segment
0.6
Loading
0.4
-3.25
Lyo13
Lyo15(T)
(T)
-3.35
Lyo10
(L)(L)
Lyo8
0.2
Lyo15 (L)
Lyo13 (L)
0
-3.45
0
100
200
Holding time (s)
Experimental scheme for creep
test by nanoindentation.
300
-0.7
-0.6
-0.5
Log (σ) (GPa)
The plot of indentation strain rate (έ) and contact
stress (hardness, σ) obtained from data
corresponding to the holding segment. Load:
1000 μN, Loading rate: 20 μN/s, Holding time
200 s.
Results – Refined wood fibers
Siqun Wang
Summary of nanoindentation results of fiber cell wall
Property/pressure
Es
GPa
H
GPa
Ci
%
n
Mean
Stdev
CV
Mean
Stdev
CV
Mean
Stdev
CV
Number
2 bars
4 bars
6 bars
8 bars
10 bars
12 bars
14 bars
18 bars
21.35
2.59
12.13
0.50
0.04
8.00
7.58
0.86
11.35
31
18.62
2.97
15.95
0.47
0.062
13.19
8.72
1.56
17.89
27
15.96
2.41
15.10
0.47
0.07
14.89
8.87
1.25
14.09
23
16.83
2.53
15.03
0.45
0.05
11.11
8.63
1.29
14.95
28
15.32
2.51
16.38
0.43
0.067
15.58
8.24
1.09
13.23
30
14.05
2.87
20.43
0.43
0.079
18.37
9.68
1.79
18.49
28
13.09
3.42
26.13
0.39
0.078
20.00
12.30
3.89
29.25
14
12.22
3.29
26.92
0.37
0.095
25.68
13.08
3.91
29.89
13
Note: Stdev: standard deviation; CV: coefficients of variation; Ci: indention creeps; n: the number of indents.
Results – Finite element analysis
Siqun Wang
Manchurian Ash
cell wall
Poplar cell wall
Indents
Adhesive transition
zone from the fiber
to matrix
Matrix
Fiber
Results – Finite element analysis
Siqun Wang
Results – Finite element analysis
Siqun Wang
Perform one simulation
when the location of the
flat punch moves to the
left or the right with every
1um
8 simulations
1um
Rigid Flat punch
radius =1um
8 simulations
Fiber
Epoxy
E=18.65Gpa,
H= 0.69 Gpa
E =4.67 Gpa ,
H =0.16 Gpa
Total 16 simulations
Results – Finite element analysis
Siqun Wang
Boundary Conditions:
• Penetration depth: 50nm applied to the indenter
• Axisymmetry BCs: applied to the center face.
• Roller BC: applied to the bottom face.
Results – Finite element analysis
Siqun Wang
Mesh
Rigid flat cylindrical punch
10
0u
m
Matrix
100um
Symmetry
plane
m
0u
0
1
m
0u
0
1
Fiber
Results – Finite element analysis
Siqun Wang
0.045
FEA
0.040
Sneddon's solution-Epoxy
0.035
Stiffness(mN/nm)
Sneddon's solution-Fiber
0.030
0.025
Epoxy
0.020
Fiber
0.015
0.010
0.005
0.000
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
8
Centerline location of flat cylinderical punch (um)
Variation of the stiffness measured from the FEA with position from the interface. The punch
radius is 1 μm.
Summary
Siqun Wang
• Nanoindentation with continuous stiffness measurement is well
suited to the evaluation nano-mechanical and time-dependent
mechanical properties of cellulose fibers in longitudinal and
transverse direction. There is no significant difference between
modulus values obtained by nanoindentation and single fiber
tensile test.
• There are some advantages using nanoindentation than tensile
test.
• Using existing nanoindentation technique it would be difficult to
calculate the exact mechanical properties without the effect of
neighboring material property in at least 8 times smaller region
than indent size.
Acknowledgements
Siqun Wang
• Haitao Xu, Yan Wu, Matthew Kant, Dayakar
Penumadu
• USDA NRI grant number # 2005-02645
• USDA Wood Utilization Research Grant
• Tennessee Agricultural Experiment Station MS#96
• Oak Ridge National Laboratory