improved nanoindentation techniques for wood...
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Improved Nanoindentation
Techniques for Wood Research
Joseph JakesUSFS Forest Products Laboratory
University of Wisconsin-Madison
Materials Science Program
Advisor at UW: Don Stone
Advisors at FPL: Chuck Frihart and Jim Beecher
Preview
C
A
C
B
A
5 µm
A: S2 cell wall lamina (SCWL)
B: Compound Corner Middle
Lamella (CCML)
C: Empty lumina
Preview
Indents in CCML Indents in SCWL
2 µm
2 µm
Specimen violates assumptions of standard analyses:
-Structural heterogeneities present
-Specimen-scale flexing
Properties are time- and pressure-dependent
Heterophase
Interfaces
Free EdgesHeterophase
Interfaces
Outline
• Specimen preparation
• Standard nanoindentation analysis
• Method to account for structural heterogeneities and
specimen-scale flexing
– Introduce a structural compliance (Cs) into the standard
analysis
– Cs behaves similar to machine compliance (Cm)
– Cs can be experimentally measured with the same
correlation (SYS correlation) used to calculate Cm
• Methods to measure rate-dependent hardness
• Ethylene glycol modified wood
Specimen Preparation
Preparing Wood Specimens for
Nanoindentation• Current literature embed wood specimens in epoxy
– Diffusion of epoxy components may be altering cell wall
• Developed microtoming technique– Create pyramid with disposable microtome blades in sledge
microtome
– Cut off apex with diamond knife in rotary ultramicrotome
C
A
C
B
A
5 µm
Diamond knife
cut
10 mm
Latewood
Standard
Nanoindentation
Analysis
Ideal Nanoindentation Experiment
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
Depth
Lo
adInfinite half-space
Oliver and Pharr (1992) J. Mater. Res. 7(6) pp. 1564.
Standard Nanoindentation Analysis
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
Depth
Lo
adInfinite half-space
chA
PH max
S
Phhc
maxmax
S
1
Pmax
hc
hc
hmax
Standard Nanoindentation Analysis
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
Depth
Lo
adInfinite half-space
S
1
Pmax
hc
hc
c
effhA
SE
d
d
s
s
eff EEE
22 1111
chA
PH max
Ideal Nanoindentation Experiment
Infinite half-space
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
Depth
Lo
ad
Real Nanoindentation Experiment
Infinite half-space
Cm
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
Depth
Lo
ad
cmtcpc
effhACChAChA
SE
11
0 50 100 150 200 250 300
Depth, h (nm)
0
200
400
600
800
1000
Lo
ad.
L(
N)
Load-depth trace
not corrected for
Cm
Method to account for
specimen-scale flexing
and structural
heterogeneities
Nanoindentation of Wood
Cellular Structure
Silicon Bridge to Validate
Specimen-scale flexing
#1 #2 #3 #4
Top view
Jakes et al. (2008) J. Mater. Res. 23(4) pp. 1113.
Side view
Silicon Bridge to Validate
Specimen-scale flexing
#1 #2 #3 #4
Modulus (Es)
underestimated when
indents placed over
unsupported region
Standard Analysis
0 2 4 6 8 10
Lmax (mN)
0
50
100
150
200
Es
(GP
a)
#1
#2
#3
#4
Jakes et al. (2008) J. Mater. Res. 23(4) pp. 1113.
P
#1 #2 #3 #4
Hypothesized additional
compliance over unsupported
region behaves similar to Cm
(independent of load)
Silicon Bridge to Validate
Specimen-scale flexing
PCPCPC pmt
eff
mtE
HPCPC
eff
ppE
H
A
PACPC
SYS Correlation
Plot Ct√P vs. √PStone et al. (1991) J. Vac. Sci.
Technol. A 9(4) pp. 2543-2547
#1 #2 #3 #4
Hypothesized additional
compliance over unsupported
region behaves similar to Cm
(independent of load)
0.00 0.03 0.06 0.09 0.12
L (N1/2
)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ct
L(
m/N
1/2
)
SYS Correlation
#4
#3
#2
#1
Cm = 21 µm/N > Cm,meas
Cm = 3 µm/N = Cm,meas
Silicon Bridge to Validate
Specimen-scale flexing
eff
mtE
HPCPC
Jakes et al. (2008) J. Mater.
Res. 23(4) pp. 1113.
P
P
eff
smtE
HPCCPC
#1 #2 #3 #4
Hypothesized additional
compliance over unsupported
region behaves similar to Cm
(independent of load)
SYS Correlation
Silicon Bridge to Validate
Specimen-scale flexing
Jakes et al. (2008) J. Mater.
Res. 23(4) pp. 1113.
0.00 0.03 0.06 0.09 0.12
L (N1/2
)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ct
L(
m/N
1/2
)
#4
#3
#2
#1
Cm = 21 µm/N > Cm,meas
Cm = 3 µm/N = Cm,meas
P
P
0 2 4 6 8 10
Lmax (mN)
0
50
100
150
200
Es
(GP
a)
#1 #2 #3 #4
Corrected Analysis
#1
#2
#3
#4
Es = 162 3 GPa
H = 12.5 ± 0.3 GPa
Average after accounting for Cs
smtmeas
effCCCA
E1
meas
max
A
PH
Silicon Bridge to Validate
Specimen-scale flexing
P
Nanoindentation of Wood
Cellular Structure
No method exists to experimentally account for
effect of free edge
Edge
Fused Silica to Validate Edge
Effects
5 µm
Edge of Specimen
0.5 µm
Edge of Specimen
Standard Analysis
Modulus (Es) underestimated
when indents approach edge
0 2 4 6 8 10 12 14
Distance from edge, d ( m)
40
50
60
70
80
Mo
du
lus,
Es
(GP
a)
0
2
4
6
8
10
12
14
Har
dn
ess,
H(G
Pa)
5 µm
Edge of Specimen
0.5 µm
Edge of Specimen
Multiple load indent
Need compliance
as function of load
Fused Silica to Validate Edge
Effects
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
Depth. h ( m)
0
2
4
6
8
10
Lo
ad,
L(m
N)
Depth, h
Lo
ad, P
eff
smtE
HPCCPC
5 µm
Edge of Specimen
0.5 µm
Edge of Specimen
SYS Correlation
0.00 0.03 0.06 0.09 0.12
L (N1/2
)
1.2
1.4
1.6
1.8
2.0
2.2
Ct
L(
m/N
1/2
)
Edge effects contribute
to Cs and is independent
of load
Increasing distance
from edge
Fused Silica to Validate Edge
Effects
eff
smtE
HPCCPC
P
P
5 µm
Edge of Specimen
0.5 µm
Edge of Specimen
eff
smtE
HLCCLC
Increasing distance
from edge
Fused Silica to Validate Edge
Effects
0 2 4 6 8 10 12 14
d ( m)
40
50
60
70
80
Mo
du
lus,
Es
(GP
a)
0
2
4
6
8
10
12
14
Har
dn
ess,
H(G
Pa)
Corrected AnalysisStandard Analysis Neglecting Cs
Standard Analysis Accounting for Cs
Es = 72.1 0.5 GPa
H = 11.1 ± 0.1 GPa
Accounting for Cs
Nanoindentation of Wood
0.0 0.2 0.4 0.6 0.8 1.0
Lmax (mN)
12
14
16
18
20
22
Mo
du
lus,
Es
(GP
a)
0
100
200
300
400
500
600
Har
dn
ess,
H(M
Pa)
Corrected AnalysisStandard Analysis
5 µm
Empty Lumen
P
Cs from Heterophase Interface
(Substrate-Adhesive)
Will the same method work for a
heterophase interface?
Adhesive, Es,adh Substrate, Es,sub
Es,sub > Es,adh
Behave similar to
free edge
Will the same method work for a
heterophase interface?
Adhesive, Es,adh
Constraining effect
from stiffer phase?Es,sub > Es,adh
Substrate, Es,sub
Cs from Heterophase Interface
(Substrate-Adhesive)
Nanoindentation of Polypropylene-
Wood Composite
Standard Analysis2 µm
Crack?
0.0
1.0
2.0
3.0
Es
(GP
a)
Nanoindentation of Polypropylene-
Wood Composite
SYS Correlation2 µm
Positive Cs suggests
this is a crack
0.000 0.010 0.020 0.030
L (N1/2
)
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Ct
L(
m/N
1/2
)
Cs = 4 µm/N
Cs = -21 µm/N
eff
smtE
HPCCPC
P
P
Nanoindentation of Polypropylene-
Wood Composite
Corrected analysis2 µm
0.0
1.0
2.0
3.0
Es
(GP
a)
Filled symbols are the
corrected analysis
Our methods are also capable of accounting for
heterophase interfaces and detecting cracks
External Compliances in Real
Nanoindentation Experiments
• Machine Compliance (Cm)– Property of nanoindenter
• Constant for every indent performed
– Standard methods established to account for Cm
• Structural Compliance (Cs)– Property of specimen and location of indent
• Specimen-scale flexing
• Heterophase interphases
– Need to use methods we developed to measure Cs
• Jakes et al. (2008) J. Mater. Res. 23(4) pp. 1113.
• Jakes et al. (2009) J. Mater. Res. 24(3) pp. 1016.
SYS Correlation:eff
smtE
HPCCPC
Methods to Measure Rate-
dependent Hardness: Direct
Observation and
Instantaneous Hardness
Based on Depth
PMMA Rate-dependence Hardness
0.1 s load to 10mN
50 s hold at 10 mN
0.1 s unload
Need method to determine decrease in
hardness with increasing creep time
50 s10 s1 s
0.5 s
0.2 s
0.1 s
0.05 s
0.01 s
Varying hold times
0.1 s load to 10mN
0.01 to 50 s hold at 10 mN
0.1 s unload
Measure areas to determine decrease in
hardness with increasing creep time
PMMA Rate-dependence Hardness
from Direct Observation
0.5 µm
50 s
hold
0.01 s
hold
0.5 µm
0.01 s hold
0.05 s hold
0.10 s hold
0.20 s hold
0.50 s hold
1.00 s hold
10.0 s hold
50.0 s hold
PMMA Rate-dependence Hardness
from Direct Observation
Puthoff et al. (2009) J. Mater. Res. 24(3) pp. 1279.
A
PH
PMMA Rate-dependence Hardness from
Instantaneous Hardness Based on Depth
Puthoff et al. (2009) J. Mater. Res. 24(3) pp. 1279.
10-4
10-3
10-2
10-1
100
101
102
d H/dt (s-1
)
2.5
2.6
2.7
2.8
2.9
3.0
3.1
log(H
)(M
Pa)
dt
AdH
ln
H
PMMA Rate-dependence Hardness from
Instantaneous Hardness Based on Depth
Puthoff et al. (2009) J. Mater. Res. 24(3) pp. 1279.
Ethylene Glycol
Modified Wood
Experimental Procedure
• Loblolly pine (Pinus taeda)– S2 cell wall lamina (SCWL) and compound corner middle lamella
(CCML) probed
• Untreated specimen tested
• Soaked untreated specimen in ethylene glycol for 3 days to modify it
• Treated specimen tested
• Indentation performed with Hysitron Triboindenter– Multiload indents with 50 s hold segment for creep
– RH controlled at ~35% with glycerol-water bath
• Structure compliance measured
• Rate-dependent hardness determined using mathematical model
Unmodified Wood
2 µm
21 1.2 mN indents
Es = 21 3 GPa
H = 380 ± 20 MPa
6 0.8 mN indents
Es = 6 1 GPa
H = 340 ± 20 MPa
2 µm
CCML SCWL
Comparison Before and After
Ethylene Glycol Modification
Untreated
Ethylene Glycol
Treated
2 µm
2 µm
SCWL SCWL
Comparison Before and After
Ethylene Glycol Modification
2 µm
2 µm
UntreatedEthylene Glycol
Treated
CCML SCWL
Ethylene Glycol Modified Wood
2 µm
18 0.4 mN indents
Es = 6 4 GPa
H = 80 ± 10 MPa
5 0.2 mN indents
Es = 2 1 GPa
H = 80 ± 40 MPa
2 µm
No residual indent! Used area based
on contact depth for calculations
CCML SCWL
10-4
10-3
10-2
10-1
100
101
102
H (s-1
)
102
103
Har
dn
ess
(MP
a)
Unmodified SCWLUnmodified CCMLEG-modified SCWLEG-modified CCML
Rate-dependence of Hardness
Jakes et al. (2008) Mater. Res. Soc. Proc. paper # 1132-Z07-21.
Ethylene Glycol Modified Wood
• Ethylene glycol entered and plasticizes both SCWL and CCML– Residual indents recovered
– SCWL and CCML swelled
– Es and H decreased
• Ethylene glycol affects H of SCWL and CCML differently at high strain rates
• Nanoindentation provides mechanical characterization of micron-size domains in wood– Accounts for structural heterogeneities and specimen-scale flexing
– Measure rate-dependent hardness over 4 decades strain rate
• Nanoindentation one tool to investigate modified wood– Chemical analyses
– Microscopy
– Bulk wood characterization
Conclusions
• Developed methods to account for specimen-scale flexing and structural heterogeneities– Cs
– Independent of load
– Measured with SYS correlation
• Rate-dependent hardness– Measured over 4-5 decades of strain rate
• Direct observation or mathematical model
• Ethylene glycol modified wood– Ethylene glycol entered and plasticized both the SCWL and CCML
Thank you!
Questions?
2 µm
2 µm