Broadband nanoindentation creep experiments in S2 cell wall laminae and compound corner middle lamellae

Transcription

1 Broadband nanoindentation creep experiments in S2 cell wall laminae and compound corner middle lamellae Joseph Jakes University of Wisconsin-Madison Materials Science Program USFS Forest Products Laboratory Advisor at UW: Don Stone Advisors at FPL: Chuck Frihart, Jim Beecher

2 Motivation Indents in CCML Indents in SCWL Heterophase Interfaces Free Edges Heterophase Interfaces 2 µm 2 µm Transverse section in loblolly pine

3 Outline Nanoindentation Broadband Nanoindentation Creep (BNC) Poly-methylmethacrylate (PMMA) BNC Loblolly pine BNC Unmodified and ethylene glycol modified Interpretation of Loblolly pine BNC Wood ultrastructure Rate processes in deformation Deformation mechanisms

4 Typical Nanoindentation Experiment P max Infinite half-space Load. L (μn) 600 Load, P S Depth, h Depth, h(nm) H = P max A E eff = S A ν s 1 ν d = + Eeff β Es Ed

5 Broadband Nanoindentation Creep (BNC) 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

6 PMMA Rate-dependent Hardness from Direct Observation 0.1 s load to 10mN 0.01 to 50 s hold at 10 mn 0.1 s unload 0.05 s 0.2 s 1 s 10 s 50 s 0.01 s 0.1 s 0.5 s Varying hold times Measure areas to determine decrease in hardness with increasing creep time

7 PMMA Rate-dependent Hardness from Direct Observation 0.5 µm 50 s hold 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 0.5 µm 0.01 s hold Puthoff et al. (2009) J. Mater. Res. 24(3) pp

8 PMMA Rate-dependent Hardness from Direct Observation H(GPa) H = P A log(creep time) (s)

9 PMMA Rate-dependent Hardness from Instantaneous Hardness Based on Depth Calculate instantaneous contact area based on instantaneous load and depth 1 Divide total depth into elastic and plastic components Calculate area from plastic component of depth (h p ) A 1/2 =k ζp h p ; Identify correct power law exponent (ζ p ) based on measurement and theory 2,3,4 Calculate instantaneous hardness from instantaneous area and load Calculate indentation strain rate 1 D.S. Stone and K.B. Yoder (1994) J. Mater. Res. 9, pp A.A. Elmustafa, S. Kose, and D.S. Stone (2007) J. Mater. Res. 22, pp D.S. Stone and A.A. Elmustafa, in Proc. of the Materials Research Society 1049 (Materials Research Society, Pittsburgh, USA, 2007) p AA Puthoff et al. (2009) J. Mater. Res. 24(3) pp

10 PMMA Rate-dependent Hardness from Instantaneous Hardness Based on Depth ζ p =1.15 ζ p =1.00

11 PMMA BNC data log(h) (MPa) ε& H = d ln dt A ε& H dε H /dt (s -1 ) Puthoff et al. (2009) J. Mater. Res. 24(3) pp

12 Loblolly Pine Broadband Nanoindentation Creep (BNC)

13 Experimental Procedure: Nanoindentation Loblolly pine S2 cell wall lamina (SCWL) and compound corner middle lamella (CCML) tested Indentation performed with Hysitron Triboindenter Semi-environmental control Multiload indents with s hold segments for creep at maximum load Structure compliance measured Rate-dependent hardness determined from direct observation and based on depth

14 Experimental Procedure: Structural Compliance Structure compliance arises from specimen-scale flexing and nearby heterophase interfaces (e.g. free edge) Cellular Structure Edge Jakes et al. (2008) J. Mater. Res. 23(4) pp Jakes et al. (2009) J. Mater. Res. 24(3) pp. 1016

15 SCWL Rate-dependent Hardness 50 s hold ζ p = μm H = P A 0.05 s hold

16 CCML Rate-dependent Hardness 50 s hold ζ p = μm H = P A 0.25 s hold

17 SCWL and CCML BNC 3.0 SCWL CCML log(h) (MPa) ε& H = H = d ln dt P A A dε H /dt (s -1 )

18 Ethylene glycol modified Loblolly Pine

19 Experimental Procedure Loblolly pine S2 cell wall lamina (SCWL) and compound corner middle lamella (CCML) tested BNC experiments performed on unmodified specimen Specimen modified with ethylene glycol (EG) Soaked in ethylene glycol for 3 days BNC experiments performed on EG-modified specimen Same specimen surface tested in modified and EGmodified experiments

20 Unmodified Wood CCML SCWL 2 µm 2 µm mn indents E s = 6.8 ± 0.4 GPa H = 350 ± 20 MPa mn indents E s = 20 ± 2 GPa H = 380 ± 20 MPa

21 Comparison Before and After Ethylene Glycol Modification SCWL SCWL 2 µm 2 µm Untreated Ethylene Glycol Modified

22 Comparison Before and After Ethylene Glycol Modification CCML CCML 2 µm 2 µm Untreated Ethylene Glycol Modified

23 Ethylene Glycol Modified Wood CCML SCWL No residual indent! Used area based on contact depth for calculations 2 µm 2 µm mn indents E s = 1.9 ± 0.7 GPa H = 80 ± 40 MPa mn indents E s = 6.9 ± 0.5 GPa H = 80 ± 10 MPa

24 SCWL and CCML BNC 10 3 Unmodified SCWL Unmodified CCML EG-Modified SCWL EG-Modified CCML H (MPa) dε H /dt (s -1 )

25 Interpretation of BNC Results

26 Ultrastructure of SCWL Tangential Lamellation Models Kerr and Goring (1975) 2 µm Boyd (1982)

27 Ultrastructure of SCWL Xylan Lignin Glucomannan Approximate Composition 35% highly oriented semi-crystalline cellulose 20% amorphous cellulose 30% hemicellulose Cellulose microfibrils (~15-25 nm across) Akerholm and Salmén (2003) 15% lignin Free volume also present

28 Ultrastructure of CCML 2 µm 200 nm Approximate Composition 80% lignin 20% hemicellulose/pectin Free volume also present TEM image of middle lamella after sodium chlorite treatment (Hafrén et al., 2000)

29 Ultrastructure of CCML 2 µm 200 nm Approximate Composition 80% lignin 20% hemicellulose/pectin Free volume also present TEM image of middle lamella after sodium chlorite treatment (Hafrén et al., 2000) SCWL and CCML have different ultrastructure and composition

30 Rate Processes in Deformation Motion of elementary defects Activation energy requirement Strain rate often given by e.g. & ε = & ε 0 exp ΔG( σ) k T B Stress σ c σ ΔG( σ ) Arrhenius rate equation Reaction Coordinate [=] volume Different mechanisms give rise to specific signatures in terms of the temperature- and rate- dependence of the flow stress

31 Examples of Plastic Deformation Mechanisms Crystallographic Amorphous Solid Polymer σ b r ξˆ screw Dislocations Shear Transformation Zones σ Strongly dependent on polymer morphology (e.g. amorphous or crystralline), stress state, and temperature (e.g. above or below glass transition temperature)

32 SCWL and CCML BNC H (MPa) Unmodified SCWL Unmodified CCML EG-Modified SCWL EG-Modified CCML dε H /dt (s -1 ) What mechanism is controlling the rate process? What type of signature can we assess?

33 SCWL and CCML BNC H (MPa) Unmodified SCWL Unmodified CCML EG-Modified SCWL EG-Modified CCML One signature to assess the effect of EG is to take the ratio H unmod /H EG dε H /dt (s -1 )

34 SCWL and CCML H unmod /H EG H unmod /H EG SCWL CCML EG has a larger relative effect at low strain rates dε H /dt (s -1 ) The ratio H unmod /H EG is similar for both SCWL and CCML. This suggests the same rate limiting step in the rate process!

35 Summary BNC technique established Experimentally assess ζ p value Account for specimen-scale flexing and edge effects Hardness over 4.5 decades of indentation strain rate measured in unmodified and EG-modified CCML and SCWL Ethylene glycol plasticizes CCML and SCWL EG lowers hardness and elastic modulus BNC data suggests effect of EG on SCWL and CCML is similar despite differences in ultrastructure

36 Thank you! Questions?

37 Experimental Procedure: Specimen Preparation Current literature embed wood specimens in epoxy Diffusion of epoxy components may be altering tracheid wall Developed microtoming technique Create pyramid with disposable microtome blades in sled microtome Cut off apex with diamond knife in rotary ultramicrotome Diamond knife cut C 10 mm A Latewood C A 5 µm B