Abstract
There is a current industrial requirement for the development of suitable test-
methodology that is capable of in-situ mechanical and physical characterisation of
the interface and interfacial region in composite materials. The most promising tool
for micro- and nano-scale measurements is the atomic force microscope (AFM)
although, for suitable test-methodology to be realised, further development is
required. The work contained within this thesis documents the development of AFM
procedures for the measurement of elastic moduli variation across the interface and
considers the physical size of the interfacial region.
The first AFM procedure is for the analysis of multiple (AFM) indentations and the
quantification of each indentation (in terms of reduced elastic moduli). Results of
interfacial testing using this procedure highlighted short transition regions of
apparently increased elastic moduli between the glass reinforcement and the
polymeric matrix. It was not possible to identify whether the increase in elastic
moduli was representative of an interphase due to the possibility that the
indentations in this region were restricted by the presence of the glass fibre-
reinforcement. A second, novel procedure was then developed to independently
verify whether or not any indentations were restricted, and identify whether a
measured transition is representative of an interphase. This procedure was based
on the principle that the any indentation performed in close proximity to the glass
fibre-reinforcement would have an uneven distribution of loading between the
surface and the indenter tip. The uneven distribution of loading resulted in torsion of
the indenter tip, which was measured during multiple indentations across the
interface of a glass fibre-reinforced phenolic composite. It was found that the
apparent increase in elastic moduli measured across the interface was directly
related to restriction due to, and contact with, the proximity of the glass fibre-
reinforcement.
Finally, the latest AFM technique with the potential for quantitative measurement, is
reviewed. The work has shown that AFM nanomechanical mapping has the potential
to be a useful supplement to liT for measuring the small-scale elastic modulus of a
polymer surface.
It was found that the technique can provide repeatable
measurements of polymer moduli for a number of different probes provided that
careful calibration procedures are used.
Author(s): Young, Timothy James
Year: 2012
Language: English
Commentary: CC BY-NC-SA V4.0, Open Access
Pages: 192
Table of Contents
1
INTRODUCTION ..................................................................................... 1
1.1
1.2
1.3
2
Background ............................................................................................... 1
Aims of this work ....................................................................................... 2
Outline of thesis ......................................................................................... 3
LITERATURE REViEW ........................................................................... 6
2.1
Introduction ................................................................................................ 6
2.2
The composite interface ............................................................................ 7
2.3
Current test methods and application to composites ................................. 13
2.3.1
Introduction ........................................................................................ 13
2.3.2
Single-fibre pull-out tests: micro-bond and micro-droplet ................... 13
2.3.3
Single-fibre fragmentation .................................................................. 16
2.3.4
Micro-indentation ............................................................................... 17
2.3.5
Discussion .........................................................................................22
2.4
Novel Mechanical test methods: Instrumented Nano-indentation ............. 24
2.5
Novel Mechanical test methods: Atomic Force Microscopy ....................... 27
2.5.1
Introduction ........................................................................................27
2.5.2
General theory ...................................................................................27
2.5.3
AFM tip-scanning modes of operation ...............................................29
2.5.4
Force-displacement AFM .................................................................. .40
2.5.5
Quantification of AFM ....................................................................... .42
2.5.6
Concluding comments ...................................................................... .46
2.6
The application of liT and AFM indentation-based measurements of the
interface/interphase in composites ..................................................................... .4 7
2.6.1
Introduction ........................................................................................47
2.6.2
Nano-scale indentation measurements in composites ...................... .4 7
2.6.3
Concluding comments .......................................................................60
2.7
Outcome of the review and refined aims ................................................... 61
3
MATERIALS AND PHYSICAL CHARACTERISATION OF THE
INTERFACIAL REGION .............................................................................. 63
3.1
Introduction ...............................................................................................63
3.2
Materials and sample preparation .............................................................63
3.2.1
Materials ............................................................................................63
3.2.2
Sample preparation methods ............................................................. 64
3.3
Experimental Methods ..............................................................................67
3.3.1
Introduction ........................................................................................67
3.3.2
Optical microscopy ............................................................................67
SEM ..................................................................................................67
3.3.3
3.3.4
Transmission electron microscopy (TEM) .......................................... 68
3.4
Optical characterisation of samples .......................................................... 69
3.5
Scanning Electron Microscope (SEM) evaluation of fracture surfaces ...... 71
3.5.1
Introduction ........................................................................................71
3.5.2
Polished cross sections .....................................................................72
3.5.3
Fracture surfaces ...............................................................................73
3.6
Transmission Electron Microscope (TEM) of pultruded rods ..................... 81
3.6.1
Introduction ........................................................................................81
3.6.2
TEM imaging of the composite interfacial region ................................ 81
3.7
Discussion ................................................................................................85
3.8
4
Concluding comments .............................................................................. 86
ATOMIC FORCE MICROSCOPY SCANNING TECHNIQUES ............. 87
4.1
Introduction ...............................................................................................87
4.2
Experimental methods ..............................................................................87
4.2.1
Materials ............................................................................................87
4.2.2
Sample preparation ...........................................................................88
4.2.3
Equipment .........................................................................................89
4.2.4
Calibration of the tip-surface force measurement.. ............................. 90
4.2.5
Operation procedure for contact mode AFM ...................................... 92
4.2.6
Operation procedure for force modulation AFM ................................. 92
4.2.7
Operation procedure for Atomic Force -Acoustic Microscopy ............ 93
4.3
Results .....................................................................................................94
4.3.1
Introduction ........................................................................................94
4.3.2
Contact mode (C-AFM) ......................................................................94
4.3.3
Force Modulation AFM (FMM) ........................................................... 97
4.3.4
Atomic force acoustic microscopy (AF-AM) ..................................... 106
4.4
Concluding Comments ........................................................................... 110
5
FORCE·DISPLACEMENT AFM AND NANO·SCALE INDENTATION
TECHNIQUES ........................................................................................... 112
5.1
Introduction ............................................................................................. 112
5.2
Experimental methods ............................................................................ 112
5.2.1
Equipment and procedures for force-displacement AFM ................. 112
5.2.2
Equipment and procedures for liT ................................................... 114
5.3
Results ................................................................................................... 115
5.3.1
Force-Displacement ........................................................................ 115
5.3.2
Instrumented nano-indentation ........................................................ 120
5.4
Discussion ..............................................................................................124
5.5
Concluding comments ............................................................................ 126
6
QUANTIFICATION OF FORCE·DISPLACEMENT AFM FOR THE
ELASTIC MODULUS CHARACTERISATION OF INTERFACES ............. 127
6.1
Introduction ............................................................................................. 127
6.2
Experimental methods ............................................................................ 127
6.2.1
Introduction ...................................................................................... 127
6.2.2
Materials .......................................................................................... 127
6.2.3
Equipment ....................................................................................... 128
6.2.4
Experimental procedure ................................................................... 131
6.2.5
Data analysis procedure .................................................................. 132
6.3
Results ................................................................................................... 138
6.3.1
Introduction ...................................................................................... 138
6.3.2
Results - Glass flake-reinforced polypropylene ............................... 138
6.3.3
Results - glass fibre-reinforced vinylester ........................................ 142
6.3.4
Results - copper intermetallic sample ............................................. 145
6.4
Discussion .............................................................................................. 148
6.5
Concluding comments ............................................................................ 149
7
NOVEL PROCEDURE FOR THE DETECTION AND IDENTIFICATION
OF MEASUREMENT ARTEFACTS IN INDENTATION TECHNIQUES .... 151
7.1
Introduction ............................................................................................. 151
7.2
Materials ................................................................................................. 152
7.3
Finite element analysis ........................................................................... 153
7.4
Experimental methods ............................................................................ 156
7.4.1
Instrumentation ................................................................................ 156
7.4.2
Data analysis ................................................................................... 158
7.4.3
Materials .......................................................................................... 161
7.5
Results ................................................................................................... 162
7.5.1
AFM indentation with torsion detection ............................................ 162
7.5.2
Comparison of AFM indentation with torsion and FEA ..................... 165
7.6
Concluding comments ............................................................................ 168
8
HIGH-SPEED NANOMECHANICAL ELASTIC MODULUS MAPPING
OF POLYMERS AND INTERFACES ........................................................ 169
8.1
Introduction ............................................................................................. 169
8.2
Experimental procedures ........................................................................ 169
8.2.1
Materials .......................................................................................... 169
8.2.2
Quantitative AFM Nanomechanical Mapping ................................... 171
8.3
Results ................................................................................................... 174
8.3.1
Introduction ...................................................................................... 174
8.3.2
Calibration ....................................................................................... 174
8.3.3
Correlation between the cantilever sensitivity and the measured elastic
moduli variation ............................................................................................. 177
8.3.4
Results of quantitative nanomechanical measurement of polymers .178
8.3.5
Application of high-speed nanomechanical elastic modulus mapping to
a polymer-polymer interface .......................................................................... 182
8.4
Concluding comments ............................................................................ 184
9
CONCLUDING REMARKS AND SUGGESTIONS FOR FUTURE WORK
............................................................................................................ 185
9.1
9.2
10
Concluding remarks ................................................................................ 185
Suggestions for future work .................................................................... 187
REFERENCES ................................................................................. 188
