Characterisation of Interfaces in Micro- and Nano-composites

199 Pages·2012·38.479 MB·English

by  Young, Timothy James

Characterisation of Interfaces in
Micro- and Nano-composites
by
Timothy James young
Submitted for the degree of Engineering Doctorate in
Micro- and NanoMaterials and Technology
Division of Mechanical, Medical and Aerospace Engineering
University of Surrey
January 2012
© Timothy James Young 2012
Acknowledgements
This thesis would not have been possible without the support of numerous
outstanding individuals from the National Physical Laboratory and the University of
Surrey. It is to these individuals that my gratitude and thanks go out.
Firstly I would like to express my gratitude for the wise words and time given to me
by my supervisors Bill Broughton, Steve Ogin and Paul Smith. Their positive
direction and support has been appreciated and has been essential for this work to
have taken place.
It is difficult to overstate my gratitude to my colleague, Miguel Monclus. Without his
continual encouragement, it is possible that this work would have remained
unfinished.
I would like to recognise the support of Julie Yeomans and Noelle Hartley, who have
successfully managed to co-ordinate the resources and framework for me to
develop as a researcher during the course of this study.
I would also like to acknowledge the input and advice from the wider National
Physical Laboratory. Robert Boyd, Martin Rides and John Nunn deserve a special
mention for their support and inspired advice.
I would like to thank my family, who have always been there for me (and more often
than not, put up with me as well).
Above all, my gratitude goes to my wife Catherine, whose love and patience has
been a constant source of motivation during the past few years.
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.
Published works
The work contained within this thesis contributed towards the following papers:
• T. J. Young, M. Monclus, W. R. Broughton, S. L. Ogin and P. A. Smith.
"Observations on interphase characterisation in polymer composites by nano
scale indentation and the use of AFM cantilever torsion to identify measurement
artefacts. Accepted for publication by Plastics Rubber and Composites, April
2011.
• T. J. Young, M. Monclus, T. L. Burnett, W. R. Broughton, S. L. Ogin and P. A.
Smith. "The use of the peak force quantitative nanomechanical mapping AFM
based method for high-resolution elastic modulus measurement of polymers".
Accepted for publication by Measurement Science and Technology, September
2011.
• M. A. Monclus, T. J. Young, D. Oi Maio "AFM indentation method used for elastic
modulus characterization of interfaces and thin layers". Journal of Materials
Science 45,3190-3197 ,2010
In addition to the aforementioned papers, the work contained within this thesis
contributed towards oral presentations at the following conferences/events:
• The 1t h International Conference on Composite Materials (ICCM-17),
Edinburgh, 2009.
• International Conference on Interfaces and Interphases in Multicomponent
Materials, Sheffield, 2010.
• Institute of Materials, Minerals and Mining, Young Person's Lecture Competition,
South East Regional and West Surrey Materials Society local heat 2010.
Table of Contents
1 INTRODUCTION ..................................................................................... 1
1.1 Background ............................................................................................... 1
1.2 Aims of this work ....................................................................................... 2
1.3 Outline of thesis ......................................................................................... 3
2 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 .........................................................................................2 2
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 ...................................................................................2 7
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 ........................................................................................4 7
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 ...............................................................................................6 3
3.2 Materials and sample preparation ............................................................. 63
3.2.1 Materials ............................................................................................6 3
3.2.2 Sample preparation methods ............................................................. 64
3.3 Experimental Methods .............................................................................. 67
3.3.1 Introduction ........................................................................................6 7
3.3.2 Optical microscopy ............................................................................6 7
3.3.3 SEM ..................................................................................................6 7
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 ........................................................................................7 1
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 ........................................................................................8 1
3.6.2 TEM imaging of the composite interfacial region ................................ 81
3.7 Discussion ................................................................................................8 5
3.8 Concluding comments .............................................................................. 86
4 ATOMIC FORCE MICROSCOPY SCANNING TECHNIQUES ............. 87
4.1 Introduction ...............................................................................................8 7
4.2 Experimental methods ..............................................................................8 7
4.2.1 Materials ............................................................................................8 7
4.2.2 Sample preparation ........................................................................... 88
4.2.3 Equipment .........................................................................................8 9
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 .....................................................................................................9 4
4.3.1 Introduction ........................................................................................9 4
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 Concluding remarks ................................................................................ 185
9.2 Suggestions for future work .................................................................... 187
10 REFERENCES ................................................................................. 188
1 Introduction
1.1 Background
In order that the UK Industry remains competitive, engineering structures and the
materials used must be carefully designed to meet high specification requirements.
Generally favoured due to their light weight and advanced properties, over the past
20-30 years there has been substantial growth in the use of composites as a
replacement for traditional materials. Composite materials are normally formed from
two materials, a bulk continuous matrix and a harder reinforcing phase, with an
interface between. This interface is critical in determining the effectiveness of the
composite in terms of its overall strength; stiffness, fracture toughness, residual
stresses and environmental resistance. Composite materials are now used in
mechanically demanding and aggressive environments and so it is important that
the mechanical properties and the overall durability may be predicted. To ensure
that the predictions are based on accurate and appropriate data, the properties of
the interface need to be understood, and more importantly measured.
The mechanical properties of an interface are often influenced by the addition of
coupling agents, such as aminosilane for a glass fibre-reinforced polymer, which
improve the chemical compatibility between the matrix and filler (improving
adhesion). Various studies, which are reviewed within this thesis, have proven that
the coupling agents can form a distinct and measurable interphase between the
matrix and reinforcement. Largely limited by the inability to measure the interphase
region, classical prediction of the overall composite properties rarely take into
account the presence of any interphase and in the majority of cases, it is assumed
to be of zero thickness (perfect bonding). Over the past 20 years, the interphase has
become an increasingly studied area of research as its responses to stresses and
strains will influence the overall properties of the composite and, critically for
industry, its failure mechanisms.
The difficulty in characterising micro- and nano-scale interfaces is highlighted by the
report "Characterising Micro and Nanoscale Interfaces in Advanced Composites" [1]
which shows that there is no current method of testing for quality assurance that is
quick to apply and independent of the fibre/matrix system.
In order to further the understanding of composites at the micro- and nano
mechanical level, development in suitable test-methodology capable of in-situ
mechanical and physical characterisation of the interphase independent of the
reinforcement type and geometry is required.
1.2 Aims of this work
The scope of this project has originated in response to the industrial requirements to
provide suitable, accurate and reliable test methodologies in order to characterise
the interface region of polymer composites. In particular, the general objectives are
as follows:
• Find and evaluate a suitable test method for the mechanical characterisation
of the interface/interphase region of polymer composites.
• Produce methodology for the selected test method which can be carried out
in industry.
Two main test methods have been evaluated for the mechanical characterisation of
the interface/interphase region, instrumented nano-indentation (lIT) and atomic force
microscopy (AFM). The focus of this work is on the capabilities of the AFM to
measure the interfacial region of the above composites. In its most simple form the
AFM consists of a small tip (with a typical radius < 20 nm) that is attached to the end
of a cantilever and then either pressed onto or dragged over the surface of the
sample. The resultant deflection of the cantilever can then be converted to an
applied force and the tip-surface interaction calculated. Advanced methods of
controlling the AFM have led to non-destructive scanning methods that are capable
of nano-scale profiling of surface features and properties as well as indentation
techniques with < nN resolution.
In order to investigate the novel nanomechanical test techniques above, results of
interfacial testing are presented for a range of polymeric composites that may have
an interphase. Three types of polymeric composites have been evaluated; glass
flake-reinforced polypropylene with various types and concentrations of sizing; two
glass fibre-reinforced vinylester samples with reported strong and weak interfacial
bonding and; a glass fibre-reinforced phenolic for which a 1 J.lm to 5 J.lm interphase
has been reported in other work [2]. From these samples, conclusions as to the
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capabilities of the novel nanomechanical test techniques and related methodologies
are presented.
1.3 Outline of thesis
This present report consists of ten chapters. The structure of the following nine
chapters is outlined below.
Chapter 2: Literature Review
This review aims firstly to discuss the nature of the composite interface and why its
properties are so critical in the performance and durability of a composite material.
An additional aim of this review is to evaluate the current test methodology and the
relevance of any measured properties to the interface/interphase. The final aim is to
evaluate potential test-methodology that may meet the aims of this thesis; namely,
the development of test-methodology capable of in-situ mechanical and physical
characterisation of the interphase independent of the reinforcement type and
geometry.
Chapter 3: Materials and physical characterisation of the interfacial region
This chapter provides an overview of the materials evaluated in this thesis, and
supplementary techniques that were used to support the results of the nano-scale
mechanical test methods. Two main types of material will be introduced, various
glass-reinforced polymeric composites and a number of reference materials used for
calibration purposes. The techniques described within this chapter are optical and
electron microscopy, which were used to image the interfacial region of prepared
composite cross-sections and fracture surfaces. The results and conclusions drawn
from these images concerning the presence of any interphase will subsequently be
used to support the results from the nano-scale mechanical test methods in the
following chapters.
Chapter 4: AFM scanning techniques
This chapter seeks to evaluate a nano-scale mechanical test technique, atomic
force microscopy, for characterising the interfacial properties of a composite at high
resolution. The capability of the AFM, in scanning modes, to produce surface
images of the mechanical and physical properties of an interfacelinterphase will be
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