Dynamically mechanical and Dynamically mechanical and Dynamically mechanical and Dynamically mechanical and nanonanonanonano----impact (fatigue) analysis impact (fatigue) analysis impact (fatigue) analysis impact (fatigue) analysis of touch screen thin films deposited on polyethylene of touch screen thin films deposited on polyethylene of touch screen thin films deposited on polyethylene of touch screen thin films deposited on polyethylene

terephthalate substrate terephthalate substrate terephthalate substrate terephthalate substrate

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aSchool of Engineering, Robert Gordon University, Garthdee Road, Aberdeen, AB10 7GJ, UKbSchool of Mechanical and Aerospace Engineering, Queen's University, Belfast, BT9 5AH, UK

cFaculty of Engineering and Environment, University of Northumbria, Newcastle upon Tyne, NE1 8ST, UK

12-15 September 2016

The 3The 3The 3The 3rdrdrdrd International Conference on Structural Nano Composites (NANOSTRUC 2016)International Conference on Structural Nano Composites (NANOSTRUC 2016)International Conference on Structural Nano Composites (NANOSTRUC 2016)International Conference on Structural Nano Composites (NANOSTRUC 2016)Robert Gordon University, AberdeenRobert Gordon University, AberdeenRobert Gordon University, AberdeenRobert Gordon University, Aberdeen

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Presentation SchemePresentation SchemePresentation SchemePresentation Scheme

1.1.1.1. IntroductionIntroductionIntroductionIntroduction

2.2.2.2. Experimental (Experimental (Experimental (Experimental (NanomechanicalNanomechanicalNanomechanicalNanomechanical/DMA)/DMA)/DMA)/DMA)

3.3.3.3. Results & SummaryResults & SummaryResults & SummaryResults & Summary

NanomechanicalNanomechanicalNanomechanicalNanomechanical LaboratoryLaboratoryLaboratoryLaboratory

NanoTestNanoTestNanoTestNanoTest®: Micro Materials Ltd.®: Micro Materials Ltd.®: Micro Materials Ltd.®: Micro Materials Ltd.

NanoSurfNanoSurfNanoSurfNanoSurf ® ® ® ® NaniteNaniteNaniteNanite S50 AFMS50 AFMS50 AFMS50 AFM

Introduction (flexible electronics)Introduction (flexible electronics)Introduction (flexible electronics)Introduction (flexible electronics)• Flexible solar cellsFlexible solar cellsFlexible solar cellsFlexible solar cells

• Touch screen panelTouch screen panelTouch screen panelTouch screen panel

• Demand for renewable energy, including flexible solar cells that give possibility to vary the

product design and technological applications.

• Commercially available polymers coated by transparent conductive oxide (TCO) layers are

used.

• Indium tin oxide (ITO) is still almost an exclusive TCO material because of its superior

electrical and optical properties.

• Deposition of films: Magnetron sputtering, chemical vapour deposition, thermal evaporation.

• Magnetron sputtering is the most common due to lower temperature deposition process.

• To analyse mechanical properties of thin film coating on flexible substrate (taking into

account that a harder top layer in nanometer range is deposited on a softer substrate) it is

essential to use appropriate tools.

• In this work, ITO and ZnO coating on polyethylene terephthalate (PET) film is investigated.

Introduction (Touch Screen)Introduction (Touch Screen)Introduction (Touch Screen)Introduction (Touch Screen)Thin films on PET Thin films on PET Thin films on PET Thin films on PET

substrate used in substrate used in substrate used in substrate used in

touch screen paneltouch screen paneltouch screen paneltouch screen panel

Find, 26 (1), 2008Find, 26 (1), 2008Find, 26 (1), 2008Find, 26 (1), 2008

Thin films:

Indium Tin Oxide (ITO)

Zinc Oxide (ZnO)

Substrate: Polyethylene

terephthalate (PET) is

thermoplastic polymer

resin of the polyester

PET

ITO

ITO

Specimen (Specimen (Specimen (Specimen (ZnOZnOZnOZnO thin film & PET substrate)thin film & PET substrate)thin film & PET substrate)thin film & PET substrate)

ZnO thin film PET substrate

ZnO

PET

Atomic force microscope image of ZnO film on PET substrate• Pulsed DC magnetron sputtering

• ITO film ~ 200-300 nm [two specimens]

• ZnO film ~ 1 microns [two specimens]

Dr Richard Fu (then at University of West of Scotland, Thin Film Centre)

Instrumented Indentation Testing?Instrumented Indentation Testing?Instrumented Indentation Testing?Instrumented Indentation Testing?

• COMMERCIALLY AVAILABLE (COMMERCIALLY AVAILABLE (COMMERCIALLY AVAILABLE (COMMERCIALLY AVAILABLE (NanoindentationNanoindentationNanoindentationNanoindentation) ) ) )

An indentation machine which can measure

- Indentation Load (P) & Depth (h) during loading, holding & unloading stage

- Hardness, Elastic Modulus

Loading

Un-loading

Indicative of the stiffness Indicative of the stiffness Indicative of the stiffness Indicative of the stiffness SSSS of contact (of contact (of contact (of contact (OliverOliverOliverOliver----Pharr Pharr Pharr Pharr

method, JMR, 1992, method, JMR, 1992, method, JMR, 1992, method, JMR, 1992, 7777, pp. 1564-1583))))

P-h curve Holding

Berkovich indenter

One of the most cited paper in Materials ScienceOne of the most cited paper in Materials ScienceOne of the most cited paper in Materials ScienceOne of the most cited paper in Materials Science

( ) ( )i

i

s

s

rEEE

22111 νν −

+−

=

)(2

)( δπ

δ AESr

=

...)( 2/1

32

2

1 +++= δδδδ CCCA

• NanoindentationNanoindentationNanoindentationNanoindentation

• NanoimpactNanoimpactNanoimpactNanoimpact

• NanoscratchNanoscratchNanoscratchNanoscratch

NanomechanicalNanomechanicalNanomechanicalNanomechanical MeasurementMeasurementMeasurementMeasurement

Experimental (Experimental (Experimental (Experimental (NanoindentationNanoindentationNanoindentationNanoindentation, , , , ZnOZnOZnOZnO on PET)on PET)on PET)on PET)

• Load: 0.1 mN, 1 mN.

• Each test was conducted for two indenter shapes (Berkovich and conical).

• Five repeated tests were done for each load range to ensure repeatability.

• An equal-displacement approach was used to keep the loading force larger than the

unloading force (i.e. PL>P

U).

• Post-test residual impressions were mapped using an atomic force microscope (AFM,

Nanosurf® Nanite, SPM S50, Liestal, Switzerland) at a scan rate of 1 Hz.

Experimental (Experimental (Experimental (Experimental (NanoindentationNanoindentationNanoindentationNanoindentation, ITO on PET), ITO on PET), ITO on PET), ITO on PET)

ITO film

PET substrate

Berkovich diamond tip

Specimen

Experimental (Experimental (Experimental (Experimental (NanoimpactNanoimpactNanoimpactNanoimpact/fatigue)/fatigue)/fatigue)/fatigue)

• Nano-impact experiments was conducted using NanoTestTM system

• Berkovich and conical nanoindenter between, 100 µN, 250 µN, 500 µN, 750 µN, 1 mN (1000 cycles, 2000 seconds, 5 repeats) at the

same location.

• Loading and unloading cycles for nano-impact tests involved linearly loading the specimen to full load in one second and then

releasing 100% of the test load in one second with zero hold time at the peak load.

• Both nanomechanical tests and AFM measurements were done in the instrument chamber at a temperature of 23 °C

Berkovich

Conical (10 µm tip radius,

60° included angle)

Experimental (Experimental (Experimental (Experimental (NanoimpactNanoimpactNanoimpactNanoimpact, ITO on PET), ITO on PET), ITO on PET), ITO on PET)

100 µN loadSharp peaks at the early stages of impacts

Self-healing properties?

Gradual failure?

Experimental (Experimental (Experimental (Experimental (NanoimpactNanoimpactNanoimpactNanoimpact, ITO on PET), ITO on PET), ITO on PET), ITO on PET)Conical indenter at 750 µN loadConical indenter at 750 µN loadConical indenter at 750 µN loadConical indenter at 750 µN load BerkovichBerkovichBerkovichBerkovich indenter at 750 µN loadindenter at 750 µN loadindenter at 750 µN loadindenter at 750 µN load

Literature examples (NanoLiterature examples (NanoLiterature examples (NanoLiterature examples (Nano----impact, impact, impact, impact, DLCDLCDLCDLC))))

100 100 100 100 µNµNµNµN 1000 1000 1000 1000 µNµNµNµN

Berkovich indenter

PilePilePilePile----upupupup

Indenter

type

Impact load (µN) No. of impacts Total test time (s)

Berkovich

100 1000 2000

250 1000 2000

1000 1000 2000

Conical

(10 µm tip

radius, 60°

included

angle)

250 1000 2000

500 1000 2000

750 1000 2000

1000 1000 2000

Literature examples (NanoLiterature examples (NanoLiterature examples (NanoLiterature examples (Nano----impact, impact, impact, impact, DLCDLCDLCDLC))))

250 250 250 250 µNµNµNµN 1000 1000 1000 1000 µNµNµNµN

Conical indenter

Backward depth deviationBackward depth deviationBackward depth deviationBackward depth deviation

K.-R. Lee et al., Diam. Rel. Mater. 2 (1993) 208.

M.-W. Moon et al., Acta Mater., 50 (2002) 1219.

Summary (Summary (Summary (Summary (NanoimpactNanoimpactNanoimpactNanoimpact))))

• For the nano-impact testing, the damage started with the crack initiation

and propagation.

• The interpretation of the nano-impact results depended on the

characteristics of the loading history.

• Under the nano-impact loading, the surface structure of film suffered

from several forms of failure damages that range from deformation to

catastrophic failures.

• It is concluded that in such type of application, the films should have low

residual stress to prevent deformation, good adhesive strength, durable

and good resistance to wear.

Further work (effect of indenter types)Further work (effect of indenter types)Further work (effect of indenter types)Further work (effect of indenter types)

• NNNN.... HHHH.... FaisalFaisalFaisalFaisal, R. Ahmed, SSSS.... GoelGoelGoelGoel, YYYY.... QQQQ.... FuFuFuFu, Influence of test

methodology and probe geometry on nanoscale fatigue

mechanisms of diamond-like carbon thin film, SurfaceSurfaceSurfaceSurface aaaannnndddd

CoatingsCoatingsCoatingsCoatings TechnologyTechnologyTechnologyTechnology, 242, 2014, p. 42–53

• Saurav Goel, Anupam Agrawal, NNNN.... HHHH.... FaisalFaisalFaisalFaisal, Can a carbon

nano-coating resist metallic phase transformation in silicon

substrate during nanoimpact? WearWearWearWear, 315(1–2), 2014, p.

38–41

• NNNN.... HHHH.... FaisalFaisalFaisalFaisal, R. Ahmed, YYYY.... QQQQ.... FuFuFuFu, Y. O. Elakwah, M. Alhoshan,

Influence of nanoindenter shape on DLC film failure during

multiple-load cycle nanoindentation, MaterialsMaterialsMaterialsMaterials ScienceScienceScienceScience aaaannnndddd

TechnologyTechnologyTechnologyTechnology, 28(9-10), 2012, p. 1186-1197

Further work (analytical models)Further work (analytical models)Further work (analytical models)Further work (analytical models)

Jia et al, Optics and Lasers in Engineering, 54, 2014, 263–268

The interfacial fracture energy of straight-sided wrinkles can be calculated:

•Critical buckling stress (σb)

•Biaxial residual stress (σr)

•Thickness of the thin film (hf)

•Half width of the wrinkle (b)

•Height of the wrinkle (δ)

•Elastic modulus (Ef)

•Poisson′s ratio (νf)

σr > σb

μ2 = π2

C1 = 3/4

Interfacial fracture energy

where ω is 52.11, and ξ = δ/h

Experimental (Dynamic Mechanical Analysis, DMA)Experimental (Dynamic Mechanical Analysis, DMA)Experimental (Dynamic Mechanical Analysis, DMA)Experimental (Dynamic Mechanical Analysis, DMA)

PerkinElmer Diamond DMA

• Dynamic Mechanical Analysis (DMA), is a method of characterizing the

viscoelasticity.

• The storage and loss moduli can be determined from the DMA technique by

applying a controlled oscillatory stress

• The resulting strain is measured, allowing the user to determine the

complex modulus of the tested material.

Experimental (FTIR): Before and after DMAExperimental (FTIR): Before and after DMAExperimental (FTIR): Before and after DMAExperimental (FTIR): Before and after DMA

• The spectrum that resulted from testing the ZnO

thin film before and after DMA can be seen in

figure.

• It shows that the absorption and transmission of

the infrared radiation was mainly unaffected by the

DMA testing.

• The molecular structure of the thin film has not

altered enough to show a significant change in its

infrared spectrum.

Experimental (FTIR): Before and after DMAExperimental (FTIR): Before and after DMAExperimental (FTIR): Before and after DMAExperimental (FTIR): Before and after DMA

• Figure shows the infrared

spectrum on the PET side of the

sample, before and after DMA

testing.

• It can be seen that there is very

little change after testing and

therefore the DMA has not caused

the molecular structure of the PET

to significantly change.

Experimental (DMA, Experimental (DMA, Experimental (DMA, Experimental (DMA, ZnOZnOZnOZnO on PET)on PET)on PET)on PET)

E' - Storage Modulus

(GPa) represent the

elastic property of the

material which varies

with time

Tan delta = E''/E'

This is the loss Tangent which

varies with time in this graph

The graph shows that with increasing time the storage

modulus decreases whereas tan delta increases until

the 42nd minute and the decreases

Glassy Viscoelastic RubberyModulus (stiffness)

is not constant as

plastics generally

creep when under

a constant load.

Experimental (DMA, Experimental (DMA, Experimental (DMA, Experimental (DMA, ZnOZnOZnOZnO on PET)on PET)on PET)on PET)

The dynamic properties (modulus and tan delta) vary with temperature. The graph shows a similar result as the previous graph varying with time.

The Tg can be found using

this graph. It is the peak of

the tan delta curve. This can

be seen clearer in the next

slide

The temperature

indicating the relaxation

in a polymer where a

material changes from a

glass to a rubber (Tg)

Tg directly relates to the

strength of the material

Experimental (DMA, Experimental (DMA, Experimental (DMA, Experimental (DMA, ZnOZnOZnOZnO on PET)on PET)on PET)on PET)

Tg=388.3 K (first peak in the loss tangent refers to glass

transition temperature)

Tan delta represents the damping which is a dimensionless property. It is a

measure of how well the material can disperse energy.

The dynamic loss modulus is associated with “internal friction” and is sensitive to different

kinds of molecular motions, relaxation processes, transitions, morphology and other

structural heterogeneities. Thus, the dynamic properties provide information at the molecular

level to understanding the polymer mechanical behaviour.

Recrystallization

Beta relaxation

Crystallization

Experimental (DMA, Experimental (DMA, Experimental (DMA, Experimental (DMA, ZnOZnOZnOZnO on PET)on PET)on PET)on PET)

Dynamic force can be described as the amount of acceleration or velocity required to move an object.

A Static force is a frictional force that pulls two things

together. The negative values indicate that this force is

in the opposite direction to the dynamic force.

Static displacement (deformation)

relates to the creep of the material.

• Creep refers to the time and temperature dependent deformation of a

material when it is subjected to a load or stress.

• Creep deformation in polymers consists essentially of two components:

reversible viscoelastic relaxation and irreversible viscous flow

Summary (DMA)Summary (DMA)Summary (DMA)Summary (DMA)

• Tan delta of DMA is described as the ratio of loss modulus (viscousproperties) and storage modulus (elastic properties) of the material and itspeak against time identifies the glass transition temperature (Tg).

• Thus, in essence the Tg recognizes changes from glassy to rubber state ofthe material and for our sample ZnO film, Tg was found as 388.3 K.

• The DMA results also showed that the Tan delta curve for Tg increasesmonotonically in the viscoelastic state (before Tg) and decreases sharply inthe rubber state (after Tg) until recrystallization of ZnO takes place.

• This led to an interpretation that enhanced ductility can be achieved bynegating the strength of the material.

Overall SummaryOverall SummaryOverall SummaryOverall Summary

1. Selections of indenter shape and loads provide a new approach to the investigation

of cohesive and adhesive failure of thin film films.

2. The failure of films starts from cohesive failure via delamination at the film-

substrate interface, resulting in peculiar decrease in contact depth (backward

depth deviation) for the conical indenter, whereas, typical increase in contact depth

(forward depth deviation) with number of fatigue cycles was recorded for the

Berkovich indenter.

3. The delamination failure indicated release of elastic stored energy (pre-existing

compressive residual stress).

4. To ensure the integrity of the thin films as this is influenced by the quality of

its adhesion to the underlying layer, residual stresses after deposition.

5. If the thin film lifts off the substrate it has been subjected to thermo-mechanical

stresses and therefore could result in the failure of the device that it is part off.

Thank you Thank you Thank you Thank you

Any Questions, Comments or Suggestions!!!Any Questions, Comments or Suggestions!!!Any Questions, Comments or Suggestions!!!Any Questions, Comments or Suggestions!!!

Robert Gordon UniversityRobert Gordon UniversityRobert Gordon UniversityRobert Gordon University

DDDDrrrr.... NNNNaaaaddddiiiimmmmuuuullll HHHH FFFFaaaaiiiissssaaaallll

School of Engineering

Robert Gordon University

N.H.Faisal@rgu.ac.uk

+44-1224-262438