Thesis

CHAPTER I

BACKGROUND AND LITERATURE REVIEW

1.1 Smart Materials

Smart materials are different from every day materials due to the flexibility in altering

its properties which can be significantly altered in a controlled fashion by external stimuli

such as stress, temperature, electricfield, and magnetic field. Representation of the smart

materials based on the response to the stimuli is shown in Table I [1].

Table I. Representation of various smart materials based on response/stimuli

1

Electrical

Magnetic

Optical

Mechanical

PiezoelectricElectrostrictive

Magnetostrictive

Magneto chromic

Negative Poisson Ratio

Mechanical

Optical

Mechanical

Thermo Chromic

Shape Memory

Thermal

Electrical

Optical

Photoconductor

Photo Chromic Optical

Optical

Mechanical

Magneto-Optic

Magneto-Rheological Magnetic

Optical

Mechanical

Electro Chromic, Electro Optic, Electro Luminescent

Piezoelectric, Electrostrictive

Electro-Rheological

Electrical

StimuliPropertyResponse

Among the above smart materials, piezoelectric materials are of great interest, as

they are commonly used as sensors, actuators and memory devices. As technology is

advancing, efforts are being made to use piezoelectric materials in combination with

other smart materials for power harvesting.

1.1.1 Piezoelectric Materials

Piezoelectric materials are smart materials used in Micro Electromechanical Systems

(MEMS). They possess a unique property called piezoelectricity, when mechanically

stressed generates a potential in volts. These materials also obey converse

piezoelectricity, where the deformation of piezoelectric materials takes place due to a

voltage applied across the material. Piezoelectric materials occur naturally and also

occur in the form of multiphase ceramics and films. Berlinite, quartz, cane sugar,

rochelle salt are some of the naturally occurring piezoelectric materials. Some of the man

made piezoelectric ceramics are barium titanate, lead titanate, lead zirconium titanate

(PZT), and lithium niobate.

Barium Titanate (BaTio3) is the first piezoelectric ceramic introduced for applications

in ultrasonic transducers as fish finders [2]. After that PZT was introduced this has better

piezoelectric properties than barium titanate. Lead Zirconium Titanate (PbZrTio3) is a

widely used piezoelectric material in sensors and actuators applications. The major

advantages of PZT over the other piezoceramics are:

High electromechanical transformation efficiency.

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Flexibility in the change of characteristics due to varying stoichiometry of the

PZT.

Better machinability and suitable for mass production.

Easy accessibility in the market because of commercial viability.

1.2 Lead Zirconium Titanate (PZT)

PZT is a pervoskite crystal with lead atoms at corner and oxygen atoms at the face

center of unit cell. The atomic size of the lead and oxygen is 1.4 Å; titanium or

zirconium atoms are located at the center of a unit cell.

Figure 1. Face centered cubic structure of the PZT.

Lead, oxygen and titanium/zirconium atoms together form a face centered cubic

array, as shown in Figure 1. PZT is manufactured by the sol-gel process and the radio

frequency magnetron sputtering process. They are usually available in the form of discs,

wires, plates and rods. To understand the properties of PZT it is important to be familiar

with the following terms below.

Lead (Pb)

Titanium/Zirconium (Ti/Zr)

Oxygen (O2)

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Dipole: A pair of opposite charges is called dipole. there moment is called dipole

moment.

Domains: The dipoles are randomly occurring in a crystal forming domains.

Poling: The method of aligning the randomly occurring domains in the direction

of the applied electric field is called poling.

Electromechanical coupling: It represents the piezoelectric efficiency of a

piezoelectric ceramic. Electromechanical coupling constant (K) for a

piezoelectric material is defined as the root mean square of the energy

accumulated within the crystal in a mechanical form. This accumulated energy

reflects the electrical output.

Piezoelectric dielectric constant: Dielectric constant is an electrical displacement

when a unity electric field is applied under no stress.

Piezoelectric distortion constant (d): Piezoelectric distortion constant is the

distortion of the piezoelectric material under the applied electric field of uniform

strength with no stress.

Piezoelectric voltage output coefficient (g): Piezoelectric voltage output

coefficient is the piezoelectric voltage output under uniform external stress

applied and no electrical displacement.

Curie temperature: Curie temperature refers to the temperature at which the

piezoelectric materials lose there polarization capability and hence, the

piezoelectric property. Curie temperature for the PZT film is 2800 C (553 K).

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Coercive field: Piezoelectricity is a property of ferroelectric materials. During

poling the domain structures are aligned along the direction of the applied electric

field. When the applied electric field is removed, polarization produces the

hysterisis loop. The reverse electric field required to cancel the remanent

polarization is called coercive field.

1.2.1 Application of PZT

The application of PZT ranges from sensor to power harvesting devices as shown

in Figure 2.

Figure 2. PZT applications.

Industrial applications of PZT for the economic benefits of the community and future

technical developments are described below.

MEMORY DEVICES

ACTUATORS

SENSORPZT

POWER HARVESTING

G

5

(a) Sensor

PZT is used for self-powered strain energy sensors (SES) to convert the applied

mechanical strain to electrical charges for charging a capacitor. Equation (1) explains the

above electromechanical relationship.

Dn = d i ,j T j+εnm E m ……………………………(1)

m, n=1, 2,3… i, j = 1,2,3….

D= electrical displacement in C/m2;

d= piezoelectric strain constant in m/V

T= stress in N/m2, E= Electric field in V/m,

ε = electric permittivity in F/m

The charged capacitor is used to transmit a signal to a remote receiver. Variation

in the signal strength is due to the amount of charges in the capacitor and the amount of

charges in the capacitor is related to the straining of PZT. The piezoelectric approach for

self powering has several advantages including: a) small size, b) low sensor cost, and c)

the ability to shape the sensor into an arbitrary geometry [3].

(b) Actuators

There are various types of actuators, like bilayer and multilayer piezo ceramic

actuators. In the case of the multilayer actuator, various PZT materials are stacked

together. PZT materials on their own are not suitable for actuation tasks; they must be

combined in certain arrangements to create viable actuators. This actuator deforms

(expands) roughly proportional to the voltage given up to about 0.1% at typically 150V,

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based on the converse piezoelectric property [4].

(c) Memory Devices

Piezoelectric materials like PZT belong to a large class of materials called

ferroelectrics. Poling of the PZT aligns the randomly oriented dipoles along the direction

of applied electricfield and hence the PZT becomes polarized. When the applied

electricfield is removed, all dipoles do not fall back to their initial state, leading to a

remanent polarization. Polarization and remanent polarization give rise to hysterisis in

PZT as shown in the Figure 3. The hysterisis property is used for memory allocations in

PZT. PZT is used as ferroelectric random access memory (Fe-RAM) in the memory

device manufacturing industries. PZT in Fe-RAM has many advantages such as non-

volatility, high speed operation and low power [5].

Figure 3. Hysterisis curve for a PZT.

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(d) Power Harvesting

Power harvesting or energy scavenging is a process of converting the ambient

energy into an usable energy for the electrical devices. Polyvinylidene fluoride (PVDF)

was implemented in vivo on a mongrel dog for power harvesting. Periodic expansion and

contraction of the rib cage in the mongrel dog strains the PVDF to generate charges [6].

PVDF was used in a self-powered damage detection unit; the charge generated during the

crack generation was stored in a capacitor, which in turn charges the transmitter to

transmit the signal to a receiver [7]. Much of the research in the field of power

harvesting has involved in the studies of optimizing the circuit design to store the energy.

Gradually researchers realized that the circuit built for optimizing the charge storage lead

to more leakage of charges [8]. PZT was used to charge the batteries ranging form 40 to

1000 mAh. PZT was vibrated at a random frequency range of 0-500 Hz; the

electromechanical efficiency was 6.57%; the typical charge cycle of one battery is shown

in Figure 4 [9].

Figure 4. Charge cycle of a nickel metal hydride battery [80 mAh].

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Physics and media group from MIT conducted three different experiments on

parasitic power harvesting. They used PZT unimorph, PVDF stave and electric generator

for energy scavenging. PZT unimorph was glued in a shoe sole, under the heel of the

foot as shown in the Figure 5. Unimorph PZT produced 2mW at 2 Hz, which was greater

then the PVDF stave; the power generated as a function of time during the brisk walk is

represented in the Figure 6 [10].

Figure 5. PZT pasted on the shoe sole under the foot heel.

Figure 6. Comparison of the power output of PZT and PVDF.

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Factors affecting the Properties PZT

Basic properties of PZT, like the ferroelectric, electromechanical coupling,

hysterisis and dielectric constant determines the wide range applications of PZT. The

importance is to be given on the factors that affect these properties. Research is in

progress to improve these properties by doping, surface treatment and nanoparticle

coating. Surface morphology of PZT is a factor in determining the leakage current of

PZT; Leakage current with smooth surfaces and rough surfaces was 10 -7A/cm2

and10-6 A/cm2 respectively [11]. The factors that affect the ferroelectric properties of

the PZT like local crystallographic orientation and the local grain-grain interactions

play an important role in determining the switching of domains. Piezoforce

microscopy was used for analyzing the polarization switching behavior at the grain

corners and boundary sites [12]. The influence of poling temperature, poling electric

field, poling time and distance between electrodes on dielectric constant and

electromechanical coupling factor of the PZT were studied; the results of the above

research are summarized in the Table II [13].

From all of the above data, the importance of PZT is seen in the field of sensors,

actuators, memory devices, power harvesting and also the factors affecting the PZT

properties. Nanoparticle coating is one of the many research areas to improve the

PZT property as explained in the following Sections.

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Table II. PZT property as a function of poling time, temperature and electricfield.

Property Condition Poling

time

Poling

temperature

Poling

electricfield

Distance

b/w the

electrode

Dielectric

constant

Higher Longer

Saturated at

high

temperature-

180o C

Higher Greater

Electromechanical

coupling

Larger Longer

Saturated at

higher

temperature-

180o C

Higher Greater

1.3 Nanoparticle Coating

Nanoparticles are the particles with the size less than 100 nm. There are several

forms of nanoparticles available in the market. Most common are carbon nanotubes,

quantum dots, ferrous nanoparticle (FNP), zinc oxide (ZnO) nanoparticle, gold [Au] and

copper (Cu) nanoparticles. Nanoparticles are widely used in the medical and engineering

fields. For example, FNP is used for drug delivery and as charge injection devices.

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Nanoparticles possess some unique properties like:

a) High surface area to volume ratio.

b) High thermal and electrical conductivity.

c) Piezoelectric and semiconducting properties.

Nanoparticle thin films have a wide range of applications, such as nanoelectronics,

magnetic storage devices, optical grating, energy harvesting and antireflective coating.

Thin film deposition of nanoparticles has been achieved successfully on a wide range of

conducting as well as non-conducting substrates such as carbon-coated copper grid,

silicon, m-plane of alumina, glass and (100) plane of NaCl single crystal [14]. Sprayable

lead sulphide (PbS) nanocrystals dispersed in a chloroform solution of a conductive

conjugative polymer, acts as a light harvesting films, shows a photovoltaic effect in the

infrared region [15]. Nanoscale mechanical energy was converted into electrical energy

using zinc oxide nanowire arrays; energy transfer efficiency was 17-30% [16].

Nanoparticle coating for power harvesting or energy harvesting is a new field and has

few literatures. Zinc oxide nanoparticles and ferrous nanoparticles possess

semiconducting, piezoelectric and magnetic properties respectively, which is of great

interest in power harvesting. Hence, understanding the properties of the ZnO and FNP is

important and it is explained in the following Sections.

1.3.1 Zinc Oxide (ZnO) Nanoparticle

Zinc oxide nanoparticles are gaining importance because of its applications in the

field of optoelectronic and photovoltaic devices; Zinc oxide nanoparticles observed under

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a SEM is shown in Figure 7. Zinc oxide nanoparticles are transparent conducting oxides.

The ZnO nanoparticle is a wide band energy [3.3eV] semiconductor as shown in Figure

8. ZnO possesses high exciton binding energy of 60 meV; high exciton energy leads to

charge emission at room temperature.

Figure 7. SEM image of ZnO nanoparticles.

The ZnO nanoparticle is the hardest of the II-VI family of semiconductors. This

means that its performance will not be degraded as easily as other compounds through the

appearance of defects [17]. Some of the physical and chemical properties of the ZnO are

described below.

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Figure 8. Band gap energy of ZnO compared with various semiconducting materials.

Table III. Physical and chemical properties of the ZnO.

Physical Properties Numerical Values

Molecular Mass 81. 389 g/mol

Specific Gravity 5. 643 g/cm3

Mohs Hardness 4

Melting Point 2240K

Specific Heat 0. 12 cal/gm

Thermal Conductivity 0. 006 Cal/cm/k

Key advantages of ZnO nanoparticles:

High piezoelectric effect (e33 = 1. 2 C/m2), among highest of all semiconductors.

High thermal conductivity of 0. 54 Wcm-1K-1 (compared with 0. 5 W/cm. K for

GaAs).

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Largest exciton binding energy of II-VI & III-V semiconductors, 60 meV

excitonic stimulated light emission upto 550 K.

More radiation resistant than GaN (2 MeV, 1. 2 x 1017 electrons/cm2).

Very large shear modulus ~45.5 Gpa (indicates stability of the crystal), compared

with 18.35 Gpa for ZnSe, 32. 60 Gpa for GaAs and 51.37 Gpa for Si.

1.3.2 Ferrous Nanoparticle

Ferrofluid (FF) is a stable colloidal suspension of magnetic nanoparticles carried

in a liquid medium. Ferrofluids with magnetite nanoparticles are super paramagnetic in

nature and possess low coercive field and high magnetic susceptibility. The viscosity of

a FF is managed by varying the magnetic field. Thermal stability of a ferrofluid is related

to the ferrous nanoparticle density. The particle appears like a catalyst and produces free

radicals, which lead to the cross linking of the molecular chains and eventual congealing

of the fluid. The catalytic action is high at elevated temperatures. Therefore, they tend to

congeal rapidly at high temperatures [18]. The mechanism of magnetization is described

by Brown and Neels mechanisms [19]. Ferrofluids with FNP are used as lubricants,

charge injection devices, high memory data storage arrays (5nm FNP could store one bit

of information) and actuators; they have an average size of 10nm. Properties of the FF

are shown in the Table IV. The carrier used may be organic solvents or water. Various

surfactants are used to prevent the agglomeration of the magnetic nanoparticles. Oleic

acid, citric acid, tetra methyl ammonium hydroxides are the common surfactants used.

The magnetic property of the FNP in the ferrofluid encouraged Sai et al [20] for FNP

coating on the PZT; it will act as an additional magnetic energy on the PZT surface. The

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response change of PZT due to FNP coating on the PZT surface is represented in Figure

9. FNP coating under the influence of a magnetic field on PZT improved the

magnetoelectric property of the PZT. Magnetoelectric property of PZT increases the

polarization due to the additional magnetic energy on PZT surface [21].

Table IV. Properties of the ferrofluid.

Edmund

Scientific

Saturation

Magnetization

Density Viscosity Surface

Tension

Volatility Flash

Point

Pour

Point

Ferrofluid 400 gauss 1.21gm/ml 6cp@ 270 29dynes/cm 9%

(1hr@500c)

920 C -940 C

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Figure 9. Comparison of the responses of PZT, FNP coated PZT and PZT treated under the magnetic field.

1.3.3 Advantages of Mixing ZnO Nanoparticles with the Ferrofluid

Ferrofluids when subjected to saturation magnetic field, whereby the applied

magnetic field overcomes the surface energy of the ferrofluid and forms ferrous cones.

These ferrous cones are used as low electron source for charge injection devices [23].

The arrays of pointed structure or the cones as shown in the Figure 10 have various

mechanical and electrical properties.

PZT with FNP Treatment

0

50

100

150

200

250

0 5 10 15 20 25

Current (Amps) (Amps)

R E S P O N S E (

mV

)

PZT under magnet

FNP coated PZT

PZT

17

Figure 10. Ferrous cones on the aluminum substrate.

Mechanical and electrical properties could be improved by coating these ferrous

cones with a wide band gap semiconductors like ZnO nanoparticles. Ferrous cones could

be particularly useful for creating emitters to be coated by wide band gap

semiconductors, which can absorb and emit photons in the ultraviolet light bands.

Coating the ferrous cones with the ZnO nanoparticles will be an alternative for the

photocathode systems and source of electrons. This technology has various advantages

as described below [23]:

a) ZnO avoids the oxidizing properties associated with ferrous cones and can be

used as photocathode systems.

b) ZnO in ferrous cones are expected to impact a wide range of electronics and

mechanical equipment such as field emission devices, photocathodes, scanning

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tunneling microscopes, atomic force microscopes and low-power propulsion

systems.

c) This method is low cost, versatile and consistent.

The challenges are:

How to coat ferrous cones with a semiconductor like ZnO for the above

described advantages?

FNP coating on PZT has increased sensitivity [20]. How will the

sensitivity of the PZT be changed by coating with a FNP and ZnO

mixture?

1.4 Importance of Tribological Analysis

Though PZT has various applications, literature on its tribological properties are

scarce. To enhance the PZT applications in the field of MEMS it is essential to study the

interfacial engineering, plastic deformation, and fracture characteristics to design smart

structures with the ability to resist failure under mechanical and thermal loads. The large

surface to volume ratios and low restoring forces, unwanted adhesion and friction

dominate the performance of the MEMS. The macroscopic contacts of MEMS are

composed of nanoscale asperities. A nanoscale asperity acts as the source of fracture or

cracks due to the high friction at the asperities contact. The characteristics of surface

morphology and properties of single asperity contacts can be studied by atomic force

microscopy (AFM). Based on the above facts, minimizing the effect of adhesion,

friction, and wear is essential for improving MEMS.

Surface morphology of PZT used in the D-RAM applications is correlated to

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fractal morphology and to evaluate efficiency of the D-RAM component [25]. PZT

domain stability, mobility and structure are a function of friction due to the fluctuating

stresses. Low friction leads to low mobility and decreases the domain walls. Change in

the sensitivity of PZT is a function of concentration of point defects, interaction between

defects and change in the real structure of the PZT. Presence of these defects is going to

affect mobility and stability of domain walls. Predicting the surface defects, interfacial

friction and energy and early elastic deformation with respect to the ramped load at the

PZT surface is essential to define its tribological characteristics [26].

1.5 Justification and Importance of This Work

Improving the piezoelectric property of the PZT is a major concern to expand the

range of applications. FNP coating on PZT is going to provide the additional magnetic

energy and improve the magnetoelectric effect of the PZT and in turn increase the

response of the PZT [21]. The effect of mixing ZnO nanoparticles in a ferrofluid is

studied by coating this mixture on the PZT and subjecting it to power harvesting.

Comparison of the power harvesting capability of plain PZT, FNP coated PZT, and (ZnO

nanoparticle + FNP) mixture coating on the PZT was carried out.

The importance of studying the atomic friction and interfacial atomic defects on

the PZT is understood to an extent, and it is a challenge for nanotribologists. There is a

necessity for development of a high spatial resolution tool to analyse these challenges.

Tribo-diagnostic tool is developed based on the phase fluctuation based mathematics to

analyse the atomic friction and to identify the atomic defects at the sliding interface. This

tool is effectively used on the silver coated PZT by considering its applications to

20

MEMS.

21

CHAPTER II

THEORY

2.1 Tribology

Tribology relates to the study of friction, wear and lubrication at the interface of

two or more surfaces in contact. Tribology is a vast subject and it is important to prevent

failures at the contact interface due to sliding, rolling, or rubbing. In 1966, the lack of

knowledge towards tribology costs 6% of the GDP to the US government, i. e. $200

million in the year 1966. One-third of the world’s energy resources used are subjected to

friction in one form or the other [27]. Research in tribology has gained importance after

these statistics were realized and extensive research on preventing the loss due to friction

and wear at the contact of the rubbing surfaces is ongoing today.

The better performance of the MEMS is possible through the study of tribology at

the microscale or nanoscale leading to a new subject known as nano/micro tribology.

The following Sections deal with the tribology at nanoscale.

2.1.1. Nanoscale Friction

Friction is a part of tribology and is defined as the force opposing the motion of

rubbing surface. There are various laws that define the factors affecting friction. In

1699, Amontons law was stated as, “friction forces are proportional to the load applied

and independent of the surface area”. After some years, Hirn [28] distinguished the

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friction variation on the unlubricated and lubricated surface. The coefficient of friction

for the lubricated surface was reduced as compared to the unlubricated solid surface.

Hence, Hirn added the effects of velocity, load and surface area on friction. In 1892,

Ewing related the origin of the friction to the surface forces and defined friction as the

opposing molecular force for the applied load, eventually leading to the molecular

displacement.

Tomlinson’s theory correlates the interaction between the two sliding elements at

the molecular level. According to Tomlinson, the energy is dissipated as the molecules

are forced into each other atomic field. This is the theory widely used to analyse the

friction at the nanoscale or atomic scale [28].

Friction at nanoscale does not obey Amonton’s law of friction. Friction at

nanoscale is proportional to the contact area, load applied, contact pressure and surface

forces at the sliding interface [27]. In many cases, simple empirical formulas used for the

describing friction at macroscale is not applied to the nanoscale because of the high

surface area to volume ratio, surface chemistry, adhesion and the roughness effects.

Hence, the friction force varies nonlinearly to the load applied and the models used to

analyze it are explained in the Section 4.1.1 [29]. For example, Figure 11 represents

friction force variation at the nanoscale over an increased load on Carbon-60 (C 60) and

Germanium Sulphide (GeS) surface. The nonlinear response of the friction vs. load is due

to the surface force effect [30].

Corrugations seen on the friction forces which are distinctively different in shape

is called as stick-slip friction. Sliding action of the surface asperities undergoes sudden

slips during sliding motion [31]. The friction due to stick-slip is studied to analyse the

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friction at nanoscale.

Figure 11. Friction force vs. load [Si tip on the GeS and C60].

2.1.2 Stick-Slip

When the two surfaces slide on each other they can have an abrupt motion or an

irregular motion. This irregular motion is due to the asperities on sliding surface which

leads to stick-slip motion. The sliding process is not a continuous one; the motion

proceeds by jerks. The asperities “stick” together until, as a result of the gradually

increasing pull, there is a sudden break with a consequent very rapid “slip” [28].

Different types of stick-slip signals are described below:

Periodic Stick-Slip: The signal with potential (friction force) maxima and minima has

narrow widths that do not overlap.

Random Stick-Slip: The signal with friction force potential maxima and minima have

wide width and overlap.

Stepped Stick-Slip: There is a step increase in the friction force potential. The saw

toothed maxima, has increased force maximum than the force amplitude before.

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Figure 12. Different types of stick-slip process.

Stick-slip friction provides information on the interface science in many ways. Some

of the theories on the stick-slip are described as below:

As the atoms in one plane are in equilibrium with the neighboring plane, when

the shear forces are applied at the interface, the atoms deform elastically

called as slip and the prior situation where atomic equilibrium existed is called

as stick. This slip may be related to loss of energy or fracture due to periodic

elastic deformation of the atomic planes [32].

Between the two sliding surfaces, asperities at the interface undergo elastic

deformation known stick” and the release of the energy leads to plastic

deformation known “slip”.

The stick-slip process is a melting-freezing process where the melting is due

to the dislocations of atoms and the freezing is related to the strain hardening

25

of the atoms [33]. In many cases the rapid slip of the upper surface relieves

some of the applied force which eventually falls below another critical value

Fk, the kinetic friction, at that point the film resolidifies and the whole stick-

slip cycle is repeated [27].

Stick-slip behavior varies with applied load, scan speed and scan direction

with respect to crystallographic directions [34].

2.1.3 Advantages and Challenges of Stick-Slip

Stick-slip study at the atomic scale helps in understanding the transitions

between different regimes of atomic scale friction and provides insight into

the origins of friction and may lead to the ability to control it [36].

Determination of variation in the amplitude of the stick-slip is a guide to

predict the intensity of the friction force. Reducing the force amplitude is

going to reduce the friction at the interface.

Understand the transition of stick-slip to continuous sliding at the atomic

friction [37].

Could the extensive study of stick-slip explain the relation between friction

and wear [29]. ?

Slip due to the atomic dislocation could be also due to the atomic defects;

hence, stick-slip could be used for imaging the atomic defects [38].

Some of these above stick-slip challenges are to be addressed to understand the

tribological behavior of the PZT. Further, these tribological characteristics could be used

for designing the PZT based MEMS for sliding applications and the relation between

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PZT electromechanical efficiency and load acting on it. AFM is used for obtaining the

stick-slip signal at the sliding interface; AFM description is given in the following

Section.

2.1.4 Atomic Force Microscopy (AFM)

AFM is used for measuring the three-dimensional surface topography and

physical properties of a surface using a profiler or sharp tip probe. The probe is placed at

a point where the surface force effect exists. The probe is raster scanned over the surface

by keeping a constant force; this raster scan displays the surface topography. AFM

consists of a cantilever beam with a probe, laser beam, piezotube scanner and a laser

photodetector. The description of the AFM is shown in the Figure 13.

Figure 13. Schematics of AFM.

The probe is very sensitive to surface forces and as the probe scans over the

sample, the probe gets deflected due to the surface forces; the feed back control system is

27

used to keep the probe and sample distance constant. The deflection of the probe will

yield a surface image; AFM has been effectively used in the field of friction analysis at

the nanoscale [38].

Force calibration mode is used in studying the interface science [tip and the

sample surface interaction]. In force calibration mode X and Y voltages applied to the

piezotube is held at zero and a saw tooth voltage is applied to the Z-electrode of the piezo

tube. The force measurement starts with the sample far away and the cantilever tip at its

rest position. As a result of the applied voltage the sample is moved up and down and a

cantilever tip deflection signal is monitored using the photodiode. The force curve, a plot

of the cantilever tip deflection signal as a function of the voltage applied to the piezotube,

is obtained. Figure 14 shows the force separation curve; the arrow head reveals the

direction of the piezo tube travel. At point 1, the tip is off the sample surface. From

point 1 to 2 there is no change in the deflection signal as the piezo tube extends, because

the force is initially zero and the sample has not come into contact with the tip. At point

2, the force between the tip and sample becomes attractive. The attractive force increases

till the point 2’ of the sample-tip, until the sample-tip force becomes repulsive. The

maximum forward deflection of the cantilever multiplied by the spring constant of the

cantilever is the “pull-off” force. At point 3, the deflection signal reaches maximum and

the piezo tube starts to retract. At point 4 the cantilever is not deflected, but due to

adhesion between the tip and the sample. The tip sticks to the sample and the cantilever

is bent down as the piezo retracts. The attractive force between the tip and sample

becomes equal to the spring force at point 5. At point 6 the cantilever has returned to its

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undeflected state. Cantilever deflection signal remains constant as the piezo continues to

retract to point 7.

Figure 14. Force curve.

AFM is used for measuring friction in the form of friction force microscopy. A 0o

scan angle corresponds to sample scanning in the “y” direction, and a 90o scanning angle

corresponds to the sample scanning perpendicular to the “y” axis in the xy-plane (along x

axis). This scan is called as perpendicular scan, where the scanning tip experiences only

frictional force.

As the tip is perpendicularly scanned over the sample, twisting of the tip takes

place. Twisting of the tip will deflect the laser beam on to the horizontal and vertical

quadrant of the photodetector. The differential signal between the left (L) and right (R)

photo detector is denoted as friction force signal and calculated from {(L-R)/ (L+R)}

[27].

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High Speed Data Capture is a function in the AFM used for collecting the friction

signal data at an unprecedented time resolution. Friction signal data at an unprecedented

time resolution is going to provide high spatial resolution. This property is important in

analyzing the atomic scale stick-slip friction. Phase fluctuation based processor is used to

analyze this friction signal data collected at an unprecedented time resolution.

2.2 Phase Fluctuation Based Processor

Phase fluctuation based processing produces better gain over the amplitude based

analysis of the signal. Studying the amplitude variation of the acoustic signal to identify

the noise and signal is a traditional method of obtaining high signal-to-noise ratio.

Sometimes the noise also has the same fluctuation as that of the signal and it is difficult

to identify the difference between the signal and noise. Amplitude based signal-to-noise

ratio does not consider the unpredictable dynamic changes in the signal. High signal-to-

noise ratio is obtained by reading the variation of phase angle with respect to time, where

deviation from the aligned phase angle is called as noise. Phase fluctuation based signal

processing provides better spectral and spatial resolution [39].

30

2.2.1 Working Principle

The working principle of phase fluctuation based processor is explained via

Figure 15.

Figure 15. Polar representation of acoustic pressure wave vectors.

Phase fluctuation is defined on three planes for self correction of the phase rate and

direction of phase angle rotation. As phase angle approaches zero, the phase angle

temporal coherence increases with gain (signal-to-noise ratio). The mathematics used in

phase fluctuation is explained based on the Figure 15.

R i, R i-1, R i-2 , where

……………………………………. (2)

Let, ti = Discrete time sample

= Phase angle at a given frequency

= Angular velocity of the vectors

= Angular acceleration of the vectors

31

………………………………………… (3)

…………………………………………… (4)

= Aligned phase angle of the vectors

…………………………………………… (5)

For a coherent signal,

=Constant; = 0

If, 0, measure of incoherence or noise

The phase fluctuation ( ) on the polar plane is defined as the phase angle of

pressure vector is out of uniform phase angle rotation with phase angle and of

the previous two phasors and , as represented by the equation 2. Uniform rate of

phase fluctuation is termed as constant angular velocity; constant angular velocity gives

zero angular acceleration. Any deviation of the angular acceleration from the zero point

towards the positive or negative side gives the information on noise.

Twisting of the cantilever tip corresponds to the friction force experienced by it.

Uniform deflection of the cantilever tip is considered to have uniform phase fluctuation

and hence, called as signal or “stick” and the deviation from this uniformity is considered

to be the surface defect or “slip”.

High spectral resolution and friction signal-to-noise ratio is essential for analyzing

the stick-slip at nanoscale and will solve the challenges mentioned in stick-slip studies.

This phase fluctuation mathematics and HSDC function in the AFM together form the

tribo-diagnostic tool.

32

CHAPTER III

EXPERIMENTAL DETAILS

3.1 Tribo-Diagnostics of PZT

The AFM with HSDC function is used for collecting the friction signal data. This

friction signal data is analysed using the phase fluctuation based mathematics coded in

Matlab. The AFM calibration and HSDC methods followed in this research for

developing the tribo-diagnostic tool are explained below.

3.1.1 AFM Calibration

Atomic Force Microscopy is used for the tribo diagnostics of the PZT smart

material. AFM supplied by the digital instruments with the nanoscope V software is used

for this experiment. Various AFM parameters used for this experiment are shown in the

Table V.

33

Table V. AFM Parameters for studying the friction force signal.

AFM Parameter’s Values

Scanning Mode Contact

Scan Area (nm2) 50 X 6. 2

Scan Rate ( Hz) 1. 99

Ramped Load (nN) 40 to 200

Scan Angle (deg) 90

Lines 256

Points/Line 256

Scanning Velocity (nm/s) 199

3.1.2 Experimental Procedures

High Speed Data Capture (HSDC)

High Speed Data Capture (HSDC) is used for obtaining unprecedented time

resolution of the interfacial friction signal. This is a data collection tool in the

nanoscope V software. HSDC can collect data at 6. 25 M Hz and 512 K Hz.

Considering high resolution requirements, the 6. 25 M Hz option was used for

collecting data [41]. The step-by-step procedure used to collect the friction signal

data is explained below.

PZT surface and silicon nitride [AFM cantilever Tip] is cleaned with

methanol for avoiding the effect of dirt on the interfacial friction.

34

The cleaned PZT surface is glued on the metallic disc and stuck on the

piezotube magnetic disc holder in AFM.

For contact mode the feed back control system with proportional gain = 3 and

integral gain = 2 is set.

The laser point is concentrated on the cantilever tip using upper and lower

knob in the AFM. In the meantime, look at the laser signal on the screen and

try to get the laser point below the center of the cross (to get ~-2 V vertical

deflection and ~0. 0 V horizontal deflection).

Nanoscope V software function “engage” is used to bring contact between the

silicon nitride tip and the silver coated PZT surface. Calculate the deflection

sensitivity using the ramp load function.

Based on the deflection sensitivity of cantilever, the load acting on the sample

is defined; the load ranged from 40 to 200 nN is applied.

At each load applied on the PZT surface, sliding of the silicon nitride

cantilever tip on the PZT surface leads to the interfacial friction.

The twisting of silicon nitride tip during scanning on the PZT surface

generates a differential signal [lateral force deflection] from the left and right

photodiodes, which provide the friction force signal.

HSDC is used to collect this data at a time resolution of 6. 25 MHz.

The friction signal data in the form of ASCII format is saved in the computer

for exporting it to Matlab for further analysis.

Phase Fluctuation Analysis: The importance and theory of the phase

fluctuation based analysis was explained in the Section 2. 2. Phase fluctuation

35

analysis mathematics was coded in Matlab. The interfacial friction signal data

collected at each load ranged from 40 to 200 nN was exported in ASCII

format to Matlab.

The Section below explains the experimental setup for nanoparticle coating to

improve the sensitivity of PZT and for power harvesting.

3.2 Coating Process

This Section explains the experimental set up for nanoparticle coating to improve

the sensitivity of PZT .

3.2.1 Coating Equipment and Procedure:

Coating Materials:

1. Piezofilm (P-5E) with resonance frequency of 1.3 kHz, impedance of 300

ohms, and capacitance of 40 nF at 120 Hz was supplied by Murata Inc. P-

5E thin sheets were used for the power harvesting experiment. The

properties of P-5E piezofilm are given in Table VI [2]. The SEM image of

the P-5E with silver coating is shown in the Figure 16.

Figure 16. SEM image of the silver coated PZT (P-5E).

36

Table VI. Property of the PZT [P-5E].

Relative

dielectric

constant

(ε33 )

Electro-

mechanical

coupling

(K33 )

Piezoelectric

constant(d33)

(10-12 m/V)

Elastic

constant

(10-12

m2/V)

Density

(103Kg/m3)

Bending

strength

(106 N/m2)

1510 62% 271 14. 3 7. 8 11. 3

2) Ferrofluid is a stable colloidal suspension of magnetite

nanoparticles of size 10 nm and coated with the surfactants like

oleic acid to prevent the agglomeration of the magnetite

nanoparticle. The ferrofluid was supplied by the Edmund

Scientific Inc; Table VII contains the properties of ferrofluid

[18].

37

Table VII. Ferrofluid properties as provided by Edmund Scientifics.

Edmund

Scientific

Saturation

magnetization Density Viscosity

Surface

tension Volatility

Ferrofluid 400 Gauss 1.21gm/ml 6cp @

270C

29dynes/cm

9%

(1hr 5000C)

3) Zinc Oxide: Zinc oxide nanoparticles were supplied by the Sigma-Aldrich

[42]. The properties of the ZnO nanoparticles are short listed in Table VIII.

Table VIII. Properties of ZnO nanoparticles.

Material Particle Size Surface Area

ZnO <100nm 15-25 m2/g

4) Ultraviolet (UV) light of wavelength 130 nm was used for curing the

coated PZT for 80 Hrs.

Experimental Procedure:

a) Aluminum Strip: Aluminum alloy 6061 is used for the experiment. It is

usually used in aircraft and automobile manufacturing. An aluminum

substrate of dimension (11X2X. 2) in3 is used as a substrate for the silver

38

coated PZT (P-5E) and nanoparticle coated PZT (P-5E) in power

harvesting.

b) The surface of the aluminum strip is cleaned with alcohol and PZT is

glued on the surface. Non-conductive epoxy resin is used as the glue.

The PZT glued aluminum is shown in the Figure 17.

Figure 17. PZT glued on the aluminum strip.

c) The nanoparticle coating equipment with the UV light, rectangular box

and magnet glued wooden block is shown in the Figure 18.

39

Figure 18. Schematic of the nanoparticle coating equipment.

d) A transparent rectangular box is fabricated using the glass and plastic

material as shown in Figure 19. At the base, a slot of size 12X2 in 2 is

machined to accommodate the PZT glued aluminum strip. The PZT

glued aluminum strip is drenched with the ferrofluid and the ferrofluid

mixed with ZnO.

40

UV Light

Magnet glued wooden board

Rectangular box with an aluminum strip

Figure 19. Rectangular box with the slot to hold the PZT glued aluminum strip.

e) Disc magnets of strength 400 gauss are glued on the small wooden plate

as shown in the Figure 20. Two wooden plates with south and north poles

of the magnets are glued. These magnet glued wooden plates are placed

with the north Pole disc magnets on the top and the south pole disc

magnets below the rectangular box. Hence, the magnetic field passes

through the aluminum strip in the rectangular box.

41

Figure 20. Magnet glued wooden piece to create the magnetic field on the coating surface.

f) ZnO nanopowder is mixed thoroughly in the ferrofluid using a vibrating

mixer. ZnO nanopowder of 6% of the volume of ferrofluid was mixed.

g) The PZT glued aluminum strip is pushed into the rectangle box slot.

h) For FNP coating, the ferrofluid is poured on the PZT glued aluminum

strip in the slot, and the ZnO nanoparticles mixed in ferrofluid solution

are used for ZnO nanoparticle coating.

i) Disc magnet glued wood pieces are placed on the rectangular box. The

magnetic lines of flux pass through the coated PZT glued aluminum, from

the south pole to north pole.

j) Ultraviolet (UV) curing of the nanoparticle coated PZT glued aluminum

is carried out for 80 hrs [20].

k) After 80 hrs of UV curing, the nanoparticle coating is polymerized onto

the PZT material.

42

l) Three each of PZT glued aluminum strips, FNP coated PZT glued

aluminum strips and FNP+ZnO coated PZT glued aluminum strips were

prepared.

The nanoparticle coated PZT glued aluminum plates are shown in Figure 21.

Sometimes the base of PZT glued aluminum strip was coated, to avoid the effect of

coating on the PZT base in the power harvesting, the coating at the base is scraped using

a scrapper.

a)

b)

Figure 21. a) FNP coated PZT glued aluminum strip; b) FNP+ZnO coated PZT glued aluminum strip.

43

3.3 Power Harvesting

This Section presents the equipment and the experimental set up for implementing

the above coated PZT and plain PZT for power harvesting.

3.3.1 Equipment and Experimental Setup

Equipment used for the power harvesting experiment are as listed below:

a) Exciter

b) Function Generator

c) Oscilloscope

a) Exciter: It is an oscillator which oscillates at the input driving frequency.

Exciter has a dc generator fixed to the output shaft. The AC-voltage from the

function generator acts as an input voltage to the exciter, which is then

converted to the oscillation of the shaft through the DC generator. This is

used for oscillating the aluminum plate with either PZT or nanoparticle coated

PZT.

b) Function Generator: It is used for the generating the sinusoidal signal at the

required frequency. It is used for exciting the exciter at the required

frequency.

c) Oscilloscope: It is used to analyze the voltage generated during the power

harvesting experiment.

44

The experimental setup used for the power harvesting experiment is

shown in Figure 22. The step-by-step procedure for the power harvesting setup is

explained below.

Figure 22. Schematic representation of the power harvesting setup.

1) The tip of the PZT glued aluminum plate is fixed on the exciter [basically

a cantilever support].

2) Exciter is connected to the function generator through connecting the

cables.

3) Alligator clips are used as connectors to determine the voltage response of

the PZT glued aluminum plate; they are connected to the oscilloscope.

Exciter

Alligator clip

Oscilloscope

Aluminum strip

Function generator

45

3.3.2 Experimental Procedure

The function generator is used for generating the sinusoidal signal at a frequency

of 90 Hz [kept constant]. As the PZT glued aluminum strip is excited at 90 Hz, due to

piezoelectric property of the PZT, a voltage is generated which is then recorded with the

oscilloscope. Similarly, the FNP coated PZT and FNP+ZnO coated PZT are used to

analyze the importance of nanoparticle coated PZT over plain PZT as related to power

harvesting.

46

.

Chapter IV

RESULTS and DISCUSSION

4.1 Tribo-Diagnostics

Based on the experimental procedure described in Section 3.1, the friction signal

data was collected and analyzed using the tribo-diagnostic tool. The tribo-diagnostic tool

possesses high spatial resolution, spectral representation of the friction signal, and pattern

representation of the stick-slip friction. The results of the tribo-diagnostic tool on the

silver coated PZT surface are described in the following Sections.

4.1.1 Effect of Friction (Atomic Scale) on Increasing Load

Friction force at nanoscale is calculated based on the surface forces during the

sliding of cantilever tip on the silver coated PZT (P-5E) surface. There are two models

used for calculating the nanofriction, the JKR model [Johnson, Kendall and Roberts] and

DMT model [Derjaguin, Muller and Toporov] [45].

The JKR model is appropriate for strongly adhering materials with short range

surface forces. The DMT model is opposite to the JKR model. The DMT model is used

for stiff, weakly adhering materials with long range forces. Since the PZT surface is stiff

and has long range forces, the DMT model is used to predict the nanofriction at the PZT

interface [44].

47

DMT Model [44], [45]:

F= Friction Force, nN

E= Reduced young’s modulus of the tip and PZT interface, Gpa

A=Contact area, nm2

L=Load applied on the PZT surface, nN

= Adhesive force, nN

LC= Pull-off force, nN

= Interfacial shear strength, Gpa

R= Tip radius =0.002 nm

= Shear strength of the PZT (P-5E) material, Gpa

= Shear strength of the silicon nitride tip, Gpa

Friction force at nanoscale is calculated as shown in the equation (6).

………………………………… (6)

Interfacial shear strength is given in the equation 7

……………………………… (7)

Effective shear strength at the interface is given in equation 8

………………………. (8)

Contact area and pull off force is given in the equation 9 and 10

………………………. (9)

48

……………………………. . (10)

Friction force is calculated based on the following data, under the load (L) of 40nN.

=52.749Gpa

=2.110Gpa……………………………… (11)

…………………………… (12)

=1.44*10-17m2……………………… (13)

From (11) and (13), in equation (6)

F=0.3 nN [at a load of 40 nN)

The above mathematics is used for calculating the friction force at each load

(80,120,160 and 200nN) and it is depicted in Table VIII and as shown in Figure 23.

49

Table VIII. Load vs. Friction Force (nN).

Load, L (nN) Friction Force, F (nN)

40 0.3

80 0.48

120 0.63

160 0.77

200 0.91

Figure 23. Load vs. friction force (nN).

50

The interfacial friction force as a function of the load with the topographic image

after each load is represented in the Figure 23. The friction force in the region of 40 to

120 nN is non-linear relates to the surface force effects at low load or high friction at low

load. Beyond 120 nN the friction force is linear till 200 nN.

4.1.2 Specgram and Fast Fourier Transform (FFT) of Friction Signal

The friction signal data [43122 data points] from the AFM was collected in 2.47 s

at a sampling frequency of 17.4 KHz. Each sond of the force signal data is magnified to

17 sec, and totally 42 sec of data is plotted for 2.47 sec. Hence, 58 ms of friction signal

data is analyzed as one second in the specgram. Each 58 ms friction signal data is

divided into 1024 points for recognizing the friction signal temporal coherence.

At 80 nN Figure 24 (a) shows the friction signal in the random stick-slip form

obtained directly by the AFM. This friction signal is represented in the form of a

specgram as shown in Figure 24(b). The magnitude of the cantilever twisting is

represented on the “y- axis” and time (s) on the “x-axis”. The dark red region in the

specgram represents a high friction region and its FFT shows high amplitude. High

amplitude relates to high intensity of twisting of the silicon nitride cantilever tip.

51

Figure 24. a) Friction force signal collected at a time resolution of 6. 25 MHz for a load of 80nN. b) Specgram and FFT of the friction signal at 12-23 Hz for a load of 80nN.

(a) (b)

a) Load: 40nN

52

b) Load: 120nN

c) Load: 160nN

53

Figure 25. a), b) c) and d) Friction signal, specgram and FFT of the friction signal , with an increasing load 40, 120, 160 and 200 nN respectively.

Spectral representation of the friction signal shows the region of high and low

friction. It is hard to differentiate the amplitude of the friction signal variation over the

load ranging from 40 to 200 nN due to lack of spatial resolution. The FFT of this friction

signal as represented above is windowed using the Hilbert transform. This windowed

signal is used for phase fluctuation analysis to obtain better spatial resolution. The above

results are an approach towards better spatial resolution of the friction signal.

4.1.3 Effect of Stick-Slip on Increasing Load

High resolution pattern representation of the silicon nitride tip scanning on the

PZT (P-5E) surface represents the stick-slip behavior of the interfacial friction. Tip

velocity is 199 nm/s; the distance traveled in 58 ms (1024 points) is 11.61 nm. Hence,

100 points represents 11.3 Å. This is represented in the stick-slip chart shown in Figure

d) Load: 200nN

54

26. As the tip is scanned over the PZT surface, slip/atomic displacement or acoustic noise

(due to atomic displacement) is observed. This is represented as a deviation of the

aligned phase fluctuation from zero or the deviation from the uniformity of tip twisting

during the tip sliding on the PZT surface. The region before this fist slip is considered as

the elastic deformation regime. The downward arrows represent the qualitative relation

between the slip and zero magnitude. The pattern as shown in the Figure 26 is a precise

observation of the positional behavior of the tip on the PZT.

Figure 26. The phase and magnitude representation of stick-slip and elastic regime before first slip at a load of 40 nN.

Elastic Regime before first slip

Slip/Atomic Defect

11. 3Å

55

a)

b)

43Å

51Å

56

c)

d)

Figure 27. a), b), c) and d): Represents the stick-slip variation and the elastic regime before first slip due to the increase in load from 80 to 200 nN respectively.

108Å

188Å

57

The pattern described in this Section shows the time and length of the first slip.

There is an increase in the early elastic regime due to increased load ranging from the 40

to 200 nN. The relationship between the elastic deformation regimes with respect to

increasing load is represented in the Section 4.1.5.

4.1.4 Hilbert Transform

The friction signal data was obtained from the AFM at a sampling frequency of

17.4 KHz. Windowing of this friction signal data is carried out using Hilbert transform at

a frequency range of 12-23 Hz. Frequency of the friction signal data is varied as a

function of the phase of the signal and provides high spectral representation. Windowed

friction signal data represents the continuous sliding and random stick-slip as represented

in the Figure 28. For 58 ms (11.61 nm) of friction signal data, the tip has scanned a

length of 11.61 nm on the silver coated PZT. In this 11.61 nm of distance covered; 11 13

Å of the sliding has continuous sliding and 91 Å of sliding has random stick-slip behavior

as shown in Figure 28. Continuous sliding and random stick-slip represents the region of

low and high friction respectively.

58

Figure 28. Continuous sliding and random stick-slip at a load of 40 nN and a band pass frequency of 12-23 Hz.

4.1.5 Early Elastic Deformation Regime at Atomic Scale

The length of early elastic deformation regime as shown in Figure 26 and Figure 27 is

calculated and plotted with the increasing load. Cubic relation between the elastic

deformation regime and load due to curve fitting is shown in the Figure 28.

If y = early elastic regime in Å

X = load in nN

y = 6.4e-005*x3 - 0.017*x2 + 2*x - 47

Stick

Slip

Continuous Sliding Random Stick-Slip

59

Figure 28. Elastic regime vs. load.

4.1.6 Effect of Interfacial Energy on Increasing Load

During scanning, magnitude of the dissipated or interfacial energy (ET) and the

energy for plastic deformation (Ep) is calculated by the equation (14. a.) and (14. b.)

= Tangential force applied on the surface, nN

L= Length of scratch, nm = 50nm

N= Number of scratches= 1024 lines

H= Hardness of the PZT surface, Gpa

Vwear= Volume of the wear, nm3

…………………………… (14. a. )

…………………………… (14. b. )

The linear relationship between the interfacial energy and load is given in the

Figure 29.

60

Table IX. Variation of interfacial energy with respect to increasing load.

Load( ), nN Interfacial Energy( ),

40 2. 048

80 4. 096

120 6. 144

160 8. 192

200 10. 24

Figure 29. Load (nN) vs. interfacial energy (10-12 J).

If, y= interfacial energy (10-12 J) and x= load in nN

From Figure 29,

y = 0. 051*x - 1. 5e-015

61

4.2 Coating

The methods of FNP coating, FNP+ZnO mixing and coating was described

earlier. Results on the geometric structure of the FNP cones and verification of the ZnO

nanoparticles in the FNP cones are described below.

4.2.1 Pattern of Ferrous Cones at Macro Scale

The magnetic field strength is higher than surface energy of the ferrofluid and

hence, the ferrofluid with ferrous nanoparticles form cones along the applied magnetic

field. The ferrous cones are at equidistance and form an array as shown in Figures 30 and

32.

Figure 30. Ferrous cones grown on a glass slide.

62

Figure 31. Array of ferrous cones.

The array of ferrous cones at the macro scale is the magnified image of the

ferrous cones at nanoscale. These ferrous cones act as a low source of electron and

contribute to the flow of charges on the PZT surface. The significance of this equidistant

array in power harvesting has yet to be determined.

4.2.2 Presence of ZnO in the Ferrous Cones.

In Section 3.2.1 the method used in mixing of the ZnO in ferrofluid is explained.

Fluroscene microscope (FM) is used to verify the presence of ZnO nanoparticles in the

ferrous cones. The FNP cones and (FNP+ZnO) cones were exposed to ultraviolet rays

for 20 min. The ferrous nanoparticle cones with the ZnO nanoparticles are shown in the

Figure 32; the tip of the ferrous cones acts as the region of charge concentration and a

low source of electrons. Figure 33 shows the emission of light from the (FNP+ZnO)

cones due to the distribution of the ZnO nanoparticles on the surface. FNP cones after 20

63

min of UV rays exposure, produced a dark picture. Qualitative analysis showed presence

of the ZnO nanoparticles in FNP cones due to the mixing procedure followed in Section

3.2.1.

Figure 32. Array of ferrous nanoparticle and ZnO nanoparticle mixed cones.

Figure 33. Shows the light emitted by the (FNP+ZnO) cones exposed to an UV light of wavelength 130 nm.

64

The intention of the ZnO coating on the FNP was to produce more charges due to

the piezoelectric property and thus control the flow of charges in the FNP cones due to its

semiconducting ability.

4.3 Power Harvesting

The above Section explains the advantages of the FNP coating and ZnO+FNP

coating in the field of charge conduction. In this Section we analyze the advantages of

these coating on the PZT in the field of power harvesting where the magnetic and

piezoelectric property is combined for increasing the electromechanical efficiency of the

PZT.

4.3.1 Effect of PZT

When the PZT glued aluminum was excited at 90 Hz, the voltage generated was

51.1 mV. The magnitude of the voltage at a frequency 90 Hz is shown in Figure 34.

65

Figure 34. The magnitude of the voltage (51. 1 mV) at an excitation frequency of 90 Hz for a plain PZT.

4. 3. 2 Effect of the FNP Coated PZT

FNP coated PZT glued aluminum was excited at 90 Hz and the voltage generated

was 115.4341 mV. This is due to the additional electron source on the surface and may

be due to the magnetoelectric effect. The magnitude of the voltage at a frequency 90 Hz

is shown in Figure 35.

66

Figure 35. The magnitude of the voltage (115.1 mV) at an excitation frequency of 90 Hz for a FNP coated PZT.

4.3.3 Effect of the (FNP+ZnO) Coated PZT

When the (FNP+ZnO) coated PZT glued aluminum was excited at 90 Hz, the

voltage generated was 366.8 mV. This is due to the additional piezoelectric effect on the

PZT surface by the ZnO in the ferrofluid coating. ZnO also acts as an additional source

of electrons, due to its ability to emit charges at room temperature. The magnitude of the

voltage at a frequency 90 Hz is shown in Figure 36.

67

Figure 36. Represents the magnitude of the voltage (366.8 mV) at an excitation frequency of 90 Hz for a FNP +ZnO coated PZT.

All the above results are plotted in the Figure 37. It shows the increase in voltage

response of the PZT due to the nanoparticle coatings. This increase is due to the

magnetoelectric effect or increase in the capacitance of the silver coated PZT (P-5E) due

to the nanoparticle coating on the surface.

050

100150200250300350400

Voltage response

(mV)

1 2 3

1=Plain PZT; 2=FNP coated PZT; 3=FNP+ZnO coated PZT

Voltage response vs. coatings on the PZT

Figure 37. Voltage response vs. coatings on the PZT.

68

CHAPTER V

CONCLUSIONS

1. The importance of High Speed Data Capture (HSDC) in the AFM was

explored, to obtain unprecedented time resolution of the friction signal at

6. 25 M Hz. Developed a tribo-diagnostic tool using HSDC and phase

fluctuation based processor. Tribo-Diagnostic tool determines stick-slip

behavior at the cantilever tip and PZT interface.

2. Specgram representation of friction signal at nanoscale identifies the

region of high and low intensity of friction with respect to time. There

were no substantial changes observed due to increase in the load form 40

to 200 nN.

3. At low load the friction on the silver coated PZT is high to the surface

force effects.

4. Relationship between Interfacial energy and load shows an exponential

relationship.

5. Windowing of the friction signal at 12-23 Hz, using Hilbert Transform

depicts the region of smooth sliding and random stick-slip. This is

important information to improvise the sliding application of MEMS.

6. Tribo-diagnostic tool used for analyzing stick-slip provides the following

information:

69

Atomic scale stick-slip transition events on the silver coated PZT

(P-5E).

The phase fluctuation pattern in Figure 26, determines the possible

defects at atomic scale, during the tip sliding on the silver coated

PZT surface.

Cubic relationship between the early elastic deformation/first slip

event with increase in the load. The reason for this behavior has

yet to be analysed.

7. ZnO and FNP coating possess semiconducting, piezoelectric and magnetic

property. Authentication of the ZnO in the FNP is carried under the

Florescence Microscope.

8. An equidistant array of ferrous cones and ferrous cones with ZnO coating

has a potential application in the field of low source electrons, surface

texturing and scanning tips.

9. Power harvesting experiment was conducted to determine the importance

of the FNP coating and (FNP+ZnO) coating on the PZT. Results, show

that (FNP+ZnO) coating on the PZT and FNP coated PZT has better

voltage response compared to plain PZT.

70

CHAPTER VI

SCOPE FOR FUTURE RESEARCH

1. Piezoforce microscopy study of the PZT and nanoparticulate coated PZT

to identify the polarized regions over a given area. This will give the

qualitative and quantitative differences in the dipole regions of PZT and

nanoparticulate coated PZT, and eventually, answering the advantages of

the nanoparticulate coated PZT.

2. The developed tribo-diagnostic tool can be used for:

Analyzing the surface defects during the sliding of the slider on

the FNP Coated alumina substrate in hard disc

Study of defects, during sliding applications of MEMS.

Defects at the sliding interface of nanotubes, nanocoating can

be analysed for identifying the origin of the fracture.

3. Experiments have to be conducted to study the hysterisis behavior of the

FNP coated PZT and analyse the effect of remanent polarization on the

plain PZT and FNP coated PZT. Studies on the change of crystal

structure, dielectric constant and electromagnetic coupling with respect to

interfacial energy and friction changes on the PZT surface is yet to be

conducted.

71

REFERENCES

1) MichaelKapps,http://isdc2.xisp.net/~kmiller/isdc_archive/fileDownload. php/?

link=fileSelect&file_id=295 , accessed on 02/02/06.

2) Murata product catalog, CAT no. P37E-18, 2004.

3) N. Elvin, A. Elvin and D. H. Choi, “ A self- powered damage detection sensor”,

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Vita

Mr. Deepak Halenahally Veeregowda, son of Mr. Veeregowda and Mrs.

Puttagangamma was born in Bangalore, India. He did his B.E. in Mechanical

Engineering from the Visweswaraiah Technological University, Belgaum, India.

After graduation he worked as a research fellow in LRDE, Defense Research

Development Organization, Bangalore, India. In January 2006 he joined the

University of Mississippi as a graduate student in the Department of Mechanical

Engineering. He was invited as a visiting scholar in the Centre for Surface

Engineering and Tribology, Northwestern University. He graduated with a Master of

Science Degree in Mechanical Engineering in December 2007.

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