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LASER INDUCED DIIIGE II LITEIUHIFDUGRIDE
SINGLE CRYSIALS

BY
NLOUPEL SOUGUIR

.A THESIS

Subnitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER.OP SCIENCE

Department of Metallurgy, Mechanics
and Materials Science

1990

655 —-— .9 2»;

ABSTRACT

LASER INDUCED DNHNGE IN LITHIUM FLUORIDE
SINGLE CRYSTALS

BY

NAOUFEL SOUGUIR

A (:02 laser with radiation wavelength A=10.6um was used
to laser-irradiate lithium fluoride single crystals on their
(001) faces. The effects of changing the laser power, the
laser pulse length, the distance from the laser focus to the
sample surface, and the sample thickness were investigated.
The samples were then etched using a dilute solution of
ferric chloride in order to reveal the slip distribution
induced by thermal stresses.

Optical and scanning electron microscopy were used to
reveal the laser-induced damage on the surface and inside
the bulk. The results proved the existence of a laser-
induced damaged zone (D.Z) on the surface. This zone
included areas of (1) plastic deformation, (2) melting, and
(3) vaporization. A ripple pattern in the shape of a helix
was observed inside the molten zone. Primary and secondary
cleavage cracks were also observed. The length of each type
of cleavage cracks depended mainly on the energy density of
the treatment. The size and the distribution of the damage
was found to be dependent on the extent of the different

laser variables and the sample thickness.

The slip distributions inside and outside the D.z were
different due to the different directions of the thermal

stresses.

I dedicate this thesis to my mother whose

sacrifices I will always remember.

ii

ACKNOWLEDGEMENTS

I would like to thank my adviser Professor K. N.
Subramanian for his substantial help and advice throughout
the project. I would also like to thank Dr P. Khan and M.
Cherif for their assistance on the CO2 Laser.

I would also like to express my gratitude to R. Vaidya
for his great assisstance in using the equipments I needed
for this project.

Finally, I'm grateful to Professors K. Mukherjee and E.
Case for their valuable suggestions towards the end of this

project.

iii

TABLE OF CONTENTS
PAGE
List of Tables ........ ..... ... ......... .............. vii

List Of Figures ............OOIOOOOOOOOOI...... ..... ... Viii

I. INTRODUCTION ..................................... 1
Thermal Properties of Ceramics ................... 5
1. Heat Capacity ................................. 5
2. Heat Conduction ........................ ....... 6
Thermal Stresses Due to Rapid Heating on the
Surface. ......................................... 6

Fracture Behavior of Lithium Fluoride. ........... 13
II. EXPERIMENTAL APPARATUS ........................... 19
IIIOEXPERIMENTAL PROCEDURE ......OOOOOOOOOOOOOOO...... 22

VI. RESULTS AND DISCUSSION ........................... 24
A. Laser Damage as a Function of Laser Power. .... 24
1) Treatment.1 ................................... 24
2) Treatment.2 ................................... 33

3) Treatment.3 ................................... 34
B. Laser Damage as a Fuction of Laser Pulse Length. 44

1) Treatment04 ......OOOOOOOOOOOOOOOOOO0.0.0000... 44

2) TreatmentOS ......OOOOOOOOOOOOO00.0.00......... 51

iv

3)

1)
2)
3)

4)

Treatment06 0.0.0.0...00...........OOOOOOOOOOOO 54

Laser Damage as a Function of Change in the distance
from the laser focal point to the surface. .... 60
Treatment.7 ................................... 60
Treatment.8 ............................ ....... 61
Treatment.9 ................................... 64

Treatment.10 ...... ........ ..................... 69

Important Observations Related to Experiment sets

A:

B, and C. ................................... .. 72

Analysis of the Helix Ripple Pattern (H.R.P). .... 78

1)
2)

3)

Slip Due to Laser Heating. .................... 85

Effect of Sample Thickness on Laser Damage

Propagation. .................................. 102
Treatment.11 .................................. 103
Treatment.12 .................................. 103

Treatment.13 ............. ..................... 103

CONCLUSIONS. ............OOOOOOOOOOOOOOOOOO0...... 111

VI. RECOMNDATIONS. ......OOOOOOOOOOOOOOOOO00.0.00... 114

APPENDIX A
Some Thermal Properties of LiF Single Crystals
APPENDIX B

Infrared Absorption Edges of Ionic Crystals

VIIOREFERENCES. ..... .... ...... ......OOOOO... .....

Vi

Table.1

Table.2

Table.3

Table.4

LIST OF TABLES

PAGE
Summary of the Results Obtained for Increasing
the Laser Power from Treatment.1 to
Treatment.3. 000.000.0000 ....... 0.0.0.000... 46
Summary of the Results Obtained for Increasing
the Laser Pulse Length from Treatment.4 to
Treatment06. .000.0.0.0...00......00000...00 59
Summary of the Results Obtained for Increasing
the Distance from Focus from Treatment.8 to
Treatment.10. 00.0.0.0...0...000000000000... 71

Laser Damage Dependency on Sample Thickness. 106

vii

LIST OF FIGURES
PAGE

Figure.1 Schematic Comparison of Laser Generated and
Explosively Generated shock waves in a solid.
(From [25]). 00.00.00000.00.0000000000000000 11

Figure.2 Crystal Structure of Lithium Fluoride.
(From [29]). 00......0000000000000.000.0000. 14

Figure.3 Projection of (110) Slip Planes onto (001)
Surface of Lithium Fluoride Crystal. (From

[30]). 15

Figure.4 Schematic Representation of Main Components of
the Laser: (1) is the Laser Control Too, (2)
is the H.F, (3) is the Beam Generator, and (4)
is the Cooling Unit. ........................ 20

Figure.5 ch Components: (i) is the Operating Panel,
(ii) is a Modular Microcomputer Card
System, and (iii) is the Wiring Board. ...... 21

Figure.6 Schematic Showing the Laser Damage for
Treatment.1, (Powgr - 190 Watts and
Fluence - 2.63x10 Joules/cm ). ............ 25

Figure.7 Optical Micrograph Showing the D.z for
Treatment.1. 000.0...00.00000000000000000000 26

Figure.8 S.E.M Micrograph Showing the 0.2 for
Treatment.1. 000.000.00.000...000.00.000.00. 27

Figure.9 S.E.M micrograph Showing the Crater Area for
Treatment.1. 00.000.000.000...000.000.0000.. 28

Figure.10 S.E.M Micrograph Showing an Evaporation Site
on the surface for Treatment.1. ........... 28

Figure.11 S.E.M Micrograph Showing the Edge of the D.z
for Treatment.1. 0.0.000...000000....00.0.00 3o

Figure.12 S.E.M Micrograph Showing Cracks Bounding
the DJ for Treatment.1. 30

Figure.13 S.E.M Micrograph Showing the Edge of the D.z
for Treatment.1. 31

Figure.14 S.E.M Micrograph Showing the Edge of the 0.2
Noted by the star in Figure.8. ............ 31

viii

Figure.15

Figure.16
Figure.17

Figure.18
Figure.19
Figure.20

Figure.21

Figure.22
Figure.23
Figure.24
Figure.25
Figure.26

Figure.27

Figure.28

Figure.29

Laser Damage Observed Inside the Bulk of the
LiF Single Crystal for Treatment.1. ........ 32

S.E.M Micrograph Showing the Laser Damage
for Treatment.2, (Power - 163 Watts and
Fluence = 2.26x10 Joules/cm ). ........... 35

S.E.M Micrograph Showing a Corner of the 0.2
for treatment.2. (This is Marked by (M) in
Figure.16). ......00.00.000.000000000000... 36

S.E.M Micrograph Showing a Crack Bounding the
D02 for Treat‘ent020 000.000.000.000000000. 37
Optical Micrograph Showing the 0.2 for
Treatment.2. 0.000.000..........0..00.0.00.. 38
Optical Micrograph Showing the Damage Inside
the Bulk for Treatment.2. ................. 38
Optical Micrograph Showing the Laser Damage
for Treatment.3, (Power - 123 Watts and
Fluence - 1.73x10 Joules/cm ). ........... 40
S.E.M Micrograph Showing the Laser Damage for
Treatment.3. ....0....000.00......00.0.0... 41
S.E.M Micrograph Showing Cracks Bounding the
D.z for Treatment.3. 0.000000...0.0..00.00. 41
S.E.M Micrograph Showing the Boundary between
the 0.2 and the H.A.z for Treatment.3. ..... 42

S.E.M Micrograph Showing the Boundary between
the 0.2 and the H.A.z for Treatment.3. ..... 42

Optical Micrograph Showing the Laser Damage

as seen Inside the Bulk for Treatment.3. 43

Schematic of Laser Damage as a Function of
Laser Power. (a) Shape of the D.z for
Treatment.1, (b) Shape of the D.z for
Treatment.2, and (c) Shape of the D.z for
Treatment.3. .............................. 45

S.E.M Micrograph Showing the Laser Damage
for Treatment.4, (Pulse Length - 540 ms and
Fluence - 2.64x10 Joules/cm ). ............ 48

Optical Micrograph Showing the Laser Damage
in the Crater Area for Treatment.4. ........ 48

ix

Figure.30
Figure.31
Figure.32
Figure.33

Figure.34

Figure.35
Figure.36

Figure.37

Figure.38
Figure.39

Figure.40

Figure.41

Figure.42

Figure.43

S.E.M Micrograph Showing Recrystallization in
the Crater Area for Treatment.4. ........... 49

Optical Micrograph Showing the Laser Damage
Inside the Bulk for Treatment.4. .......... 49

Optical Micrograph Showing the Laser Damage
Inside the Bulk for Treatment.4. .......... 50

Laser Damage for Treatment.4 as seen from the
“CR Of the sample. 000.00.000.00000000000. 50
Optical Micrograph Showing the Laser Damage

for Treatment.5, (Pulse length a 460 ms and
Fluence = 2.25x10 Joules/cm ). ........... 52
S.E.M Micrograph Showing the D.z for
Treatment050 0......00000...0.00.00.00.0000 53
Optical Micrograph Showing the D.Z for
Treatment05. 0.0000000.00000000....00000000 53
S.E.M Micrograph Showing the Laser Damage for
Treatment.6, (Pulse Length 82380 ms and

Fluence = 1.85x10 Joules/cm ). ..........z 55

Optical Micrograph Showing the D.z for
Treatment.6. ......................0....... 55
Optical Micrograph Showing the Laser Damage
Inside the Bulk for Treatment.6. . ......... 57
Schematic of the Laser Damage as a Function

of the Pulse Length. (a) Shape of the D.z
for Treatment.4, (b) Shape of the D.z for
Treatment.5, and (c) Shape of the D.z for
Treatment.6. .............................. 58
S.E.M Micrograph Showing the D.z for
Treatment.7, (Pulse Length = 400 ms, Distapce
from focu - 6.9 mm, and fluence = 2.76x10
Joules/cm ). ............................... 62

Optical Micrograph Showing The D.z for
Treatment.7. ......0...0.0.0000.00.0...000. 63
Optical Micrograph Showing the D.z for
Treatment.7. Different Lightening than for
Figur8042. .........0.....0...0.00....00.00 63

Figure.44

Figure.45

Figure.46

Figure.47

Figure.48

Figure.49

Figure.50

Figure.51

Figure.52

Figure.53

Figure.54

Figure.55

Figure.56

Figure.57

Optical Micrograph Showing the Laser Damage
for Treatment.8, (Distance from Focgs - 6.9
mm and Fluence a 2.74x10 Joules/cm ). ..... 65

Schematic Showing the Laser Damage for
Treatment.8. ..................0... ..... ... 66
Optical Micrograph Showing the Laser Damage

Inside the Bulk for Treatment.8. .......... 67

Optical Micrograph Showing the Laser Damage
Inside the Bulk for Treatment.9, (Distance
from Focu - 8.9 mm and Fluence - 2.36x10
Joules/cm ). .............................. 67
S.E.M Micrograph Showing Radial Striations as
Seen Around the Center of the Crater for

Treatment.9. .................... . ......... 68
Magnified View of Region (X) Shown in
Figure.48. .......... . ..................... 68

Optical Micrograph ShOwing the Laser Damage
on the Surface and Inside the Bulk for
Treatment.10, (Distanfe from Focgs - 14.2 mm

and Fluence a 1.80x10 Joules/cm ). 70

S.E.M Micrograph Showing the Area at the
Boundary of the Helix Ripple Pattern for

Treatment.10. 73

Optical Micrograph Showing the Bulk Damage
for Treatment.8 as Observed on the (100)
Plane. (Specimen Cleaved to Reveal the
Cross-section of the D.Z). ................
Plot of Laser Energy Density vs. D.z Width.
D.F.V a Distance from Focus is the Variable,
P.L.V - Pulse Length is the Variable, and

P.V - Power is the Variable. 76

Schematic Showing the Different Damage
Regions in Laser Irradiated LiF Single
Crystals.

S.E.M Micrograph Showing the H.R.P for
Treatment.3.

S.E.M Micrograph Showing the H.R.P for
Treatment.7.

................... ........... 79

Magnified View of the H.R.P in Figure.56. .. 80

xi

Figure.58

Figure.59

Figure.60

Figure.61

Figure.62

Figure.63

Figure.64

Figure.65

Figure.66

Figure.6?

Figure.68

Figure.69

Figure.70

Figure.7l

Figure.72

Magnified View of the Area Inside the Square
in Figure.57. .......... ....... . .......... .. 80
Magnified View of the Region Described by the

Square in Figure.58. 81

S.E.M Micrograph Showing the H.R.P for
Treatment.9. ....‘0...0...0....00....000000 81
S.E.M Micrograph Showing the H.R.P for

Treatment.10.

Magnified View
in Figure.61.

of the Area Inside the Square

...... ....................... 82

of the Area Inside the Circle

............................. 83

Magnified View
in Figure.61.

Magnified View
in Figure.63.

of the Area Inside the Square

..........00................. 83

Schematic Showing the Cross-section View of a
Ripple in the H.R.P for Treatment.10 as Shown
in Figure.640 0....00.00.000.0000000000.... 84
Optical Micrograph Showing the Dislocation
Etch Pit Pattern Outside the D.z for

Treatment.1. 86

Optical Micrograph Showing the Dislocation
Etch Pit Pattern Outside the D.z for
Treatment.2. .0............0.......00......
Optical Micrograph Showing the Dislocation
Etch Pit Pattern Outside the D.z for

Treatment.3. 87

Optical Micrograph Showing a Magnified View

of the Area Marked by (a) in Figure.68. 87

Optical Micrograph Showing a Magnified View

of the Area Marked by (b) in Figure.69. 88

S.E.M Micrograph Showing 900 (110) Slip
Lines Just Outside the D.z for Treatment.2.
(Region Above the D.z in Figure.70). ......
Optical Micrograph Showing the Dislocation
Etch Pit Pattern Outside the D.z for

Treatment . 7 . 8 9

xii

Figure.73

Figure.74

Figure.75

Figure.76

Figure.77

Figure.78

Figure.79

Figure.80

Figure.81

Figure.82

Figure.83

Figure.84

Figure.85

Schematic Representation Showing the
Dislocation Etch Pit Pattern Outside the D.z

for Treatment.7. 90

Cylindrical Coordinate System for the Thermal
Stress Field Outside the Laser Induced D.z on

the (001) Face of LiF Single Crystals. .... 94
Optical Micrograph Showing the Dislocation

Etch Pit Pattern Inside the D.Z for
Treatment.9. (Top Half of the D.z in
Figure.47). .............................0. 95
Optical Micrograph Showing the Dislocation

Etch Pit Pattern Inside the D.z for
Treatment.9. (Bottom Half of the 0.2 in
Figure.47). ............................0.”. 95
Optical Micrograph Showing a Magnified View

of the Area Marked by (c) in Figure.75. ... 96
Optical Micrograph Showing the Slip Lines in
Regions Adjacent to a Crack Inside the D.z

for Treatment.5. .......... ...... ..... ..... 96
Optical Micrograph Showing a Magnified View

of the Slip Lines Adjacent to the Crack in
Figure.78. ..... ........... ..... ..... ...... 97
Optical Micrograph Showing the Dislocation

Etch Pit Pattern Inside the D.z for
Treatment.3. ....... ................0.0.0.0 97
S.E.M Micrograph Showing Slip Lines in the
Area Between the Crater and the Edge of the

D.z for Treatment.1. .... .................. 98
Schematic Representation of the Slip Line
Distribution as Observed Inside the D.z for
Treatment03 00............0.....0...0.0.000 99
Schematic Representation of Slip Lines

Induced by (110) Cracks on the (001) Surface

of LiF Single Crystals. ... ....... . ...... .. 100

Optical Micrograph Showing Slip Lines induced

by (110) craCkSe .........0....0...00...000 101
Optical Micrograph Showing the Laser Damage

for Treatment.11, (Sample Thickness a 3.8 mm

and Fluence = 2.87x10 Joules/cm ). ........ 104

xiii

Figure.86

Figure.8?

Figure.88
Figure.89

Figure.9o

Figure.91

Optical Micrograph Showing the Laser Damage
for Treatment.12, (Sapple Thickness - 2.8 mm
and Fluence - 2.87x10 Joules/cm ). ........ 104

Optical Micrograph Showing the Laser Damage
for Treatment.13, (Sample Thicknsss = 1.6 mm ‘
and Fluence - 2.87x10 Joules/cm ). ....... 105

Plot of Sample Thickness vs. Maximum Primary
Cleavage Crack Length. .................... 108

Plot of Sample Thickness vs. Maximum Secondary
Cleavage Crack Length. .................... 109

Plot of Sample Thickness vs. D.z Width. ... 110

Plot of Absorption Coefficient vs. radiation
wavelength. (From [11]). .......... Appendix B

xiv

INTRODUCTION

Lasers, discovered two decades ago are used extensively
in today's technology particularly in metallurgy and
materials science. A laser is a device that uses the
principle of amplification of electromagnetic waves by
stimulated emission of radiation and operates in infrared,
visible or the ultraviolet region.

Laser output is powerful because atoms can be
stimulated to emit electromagnetic radiation much faster
than they would spontaneously. The laser output consists of
a nearly monochromatic, high-intensity, coherent wave [1].
Coherent light has a single frequency or one color [2].

Pulse-lasers generate high power (the shorter the pulse
is, the higher the output power of the laser) with plane
wave radiation. The focused beam from pulsed lasers can
generate energy densities at which all materials melt and
vaporise. It is possible to drill holes or to weld
materials with a laser beam, the precision being very
high [3].

An electromagnetic wave in contact with a solid material
could be reflected, scattered, absorbed, transmitted or a
combination of these phenomena, depending on the material's
structure and its electrical properties.

Different materials absorb electromagnetic radiations

in different ranges of wavelengths and are said to have

2

different absorption bands. Absorption refers to the
decrease of intensity of electromagnetic waves as it passes
through a substance [4], and is represented by an absorption
coefficient (a) which depends on the wavelength of the
incident wave interacting with the material. Scattering
also contributes to the decrease of intensity of waves
transmitted through a material. Therefore (a) for a general
case can be written as:

a - ea + as,
where (an) is due to true absorption and (as) is due to
scattering.

Absorption bands also depend on the electrical
conductivity of the solid or liquid. Good electrical
conductors like metals exhibit general absorption with a
minor dependence on wavelength in most cases: but dielectric
materials which are poor electric conductors have selective
absorption and are said to have absorption bands. However,
one material could have more than one absorption band in a
spectral region [4]. A

There is however a relation between reflection and
absorption; so high reflection of light usually means high
absorption and both have the same wavelength bands [4].

Many ceramic materials allow penetration of
electromagnetic radiation without appreciable attenuation.

The velocity (v) of propagation of a wave is reduced

slightly by the losses in the medium: it is defined as [5]:

3

v” l/JETi
where e is the dielectric constant and u is the permeability
of the medium. Such substances must be electrical
nonconductors since conduction is related to the appearance
of Joule heat [6,7]. This is an irreversible phenomena in
which the electromagnetic energy is destroyed or more
precisely transformed into heat and in consequence an
electromagnetic wave in a conductor is attenuated. In
metals, unlike in many ceramic materials, the electrical
conductivity is very high and the effect described above is
so large that they are practically opaque [8].

Scattered light from the surface can arise from various
sources: (1) Defects, whose dimensions are large compared
to the wavelength of light. (2) Isolated defects in which
dimensions are comparable with the wavelength of light. (3)
Micro-irregularities in which heights are small compared to
the wavelength of light: these produce scattering.

The prevalent source of scattering from the surface are
the micro-irregularities because all surfaces have them from
the finishing process. These micro-irregularities are
distributed over the entire surface and hence contribute a
larger fraction of the scattering, particularly in the
visible and ultraviolet, than do isolated surface defects
such as scratches, digs, and dust particles.

For smooth but wavy surfaces the scattered light is

maximum at about the specular direction (normal to the

4

surface): rougher or matte surfaces scatter the light more
equally in all directions. Surface irregularity is
specified in terms of the rms roughness. The rms roughness
(6) of a surface is the root-mean square of the heights of
irregularities on the surface. Its unit is usually (A).
The rms roughness determines the total integrated
scattering [9].

The fraction of the electromagnetic energy that is not
scattered, reflected or transmitted is absorbed by the
material and causes its thermal energy to rise.

In general, thermal energy in a solid is aquired by:

1) Vibrational energy, by which atoms vibrate around

their lattice positions with an amplitude and
frequency that depend on the temperature untii the
Debye temperature is exceeded: then they vibrate at
a relatively constant frequency, called the Debye
frequency. This vibration of the atoms around their
equilibrium positions (or lattice vibrations)
increases the energy of the crystal. One can
consider the vibrational spectrum in terms of
excitation of phonons. When (p) phonons in a given
lattice are excited, the additional energy of the
crystal is (phw), where (h) is plank's constant and
(w) is the phonon frequency[10]. .

2) Rotational energy for crystals having rotational

degrees of freedom.

5

3) The energy level of electrons.

4) Changing atomic positions by forming vacancies,
interstitials, magnetic orientation or altering the
structure of glasses at the transformation range [11].

For ceramic materials, at low temperatures, the lattice

vibrations or phonon conductivity is the major factor for
the transportation of thermal energy. However, at higher
temperatures, the formation of vacancies, interstitials,
magnetic disorder and electronic effects contribute to the
transportation of heat. The contribution from such defects

depends mainly on the structure.

mummies
Heat capacity and thermal conductivity determine the

rate of temperature change in a ceramic. They are fundamen-

tal in approximating the resistance to thermal stresses.

1. Heat Capacity

Heat capacity measures the energy required to raise the
temperature of a material and is either defined at constant
pressure or constant volume. C and CV, the heat capacity

P
at constant pressure and volume respectively are given by:

. o
Cp (dQ/dT)p (dH/dT)p in cal/mole C, (1)

0
CV = (dQ/dT)v II-(dU/dT)‘, in cal/mole C, (2),

- 2
and Cp Cv a VOT/p (3)

6

where (Q) is the:peat exchanged, (U) is the internal energy,
(H) is the enthalpy, a-dv/vdT is the volume thermal
expansion coefficient, fi=-dv/vdp is the compressibility, v
is the volume, and V0 is the molar volume. At temperatures
above a few hundred degrees centigrade, the heat capacity

varies very slowly and linearly with temperature.

2. Heat conduction
For heat conduction the determining equation is:
dQ/dt 2 KA grad T (4)
The amount of heat flowing normal to the area (A) in time
(dt) is (dQ). The temperature gradient (grad T) measures
the change in temperature as a function of position and (K)

is the thermal conductivity [11].

 

High local thermal expansion can be due to rapid

heating at the surface. The rapid heating can be provided
by lasers. The sudden rise of the surface temperature to
values far above that of the surrounding material results in
relaxation of compressive surface stresses. Subsequent
rapid cooling of the surface by self-quenching results in
tensile stresses and can cause the formation and propagation
of cracks [12].

Self-quenching is a mechanism that occurs if sufficient

laser power is available (generally more than 7.5 kw), so

7

that the surface heating is very rapid and the core remains

cool. The cold core then quenches the heated surface [13].

6 1

Typical quench rates in metals are of the order of 10 °Csec'

although the use of Q-switched lasers has recently extended

1° ”Csec"1 range [14].

the quench rates to the 10

Highly localized thermal expansion due to a rapid
heating pulse at the surface causes a stress wave to
propagate into the interior of the solid. The magnitude of
the peak stress depends on the laser pulse duration.
Cracking and fracture could occur even in a very ductile
metal with a laser pulse lasting few nanoseconds [15].

The formation of high thermal stresses during laser
processing is not desirable because it can cause micro-
cracking and a decay in the strength of the material. The
strength retained after shocking depends on the length of
the cracks present within the shocked material [16].

High-intensity laser beams (109watts/cm2) pulsed on the
surface induce pressures on the surface. The pressures
propagate into the bulk as high amplitude stress waves.
Microstuctural examination of the laser-shocked surface
shows a plastically deformed metal with a high dislocation
density and residual stess. An increase in the hardness and
strength are also registered [17].

Merkle and Bass measured the multiple pulse laser-
induced bulk damage in crystalline and fused quartz at

wavelengths of 1.064 and 0.532 micrometer [18]. They

reported that the single pulse damage threshold varied
insignificantly among materials or with wavelength. The
multipulse induced damage depended strongly upon the
material and the laser wavelength. The number of pulses
needed to produce macroscopic damage as a function of
intensity is indicative of an accumulation process in which
some minimum intensity is required to initiate the
accumulation of microscopic damage which leads to
macroscopic damage.

Musual [19] analysed the stress-strain behavior of the
near surface region of laser treated copper mirrors. They
suggested that the elastic response is maintained below a
certain surface temperature rise (JTY), which depends on the
yield stress, modulus of elasticity, poisson's ratio, and
the coefficient of thermal expansion of the metal. The
value of STY was determined to be:

. GTY z (l-u)Y/Ea (5)
where (u) is poisson's ratio, (Y) is the yield stress,
(E) is Young's modulus, and (a) is the coefficient of
thermal expansion [19,20,21].

For a temperature rise 6T above 6Ty, plastic strain
accumulates after the first single laser pulse. Musual
developed a model to show that plastic strain accumulates
during each successive laser pulse and is proportional to
the temperature rise above 26T . This plastic strain

Y
induces the formation of slip bands and intergranular slip

at the surface [19]. The slip lines are produced along
favorable crystallographic orientations [21]. These
crystallographic orientations have maximum resolved shear
stress and are inclined at 45 degrees with respect to the
direction of applied stress [19,23].

Musual also developed an expression for the pulse
fluence threshold (FY) for first-pulse plastic yield. Fy is
a function of the optical absorptance, pulse fluence, pulse
duration, thermal conductivity, density, and specific heat.
The threshold for multipulse accumulation of plastic
deformation is 2Fy.

Musual explained the thermomechanical stress induced by
lasers on the surface of a metal. He suggested that as a
result of the very rapid temperature rise on the surface,
the metal tends to expand but is stopped by ”inertial
forces" or stesses. The stresses are relieved very fast in
the direction normal to the free surface due to outward
displacement of the surface. However, strains in directions
parallel to the free surface are blocked by the surrounding
material. This can induce very large compressive stresses
in these directions. When the temperature rise and the
coefficient of thermal expansion reach a high enough level,
the compressive stresses can exceed the elastic limit of the
metlal. The metal is then plastically deformed in

compression. The compressive plastic strain, in return,

induces tensile stresses that occur during cooling. The

10

tensile stresses can induce tensile plastic strain. Thus
compressive and tensile plastic strains can accumulate
during multipulse laser irradiation of a ductile

material [19,21].

When the laser fluence exceeds the threshold for
plastic deformation by a large amount, ripple patterns,
melting, and boiling can occur in copper and aluminum as
described by Tong Jee et.al [21]. They also noted that
during multipulse operation, planes more favorable for slip
can accumulate larger plastic strain than planes that are
energetically less favorable [21].

Haessner and Seitz [22] indicated that mechanical
stress fields of nearly radial symmetry are induced by
laser-heating of (111) surfaces in copper single crystals
[22,25]. Figure.1 is a schematic comparing the laser-
generated and explosively generated stress waves in a solid
[25]. Slip systems were found to be operative only if the
radial stresses are high enough. The components of the

stress near the surface regions were found to be:

arr: -(1/2)Ea6T(R/r)2 for r>R , (6)
a¢¢- +(1/2)Ec6T(R/r)2 for r>R , and (7)
°rz' "zz' o ' (8)

where r,¢,z are the cylindrical coordinates and (R) is the
radius of the heat affected zone [22]. (GT) is the
temperature increase on the surface due to the laser

heating[22].

H

krusxitficrcube '

explosivery driven Ilyer
E .0...- t\\\\\\\\\‘mm\\\\\\\\\V

 

W
TT
TT
TT
(“Egr-

 

\\¢guenuned:flxxk
Iront

Figure.1 Schematic Comparison of Laser Generated and

Explosively Generated shock waves in a solid,
from [25].

12

Porteus et al. [23] noted that at very high
temperatures, when melting and evaporation occur,
compressive stress can be produced by a reaction pressure on
the target due to ejection of vapor [23,24]. Evaporative or
plasma-induced compression on the crater and surrounding
area results mainly in a normal stress while compression due
to thermal expansion at lower fluences is mostly parallel to
the surface [23]. Measurements for vapor particle emission
in copper and aluminum indicate that plasma-induced
compression is actually considerable at very high laser
fluences [23].

McMordie et al. [26] showed that although plasma
reflectivity was low, a very thin layer of plasma on the
surface of the target under high enough laser fluences can
greatly attenuate the incoming laser beam which eventually
would lose contact with the surface. Following this effect,
radial stresses due to thermal expansion are not active
anymore and the main form of stress acting on the surface at
very high laser fluences is the plasma-induced compression
normal to it [24,26].

The purpose of this research is to observe the laser-
induced damage in Lithium Fluoride single crystals. Lithium
Fluoride has many advantages that help assess the laser-
induced damage on the surface or inside the bulk [27].

1) Transparency. This permits the damage to be easily seen
and characterized by transmitted light and with low

I3

magnification optical microscopy.

2) The crystals can be easily cleaved for specimen
preparation. This also facilitates the study of internal
damage. Laser irradiated specimens can be cleaved to check
the damage in the cross-sectioned view.

3) The simple well defined slip systems [28,29].

Lithium Fluoride is an ionic crystal and has NaCl
structure as illustrated in Figure.2. The primary slip
system is {110}<110>. The secondary slip system is
{001}<110>. The schematic showing the six {110} planes
projected onto the (001) plane is given in Figure.3 [30].
The six solid lines in the Figure represent the traces of
the six planes. The four traces in the <100> directions
correspond to slip planes that lie at 45 degrees to the
surface. hThey are called the 45 degree slip planes. The
two diagonals in the figure are the traces of the planes
that lie at 90 degrees to the surface. They are called the

90 degree slip planes.

Fracrgre Behavior of Lirhium Fluoride.

Lithium Fluoride is a semi-brittle material. It
exhibits both elastic and limited plastic behavior [29].
Moderate plasticity exists in LiF at room temperature. At
high temperatures LiF and most NaCl crystal structured
crystals are considered ductile with five independent slip

systems within the slip systems {110}<110>, (001}<110>, and

I4

 

 

 

 

 

 

 

 

 

 

 

 

 

C9

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure.2 Crystal Structure of Lithium Fluoride,

from [29].

15

I (101)[1011 '

 

-———e»
e..—— 6—_ I
| I
l _
- l
(oi1)(011] (011)(011)

 

 

 

 

; HHO}

 

 

V
[100]

Figure.3 Projection of (110} Slip Planes onto (001)

Surface of Lithium Fluoride Crystal, from
[30].

16
(111)<110>, while at low temperatures, rock salt structures
have only two independent slip systems within the slip
systems (110)<110> [31].

Lithium Fluoride, in its brittle state, fractures by
cleavage along specific crystallographic planes. Primary
cleavage occurs along the (100) planes and secondary
cleavage occurs along the {130) planes [32]. Anisotropy of
surface energies in crystals with strong cleavage tendencies
dictates the crack path [29]. Fracture, in single crystals,
usually occurs along planes having the lowest fracture
surface energy [31].

Cleavage along primary and secondary cleavage planes
occured during laser processing of both the (100) and (110)
surfaces of LiF single crystals using a continuous wave CO2
laser [28]. It was suggested that secondary cleavage
occured to relieve the high thermal stresses. This was due
to the limited number of primary cleavage planes in LiF
[28]. Surface damage around the melt zone consisted of
cracks compromising between crystallographic cleavage planes
and non-crystallographic planes. This was attributed to
thermal stresses. This study also showed the existence of
cleavage steps indicating discontinuous crack propagation.
The crack stopped and restarted repeatedly [28].

Etching techniques applied to the LiF single crystals
indicated that cracking did not occur due to the interaction

of dislocations. A mechanism based on thermal stresses was

II
more probable since no plastic deformation occured prior to
fracture [28]. The process of laser heating, at high laser
fluences, is probably too fast to allow plastic deformation
to occur. The high concentration of energy was brought by
the laser induced large stresses. The thermal stresses were
thought to be relieved by cracking before any plastic
deformation took place [28]. It was suggested that at very
high temperatures, when the lattice vibrations (phonons)
have a high frequency, they act as barriers to dislocation
motion. Plastic deformation is then more dificult than at
low temperatues [33]. Fracture before the occurrence of
plastic deformation is typical of brittle crystals. Chaudri
et al., on the other hand, studied the effect of impact
erosion on single crystals of NaCl, MgO, and LiF [34]. They
noted that at high strain rates, the crystals behaved in a
brittle fashion. They also registered a linear increase in
the dynamic hardness as the strain rate increased.

Lee [29] used bicrystals of LiF to compare the effects
of Impact erosion and Quasi-static identation on the
fracture and deformation behavior of bicrystals. He
suggested that the most important contact parameters that
affect the mode and extent of damage produced by particle
impact were the pressure, the contact radius, and the
contact time. The pressure controlled the extent of elastic
and plastic deformation of the material, the contact radius

related the pressure or stress to the applied force while

18
the contact time determined the force history [29].

Comparing contact parameters such as contact time,
pressure induced by the particle and the contact radius for
impact erosion with those of pulsed-laser processing, the
most important parameters for pulsed-laser processing are
the pulse length, the energy per pulse, and the beam
diameter. These three parameters are essential for
estimating the laser-induced pressures on the surface and
the bulk material as shown from equations (6) and (7). The
pressures control the mode and extent of deformation in the
material.

The aim of this project is to study and assess the
laser-induced damage due to thermal stresses caused by laser
irradiation on the (001) faces of LiF single crystals. The
transparency of lithium fluoride permits the observation of

various types of damage on the surface and inside the bulk.

II. EXPERIMENTALtAPPARATUS
The COZ-laser used during this work was a TLF 2500 made
by the TRUMPF INC. It generates invisible laser light at
10.6um wave length with a maximum power of 2500 watts. The
laser power output consistency is about 12%. This laser can
be operated either in a continuous mode or a pulsed mode.
The laser machine includes four main components as shown
in Figure.4.
1) The laser control ch: Figure.5 is a schematic of the
three main components of the laser control. (i) is a
portable operating panel with graphic electronic display and
sealed keyboard. (ii) is a modular micro-computer card
system. (iii) is the wiring board to connect the card
system with the periphery.
2) H.F: The H.F generates high frequency electrical
energy. The frequency is 13.56‘MH2 and is maintained at a
constant rate by an oscillator stage. The output of the
oscillator stage is amplified by intensifier circuits and
sent to the beam generator through a coaxial cable.
3) Beam generator (resonator and supply). The resonator
includes a vacuum pump, roots pump and a gas mixing unit.
4) Cooling units. The laser contains two cooling units.
The two units offer two setting possibilities for the
desired temperature. They are responsible for cooling two
different circuits, one for Copper made components and the

other for Aluminum made components.

19

20

 

 

 

 

 

:3 HF
.

 

 

TCC

 

 

 

EASE!

 

 

 

 

 

 

 

 

 

 

 

L.__t/L___.L/

 

 

 

 

1 2 3 4

Figure.4 Schematic Representation of Main Components of
the Laser: (1) is the Laser Control ch, (2)

is the H.F, (3) is the Beam Generator, and (4)
is the Cooling Unit.

iUtSER m i
‘ L“) i i ' (I) mg
G
I O G

N
11 d .-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure.5 ch Components: (i) is the Operating Panel,

(ii) is a Modular Microcomputer Card
System, and (iii) is the Wiring Board.

III. EXPERIMENTAL PROCEDURE

The LiF single crystals used in all these experiments
were set on a metallic support with the (001) surface facing
the laser head. The laser was then set to its single-pulse
mode. The pulse length and the power were fixed using the
keyboard on the operating panel.

Five sets of experiments were conducted on LiF single
crystals.
A. The first set of experiments consisted of laser shocking
the single crystal keeping the pulse length and the distance
from the focal point of the laser as constants. The power
was the only variable.
B. For The second set, the power and the distance from
focus were kept constant while the pulse length was varied.
C. For the third set, the distance from focus was varied.
The power and the pulse length were kept constant.
D. The fourth set consisted of etching the damaged single
crystals with a very dilute solution of ferric chloride.
Etching time lasted from 12 to 16 seconds. Dislocation etch
pits produced by this method revealed the slip distribution.
This is to show the extent of plastic deformation due to the
laser-induced thermal stresses on the (001) surfaces of the
LiF single crystals.
E. The purpose of the fifth set of experiments was to
reveal the role of the sample thickness on the crack
propagation and the extent of the laser damage. The width

22

23

of the heat affected zone and the maximum length of cracks

were then recorded.

All observations and analysis relating to the above
experiments were made using reflected light microscopes and
the scanning electron microscope. The micrographs taken on
the (001) surfaces of the LiF single crystals and the
schematics representing the laser damage on these surfaces
were all bounded by the <100> directions at the bottom and
top, and the <010> directions at the right and left sides.

The diagonals have the <110> directions.

IV. RESULTS AND DISCUSSION

WWW
During this experiment the laser pulse length and the

distance from the laser focal point were kept constant. The
laser power was decreased from treatment.1 to treatment.3.

The sample dimensions were 37 mm x 14.5 mm x 3.5 mm.

1) Treatment.1

The pulse length was t-340 ms.

The laser power was P8190 watts.

The beam area at the focal point was A=0.245 mm2 .
The energy density calculated at the surface is:

I= tP/Ae 2.63x10‘ joules/cm2

a) A well-defined damaged zone (D.2) can be seen in Figures
6,7,and 8. This area with width equal to 1.35mm, included a
crater in the middle. This is where material had melted and
resolidified. The crater area is just larger than the beam
area. Observation of the S.E.M micrographs given in Figures
8,9 and 10 indicated the occurrence of melting and probable
evaporation. The crater shown in Figure.8 contains pores
that may have started as gas bubbles underneath the surface
and exploded. This is a sign of evaporation.

The area surrounding the crater consisted of material

pushed from the bulk onto the surface. This material could

24

25

Heat Affected Zone

Secondary Crack

Damaged Zone —> Primary Crack

 

. jig—b Crater

 

 

Ripple

 

Heat Affected Zone

Figure.6 Schematic Showing the Laser Damage for
Treatment.1, (Power - 190 Wasts and
Fluence a 2.63x10 Joules/cm ).

26

 

Figure.7 Optical Micrograph Showing the 0.2 for
Treatment.1.

27

 

Treatment.1

28

 

Figure.9 S.E.M micrograph Showing the Crater Area for
Treatment.1.

 

Figure.10 S.E.M Micrograph Showing an Evaporation Site
on the surface for Treatment.1.

29

be a compensation for the crater material pushed into the
bulk. Edges of the damaged zone are shown by the arrows in
Figures 11 through 14. The material in the D.z had been
stacked above the surface as can be observed by the
difference in height between the D.z edges and the heat
affected zone (H.A.Z). This may also be due to molten
material flowing away from the crater. The material from
the D.z as seen in Figures 11 and 13 descended towards the
H.A.z in steps parallel to the (001) surface.

Observation of Figures 12,13, and 14 indicate non-
crystallographic cracks bounding the 0.2 in some areas. The
separation of the 0.2 from the H.A.z as observed by the non-
crystallographic cracks at their boundary can be due to high
tensile stresses exerted on the D.z during the cooling phase
of the laser heating process. |

Ripples at the edge of the resolidified area in the
middle of the 0.2 were shown by the arrows in Figures 7,
and 8. Ripples can be a result of plastic deformation
caused by slip. Slip can occur due to the compressive
thermal stresses acting during the laser heating of the
surface. '.

b) Primary (100) and secondary (110) cleavage cracks were
observed. They are connected directly to each other, or
through non-crystallographic cracks.

c) Primary cleavage cracks were seen to jump from one (100)

plane to another parallel (100) plane as they propagate.

30

 

Figure.11 S.E.M Micrograph Showing the Edge of the D.Z
for Treatment.1.

 

Figure.12 S.E.M Micrograph Showing Cracks Bounding_
the D.Z for Treatment.1.

N

 

Figure.13 S.E.M Micrograph Showing the Edge of the D.Z
for Treatment.1.

 

:Figure.l4 S.E.M Micrograph Showing the Edge of the D.Z
Noted by the star in Figure.8.

32

 

 

Figure.15 Laser Damage Observed Inside the Bulk of the
LiF Single Crystal for Treatment.1.

 

33.

Steps were observed on the surface as shown by the letter
(F) in Figure.7 and into the bulk as shown by the letter (G)
in Figure.7 and Figure.15. The cleavage steps form in LiF
when the crack velocity falls below the critical value for
which the density of nucleated dislocations at the crack tip
increases with decreasing crack velocity. When the crack
intersected screw dislocations, the leading edge of the
crack became jagged and cleavage steps were formed as the
crack continued its propagation [35].

d) Secondary {110) cracks started from the damaged zone
(D.Z) as non-crystallographic cracks shown by arrows in '
Figure.15. Deviations from the crystal cleavage directions
can be due to competition between natural crystallographic
cleavage and thermal stress direction effects. Non-
crystallographic cracks usually propagated in planes that
were not perpendicular to the (001) surface. They induced
shaded areas in these micrographs similar to the ones
indicated by (L) in Figure.15.

e) Splattered material was observed on the surface. This
was shown by the letter (J) on the left side of the D.Z

shown in Figure.7.

2) Treatment.2
For this treatment t-340 ms, 9-153 watts, Aso.245 mmz,
and 1-2.26x10‘ J/cmz.

a) The D.Z for treatment.2 was smaller in area than in

34
treatment.1. Its width or maximum diameter was 1.18mm and
its shape was very similar to that of treatment.1 where
material had emerged from the bulk and stacked on the
surface. This can be seen in the S.E.M micrographs
presented in Figures 16,17, and 18.
b) Primary and secondary cleavage cracks were observed.
They are usually connected by non-crystallographic cracks as
shown by the arrows in Figures 19 and 20 .
c) Primary cleavage steps were observed as shown in
Figure.20. The stepping in (100) crack propagation was not
as abundant as it was for treatment.1.
d) Ripples at the rim of the resolidified area in the D.Z
were observed. These are shown by the arrow in Figure.17.
e) Secondary cracks were considerably shorter than in
treatment.1. This may indicate a lower driving force for

secondary crack propagation in treatment.2.

3) Treatment.3
Treatment.3 was repeated three times in order to get

reasonable results. This was due to fluctuations of up to
10% in power which was present during low power (around 100
watts) laser treatments.

For this treatment t-34o ms, 9-125 watts, Apo.24s mm2
and 1-1.73x10‘ J/cmz.

a) The D.Z shown in Figures 21 and 22 had width equal to

1.01mm. The area on the surface coinciding approximately

35

 

Figure.16 S.E.M Micrograph Showing the Laser Damage
for Treatment.2, Power = 16 Watts and
Fluence = 2.26x10 Joules/cm ). “

36

 

Figure.17 S.E.M Micrograph Showing a Corner of the D.Z
for treatment.2. (This is Marked by (M) in
Figure.16). ‘

37

   
 

fl)

Figure.18 S.E.M Micrograph Showing a Crack Bounding the
D.Z for Treatment.2.

 

Figure.19 Optical Micrograph Showing the D.Z for
Treatment.2.

 

Figure.20 Optical Micrograph Showing the Damage Inside
the Bulk for Treatment.2.

39
with the beam area was 0.7mm in width. No sign of melting
could be observed in this area. Melting may have been very
minor and included only a thin layer of the surface. Few
ripples at the edge of the melted zone or crater were‘
observed and are marked by the arrow in Figure.21.

The ripple pattern observed in the central area of the
crater in Figure.2l was called helix ripple pattern so as to
differentiate it from the ripples observed around the
crater. This ripple pattern will be denoted by H.R.P in the
following sections of this thesis. The H.R.P seen for
treatment.3 had a diameter equal to 0.25mm. This is only
about one third of the diameter of the crater. This
macroscopic type of laser damage was found in several‘
following laser-treatments also. Its mechanism of formation
is not fully understood. However, possible mechanisms that
could cause this are discussed in a later section of this
thesis.

The edges of the D.Z, on the other hand, are marked by
the arrows in Figures 23,24, and 25. They were bounded by
non-crystallographic cracks as seen in the case of earlier
treatments with higher laser power.

b) Only one very short (0.2 mm) secondary crack was ,
observed. This was marked by the letter (N) in Figure.21.
At this very low energy density, primary cleavage is the
only type of cleavage induced by laser thermal energy.

c) Cleavage steps were also observed. They were shown by

40

 

(Figure.21 Optical Micrograph Showing the Laser Damage
for Treatment.3,4(Power = 135 Watts and
Fluence = 1.73 10 Joules/cm . *" "‘“”~

M

 

Figure.22 S.E.M Micrograph Showing the Laser Damage for
Treatment.3.

 

Figure.23 S.E.M Micrograph Showing Cracks Bounding the
D.Z for Treatment.3.

42

 

Figure.24 S.E.M Micrograph Showing the Boundary between
the D.Z and the H.A.Z for Treatment.3.

 

Figure.25 S.E.M Micrograph Showing the Boundary between
the D.Z and the H.A.Z for Treatment.3.

43

 

 

Figure.26 Optical Micrograph Showing the Laser Damage
as seen Inside the Bulk for Treatment.3.

.44 .

the (O) in Figure.26.

The schematic given in Figure.27 is a comparison of the
shapes of the D.Z's for treatments 1,2, and 3. Increasing
the power has shown to induce deeper craters and more
material is pushed onto the surface.

A summary of the laser parameters and the laser-induced
damage as indicated by the D.Z width and the length of the
cleavage cracks for treatments 1 through 3 are presented in

Table.l.

ni_Laaer_Damase_as_a_Eunstien_ef_Laser_2ulse_Lensth
During this experiment, the laser power and the distance

from the focal point were kept constant. The laser pulse
length was decreased from treatment.4 to treatment.6 so that
the energy densities correspond respectively with those of
treatment.1 through treatment.3. The sample dimensions were

37 mm x 14.5 mm x 3.5 mm.

1) Treatment.4

For this treatment, t-540 ms, P-120 watts, A=0.245 mm2
and 1-2 . «xio‘ ave-2 .

a) The width of the D.Z in treatment.4 was 2.4 mm as
compared to only 1.35 mm in treatment.1 with the same energy
density. Its shape was almost flat as shown in Figure.28.

Since the laser-contact time with the surface (pulse length)

45

Material Overflow \flater
‘ l ISample Thickness

(a) D.Z Width '

 

 

 

 

 

 

08M Material Overflow

Sample Thickness
(b) l ‘ D.Z Width" I

 

 

Crater\ / Material Overflow

V
I Sample Thickness

(c) 6.2 Widtfi

 

Figure.27 Schematic of Laser Damage as a Function of
Laser Power. (a) Shape of the D.Z for
Treatment.1, (b) Shape of the D.Z for
Treatment.2, and (c) Shape of the D.Z for
Treatment.3.

46

Table.1: Summary of the Results Obtained for Increasing
Laser Power from Treatment.1 to Treatment.3.

 

Treatment.1 Treatment.2 Treatment.3

 

 

 

Pulse Length (ms) 340 340 340
Power (watts) 190 163 125
Distance From Focus 0.0 0.0 0.0

 

Energy Density

(104 Joules/cmz)

 

0.2 Width (mm) 1.35 1.18 1.01

 

Length of The
Longest Secondary 1.23 0.72 0.2
Cleavage Crack (mm)

 

Length of The
Longest Primary 1.14 1.10 1.10
Cleavage Crack (mm)

 

 

 

 

 

 

47

was long, the material from the bulk had moved over a larger
area.

The helix ripple pattern (B.R.P) can be seen around the
center of the crater in Figure.29. Also, in the center of
the crater, material had recrystallized slowly in
crystallographic shapes. This can be seen in the S.E.M
micrograph of Figure.30. Radial striations around this
central area were also seen (Figure.29). These may have
been left behind by the molten material flow from the center
of the crater.

b) Primary and secondary cleavages were observed. They are
very well defined straight lines in most cases.

c) The length of secondary (110) cracks reached up to

3.25 mm from inside the D.Z as compared to 1.2 mm for
treatment.1 with the same energy density.

d) Primary cleavage steps were observed. They are shown by
the arrows in Figures 31 and 32. Cleavage steps can also be
seen from the back of the sample as illustrated in
Figure.33.

e) Cracks in treatment.4 were much wider (about 10 times)
than cracks observed in treatment.1. (Compare Figure.7 and
Figure.29). -

Comparing treatment.1 and treatment.4, both with the
same energy density of 2.6x104 J/cmz, it can be concluded
that the time of laser contest with the surface was very

 

Figure.28 S.E.M Micrograph Showing the Laser Damage
for Treatment.4, Pulse Length = 540 ms and
Fluence = 2.64110 Joules/cm ).

 

Figure.29 Optical Micrograph Showing the Laser Damage
in the Crater Area for Treatment.4. '

49

 

Figure 30 S E.M Micrograph Showing Recrystallization 1n
the Crater Area for Treatment 4.

'. ggégv. 06" ‘

Dr‘

 

Figure. 31 Optical Micrograph Showing the Laser Damage
Inside the Bulk for Treatment. 4.

50

 

Figure.32 Optical Micrograph Showing the Laser Damage
Inside the Bulk for Treatment.4.

 

Figure.33 Laser Damage for Treatment.4 as seen from the
Back of the Sample.

51

critical in determining the extent of damage. Longer pulse
lengths induced slower accumulation of the damage which
resulted in much longer secondary cleavage cracks, much
wider cracks, and much wider D.Z in treatment.4 as compared

to those for treatment.1.

2) Treatment.5

For this Treatment t-460 ms, P-120 watts, A! 0.245 mmz,

and 1:2.25x1o‘ J/cs’.
a) The damaged zone had a width equal to 1.95 mm and can be
seen in Figures 34 and 35. The S.E.M micrograph shown in
Figure.36 indicated that the D.Z was not as flat as that of
treatment.4. This was due to this treatment's lower pulse
length. The crater area was larger than the beam area. The
crater is indicated by the arrows in Figure.36. Radial
striations are also seen and noted by the letter (S) in
Figure.36. Ripples around the crater can also be observed
in this Figure.

b) Primary and secondary cleavage cracks were present.
Non-crystallographic cracks were observed in treatment.5 as
opposed to treatment.4. '

c) Cleavage steps were observed. They are indicated by the
letter (P) in Figures 34 and 36.

d) Secondary cracks observed for this treatment are shorter

than for treatment.4. The longest crack measured 2.35 mm as

opposed to 3.25 mm in treatment.4. Note that the length of

 

Figure.34 Optical Micrograph Showing the Laser Damage
for Treatment.5, (Pulse leng h = 460 ms and
Fluence = 2.25x10 Joules/cm ).

53

 

 

Figure.35 S.E.M Micrograph Showing the D.Z for
Treatment.5.

 

Figure.36 Optical Micrograph Showing the D.Z for
Treatment.5.

54

primary cracks, especially those that started within the
D.Z, did not depend on the energy density. This was
observed by comparing treatments 1 through 3 and also by
comparing treatments 4 and 5. Secondary cleavage was
predominant for treatments 1, 2, 4, and 5. Its propagation
was driven by the high energy densities provided by lasers.
Primary cracks seemed to reach towards a stable, maximum
length. More energy or driving force for crack propagation
appears to contribute mostly towards the extension of

secondary cracks.

3) Treatment.6
For this treatment t-380 ms, P-120 watts, A90.245 mm2

and I-1.86x1o‘ J/cmz.

a) The D.Z of treatment.6 had a width equal to 1.62 mm.
The D.Z, as seen in the S.E.M micrograph given in Figure.37,
had a much deeper crater and more overflown material as
compared to the higher pulse-length treatments of this set
of experiments. Ripples around the crater can be observed
in Figure.38. This Figure does not indicate the existence
of radial striations observed in some of the earlier
treatments.

b) No trace of secondary cracks were observed for this
treatment. The two primary cleavage cracks are
perpendicular to each other and intersect at the center of

55

 

Figure.37 S.E.M Micrograph Showing the Laser Damage for
Treatment.6, (Pul e Length =2380 ms and
Fluence - 1.85x10 Joules/cm ). ,

   

p}???

itilJrcuug-u

Figure.38 Optical Micrograph Showing the D.Z for
Treatment.6.

56

the D.Z. This can be seen in Figure.39. This set of
primary cracks has never been observed in any of the five
earlier treatments. Primary cracks inside the D.Z usually
form a rectangle with its center coinciding with the center
of the D.Z. These cracks never crossed the center of the
damaged zone. This set of cracks, intersecting at the
center (molten zone) indicates that cracking may have
occured during the resolidification process.

c) Cleavage steps were observed. This is shown by the

arrows in Figure.39.

The shematics given in Figure.40 are a comparison of the
D.Z's shapes for treatments 4 through 6. The increasing
depth of the crater and amount of overflown material from
treatment.4 to treatment.6 indicated the effect of
decreasing the pulse-length. The decreasing of laser-
contact time with the surface induced more limited
propagation of the damage. Damage propagated over smaller

8:03. e

A summary of the laser parameters and the laser damage
as indicated by the D.Z width and the length of the cleavage
cracks for treatments 4 through 6 is presented in Table.2.

 

Figure.39 Optical Micrograph Showing the Laser Damage
Inside the Bulk for Treatment.6.

58

 

 

 

rater
Material Overflow \/—~
Sample Thickness
(a) [ ' 0.2 Width, l I

 

 

 

CratMMaterial Overflow

Sample Thickness
(b) ' 5.2 Width I

 

 

 

Material Overflow

Crater V

(c) F .2 Wi h

Figure.40 Schematic of the Laser Damage as a Function
of the Pulse Length. (a) Shape of the D.Z
for Treatment.4, (b) Shape of the D.Z for
Treatment.5, and (c) Shape of the D.Z for
Treatment.6.

 

 

I Sample Thickness

 

59

Table.2: Summary of the Results Obtained for Increasing
the Pulse Length from Treatment.4 to Treatment.6.

 

Treatment.4 Treatment.5 Treatment.6

 

 

 

Pulse Length (ms) 540 460 380

Power (watts) 120 120 120

Distance From Focus 0.0 0.0 0.0
(M)

 

Energy Density

(104 Joules/cmz)

 

D.Z Width (mm) 2.40 1.95 1.62

 

Length of The
Longest Secondary 3.25 2.35 0.00
Cleavage Crack (mm)

 

Length of The
Longest Primary 1.5 . 1.85 2.35
Clevage Crack (mm)

 

 

 

 

 

 

 

During this set of experiments the laser power and pulse
length were kept constant. The distance from the laser'
focal point was increased gradually from treatments 7 and 8
to treatment.10. The distances from the focal point were
calculated using a plot of the beam diameter versus the
distance from focus for a ten inch lens. The energy
densities of treatments 8 through 10 corresponded
respectively with those of treatments 1 through 3 and also
treatments 4 through 6. The sample used was relatively thin
and had dimensions equal to 37 mm x 14.5 mm x 1.3 mm. Note
that very thin samples, as was found in set E of
experiments, would result in more extensive damage on the
surface as well as inside the bulk. This factor has to be
taken into account when analysing the results of treatments

7 through 10.

l) Treatment.7

For this treatment t-4oo ms, 9-190 watts, a-o.275 m2
which corresponded to a distance from the focal point
d-6.86 mm. The energy density was I-2.76x104 J/cmz.

a) The D.Z in this treatment was very clear since no cracks
were present. This can be seen in figures 41,42, and 43
where the D.Z was a perfect circle as opposed to the other

treatments. The diameter of the circle was 0.96 mm. Inside

61

the D.Z, the crater, which was the area just larger than the
beam area, had width equal to 0.7mm. The edge of the crater
was shown by the arrows in Figures 41 and 43.

The helix ripple pattern (H.R.P) was also observed as
shown in Figures 41,42, and 43. The H.R.P width (0.4mm) was
smaller than the beam dimeter on the surface (0.6mm).

The radial striations observed in earlier treatments can
also be observed around the H.R.P of treatment.7. They are
marked by the (R) in Figure.43.

b) No cracking had occurred'during this treatment with
approximately the same energy density as treatment.1 and
treatment.4. This proves that the amount of damage induced
by laser is not only dependent on the energy density but
also very dependent on the extent of the different laser
variables (power, pulse length, and distance from focus).
c) Primary 45° (110) slip traces were observed around the

D.Z. They are noted by the letter (W) in Figure.43.

2) Treatment.8

For this treatment the pulse length was increased and
the power was decreased as compared to treatment.7. The
distance from the focal point, and the energy density were
kept constant. This was carried out in order to induce
cracking in the sample. It was noted from the previous
experiments that the pulse length, among other laser
variables had the most critical effect on the extent of‘Uhe

62

 

Figure.41 S.E.M Micrograph Showing the D.Z for
Treatment.7, (Pulse Length = 400 ms, Dista ce
from focu = 6.9 mm, and fluence = 2.76X10

Joules/cm ) ,.

 

 

Figure.42 Optical Micrograph Showing The D.Z for
Treatment.7.

 

Figure.43 Optical Micrograph Showing the D.Z for
Treatment.7. Different Lightening than for
Figure.42.

64

damage induced by laser.
For Treatment.8, t-500 ms, P§151 watts, d-6.86 mm, so

2, and I-2.75x1o‘ J/cmz.

APO.275 mm
a) The width of the D.Z was 2.13 mm as compared to 0.96 mm
for treatment.7. This indicated again the significant
influence of the laser pulse length and the small thickness
of the sample on the extent of damage induced. A crater
with a H.R.P in the center could be observed in Figure.44.
b) Primary and secondary cleavages were present as can be
seen in Figure.45. Primary cleavage cracks were predominant
and were very long (up to 5.2 mm in length). This can be
seen in Figure.44. They went very deep inside the bulk and
reached the back of this thin sample.

c) Cleavage steps were left behind primary cleavage cracks

as can be observed in Figures 44 and 46.

3) Treatment.9
For this treatment t-500 ms, P-151 watts, d-8.9 mm, so

3, and I-2.36x10‘ J/cs’.

APO.319 In
Features in this treatment were similar to those in

treatment.8, except smaller. (Compare Figure.44 and

Figure.47).

a) The diameter (width) of the-D.Z was 2.2 mm. A crater

with a H.R.P in the center can be observed in Figure.47.
The radial striations, believed to be left behind by

65

 

Figure.44 Optical Micrograph Showing the Laser Damage
for Treatment.8, (Distan e from Fee 5 = 6.9
mm and Fluence = 2.74x1o Joules/cm ). .

66

 

 

 

 

D.Z
—>Crater
Ripple Pattern (H.Fl.P)

 

H.A.Z
H.A.Z

 

Non-crystallographic
Crack

 

Secondary Crack

HA2

Figure.45 Schematic Showing the Laser Damage for
Treatment.8.

67

 

Figure.46 Optical Micrograph Showing the Laser Damage
Inside the Bulk for Treatment.8.

 

Figure.47 Optical Micrograph Showing the Laser Damage
Inside the Bulk for Treatment.9, (Distanc

from Focu = 8.9 mm and Fluence = 2.36x10
Joules/cm ).

 

Figure.48 S.E.M Micrograph Showing Radial Striations as
Seen Around the Center of the Crater for
Treatment.9.

 

Figure.49 Magnified View of Region (X) Shown in
Figure.48.

molten material flow from the center of the crater, can be
seen in figure.48. They were noted by the letter (X) in
this Figure. A magnified view of this area is provided in
Figure.49.

b) Primary cracks reached up to 3.7 mm in length.
Secondary cracks reached 2.4 mm in length.

4) Treatment.10
For this treatment t-500 ms, P-151 watts, d-l4.2 mm, so

2, and 1-1.s1x1o‘ J/cs’.

A=O.417 mm
a) The width of the D.Z was 2.25 mm. The D.Z did not vary
significantly in size from treatment.8 to treatment.10. A
crater with a H.R.P in the center can be observed in
Figure.50.

b) No secondary cracking was observed. Primary cracks
reached up to 3.4 mm in length.

c) Cleavage steps were very apparent. This is shown by the

arrow in Figure.50.

A summary of the laser parameters and the laser damage
as indicated by the D.Z width and the length of the cleavage
cracks for treatments 8 through 10 is presented in Table.3.

70

 

Figure.50 Optical Micrograph Showing the Laser Damage
on the Surface and Inside the Bulk for
Treatment.10, (Distance from Focus = 14.2 mm
and Fluence = 1.80X10 Joules/cm ). ' ’ ”

71

Table.3: Summary of the Results Obtained for Increasing
the Distance from Focus from Treatment.8 to
Treatment.10

 

Treatment.8 Treatment.9 Treatment.10

 

 

 

 

 

Pulse Length (ms) 500 500 500
Power (watts) 151 151 151
Distance From Focus 6.86 8.90 14.22
(mu)
Energy Density
4 2 2.74 2.36 1.80
(10 Joules/cm )
D.Z Width (mm) 2.12 2.20 r 2.28

 

Length of The
Longest Secondary 3.00 2.27 0.00
i Cleavage Crack (mm)

 

Length of The
Longest Primary 5.20 4.20 3.65
Cleavage Crack (mm)

 

 

 

 

 

 

 

and

In Figure.51, the scanning electron micrograph of the
area between the H.R.P and the edge of the D.Z is
prescribed. This area contained bubbled material and was
subdivided by what seemed like grain boundaries. This
suggested that the melted material had transfomed into a
polycrystal during the crystallization that followed the
rapid quenching of the molten surface.

The bulk damage due to treatment.8 on the (100) side
face of the single crystal after the sample was cleaved'
through the middle of the D.Z can be seen in Figure.52.
This Figure provides the three dimensional shape of the D.Z,
which is close to half a sphere with maximum radius equal to
1.1 mm (half the width of the D.Z) and minimum radius equal
to 0.7 mm (depth of the D.Z). The interface between the D.Z
and the rest of the bulk is very clear. Some material
removal can be observed at the interface. The material
removal can be due to the effect of compressive forces
exerted by the material in the D.Z during the laser pulse.
The discontinuity at the interface can suggest a loss of
crystallographic coherency between the D.Z and the rest of
the bulk. The formation of the D.Z and its separation from
the bulk in some areas can be due to 450 (110) slip.

The shape of the D.Z on the surface as described by
Figures 27 and 40, can be justified by observing Figure.52.

72

73

 

Figure.51 S.E.M Micrograph Showing the Area at the
Boundary of the Helix Ripple Pattern for
Treatment.10.

 

74

A shallow depression (crater) around the center of the D.Z
can be observed. Surrounding the crater, some material
overflow (although minor for treatment.8) can also be
observed. This is indicated by the arrow in Figure.52.
Figure.53 includes three curves of the laser energy
density versus the D.Z width for experiment sets A, 8, and C.
It can be observed that the relationship between the energy
density (I) and the D.Z width was almost linear for the
range of coordinate values chosen and for all sets of
experiments A, B, and C. However, the slope differed from
one curve to the other. The high positive slope for the
curve (P.L.V), where the pulse length was the variable,
indicated the great dependence of the D.Z width on the pulse
length as compared to its dependence on the power as shown
by the curve P.V where the power was the variable. The
slope of curve D.F.V, where the distance from focus was the
variable, was low and negative indicating that the width of
the D.Z increased as the distance from the focal plane of
the laser was increased and therefore the energy density was
decreased. The effect of the constant, high pulse length
used in set (C) of experiments, overrode the effect of
changing the distance from focus on the width of H.A.z and
therefore caused the slope of curve D.F.V to be very low.
Figure.54 is a schematic showing the different laser-4
induced damage regions on the (001) faces of the LiF single

crystals. The damage consists of (1) slip or plastic

75

 

Figure.52 Optical Micrograph Showing the Bulk Damage
for Treatment. 8 as Observed on the (100)
Plane. (Specimen Cleaved to Reveal the
Cross-section of the D. Z).

76

e<m
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O

bei—LP-Irt—lrb

.OHnswue> can he meson I >.m

can .oehueus> use he nausea amass n >.o.s
.muneauo> on» ma msoom some eoceumao I >.h.o
.nusqs u.o .u> seduces smum:m_uouao no page mm.muse«s

S. :53 Emzua Gauze mums

emu

Pinlhirlhlbill-blhbtrlh

m.p

 

 

 

C!
c:

“2
c:

‘2
e-

“2
v-

5?
cw

“?
cw

 

1TTT—l—rTYTTI—TTT‘IT—UIIlrITIIT'U‘

 

c;
r)

(M) HiCllM 2'0

 

Area of Plastic
Deformation

 

.-

Material

H———e
Area

 

of
Melting

77

 

overflow
.Fl.P m
<——><——><— 4
Area Area Area of Plastic
of of Deformation
Vapo- elting
Hzafion

 

 

 

 

 

 

Width Of The Damaged Zone

Figure.54 Schematic Showing the Different Damage
Regions in Laser Irradiated LiF Single
Crystals.

78

deformation occurring near the edges of the D.Z, (2)

melting occurring in an area closer to the center of the
D.Z, and (3) vaporization taking place in a relatively
small area around the center of the D.Z. The heat was being

conducted towards the center.of the damaged zone.

W

The helix ripple patterns were observed in treatments
that produced shallow craters. This occured when the laser
pulse length was high, the beam was focused above the
surface or at very low energy densities.

The ripples followed the shape of a helix that started
on the surface and worked its way towards the interior of
the the bulk. This feature can be observed in the S.E.M
micrographs of Figures 55 through 65. The H.R.P in all
treatments covered an area smaller than the beam area on the
surface. However, its area increased as the beam area
increased in set (C) of experiments.

The reason for this particular ripple pattern in the
center of the crater area may be due to molten material
churning. Turbulence in the molten material may have caused
it to recrystallize in the shape of the H.R.P. The H2R.P
consists of ripples, in the shape of steps, that descend
towards the interior of the bulk. One face of these steps
is a (001) plane. This is indicated by the letter (Z) in

Figure.58. The other face of the step or ripple, as may be

79

 

Figure.55 S.E.M Micrograph Showing the H.R.P for
Treatment.3.

00pm

, e

 

Figure.56 S.E.M Micrograph Showing the H.R.P for
Treatment.7.

80

 

Figure.58 Magnified View of the Area Inside the Square
in Figure.57.

N

 

Figure.59 Magnified View of the Region Described by the
Square in Figure.58.

 

Figure.60 S.E.M Micrograph Showing the H.R.P for
Treatment.9.

82

 

Figure.61 S.E.M Micrograph Showing the H.R.P for
Treatment.10.

 

.

Figure.62 Magnified View of the Area Inside the Square
in Figure.61.

83

 

Figure.63 Magnified View of the Area Inside the Circle
in Figure.61.,

 

Figure.64 Magnified View of the Area Inside the Square
in Figure.63.

84

Edge of The Ripple

 

Figure.65 Schematic Showing the Cross-section View of a
Ripple in the H.R.P for Treatment.10 as Shown
in Figure.64. -

analyzed by observing Figures 63 and 64, appear to be
bounded by alternating (100) and (010) planes as is
indicated by the ridges on the (001) faces of the ripples.
The ridges are indicated by the arrows in Figure.64. This
can also be observed in the shematic shown in Figure.65.
The (100} planes are energetically more favorable for
formation in recrystallized L1? as compared to (110) planes
based on surface energy considerations. This reinforces the
above speculations related to the shape of the ripple.

The formation of the H.R.P may also be due to internal
reflection effects of the laser beam.

Further studies will be needed to verify the above
speculations or to find the actual mechanisms responsible

for the formation of the H.R.P.

W-

In this part of the study, two samples used were etched
for 10 to 15 seconds in a very dilute solution of ferric
chloride in water.

The slip lines revealed by etching were of two types.
Thick slip lines that could be seen at low magnifications
(20x to 40X) and thin slip lines that were seen at higher
magnifications, above sax.

The thick slip lines looked like rays that emanated from
the edge of an area surrounding the D.Z where slip lines
were not observed. This can be seen in Figures 66 through

86

 

Figure.66 Optical Micrograph Showing the Dislocation
Etch Pit Pattern Outside the D.Z for
Treatment.1.

 

Figure.6? Optical Micrograph Showing the Dislocation
Etch P1t Pattern Outside the D.Z for
Treatment.2.

 

Figure.68 Optical Micrograph Showing the Dislocation
Etch Pit Pattern Outside the D.Z for
Treatment.3.

 

Figure.69 Optical Micrograph Showing a Magnified View‘
of the Area Marked by (a) in Figure.68.

 

Figure.70 Optical Micrograph Showing a Magnified View
of the Area Marked by (b) in Figure.69.

    

l——————-—-——4

1001mm

Figure.71 S.E.M Micrograph Showing 900 (110) Slip Lines
Just Outside the D.Z for Treatment.2. ‘
(Region Above the D.Z in Figure.70).

 

Figure.72 Optical Micrograph Showing the Dislocation
Etch Pit Pattern Outside the D.Z for
Treatment.7.

90

(110) Slip

5 (110) Slip

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

/

Figure.73 Schematic Representation Showing the
Dislocation Etch Pit Pattern Outside the D.Z
for Treatment.7.

 

 

91

71. However, the micrograph of Figure.72 indicated that 45°
primary slip was present in the area immediately surrounding
the D.Z. This type of primary slip is usually hard to
observe due to shallow dislocation etch pits on the surface
or due to the interference between the 900 (110) slip lines
and the 45° (110) slip lines. The 45° slip surrounding the
D.Z could be responsible for the formation of the smooth,
curved shape of the D.Z inside the bulk as was seen in
Figure.54. Figure.73 is a schematic representation of the
slip lines observed in the micrograph of Figure.72. The
thick slip lines followed primary <110> slip directions on
the (001) surface of the LiF single crystals. The
dislocation motion inducing slip can be due to mechanical
stress fields produced by the laser irradiation.

Tensile stresses caused by the rapid quenching of the
surface, were created when the laser pulse was completed.
These stresses were thought to be responsible for
controlling the slip distribution outside the D.Z [20].
According to Gorbunov et.al [20], the compressive stresses
due to the temperature gradient imposed on the surface
during the laser heating phase had the same magnitude as the
tensile stresses, however they didn't last long enough to.
control the slip distibution outside the damaged zone in
copper single crystals.

To explain the effect of thermal stresses on the motion

of dislocations and their specific slip planes, we can use

92

the same approach Haessner [22] used for copper single
crystals. He suggested a quantitative analysis to assess
the necessary forces acting on dislocations which cause

slip. He assumed

R-[? x a] x at (9)
was the component of the force
9-23: x I. (10)

which acted on a dislocation with burgers vector'h and
dislocation line't in its slip plane (unit vector 3) when an
external stress field (tensor g) was applied.

The thermo-mechanical stress field 2 produced by laser
irradiation was derived by Dailey[22). Bailey suggested a
stress field with radial symmetry (as shown in Figure.74) if
the Young's modulus (E) and the linear expansion coefficient
(8) are isotropic. The stress field on the surface (z-O) is

defined by:

°::‘-(1/2)narsr)<x/r)’ tor r>R.‘ ' (11>
a”-+(1/2)zp(sr)(R/r)2 for r>R, and (12)
arz-azz-o (13)

where (R) is the D.Z radius and (6T) is the temperature
increase on the surface-due to the laser heating [22].
Equations (11),(12) and (13) could be used to calculate
the force (H) as a function of (r) and (e) for the case of
LiF single crystals, laser shocked on (001) faces. The
0

values of (e) for which slip occurred were e- 45°, -45 ,

135°, and -135°. H should assume its maximum values at

93

these values of (e).

Thin slip lines originating inside the D.Z can be
easily observed, particularly for treatments with low energy
densities like treatments 3,5, and 9. These slip lines can
be observed at magnifications above sox as can be seen in
Figures 75 through 80. The S.E.M micrograph of Figure.81,
however, indicated the existence of slip lines inside the
D.Z for treatment.1 which had the highest energy density.

Slip lines in Figures 80,24, and 25 of treatment.3
formed a parallelogram around the rippled area inside the
D.Z. The slip line structure in this case could be
controlled by the compressive stresses as opposed to the
tensile stresses acting outside the H.A.z. Figure.82 is a
schematic representing the slip line configuration inside
the D.Z for treatment.3 with low energy density.

Figures 78, 83, and 84 indicate (110) slip lines that
intersect at a (001) crack. Slip in this case was induced
by the crack propagation, as opposed to those in indentation
experiments, where the (001) cracking was induced by the
pile up of dislocations on intersecting (110) planes [29].

Crack propagation can induce the nucleation of
dislocations in L1! when the crack velocity is less than
6x103 cm/sec [35]. At velocities less than 3x103 cm/sec,
the crack velocity in a lithium fluoride crystal oscillates
between a high and a low value as compared to a critical

velocity. As the crack slows down, dislocations are formed.

94

[no]

’0
O \ / e.o. ¢ 01 1]
...~. A 0......
......9 O) "e'.'0'.'0' r M
e e 9 0; ‘. . e
9.0... O.Q.O
9.0’ e

 

Figure.74 Cylindrical Coordinate System for the Thermal
Stress Field Outside the Laser Induced D.Z on
the (001) Face of LiF Single Crystals.

95

 

Figure.75 Optical Micrograph Showing the Dislocation
Etch Pit Pattern Inside the D.Z for
Treatment.9. (Top Half of the D.Z in
Figure.47).

 

Figure.76 Optical Micrograph Showing the Dislocation
Etch Pit Pattern Inside the D.Z for
Treatment.9. (Bottom Half of the D.Z in
Figure.47). -

96

 

 

Figure.7? Optical Micrograph Showing a Magnified View
of the Area Marked by (c) in Figure.75.

 

Figure.78 Optical Micrograph Showing the Slip Lines in
Regions Adjacent to a Crack Inside the D.Z
for Treatment.5.

 

Figure.79 Optical Micrograph Showing a Magnified View

of the Slip Lines Adjacent to the Crack in-
Figure.78.

 

rrxs it

Figure.80 Optical Micrograph Showing the Dislocation
Etch Pit Pattern Inside the D.Z for
Treatment.3.

 

 

Figure.81 S.E.M Micrograph Showing Slip Lines in the
Area Between the Crater and the Edge of the
D.Z for Treatment.1.

 

  
 
  

110) slip line

Ripple Pattern

Edge of D.Z

Figure.82 Schematic Representation of the Slip Line
Distribution as Observed Inside the D. 2 for
Treatment. 3

 

 

um

 

 

 

 

 

 

HA2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(1 10) crack (110) Slip llne

Figure.83 Schematic Representation of Slip Lines
Induced by (110) Cracks on the (001) Surface
of LiF Single Crystals.

 

 

1m

 

Figure.84 Optical Micrograph Showing Slip Lines induced
by (110) Cracks.

 

102

This absorbs energy which slows it down further. The crack
should soon stop propagating unless more force is applied to
the crystal. However, the cleaved ends of the crystal are
moving apart at constant velocity which builds up force to
drive the crack forward after it has slowed down. When this
force is high enough, the crack begins to accelerate again.
As the crack accelerates, fewer dislocations are formed
requiring less force to move it, and it continues to
acceelerate. The crack would accelerate up to the terminal
velocity if the driving force did not decrease when the
crack is moving fast again compared with the rate at which
the cleaved ends are being seperated. The crack, therefore
slows down and the cycle is then repeated [35].

According to Mott[35], the crack velocity is lowest when
the crack is longest. The slip traces induced by cracks ,
especially at regions close to the end or tip of the cracks

can be seen in Figure.86.

. l”. ; w, ,,..- ., ,e; .,.... - .. .. .,
Three laser treatments with identical laser variables
(power, pulse length and.distance from the focal plane) were
carried out on the (001) planes of three LiF single crystals
with different thicknesses. The crack lengths and the width
of the D.Z were recorded. This was to reveal the effect of
the sample thickness on the extent of laser damage.

The laser variables were kept constant at t-340 ms,

 

 

 

NB

2 and I-2.87x104 J/cmz.

P-207 watts, d-O mm, so A-0.245 mm
1) Treatment.11

For this treatment, the sample thickness was th=3.8 mm,
and the sample width was w'14.5 mm.

The D.Z width was 0.9 mm and the length of secondary
cleavage cracks reached a length of 0.7 mm. This was

measured from Figure.85.

2) Treatment.12

For this treatment, the sample thickness was th-2.8 mm,
and the sample width was 14.5 mm.

The D.Z width was 0.97 mm and the secondary cracks
reached 0.87 mm in length. This was calculated from

figure.86.

3) Treatment.13

For this treatment, The sample thickness was th-1.6 mm
and the the sample width was 14.5mm.

The D.Z width was 1.4mm.

In this treatment.13, primary cracking was prevalent as
shown in Figure.87. The primary cracks reached a maximum
length of 1.84 mm in length.

Table.4 is a summary of all the results found in this
set of experiments.

The results of Table.4 indicated an obvious relationship

 

 

 

W4

 

Figure.85 Optical Micrograph Showing the Laser Damage
for Treatment.1l, (S ple Thickness = 3.8 mm
and Fluence = 2.87x10 Joules/cm ). ‘

 

Figure.86 Optical Micrograph Showing the Laser Damage
for Treatment.12, (Sa ple Thickn es s 2.8 mm
and Fluence = 2.87x10 Joules/cm ).

 

 

N5

 

Figure.87 Optical Micrograph Showing the Laser Damage
for Treatment.13, (s ple Thickn 33 = 1.6 mm
and Fluence - 2.87x10 Joules/cm ).

106

Table.4: Laser Damage Dependency on Sample Thickness.

 

Treatment.11 Treatment.12 Treatment.13

 

Sample Thickness

 

 

 

 

 

(mm) 3.8 2.8 1.6
Pulse Length (ms) 340 340 340
Power (watts) 207 207 207
Distance From Focus 0.00 0.00 0.00
(mm)
Energy Density
4 2 2.87 2.87 2.87
(10 Joules/cm )
D.Z Width (mm) 0.90 0.97 1.40

 

Length of The
Longest Secondary 0.67 0.87 1.08
Cleavage Crack (mm)

 

 

Length of The
Longest Primary 0.90 0.90 1.84
Cleavage Crack (mm)

 

 

 

 

 

 

 

 

107

between the sample thickness and the amount of laser damage
described by the length of the cracks and the D.Z width.
Both the crack length and the D.Z width increased with
decreasing sample thickness. However the relationship
described is non-linear as both values of The D.Z width and
the length of cracks increased drastically from thickness
th-2.8 mm to th-1.6 mm. This could be observed in the
graphs provided in Figures 88 and 90. Comparison of the
graphs given in Figures 88 and 89 indicates that the
variation of the length of secondary cleavage cracks with
the sample thickness is more linear than that for the
primary cleavage cracks. It was also observed that primary
cracking predominated at small values of the sample
thickness.

Experimental observations indicated that samples with
small thicknesses may have allowed higher absorption of the
laser radiation than thicker samples. Crack propagation in
thin samples may have been made easier by less obstacles in
the crack path as compared to thicker samples. As a result,
there is less constraint for primary cleavage to propagate.
This is probably the reason for the predomination of the
primary cleavage cracking during laser irradiation of thin
samples. Also, for thin enough samples, the laser light
could be reflected after reaching the metallic support. The
reflected energy is then absorbed by the sample and causes

an increase in the amount of laser-induced damage.

 

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1 .5 2.0 2.5 3.0 3.5
SAMPLE THICKNESS (MM)

Figure.90 Plot of Sample Thickness vs. D.Z Width.

4.0

 

 

'V. CONCUDSIONS:

1. A heat Damaged zone (D.Z) is observed for all laser
treatments shown. This is an area where evaporation,
melting, and plastic deformation as indicated by slip can
occur. The mechanism of formation of this zone may be due
to the 45° (110) primary slip. The extent of each form of
damage depends on the extent of the laser variables and the
energy density. The dimensions of the D.Z and its
characteristics change as the laser energy density changes.
Rippling of the surface around the crater (area usually just
larger than the beam area) is seen even at very low energy
densities. Rippling is an indication of the plastic
deformation that occurs during laser heating.

2. A ripple pattern in the shape of a helix is observed
around the area of evaporation in the center of the D.Z for
treatments that produced shallow craters. This occurs when
the laser pulse length is large, the laser beam is focused
above the surface, or at very low energy densities.

3. Primary (100) and secondary (110) cleavages due to
laser irradiation are observed on the (001) face of the LiF
single crystals.

4. Won-crystallographic cracks are only abundant for
treatments with low values of the pulse length.

5. The pulse length has the most critical influence on the
extent of damage. A small change in the pulse length can

induce a significant change in the amount of laser damage as

“I

 

 

H2

indicated by the changes in the D.Z width and the length of
secondary cracks. Increasing the pulse length increases the
time of laser interaction with the surface, causing the
damage to propagate over larger areas.

6. Increasing the laser power, however, mainly increases
the depth of the crater and induces more material overflow
on the D.Z. Also, the cracks are longer and the D.Z is
wider but these two featues are much more sensitive to the
pulse length.

7. An increase in the distance from the focal plane of the
laser to the specimen surface, mainly decreases the length
of the primary and the secondary cracks. Primary cleavage
cracks become the prevalent sort of cleavage as the distance
from focus is increased. The width of the D.Z seems to
increase very slowly as the energy density is decreased for
the case where the constant pulse length is large. It is
suggested that repeating the treatments with changing
distance from the focus for a lower value of the pulse
length can result in larger differences in the width of the
D.Z as the energy density is decreased. In this case the
dominating effect of the pulse length would be reduced.

8. Slip is observed inside the D.Z as well as outside
(H.A.Z). The slip distribution observed inside the D.Z is
different from that observed outside. Compressive stresses
due to the temperature gradient during the laser pulse are

thought to control the slip distribution inside the D.Z,

 

 

"3

while the thermal tensile stresses due to the quenching
effect on the surface control the slip distribution outside.
9. The fluctuations in the crack velocities result in
variations in the dislocation density as the crack
propagates.

10. Propagation of cracks results in slip.

11. The sample thickness, below a certain value, affects the
extent of damage due to the laser-treatment. The D.Z width
and crack length augment as the sample thickness decreases.
This could be due to absorption of laser energy reflected
from the metallic support underneath the sample.

12. Because of the transparency of LiF, optical microscopy
was shown to reveal more of the laser damage as compared to
scanning electron microscopy. However, scanning electron
microscopy can be more advantageous in revealing the surface

damage.

 

 

 

VI. RECOMMENDATIONS

1) Transmission electron microscopy ought to be used in
order to reveal the origins of the damaged zone and the
reasons for its very well defined circular shape.

2) A thick sample should be used for the experiments with
the laser focal point being away from the sample surface.
This will eliminate the effect of thin samples on the
propagation of damage.

3) Further studies are needed to clarify the mechaniSm of
formation of the helix ripple pattern. High resolution
electron microscopy might be very useful for clarifying this
mechanism.

4) Further studies may also be necessary to clarify the
reasons for the effect of the sample thickness on the extent
of the laser-induced damage. A thick sample where one
treatment has been carried out, can be cleaved to reduce its
thickness; then one more treatment having the same laser
variables as the previous can be carried out on a spot which
location is close to that of the previous treatment. This
is to eliminate the effects of the surface roughness on the
absorption of the laser radiation. Also, two very thin
samples can be superimposed before carrying out the laser
treatment in order to check for laser-induced damage on the
surface of the bottom sample. This can help identify and
verify the effect of the sample thickness on the propagation

of damage.

"4

 

 

APPENDIX A

Some Thermal Properties of LiF Single crystals

Melting Point - 845°C [36]
Boiling Point - 1575°c (35]
Enthalpy of Fusion - 91.1 cal/gram [36]

Heat Capacity at 25°C, Cp - 9.994 cal/mol deg [37]

1143

 

Infrared Absorption Edges of Ionic Crystals

APPENDIX B

 

15—

Absorpllon coclllcienl to "l

 

10-

 

A1103

 

LiF NaF

 

 

10
Wavelength in)

20

 

 

 

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KB! _
NeCl TIC!
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no: 7
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I I
g I
I I
' I
' I
r ,’
I I
I
I
I
/
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40 60

[Figure.91 Plot of Absorption Coefficient vs. radiation

ll4b

wavelength. (From [11]).

 

 

10.

ll.

12.

l3.

l4.

 

W
p. 547, 5th edition, Vol 7, McGraw-Hill, Inc., New Yérk,
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H5

 

 

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