LASER CAVING Seminar II

Author: Rok Simič

Mentor: prof. dr. Janez Možina

Date: May 2009

Abstract Physical background of caving by ablation with one laser pulse is presented in first part of this

seminar. The light and matter interaction, including absorption of laser light, heating and ablation, is

examined. The effects of different laser parameters on laser caving/machining are presented in

second part. Further, mathematical models for three-dimensional CW and pulsed laser caving are

presented. At the end, application of laser caving in surface texturing for improving tribological

properties is mentioned.

2

Contents 1. Introduction .................................................................................................................................... 3

2. Interaction of laser light with matter .............................................................................................. 4

2.1. Absorption of laser light .......................................................................................................... 4

2.2. Heating .................................................................................................................................... 5

2.2.1. Melting ............................................................................................................................ 6

2.2.2. Vaporization .................................................................................................................... 7

2.2.3. Plasma formation ............................................................................................................ 7

2.3. Ablation ................................................................................................................................... 8

3. Caving ............................................................................................................................................ 10

3.1. Caving in one dimension (drilling)......................................................................................... 10

3.1.1. Dependence on pulse energy ....................................................................................... 10

3.1.2. Dependence on pulse duration ..................................................................................... 10

3.1.3. Dependence on pulse number ...................................................................................... 11

3.1.4. Dependence on spot size .............................................................................................. 11

3.2. Caving in two- and three-dimensions ................................................................................... 13

3.2.1. Mathematical model for CW source ............................................................................. 13

3.2.2. Pulsed source ................................................................................................................ 14

3.2.3. Surface roughness ......................................................................................................... 15

3.2.4. Producing arbitrary shaped cavities.............................................................................. 16

3.2.5. Machining with ultrashort laser pulses ......................................................................... 16

3.3. Application in Tribology ........................................................................................................ 18

4. Conclusions ................................................................................................................................... 19

5. References .................................................................................................................................... 20

3

1. Introduction Unlike ordinary light laser light beams are high directional, have high power density and better

focussing characteristics [1]. These unique characteristics of laser beams are useful in processing of

materials. When a high energy density laser beam is focussed on work surface the thermal energy is

absorbed [3,6] which heats and transforms the irradiated material into a molten, vaporized or

chemically changed state. Vaporization and plasma formation cause material removal, called

thermal ablation. Physical background of caving by ablation with one laser pulse [1,2,4,5] is

presented in first part of this seminar. The effectiveness of this process depends on thermal and

optical properties of the material. Therefore, laser machining is suitable for materials that exhibit a

high degree of brittleness, or hardness, and have favourable thermal properties, such as low thermal

diffusivity and conductivity. In recent years, researchers have explored a number of ways to improve

the laser beam machining process performance by analysing the different factors that affect the

quality characteristics. Rapid improvement of laser technology in recent years gave us facility to

control laser parameters such as wavelength, pulse duration, energy and frequency of laser. The

effects of such laser parameters on laser caving/machining [7] are presented in second part of the

seminar. Further, some mathematical models for three-dimensional CW and pulsed laser caving

[7,8,9] are also mentioned. Laser caving is actually the basis of laser machining and other

manufacturing processes such as drilling, cutting, grooving, marking, surface texturing, etc. While

laser machining is a non-contact, abrasionless technique, which eliminates tool wear and

deflections, vibrations and cutting forces, it can even replace mechanical removal methods in many

industrial applications, particularly in the processing of difficult-to-machine materials such as

hardened metals, ceramics, and composites. At the end, application of laser caving in surface

texturing for improving tribological properties [11,12,13] is mentioned.

4

2. Interaction of laser light with matter

2.1. Absorption of laser light

The physical phenomena that take place when the laser beam is incident on the matter surface are

reflection, absorption and transmission [1]. Most solids, other than those which are visibly

transparent, do not transmit radiation, so we will now discuss only reflection and absorption.

Reflection is influenced by the orientation of the material surface with respect to the beam

direction. For normal incidence of the beam the Fresnel reflectivity [3] is given by:

( ) ( )( )

2

,1

,1,

Tn

TnTR

λ

λλ

+

−=

where ( )Tn ,λ is refractive index of the material, which depends on laser beam wavelength λ and

temperature T . For conducting or absorbing dielectric materials n is a complex number. In bigger

cavities, multiple beam reflections along the wall of cavity are possible, which is schematically

represented in Figure 1. However, because of the complexity of this problem, only single reflection is

examined in this seminar.

Figure 1: Multiple reflections in a machined cavity [2].

Absorption represents the interaction of the electromagnetic radiation with the electrons of the

material. In the absence of radiation transmission and multiple reflections, we can write absorptivity

[1] as R−1 . Beer-Lambert’s law [1,6,7] relates intensity to absorption by:

zeII

µ−= 0

where 0I is the intensity (power) entering the surface and I is the intensity (power) at depth z . µ

is optical absorption coefficient [ ]1−m , which can also be interpreted as penetration depth.

5

2.2. Heating

In general, the size of the laser spot at the workpiece is small (a diameter is of fraction of a

millimeter) and the absorption depth of the workpiece is considerably smaller (fraction of a

micrometer) than the thickness of the workpiece. Consequently, one-dimensional heating situation

may become appropriate to formulate the heating problem. Figure 2 shows a schematic view of the

heating process. The Fourier heat transfer equation [4] due to a laser pulse heating can be written

as:

t

TeI

kz

T z

∂

∂=+

∂

∂ −

α

µ µ 112

2

Figure 2: A schematic view of the heating process [4].

where ( ) 01 1 IRI −= is absorbed radiation power, T temperature, µ absorption coefficient, k

thermal conductivity [ ]mKW and α thermal diffusivity [ ]sm2. Thermal diffusivity [2] is the ratio

of thermal conductivity to volumetric heat capacity:

pc

k

ρα =

where ρ is material density and pc is specific heat capacity. In upper Fourier heat transfer equation

is assumed that convection and radiation losses from the surface during the heating process are

negligible. If we now expand upper one-dimensional case to three- dimensions and consider the

laser output power intensity distribution as Gaussian, than the heat conduction equation for a solid

phase heating due to a laser irradiation pulse can be written as [5]:

t

TeeI

kz

T

zr

Tr

rr s

arz

s ∂

∂=+

∂

∂

∂

∂+

∂

∂

∂

∂ −−

α

µ µ 11 22/

1

where sk and sα are thermal conductivity and diffusivity for solid state. a is Gaussian radius

parameter. Solutions for T are shown in Fig. 3. Thermal properties, such as thermal conductivity,

specific heat for each phase and material density, that are temperature dependent, are taken as

constant in these solutions.

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Figure 3: (a) Temporal variation of dimensionless temperature at different depths inside the substrate. (b)

Dimensionless temperature distribution versus depth z inside for different heating periods.[4]

The magnitude of temperature rise due to heating causes different physical effects such as melting,

vaporization, plasma formation and ablation of the material. For the exact temperature profiles,

when all phases (solid, liquid and vapour) coexist, Fourier heat transfer equation has to be solved

independently for each phase.

2.2.1. Melting

At high laser power densities (25 /10 mW> ), the surface temperature of the material may reach the

melting point mT and material removal takes place by melting. As indicated in Figure 4a, the surface

temperature increases with increasing irradiation time, reaches maximum temperature maxT at laser

pulse time pt and then decreases. The solid–liquid interface can be predicted by tracking the

melting point in temperature versus depth “z” plots (Figure 4b). Before initialization of surface

evaporation, maximum melt depth increases with laser power density I (power per unit area) or

with increasing pulse time. Prediction of melt depth using temperature profiles obtained from

equations in previous chapter assists in determining depth of machined cavity in those materials in

which material removal takes place entirely by melting.

Figure 4: (a) Surface temperature as a function of time. (b) Temperature as a function of depth below the

surface during heating. [2]

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2.2.2. Vaporization

As the surface temperature of material reaches the boiling point, further increase in laser power

density or pulse time removes the material by evaporation instead of melting. The laser light

intensity for material vaporization without significant ionization is for CO2 lasers within 104 to 10

6

W/cm2 for metals. The vapour consists of clusters, molecules, atoms, ions and electrons. After

vaporization starts at the material surface, the liquid–vapor interface moves further inside the

material. With supply of laser energy material is removed by evaporation from the surface above the

liquid–vapor interface. The evolving vapour from the surface applies recoil pressure on the surface,

which removes the evaporated material away from the specimen. The vapour plume will absorb and

scatter the incident laser radiation. The absorption of laser light within the vapour plume obeys the

same law as absorption in solid, only with vapour absorption coefficient instead of µ for solid.

2.2.3. Plasma formation

When the laser energy density surpasses a certain threshold limit, the material immediately

vaporizes, gets ionized and forms plasma. Collisions between thermal electrons and gas-phase atoms

result in certain degree of ionization, NN e=ξ [2]. In dynamic equilibrium, where the rate of

generation is equal to the rate of recombination, ξ is given by:

TkEbe

a

i bieh

Tkm

g

g

N

/

2/3

2

2 22

1

−

=

−

π

ξ

ξ

where NNe=ξ and ae NNN += . eN and aN are the number densities of electrons and

atoms/molecules respectively, ig and ag are the degeneracy of states for ions and

atoms/molecules, iE is the ionization energy, vm is the mass of vapour molecule, bk is the

Boltzmann constant , T is gas temperature, and h is Planck’s constant.

In laser processing, this equation is only of limited value. Here, the laser light may directly ionize

species via multiphoton excitation, or via collisions with electrons accelerated within the laser field

(impact ionization). The plasma plume forms a shield over the machining area and reduces the

energy available to the workpiece. Aerosols formed due to the condensation of ionized material

vapour stick to the surface, what reduces the efficiency of machined components for applications

dominated by wear or tear load. Hence, the degree of ionization is an important parameter which

gives an indication whether plasma will be formed during the machining. Stated problems are

usually reduced or eliminated by applying additional gas stream over the substrate surface. Gas

stream transports the vaporized material and avoids radial distribution of the plasma. It takes a few

hundred picoseconds for a plasma to develop to such degree that it absorbs or reflects laser light [7].

So, the absorption of light in plasma is reduced by using short enough laser pulses (<100ps [7]).

Furthermore, short duration pulses reduce the recast layer thickness, eliminate micro-cracks and the

material removed per pulse increases due to increased energy density of short pulse [7].

8

2.3. Ablation

Material removal caused by rapid vaporization and plasma formation is referred to thermal ablation.

Ablation takes place when laser energy exceeds the characteristic threshold laser energy which

represents the minimum energy required to remove material by ablation. Thermal properties of the

material and incident laser parameters determine the location at which the absorbed energy

reaches the ablation threshold, thus determining the depth of ablation. We can say that material is

ablated as soon as boiling (vaporization) starts. Neglecting the loss of energy due to conduction,

radiation and convection, the threshold energy is given by [6]:

( ) ( )( ) VqTTcqTTcE vmblmmsth ρ+−++−= 0

where sc and

lc are specific heats for solid and liquid phase, mq and

vq are latent heats for melting

and vaporization, 0T is temperature of material before irradiation,

mT and bT are melting and

boiling temperatures, respectively. ρ is material density and V is irradiated (heated) volume of

specimen. Figure 5 highlights numerous physical phenomena present when material removal is

caused by laser light irradiation.

Figure 5: The numerous physical phenomena that are present when machining with laser pulses. [10]

Yilbas [5] introduced numerical model using energy method to describe cavity formation due to laser

heating of steel. Fourier's heat transfer equation for each phase was solved independently as well as

coupled across the interfaces of the two-phases, where both phases exist mutually (mushy zones).

Governing equations of heat transfer were solved numerically while assuming all the thermal

properties were constant. The laser output power intensity distribution at the workpiece surface

was considered being Gaussian and its centre was located at the centre of the coordinate system.

The temporal variation of laser power intensity resembling the actual laser pulse was

accommodated in the simulations. This arrangement results in an axial-symmetric heating of the

workpiece material.

9

Figure 6: Spatial distribution of laser pulse. [5]

Figure 7: Temporal variation of laser pulse. [5]

Figure 8: temperature contours in the surface

region of the substrate material during two

heating periods. [5]

Figure 9: Cavity shape obtained for variable and

constant thermal properties case at 19.7 ns of

heating duration. [5]

Figure 8 shows the temperature contours in the surface region of the substrate material during two

heating periods. Enhancement in the cavity depth is significant during the heating pulse and it slows

progressing after the laser pulse ends. Moreover, increase in the cavity depth after the laser pulse

ending is associated with the convection heating of the cavity surface by the vapour front present in

the cavity. Three-dimensional view of the cavity shape obtained by Yilbas [5] is shown in Figure 9.

Although cavity in Figure 9 resembles the actual shape of cavities produced by laser pulse heating, it

does not show other physical phenomena, such as recast layer and surface debris of melted

material, microcracks, etc. present when laser machining. These phenomena are shown in Figure 10.

Figure 10: Surface debris, recast layer and microcracks present at laser machining. [10]

10

3. Caving

3.1. Caving in one dimension (drilling)

3.1.1. Dependence on pulse energy

We define ablation rate as depth of ablated material per one pulse. It is usually expressed in

[nm/pulse] or [µm/pulse]. It has been shown that higher energy pulses create deeper cavities. In

other words higher energy pulses cause higher ablation rates. Relation between pulse energy and

cavity depth has been shown as being almost linear.

Figure 11: Ablation rate in comparison to laser pulse fluence (energy density). [7]

3.1.2. Dependence on pulse duration

The ablated depth as a function of laser pulse length is shown in Figure 12 for various (constant)

intensities, 0I . Linear relation can be seen. Increasing

0I also increases ablation velocity lhv τ∆=

(slope of solid lines). In this case, for different duration times, intensity was kept constant. This

means that longer pulses have more energy than short ones. However, if we keep energy constant

and vary pulse duration, the ablation depth decreases with increasing pulse time. Longer pulses

cause shallower depths due to loss of energy via thermal conduction.

Figure 12: Dependence of the ablated depth on the duration of laser pulses of intensities

0I . Linear interpolations are shown. [7]

11

3.1.3. Dependence on pulse number

Same linear dependence as on pulse energy was observed on pulse number lN . However, the

relation is linear at lower numbers of pulses. For deep holes, the ablation rate may strongly decrease

with increasing number of pulses. This is shown in Figure 13. The drop-off in rate observed with high

lN , can be related to different effects:

• Loss of energy by heat conduction

increases with increasing hole depth. This

becomes significant at depths comparable

to hole/beam diameter ( )wh ≈ .

• The transport of ablated species becomes

less effective with increasing depth and

favours material recondensation within

the hole.

• The attenuation of incident laser light by

the ejected material due to scattering and

secondary excitation becomes of great

relevance. Such laser-vapour/plasma

interactions are reduced or even absent

with picosecond and femtosecond pulses.

Figure 13: Ablated depth as a function of the

number of laser pulses. [7]

3.1.4. Dependence on spot size

Spot size w is easily changed/increased by moving the focus of the beam away from the substrate

surface. That increases cross-sectional area of the beam, consequently reducing the beam fluence

and intensity. So obviously, moving the focus away from the substrate surface results in lower

ablation rate. However, if we keep fluence constant, the ablation rate decreases with increasing spot

size. This effect originates from the attenuation of the laser beam by expanding plasma. Attenuation

decreases with decreasing spot size because 3D transport of ablation products becomes effective.

Figure 14a shows the ablation rates for LiNbO3 as a function of laser fluence for different laser-beam

spot sizes. For nanosecond pulses, the ablation rates are higher for smaller spot sizes. When

diameters reach 80 µm or more the ablation rate becomes independent of w . Similar observations

were made with various other materials and longer pulses. This effect originates from the

attenuation of the incident laser radiation by expanding plasma plume. For shallow cavities, the

attenuation decreases with decreasing spot size because 3D transport of ablation products becomes

effective. For pico- and femtosecond pulses, almost no plasma plume can develop during pulse time,

and the attenuation is reduced or even avoided. This fact is demonstrated in Figure 14b.

12

Figure 14: Ablation rate versus laser fluence for various spot diameters (d=2w). (a) 11 ns pulses. (b) 1 ps and

15 ns pulses. [7]

13

3.2. Caving in two- and three-dimensions

3.2.1. Mathematical model for CW source

An analytical approach to determining the relationship between the groove depth and process

parameters [1,9] is presented in this chapter. Since the erosion front presents a complicated three-

dimensional shape and is difficult to solve analytically, this approach assumes that the erosion front

can be divided into a set of infinitesimally-small control surfaces (Figure 16); each control surface is

linear on each side with an inclination angle θ from the x axis and φ from the y axis.

Figure 15: Three dimensional view of erosion front and control surface.

Energy is introduced into the control volume due to beam irradiation, and it is dissipated trough

material ablation and heat conduction, as shown by the heat balance relation [1]:

( ) ( )dxdyvLdAn

Tk

dAaI

on

θρφθ

tantantan1 22

+

∂

∂−=

++ =

where a is absorptivity and ( )0TTcqqL vmv −++= is specific latent heat for heating, melting and

vaporization. Several assumptions are made for this analysis. First, thermal properties of material

are constant. Second, beam-material interaction results in material removal at the vaporization

temperature. Any molten material is assumed to be entirely removed by a gas jet. Third, conduction

into the workpiece occurs only normal to the groove surface. Fourth, laser beam has a Gaussian

distribution. From these assumptions, the integral relationship for groove depth can be derived from

the control volume energy balance [1]:

( )( )( ) 0

0

2

22

2exp

, SdxLTTcv

w

yx

w

aP

yxS

x

sp

++−

+−

= ∫∞−

ρ

π

For the case where multiple beam passes are used, 0S is the surface depth created by the previous

beam pas. The surface temperature sT and the slope φ in the y direction are both functions of x

and y . For the centerline (where 0=y and 0=φ ) change in groove depth ( )0,xSD =∆ can be

calculated [1]:

( )( ) ( )( )LTTcvd

aP

LTTcvd

aPD

spsp+−

=+−

=∆00

128.12

ρρπ

14

3.2.2. Pulsed source

Mathematical model presented in previous chapter assumed that material is irradiated with

constant moving source such as CW laser light. However, situation is a bit different for pulsed laser

sources. Trace produced by moving pulsed laser source consists of numbers of small cavities/craters

made by single pulse. Here, we can apply simple model [9]. Lets assume irradiation source is

travelling along x direction with velocity xv and pulse frequency f . Distance between successive

crater centres is then fvD xk /= and is called interpulse displacement. Every pulse creates a crater

of diameter krD . A parameter G , showing the rate at which two successive craters intersect, was

introduced as:

k

kr

D

DG =

Figure 16: Pulse intersection: 0 < G < 1. [9]

Figure 17: Pulse intersection: 1 < G < 2. [9]

Figure 18: Pulse intersection: G = 2. [9]

Figure 19: Pulse intersection: 2 < G < 3. [9]

15

In Figures 16-19 different intersections of successive pulses are presented. The ablation is simplified

and cylindrical shaped craters are used to demonstrate production of larger cavities or grooves. In

fact, craters should have the shape similar to that shown in Figure 9. Despite upper simplification,

the model is good enough to show the roughness of the surface produced by a sequence of pulses.

The roughness would be even more obvious, if actual shape of pulses would be used.

3.2.3. Surface roughness

In laser caved workpieces the surface quality is most important. It is characterized by a nearly

periodic pattern, which determines the roughness. Surface roughness is aspect of surface texture

irregularities. The description by a one dimensional parameter is employed very often. We evaluate

the surface roughness by the Arithmetic mean roughness aR , defined as [10]:

( )∫=l

dxxzl

0

a

1R

where ( )xz is surface profile along the measured axis x and l is reference length. Changes of

operational laser parameters have strong influence on surface roughness and removal rate. In

pulsed laser caving the interpulse displacement has a strong influence on the caving results. In

Figure 20 the effect of the interpulse displacement on the roughness of the surface is presented. As

one would expect, the surface roughness increases with interpulse displacement. It is very surprising

that with very small interpulse displacements the aR is also large. The influence of laser pulse

energies on aR is also shown in Figure 20. It was proven, that surface roughness is lower for low

laser pulse energies. At high removal rates the surface quality degrades. In fact, good surface quality

can only be achieved with poor removal rates. Lower ablation rates can be achieved , not only by

lowering pulse energy, but also by moving laser beam focus position away from the workpiece

surface, which consequently decreases beam’s fluence. The effect of focus position on ablation

depth and surface roughness is presented in Figure 21.

Figure 20: Surface roughness dependence on the

interpulse displacement at different laser pulse

energies. [10]

Figure 21: Influence of the focus position on the

cave depth and surface roughness. [10]

16

3.2.4. Producing arbitrary shaped cavities

Facts, that were presented in previous chapters, can be used to show, how arbitrary shaped cavities

are produced. Figure 22 illustrates the machining of such arbitrary cavity. For deep cavities that

require more layers to be removed, the machining with higher pulse energies and thus higher

removal rates can be used for the initial layers. To obtain final shape of the cavity, precise machining

with low energy pulses is required. Low energy pulses and thus low ablation rates allow us to

produce surfaces with high precision and achieve low surface roughness.

Laser machining also allows us to produce cavities with overhanging side walls. Such cavities can

serve as small oil containers on surfaces where good lubrication is needed. To produce the

overhanging wall, laser beam has to form an angle with the surface normal. Absorption of such

incident beam is complicated, because it depends on beam’s polarization, angle of incidence, angle

of reflection, etc. However, some examples were made, and cavities with the overhanging wall

angles of o50≈pα were produced when the beam tilt angle was

o60≈α [8].

Figure 22: Production of arbitrary shaped cave.[8]

Figure 23: Cave with overhanging wall. [8]

3.2.5. Machining with ultrashort laser pulses

Ultrashort laser pulses are referred to picosecond and femtosecond laser pulses. Usually, extremely

high peak powers are achieved, although low energy is required due to short duration time.

Ultrashort laser ablation has been proved to be a powerful technique for caving thermally high-

conductance materials. The duration of the laser pulse is so short that the energy does not have the

time to diffuse away. It piles up rapidly, causing the material to directly vaporize and even ionize.

With nanosecond pulses heat diffusion out of the ablated volume results in a heat-affected zone

(HAZ). In the simplest approximation, the width of the HAZ can be estimated from the heat

penetration depth td p α4= , where t is pulse duration time. Thus, with ultrashort pulses, the

HAZ can be significantly reduced or even avoided. Another feature of ultrashort laser pulses is that

plasma shielding is not relevant. It takes about 100 ps for the development of high-density plasma

[7], which becomes opaque for further transmission of laser energy due to absorption and

17

reflection. Laser pulses shorter than 100 ps therefore avoid plasma shielding. Because both the

influence of heat conduction within the material and screening of the incident laser light by plasma

are strongly diminished with picosecond pulses, and can even be ignored with femtosecond pulses,

machining is much more accurate and requires less energy. High accuracy of ultrashort laser pulse

machining in comparison with nanosecond pulses machining is shown in following Figures 24-25.

Figure 24: Holes fabricated in a 100 µm thick steel foil. (a) 3.3 ns pulses with 4.2 J/cm2. (b) 200 fs

pulses with 0.5 J/cm2. [7]

Figure 25: Ablated NaCl surfaces. (a) With nanosecond pulses (16 ns, 4.2 J/cm2, N=15) cracks are

reaching deep into the surrounding material. (b) With femtosecond pulses (300 fs, 0.5 J/cm2,

N=500) well defined patterning without any cracks within the surrounding material is possible. [7]

18

3.3. Application in Tribology

Laser surface texturing (LST) has emerged in recent years as a potential new technology to enhance

lubrication and reduce friction in mechanical components. LST produces a large number of micro-

dimples on the surface and each of these microdimples can serve either as a micro-hydrodynamic

bearing in cases of full or mixed lubrication, a micro-reservoir for lubricant in cases of starved

lubrication conditions, or a micro-trap for wear debris in either lubricated or dry sliding. Perhaps the

most familiar and earliest commercial application of surface texturing is that in engines. Friction loss

in an internal combustion engine is an important factor in determining fuel economy and

performance of the vehicle. Approximately 40-50% of the friction losses in an internal combustion

engine are due to the piston/cylinder system [11,13]. Different studies [11,12,13] pointed out that a

friction reduction of 30% and even more is feasible with a textured surface. Also, a dimple size of

approximately 100 µm and a dimple density of 5–20% was recommended [11]. Such textured

surface is presented in Figure 26b, where very little damage can be seen due to friction. In Figure

26a a much greater damage of untextured surface due to higher friction is shown. Different test rigs

and results are shown in Figures 27-30.

Figure 26: Different surfaces after a long sliding test. (a, left) Damage of untextured surface

due to high friction. (b, right) Dimpled surface with almost no damage due to friction. [11]

Figure 27: Seal model. [12]

Figure 28: Friction torque versus sealed pressure

for nontextured and textured seals. [12]

19

Figure 29: Test rig for simulation of piston-cylinder sliding. [13]

Figure 30: Time-averaged friction force vs. crank angular velocity for

external normal pressure 0.5 Mpa. [13]

4. Conclusions Interaction of laser light and matter was presented in the first part of the seminar. The absorbtion of

radiation, heating, vaporization and plasma formation were examined for nanosecond pulses. The

dependence of ablation characteristics on different pulse parameters was also examined and some

models were presented. It has been shown that higher laser pulse fluences and longer pulses with

smaller diameters result in higher ablation rates. However, higher ablation rates also cause

increased surface roughness. Therefore, lowest surface roughnes and thus best surface qualities can

be achieved with low ablation rates. However, ultrashort pulses are also useful for producing quality

surfaces. Their short duration reduces thermal conduction within the material and avoids plasma

scattering. At the end, application of surface texturing with small cavities (dimples) was presented. It

has been proven, that friction can be reduced with appropriate surface texturing.

20

5. References

[1] G. Chryssolouris, Laser Machining: Theory and Practice, Springer-Verlag, 1990.

[2] A. N. Samant, N. B. Dahotre, Laser machining of structural ceramics—A review, Journal of

the European Ceramic Society, 29 (2009), 969–993.

[3] J. Byskov-Nielsen, P. Balling, Laser structuring of metal surfaces: Micro-mechanical

interlocking, Applied Surface Science, 255 (2009), 5591–5594.

[4] B.S. Yilbas, N. Al-Aqeeli, Analytical investigation into laser pulse heating and thermal stresses,

Optics & Laser Technology 41 (2009), 132– 139.

[5] B.S. Yilbas, S.B. Mansoor, Laser pulse heating and phase changes in the irradiated region:

Temperature-dependent thermal properties case, International Journal of Thermal Sciences

48 (2009), 761–772.

[6] J. T. Luxon, D. E. Parker, Industrial Lasers and Their Applications, Prentice-Hall, 1985.

[7] D. Bäuerle, Laser processing and Chemistry (Third Edition), Springer, 2000.

[8] A. Horvat, Lasersko Dolbenje, Magisterij, Fakulteta za strojništvo, Ljubljana, 1996.

[9] G. Rakovec, Lasersko Označevanje v Maloserijski Proizvodnji, Magisterij, Fakulteta za

strojništvo, Ljubljana, 1991.

[10] A. Horvat, L. Grad, J. Možina, Surface Roughness in Pulsed Nd:YAG Laser Caving Of Alumina

Ceramic, Lasers & Engineering, Vol. 6 (1996), 151-159.

[11] M. Wakunda, et al, Effect of surface texturing on friction reduction between ceramic and

steel materials under lubricated sliding contact, Wear 254 (2003), 356–363.

[12] I. Etsion, State of the Art in Laser Surface Texturing, Journal of Tribology 127 (2005), 248-

253.

[13] G. Ryk, et al, Experimental Investigation of Partial Laser Surface Texturing for Piston-Ring

Friction Reduction, Tribology Transactions 48 (2005), 583-588.

[14] Micromachining Handbook:

http://www.cmxr.com/Industrial/Handbook/Index.htm , april 2008