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Chapter 2.1 Principles of Laser Cleaning in Conservation Salvatore Siano Istituto di Fisica Applicata “Nello Carrara”,
Consiglio Nazionale delle Ricerche, Sesto Fiorentino, Italy Contents 2.1.1
Introduction The properties of monochromaticity,
collimation, and coherence of laser light and the associated
interaction features have favoured the development of a variety of
applications in several fields, such as for examples industrial [1-3],
biomedical [4-6], and cultural heritage [7-9]. A general distinction
between applications not involving relevant and permanent physical
changes to the irradiated material and the ones producing irreversible
modifications can be done. The former are usually aimed at
characterisation purposes, whereas the latter include both diagnostic
and processing techniques. Laser ablation is one of the most important
irreversible irradiation effect (see for example [10,11], and the other Proceedings
of the COLA Conferences), which can be induced on optically absorbing
materials or in their close proximity. Laser cleaning is a particular
case of laser ablation where a specific material layer or substrate is
uncovered through the removal of undesired layers or
incoherent particle distributions. Laser cleaning processes
are exploited in different fields. Thus for examples, besides the
conservation of artworks discussed in the following, it is being used
in a number of industrial needs, such as semiconductors cleaning in
microelectronics, die cleaning in plastic pressure casting, paint
stripping in the aircraft maintenance, and other. Even some laser
surgery treatments, such as for example the removal of tattoos, can be
classified as a cleaning application. The first observation of the laser ablation and
cleaning phenomena dates back to the origins of laser technology [12,16]. Significant advancements on
the understanding of the physical mechanisms through systematic
phenomenological studies and the diagnostics of the laser-material
interaction were achieved starting from beginning eighties. The main
results were provided by researches related with medical surgery and
microelectronic industry, which resulted of fundamental
importance also in other fields of application including the
present one. After a short historical note, the following
paragraphs report a brief review of the basic mechanisms involved in
the laser cleaning process in the restoration of art and historical
artefacts. It is based on the suitable framing of the general
achievements mentioned above within the present application domain and
on the specific insights related with important conservation problems
that were approached by laser during the last decade.
The application of the laser cleaning in the
conservation of artworks was proposed by J.F. Asmus
since the beginning of seventies [17-19] through a set of practical
tests carried out in Venice on encrusted stone artefacts. As reported
by the involved conservation scientists and restorers [20-23], the
novel approach did not overcome the experimental stage for several
years mainly because of the technological limits of the pulsed laser
sources available at that time: Ruby and Nd:YAG
lasers with low pulse repetition rate, absence of versatile beam
delivery systems, very low reliability for long time operations, and
high costs. During the eighties the technological level of
the laser devices increased significantly but the costs still were out
of scale for the specific field of application, even more whether
considering the relatively low productivity, as compared with
traditional chemical and mechanical cleaning techniques. The
“surviving” of the novel conservation approach for
more than a couple of decades, against unfavourable performances along
with the caution, scepticism, or indifference of the most of the
conservation community, has to be completely attributed to the
perseverance of Asmus
and his co-workers [24-30]. Mainly thanks to the stimulus provided to the
research of innovative technologies dedicated to the study and
safeguard of cultural heritage by European Framework Programmes and
various National Innovation Programs, the situation drastically changed
during the nineties. Several research centres, conservation
institutions, and restoration enterprises initiated constructive
interactions aimed at developing laser systems and methodologies
dedicated to different classes of materials and deterioration
problems. Up to 1995 the scientific results were reported in some
conservation meetings, disciplinary congresses, and journals without
the possibility of real interdisciplinary debates and experience
exchanges on the topic [18-49]. The institution of the international
conference LACONA (Laser in the Conservation of Artworks) [50] held for
first time in Crete (1995) has represented a very important step for
consolidating and refining the positive results already achieved on
encrusted stone and extending the application to other conservation
problems by increasing the scientific level and improving
interdisciplinary approach. The conference has become a fundamental
reference for the development and dissemination of the laser techniques
in the conservation field. Physical investigations on the laser cleaning
processes of stones, metals, paintings, paper, parchment etc.,
and thorough evaluations of the irradiation effects were reported
throughout the six editions of the LACONA conference [50-54] held up to
now. In recent years, two more congresses were born within the
disciplinary context of the applied optics dedicated to the application
of laser techniques in conservation (SPIE conference Laser Techniques
and Systems in Art Conservation Munich 2001-05 [55-57] CLEO 2005
Symposium on: Laser technology for the preservation of cultural
heritage). Furthermore, specific sessions on the topic started to be
included in congresses on laser technology and applications
(International workshop on: New trends in laser cleaning 2001-05, COLA
2003 [10], and other). Some contributions can be also found in the
proceedings of interdisciplinary conferences such as Science
and Technology for the Safeguard of the Cultural Heritage in the
Mediterranean Basin (since 1995 [58]), International Congress on
Deterioration and Conservation of Stone, and other. The
gradual acceptance within the scientific and conservation
communities of what can be defined as a kind of revolution in
the conservation procedures is also documented by the increasing number
of studies published in various applied physics, applied chemistry, and
interdisciplinary journals. Anyway, as a result of this collective effort,
which involved the scientific community of many countries, the most
important advancement is represented by the transition of laser
techniques from the laboratory experimentation to the everyday practice
of important conservation institutions and restoration enterprises.
Along the last decade There are no special recipes to speedup the
acceptance and dissemination process. The only effective ways
are the ones based on rigorous approaches to the material
characterisation, physical evaluation, and validation associated with
specific conservation objectives, whose definition pass through a
strict interaction of the scientific component with the historical,
artistic, and management ones. The present section aims at providing a first
introductory picture of the main physical phenomena, which should be
taken into account for optimising laser cleaning treatments, while the
state of the art for the various materials and conservation problems is
summarised in some details in the next sessions of this chapter.
2.1.3 Laser Systems and Parameters Only pulsed lasers are used in the cleaning of
art and historical objects. Up to a few year ago, the most employed
systems in stone cleaning were based on Q-Switching (QS) Nd:YAG lasers emitting at the
fundamental harmonic (1064 nm) pulses of typical duration of 8-20 ns
and energies between 0.1-1 J/pulse. Usually, also the Free Running (FR)
regime is available on these commercial laser systems providing pulse
durations of 200-500 µs and higher pulse energies up to 2 J
but, as explained in the following, this range of pulsewidth is not effective for
the most of the cleaning problems. A novel class of Intermediate Pulse
Duration (IPD) Nd:YAG
lasers was proposed and commercialised in the last years. These systems
are based on Short Free Running (SFR) and Long Q-Switching (LQS)
regimes providing pulse duration between 50 ns-3 µs and
20-120 µs, respectively, with similar energies as FR
and QS lasers [59-60]. IPD laser systems are always fibre-coupled (1
mm typical core diameter) and equipped with versatile handpieces providing very
homogeneous and a finely controllable irradiation spot. The market also
offers QS lasers coupled into relatively thick fibres (1.5 mm core
diameter). The output energy from a single fibre is limited to around
200 mJ because of
damage risks due to the very high intensity and associated
non-linear absorption phenomena (optical breakdown). Only a few of
these systems are presently in use, whereas the most of commercialised
QS laser systems are coupled in an articulated arm, which allows to
propagate higher pulse energies (up to 1 J) but with a low beam quality
because of possible hot spots and fringe structures usually associated
with Nd:YAG laser beams. The handpieces
have a lower spot control and versatility with respect to fibre optical
beam deliveries. The success of this class of systems is due to the
relatively high cleaning efficiency, which favoured their employ for
cleaning intervention on relatively large areas. During the last years the mentioned limits
along with the problem of yellow appearance associated with the
cleaning of whitish substrates [61-63] have significantly slowed the
commercial spread and hence the use of QS systems, which provided room
to a relevant penetration of the market by the SFR lasers, which was
favoured by a number of basic studies and successful example
applications [47,63-71]. Multi-wavelength QS Nd:YAG
laser systems were also proposed for overcoming the problem of the
yellow appearance and to approach the cleaning of wall paintings. In
particular, positive results were documented for the second harmonic
(532 nm) [71-74] and the combination of first and third harmonics [75].
Despite the higher technological complexity and costs of the
multi-wavelength lasers, which are big obstacle to a wide spread, it is
worth nothing that the double wavelength solution was
successfully applied to clean sculptural elements of the frieze of the
Parthenon [76]. Two more class of lasers were proposed for the
cleaning of paintings through extensive laboratory tests and some
practical applications. They are KrF
excimer laser (248
nm, around 30 ns) and FR Er:YAG (2.94
µm, 250 µs) [77-80]. Different order of problems,
concerning side effects and for the former also costs and lack of
portability, are preventing the acceptance and widespread of cleaning
treatment based on these laser systems. The present perspective appears
hence not so promising. Even less whether considering the positive
results recently achieved by Nd:YAG
laser systems [74, 81-85] that already dominate the present field of
application.
2.1.4
Linear Laser-Material Interaction Laser ablation of a material stratification is
a strongly non-linear process occurring when the irradiation
fluence (pulse energy per unit area: F0
= E/A) or in some cases
intensity (peak power per unit area: I0
= P/A) overcomes a critical
threshold, which is an intrinsic property of the material structures
under irradiation. In the domain of interest fluence and intensity are
usually expressed in mJ/cm2 (m=10-3)
or J/cm2 and MW/cm2 or
GW/cm2 (M = 106, G = 109),
respectively. The dynamical development of the laser ablation
involves optical, photothermal
and photomechanical phenomena depending on the laser
parameters and material properties. In order to understand the
different ways in which pulsed laser irradiation can produce material
removal, it is useful to introduce separately these different phenomena
starting from their linear regimes occurring at relatively low
fluences, i.e.
significantly lower than the critical ones for inducing any
irreversible effect to the irradiated material.
2.1.4.1
Absorption and
Scattering The incidence of a laser beam on a material
(here and elsewhere if not specified differently the material is
assumed to be homogeneous) is accompanied by absorption and scattering
phenomena producing attenuation and spatial redistribution (diffusion)
of the beam energy. In the case of a layer of dielectric material (as
for examples black crust, whitewashes etc.), it is useful
distinguishing among back scattered, absorbed, and
forward-scattered radiation and to introduce the reflectance (R=Er/E),
absorbance (A=Ea/E),
and transmittance (T=Et/E)
parameters of the material layer (Fig. 2.1.1):
When irradiating thick or very absorbing
materials T ≈ 0, but in practical
cases of adjacent material layers, often the transmittance must be
taken into account in the energy balance (Fig. 2.1.1c). The flux of energy, which propagates into the
material, F, undergoes to a typical exponential
attenuation law along the optical axis, z:
where Fa=(1-R)F0
and µ is the effective
absorption coefficient, whose reciprocal represents the optical
penetration depth δ = 1/µ, also named optical
extinction length. It is the
length of the optical path along the z-axis, which
produces an attenuation of the energy flux at the material surface Fa
of a factor 1/e.
For very absorbing material δ = 1/µa, where µa is absorption coefficient,
whereas at the opposite limit, when the propagation is dominated by the
scattering, the diffusion approximation provides the following
expression:
where µs is the scattering coefficient,
g the
anisotropy parameter, which represents the
integral average of cosΘ on the scattering phase function.
These are fundamental parameters of the material depending on its
composition and microstructure, as well as on the laser
wavelength δ. The penetration parameter δ allows estimating the irradiated (or
absorption) volume, V=d×δ, where d is the laser spot
diameter. For visible and near infrared wavelength the typical values
of δ range from several nanometres of a metal, to ~10-100 µm of a fairly homogeneous
black crust or a brown patination,
up to several millimetres of calcite or gypsum. When the thickness of
the irradiated material layer, l, is lower
than δ it could be useful to consider δ = l. Fig. 2.1.1 displays a qualitative
representation of the energy re-distribution in homogeneous
material layers in the case of absorption (Fig. 2.1.1a) and
scattering limits (Fig. 2.1.1b), along with a composite situation of an
absorbing layer on a diffusing substrate (Fig. 2.1.1c). Anyway, in
cases of practical interest the irradiated material layers are strongly
inhomogeneous. Thus besides the mentioned approximation limits
also their superposition within the same layer must be taken into
account as possible description of the optical propagation regime.
Fig. 2.1.2 shows qualitative examples of optical propagation
into real stratigraphies
where the outer layer is absorbing (Fig. 2.1.2a), diffusing
(Fig. 2.1.2b) or both (Fig. 2.1.2c).
Despite the strong variability that can be
encountered from zone to zone of the same artefact or also within the
depth of a relatively thick stratification and the difficulty to
determine the real energy distribution, as well as to measure µa, µs, and phase function, useful estimations of the
irradiation effects can be derived from relatively simple reflectance
and transmittance measurements (Fig. 2.1.3). These allow
measuring EA, the absorption
volume, and hence the energy density inside the material (ε = EA
/V expressed in J/cm3), which is the fundamental
parameter for any irreversible phenomenon associated with the laser
irradiation.
Table 2.1.1 reports the results of the
reflectance measurements in dry and wet conditions for two stone
typologies: white Table 2.1.1: Examples of reflectance
measurements of stone quarry samples and fragments from historical
facades of
As it can be seen, the samples from historical
façades exhibit a significant increase of reflectance while
stepping from the black crust, to the underlying layer (Ca-oxalates
film or surface sulphation).
This optical feature is very useful in laser cleaning treatments since
it favours a self-terminating behaviour of the laser ablation within
relatively wide operative fluence ranges. Anyway, as stated above, the
most important parameter for any induced effect is energy density
realised into the irradiated materials, As shown by reflectance and penetration
measurements, the wetting of the irradiated surface produces and
increase of both A and d parameters. For the present examples the two
effects are balanced, the wetting does not produces a relevant
variation of the energy density within the irradiated volumes
(i.e. εwet ≈ εdry). On the other hand water assists plays an
important thermal role.
In the most of the cases the absorbed energy Ea
is dissipated through the thermal channel, whereas only at high
irradiation intensities or short VUV wavelengths also the direct
ionisation and molecular photodissociation
can play an important role. Hence, the main direct effect of the laser
irradiation is a temperature rise within and in proximity of
the irradiated volume. Theoretical estimations of the thermal
distributions induced in homogeneous materials can be derived through
the heat conduction equation [86] under the assumption of constant
optical and thermal parameters. This hypothesis holds whenever the
temperature peak are
lower than the critical ones for any irreversible phenomenon
(discoloration, carbonisation, vaporisation etc.). Let us consider a homogeneous laser beam of
intensity I(t)
incidents on the surface of a semi-infinite conductive material and a
surface photothermal
conversion of the absorbed energy (δ=0). The one-dimensional solution of the
conduction equation allows achieving the temperature rise ΔT induced by the intensity Ia(t)=(1-R)I0(t):
where K and D
are the thermal conductivity and diffusivity
of the material, respectively. D=K/ρCp, with ρ and Cp
density and specific heat
of the material. Besides the material parameters the use of eq. 4 requires the knowledge of
the temporal profile of the laser pulse. As a first approximation it
can be often assumed as Gaussian in the nanoseconds range and top-hat
for longer pulse durations. For this latter case the expression of the
surface temperature assumes the following well-known form:
where τ is the laser pulse duration
(FWHM). As a general behaviour this equation states that the surface
temperature increases when the pulse duration decreases, which is
particularly important in the cleaning of metal artefacts. As an
example Fig. 2.1.4 reports the temperature rise at gold-air
interface provided by equations 4 for Gaussian profiles and Fa
= 0.15 J/cm2. As it can be seen, the temperature
peak decrease from 454 °C to 148 °C when the pulse
duration increases from 6 ns to 100 ns, which corresponds to a scaling
law around τ-0.4, only slightly different from the τ-1/2 dependence associated with top-hat pulses (eq. 5). The parameter representing the heat propagation
into the material is the thermal diffusion length: For top-hat laser pulses, it is the propagation
distance of the thermal wave producing an attenuation of the peak
temperature to about 0.1 of the maximum surface value.
Fig. 2.1.5 displays the thermal diffusion length of copper,
sandstone, limestone, and water. Considering for example τ=200 µs (FR lasers) eq. 6 provides zth=305,
38, 30 and 11 µm, respectively.
Laser irradiation is indicated as thermally
confined whenever zth
<< δ, or also τ << τth where:
is thermal relaxation time,
representing the transit time of the thermal wave throughout the
irradiated volume along the optical axis. In this condition
the thermal effects are strictly circumscribed within the irradiated
volume. The temperature rise in an absorbing material (µs << µa) is provided by energy balance:
When considering the ideal case of a
homogeneous insulating material, the thermal confinement condition
usually represents a good approximation for pulse duration up to the
microseconds range, whereas only for a few materials, as for example
pure black carbon or graphite, the conduction limit can provide useful
estimations. On the other hand, relatively complex features are
expected in real stratifications similar to the one of Fig. 2.1.2c
because of the superposition of the conduction and confinement regimes
described above. Furthermore, also the thermal role of water should be
taken into account in water-assisted conditions. In porous materials,
such as the one encountered in deterioration stratifications,
the wetting produces a relevant reduction of peak temperature within
the irradiated volume since the imbibitions improves the thermal
conduction and increases the average specific heat. To complete these introductory notes on the photothermal effect it is useful
to mention also the possibility of cumulative heating
occurring whenever the pulse repetition rate, f,
is too high for allowing a complete thermal relaxation within the time
interval between two consecutive laser pulses. The saturation
temperature rise generated by cumulative heating is
proportional to the average power Pav=f×E
and inversely proportional to the laser spot radius, r:
At the operative fluences
of SFR lasers for black crust removal from white marble this
contribution could start to be not negligible above 10-20 Hz. Thus for
examples direct temperature measurements during 80 s
irradiation of aged white marble provided an inferior limit of the
saturation temperature of 130 °C [8]. In general the cumulative
heating should be avoided or at least minimised in order to reduce the
risks of thermal side effects such as discoloration, structural damages
due to thermal dilatation, and other.
Pulsed irradiation can generate acoustical
transients, which propagates into the irradiated materials structures
along distances much larger than δ and zth.
The basic mechanisms can be very different depending on the physical
properties and laser parameters. For solid absorbing materials laser
intensities of order of 106-108
W/cm2 the photoacoustic
effect is usually originated by the thermoelasticity.
All the materials in different extents exhibit
a volume variation when heated, which is reversible within specific
temperature and pressure range, i.e. thermoelasticity
domains. The parameter characterising the effect is the thermal
expansion coefficient, β, representing the relative volume variation
produced by unit temperature variation. Laser irradiation with pulse duration short
enough to realise the thermal confinement condition (τ << τth) generates fast thermal transients within the
irradiated volume and hence associated pressure rises. This produces a
pressure wave, which propagates into the medium with a quasi-sonic
speed.
The general problem of the thermoelastic generation in
homogeneous absorbing materials can be approached through the classical
method of the Green function [87-89]. Two different solutions are
determined for rigid (r) and free (f)
boundary conditions, respectively:
where
where Zaw
= τa,m ca,m
are the acoustic impedances of air and solid material, respectively.
Since it is Zm
>> Za
eq. 11 provides Ra
c ≈ -1, that corresponds to a total reflection at
the water-air interface, which is accompanied by a phase inversion
producing the rarefaction peak described by eq.
11. The transit time along the distance d is named elastic relaxation
time, τel:
If τ<τtel, the pressure into the irradiated volume
increases along the whole laser pulse duration since the elastic
relaxation occurs at longer times. This defines the inertial
or pressure confinement condition, which
gives rise to a high-pressure gradient at the interface. In principle,
this condition can be realised when using short pulse duration (ex. a
few nanoseconds) on materials exhibiting relatively large optical
penetrations (ex. several tens of microns).
2.1.5
Non-Linear
Laser-Material Interaction The increase of the laser fluence or intensity
produces the transition to non-linear interaction regimes resulting in
microscopic and macroscopic irreversible changes to the irradiated
material structures, which invalidate the description of the optical,
thermal, and mechanical features reported above. The extreme stage is
represented by the generation of a multiphase flow including solid
particles, vapour, gas, and in some cases also a plasma phase, which
characterises the so called ablation plume. Before approaching the
description of the different possible ablation channels let us list
some of the main non-linear interaction effects, which are involved at
sufficiently high fluence or intensity levels. Variation
of the macroscopic parameters —
Discoloration, optical trapping, and atomic scale effects are
responsible for significant variations of the optical
parameters, —
For amorphous solids a temperature rise usually produces an increase of
K and D, —
The thermal expansion coefficient depends on the temperature, —
Porosities, surface roughness, microfractures
and other structural features can affect in different extend the
description of the laser induced effects based on macroscopic material
parameters, —
Laser induced structural modifications can produce strong non-linear
effects, —
Material removal is an efficient cause of cooling of the material
substrate. It produces strong discontinuities in the optical, thermal
and mechanical propagation phenomena. Atomic
and molecular scale —
Multiphoton
absorption, —
Saturation of specific absorption bands, — Photodissociation, — Photoionisation, — Plasma formation.
2.1.6
Note on
Laser-Induced Plasma Plasma is a macroscopically neutral gaseous
phase where a relevant fraction of particles (of order of 10% or more)
is ionised. Laser irradiation of absorbing materials at high intensity
(above 108 W/cm2) can
induce the onset of plasma in proximity of the surface through the
so-called optical breakdown phenomenon [90]. It
develops from a number of initial electrons, mostly generated by multiphoton ionisation of atoms
and molecules. The energy of these free electrons is increased by
absorption of incident photons (inverse bremsstrahlung)
up to the ionisation energy levels, thus driving an avalanche
multiplication. Depending on the intensity and temporal profile of the
laser pulse, after some or several nanoseconds the electron
density reaches values of order of 1018-1020
cm-3 and the electron temperatures
rises up to ~104 K [91]. The plasma plume is very opaque at long
wavelength, such as the ones on CO2, Er:YAG
and Nd:YAG lasers, being the plasma optical extinction length δp In the practical cases of laser cleaning,
plasma formation can occur when using short QS Nd:YAG
lasers (ex. 5 ns) at relatively high fluences
and hence intensity (ex. 3 J/cm2, 6×108 W/cm2)
to remove hard or weakly absorbing stratifications. Anyway, these
operative conditions are often ineffective and harmful for the
integrity of the substrate. Whereas, at the typical operative fluences (0.5-1 J/cm2)
in water assisted conditions, the occurrence of a dense plasma phase is
unlike, even though some not significant ionisation phenomena, favoured
by inhomogeneous beam and strongly absorbing components (hot spots and
black carbon particles), could accompany the ablation dynamics [92]. Similarly, no dense plasma is expected for
cleaning with LQS and excimer
lasers because of the low operative intensities. For example the
maximal operative fluence in stone and metal cleaning with LQS Nd:YAG
laser pulses of 20-120 ns is around 3 J/cm2,
which corresponds to 2.5-15 107 W/cm2,
while excimer laser
removal of varnishes is carried out around 0.5 J/cm2
(about 1.7×107 W/cm2).
SFR lasers at fluences
as high as 20-30 J/cm2 can produce the formation
of rarefied plasma, which is optically
thin and hence does not influence significantly the ablation
dynamics driven by direct laser-material interaction [64].
A general scheme of the different laser
ablation channels is reported in Fig. 2.1.7, for pulse duration ranging
between a few nanoseconds up to hundreds of microseconds. Let us
distinguish ablation occurring below and above the vaporisation
threshold of the irradiated material. At fluences
below the minimum one for vaporisation the only possible ablation
mechanisms are of photomechanical type, apart from the special case of
VUV wavelengths, where the direct molecular bond breaking can provide a
relevant contribution. The two main channels are based on pressure
confinement (t<tel), where the impulsive ejection is generated by
the high-pressure gradient at the interface, and primary spallation [93-94]
occurring when the rarefaction peak exerts a strength larger than the
specific breaking load of the material. These two channels are very
interesting for laser cleaning applications because they involve
moderate temperatures and are very efficient. In real cases of
inhomogeneous multilayer stratifications, the mechanisms
of the photomechanical ablation gets rather more complex.
They include coherent superposition effects of pressure transients
generated at the absorption centres, secondary and water
mediated spallation.
These latter two mechanisms are frequent in cleaning applications. As
schematically illustrated in Fig. 2.1.8, in secondary spallation a relevant part of
the radiation is absorbed underneath the outer layer of the irradiated
stratification, which is removed by interface pressure development in thermoelastic or vaporisation
regimes. Instead the concept of water-mediated spallation
aims at representing the significant role of water assists in the
mechanical coupling and propagation of the pressure wave. Above the vaporisation threshold only fast
thermal explosion induced by laser pulses in the
nanoseconds range is the most properly called laser ablation,
where the thermal confinement is usually assumed as rigorously
verified, but in the present concern it is useful to extend the concept
to plasma-mediated material removal, of importance
in some specific high intensity treatment, and to quasi-continuum
vaporisation produced by SFR and FR Nd:YAG
lasers. Fast thermal explosion and plasma-mediated ablation are usually characterised by fluid dynamic regimes and strong recoil stresses released to the material surface [95-98]. For pulse duration in the order of microseconds, pure vaporisation and plasma channels do not exhibit strong differences because, as mentioned above, the plasma phase is expected to be very rarefied. For this reason in Fig. 2.1.7 they were merged in a single channel. The front of the ablation plume proceeds with a quasi-sonic speed, so it cannot drive a shock wave and then a significant recoil stress [64]. Finally, not thermally confined (τ>τth) slow vaporisation is the ablation mechanism at pulses duration of 200-500 µs, usually provided by commercial Nd:YAG laser systems.
The ablation rate represents the single pulse efficiency of the laser removal at a given fluence. It is usually measured in µg/pulse or µm/pulse, while the ablation efficiency in µg/J·cm-2/J or µg/J·cm-2. The knowledge of the dependence of the ablation rate on laser fluence is of practical importance in order to precisely control the removal of the stratification. The typical behaviour for ablation above the vaporisation threshold is the one reported in Fig. 2.1.9a, where the fluence and rate scales were arbitrarily assumed as the ones of the experimental data of Fig. 2.1.9b.
Laser ablation starts to be observed above the
minimum fluence Fth
named ablation threshold. Above this value the
removal is almost linear up to the saturation fluence
Fs, indicating where
the efficiency is significantly reduced by dissipative phenomena as in
particular ionisation and plasma formation. The maximum efficiency is
achieved at the intermediate fluence Fm.
The maximum rate in a pure vaporisation regime is equal to the optical
penetration depth, δ. The so-called blow-off
model provides a theoretical estimation of the rate curve
[99]. It is based on the assumption the material is immediately removed
when the laser heating overcomes the critical energy density
of the irradiated material, εcr, which allows achieving the following
estimation of the ablation rate and threshold:
The condition zabl=δ allows determining the saturation fluence Fs=ε·Fth ≈ 3·Fth, which can be considered as a limit operative
fluence. Equations 13 are derived under the assumption
of thermal confinement, i.e. from eq.
2 and eq. 8. It is
important to note that in the present approximation Fth
does not depend on the laser pulse duration. Conversely, for
long pulses τ > τth) eq.
5 (conduction limit) provides the following expression:
which shows a dependence on pulse duration of
type τ1/2. Thus for example if Fth
is the ablation threshold at 50 µs, the threshold at 500
µs will be 3.3 Fth.
The strong reduction of efficiency explains why long pulses are rarely
used in laser cleaning. For real stratifications it is useful to
introduce concepts like cleaning fluence of cleaning
threshold Fcl,
which is the minimum laser fluence providing the desired cleaning result in self-terminated
cleaning treatments, since Fth
can exhibit strong variations from point to point and within the
stratification depth. Anyway, their fluence and pulse duration
dependence are still roughly estimated by eqs.
14 and 15.
In this section we provided an introductory
description of the main physical concerns involved in the lasers
cleaning of artworks. Several insights on the various topics and
examples of application aimed at optimising specific laser cleaning
treatments can be found in the following sections and in the reported
literature. At the same time it is worth noting that further studies
are needed in order to provide thorough quantitative descriptions of
the interaction dynamics occurring in real inhomogeneous
multilayer stratifications.
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Steen W. M., Laser
material processing, 3rd edition, Springer-Verlag,
[2]
Migliore L. (Ed.), Laser
Material processing, Marcel Dekker
Inc., [3]
Chryssolouris G., Laser
Machining, theory and practice, [4]
Palumbo G., Pratesi R. (Eds.), Lasers and current
optical techniques in biology, European Society for Photobiology, Honk
Kong, 2004 [5]
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Istituto di Fisica Applicata “Nello Carrara” Consiglio Nazionale delle Ricerche Sesto Fiorentino Italy |
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