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Journal of Radiation Research and Applied Sciences xxx (xxxx) xxx–xxx
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Contents lists available at ScienceDirect
Journal of Radiation Research and Applied Sciences
journal homepage: www.elsevier.com/locate/jrras
A review of incorporating Nd:YAG laser cleaning principal in automotive
industry
Mohammad Khairul Azhar Abdul Razaba,∗, An'amt Mohamed Noorb, Mohamad Suhaimi Jaafarc,
Nor Hakimin Abdullahb, Fatanah Mohamad Suhaimid, Mazlan Mohamedb, Noraina Adame,
Nik Alnur Auli Nik Yusufb
a
Medical Radiations Programme, School of Health Science, Universiti Sains Malaysia, Health Campus, 16150, Kubang Kerian, Kelantan, Malaysia
Advanced Materials Research Cluster, Faculty of Bioengineering and Technology, Universiti Malaysia Kelantan Jeli Campus, Locked Bag No. 100, 17600, Jeli, Kelantan,
Malaysia
c
Medical Physics Laboratory, School of Physics, Universiti Sains Malaysia Main Campus, 11800, Minden, Penang, Malaysia
d
Advanced Medical and Dental Institute, Universiti Sains Malaysia, Bertam, 13200, Kepala Batas, Penang, Malaysia
e
Computational Chemistry and Physics Laboratory, School of Distance Education, Universiti Sains Malaysia Main Campus, 11800, Minden, Penang, Malaysia
b
A R T I C LE I N FO
A B S T R A C T
Keywords:
Laser removal mechanisms
Coating system
Ablation
De-composition
Particulate matter
Laser cleaning has been identified as an ideal technology to replace conventional chemical techniques in the
motorcar coating removal process to maintain the sustainability of our environment. This is due to the unique
characteristics of this cleaning technique, such as being versatile, precise, controllable, lack of waste generation,
and environmental friendly process. This laser technique can remove the coating layers without using chemical
products and prevents the metal substrate surfaces from defect. This paper reviews the potential of incorporating
pulsed Nd:YAG laser and its principals in coating removal for automotive industry with respect to these characteristics.
1. Introduction
Nd:YAG laser consists of transparent dielectric crystals or amorphous glasses that act as “hosts”, where some of the ionic atom species
are interspersed or “doped” within the host (Silfvast, 2003). In this case,
yttrium aluminum garnet (Y3Al5O12) is doped with neodymium, which
causes the lasing process in an active medium (Steen & Mazumder,
2010). The lasing action comes from the energy jumps between electronic energy levels of the Nd3+ ions in the lattice when approximately
one percent of yttrium is substituted by the alternative rare-earth
neodymium, which is clearly shown by neodymium's energy level seen
in Fig. 1. The transition will occur from the upper lasing level at the
metastable state to a lower lasing level at the terminal state. This
transition produces laser radiation in the near infra-red (NIR) spectral
region at a wavelength of 1.06 μm (Elijah, 2009).
Laser coating removal involves several complex mechanisms
namely thermal ablation, mechanical effect, and combination of
thermal ablation and mechanical effects (Zhang et al., 2018). The mechanisms generally depend on the laser beam's characteristics and its
delivery methods. However, the factors that mostly influence the
mechanisms are the laser parameters as well as the physical and chemical properties of the coating material (Koh, 2006). Quality laser
coating removal can be obtained via properly controlled laser power or
fluence, pulse width, repetition rate, and beam size (Coutouly, Deprez,
Breaban, & Longuemard, 2009). This is essential to; (a) optimize the
coating removal process, (b) to minimize the risk of substrate damage,
(c) to reduce health implications to the practitioner, (d) to sustain the
environment, and (e) to cut the cost of labor. Thus, important factors
related to the laser coating removal technique must be addressed.
Chemical based stripping and grit blasting are the current techniques practiced by automotive industry for car re-coating. However,
neither of these techniques is ideal, as both would result in environmental imbalance due to the production of large amounts of waste.
Additionally, this process is unfavorable due to its higher cost, as shown
in Table 1 (Arthur, Bowman, & Straw, 2008; Naguy & Straw, 2010;
Walters, Campbell, & Hull, 1998; Wolf, 2009).
The ideal coating removal process in the automotive industry should
have the following characteristics; a. reproducible clean surfaces, b.
controllable and accurate removal area, and c. minimal detrimental
effects on car's metal substrate. The laser based coating removal process
Peer review under responsibility of The Egyptian Society of Radiation Sciences and Applications.
∗
Corresponding author.
E-mail address: khairul.azhar@usm.my (M.K.A.A. Razab).
https://doi.org/10.1016/j.jrras.2018.08.002
Received 1 May 2018; Received in revised form 5 July 2018; Accepted 2 August 2018
1687-8507/ © 2018 The Egyptian Society of Radiation Sciences and Applications. Published by Elsevier B.V. This is an open access article under the CC
BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Razab, M.K.A.A., Journal of Radiation Research and Applied Sciences (2018),
https://doi.org/10.1016/j.jrras.2018.08.002
Journal of Radiation Research and Applied Sciences xxx (xxxx) xxx–xxx
M.K.A.A. Razab et al.
Nilaya & Biswas, 2010). Laser cleaning for coating removal has a high
potential in replacing wet chemical methods that consume water and
are costly (Kumar et al., 2014; Steen & Mazumder, 2010). Another
advantage of applying laser cleaning is the absence of mechanical damages on the metal substrate, which simultaneously increases the effectiveness of coating removal (Büchter, 2018; Veiko, Mutin, Smirnov,
Shakhno, & Batishche, 2008). The unique characteristics of laser
cleaning include being versatile, precise, controllable, selective, and an
environmentally friendly process. It is capable of removing the coatings
from a substrate's surface, and it has been utilized in various industries
(Lee & Watkins, 2000; Madhukar et al., 2013a).
The current experimental results showed that laser cleaning can be a
good alternative for the conventional chemical cleaning process in
coating removal (Chen, Kwee, Tan, Choo, & Hong, 2010; Madhukar,
Mullick, & Nath, 2013b, 2013a; Naguy & Straw, 2010; Xiaoguang et al.,
2017). Hence, a systematic and effective coating removal system is
urgently needed for a thorough old coating removal in the car recoating process (Shu-Dong et al., 2012; Tsunemi et al., 1996). Recoating of the metal substrate following laser cleaning has been proven
to provide superior adhesion due to less surface roughness, and fine
microstructures compared to other conventional cleaning techniques
(Razab, Jaafar, Abdullah, Amin, & Mohamed, 2016b; Shamsujjoha
et al., 2015b; Xiaoguang et al., 2017).
2.1. Car coating removal
Car coating generally consists of a mixture of numerous materials,
such as resin, pigments, solvents, and additives (Madhukar et al.,
2013b). Resins, such as acrylic, alkyd, epoxy, urethane, and cellulose
are sticky materials that act as binders in a coating system (NZIC, 2014;
Tracton, 2007). A car coating system needs a combination of multilayer coatings due to its inability to provide good adhesion, anticorrosion, and environment resistance in one single layer. The chemical
compositions, layer sequences, and layer thickness vary according to
the manufacturer, color, and type of the coating system (Nieznañska,
Ziêba Palus, & Koœcielniak, 1999; Trejos, Castro, & Almirall, 2006).
Typical compositions of a car coating system consist of pre-treatment,
first primer coat, base coat, and a top clear coat, as shown in Fig. 2
(Deconinck, Latkoczy, Günther, Govaert, & Vanhaecke, 2006; Hobbs &
Almirall, 2003; Streitberger & Dossel, 2008; Trejos et al., 2006). Generally, the chemical compositions for each coating layer, with the exception of the top clear coat, will contain C, O, Al, Si, Ti, and other
elements. These remaining elements are unique for each type of layer,
thus helps in its identification (NZIC, 2014; Nieznañska et al., 1999).
Conventional car coating removal usually utilizes chemicals that
contain methylene chloride, phenolic compounds, activated acids,
bases free from phenols, and chromates (Malavallon, 1995; Young,
Clayton, Yesinowski, Wynne, & Watson, 2014). The stripping methods
may also employ cleaning agents, such as chlorofluorocarbons (CFC's),
and other chemical solvents that are now deemed harmful to the environment or pose hazards to workers (Lu, Takai, Komuro, Shiokawa, &
Aoyagi, 1994b, 1994a; Walters et al., 1998; Wolf, 2009). These techniques have also raised waste quantity by producing extra waste slurry
(Schmidt, Li, & Spencer, 2001). Conventional chemical stripping has
been scrutinized by the Environment Protection Agency (EPA) in the
USA due to the production of large volumes of hazardous wastes, which
cause subsequent disposal problems (Pole, Agarwala, & Rajeshwar,
2006). In addition, cleaning techniques that use chemical agents may
cause serious damage to the deeper layers of the metal substrate
(Sanjeevan & Klemm, 2005). Irreversible damage to the metal can occur
when chemical liquids, such as methylene chloride, and phenol reach
the substrate's surface (Georgiou, 2004).
Previous studies have found that there were no significant effects in
the laser coating removal efficiency between archaic and new coating
systems (Mongelli, 2005). However, the removal process depends on
pigment color, oxidation, porosity, and the surface roughness of the
Fig. 1. Schematic process of neodymium energy level: energy jumps between
electronic energy levels of the Nd3+ ions in the lattice medium to produce laser
radiations (Steen & Mazumder, 2010).
was found to offer these advantages (Kumar et al., 2014; Madhukar,
Mullick, Chakraborty, & Nath, 2013a; Zhang et al., 2018). The use of
laser technology to remove coated layers from a surface has been studied for many years with a variety of lasers, coatings, and substrates
(Schmidt, Li, & Spencer, 2003). Such superior characteristics have been
proven to be successful in cleaning polymers, rubber tire molds, large
mirrors, artworks and historical heritage pieces, as well as semiconductors (Büchter, 2018; Lu, Aoyagi, Takai, & Namba, 1994a; Raele,
Pretto, & Zezell, 2017; Shu-Dong et al., 2012; Zanini & Bartoli, 2018).
However, to the best of our knowledge, laser-cleaning techniques in
the automotive industry have received less attention. Furthermore,
their efficient processes, and the ideal laser maneuver are not well
documented due to the lack of exposure to the public, although it is
practiced by the Air Force Research Laboratory (AFRL) in the United
States of America (USA) (Mongelli, 2005). Studies by Razab et al.
(Razab, Jaafar, Rahman, & Saidi, 2014d) have shown that a low power
pulsed Nd:YAG laser coating removal on selected Malaysian and nonMalaysian car substrates can be achieved with certain laser parameters.
In addition, the optimum parameters obtained for this process produce
lesser environmental pollution compared to the current techniques
applied in the automotive industry (Razab, Jaafar, Rahman, Mamat, &
Ahmad, 2014b). In supporting the research outcomes, this paper proposes that laser cleaning is incorporated into automotive industry. We
hope that this review paper will result in a fundamental study of replacing the hazardous chemical stripping technique in the near future.
2. Laser cleaning
Laser cleaning can be defined as the removal of unwanted layers or
the extended contamination of the unwanted layers from a solid substrate (Bäuerle, 2011). It involves cleaning off foreign organic impurities, namely coatings over a solid or metal substrate (Kane, 2006;
2
Journal of Radiation Research and Applied Sciences xxx (xxxx) xxx–xxx
M.K.A.A. Razab et al.
Table 1
Percentage of cost comparison between current technology and laser removal techniques of four available applications (Wolf, 2009).
Number of application
Available stripping application
Current stripping technique
Cost of stripping with current technology
Cost of stripping with laser technique
1
2
3
4
Aircraft
Storage tank
Ground vehicles
Navy parts
Chemical stripping
Sand blasting
Media blasting
Burn-off oven
100%
95%–100%
100%
100%
71.36%
42.79%
54.9%
126.3%
Note: Data reviewed in 2009, United State of America (USA).
Fig. 2. Cross section of the metallic appearance in a typical multi-layered car
coating and the approximations of the thickness of each layer in a sequential
order (Trejos et al., 2006).
coating system (Koh & Sarady, 2001). Certain pigment colors in the
coating system are able to absorb low peak irradiance laser energy,
which can be used to determine the effectiveness of the coating removal
process (Arthur et al., 2008). Nevertheless, all coating colors are considered to be removed instantly in the IR region due to the high peaked
irradiance laser energy produced. This leads to the high absorption of
laser beam by the main bond in the polymeric coatings of CeC, and
CeO bonding (Arthur et al., 2008; Cottam, Emmony, Cuesta, & Bradley,
1998; Georgiou, 2004; Li, Steen, Modern, & Spencer, 1994; Razab,
Jaafar, Abdullah, Amin, & Mohamed, 2016a). Moreover, the proper
controllability of the laser's parameters may allow this technique to
remove any single coated layer without damaging the following layer of
the coating system (Georgiou, 2004; Siggs, 2010).
Our team from Universiti Malaysia Kelantan, and Universiti Sains
Malaysia have been studying the efficiency of the Nd:YAG laser coating
removal system over selected Malaysian and non-Malaysian coated
cars. The study has concluded that cleaning efficiency depends on the
laser parameters, laser types, and the coating materials being irradiated
(Razab et al., 2016a, 2016c, 2014d; Razab, Suhaimi Jaafar, Rahman, &
Affandi Saidi, 2014f). The findings are based on Infinite Focus Meteorology (IFM) scanning images as shown in Fig. 3. Our findings also
revealed the percentage of aluminum flakes embedded in the base coat
of the coating system enhanced the thermal distribution and influenced
coating removal efficiency (Razab et al., 2016a).
Theoretically, coating removal efficiency is the function of laser
parameters, which include beam intensity, pulse width, repetition rate,
and beam size of the irradiation area (Büchter, 2018). The efficiency of
the laser coating removal process can be defined by Equation (1)
(Roberts, 2004; Zhou et al., 2001).
∈ = V / E = d / nF.
Fig. 3. Infinite Focus Metrology (IFM) image for pulse width 100 ms, repetition
rate 1.0 Hz, beam size 5 mm and laser fluence 180 J/cm2 on a sample of
Malaysian car coated substrate with 5× magnifications: a. Uniform crater
surface obtained b. Contour was easily identified and obtained in smooth pattern as well as its smooth ‘wavelike’ ring c. Unaffected of surface painted layer
(Razab et al., 2014d).
both (Madhukar et al., 2013a; Sanjeevan & Klemm, 2005). Three possible types of interaction processes may occur during the ablation and
thermal decomposition mechanisms in coating removal, which are
photothermal, photochemical, and photomechanical interactions
(Coutouly et al., 2009).
2.2. Laser ablation and thermal decomposition
Two basic mechanisms have been considered in laser coating removal known as laser ablation and thermal decomposition processes
(Mongelli, 2005). Both mechanisms offer advantages for coating removal, in terms of technical aspects, such as no direct mechanical or
chemical contact to the substrate, in situ cleaning, less cleaning time,
nonuse of toxic solvents and chemical products, no damages to the
metal substrate, and controllable cleaning (Madhukar et al., 2013a;
Morais et al., 2010). However, these advantages become the opposite if
the mechanism is not properly controlled with the right operational
laser parameter setup. This is crucial since laser ablation and thermal
decomposition are irreversible processes and may pose as a threat to the
removal area (Cooper, 1998; Morais et al., 2010; Sanjeevan & Klemm,
2005).
Laser ablation is a mechanical process, which can be achieved by
applying high intensity pulse and CW laser irradiation (Bian, Yu, Zhao,
Chang, & Lei, 2013; Brown and Arnold., 2010). The study of ablation
plumes via photography was initiated by Ready and his team in 1963.
Then, a number of related papers appeared in the same year discussing
the first ablation studies of biological material as well as the use of
(1)
−1
Note: (a). ∈ is the average coating removal efficiency (μm cm J ).
(b). V is the volume of coating removed. (c). E is the total laser energy.
(d). d is the depth of coating removed. (e). F is the laser's fluence. (f). n
is the number of laser shoots.
Laser coating removal can occur through ablation and thermal decomposition mechanisms, either photothermally or photochemically, or
2
3
Journal of Radiation Research and Applied Sciences xxx (xxxx) xxx–xxx
M.K.A.A. Razab et al.
Fig. 6. Graphical illustration of the laser thermal decomposition (Mongelli,
2005). (a). Laser energy is deposited into the top clear coat. (b). The laser energy induces the thermal decomposition mechanism.
coating layer and burning it off. This process can be achieved by applying thermal energy from the low power of CW or long pulsed lasers
for a certain period of time until the coating layer is successfully removed (Mongelli, 2005). An illustration for this mechanism is in Fig. 6.
2.3. Laser parameters
Fig. 4. Schematic illustration of the laser ablation. The laser beam interacts
with the coating layers, which induces the surface plasma and shockwave
(Prinsloo et al., 2007).
It is important to select appropriate laser parameters in a laser
cleaning process (Sanjeevan & Klemm, 2005). Several parameters must
be considered during the coating removal process, which are laser
wavelength (λ), laser fluence (F), pulse width (PW), and repetition rate
(RR) (Brygo et al., 2006). Inappropriate selection of laser parameters
may lead to overexposure, which may result in metal substrate damage
due to the high energy density of the laser beam, whereas underexposure can leave residual contaminations on the surface of the substrate (Lee & Watkins, 2000; Li, Zhang, Zhou, Zhu, & Liu, 2018).
However, the damage to the substrate material can be eliminated by
applying optimum operational laser parameters without external distraction during the cleaning process (Han et al., 2017; Heidelmann,
2011). The parameters and their derivation formulas in characterizing
pulsed and CW laser beam output are listed in Table 2.
Laser radiation absorptivity into the coating material depends on
the wavelength for CW laser beam, but not for the pulsed laser beam.
There is no clear relationship between laser radiation absorption and its
wavelength due to domination applications of the pulse duration laser
beam (Siggs, 2010). However, a study at the University of Southern
California revealed that by increasing the laser's wavelength, the
rotational and vibration approaches of molecular emission bands to
measure the temperatures of ablation plumes (Miller & Haglund, 1997).
However, the use of laser ablation to remove the outside layer of a
material was first proposed in 1987 (Prinsloo, Van Heerden, Ronander,
& Botha, 2007). In this case, the laser beam transforms the first microns
of the coated layer into highly compressed plasma. This will generate a
shock wave that ejects the layer into fine particles. The substrate behind
the layer is preserved by keeping the laser energy density below the
damage threshold. Although the exact mechanism of the laser coating
removal is not fully known, it can be easily demonstrate, as seen in
Fig. 4.
In this process, the coating is ablated and the effluent is ejected from
the metal substrate at a high velocity. This condition will produce
pyrolysis gases and inorganic materials as the by-product (Mongelli,
2005). The more detailed mechanism of the ablation process is shown
in Fig. 5.
Meanwhile, thermal decomposition is the process of heating up the
Table 2
Ordinary symbol, unit and definition of each laser parameter to characterize
beam output (Semrock, 2001).
Fig. 5. Graphical illustration of the laser ablation mechanism (Mongelli, 2005).
(a). Laser vaporizes the coating layer and creates a plasma formation. (b).
Plasma creates a shock wave and crack network. (c). The top clear coat has been
removed in the ablation mechanism.
Symbol
Definition
Unit
Formula
τp
RR
DC
a
P
E
b
A
c
I
d
F
Pulse duration
Repetition rate
Duty cycle
Power
Energy
Area of laser beam
Intensity
Fluence
s
Hz
–
W
J
cm2
W/cm2
J/cm2
τp = DC/RR
RR = DC/τp
DC = RR × τp
Ppeak = E/τp
E = Ppeak × τp
A = (π/4) × (diameter)2
I = P/A
F = E/A
a
b
c
d
4
(a). Pavg = E × RR. (b). Pavg = Ppeak × DC.
Gaussian shape laser beam.
(a). Ipeak = F/τp. (b). Iavg = F × RR.
(a). F = Ipeak × τp. (b). F = Iavg/RR.
Journal of Radiation Research and Applied Sciences xxx (xxxx) xxx–xxx
M.K.A.A. Razab et al.
∞
amount of energy absorbed by a coating system is reduced (Mongelli,
2005). This is because the highest laser absorption and the photochemical dissociation on the surface are usually achieved at short radiation wavelengths (Lee, Yu, & Koh, 2003; Lu et al., 1994b).
Meanwhile, increasing the laser fluence will increase the removal
efficiency of the unwanted coating layers and contaminants (Bian et al.,
2013; Lee et al., 2003; Raciukaitis, Brikas, & Gedvilas, 2008b). However, laser irradiation should reach the maximum layer thickness until
all the coating system has been removed (Siggs, 2010). Partly, the level
of laser fluence needed to ablate the coating layers depends on the
threshold fluence (FT) of the coated material, and the repetition rate of
the laser pulse (Brygo et al., 2006; Han et al., 2017; Kumar et al., 2012;
Razab, Jaafar, Rahman, & Saidi, 2014c, 2015).
Laser intensity or laser power per beam spot is also an important
factor for heat transfer into the coating material (Heidelmann, 2011;
Weber et al., 2014). Increasing the power of the laser will increase the
possibility of a coating removal to occur, hence reduces the interaction
time of the laser beam with the substrate's surface. This condition will
reduces the thermal effects induced by the laser beam on the metal
substrate (Daurelio, Chita, & Cinquepalmi, 1999). The principal formation of the typically used Gaussian shape beam of transverse electromagnetic, TEM00 mode in coating removal is shown in Fig. 7.
Note: (a). L = 2zR is the length (depth) of focus (zR is the Rayleigh
length of laser focus [μm]). (b). θ is the beam divergence angle. (c). 2ωo
is the beam waist. (Bäuerle, 2011).
The Gaussian shape laser beam intensity within the focal plane is
given by Equation (2):
I (r) = Ioexp (- r
2
/ ωo2).
∞
∫ I (r ) = ∫ 1/2πrdP /dr ,
0
0
∞
P=2π ∫ rI (r ) dr ,
o
= π wo2 Io.
(3)
This equation always refers to the effective power incidence onto
the targeted coating surface (Bäuerle, 2011). Moreover, the source to
target distance (STD) is influenced by the intensities of the laser beams
produced by the laser source. Changing the STD value can lead to a
varied size of the laser beam. This is because a longer STD will diverge
the beam size, thus resulting in a reduced beam concentration. Hence,
the beam size can be easily controlled by manipulating the STD to
optimize the laser beam concentration for optimum coating removal
process (Mongelli, 2005).
On the other hand, pulse width (PW) can be defined as the irradiation time for the pulsed laser system in processing materials (Siggs,
2010). Increasing the pulse width will increase the melting effect
around the crater depth of the coating system. Despite the low optical
absorption of the coating, laser fluence is required to reach the
threshold fluence, which strongly depends on the pulse width parameter (Siggs, 2010). Threshold fluence is lower at shorter pulse width
due to changes in the absorption coefficient (α) of the coating system at
high intensity laser beams (Brygo et al., 2006; Kumar et al., 2012).
However, Zhigilei and his team concluded that the pulse width did not
have any significant effect on the threshold fluence (Zhigilei, Leveugle,
Garrison, Yingling, & Zeifman, 2003).
For a thin coating laser ablation, the irradiation time is defined as
the time required to remove a certain coating thickness at a certain
point by focusing the laser beam on a targeted surface (Zhou, Imasaki,
Furukawa, Nakai, & Yamanaka, 2002). The total irradiation time is
directly proportional to the intensity of the laser beam and the correlation is given by Equation (4) (Liu & Garmire, 1995):
(2)
Note: (a). ωo is the radius of the laser focus of the Gaussian beam.
(b). r is the radial distance of the laser beam. (c). I (r) is the transmitted
intensity of laser output in the function of r. (d). Io is the initial laser
intensity (W/cm2).
Since the transmitted laser intensity, I is equal to the laser power, P
(W) divided by beam area, A (cm2), thus by integrating Equation (2) to
the function of r, the total laser power of the Gaussian beam is given by
Equation (3):
Δ t = C × I n = C × (P/A) n.
(4)
Note: (a). Δ t is the total irradiation time (s). (b). I is the transmitted
laser intensity (W/cm2). (c). P is the laser power (W). (d). A is the area
of the beam spot (cm2). (e). x is the coating thickness (μm). (f). C and n
are two empirical constants that are dependent upon the target conditions and laser parameters, which can be evaluated based on the experiments (Liu & Garmire, 1995; Zhou et al., 2002).
It was found that a high repetition rate (RR) laser could reduce the
threshold fluence of the coating system due to heat accumulation, and
have a significant impact in laser coating removal efficiency (Daurelio
et al., 1999; Raciukaitis, Brikas, Gecys, & Gedvilas, 2008a; Roberts,
2004; Weber et al., 2014). The crater depth of the removed coating can
be increased by increasing the repetition rate and the constant fluence
and pulse width (Brygo et al., 2006). However, the accumulation of
particles that were released during the coating removal process may
also be increased as the repetition rate increased, thus decreasing the
ablation efficiency due to the absorption of the laser beam in the produced plume (Brygo et al., 2006; Madhukar, Mullick, Shukla, Kumar, &
Nath, 2012; Raciukaitis et al., 2008a). The relationship between the
repetition rate (RR) and the pulse duration (τp) is represent by Equation
(5) (Semrock, 2001).
I = P/A,
RR = DC / τp.
(5)
Note: DC is the duty cycle of the laser and the unit is dimensionless
(Madhukar et al., 2012).
Fig. 8 illustrates and summarizes the general parameters of the
pulsed laser beam output based on laser power (P) versus time of irradiation (t) (Semrock, 2001; Zhou et al., 2002).
Note: (a). Ppeak is the peak power of a pulsed laser beam. (b). Pavg is
the average power of the accumulated pulsed laser beam. (c). PW is the
Fig. 7. Principal of Gaussian beam formation (Bäuerle, 2011). (a). Laser beam
is first expanded, and then focused by a lens. (b). Intensity distribution and
formation of Gaussian shape laser beam near the focal plane.
5
Journal of Radiation Research and Applied Sciences xxx (xxxx) xxx–xxx
M.K.A.A. Razab et al.
intensity to the coating layers has exceeded the ablation threshold of
the coated material (Metsios, Dai, Chard, McComb, & Kwon, 2018;
Poprawe, 2011; Targowski et al., 2009). This can be described by the
Beer – Lambert absorption law, as given in Equation (9) (Kumar et al.,
2012; Lawrence, Minami, Li, Edwards, & Gale, 2002; Schmidt et al.,
2003).
d = [ln (F / FT)] / α.
Note: (a). d is the depth of the removed coating. (b). α is the absorption coefficient (cm−1). (c). F is the laser fluence. (d). FT is the
threshold fluence for paint removal to occur.
The threshold fluence (FT) for a certain coating system can be easily
derived from the interception of a fitted linear line through the X-axis of
the d versus F graph. Furthermore, by rearranging Equation (9), the
absorption coefficient can be determined using Equation (10).
Fig. 8. An illustration of pulsed laser parameters for waveform propagation of
pulse laser irradiation, peak power, average power, pulse width, and pulse
duration.
α = [ln (F / FT)] / d.
time to obtain a complete pulse, and defined as the pulse width. (d). 1/
RR is the time interval between two pulses and defined as pulse duration (τp) (Zhou et al., 2002).
γ = FT α.
Pulsed laser in the infrared (IR) region tends to vibrate and excite
free electrons within the coatings and the metal substrate, which can
dissipate the laser energy into heat in a short time (Bäuerle, 2011).
Thus, energy absorbed during laser coating removal is mostly transformed to heat (Poprawe, 2011). If a constant flux, F0 is absorbed at the
surface and there is no phase changes in the material, the heat flow in
one dimension is given by Equation (6) (Steen & Mazumder, 2010).
(0, t) = 2F0/κ
(D t/π)1/2.
3.2. Thermal in coating material
Rapid vaporization or thermal stress in a coating spallation may
occur when the laser beam's energy has been absorbed high enough and
exceeds the thermal threshold of coating adhesion (Targowski et al.,
2009). Significant vaporization was correspond to the critical temperature that has been reached when the coating layers start to ablate
(Brygo et al., 2006). During the process, heat transfer is considered to
contribute in coating ablation and spallation as long as the coated
layers are not totally removed (Coutouly et al., 2009). For coating
materials with low thermal conductivity, thermal effect is presented by
the thermal confinement regime, as shown by Equation (12) (Brygo
et al., 2006).
(6)
LT = √ (D.τp).
(0, t) = 2
βI0 /κ (D t/π)1/2.
(12)
Note: (a). LT is the thermal diffusion length. (b). D is the thermal
diffusivity of the coated material. (c). τp is pulse duration.
From Equation (12), LT is directly proportional to τp. Thus, the
application of long pulsed duration laser beam is considered to give a
major influence to the thermal diffusion length in thermal confinement
regime of coated material (Brygo et al., 2006; Liu & Garmire, 1995;
Razab, Jaafar, & Rahman, 2014a; Rode et al., 2008). Properties of the
coating and laser beam size were also the main factors in influencing
thermal distribution into coated material during laser cleaning (Razab
et al., 2014a). However, from our findings, long pulsed Nd:YAG laser
will give tremendous effects to overheat the coating structure if not
properly controlled as shown in Fig. 9.
(7)
Note: (a). R is the reflectance. (b). β is the absorptivity. (c). I0 is the
incident flux.
Hence, by integrating Equation (7) into Equation (6), the surface
temperature of the coating and the metal substrate in laser coating
removal is given by Equation (8).
T
(11)
These equations have revealed the interaction characteristics of the]
coated substrate with laser irradiation in terms of threshold fluence, FT
(J/cm2), absorption coefficient, α (cm−1) and thermal loading, γ (J/
cm3) (Razab et al., 2014c, 2015).
Note: (a). D = κ/ρCp, T (0, t) is the temperature at the surface after
time t. (b). κ is the thermal conductivity. (c). D is the thermal diffusivity.
(d). ρ is the density. (e). Cp is the specific heat of the material. (f).
π = 3.14159.
This equation is true by considering the following assumptions; a.
the laser beam is uniform with no transverse variation in intensity, b.
the coated surface is uniform and planar, c. the beam diameter is much
larger than the coating thickness and thermal diffusion length and d.
the beam energy is absorbed at the surface.
Since the vaporization of the coated layers depends on the differences between the absorptivity of the metal substrate and the coating
requiring removal, the constant flux, F0 is given by Equation (7).
F0 = (1 –R) I0 = βI0.
(10)
Moreover, identifying the threshold fluence and α for a coating
system will lead to the determination of thermal loading, γ, which can
be easily expressed as in Equation (11) (Schmidt, Li, & Spencer, 2000,
2001).
3. Thermal induced in laser coating removal
T
(9)
(8)
3.1. Heat affected zone
3.3. Thermal in metal substrate
The thermal effect in a restricted laser ablation is known as the heat
affected zone (HAZ) (Targowski, Ostrowski, Marczak, Sylwestrzak, &
Kwiatkowska, 2009). HAZ results in the remaining fraction of the directly absorbed laser energy by the irradiated target during laser ablation mechanism (Vorobyev & Guo, 2007). HAZ is minimal in short
laser interactions due to the fully absorbed laser energy by the coated
material, hence the laser ablation is considered to finish before the heat
diffusion starts to occur (Targowski et al., 2009).
Coating removal mechanism begins after the absorption of laser
Metal substrate damage in coating removal is mainly due to thermal
effects induced strain and stress (Labuschagne & Pityana, 2005).
Thermal effects on metal substrate is critical when the heat is directly
dissipated through the metal surface since the entire coating layer has
been removed (Coutouly et al., 2009). Direct exposure to high temperature gradients in metal substrate leads to metal surface cracking,
depletion of certain material components, thermal stress, ductility
6
Journal of Radiation Research and Applied Sciences xxx (xxxx) xxx–xxx
M.K.A.A. Razab et al.
surface coating layers (Kusch, Heinze, & Wiedemann, 2003; Ratautas
et al., 2012). The Clean Air Act (CAA) of the USA has declared particulate matter as one of six major air pollutants, which led to the establishment of National Ambient Air Quality Standards (NAAQS) by
EPA (Costa et al., 2004).
4.1. Particulate matter
Particulate matters could exist as solid particles or liquid droplets in
a wide range of sizes (Kadiyala, 2008; McInnes, 2009). Aerosol particulate matters with diameter greater than 1.0 μm and equal or less than
10 μm are known as PM10.0. PM10.0 can accumulate in the upper respiratory tract to the lower respiratory system, and can cause illnesses.
In addition, aerosol particulate matters with a diameter of less than
1.0 μm is known as PM1.0 (Brunekreef & Holgate, 2002; McInnes,
2009). Particulate matters not only influence the efficiency of the laser
removal process but worst is that direct exposure to these aerosols will
increase health risks to the practitioner (Dewalle, Vendel, Weulersse,
Hervé, & Decobert, 2007, 2010). Primary route of particulate matter
exposure is through inhalation, and secondarily is through ingestion
(McInnes, 2009).
Acute and chronic exposure to aerosol particulate matter can also
lead to detrimental health effects (Kadiyala, 2008; McInnes, 2009). The
main entry for particulate matters is the lungs, thus the interaction of
particulate matters with the respiratory epithelium and alveolar macrophages will induce pulmonary diseases (Costa et al., 2004). This
condition becomes the main factor to aggravate lung diseases, such as
asthma and chronic obstructive pulmonary disease (COPD), which
leads to lung cancer and cardiac problems (McInnes, 2009; Pope et al.,
2002). Ultrafine particulate matter is identified as highly toxic, and
poses the largest health risk due to its ability to migrate and deposit
deep into the lung, and jeopardizes the whole respiratory system (Costa
et al., 2004; Donaldson, Stone, Clouter, Renwick, & MacNee, 2001).
Concentration of particulate matter or aerosol by-products released
from the laser coating removal process is dependent on physical and
chemical compositions of the ablated coating material and the laser
parameters (Razab et al., 2014b). The aerosols generated during the
laser coating removal process can be easily characterized in terms of
size distributions (PM1.0 and PM10.0) and the average particle mass
concentrations. By assuming that the spatial distributions of laser energy is homogenous for each shot on the coated material, it is possible
to normalize the measurements with respect to the interaction of surface area (Lee & Cheng, 2006). Thus, each measurement of particle
mass is related to one laser shot and 1.0 cm2 of ablated coating.
Equation (14) depicts the averaging method which can be used to acquire the mass of particles, N (Dewalle et al., 2010).
Fig. 9. IFM image for pulse width 200 ms, repetition rate 1.5 Hz, beam size
3 mm and laser fluence 290 J/cm2 on a sample of Malaysian car coated substrate: a. Non-uniform crater surface b. Bumping contour pattern and roughen
‘wavelike’ ring c. Effected of surface painted layer (Razab et al., 2014a).
deterioration and fatigue life extension effects (Bäuerle, 2011;
Labuschagne & Pityana, 2005; Shamsujjoha, Agnew, Brooks, Tyler, &
Fitz-Gerald, 2015a). A study conducted by Shamsujjoha et al. has
proven the tensile residual stress and surface roughness of the metal
substrate undergoing laser cleaning will induce metal fatigue due to
heat dissipation (Shamsujjoha et al., 2015a).
Metal substrate temperature will keep rising during laser coating
removal process and will drastically rise when all the coating layers
have been removed. Deviation of metal substrate temperature, ΔT (°C)
depends on the thermal diffusivity of the coated material as well as the
laser fluence, pulse width, repetition rate, and beam size of the irradiation (Razab et al., 2014a). The average temperature deviation, ΔTAV
can be determined for each laser parameter applied on the metal substrate by using Equation (13).
ΔTAV = Σ ΔT / nshots.
(13)
Note: (a). ΔTAV ( C) is the average of the temperature deviations
measured on a metal substrate sample. (b). Σ ΔT (°C) is the total temperature deviations for nshots measured from temperature base (Tbase).
(c). nshots is the number of laser irradiation obtained on a substrate
sample. (d). Tbase (°C) is the temperature of the substrate sample measured prior to the initiation of each laser irradiation.
°
N = df × (CAv – Cnoise) × Q × Δt / (nshots x A).
(14)
Note: (a). N is the mass density of particulate matter per one laser
shot and 1.0 cm2 (mg/shot/cm2). (b). df is the dilution factor for the
measurement device. (c). CAv (mg/m3) is the average concentration
issued from the measurement device during Δt. (d). Cnoise (mg/m3) is
the average concentration issued before laser shoots of the measurement device. (e). Q (m3/min) is the constant air flow rate of the device.
(f). Δt (min) is the time interval for the particulate matter accounted.
(g). nshots is the number of laser shoots within Δt. (h). A (cm2) is the
irradiated crater area. (i). df is the dilution factorbuilt into the device.
4. Health and environment implications
Health-related effects and environmental pollution in laser coating
removal are considered to be minimal in terms of producing toxic
waste, air contaminants, particulate matter, various metal particles, and
hazardous waste compared to conventional chemical techniques.
However, the severe implications to these conditions still exist
(Anthofer, Lippmann, & Hurtado, 2013; Mongelli, 2005; Wolf, 2009).
This is due to the tiny population of spherical and aggregate particulate
matter (PM) that ranges from nano to submicron particles are released
from the interaction between the laser beam and the coated layers
(Dewalle, Vendel, Weulersse, Hervé, & Decobert, 2010, 2007; Dudoitis,
Ulevičius, Račiukaitis, Špirkauskaitė, & Plauškaitė, 2011; Madhukar
et al., 2013b, 2013a; Ratautas, Gedvilas, Voisiat, Raciukaitis, &
Grigonis, 2012). The emission rate of this hazardous substance is highly
dependent on the thickness and physical condition of the unwanted
Air quality guidelines
Previously, there are no threshold concentration of particulate
matter proposed due to the inability to define a threshold below the
adverse effects expected (Brunekreef & Holgate, 2002). However, the
World Health Organization (WHO) has proposed specific guidelines for
each pollutant of particulate matters based on current scientific findings
and interim target values based on selected cities around the world in
7
Journal of Radiation Research and Applied Sciences xxx (xxxx) xxx–xxx
M.K.A.A. Razab et al.
Table 3
WHO AQG and interim targets for 24-h mean concentrations recommended for short term exposure (WHO, 2006).
NIL
Interim target 1 (IT-1)
PM10.0 (mg/m3)
−3
150 × 10
−3
PM2.5 (mg/m3)
−3
75 × 10
−3
Interim target 2 (IT-2)
100 × 10
50 × 10
Interim target 3 (IT-3)
75 × 10−3
37.5 × 10−3
Air quality guideline (AQG)
50 × 10−3
25 × 10−3
Basis for the selected level
Based on published risk coefficients from multi-center studies and meta-analyses (about 5% increase of shortterm mortality over the AQG value).
Based on published risk coefficients from multi-center studies and meta-analyses (about 2.5% increase of shortterm mortality over the AQG value).
Based on published risk coefficients from multi-center studies and meta-analyses (about 1.2% increase in shortterm mortality over the AQG value).
Based on the relationship between 24-h and annual particulate matter levels.
Note: No 24-h mean concentrations recommended by WHO AQG has been specifically found for PM1.0.
Appendix A. Supplementary data
2005 (Krzyzanowski & Cohen, 2008; WHO, 2006). Table 3 list the 24-h
mean concentrations of allowable particulate matters for air quality
guideline (AQG) (WHO, 2006). The recommended values for the annual
and 24-h mean concentrations are 20 × 10−3 mg/m3 and
50 × 10−3 mg/m3 for PM10.0, and 10 × 10−3 mg/m3 and
25 × 10−3 mg/m3 for PM2.5 (Krzyzanowski & Cohen, 2008; WHO,
2006).
However, WHO AQG of these particulate matter data were officially
done in open air and not in the workplace (WHO, 2006). There should
be a threshold limit value (TLV) in the workplace as suggested by Kusch
et al. (2003), which is 6 mg/m3 for totally independent chemical
compositions of respirable dust (Kusch et al., 2003). This is because
health risks caused by these particles are highly dependent on air
ventilation and volume of particulate matters at the workplace.
Meanwhile, our findings of PM1.0 and PM10.0 released during Nd:YAG
laser coating removal process in close fabricated room for 10 min
counting using Dustrak 8520 were far exceeding from the recommended values suggested by WHO (Razab, Suhaimi Jaafar, &
Rahman, 2014e; Razab et al., 2017, 2014b). However, air contaminants
generated by laser irradiation are usually toxic, allergic, carcinogenic,
and can cause severe diseases after many years of exposure (Ostrowski,
Marczak, Ostendorf, Strzelec, & Walter, 2007). Since the chemical
compositions of PM released during the laser coating removal process
are not clearly known, detailed precaution and protection to the staff in
charge should be strictly considered. Smooth air ventilation in workplaces must be properly set up and personal protective equipment (PPE)
should be strictly worn by the practitioners.
Supplementary data related to this article can be found at https://
doi.org/10.1016/j.jrras.2018.08.002.
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Fundings
Fundamental Research Grant Scheme (UMK, MOHE) (R/FRGS/
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