P-13
Proceedings of the 3rd International Conference on the Frontiers of Plasma Physics and Technology (PC/5099)
LASER ABLATION OF MULTI-LAYERED TARGETS WITH SHORT
LASER PULSES.
B. Gaković1, T. Desai2, M. Trtica1, D. Batani2, M. Busolli2, J. Stašić3, P. Panjan3, M.
Čekada3
1
Vinča Institute of Nuclear Sciences, 11001 Belgrade, Serbia.
2Universita degli Studi Milano Bicocca, Dipartimento di Fisica “G. Occhialini”,
20126 Milano, Italy.
3
Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia.
Abstract
Interaction of Nd:YAG laser, operating at 1064 or 532 nm wavelengths and pulse duration of
40 picoseconds, with titanium implant has been studied. Surface damage threshold were
estimated to be 0.9 and 0.6 J/cm2 at wavelengths of 1064 and 532 nm, respectively. The
titanium implant surface modification was studied at the laser fluence ~ 4.0 and 23.8 J/cm2 (at
1064 nm) and 13.6 J/cm2 (at 532 nm). The following titanium implant surface morphological
changes were observed: (i) both laser wavelengths cause damage of the titanium in the central
zone of the irradiated area (ii) appearance of a hydrodynamic feature in the form of
resolidified droplets of the material in the surrounding outer zone with the 1064 nm laser
wavelength, and (iii) appearance of wave-like microstructures with the 532 nm wavelength.
Generally, both laser wavelengths and the corresponding laser energy densities can efficiently
enhance the titanium roughness. Implant roughness is expected to improve its bio-integration.
Keywords: Picosecond Nd:YAG laser; Titanium implant surface; laser-induced damage,
surface modification.
1. Introduction
In the recent years Titanium and titanium-based alloys are being extensively used in
biomedical applications. Titanium also exhibits excellent physical and chemical properties. It
has a high melting point, high erosion resistance etc. [1]. The main applications of titanium
and titanium-based alloys implants in medicine are based on the fact that they possess a high
level of bio-compatibility and bio-integration with human body [2-9]. They are frequently
used for joint replacement parts for hip, knee, shoulder, spine, wrist, etc. In dental field these
implants can be successfully employed, among other, for replacement of lost teeth [6].
Titanium of CP grade was typically utilized for titanium implants [3,5,6,9] whereas Ti6Al4V
[4] used for alloy-based implants. A comparative analysis of three types of metal implants,
including titanium, in a case of fracture treatment has been reported [2] according to which
the behavior of titanium was superior compared to stainless steel and cobalt-chromiummolybdenum implants. Generally, titanium and titanium based implants show long-term
durability. It is well known that the success of bio-integration with the surrounding host
tissues depends on the efficiency of the cell/tissue-implant interaction [4,6,7]. The quality of
this interaction further depends on the state of the implants surface [5-7]. The implant surface
must be contaminant-free, while roughness is its desirable morphological feature as it plays a
significant role in the tissue integration [5-7]. Rough surface, for example, promotes growth
of the bone tissue around the implant [5,6]. One of the possible methods for enhancement of
roughness is laser treatment. The knowledge of titanium surface, from previous mentioned
reasons, is highly desirable. The present work deals with laser modifications of titanium
implant surface.
Other medical applications of titanium and titanium-based alloys include those in
medical-engineering, e.g. artificial heart valves, surgical instruments, components of highspeed blood centrifuge, etc. [8].
Interest in the studies of laser beam interaction with titanium has relatively increased,
especially in the last decade. Different types of laser systems including excimer [6,7],
Nd:YAG [10-13], Ti:Sapphire [14], cw CO2 [8,15] and TEA CO2 [16,17] have been
employed for these purposes.
Interaction of titanium with Nd:YAG laser beam pulsed in the picoseconds time
domain has not been sufficiently described in literature [13] unlike nanosecond domain [1012]. In the present paper our emphases is to study the effects of a picosecond laser emitting in
the near-infrared (1064 nm) and also in visible regions (532 nm) on a titanium implant
surface. Special attention has been paid to morphological surface modifications of titanium at
both the wavelengths. Our initial results on the topic have been published [13].
2. Experimental set up:
The titanium implant surface was prepared by a standard metallographic procedure.
This included polishing, rinsing and drying. Bulk dimensions of the rectangular shaped
samples were typically 15mm x 15mm x 4mm. The surface roughness of the samples was
evaluated using AFM and it was observed to be less than 0.6 µm. These samples were similar
to our earlier experiment described in ref. 13.
Figure 1. XRD spectrum of non-irradiated Ti implant surface with the characteristic intensities of α- titanium
marked. Ni-filtered Cu Kα radiation was used.
Various analytical techniques were used for characterization of the titanium implant
samples before and after the laser irradiation. Identifying crystal phases was done by an X-ray
diffractometer (XRD; Siemens D500, software- Diffracplus). Surface morphology was
monitored by Optical Microscopy (OM), Scanning Electron Microscopy (SEM; JOEL-JSM6500F) as well as by Atomic Force Microscopy (AFM; TM Microscopes-Auto-Probe CP
Research). The SEM was connected to an Energy Dispersive Analyzer for determining
surface compositions of the targets. Profilometry (Taylor-Hobson Talysurf 2) was used for
specifying the geometry of the ablated area.
2
Figure 2. Picosecond Nd:YAG, 1064 nm laser-induced Ti implant morphology changes as recorded by SEM for
laser energy density 23.8 J/cm2 for B,C and 4.0 J/cm2 for D; A)- The view of the Ti surface prior to laser action;
B)- Ti after 1 laser pulse, B1- the entire spot, B2- the centre of the base, and B3- the near periphery of the laser
irradiated area; C)- Ti after 5 laser pulses, C1- the entire spot, C2- centre of the base, C3- the near periphery, and
C4,C5- farther periphery of the damage area; D)- Ti after 30 laser pulses, D1- the entire spot, D2,D3- the centre,
D4- the near periphery, and D5- farther periphery of the damage area.
Samples were irradiated by focusing the laser beam using a quartz lens of 12 cm focal length.
During the irradiation process the laser was operated in the fundamental transverse mode. The
angle of incidence of the laser beam with respect to the surface was 900. The irradiation was
carried out in air, at a pressure of 1013 mbar and standard relative humidity.
Experiments were performed at the University Milano-Bicocca using an active-passive modelocked Nd:YAG system [18]. It includes a laser oscillator, an amplifier, and a nonlinear
crystal (KD*P). Pulse duration of about 40 picoseconds is obtained by using a saturable
absorber dye and an Acuosto-Optic standing wave modulator. The laser was operated with a
typical repetition rate of 2 Hz. The choice of irradiation wavelength was either 1064 or 532
nm, depending on the experimental requirement.
3. Results and discussion
X-ray diffraction analyses of titanium prior to laser irradiation have confirmed its
crystalline structure, Figure 1. The sample exhibited a hexagonal structure with prominent
(002) and (101) orientations. These orientations are characteristics to the α- phase titanium.
The titanium implant surface has typical white-silvery metallic color. Elemental analysis of
the sample surface, carried out prior to irradiation by EDS (Table 1- spectrum 3, Figure 6),
showed the following content: titanium 93.41%, balanced to 100% by O (4.34%), C (2.15%)
and Al (0.10%). All percentage data are by weight. The complete elemental analysis was
normalized.
Investigations of the morphological changes induced by laser on titanium implant
sample have shown their dependence on laser beam characteristics: primarily on laser fluence,
peak power density, pulse duration, number of accumulated pulses and wavelength.
3
Figure 3. Two-dimensional profilometry view of the titanium implant surface with one and five successive laser
pulses (refer to Figs. 2B1 and 2C1).
Morphological changes of the titanium for 1, 5, and 30 accumulated laser pulses at 1064 nm,
and for 30 accumulated pulses at 532 nm, are presented in Figs. 2 and 4, and Fig. 5,
respectively. The laser fluence, during the experiment, were 4.0 and 23.8 (for 1064 nm), and
13.6 J/cm2 (for 532 nm) and they induced significant surface modifications of the titanium
implant surface. The results of the induced modification are presented in the following text.
Results of Nd:YAG laser (1064 nm)
After 1 or 5 pulses at 23.8 J/cm2 (Fig. 2B and C) the damage of the titanium implant
can be clearly recognized. In the central part of the damaged area, a relatively smooth region
is visible, Fig. 2B1 and 2C1. At large magnifications, Fig. 2B2 and C2, some structures on
nano scale can be clearly recognized. On the periphery of the damaged zone, hydrodynamic
effects like resolidified droplets of materials, become observable, Fig. 2B3 and 2C3-C5. The
damage threshold, which is defined, as the minimum laser energy required creating a
detectable damage on the implant target, has been determined. Our results show that the
damage threshold for Nd:YAG (1064 nm)-titanium interaction is about 0.9 J/cm2. For a
nanosecond Nd:YAG laser titanium interaction damage threshold damage has been reported
to be about 1.5 J/cm2 [19]. A profilometer analysis of the sample after the action of 1 and 5
laser pulses is presented in Figure 3. Irradiation due to several consecutive pulses caused
deeper damage. The depth of irradiated area was about 50 and 240 microns for 1 and 5
accumulated laser pulses, respectively (Fig. 3). After 30 laser pulses at 4.0 J/cm2 (Fig. 2D),
the following effects are observed: (i) damage of the titanium implant sample in the central
zone (Fig. 2D1), (ii) appearance of cracking (Fig. 2D2 and D3), and minor hydrodynamic
effects on the periphery.
Three dimentional topographical view of some of the areas of Fig. 2D recorded by
atomic force microscope is presented in Figure 4. As expected, cumulative action of 30 laser
pulses induced surface morphology modifications in the central (Fig. 4B1) as well as
periphery regions (Figs. 4B2 and B3) are clearly observed.
During laser interaction with metals, including titanium, the laser energy is absorbed
primarily by free electrons [20,21]. The absorbed radiation energy in the skin layer involves
thermalization within the electron sub-system.
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Figure 4. 3-D surface images obtained by AFM. A) - Implant surface before laser treatment. B) - Ti implant
after 30 laser pulses; B1- centre, B2,B3- near and farther periphery of the damage area, respectively. Laser
energy density was about 4.0 J/cm2.
Due to the electron thermal diffusion, energy transfers to the lattice and heat is
transported into metal. For the laser pulses with laser fluence higher than the damages
threshold the thermal diffusivity cannot be neglected. Generally, depending on the fluence, a
series of effects such as melting, vaporization of the molten materials, dissociation and
ionization of the vaporized material, etc., can be generated on the target/implant. In our
experiments the titanium was irradiated in two different laser energy density regimes to
investigate the effects in both LRED: (i) Irradiation with higher fluence, 23.8 J/cm2, here the
absorbed energy exceeds the latent heat of melting and melt pool was formed on the target
surface and spills over the liquid metal towards the periphery (Figs. 2.B1 and C1). The
situation was similar for one and five pulses but the depth of irradiated area, in later case, was
increased (Fig. 3). (ii) Irradiation with lower fluence, 4.0 J/cm2, resulted in lower ablation
rate. The morphological changes, such as micro-cracking and nano-grains, were mainly
consequence of rapid heating and cooling of the implant target surface layer, (Figs. 2.D3);
also, minor fraction of material was ejected at the periphery in this case. A direct consequence
of these effects is the increase in the surface roughness of the implant target.
Figure 5. Picosecond Nd:YAG, 532 nm laser-induced Ti implant morphology changes as recorded by SEM for
laser energy density 13.6 J/cm2; A) - The view of the Ti implant surface prior to laser action.
B)- Ti after 30 laser pulses; B1- the entire spot, B2, B3- the centre, B4- the near periphery, and B5,B6- farther
periphery of the damage area.
5
The process of the Nd:YAG laser interaction with titanium during the first and
subsequent pulses was accompanied by the appearance of spark-like plasma in front of the
sample.
Results of Nd:YAG laser (532 nm)
With the exposure of the titanium implant with 30 laser pulses of energy density 13.6
2
J/cm at 532 nm the modified surface is shown in Figure 5. With 30 accumulated laser pulses
a more pronounced surface modification was obtained than 5 pulses [13]. Surface changes can
be summarized as follows: (i) ablation of the titanium in the central zone, Fig. 5 B1,2,3, (ii)
appearance of a wave-like periodic microstructure, in near periphery, Fig. 5B4 and, (iii) A
characteristic nano scale structures appeared in farther periphery, Fig. 5B5,6. Further, with
higher number of accumulated laser pulses, e.g. 50 and 100 pulses resulted in a prominent
wave-like microstructure. Under these conditions the central part of the damage area was also
deepened.
Damage threshold for Nd:YAG (532 nm)-titanium interaction is estimated to be about
0.6 J/cm2 which is relatively lower than the corresponding value for 1064 nm laser radiation
(0.9 J/cm2). This can be explained due to the lower reflectivity of titanium (0.49) for shorter
radiation and (0.55) for longer radiation [22]. As a consequence, a better coupling between
laser energy and target is achieved, reducing the damage threshold value for 532 nm laser
radiation interaction.
Elemental analysis of the titanium sample in the central region (spectrum 1) and
farther periphery (spectrum 2) is shown in figure 6, showed that oxygen contents were
different, i.e. 6.04% and 8.85%. The increased oxygen content in the irradiated region with
respect to the non-irradiated area (the initial oxygen concentration in non-irradiated area was
4.34%, spectrum 3, Fig. 6), imply that possible oxidation of the titanium sample has taken
place. Presence of titanium oxide(s) on the implant surface can be very desirable: (i) the
composition of the oxides, including their distribution, can resist the corrosion of the implant
sample [4], (ii) titanium oxides can improve implant adhesion properties, (iii) oxidation of
titanium implant target can lead to local hardening and improve the wear resistance.
Additionally, titanium oxide(s), due to their bio-inertness, could act as a diffusion barrier.
Figure 6. An EDS analysis of the Ti implant surface in the marked regions. Laser wavelength, 532 nm, 30
cumulative laser pulses, laser energy density was about 13.6 J/cm2. Spectra 1, 2 and 3 correspond to irradiated
area, periphery of irradiated area and non-irradiated area.
6
At the periphery, periodic wave-like microstructures were recorded as fringes, Figs.
5B4 and 6 (spectrum 1). Laser beam properties like polarization and coherence, angle of
incidence to the sample, etc. affect the appearance of microstructures [23]. In the present
experiment p-polarization was employed. Generally speaking, the theory of the formation of
wave-like microstructures is rather complex and is dealt in detail in reference 20.
It should be noted that the laser radiation at 532 nm creates plasma in the first laser shot and
in all the subsequent laser pulses.
4. Conclusion
A study of morphological changes on titanium implant surface induced by a
picoseconds Nd:YAG laser, operating at 1064 or 532 nm, is presented.. Laser energy densities
around 4.0 J/cm2 and 13.6 J/cm2 for laser wavelengths 1064 and 532 nm, respectively can
induce morphological changes on the titanium implant surface. Laser action at 1064 nm
exhibits pronounced damage boundaries on the titanium implant surface, than at 532 nm
which were partially diffused. Also, the both laser wavelengths increase the titanium surface
roughness.
Both laser wavelengths show different features of surface morphology. Under the
present experimental conditions, it is clear that wavelength has an important role in producing
the differences. Since the morphological changes are so distinct, we cannot conclude which is
advantageous for further applications and hence bring both effects to the knowledge of the
researchers.
The morphological modifications of the titanium target surface can be summarized as
follows: (i) intensive damage to the implant surface in the central zone of the irradiated area
was recorded with both laser wavelengths. (ii) appearance of a hydrodynamic feature at the
periphery, like resolidified droplets of the material, were characteristics typically to
irradiation at 1064 nm, whereas wave-like microstructures were recorded only with the 532
nm irradiation. The increased oxygen concentration in the sample upon irradiation indicates
possible titanium surface oxidation.
It can be concluded that Nd:YAG laser wavelengths in 1064 and 532 nm and
picoseconds range at reported energy densities, can be effectively applied for enhancing
titanium target/implant roughness, thus improving its bio-integration. Surface modification of
titanium using laser radiation is practically instantaneous implying that large quantity of
implant surfaces can be processed in short times. Appearance of plasma in front of the
implant surface indicates high temperature is created on the surface. This offers a sterilizing
effect facilitating contaminant-free conditions.
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