Laser Induced Micro/Nano …
443
produced micro/nano textures could be used for different applications such as optical
properties, wettability, sensors, and biomedical implants [12–15].
2.1 Interaction of Laser with Metal Surfaces
The irradiation of metal surfaces with laser results in various physical processes
occurring on the surface such as heating, melting, vaporization, etc. Furthermore,
the conversion of laser (electromagnetic wave) energy into heat significantly varies
depending on the nature of the materials whether metals or non-metals. The absorption of laser energy by the metals during laser processing takes place through the
photon-electron interactions with free or bound electrons. Therefore, the energy
state of the electrons in the conduction band raises by following the classical thermal
conduction mechanisms. These excited electrons can follow two processes (1) collide
with lattice phonons and with other electrons or (2) give back their radiation by spontaneous emission. The relationship between the electron density of a metal N e and
optical properties is denoted as plasma frequency ω p =
√
N e e
2 /m e ε 0 , where m e is
the mass of an electron and ε 0 the permittivity of free space [16]. Since the mean
free time of the electrons in the conductor is about 10
−13 s, the thermal equilibrium
is quickly achieved during the process.
The absorbed energy density of laser during the interaction with the metals is determined by various factors viz., absorbed power density, pulse duration, the reflectivity
of the surface, thermal diffusivity, vapor pressure, and optical absorbance. These
factors further decide the penetration of laser with substrate material and the resulting
properties. At low and high laser power densities, the interaction of laser with metals
is thermal and explosive characters, respectively. The materials are heated, melted
until reaching the vapor pressure, and material removal take place when it reaches
the vapor pressure during the laser interaction. At higher laser intensities several nonequilibrium processes to take place, these results in the large mass removal from the
surface. Another important thing to be considered is the wavelength of laser beam
since the absorption depths of the materials depend on the wavelength and choosing
the appropriate wavelength laser will be effective to local modification of surface
properties without altering the bulk materials. Therefore, it is important to control
the laser processing parameters to achieve the desired surface nature with targeted
properties.
2.2 Selective Laser Melting (SLM) of Metallic Implants
The SLM is an additive manufacturing (3D printing) process, where a high-power
laser beam scans the metallic powder materials, and completely melt them to
fabricate the desired geometry (Fig. 2a). This method is generally employed to
produce new alloys consisting of a millimeter or sub-millimeter scale structures
443
produced micro/nano textures could be used for different applications such as optical
properties, wettability, sensors, and biomedical implants [12–15].
2.1 Interaction of Laser with Metal Surfaces
The irradiation of metal surfaces with laser results in various physical processes
occurring on the surface such as heating, melting, vaporization, etc. Furthermore,
the conversion of laser (electromagnetic wave) energy into heat significantly varies
depending on the nature of the materials whether metals or non-metals. The absorption of laser energy by the metals during laser processing takes place through the
photon-electron interactions with free or bound electrons. Therefore, the energy
state of the electrons in the conduction band raises by following the classical thermal
conduction mechanisms. These excited electrons can follow two processes (1) collide
with lattice phonons and with other electrons or (2) give back their radiation by spontaneous emission. The relationship between the electron density of a metal N e and
optical properties is denoted as plasma frequency ω p =
√
N e e
2 /m e ε 0 , where m e is
the mass of an electron and ε 0 the permittivity of free space [16]. Since the mean
free time of the electrons in the conductor is about 10
−13 s, the thermal equilibrium
is quickly achieved during the process.
The absorbed energy density of laser during the interaction with the metals is determined by various factors viz., absorbed power density, pulse duration, the reflectivity
of the surface, thermal diffusivity, vapor pressure, and optical absorbance. These
factors further decide the penetration of laser with substrate material and the resulting
properties. At low and high laser power densities, the interaction of laser with metals
is thermal and explosive characters, respectively. The materials are heated, melted
until reaching the vapor pressure, and material removal take place when it reaches
the vapor pressure during the laser interaction. At higher laser intensities several nonequilibrium processes to take place, these results in the large mass removal from the
surface. Another important thing to be considered is the wavelength of laser beam
since the absorption depths of the materials depend on the wavelength and choosing
the appropriate wavelength laser will be effective to local modification of surface
properties without altering the bulk materials. Therefore, it is important to control
the laser processing parameters to achieve the desired surface nature with targeted
properties.
2.2 Selective Laser Melting (SLM) of Metallic Implants
The SLM is an additive manufacturing (3D printing) process, where a high-power
laser beam scans the metallic powder materials, and completely melt them to
fabricate the desired geometry (Fig. 2a). This method is generally employed to
produce new alloys consisting of a millimeter or sub-millimeter scale structures