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S. Arthanari et al.
in a random microstructure. However, the orientation of ripples was the same as
in the case of laser scans, and the irradiation area was expanded as the number of
pulses was increased up to 500. An increase in pulse numbers above 500 resulted
in coarse and poorly defined features. The formation of micro and nano ripples due
to the laser irradiation was explained based on several factors. The energy transfer
to the metal lattice due to the ejected electron during the laser irradiation and initial
few pulses result in the formation of random nano-protrusions. Formation of nano
protrusions could be attributed to several factors including the laser ejected species,
plasma confinement, and materials re-deposition within several picoseconds during
rapid cooling condensation and plume collapse process. An increase in laser pulse
leads to the expansion of the nano-protrusion area and the surface plasmons (SPs)
both localized and propagating along the surface would be excited by coupling the
incident light to such nano-protrusions. The high cooling rate during the laser treatment results in a variety of surface tension resulting in surface curling, and highly
localized heating, as well as large thermal gradients. This caused the development of
severe strain fields in the peripheral area of the irradiated surface, finally, resulting in
the formation of micro-ripples and isolated melt (cluster structures) on the surface.
A hybrid laser surface modification such as melting, and texturing of Mg-6Gd0.6Ca alloy had a significant variation in the microstructure, corrosion performance
and in vitro biocompatibility. The as-received microstructure mainly consisted of
equiaxed α-Mg phase as the matrix and β-Mg 5 Gd phase distributed in the matrix
along the extrusion direction (Fig. 5A-a). After laser surface modification, solidification microstructure was changed to the α-Mg phase and the β-phase precipitation along grain boundaries was not noticed. The high-temperature laser treatment
resulted in the dissolution of the second phase which was confirmed from the XRD
results (Fig. 5A-b) and the thickness of the laser-melted layer was about 360 μm. The
corrosion results indicated that for the as-received specimen, initial galvanic corrosion occurred preferentially in the α-Mg matrix around the β-phase and extended
into deep corrosion holes. After the laser surface modification, the uniform corrosion
was noticed for the laser melted alloy due to the dissolution of β-phase in the melted
region [41]. As a result of the significant reduction of the second phase concentration, and resulting galvanic influence of the laser melted surface, the hydrogen
evolution volume (Fig. 5A-c) was significantly decreased, and the corrosion rate
was much lower than that of the as-received alloy. The improvement of corrosion
resistance in HBSS was mainly attributed to the dissolution of the β-phase Mg 5 Gd
and homogeneous solid solution in the laser melted layer.
The MC3T3-E1 cell culture studies revealed that as-received specimens showed
cytotoxicity due to a high degradation rate and higher hydrogen volume accumulation
on the surface. Further, no cells survived on the surface during the culturing periods.
Interestingly, the laser-modified specimens exhibited good biocompatibility owing
to their enhanced corrosion resistance. In vitro cell culture studies showed that the
MC3T3-E1 cells survived on the surface of laser-modified specimens, and significant
improvements were illustrated by good adhesion property, spreading performance
and proliferation capacity (Fig. 5B). LIPSS was produced on to the melted surface
using a Ti: sapphire chirped-pulse regenerative amplification laser and a wavelength
S. Arthanari et al.
in a random microstructure. However, the orientation of ripples was the same as
in the case of laser scans, and the irradiation area was expanded as the number of
pulses was increased up to 500. An increase in pulse numbers above 500 resulted
in coarse and poorly defined features. The formation of micro and nano ripples due
to the laser irradiation was explained based on several factors. The energy transfer
to the metal lattice due to the ejected electron during the laser irradiation and initial
few pulses result in the formation of random nano-protrusions. Formation of nano
protrusions could be attributed to several factors including the laser ejected species,
plasma confinement, and materials re-deposition within several picoseconds during
rapid cooling condensation and plume collapse process. An increase in laser pulse
leads to the expansion of the nano-protrusion area and the surface plasmons (SPs)
both localized and propagating along the surface would be excited by coupling the
incident light to such nano-protrusions. The high cooling rate during the laser treatment results in a variety of surface tension resulting in surface curling, and highly
localized heating, as well as large thermal gradients. This caused the development of
severe strain fields in the peripheral area of the irradiated surface, finally, resulting in
the formation of micro-ripples and isolated melt (cluster structures) on the surface.
A hybrid laser surface modification such as melting, and texturing of Mg-6Gd0.6Ca alloy had a significant variation in the microstructure, corrosion performance
and in vitro biocompatibility. The as-received microstructure mainly consisted of
equiaxed α-Mg phase as the matrix and β-Mg 5 Gd phase distributed in the matrix
along the extrusion direction (Fig. 5A-a). After laser surface modification, solidification microstructure was changed to the α-Mg phase and the β-phase precipitation along grain boundaries was not noticed. The high-temperature laser treatment
resulted in the dissolution of the second phase which was confirmed from the XRD
results (Fig. 5A-b) and the thickness of the laser-melted layer was about 360 μm. The
corrosion results indicated that for the as-received specimen, initial galvanic corrosion occurred preferentially in the α-Mg matrix around the β-phase and extended
into deep corrosion holes. After the laser surface modification, the uniform corrosion
was noticed for the laser melted alloy due to the dissolution of β-phase in the melted
region [41]. As a result of the significant reduction of the second phase concentration, and resulting galvanic influence of the laser melted surface, the hydrogen
evolution volume (Fig. 5A-c) was significantly decreased, and the corrosion rate
was much lower than that of the as-received alloy. The improvement of corrosion
resistance in HBSS was mainly attributed to the dissolution of the β-phase Mg 5 Gd
and homogeneous solid solution in the laser melted layer.
The MC3T3-E1 cell culture studies revealed that as-received specimens showed
cytotoxicity due to a high degradation rate and higher hydrogen volume accumulation
on the surface. Further, no cells survived on the surface during the culturing periods.
Interestingly, the laser-modified specimens exhibited good biocompatibility owing
to their enhanced corrosion resistance. In vitro cell culture studies showed that the
MC3T3-E1 cells survived on the surface of laser-modified specimens, and significant
improvements were illustrated by good adhesion property, spreading performance
and proliferation capacity (Fig. 5B). LIPSS was produced on to the melted surface
using a Ti: sapphire chirped-pulse regenerative amplification laser and a wavelength