Publications

A nanosecond pulsed Nd3+:YAG laser was used to generate grooves as well as dimples on the surface of the moly-chrome film. Interaction of laser, operating at the laser wavelengths of 1064 nm, 532 nm and 355 nm, respectively on the target material were investigated. The principal finding is based on the geometrical parameters such as width, depth and recast layer in the case of the groove. Various roughness parameters inside the grooves were studied. Whereas in the case of the dimple, various parameters like dimple diameter, depth and recast laser were analyzed. A study to find the optical reflectivity of the moly-chrome material at different laser wavelengths was conducted. The groove and dimple formed with the laser wavelength of 1064 nm showed a high material removal and re-cast layer compared to 532 nm and 355 nm. The roughness Ra inside the groove formed with the laser wavelength of 1064 nm and 355 nm exhibited a percentage increase of around 68% and 20% compared to the 532 nm.

Continuous micro-channels were formed on the surface of the automotive piston rings using two different lasers, namely nanosecond and femtosecond pulsed laser. Tribology measurements were conducted to compare the friction properties of the samples fabricated with the laser pulse width of nanosecond and femtosecond order. The femtosecond laser ablation comparatively minimized the re-deposition layer, heat affected zone around the micro-channel compared to the nanosecond laser ablation. Owing to the localized material ablation, the compressive residual stress was not altered with femtosecond laser processing. The friction test was conducted using reciprocating type friction measurement setup for the loads ranging from 10 to 130 N. The friction coefficients of all the textured samples were reduced compared to the non-textured samples. Furthermore, it is observed that the average friction coefficient was low with the femtosecond laser textured samples compared to that of the nanosecond laser textured samples of the same dimensions. The results show that the thermally induced defects while using the nanosecond laser ablation might have influenced the tribological properties.

In the present work, pulsed laser assisted texturing of the piston ring and its influence on tribology characteristics were studied experimentally and theoretically. Influence of laser wavelengths of 532 nm and 1064 nm on surface morphology of dimples, formed on the piston ring samples was studied. Subsequently, the laser wavelength of 532 nm was used for texturing to generate dimples with different sizes, aspect ratio and area density. Tribological characteristics of textured samples consisting of dimples with size, aspect ratio and area density ranging from 40 μm to 130 μm, 0.1 to 0.3 and 5% to 38%, respectively were measured experimentally by using reciprocating tribometer. The results showed that the aspect ratio concerning minimum friction varied with dimple size. The results also indicated that the area density of 16% showed low friction compared to other fractions for all the dimple diameters considered for the analysis. A reduction in cylinder liner wear rate of 72% was observed when tested with textured ring sample of the desired dimension compared to the liner tested with the non-textured ring. A theoretical model based on Reynolds equation was used to determine lubricant film thickness between textured surface and a non-textured counter surface. The experimental studies on friction coefficient were then compared with the theoretical results on film thickness.

Recently, there is an increasing interest to create micro-channels on metal thin films for diverse applications, such as biomedical, micro channel heat exchangers, chemical separation processes and microwave antenna. Nanosecond (ns) Nd3+:YAG laser has been studied for generating micro-channels on Cu thin film (35 μm) deposited on polyimide substrate (50 μm). A pulsed Nd3+:YAG laser (532 nm / 355 nm) based scribing was performed in air and water ambiancePlasma shielding phenomenon is observed to influence the depth of microchannel at higher energies. A novel pump-probe experiment has been conducted for verifying the plasma shielding effect in air. In underwater scribing the recast layer was reduced significantly as compared to that in air. Laser scribing of Cu thin film followed by chemical etching using FeCl3 was studied. However, the approach of chemical etching resulted in undercut and thinning of Cu film. Alternatively, laser material processing in NaCl solution was studied. Cl− ions present in the solution reacts with Cu which is removed from the sample via laser ablation and forms CuCl2. Formation of CuCl2 in turn improved the surface morphology of the channel through localized etching. The surface roughness parameter Ra was less than 400 nm for NaCl solution based scribing which is smaller compared to air and underwater based methods which are typically around 800 nm or above. Preliminary studies using femtosecond (fs) laser based Cu scribing in air with the fluence of 0.5 J/cm2 resulted in a crated depth of 3 μm without any recast layer.
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Laser scribing is carried out using a Q-switched (Brilliant B, Quantel) Nd3+:YAG laser system to
scribe micro channels on copper coated on polyimide film, where copper thickness is approximately
35 microns and polyimide film thickness is 50 microns. Chemical etching is performed using FeCl3
solution for the laser scribed micro channels and from the experimental results it is observed that
depth of the channel after etching is increasing with a reduction in the recast height. It is observed
that with the increase in concentration of FeCl3 and the etch time, the material removed from the
copper target increased. The height of recast for the 50 μm wide micro channel scribed using 20 mJ
of energy and a laser wavelength of 532 nm reduced from 10 μm to 5 μm in case of 10% FeCl3
etched for 1 min. However the overall thickness of the copper thin film is observed to reduce from
35 μm to 30 μm. Hence a hybrid technique using NaCl as the scribing medium is developed, so that
CuCl2 formed in the process of scribing helped in achieving a localized etching inside the channel
without affecting the total target thickness.