Publications

Silicon material has been extensively used in electronics, optoelectronics, and Microelectromechanical Systems (MEMS). In spite of good mechanical behavior exhibited by the silicon material, their brittleness causes difficulty in machining, which greatly affects the surface integrity of the machined parts. This demands a need for further investigations in this area for improving the material removal rate and surface integrity. Ultrashort-pulsed laser machining techniques have brought new perspectives for efficient machining of semiconductor materials in terms of surface integrity and energy consumption. In the present work, the effect of number of pulses and repetition rate at 1064 and 532 nm wavelengths of a picosecond laser has been investigated for dimensional quality and subsurface defects of holes. Further experiments were conducted using a femtosecond laser to understand the effect of repetition rate, pulse duration, pulse energy, and scanning speed on the quality of kerf surface.

A plasma shielding phenomenon and its influence on micromachining is studied experimentally and theoretically for laser wavelengths of 355 nm, 532 nm and 1064 nm. A time resolved pump-probe technique is proposed and demonstrated by splitting a single nanosecond Nd3+:YAG laser into an ablation laser (pump laser) and a probe laser to understand the influence of plasma shielding on laser ablation of copper (Cu) clad on polyimide thin films. The proposed nanosecond pump-probe technique allows simultaneous measurement of the absorption characteristics of plasma produced during Cu film ablation by the pump laser. Experimental measurements of the probe intensity distinctly show that the absorption by the ablated plume increases with increase in the pump intensity, as a result of plasma shielding. Theoretical estimation of the intensity of the transmitted pump beam based on the thermo-temporal modeling is in qualitative agreement with the pump-probe based experimental measurements. The theoretical estimate of the depth attained for a single pulse with high pump intensity value on a Cu thin film is limited by the plasma shielding of the incident laser beam, similar to that observed experimentally. Further, the depth of micro-channels produced shows a similar trend for all three wavelengths, however, the channel depth achieved is lesser at the wavelength of 1064 nm.

Laser surface processing is increasingly used to improve the tribological properties of sliding
surfaces over the last few decades. Texturing in the form of dimples which are evenly spaced helps
to reduce friction to a greater extent. But optimizing the size of the dimples with appropriate area
density and aspect ratio is a big challenge for the tribologist. This paper aims at the investigation of
producing textured surface using nanosecond pulsed Nd3+: YAG laser on the surface of piston rings
and also to study its frictional properties using a reciprocating tribometer. Experimental analysis on
the effect of diameter area density and aspect ratio of the dimples has been demonstrated to improve
frictional performance under different lubrication regimes. Friction tests were carried out with both
non-textured and textured piston rings by varying the normal load applied on it using synthetic oil
as a lubricant at an oil bath temperature of 180°C. A maximum frictional reduction of up 69% was
observed with the nanosecond pulsed laser textured sample at high loading conditions. But
nanosecond pulsed laser texturing is always accompanied by recast layer, heat affected zone and
micro cracks which may affect the surface properties and in turn the frictional properties of sliding
surfaces. Preliminary studies on surface texturing was performed using a femtosecond pulsed laser
to improve the surface morphology and tribology characteristics of a piston ring surface.

Phase-field method uses a non-conserved order parameter to define the phase state of a system and is a versatile method for moving boundary problems. It is a method of choice for simulating microstructure evolution in the domain of materials engineering. Solution of phase-field evolution equations avoids explicit tracking of interfaces and is often implemented on a structured grid to capture microstructure evolution in a simple and elegant manner. Restrictions on the grid size to accurately capture the interface curvature effects lead to large number of grid points in the computational domain and render the simulation computationally intensive for realistic simulations in 3D. However, the availability of powerful heterogeneous computing platforms and super clusters provides the advantage to perform large scale phase-field simulations efficiently. This paper discusses a portable implementation to extend simulations across multiple CPUs using MPI to include use of GPUs using OpenCL. The solution scheme adapts an isotropic stencil that avoids grid-induced anisotropy. Use of separate OpenCL kernels for problem specific portions of the code ensure that the approach can be extended to different problems. Performance analysis of parallel strategies used in the study illustrate the massively parallel computing possibility for phase-field simulations across heterogeneous platforms.