Influence of Environment on Excimer Laser Irradiation of Metals
- PDF / 2,353,705 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 15 Downloads / 203 Views
INFLUENCE OF ENVIRONMENT ON EXCIMER LASER IRRADIATION OF METALS Julian P. Partridge, Joseph Pellegrino, Craig Murphy, and Peter R. Strutt. The University of Connecticut, Institute of Materials Science, Box U-136, Storrs, CT 06268. ABSTRACT Irradiation of metals in gaseous and liquid nitrogen by KrF excimer laser pulses shows the way in which vapor or plasma pressure fields determine the nature of the rapidly solidified surface topography. Wave patterns formed on aluminum by irradiation in N 2 gas arise from recoil pressure differences produced by highly localised differential surface evaporation. The solidification time, calculated from the observed ripple wavelength, is in good agreement with that deduced from a heat flow calculation. For irradiation in liquid N 2, the homogeneous pressure within the center of the dense plasma zone results in a smooth surface. Beyond this region the progressively diminishing pressure induces surface ripples, fragmentation and droplet formation. Intriguingly, immersion of Ni, Ti, Zn, and Al in liquid N 2 (to a depth of 2 to 4 mm) considerably increases the area and depth of irradiation-modified regions. INTRODUCTION The phenomena which occur when an intense pulsed laser-beam irradiates a metal depends upon the surrounding environment. If this be gaseous in nature, the plasma formed at higher power densities, as well as influencing the energy coupling efficiency will generate a short-duration zone of high pressure. Analyses of the mechanisms involved have been studied by various workers, such as Pirri et all for 10.6 Aim wavelength radiation from the CO2 laser and Ursu et aV2 for excimer laser irradiation of aluminum. Interestingly, a much greater magnitude pressure 3 pulse (well into the GPa range) is produced when the surrounding medium is a liquid . Although this type of phenomenon is well known3,4, it is only recently that the effect has been recognized for potential use in surface modification. For example, Nakamura et al in Japan' have irradiated niobium immersed in liquid nitrogen for surface nitriding. Naturally, one important aspect of laser surface processing is the creation of surface topographical patterns by processes such as (i) differential surface vaporization and melting resulting from spatial distributions of the radiation intensity, and (ii) hydrodynamical processes within the melt layer following cessation of the radiation. This paper is concerned with the characterization of topographical features developed by momentary melting for irradiation in liquid nitrogen using 248 nm wavelength radiation. Of specific interest is a comparison of the topographical features developed in aluminum by irradiation within (i) gaseous and (ii) liquid nitrogen. EXPERIMENTAL DETAILS The current experiments using a KrF excimer laser were restricted to single-pulse events of fixed wavelength (248 nm), pulse width (22 ns), and incident pulse energy (110 mJ). Substrates consisted of polished discs of Ni, Ti, Al, and Zn with approximate dimensions 10 mm diameter x 5 mm thickness, one su
Data Loading...
