Surface hardening of SUS304 by irradiation with a KrF excimer laser in SiH 4 gas ambient

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Eiichi Tamura and Keigo Nagasaka Department of Physics, Science University of Tokyo, Kagurazaka, Shinjuku, Tokyo, 162, Japan (Received 21 April 1989; accepted 14 September 1989) Surface hardening of SUS304 resulting from the process of doping and deposition of Si by irradiation of a KrF excimer laser beam in a SiH4 gas ambient is investigated, and variations of the surface hardness are examined for different numbers of laser pulses and the laser fluences. The hardening is due to Si incorporation in high concentration. The continuous distribution of Si atoms across the surface layer suggests that a very high adhesion strength of the deposited Si films can be formed. The specific process for surface modification is referred to as laser implant-deposition (LID).

I. INTRODUCTION

Surface modifications of mechanical and chemical properties of metals such as hardness, wear, friction, and corrosion have been widely investigated, since diversified materials with excellent properties are required in mechanical manufacturing. These studies have shown that although metal alloying, oxidation, or nitridation by ion implantation has merit,1"5 ion implantation with a dose high enough to modify the surface creates many problems, such as sputtering,6 defects,7 and contamination by oxygen8 and carbon.9 An alternative method, UV pulsed excimer lasers, has several advantages10"12: neither sputtering nor defects have been observed [using high-resolution cross-section transmission electron microscopy (HRXTEM)], and contamination is reduced.1213 In addition, selective surface modification can be achieved by limiting the area of the laser beam irradiation by using a mask14'15 or a scanning focused beam.16 However, knowledge is limited as to the mechanical properties of the metal surface treated by laser processing. Deposited metal thin films for laser alloying and liquid phase dopant sources for laser oxidation and nitridation are primarily discussed in the literature, but gas phase dopant sources are also attractive in order to provide a wide variety of laser modifications of the metal surfaces. In the present study, surface hardening of SUS304 by irradiation with a KrF excimer laser in SiH4 gas ambient is described, with SiH4 gas used as a dopant source. In this case, most of the laser energy is deposited at the sample surface and is immediately transformed into heat, since the absorption depth of UV light into metals is of the order of 10 nm.12 In addition, the SiH4 gas has no optical absorption and the optical transmission of the window made of synthetic quartz is 93% at the wavelength of KrF excimer laser. By laser J. Mater. Res., Vol. 5, No. 2, Feb 1990

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heating, the surface might be melted and simultaneously the adsorbed gas molecules on the sample surface subjected to thermal dissociation. The dissociated Si atoms distribute rapidly in the molten region by liquid phase diffusion, and then solidification occurs from the liquidsolid interface. Such a rapid thermal reaction inc