Effect of excimer laser alloying of Ti on the sliding friction of AISI 304 stainless steel
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J.-P. Hirvonen Department ofPhysics, University ofHelsinki, SF-00170 Helsinki, Finland
M. Nastasi Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87544 (Received 27 May 1988; accepted 2 August 1988)
Dry sliding friction measurements made on titanium layers evaporated on AISI 304 stainless steel in the as-deposited and excimer laser mixed form show a dependence on the film thickness and the amount of mixing. The effect of laser mixing is dependent on the incident fluence with high fluences and/or large numbers of pulses producing surfaces with poor frictional properties. The optimum total fluence depends on the thickness of the surface layer, a result consistent with thorough mixing of the alloyed layer without the surface damage that results from large numbers of pulses.
I. INTRODUCTION Beam techniques of various kinds have been shown to improve the surface properties of engineering materials. In particular, ion beam techniques' and laser processing2 have shown that surface processing can enhance the properties of materials and decouple the properties of the surface from those of the base material. Previous studies of Ti and C ion implantation in Fe surfaces3 and ion mixing of Ti and Fe surface multilayers on engineering materials4 have demonstrated the effectiveness of these surface techniques for improving the wear and friction properties of materials. However, ion beam techniques are relatively slow and require substantial investment in capital equipment. We have previously shown5 that excimer laser mixing of Ti layers on AISI 304 stainless steel can produce amorphous layers of a composition similar to that of the Ti 50 Fe 50 materials. Excimer lasers are well suited to metal surface processing as the wavelength in the ultraviolet couples well to most metal surfaces, yielding an improvement in efficiency of several orders of magnitude compared to CO 2 laser processing. Since the melt depth is much smaller for ultraviolet processing (/im's) than for infrared processing (100's of/im's), the resultant surface finish is much smoother. Finally, because of the shallow melt depth and short pulses involved, cooling rates are of the order of 109 K/s, making metastable structures accessible. Excimer laser techniques can process large areas in a short time with only moderate capital equipment. In this paper, we report the effect of excimer laser mixing of Ti layers of different thickness on the dry sliding friction coefficient of AISI 304 stainless steel. The time history of the temperature of the melted 1104
J. Mater. Res. 3 (6), Nov/Dec 1988
http://journals.cambridge.org
surface layer of metals can be calculated directly from the fundamental optical and thermal constants of the material. If we assume that the laser pulse has a square temporal profile, so that power incident is either a constant PQ or zero, the temperature T of the material at any depth z and time t is given by6 T(z,t)
=
\2yJKt T(z,t)
= (1 -
,
for
t>t0,
where R = reflectivity, K = thermal diff
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