Excimer Laser Mixing of Titanium Layers on AISI 304 Stainless Steel
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EXCIMER LASER MIXING OF TITANIUM LAYERS ON AISI 304 STAINLESS STEEL T. R. JERVIS, M NASTASI, and T. G. ZOCCO
Materials Science and Technology Division, Mailstop E549, Los Alamos National Laboratory Los Alamos, NM 87545 ABSTRACT
We have used excimer laser radiation to mix single layers of Ti directly on annealed substrates of 304 stainless steel. This process produces much thicker surface alloy layers and is much faster than ion mixing techniques. Liquid state diffusion results in mixed layers of the order of 1.0 ptm thick from 0.5 rnm Ti overlayers. In single pulse experiments, fluences of up to about 4 times melt threshold could be used before ablation occurred. The fluence in these experiments determines the melt duration and therefore the amount of mixing. In order to achieve greater mixing without ablation of the surface, multiple pulses at lower fluence were used. Profiles of laser mixed surface layers and microstructural characterization of the layers are presented. INTRODUCTION
Surface amorphous layers of Ti-Fe alloys on stainless steel have been shown to have superior wear and friction properties when produced by ion mixing of multiply deposited layers and by ion implantation[1,2]. However, because of the kinetic limits on diffusion in ion mixing, multiple layers must be used and the dose required for effective mixing can be quite large. Likewise, the dose required to create and amorphous surface layer by ion implantation is quite large. Alternatively, laser cladding using C02 laser radiation can be used to apply surface layers of various alloys to bulk materials[3]. In this case, power levels of the order of kilowatts are required to melt the surface using cw radiation, and the cooling rates are sufficiently slow that phase segregation can occur. Further, the surface which results is rather uneven and remachining is required for tribological applications. An alternative to these techniques is laser mixing of surface layers using pulsed excimer radiation in the ultra-violet (UV) portion of the spectrum. Because metals absorb strongly in the UV, coupling of energy to the surface is much more efficient and melting occurs at fluences of about 1.0 J-cm"2 . Because excimer lasers are intrinsically about as efficient as CO2 lasers in converting electrical energy to photons, Laser power requirements are more modest. In the case of metals processed by excimer laser pulses, the thermal diffusion length 8 th, given by: kth = 2 rD~th T
where Dth is the thermal diffusivity and T is the pulse length, is much greater than the absorption depth 8opt. For AISI 304 stainless steel, Sth is 0.7 gm, while 8o t is more than an order of magnitude smaller. The figures for Ti are similar. In this case, athough the energy is deposited at the surface, the thermal effect is distributed more or less uniformly through the thickness Sth, and the surface temperature can be written as: Ts = 1.13 F(1-R) p Cp 8th
where F is the incident fluence, R the reflectivity, p the density and cp the heat capacity[4]. Combining these with the heat
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