domingo, 14 de marzo de 2010

Láser Surface Alloying (LSA)

When a thin metal film A on a metal substrate B is exposed to láser radiation, the combination can be alloyed through melting to yield a new modified surface layer. This LSA process can be understood with reference to the schematic cross-sectional views. A láser pulse causes film A to melt and the resulting liquid/solid front sweeps past the original A-B interface; interdiffusion of film and substrate atoms occurs as irradiation terminates. The máximum melt depth is reached, where the atomic mixing is quite vigorous. Resolidification then begins and the solid-liquid interfacial velocity, which is initially zero, increases very rapidly. Interdiffusion continúes within the liquid but the resolidified metal cools so rapidly that atoms are immobilized and frozen in place.

 The final result is an alloy of nominal composition A^B,^ which is not necessarily homogeneous. As an example, consider the surface alloying of a thin Au film on a Ni substrate. Typical Q-switched láser pulses genérate total melt times (tM) ranging from 50 to 500 nsec and produce atomic mixing over the diffusion distance ~ 2 \jDtM. The diffusivity of atoms in liquid metáis is rarely outside the range of 10~5 to 10~4 cm2/sec so that compositional change can be expected over a distance of 140-1400 A. The higher estímate roughly agrees with the Au concentration profile data .  In this example the total melt depth is about 4500 A and considerably exceeds the initial Au thickness. If, on the other hand, tM is lengthened to ~ 50 f¿sec, a situation arising when using a cw-C02 láser pulse, then the molten pool is deeper and the diffusional length exceeds 4500 A. Complete homogenization clearly occurs in this case.



Considerable LSA experimentation has been conducted on assorted binary alloy systems. They include (1) those in which the two components are mutually soluble in both liquid and solid states, e.g., Cr-Fe, Au-Pd, W-V; (2) those where there is appreciable liquid solubility (miscibility) but limited solid solubility, e.g., Cu-Ag, Au-Ni, Cu-Zr; and (3) those that exhibit both liquid and solid phase immiscibility, e.g., Pb-Cu, Ag-Ni, Cu-Mo. Category 1 lends itself to thermodynamically favored interdiffusional mixing. Except for rare  or  expensive  alloying  elements,   however,   láser  processing  in  these systems offers few advantages over conventional bulk alloying processes. Thermodynamic obstacles to mixing in category 3 are not easily overeóme even with LSA methods. 

Only when the films are thin enough and the melt temperature high enough is there the chance that a single-phase liquid will form, which then can be quenched to retain metastable phases. Otherwise, predictable phase separation will oceur. Intermedíate category 2 offers the greatest potential for quenching in metastable and amorphous phases. It is this class of binary systems which had been previously studied by vapor-quenching methods over a decade earlier. The objective was the same as LSA —to extend solubility of terminal phases, freeze in metastable phases and produce amorphous phases by suppressing crystallization. Láser scanning methods offer the best means of achieving these ends over large surface áreas.

Ronellys Flores---CRF---libro the materials science of thin films



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