Laser-induced structural transformations such as transformation hardening, annealing, recrystallization, glazing, shock hardening, etc., are based on the high processing temperatures that can be reached during short heating and cooling cycles under high-power pulsed laser or rapidly scanned cw-laser irradiation. Short processing cycles permit material transformations within thin films and surfaces without significant influence on the substrate or the underlying bulk material. In the case of surface absorption, which is a good approximation with many applications, the thickness of the heated zone is approximately described by the heat diffusion length, l T . The time for heating the material to a certain temperature and depth, and the time for cooling can both be calculated from the equations given in Chaps. 6–9. The thickness of the modified layer, Δh ti l T , decreases with decreasing pulse length. With ultrashort laser pulses Δh becomes so small that cooling rates up to more than 1012 K/s can be achieved. If τ l < D/v 0 2 ≈ 10−12 to some 10−14 s (v o is the sound velocity), the finite velocity of the heat front must be taken into account (Sect. 2.2). In any case, with such cooling rates it is possible to freeze non-equilibrium phases, suppress nucleation, etc. There is, however, a limitation. The transformation temperature must be sustained during a time which is longer or at least comparable to the time required for the phase transformation to take place. Furthermore, with many systems, successful laser processing is related to strong temperature gradients which induce internal stresses, redistributions of defects, different types of transport phenomena, etc.
KeywordsHeat Affected Zone Interface Velocity Laser Annealing Transformation Hardening Strong Temperature Gradient
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