Ablation and Spallation of Metals by Femtosecond Laser Pulse

摘要

Irradiation of metals by femtosecond (fs) laser pulse causes pressure buildup within a thin surface layer of thickness d_T in the range 50-200 nm depending on particular metal. As a result, an acoustic wave, consisting of a compressive wave front and rarefaction tail, forms propagating into the bulk. Such a two-part wave with positive and negative pressure parts is generated during acoustic decay of the pressurized layer followed by reflection of acoustic wave from the free vacuum boundary. Elapsed time for creation of the two-part pressure wave t_s ～ d_T/c_s ～10-50 ps, where c_s is speed of sound. Stretching due to the negative pressure of the two-part wave can cause detachment and run away (ejection, spallation) of a layer of condensed metal from the target if the tensile stress exceeds the metal's dynamical strength σ~*, which depends on the metal temperature and strain rate produced by the wave. In laser physics this phenomenon is called thermo-mechanical ablation. Simulations of aluminum, gold and nickel targets show that for σ＞σ~* the surface layer is in molten state after laser energy absorption. Strong tensile stress in negative pressure wave results in bubble nucleation in a hot molten layer. This phenomenon is called cavitation. Cavitation is followed by inertial expansion into vacuum of a liquid-vapor mixture accelerated by rarefaction wave. Corresponding motion of the free surface is observed in fs laser experiments. The experimental tensile strength is obtained from the decrease in velocity during deceleration of the ejected layer. Simulations show a temperature dependence of the tensile strength σ~*(T) for liquid metals that agrees well with experimental data. As the two-part pressure wave propagates into the bulk, the positive pressure wave transforms to a shock due to nonlinearity of the compression wave resulting in focusing of characteristics. In the case of a free-standing metal film, the shock wave is reflected from the rear-side and, if the tensile strength of the reflected wave exceeds the dynamical spall strength of the solid metal, causes spallation. Thus, the tensile strengths for both molten and solid phases are obtained for several metals from fs laser experiments and simulation. Thickness d_T and sonic time t_s are extremely small in comparison with scales for more usual flyer plate impact experiments and experiments with nanosecond (ns) lasers. Therefore our measurements of strengths correspond to very high strain rates V/V in the range ～10~9 s~(-1), approaching ideal or limiting strength. E.g., the strengths for Al are of the order of few GPa. Our laser pump-probe diagnostic technique has time resolution, which is 2-3 orders of magnitude higher than more usual VISAR, ORVIS technique used in flyer plate impact and ns laser experiments. Authors (NAI, SIAsh, VAK, SIAn, MBA) acknowledge the support from RFBR grant No. 10-02-00434-a.