AbstractThe behavior of highly anisotropic materials under short pulses of high
power laser irradiation has been studied by irradiating highly oriented
pyrolytic graphite (HOPG) with 30 nsec Ruby-laser pulses with energy
densities between 0.1 and 5.0J/cm2. Raman spectroscopy has been
used to investigate the laser-induced modifications to the crystalline
structure as a function of laser energy density of the laser pulse. A Raman
microprobe was used to investigate the spatial variations of these
near-surface regions. The irradiation of HOPG with energy densities above ~
0.6J/cm2 leads to the appearance of the ~ 1360 cm-1
disorder-induced line in the first order Raman spectrum. The intensity of
the ~ 1360cm-1 line increases with increasing laser energy
density. As the energy density of the laser pulse reaches about
1.0J/cm2, the ~ 1360cm-1 line and the ~
1580cm-1 Raman-allowed mode broaden and coalesce into a broad
asymmetric band, indicating the formation of a highly disordered region,
consistent with RBS-channeling measurements. However, as the laser energy
density of the laser pulses is further increased above 3.0J/cm2,
the two Raman lines narrow and can again be resolved suggesting
laser-induced crystallization. The Raman results are consistent with high
resolution electron microscopy observations showing the formation of
randomly oriented crystallites. Raman Microprobe spectra revealed three
separate regions of behavior: (i) an outer unirradiated region where the
material appears HOPG-like with a thin layer of material coating the
surface, (ii) an inner irradiated region where the structure is uniform, but
disordered, and (iii) an intermediate region between the other regions where
the structure is highly disordered. The changes in structure of the inner
region are consistent with the behavior observed with RBS and conventional
Raman spectra. The identification of an amorphous carbon-like layer on the
outer region is consistent with a large thermomechanical stress at the
graphite surface, introduced by the high power laser pulse, and known to
occur in metals.
Effect of laser energy density on microstructure and properties of SLM 24CrNiMoY alloy steel
