Infrared (IR) radiometry and time resolved reflectivity (TRR) methods can be used for investigation of laser
pulse effects on materials in nanosecond time scale. The methods in combination are capable to quantify
object temperature and detect phase transformations in the solid state, melting and plasma formation from
vapour. Measurements with different laser pulse energy densities provide threshold of the transformation. The
melt duration can be also determined. The experimental system is described. It contains KrF excimer laser
with homogenizer and variable attenuator, fast IR detector for radiometry, continuous probing laser with Si
photodiode for reflectivity measurement and UV detector for pump laser pulse reflection measurement. The
system was applied to investigation of responses to laser light of silicon and different pure metals and alloys. The
range of energy densities used was 1-5500 mJ.cm<sup>-2</sup> and measurements were done with temporal resolution of 6
ns for radiometry and 1 ns for reflectivity.
A high number of papers were published on the simulation of laser/surface interaction at the level of nanosecond scale.
Several assumptions on thermal properties data, laser spot homogeneity, were assumed for describing as well as possible
the boundary conditions, the mathematical writing and finally the numerical or the analytical results. A few tentative of
surface temperature monitoring during laser processing were proposed for the numerical validation. Also, simulation of
the melting kinetics is rarely directly compared to in situ experiments. It is very hard to determine the time duration of a
melting pool by in situ experiments. It should be the same for the surface temperature.
A new method to plot the thermal history of the surface by using a combination of the Time Resolved Reflectivity (TRR)
and the Pulsed Photo-Thermal (PPT) or Infrared Radiometry (IR) methods is proposed in this paper. Surface
temperature, melting kinetics, threshold of melting and threshold of plasma formation are determined in the case of KrF
laser spot in interaction with several materials. In the first step, the experimental setup including fast detectors (IR, UV,
Vis.) and related optical devices is described. In the second step, typical results (TRR and IR spectra) for monocrystaline
silicon are presented and discussed. Namely, phase change transitions (melting and resolidification) are detected versus
fluence change and number of laser shots change. TRR and IR spectra of metallic surfaces (Cu, Mo, Ni, Stainless steel
15330 and 17246, Sn, Ti), are measured. For each sample the surface temperature during heating, the threshold of
melting, melting duration and the threshold of plasma formation are directly deduced.
Laser treatments of various metals are studying depending on the laser wavelength, pulse time duration and shape, and fluence (laser/metal interaction regime). Low fluence excimer UV laser melting process of gold layer is shown to improve the corrosion resistance of multilayer (Au/Ni/Cu alloy) electrical contacts. For this application the homogenity of the laser beam as well as the initial Cu substrate roughness are found to be limiting parameters of the process. Carburization of Al alloy, performed in C<sub>3</sub>H<sub>6</sub> atmosphere with a KrF laser induces the incorporation of carbon atoms over about 4 μm depth. The crystalline Al<sub>4</sub>C<sub>3</sub> synthesized at the surface leads to a strengthening of the light Al alloy, which is of great interest for application in car industry. The study shows that diffusion of C atom in the target is possible because of a plasma presence on the surface which supports the molten bath life time and induces dissociation of the ambient gas. In the last example of laser metal surface treatment presented in that paper, a commonly used steel is treated in air with different lasers at a fluence above the plasma formation threshold. It is seen that the machining oils covering the surface before the treatment can be efficiently removed and that new compounds (nitride, carbide and oxides) are formed at the surface.
The laser surface treatment is applied to a multilayer component (copper alloy plated with two thin coatings, nickel and gold). The aim of the study is to melt the whole gold layer (thick < micrometers ) without damaging the underlying layers. The gold melting must be homogeneous and the process must be fast to avoid heat diffusion in the depths. For these reasons, the laser has been chosen for surface treatment. The application of this laser surface treatment is to improve the corrosion of resistance of electrical contacts due to columnar microstructure of gold deposited by electrolytic process. Tests of corrosion are carried out in the humid synthetic air containing low contents of pollutants (NO<SUB>2</SUB>, SO<SUB>2</SUB> and Cl<SUB>2</SUB>). An numerical study has been realized to find the best laser conditions to melt the whole gold layer.