Monitoring of the thermal melting of a silver surface layer with use of probing laser light

Optical monitoring of the thermal melting of materials caused by high power laser radiation uses the dependence of the reflectivity on the surface temperature. The thermal melting of the surface layer is considered as the nucleation followed by the nuclei coalescence. It cannot be detected using the reflectivity. In this article, the melting of the surface layer is studied experimentally for silver samples by measuring temperature of the samples and registering patterns of probing laser light scattered and specularly reflected from the surfaces. The samples having unidirectional roughness of a surface were heated in an electric furnace. The surfaces were oriented vertically and illuminated by the probing laser beam at the wavelength of 660 nm. The power and the divergence of the incident beam were approximately 50 mW and 0.3 mrad. The study has helped clarify the features of the patterns which allow detect the melting of the surface layer. Also we found a phenomenon similar to the directional crystallization. A rod of silver grew out the molten silver against the direction of the laser beam.