Two-component finite-volume scheme for modelling the initial stage of Al₂O₃-TiC laser processing

Abstract

Processing Al2O3-TiC ceramics with excimer laser radiation is accompanied by the formation of columnar structures that significantly increase the surface roughness. Once nucleated, the surface structures may grow towards the direction of laser beam propagation because the radiation is reflected from the lateral column sides and concentrates between columns. The origin of the column nucleators is the subject of controversial theories, involving mechanisms from Raleigh-Taylor hydrodynamic instability to capillary waves. We demonstrate that, for the case of initially inhomogeneous ceramic microstructures, nucleator development may be described by conventional fluid-dynamics phenomena. With a simple finite-difference scheme (incorporating the conservation equations for alumina and titanium carbide masses, momentum and internal energy) we trace the evolution of a single cylindrical TiC grain at the surface of an alumina matrix. Though the fusion temperature of TiC is higher than that of Al2O3, melting starts with titanium carbide: due to its much higher absorptivity, radiation is absorbed in an extremely thin upper layer of the TiC grain, which is heated up and melted nearly instantly. The melting of TiC is accompanied by expansion, with the tendency for small surface swellings to form a surface globule similar to those observed experimentally.