Investigation of the effects of nonconventional beam geometries in laser surface treatment of ceramics: Experimental analysis

Laser surface treatment of ceramics by melting and resolidification generally leads to the formation of fully densified and homogeneous surfaces, with enhanced mechanical and physical properties. A common problem associated with the process is the formation of thermally induced cracks on solidification. This, however, can be eliminated by controlling the cooling rates and thermal gradients during processing. The technique of scanning the laser beam at relatively slow speeds to modify the thermal shock conditions to which the surface is subjected, has been successfully applied to the development of crack-free surfaces in Al2O3-based refractory ceramics. The relatively slow speed, however, limits the practicability of the technique. Novel work on the effects of various nonconventional beam geometries on cooling rates and, hence, on interaction times and processing speeds, during laser surface treatment of ceramics has been carried out. A commercial finite element analysis package was used to simulate the material-beam interaction conditions for various beam geometries, including rectangular, line, triangular, pi, and rhomboid. Extensive experimental work has verified the theoretical finite element analysis results, concentrating on the pi and rhomboid beam geometries. The time-temperature histories of the aforementioned geometries were measured in-process, and compared with the modeling results; this confirmed that the processing speed for the rhomboid geometry had to be increased by a factor of about 5 and that for the pi geometry by a factor of about 2.5 compared to the standard circular beam geometry to produce similar cooling rates. The characteristics of the surfaces treated with the pi and the rhomboid beam geometries at these speeds confirmed the formation of crack-free, fully densified and homogeneous surfaces. The treated zones consisted of a biphasic microstructure, including a crystalline Al2O3-Cr2O3 solid solution and aluminosilicate glass. These features, as well as the mechanical properties, were similar to those of the surfaces treated with the conventional circular beam geometry.