Metallographical analysis of steel and hard metal substrates after deep-drilling with femtosecond laser pulses

Investigations on the evolution of the ablated depth over a large number of incident femtosecond laser pulses and the occurrence of structure and hardness changes in the immediate vicinity of the laser induced craters (for two sorts of steel and a hard metal substrate) are reported in this work. Experiments were performed in air with a Ti:sapphire laser (800 nm, 100 fs) at mean fluences of 2, 5 and 10 J/cm2. After the laser irradiation, cuts were made through the processed samples and pore cross-sections were obtained. Through their metallographical analysis it was possible to evidence crystalline structure changes in the immediate vicinity of the laser induced microholes; the extent of modified material zones increased for higher laser fluences. In such zones, a chemical composition quasi similar to that of the raw material was found and the performed nanohardness tests revealed hardness increases.