The Thermal Effect in Ablated Stainless Steel by Femtosecond Laser and Nanosecond Laser

Laser was widely used in the high precision micro-machining of various materials. However, the thermal effect in the laser processing is not easy to be avoided. Therefore, many applications and researches, in recently, use ultraviolet laser with nanosecond duration and near-infrared laser with femtosecond duration. The both types of laser have the reduced thermal effect during laser processing and thereby increase the precision such as minimization of burr, improvement of surface roughness of structure, promotion of repeatability in dimension etc…. However, the thermal effect in the laser micro-machining was not understood yet. It is due to almost the explanation only depending on the simulation. Seldom researcher using the experimental evidence shows the temperature. This issue to uncover the machining mechanism is vital to understanding the limitation and advantage in application of laser micro-machining applications. In this thesis, the property to form the different type of oxide in the different condition will be used to disclose the temperature on the stainless steel surface during femtosecond laser processing and nanosecond laser processing. The experimental method is using optical microscope and Raman spectroscope to measure and to analyze the products on the stainless steel surface within the ablated region. Raman experiment shows that femtosecond and nanosecond lasers create different iron oxides during the micro-machining. These oxides provide a clear evidence of processing temperature which is important to understand the laser-machining mechanism.