Investigation of Interaction Mechanisms in Laser Processing using Digital Speckle Photography

摘要

When cutting or drilling advanced engineering ceramics - such as Al{sub}2O{sub}3, TiB{sub}2, SIALON:s, PCBN and cemented tungsten carbide - with an IR laser beam, the dominant interaction mechanism is thermal. This causes thermally induced strains around the focused laser spot and interaction zone. These strains often lead to increased residual stresses and the formation of cracks. By using the second and fourth harmonics of a diode-pumped and AOQ-switched Nd:YAG-laser with pulse duration of about 100 ns, the risk for cracking has been significantly reduced. This indicates a change from thermal to chemical dissociation or photo-ablation as the dominant interaction mechanism. By using digital, defocused speckle photography in situ and in real time, it has been possible to visualize the strain field around the focused spot of the processing beam, in the form of "movie sequences". The equipment for the speckle photography consisted of a c.w. HeNe laser for illumination and a CCD camera with band pass filter for 633 nm. Digital speckle photography is a highly sensitive method for visualizing surface strains. In addition to investigation of topological changes by speckle photography, the cutting and drilling processes with different wavelengths are also being investigated.