2. Interaction of Lasers with Material Extensive studies on the interaction of lasers with a variety of materials have been made over the past 10 to 15 years [9, 10, 11, 12]. A brief review and salient highlights of this interaction is worth recapitulating. • When a laser beam falls on a surface, it interacts with the electrons of the material. These absorb light quanta and are raised to higher energy states. The electrons transform this increased energy into lattice vibrations and heat typically within periods of 10“12 sec. This means that for most practical purposes lasers can be treated as impulsive heat sources at the surface of the object [11]. • Depending on the surface factors (absorptance, reflectance), material properties such as thermal conductivity, density and laser characteristics such as fluence and pulse length, the laser radiation is absorbed in a relatively thin surface layer. The thickness of this layer can be calculated approximately or exactly as a function of time [8, 12, 13, 14, 15] given appropriate boundary conditions. • Neglecting convection, emission of thermal radiation from the heated surface and radially directed heat conduction, the problem is transformed into a longitudinal heat conducting problem where heat is transferred longitudinally (in the direction of the laser beam) from the relatively thin absorbed layer to the rest of the material [8, 12, 13, 14, 15]. • The laser irradiation results in a temperature distribution of the object which is a function of its thickness, the duration of irradiation, the specific heat of the object, the density of the object, the thermal conductivity and the absorptivity of the object. • Depending on the laser and material characteristics, simple heating, partial melting, partial vaporisation with melting and plasma formation with shock wave propagation are all important phenomena governing failure modes of interests [11]. Differential heating can also produce extreme local stresses which may lead to fracture. • Depending on the power flux of the laser beam and the pulse duration, the surface layer can melt resulting in a further alteration in the temperature profile across the thickness of the object [12]. • This molten layer moves inwards with continued irradiation till the material is completely molten. • The front part of the surface can vaporize while some part is molten. The vapour can interact with the laser. Continued irradiation will further remove material as vapour and a complete burn-through can occur [14]. • In certain situations due to the temperature inside the material being higher than the surface temperature, vapour may form inside and explode outward resulting in removal of the in-between material [10]. • During vaporization, at flux energy levels above a certain threshold, plasma is formed. This gets heated and enhances the coupling of the laser energy to the surface. Shock waves are generated from this interaction which propagate back towards the surface. The surface can break up because of this impulse impact [17, 18, 19, 20]. Fracture of ceramic surfaces have been observed during laser irradiation [21].
RkJQdWJsaXNoZXIy MTU5NjU0Mg==