Chapter 2
Laser ablation surface pre-treatment
The effectiveness of the chemical pre-treatments in making the substrates surfaces suitable for the bonding, by increasing their surface free energy or by altering the surface morphology in such a way to promote the interlocking effect, has been discussed in Chapter 1. In many works [107] [108] [109], however, the high environmental hazards and the risks for the operator innate in the chemical pre-treatments are underlined. Some strategies were developed in order to reduce the dangerous materials or phases of the work, as attempted for instance by Critchlow et al [110] with respect to the topic of the replacement of chromic acid anodizing, but the fact that often these alternatives are not always cost-effective must not be underestimated. On the other hand, the grit blasting is rather easy to be performed and brings some enhancement in the surface morphology, although it does not reach to achieve the performance levels provided by the most of the chemical and electro-chemical treatments for adhesively bonding purposes.
A valid alternative to the grit blasting which has been more and more studied in the last years is the laser ablation. It presents many benefits related to the flexibility of the process and the repeatability of the results. Moreover, it assures the possibility to circumvent the poor control over the resulting surface morphology obtainable by means of the traditional chemical and mechanical pre-treatments [111] [112].
In this Chapter the main characteristics of the laser ablation process will be touched, as well as the main documented goals which is possible to achieve with respect to the adhesion quality and to the bonded joint strength will be discussed.
energy (called pump) and a series of mirrors usually referred to as the optical resonator or, erroneously, optical cavity. According to [113], a laser is definable as a coherent, convergent and monochromatic beam of electromagnetic radiation with wavelength ranging from ultraviolet to infrared.
The classical theory dealing with electromagnetic radiation considers it as a wave propa-gating in a perpendicular direction to the ones associated to the oscillations of the electric and the magnetic fields, respectively. For a plane polarized light, the oscillation of the electric field is not randomly directed but is is strictly confined to a specific plane. The so-called intensity of radiation is the energy per unit area perpendicular to the propagation direction.
By dividing the frequency by the energy, the characteristic wavelength of the radiation is available, whose value is the discriminative element which divides the electromagnetic spectrum into several ranges, each one corresponding to a different type of radiation (e.g.
radio waves, micro-waves, infrared radiations, visible light, ultraviolet radiations, x-rays, gamma-rays).
To better explain the occurrence of some events, an alternative approach called quantum theory was developed, according to which the electromagnetic radiation is considered as composed by particles referred to as photons, each one possessing a specific amount of energy.
The three main mechanisms through which a laser works are the population inversion, the stimulated emission and the amplification. The first one, called pumping, is the process which leads the electrons from lower to higher states of energy, therefore perturbing the thermal equilibrium of a material by "inverting the population" in terms of typical distribution of electrons. The main mechanisms involved in the population inversion for a typical lasing system with three and four levels of energy, respectively, are shown in Fig. 2.1.
Fig. 2.1 Schematic representation of the population inversion phenomenon in a laser system employing three (a) and four (b) levels of energy,respectively [113]
2.1 Basic concepts about laser ablation 45
Basically, the adsorption of an electromagnetic radiation from an external source triggers the pumping phenomenon, leading the electrons from the low state of energy E0to an higher lever (E2in the three levels case, E3in the four levels case), from which they undergo a rapid radiationless decay to a lower level of energy (E1in the three levels case, E2in the four levels case). The subsequent decay towards a further lower state of energy (the original E0in the three levels case, the level E1 in the four levels case) is accompanied by the emission of a laser radiation, provided that the pumping intensity is beyond the so-called laser threshold. In solid-state lasers (e.g.: Nd:YAG) the population inversion is performed by means of gas-filled flashlamps or by using diode-lasers as pumping source. In the gas lasers, indeed, the pumping occurs electrically.
The stimulated emission is the phenomenon associated to the interaction of a photon and the excited atom of the laser medium whose electrons, submitted to population inversion, are going to decay from a high to a low state of energy, as it is exemplified by Fig. 2.2.
Fig. 2.2 Schematic representation of the stimulated emission [113]
When the stimulated photons are in the same phase and state of polarization, they are able to increase the amplitude of an incoming photon, resulting in an amplification of the light. This is performed in the so-called optical cavity, at whose ends a set of mirrors is adequately placed. The laser medium is positioned between the mirrors and typically one mirror is completely reflecting (in order to boost the amplification) while the other one is partially transmitter. In Fig. 2.3 an amplification phenomenon from the reflective mirror (on the left) to the partially transmitter one (on the right) is shown.
Fig. 2.3 Schematic representation of the amplification phenomenon [113]
The main properties of the laser beams are listed and defined below.
• Monochromaticity: property related to the narrow bandwidth of the lasing light.
• Collimation: property depending on the capability of the beam to focus on small areas even at high distance.
• Coherence: the degree of orderliness of waves, which measures the correlation between the light waves in two different points at two different times.
• Radiance or brightness: the amount of power emitted per unit area per unit solid angle by the laser beam.
• Focal spot size - the distance between the beam axis and a point at which the intensity drops to 1/e2from its value at the center of the beam (as defined in [114]), on which the property called irradiance of the beam strictly depends.
• Transverse modes: specific spatial profiles apparent from the observation of a cross sec-tion of the laser beam and represented as electromagnetic modes. Fig. 2.4 graphically depicts the concept.
Fig. 2.4 Examples of cylindrical (panel a, where the two subscripts point out the number of dark rings and the number of dark bars across the pattern, respectively) and rectangular (panel b, where the two subscripts point out the number of dark bars in the x- and y-directions, respectively) transverse mode patterns [115]
The relation ratio between the beam diameter ds and its value associated to the fun-damental mode d0 is expressed by a factor M, and the same relation is valid for the ratio between the multimode divergence φsand the fundamental mode parameter φ0.