density, ED, which is the amount of energy per unit area transmitted by the laser to the surface [150][151] and is defined as:
ED= P
vφs (3.2)
In general, as one would expect, the depth of the grooves produced by the laser beam increases with ED, because a higher amount of emitted energy is able to melt a higher amount of material on the surface.
3.2 Surface assessment
3.2.1 Surface topology observation
To conduce an in-depth investigation over the changes induced by the laser treatment on the surface topography, using traditional optical microscope is not always sufficient, because of the poor depth of field which is typical of this tool. It is far preferable to employ the Scanning Electron Microscopy (SEM) technique, which exploits the interaction between the sampled surface and a focused beam of electrons impacting on it to record the different electrical signals emitted by every impacted point of the surface. In this way, it composes an high-resolution image which succeeds in focusing details which do not lie on the image plane and which would appear blurred if using the traditional optical microscope. The use of SEM allows to obtain high magnification images of the treated surfaces, allowing to visually compare them with two reference cases (as produced and grit-blasted).
SEM images provide an immediate snapshot of the enhancement in roughness which occurs passing from the as-produced to the grit-blasted and finally to the laser-treated samples.
Sometimes it is useful to submit fracture surfaces to the SEM observation after the breaking of the joint. This can be an aid in identifying the mechanism (adhesive or cohesive) and the locus of failure, even if the amount of charge accumulated on non-conductive surfaces, like the one of the adhesive on which cohesive failure occurred, results in scanning faults, which is why it is always recommendable to coat non-conductive samples with an ultra-thin alloy of electrically conducting material.
In this work, SEM analysis are carried out using a Field-Emission SUPRA40 Zeiss SEM equipped with a GEMINI FESEM detection column and an Oxford Instrument EDX micro-analysis setup to analyse the laser treated surfaces before the production of the bonded joints to evaluate the differences brought by the ablation process with respect to the two aforemen-tioned case and, within the laser ablated samples, by using different process conditions and parameters. The EDX microanalysis instrument embedded into the SEM equipment is also
used to get information about the chemical modifications induced by the treatment. Finally, a 50x optical microscopy is used to observe the surfaces of the joints after the breakage.
3.2.2 Surface morphology evaluation
The simple observation of the treated surfaces allows to get an idea of the surface topography, but, in order to assess the surface morphology not only from a qualitative point of view but in a way that allows to put the study cases in order with respect to it, it is always desirable to flank the SEM analysis with the determination of numerical indexes able to summarize the main characteristics of the surface morphology.
The first and primary parameter which aids in quantitatively characterizing a surface is the roughness. In this work, the roughness of the surface is measured according to ISO 25178-2 [130], in which areal methods to assess the surface texture are presented. In particular, in this work the average surface roughness, Sa, is preferred to the classic linear profile roughness, Ra, because of the strongly anisotropic nature of the surface texture which would make the classic parameter unrepresentative of the state of the surface. The surface roughness can be evaluated as:
Sa= 1 A
Z Z
|z(x, y)|dxdy (3.3)
In Eq. 3.3 A is the area whose roughness is to be calculated, z is the coordinate in the normal-to-the-surface direction while the x-y plane is the one on which the surface lies. Fig.
3.2 illustrates the definition of surface roughness.
Fig. 3.2 A graphic illustration of the definition of Sa
3.2 Surface assessment 83
The main disadvantage in using such a parameter to summarize the surface morphology is that it can provide information only about the deviation of the profile with respect to the medium plane, but it tells nothing about the distribution of peaks and valleys or the homogeneity of the texture. For this reason, another surface parameter is taken into account and supported the surface roughness in comprehensively describing the characteristics of the morphology, namely the Pearson’s first coefficient of skewness Ssk. According to [130], the surface skewness is calculated as:
Ssk = 1 S3qA
Z Z
z3(x, y)dxdy (3.4)
In Eq. 3.4 Sqis the root mean square height of the surface, defined in Eq. 3.5.
Sq= r1
A Z Z
z2(x, y)dxdy (3.5)
The surface skewness helps in understanding how the peaks and valleys are distributed over the surface with respect to the mean plane of the surface itself, resulting in height of the peaks higher than the depth of the valleys in case of positive values of Ssk and in the opposite situation in case of negative values of the index, as shown in Fig. 3.3.
Fig. 3.3 A graphic illustration of the definition of Ssk
To assess the surface properties from a quantitative point of view, many techniques are available, including scanning probe or scanning tunnelling microscopy. In this work, the employed method to obtain information about the deviation of the profile and the distribution of peaks and valleys occurring naturally or induced by the laser treatment with respect to the mean plane of a processed surface is the laser profilometry. It consists in scanning the surface with a CCI Taylor-Hobson 3D optical profilometer with a resolution of 340 nm on
the longitudinal x-y plane and 1 nm on the vertical z-axis. This approach is by far more advantageous with respect to a classic contact profilometer because the tip of the diamond stylus does not succeed in penetrating in surface structures shaped by very narrow and deep grooves. On the other hand, if using a non-contact profilometer attention must be paid to the cleaning of the surfaces, because the presence of contaminants can alter and disturb the measure.
3.2.3 Wettability measurements
Some measurements of the modification of the static water contact angel are performed by means of a Dataphysics OCA20 (Optical Contact Angle) instrument. The sessile drop test is employed and the drop volume is set to 1 ml. The test is performed at room temperature. Fig.
3.4 provides an example of the method used for evaluating the contact angle from the tangent curve to the drop profile in the contact point between sessile drop and substrate surface.
Fig. 3.4 Example of determination of tangent curve to the drop profile starting from which the contact angle is assessed [152]