Scanning electron microscopy (SEM) 72

Surface topography is one of the most important surface character-istics of metallic substrates and the usual manner of investigation is the use of scanning electron microscope to provide a high magni-fication image of the material under investigation.

Figure 3.9: Characteristic morphology of as produced stainless steel.

Optical microscopy is not really sufficient, not because it lacks the range of magnification of a SEM, although this is an important feature, but because of its poor depth of field and depth of focus.

In optical microscopy, features not in the image plane appear out of focus (i.e., blurred), whereas a SEM is able to accommodate very large depth of field.

The importance of surface topography is illustrated by the im-ages of Fig.3.9 and Fig.3.10 where are shown steel substrates as produced and grit blasted respectively and the enhanced rugosity that is provided by such simple mechanical treatment is clearly seen.

This effectively increase the degree of interfacial contact area between the adhesive and the substrate and, at a very simplistic level, may enhance the level of adhesion and durability so obtained.

Figure 3.10: Characteristic morphology of grit-blasted stainless steel.

It is sometimes convenient to use a SEM to examine failure sur-faces of joints. The micrograph of Fig.3.11 is the failure surface of an aluminum substrate bonded with a structural adhesive (Henkel Loctite Hysol 9466 epoxy adhesive). The definition of the locus of failure is a rather complex task and depends on the level of sophis-tication of the assessment methods available.

Some pre-treatments lead to characteristic morphologies on a very fine length scale which can be clearly defined only by high resolution SEM. The typical example of this kind of morphology is the acid anodized aluminum.

In the present work, using laser irradiation a fraction of the laser beam energy is absorbed by the material, thus promoting material ablation, i.e. removal by vaporization. In order to analyze surface morphological modifications made by the laser process, SEM analy-ses (Cambridge Stereoscan, 20 keV electron beam, current v 3.4µA, spot size v 1.4 mm²) were carried out on as-produced and surface pre-treated samples.

Figure 3.11: Failure surface of a grit-blasted aluminum surface bonded with a structural adhesive.

3.3.2 Stylus profilometry

In any assessment of surface roughness it is desirable to move away from the qualitative images of SEM to an approach which can pro-vide a quantitative assessment of surface roughness. This can be achieved by a variety of techniques including the scanning probe mi-croscopies of scanning tunnelling microscopy, but the most straight-forward method is the use of stylus profilometry. This is a standard

Figure 3.12: Definition of surface roughness.

metrology tool which is widely used in engineering to assess the sur-face profile (or roughness) of a machined component. The concept is simple in that a diamond stylus is dragged across the surface and records the short range undulations (roughness) and long range undulations (waviness) of the surface in a graphical manner, either from direct deflection of the stylus or by using an interferometric ap-proach. The interpretation of roughness data is considered at length in the relevant national standards and international standards, but the most important terms are roughness average (also known as cen-ter line average), Ra, and RMS (root mean squared) roughness, Rq. The term Rz to define maximum excursion of the profile from the hypothetical center line is sometimes used but is not very helpful.

The terms Ra and Rq can be defined, by reference of Fig.3.12 as follow:

Additional parameters are available to describe the bearing area, the autocorrelation coefficient as well as many others which are de-scribed in the standards. The main disadvantage of such an ap-proach is that although it gives quantitative information regarding the deviation from the center line profile, it tells us nothing about the distribution of heights, the length scale of the surface profile, or the variations as a function of distance along the length of the scan. For this reason, profilometry, when used for reason other than to check the profile of a machined component for metrology pur-poses, should always be combined with a microscopic technique to visualize the surface (e.g., SEM, AFM).

In this work of thesis, contact profilometry was undertaken us-ing a SM RT-150 stylus-based profilometer to have a quantitative assessment of surface morphology. Each specimen was acquired 12 times in order to obtain 12 samples for each substrate. Each re-sulting dataset, consisting of a height map sized 64x64 points with uniform spacing dx = dy = 12.5µm, was leveled by subtraction of the least-squares mean plane and used to evaluate the princi-pal 3D field parameters for surface finish assessment, as defined

Figure 3.13: Core Roughness depth Sk.

in ISO/FDIS 25178-2. In particular, the Sk field parameter (core height) was employed as a measure of the surface roughness. Sk

represents the core (or kernel) roughness of the surface over which a load may be distributed during most of the functional life of the surface. It is a measure of the nominal roughness and may be used to replace the mean roughness, Ra, and similar parameters. Sk, is a measure of the “core” roughness (peak-to-valley) of the surface with the predominant peaks and valleys removed, as shown in Fig-ure 3.13. [83] It is more robust than Ra especially when localized singularities (e.g. very high peaks or deep pits) are present on the surface.