Artefacts from slicing
Observation of the block face using the BSE signal is sensitive to the microscale surface roughness since this changes the electron scattering angle. The surface roughness caused by ultramicrotoming the AA2024 aluminium test-piece has been assessed by atomic force microscopy (AFM) and Kelvin probe microscopy (KPM) in Fig. 3. The surface roughness recorded by AFM is around 1.2nm over the green region in Fig. 3(a), which does not contain any gross knife marks. The Volta potential in the corresponding KPM map confirms that the roughness is mainly associated with the intermetallics. It is noteworthy that other commonly used surface preparation techniques, such as electropolishing or mechanical polishing, typically result in much greater average roughness values (about 100nm and 230–280nm respectively). The knife marks are caused by smearing of the block face by the lift out of hard intermetallics that adhere to the knife edge. In this case the hard, mainly copper- and magnesium-containing intermetallics are randomly distributed in the matrix and, therefore, the associated knife marks cannot be avoided. However, as shown in Fig. 4, image processing can effectively remove these linear features.
Other artefacts typical of slicing by ultramicrotomy, namely chattering and cc-5013 of chip materials, are often observed. Chattering is evident from the unevenness of the slice, which is caused by the mechanical instability of the knife and the specimen during slicing. Slow speed cutting can reduce chatter. The compression of the chip material arises from the strong force applied between the cut surface and the chip. The compression rate (cutting speed) influences the chattering of the block face. The associated surface texture is clearly evident in each grain in Fig. 4. Suppressing such chatter is the biggest challenge when slicing metallic samples and this is discussed below.
Principles of cutting
Merchant [43,44] developed an orthogonal cutting model that assumes that the knife is sharp and no rubbing occurs between the knife and the specimen. A sharp knife is defined as one where the slice thickness is more than 10 times the radius of the knife-edge. The radius of the diamond knife with an edge of 45 degrees is measured to be approximately 3nm . Using the Merchant cutting model, the force, and angle and velocity relationships can be estimated for slice thicknesses greater than 30nm, as displayed in Figs. 5 and 6 respectively.
The resultant force, R, can be resolved into two force components, namely the cutting force along the cutting direction, Fc, and that perpendicular to the cutting direction, Ft. These forces are expressed as follows:where φ is the shear angle, α is the tool rake angle (41°, with a 4° clearance angle (the angle between the back face of the knife and the block face)) giving a 45° knife angle, β is the angle between the resultant force and the normal to the rake face, τm is the shear strength of the material, T is the thickness of the slice removed and b is the width of the cut tool (diamond knife). The resultant force, R, can also be resolved into a friction force, F, and a normal force, N, perpendicular to it or equivalently with respect to the shear plane into a shear force, Fs, and a force normal to the shear force, Fn.
The shear angle, φ, is an important variable in metal cutting analysis because it defines the deformation characteristics of the chips. The relationship between the angles is then given by the following equation:
The compression rate, c, can be determined from the length of cut or chip thickness, as follows:where l is the length of the cut, lchip is the length of the chip, T is the depth of the removed slice, Ts is the chip thickness and c is the compression rate. It is difficult to evaluate the compression rate from the slice length or thickness because it is not possible to collect the individual chips (representing the removed slices) from within the SEM. Therefore, the compression rate was determined from the shear angle and chip thickness measurements obtained by stopping the cutting process for different slicing thicknesses and examining the attached chips in the SEM (Fig. 7). Once the shear angle has been determined, all the forces may then be estimated enabling understanding of how damage is introduced into the block face. It is noteworthy that for slice thicknesses ranging from 50nm to 10µm no significant change in the shear angle was measured (Fig. 7). This confirms that the cutting forces have a linear relationship with the thickness for slices thicker than 50nm.
Artefacts from slicing