Metallography

Influence of Specimen Preparation on SEM Images and Comparison to LOM Images

SEM and LOM are sometimes viewed as competitive methods; but, they are actually complementary. Some people believe that specimen preparation is less critical for SEM work than for LOM work; but, the reverse is true. Image forming mechanisms for the LOM and the SEM are different – we should make the most of these differences. The LOM is a more efficient tool for fast, low magnification (≤ 2000X) examination of a specimen – use it first to determine if, and where, one should follow with higher magnification SEM examination and EDS or EBSD work.

Prepare every specimen for SEM examination with the quality required for EBSD work; this is actually quite easy with the right equipment, consumables and methods. Examples of preparation methods for many metals/alloys and other materials can be found in Struers publications and web site. Examples of LOM and SEM images of specimens that were properly prepared or poorly prepared will be shown.

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Guidelines

• Low magnification evaluation; deeper etch
• Time is less reliable than appearance
• Best results occur when etching right after polishing, time can create a passive surface
• High magnification evaluation; shallow etch
• If you are not satisfied, an etch-repolish-etch sequence may improve the results

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Hexagonal-Close Packed Crystal Lattice

Each of the interior atoms has a slice in an adjoining cell. In turn, there are three slices within this cell. Thus the three whole atoms are contributed to the cell. The two atoms centered on the top and bottom faces are each half within the cell. These two halves combined contribute one whole atom. Each of the twelve corners has one-sixth of an atom. Thus all twelve corner atoms combined contribute two whole atoms.

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Towards A More Quantitative Measurement of the Deformation During Metallographic Specimen Preparation Using EBSD and FIB

By Philippe T. Pinard, Pierre Hovington, Marin Lagacé, George F. Vander Voort, Raynald Gauvin

• Materials and Mining Engineering Department, McGill University
• Institut de recherche d’Hydro-Québec, Varennes, Québec
• Consultant, Struers Inc., Westlake, Ohio, USA

Surface deformation during metallographic preparation have been previously studied using light optical microscopy (LOM) and transmission electron microscopy (TEM) [1]. With its submicron resolution, electron backscattered diffraction (EBSD) can provide quantitative deformation analysis at a smaller length scale than LOM while provide higher statistics than TEM. This work aims to determine the level of deformation produced during different metallographic preparation steps of common materials. As a first iteration, the deformation profile induced by 80, 240 and 600 ANSI grit SiC papers on commercially pure iron (BCC), copper (FCC) and titanium (HCP) was measured.

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New technologies have improved the speed and accuracy of specimen preparation for materials such as ceramics, cermets, nitrides, borides and sintered carbides.

Microscopic examination is an important inspection technique widely used in the research, quality control and failure analysis of very hard materials. To properly reveal the microstructure of these very hard materials, it is critical to use the proper equipment (usually automated), along with suitable high-quality consumable products, such as cutting blades, mounting compounds, abrasives, lubricants and working surfaces with proven preparation methods.

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Sectioning of Specimens

“Chop” style cuttershave variable pressure over the cut which degrades the microstructure. Minimum area of contact cutting (MACC) maintains pressure more constantly yielding less damage.

Chop vs. Orbital Cutting

With the chop cutter the contact area, when cutting a round bar,is very small initially, increases to a maximum, then decreases to a point contact. Thus, the pressure varies through the cut. This reduces cutting efficiencyand increases heat generation. Orbital cutting reduces this variation. When the wheel lifts out of the cut, coolant flows into the cut which reduces sectioning damage.

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The paper describes the approach taken in the author’s first metallographical study of plasma sprayed coating. After consideration of the preparation problems known to be associated with porous materials, the coating materials and the substrate material, and coatings in general, literature on metallographic studies of plasma sprayed coatings was identified, obtained and reviewed.

A coating section of each type was carefully removed from the substrate, refrigerated in liquid nitrogen, broken and examined with the SEM to reveal the nature and degree of porosity and bonding defects. Specimens were prepared using two different approaches and two variations of one of the approaches. Polishing was conducted using DP-Plan cloths and two different diamond sizes followed by two stages of diamond polishing with conventional cloths and finally using colloidal silica on a vibratory polisher. Vibratory polishing time must be critically controlled for two of the three coatings. Conventional etchants were tried to enhance microstructural detail but the Pepperhoff interference layer method was found to be more useful. By George Vander Voort