Gatan, Inc. Application Note: PECS Slope Cutting of Cast Iron

Application Note: PECS Slope Cutting of Cast Iron

By Richard Mitro, Gatan Inc.

Cast iron with spheroidal graphite (SG) is made by adding trace amounts of magnesium to the same raw material as grey iron melt before casting. The graphite forms in a spherical shape instead of flakes. The challenges when preparing samples for analysis are retaining the graphite in its original shape and size and keeping the graphite nodules retained. A common preparation error in mechanical polishing is the insufficient removal of smeared matrix metal after grinding, which can obscure the true shape and size of graphite.

Gatan PECS with Slope cutting for preparation of cast iron

The Model 682.40000 SC-Tool in conjunction with the PECS offers a solution to mechanical polishing problems and was used here to produce a cross sectional cut across the sample surface while minimizing mechanical deformation or damage. Since analysis of a specific area was not required, an angled cut (45^o) was made to provide more surface area for observation of the bulk sample. Angled cuts also minimize re-deposition of materials on the cut surface. The cutting head assembly with the sample was rocked over an angle of 75^o to reduce/eliminate striations in the cut surface.

Preparation: A section was roughly cut (12mm x 6mm x 1.5mm Thick) from a bulk sample using a diamond saw with the top as cast surface retained even after slope cutting and etching. The sample was then mounted to the 45^o cutting stub without additional preparation steps.

Cutting parameters:

Ion gun voltage 6 keV, Ion beam current 300 uA, 75^o rock in right angle orientation, Time 5 hours This resulted in a cut surface measuring 30um – 70um in depth across the entire 12mm width of the sample. The cut surface revealed the presence of two distinct phases the dark spherical graphite phase and the lighter matrix as shown in Fig. 1.


	Fig. 1 SEM image showing the top surface, the cut surface and front sputtered region.

An important feature of the PECS and SC tool is that after cutting to the preferred depth, the sample may be ion beam polished and selectively etched. This enables decoration of the cut surface to reveal internal grain/phase structure and topographical differentiation. The quality of the graphite matrix clearly shows slope cutting preserves structures that conventional mechanical polishing can damage.

The difficulty in the metallographic preparation of cast irons is to retain the true shape and size of the graphite in its flake, nodular or tempered form. During grinding the matrix is smeared over the graphite and unless it is followed by a very thorough diamond polish, the graphite is not seen in its true form. Slope cutting and etching eliminate this problem as shown here. Etching of the sample, after cutting (Fig. 2) revealed the lighter matrix of pearlite microstructure and also the grain boundaries between the various bi-crystals of cementite and ferrite (Fig. 3).

Etching parameters:

Etching was done in two steps; first polishing was performed at a grazing angle to provide a flat surface and second perpendicular etching to enhance the pearlite microstructure.

Step 1: Ion gun voltage 6 keV, Ion beam current 300 uA, 80^o fixed angle, Time 30min

Step2: Ion gun voltage 6 keV, Ion beam current 300 uA, 0^o fixed angle, Time 8min


	Fig. 2 SEM image showing the top surface and cut face of the graphite node, the pearlite microstructure is also seen.


	Fig. 3 SEM image showing the grain boundaries between the various bi-crMMystals of Ferrite and Cementite.

Conclusions:

PECS slope cutting along with selective etching resulted in a clean cut-surface, 30-70 microns deep artifact free and undamaged graphite nodes. Etching resulted in the emergence of a pearlite microstructure revealing thin plates of cementite well differentiated from the ferrite matrix. The grain boundaries between the various bi-crystals of cementite and ferrite were made evident as well. Thus, slope cutting combined with selective etching successfully resulted in revealing the internal structure combining material contrast with topographical contrast.

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