Triple-beam Ar-Ion-Milling with a Rotary Stage to Decorate Grain Boundaries and Substructures in Rock Salt

March 03, 2016

Decoration of grain boundaries in polycrystalline rocks has a long tradition in Structural Geology as in a monomineralic rock the recrystallized grain size is a good indicator for the paleostress conditions. Understanding the mechanical properties of rock salt and its deformation behavior is of major importance for the prediction of long-term stability of nuclear waste repositories, and our understanding of the dynamics of salt-related sedimentary basins which host the majority of oil and gas accumulations on Earth.

Common methods to expose grain boundaries and substructures in rock salt are chemical etching procedures using brine or water or the gamma irradiation method [1]. Here we present an alternative method to decorate microstructures in rock salt: Ar-ion polishing and contrast enhancement using the Leica EM TIC 3X with a rotating sample stage.

Sample

Sample material was synthetic rock salt produced by cold-pressing at room temperature in an extrusion machine (from 0 to 4.5 GPa in 15 s) from a salt mush made of pure NaCl-powder (NaCl content >99.9 %, grain size <10 μm) and NaCl-saturated solution. The resulting cylinder with a diameter of 2 cm was cut into 4 mm thick cylindrical slabs using a diamond blade rock saw. These samples were stored at room temperature and atmospheric pressure in an air tight vessel for more than 10 years.

Methods

In the first step the sample slab’s surface was carefully ground and polished with P2000 and P4000 SiC grinding paper and subsequently imaged in the SEM (Figure 1a). Then the surface was swabbed with H2O and after approx. 2 sec it was quickly dried with a hot air blower and also imaged in the SEM (Figure 1b). The same surface was then re-grinded using the same protocol of the first step followed by Ar-ion polishing using the Leica EM TIC 3X with a rotary stage at low angle with respect to the sample surface and 6 kV acceleration voltage (Figures 1c, 2a1 and a2). The sample movement is a superposition of sample rotation and sample translation to achieve a large homogeneously polished area. Finally, the surface was ion milled at higher angle and 3 kV acceleration voltage (Figures 2b1 and b2) to decorate the grain boundaries.

Results

Surface grinding using P4000 SiC grinding paper produced a rough but regular surface with pronounced grooves in grinding direction (Figure 1a). At high resolution crystals were observed on top of the grooves, presumably resulting from precipitation and recrystallization on the sample’s surface caused by room humidity conditions (approx. 40 %). Water etching entirely removed the surface grooves (Figure 1b). Moreover, it produced a smooth surface and exposed structures like pores and grain boundaries that could not be recognized after exclusively grinding. Investigating the sample at high resolution it could be speculated that even subgrain boundaries and different crystal lattice orientations could be distinguished (Figure 1b2). Proving this, however, would require EBSD analysis.

Disadvantage of surface etching using pure H2O was that the process was difficult to control. This was visible in pronounced dissolution of e.g., pore edges. Even using the far more sophisticated etching method, established by [1], with slightly undersaturated NaCl solution under controlled conditions, would be highly influenced by the presence of pores and the defect structure of the grains, and hence needs to be adjusted to the characteristics of each single type of salt sample.

Ar-ion polishing using the Leica EM TIC 3X and the rotary stage at low angle towards the triple beam produced a smooth surface (Figure 1c and Figures 2a1, a2) of the entire surface area (∅ 2 cm). SEM imaging allowed determination of the grain structure based on the density contrast caused by different mineral orientations. The grain boundaries are exposed as thin grooves and are only visible at very high magnification (Figure 2b1). The pores showed sharp etches and some of them were filled with small salt grains, which were presumably grinding dust.

Grain boundary decoration was done by means of etching using the rotary stage at high angle with respect to the orientation of the triple beam (Figures 2b1 and b2). This treatment caused the exposure of grain boundaries as grooves that were visible also at low magnification in SEM. The bulk grains showed a distinct surface pattern that varies from grain to grain and could be interpreted to be dependent on the crystal lattice orientation. However, this assumption could only be validated employing EBSD analysis.

Hand-polished

a1
a2

Water-etched

b1
b2

Ar-Ion polished

c1
c2

Fig. 1: Secondary Electron Micrographs of a stepwise a) hand-grinded, b) water-etched and c) hand-grinded and subsequently Ar-ion polished polycrystalline sample of rock salt (left column: low resolution; right column: high resolution).

a1
b1
a2
b2

Fig. 2: Secondary Electron Micrographs a stepwise (a) low angle Ar-ion polished rocks salt and subsequently (b) high-angle Ar-ion etched surface of polycrystalline rock salt.

Outlook

Grain boundary decoration using the Leica EM TIC 3X is an efficient and promising method to expose grain boundaries and substructures in polycrystalline rock salt. As the irradiation process is a mechanical process it could have advantages compared to the traditional chemical etching in terms of reproducibility and transferability to other samples and materials. The method will be evaluated and validated using complementary methods such as EBSD and/or the gamma irradiation method [1]. The applicability of the method to other monomineralic rocks such as quartzite is likely and worth to explore as these kinds of rocks are far more difficult to prepare with chemical etching.

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