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Steel Surface Preparation for SPM


Figure 5 : Correctly polished stainless steel surface revealed in a composite contact mode HS-AFM topography map of a 300 μm × 67 μm area. Polishing followed the outlined sample preparation method. The map is 90 megapixels, with a pixel size of 10 nm, and took 4 minutes to collect. Lines of precipitates in the grains’ boundaries are shown with a height of about 10 nm above the average plane of the surface.


the sample from the resin puck. T is should be done now, as no further polishing is required. Depending on resin type, it may be possible to dissolve the resin, or it may need to be cut away. For thin samples mounted in non-adhesive resin, typically it is only necessary to carefully cut around the edge of the sample using a scalpel or razor blade. For thicker samples, it may be necessary to reduce the size of the resin mount using a hack-saw and vice, before removing remaining resin from the sample with a scalpel or fi ne-nose wire cutters, taking care not to touch the polished face. Where possible, avoid getting dust/ resin residue on the specimen face, and always use gloves and tweezers for specimen manipulation to reduce the potential for contamination. Rinse the specimen again with DI water followed by sonicating with acetone to remove any trace resin particles, then blow dry with N 2 . Figure 2 shows an eff ective polish, but salt particles have formed during drying because standard tap water was used and no blow-drying was employed. Figure 3 shows the streaking caused by movement of loose debris, such as resin dust or residual colloidal silicon particles, across a well-polished specimen surface.


Using a clean beaker, sonicate the specimen in acetone for 2 minutes. Thoroughly clean the beaker and specimen with sequential washes of DI water and detergent, ethanol, and acetone. Repeat the acetone sonication. As most resins are soluble in acetone, this two-step sonication is necessary to dissolve any resin residues adhering to the specimen. Next, clean the specimen and beaker using the same sequential washing process as before and repeat the sonication process with ethanol. Clean the specimen and beaker using the same sequential washing process as before and repeat the sonication process with isopropanol. The use of organic solvents is important to ensure the thorough removal of any oil-based residues leſt from diamond polishing ( Figure 4 ). Clean the specimen and the beaker and repeat the sonication with DI water. Wash the sample thoroughly with isopropanol and DI water, followed by blow-drying with N 2 to prevent the formation of drying marks.


Storing and mounting for AFM . Store the specimen in a dust-free environment to ensure no contamination prior to analysis. Sealable plastic bags should not be used, as it is possible for some of the plastic to transfer onto the sample and aff ect the quality of the surface. When mounting the sample onto the SPM,


2016 May • www.microscopy-today.com


do not use cyanocrylate-based glues (superglues) because the vapor given off can settle on the sample. Figure 5 shows a composite contact mode high-speed AFM [ 6 ] image of a large area of steel that has been correctly prepared using the method described, revealing no surface debris or scratches but exhibiting signifi cant grain boundary precipitation inherent in the sample. A contact mode high-speed AFM [ 7 ] was used to assess areas of samples far larger than can be imaged using a traditional AFM. T is is achieved by stitching together the video rate images collected using the high-speed AFM as the microscope moves across the surface. T e resulting image has the same lateral resolution as a conventional AFM but is capable of imaging over millimeter-sized areas.


Conclusion


SPM images are especially sensitive to artifacts arising from surface contamination. T e method described in this article has been used to prepare a range of steels and has been shown to work consistently over large sample areas. High-speed AFM (HS-AFM) data show that with correct sample preparation, steel samples can be prepared that are free from scratches, debris, and organic and salt contamination over areas millimeters in size whilst leaving material features such as grain boundaries visible.


Acknowledgements


Author ADW wishes to acknowledge the Engineering and Physical Sciences Research Council PROMINENT consortium [EP/I003282/1] for funding; AMU was supported by EDF Energy; and authors OP and LP were both funded by the Royal Academy of Engineering. T e authors would also like to thank EDF Energy and Outukompu steel for provision of the sample materials and the University of Bristol’s NSQI Centre for hosting the high-speed AFM.


References [1] AD Warren et al ., Ultramicroscopy 148, ( 2015 ) 1 – 9 . [2] MM Nowell et al ., Microsc Microanal 11 ( S02 ) ( 2005 ) 504 – 05 . [3] U Hubner et al ., Appl Phys A-Mater 76 ( 2003 ) 913 – 17 . [4] Y-S Lo et al ., Langmuir 15 ( 1999 ) 6522 – 26 . [5] J Vesenka et al ., Rev Sci Instrum 65 ( 1994 ) 2249 . [6] P Klapetek et al ., Nanotechnology 26 ( 2015 ) 065501 – 10 . [7] O Payton et al ., Nanotechnology 23 ( 2012 ) 205704 – 10 .


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