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Preparation of Stainless Steel Surfaces for Scanning Probe Microscopy


Alexander D. Warren,1* Ana I. Martinez-Ubeda,1 Oliver D. Payton,1,2 Loren Picco,1 and


Tom B. Scott1 1 Interface Analysis Centre , HH Wills Laboratory , University of Bristol , Bristol , BS8 1LT , UK 2 Engineering Maths , Merchant Venture School of Engineering , University of Bristol , Bristol


* xander.warren@bristol.ac.uk


Abstract: A surface preparation route is presented that is designed to give high-quality fi nishes to austenitic stainless steels for analysis with advanced scanning probe microscopy techniques. The method details a series of polishing and cleaning steps suitable for novices and experts alike. The steps taken are justifi ed throughout and illustrated with examples of potential defects.


Introduction


The properties of metals and alloys depend on their microstructure, including grain size, number of phases, amounts of phases, and phase distributions within the sample at the micrometer scale. Microstructure is elucidated through metal- lographic examination. Typical preparation steps for specimens of this type include mounting in a thermosetting resin, polish- ing to a mirror-like finish, and in some cases etching with a weak acid (chosen to selectively dissolve grain boundaries or specifi c phases). Conventional techniques used for subsequent examination include light optical microscopy (LOM), scanning electron microscopy (SEM), and electron backscatter diff raction (EBSD). T e latest developments in metallographic analysis are focused at the nanometer scale and require precise sample preparation. Scanning probe microscopy (SPM) techniques such as atomic force microscopy (AFM) are well-suited for the analysis of metallographic features at sub-micrometer length scales including the following: precise determination of carbide morphology, topographic analysis of creep cavities and corrosion, and determination of phase distributions using stiff ness or magnetic data [ 1 ]. T e suite of SPM techniques are non-destructive, allowing the same area of interest to be examined later by other techniques. T is saves considerable time compared with destructive metallographic techniques and enables direct comparisons of the resulting data.


In the experience of the authors, there is a degree of confusion regarding the most eff ective way to prepare stainless steel specimens to achieve a fi nal fi nish suitable for SPM analysis. Although sample preparation notes for techniques such as EBSD imaging have been published [ 2 ], they oſt en fail to specify cleaning methods to remove the fi ne layers of organic material and stray particles (for example, colloidal silicon or diamond), which sequential polishing typically generates. Such preparation procedures are suffi cient for sample inspection with techniques where the information is coming mainly from a volume beneath the actual surface (for example, SEM, BSE, EBSD, etc.) because nano-scale surface contamination does not signifi cantly distort the signal, and the contamination is oſt en not observed. However AFM, like many SPM techniques, requires near-pristine specimen surfaces to maximize results, and conventional metallographic preparation may not be suitable. If the specimen or SPM probe tip becomes contaminated with an organic residue, such as that


52


leſt by some polishing lubricants, the recorded data could be distorted [ 3 – 5 ].


In this article we set out a methodology that has been found


to be eff ective in the preparation of AISI Type 300 series austenitic and SAF 2205 (duplex) stainless steels. T e preparation route described has been shown to give a reproducible high-quality surface polish, with the cleaning stages being effective at removing residues and particulates. T e principal diff erences compared with other sample preparation procedures [ 2 ] are the time spent on each grinding/polishing step, the number of fresh SiC papers used, and the rigor of the post-polishing sample cleaning. It is important not to skip any step because each is required to remove mechanical damage induced by the previous step. Although the technique minimizes preferential polishing on soſt er phases, this will still occur to some degree and lead to a higher variation in surface topography, particu- larly in fi ne-grained or heavily aged precipitation-hardening materials. T e latter is beyond the scope of this paper.


Materials and Methods Cut the sample to size using an automated diamond disc


cut-off saw (for example, a Struers Accutom - Struers, Rødovre, Denmark) with the sample fed at 0.05 mm/min to minimize damage to the cut surfaces. The sample is then set in resin (cold set, non-adhesive resins are preferred (for example, Struers Clarocit)) prior to polishing. T e use of a programmable automatic polisher (for example, Struers Tegrapol-15 polisher) with the sample holder rotating in the same direction as the


Figure 1 : Topographic AFM map showing a specimen with large scratches caused by a poor coarse polish. Later stages of polishing were more thorough, revealing the grain boundary precipitation in the center.


doi: 10.1017/S1551929516000341 www.microscopy-today.com • 2016 May


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