A METHOD FOR SHARPENING FIM-SPECIMENS

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A METHOD FOR SHARPENING FIM-SPECIMENS

U. Rolander

To cite this version:

U. Rolander. A METHOD FOR SHARPENING FIM-SPECIMENS. Journal de Physique Colloques,

1986, 47 (C7), pp.C7-449-C7-452. �10.1051/jphyscol:1986775�. �jpa-00225970�

JOURNAL DE PHYSIQUE

Colloque C7, suppl6ment au n o 11, Tome 47, Novembre 1986

A METHOD FOR SHARPENING FIM-SPECIMENS

U. ROLANDER

Department o f Physics, Chalmers U n i v e r s i t y of Technology, 5-412 96 Gbteborg, Sweden

Abstract

A method is presented which makes it possible both to sharpen also extremely blunt specimens and to perform controlled back polishing without making the specimens blunter.The method seems to have quite general applications. It has been applied to such different materials as Tic-Ni-based and WC-Co-based cemented carbides, low alloyed and high alloyed steels, low alloyed zirconium and pure titanium.

1. Introduction

One major weakness of the atom probe, when used in metallurgical reasearch, is that a very limited volume of the sample is accessible for analysis. The need to place the volume of interest at the very vertex of the specimen tip combined with the requirement of a tip radius not exceeding -50 nm makes sample preparation extremely tedious. More time is often spent on sample preparation than on analysis and evaluation of spectra, even if the preparation process is well established.

One way of making a selected volume accessible for analysis is by controlled back polishing, i.e. by electropolishing the sample using pulses of typically 0.5 ms duration and examining the specimen in an electron microscope after each puls. One puls removes typically 50 nm of the tip so with the aid of the TEM it is possible to establish the exact position of the tip vertex, in the original material.

Unfortunately the specimen usually becomes more and more blunt with each puls. This can cause problems if the features of interest in the material are scarce.

In this paper a method is presented which makes it possible both to back polish specimens without risking that they become blunt, and to sharpen even extremely blunt specimens, e.g. those which have failed in the atom probe. It is based on a method which was developed for performing controlled back polishing on specimens made of WC-Co-based cemented carbides (ref.l), but has been applied on such different materials as WC-Co-based and Tic-Ni-based cemented carbides, low-alloyed and high-alloyed steels, low-alloyed zirconium and pure titanium.

2. Description of the method

The experimental setup is shown in Fig.1. The specimen is electropolished in 5% sulphuric acid in methanol, cooled to -20°C (the same electrolyte is used for all materials). 40 volts DC is applied in pulses of 0.5 to lOms duration depending on the initial quality of the specimen. The method seems to be identical with ordinary back polishing but has led to some rather surprising results:

The specimen becomes sharper with each puls until a final minimum tip radius is obtained. This minimum radius stays constant during further polishing.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986775

JOURNAL DE PHYSIQUE

Figure 1. Experimental setup used for back polishing FIM-specimens.

Many short pulses lead to a smaller radius than one long. This indicates that the initial transient part of the process is of vital importance and thus the ordinary theory for electropolishing is not applicable.

If the applied voltage is decreased the radius will increase wilh each puls in the same fashion as during ordinary back polishing.

The minimum tip radius is strongly dependent on the tip angle. For tip angles less than 25O, tip radii less than 20nm can be obtained.

3. Applications of the method 3.1 Sharpening a fractured specimen

Fig.2 demonstrates the sharpening of a Tic-Ni-based cemented carbide specimen (a) which has failed in the atom probe. First 20 pulses of lOms duration is applied (b), then 6 pulses of lms duration (c) and finally one 0.5ms puls (d). The specimen is now ready for back polishing using 0.5ms pulses. At this stage the voltage may be lowered in order to reduce the polishing speed. This, however, will eventually lead to a blunt specimen.

3.2 Sharpening a blunt (back polished) specimen

Fig.3 shows an electropolishing sequence for stainless steel. A blunt specimen (a) is polished in steps of four lms pulses (b, c and d).

3.3 Controlled back polishing

Fig.4 demonstrates how a TiClTiC-grain boundary is made accessible for analysis using a 0.5ms puls and 10 volts.

4. Discussion

The method described above has so far been a~vlied on the followine materials:

WC-Co-based cemented carbides Tic-Ni-based cemented carbides low-alloyed steel

high-alloyed steel low-alloyed zirconium pureTitanium

For all these materials satisfactory results have been achieved. However, small variations in

behaviour have been noticed between the different materials. The polishing conditions reported above

are optimized for cemented carbide specimens but no attempt has been made to find the optimal

conditions for the other materials. It is thus highly probable that the method can be improved further for these materials.

The method has so far not been tried on any other materials than those listed above so very little can be said about its limits, but it seems likely that several more applications can be found.

5. Conclusions

Blunt specimens made of cemented carbides, steel, titanium or zirconium can be sharpened by back polishing if the polishing conditions described above are used.

Sharp specimens can be back polished in a controlled way with the same method.

The minimum tip radius which can be obtained depends strongly on the length of the applied pulses, on the tip angle of the specimen and on the applied voltage.

The limits of the method in terms of for which materials it can be used have not been investigated.

Although the method gives satisfactory results for all the materials it has been tried on, it can probably be further improved in the case of steel, zirconium and titanium.

Acknowledgements

This work has been financially supported by the National Swedish Board for Technical Development (STU) and AB Sandvik Hard Materials.

Reference

1. M Hellsing, A. Henjered, H. NordCn and H.O. Andren, "Science of Hard Materials", eds. R.K. Viswanadham, D.J. Rowcliffe and J. Gurland,

Plenum Press, New York (1983) p. 931

Figure 2. Electron micrographs showing different stages of an electropolishing sequence.

A Tic-Ni-based cemented carbide specimen (a) which has failed in the atom probe

is sharpened by first applying 20 pulses of lOms duration (b), then 6 pulses of

lms duration (c), and finally one 0.5ms puls (d).

C7-452 JOURNAL DE PHYSIQUE

Figwe 3. Electron micrographs showing an electropolishing sequence for high alloyed steel.

Between each micrograph four lms pulses of 40volts have been applied.

Figure4. The TiCfliC grain boundary in (a) is made accessible for analysis by

back polishing the specimen using a 0.5ms puls and 10 volts (b).