Triple Ion Beam Cutting of Diamond/Al Composites for Interface Characterization

June 21, 2014

The microstructure of new materials with heterogenic components is extremely difficult to study. As mechanical polishing often not leads to smooth surfaces, a novel ion beam cutting technique has proven to be very effective. In this article, the applicability of a novel triple ion beam (TIB) cutting technique for a clean metallographic preparation of the diamond/Al composites will be described.

With the advent of numerous electronic instruments, it has become imminent to develop novel materials to counter thermal expansion and to efficiently evacuate heat from electronic components for their optimal performance. Recently, a diamond particle reinforced Al matrix (i.e. diamond/Al) composite has been developed with which high thermal conductivity can be achieved. However, the heterogenic nature of the components makes it extremely difficult to study the microstructure. Hence there is imminent need for an innovative preparation method for such a heterogeneous material. In the following, the applicability of a novel triple ion beam (TIB) cutting technique for a clean metallographic preparation of the diamond/Al composites will be described.

Advantages of three incident ion beams

An aluminum powder was used as material for the matrix. Synthetic diamond particles with different average sizes of about 30 and 200 nm were employed to obtain two sets of diamond/Al composites. To highlight the efficiency of the TIB cutting, standard mechanical polishing was carried out on a tripod polisher using diamond-particle lapping polymer films. Alternatively, the new TIB technique was tested by using a Leica EM TIC 3X TIB slope cutter to achieve high quality surface finishing. Instead of one ion beam the Leica EM TIC 3X uses three incident ion beams which intersect at the center edge of the mask to form a milling sector of 100° (Figure 1). During milling the sample remains immobile contrary to the habitual oscillations in some other methods so that the heat transfer between sample and stage is much better compared to that of an oscillated sample. The attached binoculars allow real-time observation to control milling time until a flat surface containing diamond/Al interfaces is acquired.

Fig. 1: a) Schematic drawing of TIB ion milling; b) photograph of the standard sample holder of the Leica EM TIC 3X instrument used in this work. In a), 1: sample, 2: mask, 3: sample surface, 4: crossover point of ion beams, 5: area of interest, 6: direction of observation and J1, J2 and J3: ion beams.

Triple Ion Beam technique provides smooth surface

The large difference in hardness of the diamond and Al creates an un-even surface at which the final roughness remains around 5–15 nanometers despite repeated polishing. The main drawbacks of mechanical polishing are un-even surface because of the inherent material properties, surface contamination caused mainly by use of polymer sheets leading to possible wrong conclusions on EDX analysis and un-even surfaces render FIB preparation difficult for a preferred site selection. Therefore, such a surface is not even qualitatively suitable for metallographic investigations. Comparatively, the TIB prepared sample exhibits a quite smooth surface, even revealing grain contrast in the Al matrix (Figure 2).

 

Fig. 2: SEM secondary-electron images showing a) mechanically polished, b) TIB polished surfaces of the 30 micron diamond/Al composite and c) higher magnification image of the rectangular area in b); d) SEM/EDX spectrum of an interfacial particle, e) TIB polished surface of the 200 micron diamond/Al composite, inset: an overview of the polished (arrowed) and unpolished (double arrowed) surfaces. The surfaces in a) and b) were tilted 45° with respect to the usual horizontal position to reveal surface roughness.

Almost no artifacts during ion milling

Initially, it was suspected that the TIB method tested in the present study may introduce artifacts during ion milling. However, it was proven that diamond and Al can be milled equally due to the ion incidence being parallel to the prepared sample surface which leads to flat and polished surfaces almost free of any preparation artifact.

SEM/EDX analysis shows that the interfacial particles in the 30 nanometers diamond/Al sample are rich in Al, O and C, while the contents of O and C on the Al matrix adjacent to the interface are negligible. This presence can have a profound effect on the global thermal properties of the composite. As there is no alumina supporting elements in the present work, the Al2O3 particles most probably originate from the Al2O3 layer present on the original Al powder particles.

Conclusion

The novel TIB technique was adapted to acquire near-perfect and artifact-free surfaces for interface characterization. The as-prepared surfaces strongly reduce uncertainties in SEM imaging and spectroscopy and allow characterization to unambiguously reveal sub-micrometer Al2O3  particles and clean interfaces, which are present in the 30 and 200 diamond/Al composites, respectively. The obtained surfaces are also ideal for preferential selection by FIB for further TEM characterization.

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