You can glue the microspheres on a support using an epoxy adhesive, and test a number of them by nanoindentation. If you don't have access to this instrument, an AFM after calibration should lead to the same results. Doing this, you'll be able to measure the modulus of the spheres. If you want to have the properties of the shell in the perpendicular (tangent) direction, you can do the same after embedding the spheres in an epoxy matrix and polishing the whole. Doing this, you'll have access to the modulus along the thickness of the shells.

Alain

I thought of nanoindentation, too, however, would have to check whether I have access to the equipment. Besides, what concerned me was the question of whether the fact that the shell is very thin and there's a cavity behind it of course would not cause problems. Can such effects easily be compensated? I have no personal experience with nanoindentation really.

CSM equipments (Switzerland) is a supplier of nanoindenters and can probably test your samples for free. If the properties through-thickness are doubtful, then my suggestion of embedding the spheres in an epoxy resin and polishing after hardening is the best option. A nanoindenter is enough accurate to measure the properties inside the thickness of the shell. Moreover, you can select the zone of interest.

If you don't have access to nanoindentation, you'll find a lot of AFM suppliers eager to sell their instruments and ready to do free tests for you.

Alain

Dear Alain,

thanks again or your reply. AFM I have at hand at Fraunhofer IFAM here in Bremen and probably elsewhere at the University, I'll definitely check whether they have calibrated their equipment in the required way. And yes, of course, embedding and polishing would be a good alternative for testing if doubts in terms of testing in radial direction should prevail.

Thanks again, and best regards -

Dirk

I think nanoindentation with special indenter may be used to test it, through large amount of tests and statistical analysis. Nanoindentation is normally used for hardness and modulus measurement, I am not sure whether it can be used to test strength with normal indenter. Besides, if the wall material is homogeneous, and the wall thickness is known, the strength can be computed by numerical methods.

Dear Dr. Ma,

numerically computed would mean based on the supplier specification of sustainable hydrostatic pressure? Problem I have with that is that they only give the statistical information that "90% of spheres survive", and no further information about the distribution - thus I am really looking for suitable experimental techniques. In terms of the hardness and modulus results to be got from nanoindentation, it would indeed be interesting to know to what level of accuracy hardness results would allow a statement on strength. Or do you actually refer to numerical methods in terms of using them for deriving strength data from nanoindentation measurements?

Best regards,

Dirk

If you use AFM, then it is of highest importance that the tip is calibrated with a material having mechanical properties as similar as possible to those you expect for your microspheres. In other words, do the calibration with glass or silica, not with a polymer for example. I have seen huge errors when the calibration was done with a non-suitable reference, in the case of the properties of the material to be tested were totally unknown. But once the correct reference was used, the results were close to those given by a nanoindenter and the errors were within the experimental scattering.

Alain

Standard indentation techniques will not provide accurate strength data, you will not directly measure it and try to correlate a hardness value to a strength value. You can try a micro compression tester which is basically an indentation system with a flat tip. This brings a loading on the sphere that allows to compute a more or less straightforward solution for the load at the failure location and therefore a relatively accurate strength value.

I do not have experience in mechanical testing of hollow spheres. However, I found the attached paper interesting in response to your question. The authors utilized the nano-indentation set-up, described above by others, and made it appropriate to test the microballoon. I hope this would help.

Dear Mehdi,

that is indeed interesting, thanks! I was going to ask whether anyone might know of papers that have already addressed the topic.

Kind regards,

Dirk

Dear Dirk,

I have seen this paper last year, reviewing the different types of hollow and porous spheres, as I wanted to find a new filler material for developing my own syntactic foam. Actually, I am working on expanded perlite and another potential filler material now and it is my interest to test the mechanical properties of the particles as well.

All the best,

Mehdi

This paper along with its reference may also be helpful.

AFM nanoindentation with a tipless cantilever.

Please ask Dr. Manfred Fries, IKTS Dresden (Tel. 0351 2553810). As far as I know, they have a measurement equipment for single granule strength (similar to a device I developed many years ago for my doctoral thesis). Please refer to my recommendation.

Regards - Ralph Lucke

Dear Dr. Lücke,

thanks for the hint, I may follow-up on this, though knowing now a little better what to look for, I still hope to find something in Bremen, be at at Fraunhofer IFAM or the University.

Best regards,

Dirk Lehmhus

Dirk -- did you ever find an answer to this question?

John