Some aspects are relevant, such as surface roughness of samples. Did you follow the established by ISO 14577 for your sample roughness? I also think that the holding time is relatively short...

With regards.

Thank you for the answer. For sample preparation I used diamond polishing paste followed by electropolishing. Maybe it isn't enough and some small scratches are left on the surface.

But why do you think that holding time is short?

Well, as your load is relatively low, check the relation between indentation depth and roughness. Usually, the rule is indentation >= 20 Ra.

With respect to holding time, I recommend https://www.researchgate.net/publication/229122248_Investigation_of_creep_behaviour_under_load_during_indentation_experiments_and_its_influence_on_hardness_and_modulus_results?ev=prf_pub

Note that soft metals have longer holding time...

Besides this, another aspect should avoid, is the thermal drift. A broad literature can be found. For example: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-14392007000400019&lng=en&nrm=iso&tlng=en

They treated this topic and other important procedure aspects. Good luck!

Article Investigation of Creep Behavior Under Load During Indentatio...

Thermal drift seams to be the first thing to check. You should have a look at how this problems is addressed by your indentation device's software and also let you sample for some time (~12 hours) in your machine before running the tests.

Regards

Not sure if I am interpreting your loading curve correctly, but it is possible that some "creep" or inelastic deformation of the material is taking place.

I am also not sure but I understood that the displacement was going backwards which (if I am right) would exclude creep during the holding step at at maximum load. Maybe we need to have more data here, especially a displacement vs. time curve.

Regards.

I am agree with opinion about creep which influencing on the curve's "anomaly" . It's interesting to perform experiment with repeated indentation on the same samples. Thus we can see the creep accumulation.

I guess it couldn't be creep because of the "negative" displacement. It seems to me that surface roughness and thermal drift are the main possible reasons for that curve appearance.

Thank all of you for the usefull suggestions.

Hi Kateryna, you needs check the quality of the surface, the contact and mainly review again the setup of the indenter,parameters, the apparent contact is up to 1000nm?. also review quality of the tip, and I recommend you start using similar parameters from others researh works.

Is this phenomenon repeatable, or, it is an exceptional event?

It was repeatable, the loop at the curve was more or less visible. For consistency, the whole curve is in attachment.

The curve looks normal in general. How did you mount the sample to the stage? And, have you tried to perform the indent at different loads, say, much lower loads? Your indent load is rather and I don't think surface roughness is a big issue.

Maybe you could show us several load vs. displacement curves plotted on the same figure to discuss the repeatability and also provide a close view of the zone of the curves where the load in held constant. Even if the whole curves are repeatable, the issue we were discussing before may not be.

By the way, your curve exhibits a lot of reversible work during the unloading for a steel sample. Are you using a Berkovich indenter or something blunter ? How is your sample held ? can you plot the square root of the load vs. the displacement ?

Regards.

Hi! What kind of hardness tester are you used? I met something similar during nanoindentation of ITO/Si structures subjected to small electrical currents. I thought that is because of thermal expansion of sample. PS The measurements was performed on CSM NHT.

Xuwen, the sample was mounted to the stage by hot melt glue. My goal was to investigate the mechanical properties at 100 mH, so I don’t try another loads. Actually, of course, it’s interesting to compare the curves obtained at different loads, so I will try it.

Ludovic, I used Berkovich indenter. Here are more curves.

Ludovic, what plot do you mean? Here is √P(h)

Evghenii, I used the same hardness tester as you.

The plot points to nonlinear response, by the end of the experiment, close to the maximum applied load. It presents a small anomalous horizontal trace, disregarding experimental noise. This suggests that the system is responding with (at least) two responses in that region to a single value of applied load. These responses are responsible for a (small) region of ambiguity where experimental information is lost. To correct this ambiguity, which I think can be safely considered an experimental artifact, it is convenient to identify and remediate the source of nonlinear behaviour. Unfortunately, the underlying cause of this kind of behaviour is usually unpredicted and/or unusual, being therefore difficult to troubleshoot. The possible causes for the nonlinear response trouble can be of varied nature. They can be related to possible electronic malfunction of the equipment (possibly involving sensors, transducers, or signal amplifiers). Other kind of causes may also be involved, such as lack of lubrication and elastomers or springs having lost elasticity. Qualified professional assistance from the equipment’s provider can be necessary.

Kateryna, did you check for a layer on your surface? Looks a little bit like your tip would stick to the surface ...

Do you still measure some let's say good curves and some bad ones? Or did the behaviour of your system change and now remains in the bad condition?

what is the K of the cantilever? have you tried to vary the loading rate? In my personal opinion you are seeing just an artefact due to the setting parameters you are using.

Michael, Roberto, I had these kind of curves while doing nanoindentation via CSM instrument. Recently I did experiment via G200 Aglient and all the curves look normal. I still don't know why it had happened before, maybe I should have changed some parameters..

It looks like that in your plot one should change the coordinates, did you check it?

Hi Marchello!

Thank you for the very detailed answer!

it looks like something is slipping/noise/drift, like Marcello was suggesting.

Plotting L-D curve is difficult when we do nanoindentation, nanoscrach tests. I am not sure you are getting the same kind of curve all the times. It is better to do at least 30 indentations to confirm the nature of curve. I can help, if you send the raw data. 

For more information about load-displacement plots go through below links.

http://onlinelibrary.wiley.com/doi/10.1002/app.43532/full

http://www.sciencedirect.com/science/article/pii/S0032386116301227