You should indeed make sure before anything else that the materials are identical except the grain size. If the porosity changes, thus what you observe is more likely related to the change of porosity than to the grain size. Have a look at Rice's papers, explaining the effects of the porosity and of the pore shape on the mechanical properties.

Alain

I am sorry for my late response, Sir. Thank you for your opinion. In my experiment, the difference in grain size of the samples is larger than that in porosity. For example, the lowest and highest density are 98% and 99.5%, relatively. But the corresponding grainsize are 4 um and 15 um relatively (nearly four times). I want to add some more details that I had sintered them from the same starting materials which have different particle sizes.

Could you please give me the title of the papers of Rice, so that I can google it.

Thank you very much!

Here are a few papers, but I know that there are many others, and probably not so old, from the same author :

Rice RW. J Mater Sci 1996;31:1509.

Rice RW. Key Eng Mater 1996;115:1.

Rice RW. J Mater Sci 1996;31:102.

Alain

Thank you so much, Sir!

Strength is increased with decreased in grain size. This is happening due the strain hardening process.