Generally, most papers regarding HAP, sintered at a relatively high temperature, say that it will induce bigger grain size and reduce the hardness. While the relatively high temperature will enhance densification and linear shrinkage up to a critical temperature before it degrades, that can also associated by phase change to beta or alpha TCP. However, I have still to find a solid explanation to relate grain size and mechanical strength. Can anyone explain and discuss this?
Smaller the grain size higher the surface area for interaction with the neighboring grains with directly shows higher mechanical strength vice versa in case of bigger grain size.
Check this
http://www.doitpoms.ac.uk/tlplib/mechanical-testing/grainsize.php
good luck
This is a good question, and easier to answer for materials where the composition and phase is retained upon heating. A paper from 2004 showed the effect of grain size on hardness. As you know, the hardness is related to the yield strength, and so an increase in hardness will provide an increase in the yield strength. Please see the following article.
See Fig 9 in Biomaterials 25 (2004) 1395–1405
Beware of changes in the composition and porosity, that can contribute to changes in the mechanical properties
Biomaterials 25 (2004) 1385–1394
One should also keep in mind the effect of secondary phases, and dehydroxylation - issues that have not yet been assessed.
I read something bigger grain size will have lesser grain boundaries. What is your opinion on this?
Very quickly, a smaller grain size implies more grain surface area per grain volume. A smaller grain size has a higher yield strength. Have a read of Biomaterials, 2011, 7, 4235.
HAp-based materials are specific systems since undergo dehydroxylation and phase transformations, depending on Ca/P ratio, sintering temperature and some other processing parameters. Mechanical response are often targeted issue for these materials, but still being away from load-bearing application.
Generally. it is shown that hardness would be enhanced by grain size decrease, but it will happen basically if you compare dense materials with average grain size on micrometer and nanometer level. As of the fracture toughness, the sensitivity on grain size is higher, and if you go below 100 nm with uniform grain size distribution, you can expect significant fractre toughness improvement. All of this is valid for fully dense materials since density is the most important for mechanical response.
Thanks Miodrag.
Yes, fracture toughness is important and you have correctly indicated that fracture toughness increases with a decrease in the grain size. In the paper that was mentioned, there was further work that even looked at the amorphous phase. The fracture toughness increases with a decrease in grain size, and theoretically attains a high value for the amorphous phase. The study shows the the load to crack initiation is at least 5 times higher in an amorphous calcium phosphate compared to hydroxyapatite.
Yes Karlis, I know your papers regarding mechanical properties, good work, giving the oportunity to think about tailoring the system that imparts two tougness components: the resistance to crack formation and the resitance to crack growth.
I think that attention needs to be paid on grain boundary engineering by combining amorphous/crystalline nature of gb, since as I remember you indicated very low fracture toughness of HAp single crystal (0.45 MPam1/2 I think).
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