what are the consequences of heating hydroxyapatite coatings in air at 900 oC
Hello,
There are many papers on the effect of heating at high temperature on hydroxyapatite. Although the precise effects strongly depend on the origin of the samples (natural vs synthetic) and many other parameters including hydration state, stoichiometry, crystal size, temperature gradient, chemical atmosphere etc, in brief, you get crystalline phase transitions and particle growth. The phase transition generally starts around 600 oC and so does the particle growth. This is generally associated with important color changes (dark brown up to 600 oC, then somewhat lighter up to 900 oC and an intense gray white at some point after).
Of course, this has a major impact on the mechanical properties, the material becoming harder and more brittle. Depending on your substrate, if the coating has a very different thermal expansion coefficient, you're very likely to see some cracks appearing and, even if this doesn't happen, you'll probably have a high amount of locked stresses, which means that your coating will be more fragile. So if you can, try matching the substrate as close as possible to the coating. The wear properties are also likely to change.
More precise details would require some precisions on your system, but the general idea is there.
Good luck !
PS: if you try it out, it would be nice if you could provide some basic feedback, even if the results are not expected.
Hi Charanjeet
Great question. Calcining your hydroxyapatite films at 900 oC will densify and increase the mean particle size of the hydroxyapatite crystallites in your films (i.e. increase the film crystallinity), as noted by Aurelien Gourrier.
High temperature treatments may also thermally decompose your hydroxyapatite
Ca10(PO4)6(OH)2 ---> 3Ca3(PO4)2 + CaO + H2O
Best
Geoff
Dear Singh,
It is generally related to the production and post procedure of the powders.
For the chemically prepared HA, first physically adsorbed than chemically adsorbed water are evolved. After that decarbonisation and partial dehydroxilation will be occured.
For detail information, pls check:
A review on the thermal stability of calcium apatites DOI: 10.1007/s10973-011-1877-y
Regards,
Fatih.
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