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.

As an example, I have done a project on coating HA on Ti. The heat treatment enhances the coating strength of HA and its composite, therefore I recommend you to read my paper and very interested in having your comments.

EPD is an interesting technique by which particles in a suspension are deposited onto an electrode by their migration under the influence of an applied electric field [23]. In this work the suspensions of the NHA-Di Nano powders at a total concentration of 10 g solid/100 mL were prepared in isopropanol (99.9% Merck) and then treated in an ultrasonic bath for 100 min and homogenized with a magnetic stirrer for 30 min before being coated in order to ensure an excellent dispersion of the particles. An amount of 1.6 ml triethanol amine (TEA) was added as a dispersant (99.9%-Merck). Ti sample and graphite electrodes were used as the anode and cathode, respectively. The electrodes were placed parallel to each other in suspension, with a separation of 10 mm approximately, connected to a DC power supply. After deposition, the green form coatings were dried at room temperature in air, and then sintered in the tube furnace at 850 °C for 3 h in atmosphere at a heating rate of 10 °C/min and left to cool in the furnace. Key EPD deposition (process) parameters (namely applied DC electric field (V/l), deposition time and solid concentration in suspension) were optimized by visual inspection of the deposited films by AFM, SEM and XRD. The examination of the EPD process was done for 30, 40, 50 and 60 V using a D.C volt regulator in constant times 5, 7, 10 and 12 min. The coating bio-layers were not uniform in 30–40 V and pores could be visually observed. Therefore, the optimum parameters for EPD process were founded to be at 50 V for 10 min.

The published papers are as below:

https://www.scientific.net/AMR.829.314

http://www.sciencedirect.com/science/article/pii/S0272884215011815