Thermal and physical characterisation of apatite/wollastonite bioactive glass–ceramics

Glass–ceramics, containing apatite and wollastonite (A/W) crystals in the MgO–CaO–SiO2–P2O5 glassy matrix, show the ability to form tight chemical bonds with living tissues when implanted in the body, as demonstrated by Kokubo and co-workers. However, the medical applications are mainly limited to non-load bearing conditions because of their poor mechanical properties. To overcome this drawback, a coating of the A/W glass–ceramic could be deposited onto a titanium substrate, in order to combine the good bioactivity of the bioceramic and the good mechanical strength of the titanium alloy base material. In this study, A/W powders obtained from commercial raw materials were thermally sprayed by APS (atmospheric plasma spraying) on Ti–6Al–4V substrates. Since in the as-sprayed conditions the coating microstructure was defective because of pores and cracks, thermal treatments on A/W plasma-sprayed coatings were conducted to enhance the coating microstructure. In order to gain a deeper insight, A/W bulk and sintered samples of the same composition were prepared and subjected to the same thermal treatments. In addition, the crystallisation behaviour of A/W bioactive glass–ceramic was investigated. The frits thermal behaviour was characterised by means of hot stage measurements and DTA analysis. DTA analysis on the A/W base glass, revealed two crystallisation peaks at about 1150 and 1193 K, corresponding to the crystallisation of oxyapatite and β-wollastonite phases, respectively. The two corresponding activation energies, calculated with the Kissinger equation, were also reported.The microstructure and the crystallinity of the A/W glass–ceramics were evaluated depending on the thermal treatment. The morphology and the microstructure were observed by SEM and the crystalline phases were detected by X-ray diffraction. Finally the porosity was determined via image analysis.