Enamelled coatings produced with low-alkaline bioactive glasses

Enamelling is a relatively easy and inexpensive technique to produce glass coatings. In this contribution, three different low-alkaline bioactive glasses, modified with Na2O and/or K2O for a total alkaline content of 4.6 mol%, were enamelled on Ti6Al4V substrates for potential orthopaedic applications. The glasses in powder form were applied by means of a precipitation-based method and thermally treated in the 800-850 °C range; in particular, the enamelling temperature required to obtain uniform coatings increased with increasing K2O amounts. The SEM observation revealed that the coatings were about 100 μm thick, with a crack-free interface with the metal substrate mediated by the development of titanium oxides. Even if the low-alkaline glasses are characterised by a high crystallization temperature, the coatings underwent a partial devitrification, especially in the presence of K2O. However, the development of bioactive crystalline species, such as wollastonite, was beneficial, in that the new phases not only improved the local mechanical properties (in terms of Vickers microhardness, from 232.1 ± 76.8 HV for the Na2O-modified glass coating to 317.9 ± 48.8 for the K2O-modified one), but still preserved the apatite-forming ability in a simulated body fluid.