Different approaches to produce coatings with bioactive glasses: Enamelling vs plasma spraying

Two alternative approaches, enamelling and plasma spraying, were tested to deposit coatings made with two different bioactive glasses: the established Bioglass® 45S5, which is considered as a term of comparison, and the experimental BioK. The strong points and weaknesses of the two methods were highlighted. From the analysed samples, it resulted that the enamelling approach works well on thermally stable substrates and creates a strong bond, characterized by a compositional gradient, with alumina substrates. However, the coating thickness must be carefully controlled to limit the thermal residual stresses and the glass formulation should be designed to reduce the glass tendency to crystallize. Instead plasma spraying is suitable for any kind of substrate and is highly automatizable, but the equipment is relatively expensive and the coatings are likely to retain some defectiveness, which makes a post-deposition thermal treatment necessary. Both enamelling and plasma spraying may induce crystallization phenomena, depending on the glass formulation. The introduction of potassium oxide in the glass composition, such as in the BioK, may be useful to hinder the crystallization.