STUDY OF ATMOSPHERIC PLASMA SPRAYING PARAMETERS TO OBTAIN DENSER BIOACTIVE GLASS COATINGS

Atmospheric Plasma Spraying (APS) parameters were studied to understand their influence on the splat stacking and the porosity formation in bioglass coatings. Commercial Schott 45S5 bioglass powder was sprayed onto 304L stainless steel substrate. The plasma spraying was performed by using a F4MB Oerlikon Metco gun with a nozzle of 6 mm in diameter. Three Ar/H₂ mixtures were used as plasma gas (84/16, 88/12 and 95/5 vol.%), the current intensity was varied from 450 to 650 A and the spray distance from 80 to 110 mm. Phases and structural characteristics of the coatings were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Porosity was evaluated by image J software. The process simulation carried out by Jets & Poudres software for each set of spray parameters shows suitable Sommerfeld number to obtain a good splat stacking. However, Na₂O and P₂O₅ volatilization occurred experimentally during spraying of 45S5 bioglass promote the porous formation in the coating structure. Denser 45S5 bioglass coatings were obtained under suitable relationship between plasma gas mixture ratio, the current intensity and the spray distance to achieve thermal energy input enough to overcome the glass transition temperature and to avoid the evaporation of volatile oxides.