Deposition mechanisms in high velocity suspension spraying: Case study for two bioactive materials

Comparison between Suspension Plasma Sprayed and High Velocity Suspension Flame Sprayed bioactive coatings

“…The former, by contrast, are obtained when aggregates of primary particles are heated above the glass transition temperature. In this case, indeed, the aggregates sinter in a single, larger droplet containing internal gas porosity, as previously depicted in [34]. As mentioned previously, both kinds of particles reached the substrate with relatively low velocity and temperature, which prevented complete flattening.…”

“…The HVSFS system has already been described in [30][31][32][33][34][35][36]. It consists of a modified gas-fuelled HVOF torch (Top Gun-G, GTV GmbH) equipped with a cylindrical combustion chamber and a liquid injection nozzle (ending up with a 0.9 mm diameter orifice) replacing the standard dry powder feeding port.…”

“…As the Stokes number decreases below unity, particles increasingly tend to copy the flow streamlines. As detailed in [34], the Stokes number for a thermally-sprayed particle facing the stagnation flow can be expressed as (1):…”

“…As the latter is axially injected inside the combustion chamber of the torch, complete mixing of the suspension drops with the gas is favoured. The solid particles (released after solvent vaporisation) are heated by the gas in the combustion chamber and in the expansion nozzle and, as the gas stream attains supersonic velocity upon expansion to ambient pressure outside the torch, they are also dragged to extremely high velocities (up to 900 m/s according to some recent estimates [33,34]). Extensive flattening onto the substrate is achieved, resulting in very dense layers with high cohesive/adhesive strength, which has already been experimentally verified both for calcium-phosphate coatings (tricalcium phosphate [34] and hydroxyapatite [31]) and for bioactive glass coatings [35].…”

“…The solid particles (released after solvent vaporisation) are heated by the gas in the combustion chamber and in the expansion nozzle and, as the gas stream attains supersonic velocity upon expansion to ambient pressure outside the torch, they are also dragged to extremely high velocities (up to 900 m/s according to some recent estimates [33,34]). Extensive flattening onto the substrate is achieved, resulting in very dense layers with high cohesive/adhesive strength, which has already been experimentally verified both for calcium-phosphate coatings (tricalcium phosphate [34] and hydroxyapatite [31]) and for bioactive glass coatings [35]. The latter, even though somewhat prone to crystallise at high temperature, offer several advantages with respect to HA [13,[36][37][38], since they do not decompose during thermal spraying and they possess a higher bioactivity index than HA.…”