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

Two different oxide-based bioactive materials, namely tricalcium phosphate (TCP) and K2O–CaO–P2O5–SiO2 bioactive glass (“Bio-K”), were processed by the High Velocity Suspension Flame Spraying (HVSFS) technique. Single splats were deposited onto polished cold glass plates using a custom-built experimental set up. In addition, complete coatings were deposited onto pre-heated Ti plates. Deposited splats and coatings were examined by scanning and transmission electron microscope (SEM, TEM) techniques and their phase composition was investigated by micro-Raman spectroscopy. The 3-D profiles of single splats were also reconstructed through atomic force microscopy (AFM). Based on the experimental results, explanations of the deposition mechanisms were put forward, discussing the different heating and melting behaviour of particles and agglomerates. The critical particle sizes needed to obtain deposition of molten splats at impact, and their solidification behaviour onto cold vs. hot surfaces were studied as well. Most importantly, the splats can only be formed by molten droplets comprised within a given size range (“sprayability window”). Smaller droplets possess insufficient thermal and mechanical inertia to overcome the stagnation flow in front of the substrate; larger ones are mostly unmelted. This “sprayability window” depends on the deposition process itself and on the physical properties of sprayed materials.