Correlating Electrospray Characteristics with Surface Morphology of Calcium Phosphate Coatings Deposited Using Electrostatic Spray Deposition (ESD)

摘要

In situ measurements of electrospray droplet sizes and velocities were performed by Phase Doppler Anemometry during Electrostatic Spray Deposition (ESD) of calcium phosphate (CaP) coatings. Numerous processing parameters were varied (nozzle-to-substrate distance, deposition temperature, nozzle geometry, and composition of the precursor solution), whereafter the morphological characteristics of these ESD-derived CaP coatings were correlated with measured droplet characteristics. Equal droplet sizes and velocities were measured for nozzle-to-substrate distances up to 40 mm and deposition temperatures up to 400 °C, indicating that electrospray droplets did not shrink at all during droplet flight using an involatile solvent butyl carbitol with a high boiling point (T{sub}(b) = 231 °C). Nevertheless, coatings with considerably different surface morphologies were obtained under these conditions, varying from microporous structures with coalesced pore walls to morphologies revealing isolated rings on top of dense or grainy underlayers. The chemical composition of the precursor solutions and the mixing characteristics of the calcium and phosphate precursor components strongly influenced the initial droplet sizes, precipitation kinetics of the CaP solute, and subsequent coating morphology. Unique, reticular coating morphologies were deposited at a deposition rate of 3.2 μm/hour, which have a graded structure consisting of a dense underlayer, a submicron-porous intermediate layer, and a roughened toplayer revealing droplet-derived features such as isolated rings or coalesced, hollow surface pits.