Post-deposition laser treatment of plasma sprayed titanic-hydroxyapatite functionally graded coatings

The viability of a high power diode laser source as effective post-deposition treatment technique of functionally graded titanic-HA coatings was checked. In particular, several laser treatments were performed on various coatings plasma-sprayed under different conditions to verify the presence of an operative window large enough for practical purposes and, subsequently, to identify the most promising settings of the laser parameters. Laser power as low as 80-100 W and focus distance as high as -4 mm were found to be the most feasible choice to improve the overall coating properties as well as to inhibit undesired secondary reactions between calcium phosphates and titania. Finally, the best set of the laser parameters were applied to a pure HA coating and to a titanic-HA graded one, plasma-sprayed under the same conditions, to perform a comparative evaluation. The microstructural characterization by scanning electron microscopy, X-ray diffraction and the local mechanical investigation by Vickers micro-indentations proved that the degree of crystallinity of HA at the outermost layers of the graded coating could be improved without significantly altering the compositional and functional gradient. Furthermore, the properties of the pure HA coating, as-sprayed and laser treated, were found to be substantially less advantageous than those of the titanic-HA functionally graded coating (higher microstructural defectiveness; inferior degree of crystallinity of HA at the working surface; lower Vickers hardness), thus confirming the beneficial effect of the compositional gradient. (C) 2009 Elsevier Ltd. All rights reserved.