Stoichiometric transfer in pulsed laser deposition of hydroxylapatite

Hydroxylapatite (HA, Ca-10(PO4)(6)(OH)(2)) is a calcium phosphate used as coating for dental and orthopaedical implants because its composition and structure is similar to the mineral part of bone. As an alternative to traditional plasma sprayed coating technique, pulsed laser deposition (PLD) has been applied due to its ability to reproduce complex stoichiometries. A hydroxylapatite target was ablated with an ArF laser in a water vapor atmosphere to investigate in which range of fluences the stoichiometric transfer to a titanium substrate is possible. The Ca/P ratio of the coatings was measured by energy dispersive spectroscopy (EDS), while their OH- and CO32- content was evaluated by Fourier transform infrared spectroscopy (FT-IR) spectroscopy. The irradiated target surface was analyzed by scanning electron microscopy (SEM) and the ablation rate measured with a profilometer. While at higher fluences all the target material is congruently ablated and stoichiometry is transferred to the coatings, at lower fluences (< 1.2 J cm(-2)) preferential ablation of Ca and strong out-difussion of CO32- impurity as CO2 takes place at the target. The incongruent melting of the hydroxylapatite target at low fluences provokes its enrichment in Ca. The higher Ca concentrations arriving to the substrate, together with the higher CO2 partial pressure, yields enhanced substitution of PO43- by CO32- and increasing of the Ca/P ratio at the coating. (C) 2000 Elsevier Science B.V. All rights reserved.