Evolution of the deposition rate during pulsed laser deposition of hydroxyapatite coatings and its relation with target morphology

The evolution of the deposition rate during pulsed laser deposition of hydroxyapatite coatings with a KrF excimer laser was studied. This evolution was related to the changes produced on the target morphology during laser irradiation. Laser fluences in the 1.0-5.2 J/cm(2) range, typical for the deposition of hydroxyapatite coatings, were investigated. Deposition rates were measured through a quartz-crystal microbalance. and the target surface was observed by scanning electron microscopy. At the lowest fluences, the deposition rate decreases as the number of pulses increases. At the same time, a cone structure is developed on the target surface. At the highest fluences, the deposition rate increases with the number of pulses. In this case, the target surface becomes rough but cones are not formed. In all cases, an enlargement of the effective irradiated area results. This enlargement produces the effect of fluence dilution and also an increase of the emitting area. These opposite effects could account for the different evolutions found for the deposition rate. At low fluences the effect of fluence dilution would predominate over the increase of the emitting area, and at high fluences the second effect would predominate over the first.