Electric discharge assisted pulsed laser deposition of hydroxylapatite

Hydroxylapatite (Ca10(PO4)6(OH)2, HA) is a calcium phosphate used as coating for dental and orthopaedic titanium implants, because its composition and structure are similar to the mineral part of bone. Pulsed laser deposition has been successfully applied, but requires temperatures higher than 450 deg. C, which may affect the coating-substrate interface. A HA target was ablated with an ArF laser in a water vapour pressure of 45 Pa. The material was deposited on a titanium substrate, placed at 4.7 cm from the target. In this region an autonomous electric discharge was maintained with a DC power supply (0-1000 V, 800 W), giving an ionisation current between 0 and 80 mA. The presence of the HA phase in the coatings was assessed by X-ray diffraction. Their Ca/P ratio was measured by energy dispersion of X-rays, while the OH- and CO32- contents were evaluated by Fourier transform infrared spectroscopy. At higher ionisation currents, crystalline HA coatings can be obtained at lower temperatures (as low as 300 deg. C). This effect is due to both the higher incorporation of OH- in the coatings (higher H2O dissociation by the ionisation current) and the higher mobility and ionisation of the particles on the substrate (provided by the electron bombardment of the coating during its growth). It is only necessary to use a high ionisation current during the first growth steps in order to provide the seeds for crystallisation