Characterization of hydroxyapatite films obtained by pulsed-laser deposition on Ti and Ti-6AL-4v substrates
Introduction
Numerous developments of bioactive biomaterials containing calcium phosphates and hydroxyapatite (HA) took place during the 1980s. Because both are highly biocompatible and well matched with the physical properties of HA, titanium substrates are chosen by preference. HA ceramics have been used in medical implants for direct and rapid bonding to living bone tissue. In the periodontic field, hydroxyapatite was used as synthetic graft material to treat intraosseous defects. The chemical interaction between hydroxyapatite and bone tissue permitted some success with this therapy. The brittle nature of HA requires it to be coated onto more resistant substrates such as titanium or Ti-6Al-4V. The HA coating permits a faster osteointegration due to its bone tissue bonding properties. Several coating techniques have been developed: plasma spray, ion beam-assisted deposition or electrophoresis deposition [1], [2], [3]. The first clinical results with HA coated titanium dental implants were encouraging, showing a good rate of success, even with poor bone quality. However, after longer periods of implantation, mechanical failure occurred at the HA/metallic substrate interface [4]. The HA-coating dissolves because of amorphous or poor crystallized structure [5], [6], enhancing loss of adherence with the titanium surface and dramatic late implant failure [6], [7]. Hence, the stability of the HA coating determines the success of this type of implant. This coating could be stabilized with a thin deposit of highly dense and crystalline HA, able to establish a strong bond both with the bone matrix [8]. Another identified problem was poor mechanical bond strength between the coating and the substrate [1].
The laser ablation technique or pulsed-laser deposition (PLD) is an alternate process available to obtain a thin layer of coating over a metallic substrate. The research in PLD is directed towards the creation of highly crystalline apatite films at low substrate temperature [9].
The aim of this study was to determine the specific parameters of the technique in order to produce crystalline and adherent HA thin films on Ti and Ti-6Al-4V substrates. The parameters assessed were: the nature and pressure of the gases in the ablation chamber; the temperature of substrate; the laser beam parameters and the distance between the target and the substrate. Three different techniques, SEM for morphology, AFM for topography roughness and a micro-scratch tester for coating adhesion were used to characterize the thin HA layers produced.
Section snippets
Pulsed-laser deposition (PLD) technique
A Nd:YAG laser (B.M. Industries, Div of Thomson Components. France) with a wavelength of (λ) 266 nm (used frequency doubling KDP crystals), a pulse duration of 5 ns at a 5 Hz repetition rate and an energy of about 2 J was used. The laser beam was focused on the target at an incidence angle of 45 degrees. The target was rotated during the deposition. The laser wave was directed at the target creating a plasma spray of the impulse slacken in the enclosure of the ablation chamber. The high vacuum in
Rutherford backscattering (RBS)
The results obtained with fourteen samples under different coating conditions are presented in Table 1. (The calcium to phosphorus ratio, determined using RBS, is systematically calculated.) When a Ca/P ratio equal to HA was found on a Si substrate, parameters were refined on Ti and Ti-6Al-4V substrates.
The chemical composition of the films was related to the oxygen content of the gas environment during deposition similar to Cotell et al. [10] findings. The temperature of the substrate and the
Discussion
According to the literature [12], [13], the amorphous-to-crystalline HA conversion is dependent on both temperature and water vapour pressure. The correct Ca/P ratio (1.66) of HA films is obtained at a substrate temperature of 575 °C by depositing the film in a gas environment containing an oxygen-like species atmosphere (water vapour at 0.4 mbar). The water vapour pressure seems to be the determining factor for the fixing of phosphorous in HA films [10], [14]. At this temperature, it appears
Conclusion
In odontology, obtaining such HA layers should undoubtedly allow more consideration of hydroxyapatite-covered metal implants. The hydroxyapatite would have the double role of a bioactive coating, improving osteointegration, and creating a barrier to the release of elements such as Ti and V. Pulsed-laser deposition can be used to form a thin HA coating (0.05–3 μm) on metallic substrates. These films are adherent to the substrate and have a high degree of crystallinity inducing biological behavior
Acknowledgements
The authors would like to give their sincerest thanks to Professor Jacques Perrière for his contribution at this work, particularly for the use of the PLD system (Groupe Physique du Solide, Universités Paris 6 et Paris 7, France).
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