Hydroxyapatite coatings grown by pulsed laser deposition with a beam of 355 nm wavelength
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Hydroxyapatite coatings grown by pulsed laser deposition with a beam of 355 nm wavelength J.M. Ferna´ndez-Pradas,a) L. Cle`ries, G. Sardin, and J.L. Morenza
` ptica, Av. Diagonal 647, Universitat de Barcelona, Departament de Fı´sica Aplicada i O E-08028 Barcelona, Spain (Received 20 July 1998; accepted 13 September 1999)
Calcium phosphate coatings, obtained at different deposition rates by pulsed laser deposition with a Nd:YAG laser beam of 355 nm wavelength, were studied. The deposition rate was changed from 0.043 to 1.16 Å/shot by modification of only the ablated area, maintaining the local fluence constant to perform the ablation process in similar local conditions. Characterization of the coatings was performed by scanning electron microscopy, x-ray diffractometry, and infrared, micro-Raman, and x-ray photoelectron spectroscopy. The coatings showed a compact surface morphology formed by glassy grains with some droplets on them. Only hydroxyapatite (HA) and alpha-tricalcium phosphate (␣–TCP) peaks were found in the x-ray diffractograms. The relative content of ␣–TCP diminished with decreasing deposition rates, and only HA peaks were found for the lowest rate. The origin of ␣–TCP is discussed.
I. INTRODUCTION
One approach to enhancing the biocompatibility of implants is to coat their surfaces with a biocompatible material. Such coatings increase the life of the implant and, if bioactive, they allow direct bonding between bone and implant and accelerate the process of healing and growth of bone. Calcium phosphates, especially hydroxyapatite (HA), are at the forefront of biomaterials due to their similarity to the inorganic component of bone. The different phases of calcium phosphate have different behavior when implanted in the body. Although HA is the most stable, other phases with lower stability are resorbed and offer a dynamic surface. The deposition of HA coatings on titanium implants relied on the plasma-spray technique. It was not until the beginning of the nineties that pulsed laser deposition (PLD) was proposed as a new technique to improve the quality of the HA coatings.1– 4 The products of the laser– target interaction are atoms, molecules, small aggregates, and large particulates or droplets, with a high ratio of ions. By varying the laser beam characteristics and the nature of the target, the presence and proportion of these populations can be changed.5,6 Two approaches have been pursued in order to obtain hydroxyapatite coatings by PLD. In one approach, the material from the target is mainly emitted in form of micrometer-size particulates that are collected at the substrate to form the film.7,8 In
the other approach, atoms, molecules, and small clusters are transferred from the target to grow the HA material.1,9–12 The PLD technique allows reproducible production of coatings with different composition and phases and even with hydroxyapatite as the only phase. Thus a thorough study of the influence of the involved technological parameters on the proper
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