Mechanical properties of pulsed laser-deposited hydroxyapatite thin films for applications in biomedical implants
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Y8.13.1
Mechanical properties of pulsed laser-deposited hydroxyapatite thin films for applications in biomedical implants Hyunbin Kim, Yogesh K. Vohra, William R. Lacefield1, Renato P. Camata Department of Physics, University of Alabama at Birmingham, Birmingham, Alabama 35294 1 Department of Prosthodontics and Biomaterials, University of Alabama at Birmingham, Birmingham, Alabama 35294
ABSTRACT
We have obtained nanostructured hydroxyapatite thin films on titanium alloy substrates by pulsed laser deposition. Deposition was carried out using a KrF excimer laser (248 nm) with the energy density of 4 - 7 J/cm2 at substrate temperatures in the 550°C - 650°C range. The crystallinity of the coatings was probed by X-ray diffraction. Phase transitions from hydroxyapatite to other calcium phosphate compounds were observed with varying the substrate temperature during the growth process. Scanning electron microscopy revealed thin films made up of partially sintered nanoscale grains. The average size of nanoscale grains increased significantly with film thickness, suggesting a growth mechanism involving the coalescence of nanoscale grains. As the laser energy density increases, the hydroxyapatite crystallites in the coatings are oriented preferentially along the c-axis perpendicular to the substrate. Mechanical properties of the highly c-axis oriented coatings such as hardness and Young’s modulus were studied by using nanoindentation technique.
INTRODUCTION
Hydroxyapatite, Ca10(PO4)6(OH)2, is the main mineral constituent of bone tissue in humans. Because of its bioactivity and biocompatibility, it has been commonly coated on orthopedic and dental metal implants by plasma spray methods in order to improve longterm implant stability and osteointegration [1, 2]. Over the past decade, numerous studies have shown that pulsed laser deposition (PLD) can deposit hydroxyapatite (HA) coatings with better control over chemical composition and crystallinity as compared to those obtained by plasma spray [3-5]. This enables the optimization of the dissolution rate, abrasive resistance, and mechanical integrity of the coatings. In particular, improvement of the mechanical properties of HA coatings is necessary for load-bearing implants and other devices that undergo bone insertion. We have used PLD to deposit nanostructured HA thin films on titanium alloy substrates and analyzed their mechanical properties.
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EXPERIMENT We obtained the HA coatings of thickness 1 - 3 µm in Ar/H2O atmosphere by using a KrF excimer (248 nm) laser with the energy density of 4 - 7 J/cm2 and a repetition rate of 30 Hz. The base pressure was 2.5 × 10-5 - 9 × 10-7 Torr and increased to 600 mTorr with 99.999 % Ar gas and deionized water vapor before deposition and kept it during deposition. The substrates temperature was varied in the range of 550 - 650 °C. The HA targets were pelletized by compressing HA powder (Plasma biotal Inc.) at the pressure of 2500 psi and sintered at 1200 °C in Ar/H2O atmosphere for 1.5 hrs. The targets were preablated for 1 -
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