Mechanical Properties and Microarchitecture of Nanoporous Hydroxyapatite Bioceramic Nanoparticle Coatings on Ti and TiN
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Mechanical Properties and Microarchitecture of Nanoporous Hydroxyapatite Bioceramic Nanoparticle Coatings on Ti and TiN Andrei Stanishevsky1, Shafiul Chowdhury1, Nathaniel Greenstein1, Helene Yockell-Lelievre2, and Jari Koskinen3 1 Physics, University of Alabama at Birmingham, 1300 University Blvd, Campbell Hall #310, Birmingham, AL, 35294 2 University Laval, Quebec, G1K 7P4, Canada 3 VTT Technical Research Centre of Finland, Espoo, P.O.Box FIN-1000 02044 VTT, Finland
ABSTRACT Hydroxyapatite (HA) based bioceramic materials are usually prepared at high sintering temperatures to attain suitable mechanical properties. The sintering process usually results in a material which is compositionally and morphologically different from the nonstoichiometric nano-crystalline HA phase of hard tissue. At the same time, HA particulates used as precursors in ceramic manufacturing are often very similar to the natural HA nanocrystals. It has been shown that synthetic nanoparticle HA (nanoHA) based materials improve the biological response in vitro and in vivo, but information on the mechanical properties of these materials is scarce. In this work we studied HA nanoparticle (10 - 80 nm mean size) coatings with 30 - 70% porosity prepared by a dip-coating technique on Ti and TiN substrates. It has been found that the mechanical properties of HA nanoparticle coatings are strongly influenced by the initial size, morphology, and surface treatment of nanoparticles. The nanoindentation Youngís modulus and hardness of asñdeposited nanoHA coatings were in the range of 2.5 - 6.9 GPa and 80 - 230 MPa, respectively. The coatings were stable after annealing up to at least 600 oC, (with Youngís modulus reaching 23 GPa and hardness reaching 540 MPa), as well as in simulated body fluids. INTRODUCTION Hydroxyapatite (HA) bioceramic in the form of coatings, powders, and 3-D objects is one of the most used inorganic biomaterials. The widespread use of HA arises due to the chemical similarity between synthetic HA and the mineralized tissue of human bone, although the composition and structure of most synthetic HA materials are often quite different from those of bone mineral represented by amorphous phase together with rod-shaped crystallites or elongated 2-5 nm thick platelets with a 20-40 nm length [1-4]. Clinical studies in vitro and in vivo have proved that synthetic HA is superior to many other biomedical materials, particularly Ti-alloys. The HA coatings on metallic implants usually have a good stability to dissolution in body fluids, and suitable surface morphology to promote cell attachment and tissue growth. Numerous techniques to prepare HA coatings have been used, including plasma spray, electrophoresis, PLD, sputtering, and sol-gel [5,6]. Because HA has a complex crystal structure, it is sensitive to nonstoichiometry and impurities during synthesis, calcination, and sintering. As a result, the HA materials often lack compositional purity and homogeneity. Their densification typically requires high temperatures, resulting in gr
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