Papers by Author: Aliasghar Behnamghader

Abstract: Surface degradation and cracking of dense hydroxyapatite were evaluated through Vickers micro indentation using indentation loads ranged from 25 gf to 2000 gf. Crack lengths, imprint diameters and the number of lateral cracks and chips were measured using SEM. The crack length-indentation load data were analyzed with regard to the specific relations of Palmqvist and fully developed radial cracks. Crack type transition load from Palmqvist to median crack was experimentally assessed through serial sectioning technique. The analytical estimated transition load, based on the theoretical relation of the indentation load and crack lengths showed a good agreement with one obtained from experimental itinerary. Palmqvist and median cracks were identified in low and medium indentation loads, respectively. High indentation load could also lead to the formation of lateral cracks and chips. The tendency for lateral cracking was evaluated taking into account the number of lateral cracks and chips. The chips were found to be appeared just after test in higher indentation load, whereas in medium loads they could be detectable only after several weeks.

Abstract: In this research hydroxyapatite powder was synthesized by wet chemical method using calcium nitrate and diammonium hydrogen phosphate precursors at 2, 20, 50 and 90⁰ C. Phase composition, morphological aspects and particle size have been investigated using X-ray diffraction method (XRD), Fourier transform infra red spectroscopy (FTIR) and scanning electron microscopy (SEM). For the low synthesis temperature, products with low degree of crystallinity were obtained and the phases like calcium deficiency hydroxyapatite (CDHA) and hydroxyapatite were formed. In higher temperature ranges, higher degree of crystallinity was measured. For these samples, crystallite sizes estimated by scherrer formula and measured directly by SEM were found to be less than 100 nm. The synthesis temperature of 90°C has showed better results from crystallinity point of view. Considerable crystallinity of the powders produced in this research is almost comparable to that of conventional heat treatment used for enhancing the crystallinity.

Abstract: Mechanical behavior and fracture mechanisms of plasma sprayed hydroxyapatite coatings on Ti-6Al-4V substrate were assessed taking into consideration two variables: the coating thickness and the substrate roughness. The results show that the specimens having a substrate arithmetic average roughness parameter Ra = 2.29 µm is favorable with respect to Ra = 1.23 µm. For coating thickness above 105 µm, cracks can be observed in the coating/substrate interface and the higher critical load Pc2 (used generally in comparative evaluation of adherence) decreases. A 90 µm coating thickness sprayed on a substrate having an arithmetic average roughness parameter Ra equal to 2.29 µm seems to be the best compromise between microstructure, mechanical resistance (high critical loads and fairly good contact quality) and long term stability in the physiological medium (low dissolution rate) for an orthopedic application.

Abstract: The aim of this study was to investigate the sol gel synthesis of HA/FA nanoparticles and the possible formation of TCP phase or unstoichiometric calcium deficient hydroxyapatite (CDHA) from the precursors with a Ca/P ratio of 1.62. In order to prepare the sol, the solutions of Triethyl phosphite, ammonium fluoride and calcium nitrate in ethanol were used respectively as P, F and Ca precursors. The crystallinity, particle and crystallite size, powder morphology, chemical structure and phase analysis were investigated by SEM, XRD, FT-IR and Zeta sizer experiment. A multiphase compound containing hydroxyapatite (HA) and fluoroapatite (FA) nanoparticles and calcium deficient hudroxyapatite (CDHA) agglomerates was obtained. The size of the crystallites estimated from XRD patterns using Scherrer equation and the crystallinity of HA phase were about 5 nm and 66% respectively. The zeta sizer experiments for the dispersed particles in its own conditions showed an average size of 98 nm.