Precipitation of hydroxyapatite nanoparticles: effects of precipitation method on electrophoretic deposition.

Electrophoretic deposition is a low-cost, simple, and flexible coating method for producing hydroxyapatite (HA) coatings on metal implants with a broad range of thicknesses, from < 1 microm to > 500 microm. As for many other HA coating techniques, densification of electrophoretically deposited coatings involves heating the coated metal to temperatures above 1000 degrees C. Metal substrates tend to react with HA coatings at such temperatures inducing decomposition at temperatures below 1050 degrees C (decomposition for pure HA normally occurs above 1300 degrees C). Therefore, densification of these coatings needs to be conducted at temperatures lower than 1050 degrees C, and this necessitates the use of high-surface-area HA nano-precipitates, rather than commercially available pre-calcined powders, which densify at temperatures typically higher than 1200 degrees C. HA nano-precipitates were prepared by three methods and deposited on metal substrates by electrophoresis: (1) the acid base method, which produced plate-like nano-particles with a 2.5:1 aspect ratio, and severely cracked coatings; (2) the calcium acetate method, which produced needle-like nano-particles with a 10:1 aspect ratio, and slightly cracked coatings; (3) the metathesis method, which produced rounded nano-particles with a 2:1 aspect ratio, and high-quality crack-free coatings. The results suggested that the less equiaxed the nano-particles, the more cracked the coatings obtained by the electrophoretic deposition technique.

Hydroxyapatite-coated metals: interfacial reactions during sintering.

Electrophoretic deposition (EPD) is a low cost flexible process for producing HA coatings on metal implants. Its main limitation is that it requires heating the coated implant in order to densify the HA. HA typically sinters at a temperature below 1150 degrees C, but metal implants are degraded above 1000 degrees C. Further, the metal induces the decomposition of the HA coating upon sintering. Recent developments have enabled EPD of metathesis-synthesised uncalcined HA which sinters at approximately 1000 degrees C. The effects of temperature on HA-coated Ti, Ti6Al4V, and 316L stainless steel were investigated for dual coatings of metathesis HA sintered at 1000 degrees C. The use of dual HA coatings (coat, sinter, coat, sinter) enabled decomposition to be confined to the "undercoat" (HA layer 1), with the surface coating decomposition free. The tensile strength of the three metals was not significantly affected by the high sintering temperatures (925 degrees C < T < 1000 degrees C). XRD/SEM/EDS analyses of the interfacial zones revealed that 316L had a negligible HA:metal interfacial zone (approximately 1 microm) while HA:Ti and HA:Ti6Al4V had large interfacial zones (>10 microm) comprising a TiO2 oxidation zone and a CaTiO2 reaction zone.

Solution ripening of hydroxyapatite nanoparticles: effects on electrophoretic deposition.

Electrophoretic deposition is a low-cost, simple, and flexible coating method for producing hydroxyapatite (Hap) coatings on metal implants. However, densification requires heating the coated metal to high temperatures, which, for commercial HAp powders, generally means at least 1200 degrees C. At such temperatures, the metal tends to react with the HAp coating, inducing decomposition, and the strength of titanium and stainless steel implants is severely degraded. With the use of raw uncalcined nanoparticulate Hap, densification can occur at 900 degrees -1050 degrees C; however, such coatings are prone to cracking due to the high drying shrinkage. This problem was solved by precipitating nanoparticulate HAp by the metathesis process [10Ca(NO3)2 + 6NH4H2PO4 + 8NH4OH] and optimizing the approximately 30 nm of nanoprecipitates by an Ostwald ripening approach, that is, by boiling and/or ambient aging in the mother liquor. While the as-precipitated nanoparticles produced severely cracked coatings, 2 h of boiling or 10 days of ambient aging ripened the "gel-like" mass into unagglomerated nanoparticles, which produced crack-free coatings. Since boiling enhanced particle size but ambient aging did not, crack elimination probably was due to the transition from the highly agglomerated gel-like state to the dispersed nanoparticulate state rather than to particle growth. Furthermore, boiling only reduced the amount of cracking whereas aging completely eliminated cracking.

Interfacial bond strength of electrophoretically deposited hydroxyapatite coatings on metals.

Hydroxyapatite (HAp) coatings were deposited onto substrates of metal biomaterials (Ti, Ti6Al4V, and 316L stainless steel) by electrophoretic deposition (EPD). Only ultra-high surface area HAp powder, prepared by the metathesis method 10Ca(NO3)2 + 6(NH4)2HPO4 + 8NH4OH), could produce dense coatings when sintered at 875-1000degreesC. Single EPD coatings cracked during sintering owing to the 15-18% sintering shrinkage, but the HAp did not decompose. The use of dual coatings (coat, sinter, coat, sinter) resolved the cracking problem. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) inspection revealed that the second coating filled in the "valleys" in the cracks of the first coating. The interfacial shear strength of the dual coatings was found, by ASTM F1044-87, to be approximately 12 MPa on a titanium substrate and approximately 22 MPa on 316L stainless steel, comparing quite favorably with the 34 MPa benchmark (the shear strength of bovine cortical bone was found to be 34 MPa). Stainless steel gave the better result since -316L (20.5 microm mK(-1)) > alpha-HAp (approximately 14 microm mK(-1)), resulting in residual compressive stresses in the coating, whereas alpha-titanium (approximately 10.3 microm mK(-1)) < alpha-HAp, resulting in residual tensile stresses in the coating.

Sintering effects on the strength of hydroxyapatite.

Mechanisms underlying temperature-strength interrelations for dense (> 95% dense, pores closed) hydroxyapatite (HAp) were investigated by comparative assessment of temperature effects on tensile strength, Weibull modulus, apparent density, decomposition (HAp:tricalcium phosphate ratio), dehydroxylation and microstructure. Significant dehydroxylation occurred above approximately 800 degrees C. Strength peaked at approximately 80 MPa just before the attainment of closed porosity (approximately 95% dense). For higher temperatures (closed porosity), the strength dropped sharply to approximately 60 MPa due to the closure of dehydroxylation pathways, and then stabilized at approximately 60 MPa. At very high temperatures (> 1350 degrees C), the strength dropped catastrophically to approximately 10 MPa corresponding to the decomposition of HAp to tricalcium phosphate and the associated sudden release of the remaining bonded water.