Coatings of hydroxyapatite - nanosize alpha alumina composites on Ti-6Al-4V

Composites of hydroxyapatite (HA) and nanosize-alpha-(alpha)-Al2O3 with 5 wt.% CaF2 were coated on Ti-6Al-4V rods by cold pressing and sintered in air or argon atmosphere at 1100 degrees C to improve the bonding between the metal and the ceramic. 25 and 40 wt.% alpha-Al2O3-5 wt.% CaF2-HA composites gave the best results of strong bonding. 25 wt.% alpha-Al2O3-5 wt.% CaF2-HA and Ti-6Al-4V showed the same thermal expansion coefficients. The thermal expansion coefficient of the 40 wt.% alpha-Al2O3-5 wt.% CaF2-HA composite was slightly smaller than that of Ti-6Al-4V, which created a compressive stress on the coating, resulting in improved bonding between the metal and the ceramic

Hydroxylapatite-zirconia composites: Thermal stability of phases and sinterability as related to the CaO-ZrO(2)phase diagram

Composites of hydroxylapatite ( HA) with pure zirconia, and 3 and 8% Y2O3 in zirconia, were pressure-less sintered at temperatures from 900 to 1300 degrees C, and hot-pressed at 1200 degrees C in argon gas atmosphere for 1 h. The reactions and transformations of phases were monitored with X-ray diffraction and thermal analysis. At sintering temperatures higher than 1000 degrees C, calcium from HA diffused into the zirconia phase, and the HA phase decomposed to tri-calcium phosphate (TCP). Above about 1200 degrees C, CaZrO3 was formed. These reactions and transformations were interpreted in terms of the ZrO2-CaO phase diagram. On the other hand, zirconia and hydroxylapatite phases in hot pressed composite remained mainly stable suggesting that air in the sintering environment increased the reactivity between the phases. The highest densification was found in a composite initially containing 10% monoclinic ZrO2 sintered at 1300 degrees C. The densification of the composites decreased at lower sintering temperatures and higher zirconia contents upon air-sintering. (C) 2005 Springer Science + Business Media, Inc

New approach to the synthesis of nanocrystalline boron carbide

Herth S, Joost WJ, Doremus RH, Siegel RW. New approach to the synthesis of nanocrystalline boron carbide. JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY. 2006;6(4):954-959.The use of nanoparticles in ceramic matrix composites provides lower sintering temperatures and higher densities at a given temperature than common coarse-grained materials. Nanocrystalline B4C was synthesized by an inexpensive carbothermal reduction method using carbon black and B2O3 as precursor. Full conversion was achieved at 1623 K for annealing times of 480 minutes or with a large excess of B2O3 and oxidation of the remaining carbon after 30 minutes of annealing. The average particle size of the synthesized B4C powder was 260 nm, which was reduced to 70 nm after separation of the small particle fraction from the larger particles by sedimentation. A mixture of the as-prepared powder and commercial coarse-grained B4C yielded an increase of the density of low temperature hot pressed samples by 25% in comparison to pure commercial B4C. Possible chemical reactions and mechanisms in the synthesis of B4C were examined with the Gibbs free energies of reactions. The most likely reaction was the reduction of B2O3 vapor at the surfaces of the carbon particles after its vapor transport from the liquid B2O3. An observed reduction of B4C yield above 1623 K was probably caused by loss of B2O3 vapor from the reaction mixture