N. V. Petrakova

Effect of hot pressing temperature on the microstructure and strength of hydroxyapatite ceramic

A hot pressing method was applied for production of a dense ceramic with homogeneous structure and enhanced strength characteristics. Comparative investigations were carried out for properties of a ceramic obtained by traditional sintering in air and sintering under pressure in a temperature range of 900–1300°C. The hot pressing made it possible to decrease the temperature of achieving the vitrified state by at least 200°C. The microhardness of such materials at 1000°C is 4.2 GPa at an average crystal size of 80–150 nm and open porosity of 4%.

Low-temperature oxidation of MoSi2–Si3N4 composites

Composites in the MoSi2 + Si3N4 system containing 1.0, 2.5, and 5.0 wt % Si3N4 were synthesized by hot pressing at the temperature of 1650°C and the pressure of 30 MPa. Silicon nitride powders of two types of were used as reinforcing additives: one with isometric equiaxed crystals and one with fibrous structure. The samples were characterized by microstructural and phase analyses; the relative density and the flexural strength were determined. Composites with the flexural strength of up to 410 MPa were synthesized. The resistance to low-temperature oxidation of MoSi2 + Si3N4 composites in air at the temperature of 750°C was studied. The increase in the resistance to oxidation of composites with silicon nitride in air compared to pure molybdenum disilicide for both types of silicon nitride powders was established. For all the composites obtained, the parabolic oxidation rate constants were calculated.

Copper-substituted tricalcium phosphates

Copper-substituted tricalcium phosphates (CuTCP) with different copper contents were developed using precipitation of copper-containing amorphous calcium phosphates (ACP) from salt solutions followed by heat treatment. Porous CuTCP ceramic was obtained using negative replicas. Using a set of investigation methods (powder X-ray diffraction, IR spectroscopy, ESR spectroscopy, and scanning electron microscopy), all copper-substituted tricalcium phosphates were found to have the whitlockite structure with copper incorporated in TCP in the 2+ oxidation state. The resulting material is promising for the use in regenerative medicine owing to higher solubility in body fluids compared with TCP and combination of bactericidal properties and the lack of cytotoxicity.

Thermochemical gypsum conversion forming calcium phosphates

A study was carried out on effect of the conditions of physicochemical conversion of a porous gypsum intermediate product to calcium phosphates. Depending on the initial conditions of sample preparation, the materials around hydroxyapatite and dicalcium phosphate were obtained. The conducted thermal analysis and dilatometric studies made it possible to propose the conditions of thermal treatment preventing shrinkage and deformation of samples. After thermal treatment at 1000°C, fine crystalline porous materials were obtained with crystal size between 0.5 and 2 μm and pore size up to 50 μm on the basis of β-tricalcium phosphate with strength up to 2.8 MPa. The developed pore structure and fairly high strength make it possible to use the developed materials based on tricalcium phosphate to obtain biomaterials for the replacement of defects in bone tissue.

Molding Features of Silicon Carbide Products by the Method of Hot Slip Casting

The effect of the content of temporary process binder (paraffin, oleic acid, and beeswax) in the range of 10–18% on the molding of the silicon carbide products using hot slip casting has been studied. It has been determined that the flawless compact blanks based on SiC with Y3Al5O12 can be prepared at the TPB content of 14%. The conditions of four-stage heat treatment of the specimens during removal of TPB have been determined. The bending strength of the ceramic specimens prepared by sintering of the blanks at 1800°C is 437 ± 0.2 MPa.

Compaction of hydroxyapatite nanopowders by hydrostatic pressing

In this work, we show how the degree of compaction upon pressing of two hydroxyapatite nanopowders, whose average particle sizes are 18 and 65 nm, influences the density and strength of ceramics on their basis. The density and the flexural strength are found to be higher for a ceramic based on the powder with the particle size of 65 nm at the same density of precursors. The flexural strength of ceramic on its base was 20–25% higher than for ceramic of a powder with the average particle size of 18 nm at the identical density of precursors. The additional hydrostatic pressing increases the densities of specimens by 10–12%. The final hydroxyapatite ceramics are characterized by a fairly high bending density of 170 MPa and homogeneous structure with the average crystallite size of 0.4–0.6 μm.

Microstructure and properties of α-tricalcium phosphate-based bone cement

This paper examines the physicochemical properties and microstructure of brushite calcium phosphate cements possessing strength acceptable for application in surgery (15–20 MPa) and ensuring an optimal acidity (pH 6.5–7.5) of solutions in contact with them. Holding in a physiological saline produces significant changes in the microstructure of the cement relative to that before immersion in the solution: it causes a transformation of the most soluble components into platelike hydroxyapatite crystals.

Phosphorylated fabric containing particles of calcium phosphates and chitozane

The synthesis of calcium phosphate particles on cellulose fibers of cotton fabric is developed. The possibility of obtaining particles homogeneously distributed over the fiber surface through synthesis including the procedures of cellulose phosphorylation in orthophosphoric acid solution and treatment in calcium salts followed by treatment in ammonium hydrophosphate is established. The method can serve as a basis for the development of the technology of antiburn wound-healing bandages.

Ceramic calcium phosphate materials obtained by technology adapted to three-dimensional printing on inkjet printer

The formation of microstructure and the mechanical properties of medical application ceramic materials obtained by technology adapted to three-dimensional printing on an inkjet printer are investigated. The technology is based on the preparation of a calcium phosphate powder containing a uniformly distributed organic additive that glues together powders upon interaction with a solvent. The obtained raw intermediate product is burned to obtain a ceramic. Powders containing 10 wt % polyvinyl butyral whose solvent is ethanol had the best characteristics (powder friability, compression strength of raw product, and time of hardening after interaction with solvent).

A mechanism of removal of a glass envelope from a “thick” amorphous Co-alloy wire prepared by the Ulitovsky-Taylor method

Optical and scanning electron microscopy are used to study a mechanism of removal of a glass envelope from a “thick” (D = 96 μm) amorphous wire prepared from a model Co alloy by the Ulitovsky-Taylor method. It is shown that the mechanism of destruction and removal of the glass envelope during pulling of the wire through a cylindrical tool with a hard rough coating is determined by the type and level of stresses that act in the amorphous metal core. The stresses that occur in the longitudinal direction owing to the difference in the linear expansion coefficients of the metal and the glass provide an easy chipping of long (5–15 wire diameters) fragments of the glass envelope. The local stresses induced by the bending of the wire even in the elastic region contribute to the effective destruction of the remaining small fragments of the glass envelope along the directions of the preferred formation of shear bands. High bending stresses arising from the plastic deformation of the amorphous metal core increase the efficiency of removal of the glass envelope, but lead to a decrease in the mechanical and magnetic properties of the metal core owing to the nucleation and propagation of a network of shear bands.