The paper examines the phase interaction in mechanical mixtures of thermostable hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) powders depending on the annealing temperature, initial composition, and particle size with x-ray interpretation of phase changes. It is established that HA transforms into β-TCP through a dehydration reaction during annealing of the mechanical mixtures of HA and β-TCP powders. The dehydration reaction proceeds through interphase activation in which the limiting stage of nucleation disappears. The phase boundaries in the two-phase HA-TCP mixture are more permeable for water vapors than single-phase boundaries owing to their disordered structure.

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The paper examines the compactability and sintering of nanosized synthetic hydroxyapatite (HA) powders for increasing the durability and strength of HA ceramics. The granulation of HA powders and limitation of the applied pressure prevent delamination in pressing. Porous HA samples show homogeneous nanoporous structure, high compactability, and excellent sinterability, which is better than that of nanosized powders.Calcium phosphate materials are used in surgery and other modern engineering areas to repair bone defects and place coatings on metal implants, for prolonged drug delivery and nonviral gene transfer, and as chromatographic sorbents, catalyst carriers, gas sensors, and ion conductors. The development of materials for such various applications involves controlling their structure and porosity based on the study of sintering and pressing processes. For example, the biological applications of calcium phosphates are based on hydroxyapatite (HA) Ca 10 (PO 4 ) 6 (OH) 2 , which is the closest analog of the bone mineral in chemical composition and crystal structure, and many studies on calcium phosphate biomaterials are focused on reproducing the morphology and structure of particles and pores found in natural bone [1].The main bone phase is represented by HA in which the Ca : P ratio is close to 1.5 (as different from stoichiometric HA in which Ca : P = 1.67), and primary particles constitute lamellar crystals about 1.5-3.5 nm × × 5.0-10.0 nm × 40.0-50.0 nm with complex nanostructure [2]. The volume fraction of these crystals is different: from 97% in dental enamel or 40% in the densest cortical bone to 10-20% in cancellous (spongy) bone. Collagen and other organic substances constitute the rest.Hydroxyapatite (either synthetic or organic from bones) is the main component of most biomaterials used to restore bone tissue. In spite of the great variety of inorganic biomaterials proposed recently, none of them can equal HA in the affinity to the living body, minimization of the body's negative reaction to the implantation of even substantial amounts of this material, osteoconductivity, i.e., ability to bond with bone and support bone growth, and rate and strength of bonding with bone. For many years of application, no difference has been noted in the body's