The paper presents the main results of development and optimization of the synthesis of hydroxyapatite and the application of the micro-plasma spraying technique for biocompatible coatings. The hydroxyapatite synthesis was optimized using the mathematical modelling method. Synthesized hydroxyapatite was studied by IR spectrometry and X-ray diffraction analysis for assessment of the compatibility of the chemical and phase composition to the bone tissue. The Ca/P ratio of the obtained hydroxyapatite was 1.65, which is close to that of bone tissue (1.67). To increase the adhesion strength of the HA coating to the surface of the titanium implant, it was suggested to apply a titanium sublayer to the implant surface. Microplasma spraying (MPS) of biocompatible coatings from titanium wires and synthesized HA powders onto substrates made of medical titanium alloy has been carried out. Microplasmatron MPN-004 is used to obtain the two-layer coatings for titanium implants. The two layer coating includes a sub-layer of a porous titanium coating with a thickness in range from 200 up to 300 μm and the porosity level of about 30%, and an upper layer of HA about 100 μm thick with 95% level of HA phases and 93% level of crystallinity. The pore size varies from 20 to 100 μm in both coatings. The paper describes the technology and modes of microplasma deposition of two-layer coatings, including the mode of gas-abrasive treatment of the surface of implants made of titanium alloy before spraying. The synthesized HA powder and the Ti/HA coatings were investigated by optical microscopy and scanning electron microscopy with the energy dispersion analysis and the X-ray diffraction analysis.

This paper presents new results of microplasma spraying (MPS) of laboratory-synthesized hydroxyapatite (HA) powder coatings onto trabecular substrates obtained by selective laser melting (SLM) of a certified titanium medical alloy powder. The aim of the study was to establish the possibility of combining the technologies of MPS and additive manufacturing (AM) for the possible production of custom-designed implants with increased surface biocompatibility, as well as to establish the MPS parameters that ensure chemical purity of the HA coating and satisfactory adhesion of the coatings to the substrate. The structural-phase compositions of the initial HA powder and the plasma-sprayed HA coating were studied by X-ray diffraction analysis and transmission electron microscopy, and the adhesion strength of the coating was tested according to the F1147 standard of the American Society for Testing and Materials (ASTM). The main results of the study are the following: the application of the MPS technology for HA coating with an average thickness of 150±50 μm on trabecular substrates obtained by the SLM method has been shown. The parameters of MPS of HA coatings onto titanium implants with a trabecular surface have been established. It is also proved that using the appropriate MPS parameters, it is possible to obtain a HA coating with a 95% level of HA phases, 93% level of crystallinity, and the adhesion strength to the trabecular substrate of 24.7±5.7 MPa, which complies with the requirements of the international medical standard (International Organization for Standardization [ISO] 13779-2:2018). These results are of significance for a wide range of researchers developing plasma spray technologies for the manufacture of biocompatible coatings.

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The deformation twinning leads to a reduction of the coefficient of strain hardening, i.e. twinning plays an important role in deformation of steel. The contribution of twinning to the total deformation is significant (up to 1/3 of the overall deformation). The final effect of twinning on strain hardening rate depends on the number of factors introduced by the twinning itself in the process of sliding. Calculated Schmid factors for slip systems operating in the matrix and in the twin. The analysis of the twinning geometry in particular grains showed that twinning is primarily developed for systems with the highest Schmid factor. However, twinning was also observed with a minimum and even zero Schmid factor. The obtained results indicate the importance of the orientation of slip planes in the newly formed micro-twins and Schmid factors for twinning. The important role of crystallographic texture in the physics of strain hardening was also shown.

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