We are the first to perform experiments on strengthening plasma coatings on a cylindrical surface by friction stir processing with two oppositely located tools. A 65Kh25G13N3 steel powder (Fe–23.7

The structure and microhardness of an austenitic steel coating plasma sprayed onto a cylindrical surface are studied after friction stir processing (FSP) with two steel tools at a pressure of 30 MPa. During FSP, the coating temperature is found to increase to 1235°C, which makes it possible to plastically deform the coating and to compact it by destroying the initial structure and welding sprayed particles. As a result of FSP, the microhardness of the coating increases from 3.2 to 4.6 GPa.

The content of oxygen and nitrogen has been determined in plasma coatings sprayed from the powder of Co and its alloys Co–32Ni–21Cr–8Al–0.5Y and Co–27.9Cr–7.04Al–3.25Si–2Y (wt

The structure of hydroxyapatite (HA) plasma coatings obtained by changing the power of the plasma torch (P) from 20 to 26 kW was studied by X-ray diffraction analysis. All coatings contained the decomposition products of hydroxyapatite in the plasma flow, phases of tetracalcium phosphate (TTCP) and CaO. Their number, which increases with increasing P and plasma temperature (Tp), is determined by the partial loss of phosphorus by the initial HA powder in plasma and the shift in the chemical composition of the coating to higher calcium contents. An increase in P and Tp is accompanied by an increase in the intensity of the HA basic texture. In the range of 24 < P < 26 kW, the intensity ratio of X-ray reflections HA δ = I(200)/I(211) exceeds 3.0 in its value, while at P = 20 kW δ <1.5. The presence of orientation relationships between the predominant crystallographic orientations of HA and TTCP crystals in coatings is associated with the similarity of their structures, the proximity of lattice periods, and the presence of identical fragments in their structures. The possibility of the formation of mixed-layer structures involving these compounds is discussed. The following factors are considered as the factors that have the greatest influence on the nature of the dependences obtained: the substrate temperature, the particle size in the initial HA powder, and the composition of the plasma gases.

Four plasma cermet coatings with similar compositions based on TiC carbide and NiCrMo matrices with additional Cr3C2, WC carbides, and carbon were studied. The average oxygen content for four cermets increases from 0.51 % for powders to 0.86 % for coatings with a minimum plasma power and does not increase with its increase. The maximum average nitrogen content in the coatings, 0.34 %, is determined by the nitrogen content in the plasma. When spraying coatings, the loss of carbon, relative to the initial powder composition, is 2.79 – 3.76 %, less than in the manufacture of powders for spraying 4.3 – 6.6 %. Carbides Cr3C2, WC, matrix elements and the content of additional carbon determine the formation of the annular zone around TiC carbide, increase the total carbide content in the coating from 60 to 74 – 83% and the microhardness of the coating is 18 GPa with an indenter load of 200 gf.

Studies were carried out on the oxygen and nitrogen content of nickel-based powders and plasma coatings therefrom. The Ni-based sprayed materials were divided into four groups according to the mechanism of their interaction with oxygen. The first group includes Ni, the second alloys alloyed with Cr, the third additionally alloyed with Al, the fourth alloys having more complex alloying with Cr, C, B, Si elements that actively interact with oxygen, including with the formation of gaseous oxides. Alloying 20 % Cr reduces the oxygen content of the coating by 35 %, alloying 17 % Cr – 10 % Al reduces the oxygen content of the coating by 57 %, more complex alloying 13.5 % Cr – 2.7 % Si – 1.65 % B – 0.36 % C — reduces the oxygen content of the coating by 92 %. Increasing the plasma jet power increases the oxygen content of the Ni20Cr coating from 0.55 to 1.6 %. Increasing the plasma jet power and preheating the substrate prior to sputtering results in a limited increase in the oxygen content of Ni – 17 % Cr – 10 % Al – 1 % Y and Ni – 15 % – Cr 4 % – Fe 0.8 % C – 4.1 % Si — 3.1% B coatings using a plasma nozzle to eliminate the thermal effects of the plasma jet. The Ni – 17 % Cr – 10 % Al – 1 % Y coating sprayed with the nozzle contains 0.48 % O and 0.14 % N instead of 2.31 % O and 0.37 % N when sprayed without the nozzle. An increase in the oxygen and nitrogen content in the coatings Ni, Ni – 20 Cr and Ni – 40 Cr together with the formation of a liquid-hardened structure in these coatings determines an increase in microhardness relative to the value for the powder by 1.6 – 1.9 times.

We have developed and investigated the process of friction treatment of a plasma coating made of R6M5 steel on a cylindrical substrate. Processing for up to 70 s is carried out by cyclic application of a pressure of 30 MPa by two tools made of R18M5 steel on a coating rotating at 900 rpm, including with additional movement of the tools along the generatrix of the cylindrical substrate. As the friction treatment time increases, the surface temperature of the coating rises to 1202 °C, which is sufficient for plastic deformation of the coating material. The microhardness of the coating after plasma spraying is 3.13 GPa after friction treatment increases to 7.64 GPa. A large degree of deformation of the upper layers of the coating under the action of tools determines the increase in the microhardness of the coating from the substrate to the free surface from 5.85 to 7.64 GPa.

The analysis of the literature on bioactive surfaces of volume implants and plasma coatings on their surface is performed. The influence of the porous sizes on the process of growing new bone in an implant surface is described. The traditional porous structure of a plasma titanic coating, which is formed owing to a decrease in extent of deformation of spraying particles on a substrate, is not optimum for growing of a new bone tissue. Modern dense hydroxyapatite (HA)-titanium coatings do not have the necessary porous structure, and transfer of cyclic loading through an external layer is not reliable. At the same time, comparative studies of 17 surfaces of implants indicate obvious advantages of plasma HA coatings during implantation, which are not dependent on the degree of their crystallinity. Three-dimensional capillary-porous (TCP) titanium coatings in the form of ridges and cavities with additional HA plasma and microplasma coatings are prospective for medical application owing to division of dense and porous volumes of a coating, increase in the area of the border with a new bone tissue, higher shear, and the possibility of spraying of the HA coating with equilibrium structure at a temperature of the surface of titanium of 550°C. The analysis of HA coating content phase is made depending on the modes of plasma spraying.

We have analyzed the surface of a plasma three-dimensional capillary-porous (TDCP) Ti coating with an additional bioactive hydroxyapatite coating formed by microplasma oxidation (MPO). Scanning electron microscopy (SEM) was used to analyze the open porosity of these composite coatings. The surface of the TDCP Ti coating consists of ridges and valleys. The main porosity of the TDCP Ti coating is concentrated within the volume of valleys and reaches 43%. The presence of macro- and micropores 0.08–600 μm in size makes TDCP Ti coatings promising for using in a variety of fields, including as the surface coatings of intraosseous implants.

— Relatively thin (~30 μm) and thick (100 μm) plasma-sprayed hydroxyapatite (HA) coatings grown on unheated and preheated (550°C) titanium substrates have been characterized by X-ray diffraction and scanning electron microscopy. The microstructure of the coatings grown on unheated substrates contains a considerable density of defects and is fragmented. The coatings grown on preheated substrates have a denser, perfect microstructure. The presence of the CaO and tetracalcium phosphate phases in the thick coatings grown on preheated substrates suggests that the temperature in the spraying zone approaches the partial decomposition temperature of HA. The formation of a well-defined basal texture ( I (002)/ I (211) = 2.72) is favored by the high temperature maintained during the growth of these coatings. We discuss the effect of specific structural features on the strength of coating–substrate bonding and analyze the correlation between the texture factor and coating adhesion/cohesion.

Five cermet coatings based on carbides 45TiC – 10Cr3C2 – 5WC with different contents of additional carbon were formed by plasma spraying with local protection: 0; 1.4; 2 and 2.8 %. In four cermets, the matrix was based on Ni – 20Cr. In one cermet, the alloy used was 38.5Co – 32Ni – 21Cr – 8Al – 0.5Y. All matrices were additionally introduced Mo. Powders for spraying were obtained by crushing cakes. In the particles of the obtained powders, carbides are distributed relatively uniformly; in coatings, this is noticeable to a lesser extent. After liquid-phase sintering, WC and Mo are not fixed in cermets; part of the Cr3C2 carbide passes to another structural state. The initial carbides in the cake and coating partially dissolve and, upon solidification and together with matrix elements and additional carbon, form an annular zone around the initial TiC carbide, decreasing its lattice period, X-ray fixes TiMoC2 carbide, the content of which is higher than the content of TiC carbide in the initial mixture. The content of the initial carbides in the coatings, measured by optical microscopy, decreases from 71 vol.% In the powder to 48 vol.% At the minimum plasma power and up to 36 vol.% At the maximum power. The average total TiMoC2 content of carbides in coatings according to x-ray data for four cermets is 76 %, higher than their content in spraying powders, 72 %, due to higher spray hardening rates. The average microhardness for all coatings is 22.01 GPa with an indenter load of 20 gf, which is lower than the average microhardness for all powders, 23.51 GPa. With an indenter load of 200 gf, the average microhardness for all coatings of 15.88 GPa corresponds to the average microhardness for all powders, 15.17 GPa.

Three-dimensional capillary porous (TCP) plasma coatings, which are formed by spraying a bronze powder with a particle size of 20–32 and 71–100 μm on threaded profiles with angles of 34° and 60° and a depth of 2 and 1.85 mm, respectively, have been developed and studied. The coatings are designed to form effective heat-exchange surfaces with a change in the state of aggregation of a refrigerant. TCP coatings with a porosity of 15–32%, a thickness of up to 400 μm, a ridge width up to 160 μm, and a valley width up to 100 μm are studied. The structure of a bronze TCP coating makes it possible to increase the contact line normalized relative to the initial thread line to 14.805 and that relative to the cylindrical surface before threading to 24.24.

Phosphate coatings have been produced by plasma-spraying hydroxyapatite (HA) and tricalcium phosphate (TCP) powders onto Ti substrates at initial temperatures of 20, 300, and 550°C, followed by hydrothermal treatment (HTT) at 650°C, and the variation in the phase composition of the coatings has been examined in relation to the phase composition of the plasma-sprayed powders: 100 wt % HA, 100 wt % α‑TCP, and 100 wt % β-TCP. The as-prepared coatings produced by plasma-spraying the HA powder consisted of 87–91 wt % HA and 9–13 wt % CaO, and after HTT their phase composition was 89–93 wt % HA and 7–11 wt % CaO. The coatings produced by plasma-spraying the α-TCP powder at initial substrate temperatures from 20 to 550°C consisted entirely of a crystalline α-TCP phase. The coatings produced by plasma-spraying the β-TCP powder consisted of both β-TCP and α-TCP, and the content of the latter phase decreased from 100 to 80% as the substrate temperature was raised from 20 to 550°C. After HTT, the coatings contained 26–28% HA, independent of the phase composition of the starting TCP powders.

An analysis of the surface of a plasma three-dimensional capillary-porous (TCP) Ti coating with an additional bioactive hydroxyapatite coating formed by microplasma oxidation (MPO) has been performed. Scanning electron microscopy (SEM) analyzed the open porosity of these composite coatings. The surface of the TCP Ti coating consists of ridges and depressions. The porosity of the TCP Ti coating of the depressions reaches 43 %. The presence of macro- and micropores with a size of 0.08 – 600 µm makes the TCP Ti coatings promising for use in a number of areas, including on the surface of intraosseous implants.

Porous coatings are actively used in engineering practice. The porous coating determines the reliability of the operation of the intraosseous implant and the heat transfer process when the aggregate state of the refrigerant is changed. The choice of method for quantitative analysis of porosity is determined by the structure of the coating. In this study, plasma coatings sprayed from powders were analyzed. A porosity of 10.3% inside the alumina coating was analyzed by mercury porosimetry. The main volume of the porosity of the coating is formed by pores ranging in size from 0.13 to 0.36 μm; their share in the total volume is 68.29%. The remaining volume is distributed in sizes of 0.04–0.12 mm, 0.58–4.66 mm, and 5.66–18.2 mm. Microtomography makes it possible to get a more complete general idea of the macro- and microstructure of coatings, establishing the mechanisms of its formation, and to obtain data on the real shape of the pores. A quantitative description of the visible pores of three-dimensional capillary-porous 3CaO⋅Al 2 O 3 coatings in the form of ridges and depressions was determined by raster image analysis using special STIMAN programs. The porosity of this coating is 39.7% of the distribution over four pore groups: 0.74–3.56, 4.34–11.57, 14.08–55.65, 67.73–267.71 mm.

Plasma coatings of hydroxyapatite (HA) were formed on Ti substrates in modes to obtain high mechanical properties, structural stability, and phase composition. Preheating the titanium substrate to 550°C increases the content of the equilibrium HA phase in the coating to 92%. By the DSC method, there is no local thermal effect of heat release at 723°C, as in the case of a coating sprayed onto an unheated substrate, and there is no halo in the X-ray diffraction pattern in the region of the main HA reflections. Hydrothermal treatment (HTT) of the HA coating at 650°C increases the HA content to 98%, regardless of the temperature of the preheating of the Ti substrate. Regardless of the state of the coatings, there is a gradual release of heat in DSC studies in the range of 450–1000°C, which increases after hydrothermal treatment. This phenomenon requires additional research. The crystallite size in the sprayed coatings of 42.1–43.1 nm increases to 64.4–68.3 nm after HTT is comparable to the crystallite size of 57.4 nm in the sprayed powder. After HTT of coating, the tricalcium phosphate phase is absent.

The microstructure and microhardness of eleven volumetric cermets based on TiC carbide with nickel- and cobalt-based matrices after liquid-phase sintering at a temperature of 1400°C were studied. It is proposed to use the research results for the subsequent production of powders for plasma spraying of coatings. The compositions of the matrices and the contents of additional hardening phases and carbon were selected taking into account the specific features of the formation of plasma coatings: a decrease in the carbon content and high solidification rates of the sprayed particles with the formation of additional nanosized carbides and an increase in the volume fraction of carbides from 70 to 88%. The traditional composition for cermets with TiC carbide, NiCr–Mo, and industrial powders such as PG-SR2 grade (PN-CrNi80Si2B2) with the composition (wt %) Ni, 13.5 Cr, 2.7 Si, 4.5 Fe, 0.37 C, 1.65 B and TAFA 1241F grade with the composition (wt %) Co, 32 Ni, 21 Cr, 8 Al, 0.5 Y were used as the matrix. The ring zone on TiC carbide is formed by sintering with the participation of WC, Cr 3 C 2 , TiN, matrix phases, and 1–2.8 wt % of additional carbon in the composition of cermets. As a result, the initial volume fraction of TiC carbide increases from 70 to 88%. Additional carbon is also consumed to decrease the oxygen content at the sintering stage (the reduction of oxides). Upon sintering, the cermets have high microhardness values of 1940–3210 kgf/mm 2 at an indentation load of 20 G and lower values at an indentation load of 200 G, which can be explained by the scale factor. The calculated maximum contribution of the hardness of the hardening phases to the hardness of the cermet was assessed for cermets with a Co matrix at 3681 kgf/mm 2 .

Hydroxyapatite (HA) coatings were sprayed by an arc plasma gun with argon-nitrogen plasma at the power of 25 kW from the powder with particle size of 25–63 μm at the distance of 95 mm. Before spraying of the coatings, the samples were preheated in a resistance furnace in air to the temperatures within the range from 20 to 600°C. Adhesion of the HA plasma coating to a titanium substrate was determined on pin samples. The maximum mean value of adhesion was observed when the titanium substrate was preheated to the temperature of 550°C. The results of the study were discussed by reference to the way of increase in activity of the titanium substrate at its preheating for increase in the HA coating adhesion and formation of an equilibrium phase state in the HA coating necessary for long-term usage of the implants. The obtained results will be used for formation of the optimal structure of the bioactive coating consisting of a three-dimensional capillary-porous titanium coating (3D CP Ti) in the form of crests and hollows with porosity of 50% and HA coating sprayed on its surface at the temperature of 550°C. Such mode of the spraying provides formation of a dense, strong, and stable HA coating on endosseous implants.

In this study, biocompatibility of plasma-sprayed 3D CP Ti-coated wire implants has been evaluated. Plasma sprayed 3D CP Ti coatings have a porous structure consisting of ridges and cavities whose height is equal to that of the thickness of the coating. The porosity of plasma-sprayed 3D CP Ti coatings was found to be higher in cavities and it made up 46%. To study the clinical success of these implants, post-mortem computed tomography (CT) of distal femurs of animals (mainly dogs) was carried out. In vivo studies failed to demonstrate an increase in the amount of new bone tissue around uncoated implants. The amount of new bone tissue around implants with additional ceramic coatings made up 60% after 8 weeks. The amount of new bone tissue around 3D CP Ti-coated implants was only 5% after 8 weeks; however, after 16 weeks, an intensive growth of bone tissue around implants was observed. The amount of new bone tissue around implants with an additional ceramic coating was insufficient after 16 weeks. Forty-eight weeks after implant placement, the distribution of bone tissue in 3D CP Ti, 3D CP Ti-HA- and 3D CP Ti-HA-CaP-coated implants was 74.5%, 89.2% and 92.5%, respectively. The amount of bone tissue around uncoated implants did not exceed 10%. 3D CP Ti-HA coatings demonstrated better osseointegration compared with 3D CP Ti coatings and were found to have higher shear strength (9.8 +/- 1.54 MPa) at early stages of osseointegration.