Ultrahigh- Amorphous highly conductive coatings Ti-Al-C, (Ti,Mo)-Al-C and (Ti,Cr)-Al-C were deposited on titanium alloy substrates by hybrid magnetron using T₂AlC and Ti₃AlC₂ MAX-phases-based targets and in parallel cathode-arc evaporation of Mo or Cr targets. The (Ti,Cr)-Al-C coating demonstrated the highest long-term oxidation resistance, and after heating in air at 600 °C for 1000 h, its surface electrical conductivity became even slightly higher after long-term heating: increased from s= 9.84×10⁶ S/m to s= 4.35×10⁵ S/m, which is explained by the crystallization of the amorphous coating during heating process. The nanohardness and Young's modulus of the coating after deposition were within 15 GPa and 240 GPa, respectively. The (Ti,Cr)-Al-C coating showed the highest electrochemical corrosion resistance among all deposited coatings in 3.5 wt.% NaCl aqueous solution at 25 °C: corrosion potential E_corr = 0.044 V vs. saturated calomel electrode, corrosion current density i_corr = 2.48×10^-9 A/cm². The hybrid magnetron deposited (Ti,Cr)-Al-C coatings can be used to protect interconnects in lightweight molten carbonate fuel cells elements.

Ultrahigh-temperature, corrosion-resistant materials based on HfB₂ (melting point of HfB₂ - 3380 ^oC) have high thermal conductivity, high level of mechanical characteristics, high corrosion resistance in oxidizing atmosphere due to the ability to form protective, oxidation-resistant scales at elevated temperatures. They are promising for many ultrahigh-temperature applications, for example, for the manufacture of nozzles for aircraft and rocket engines that are in contact with aggressive gases at high temperatures, as well as for the manufacture of wing edges and fairings for supersonic aircraft, etc. It is known that the addition of SiC to HfB₂ can increase the mechanical properties of composite. The results of present investigations (obtained in the framework of III-5-23 (0786) grant from the National Academy of Sciences of Ukraine) showed that on the densification, mechanical characteristics and resistance toward ablation important role play sizes and quality of SiC initial powder used as addition. Such effect we observed both for the composites prepared under hot pressing conditions (30 MPa pressure) and conditions of high pressure (2 GPa) – high temperature. Our previous studies have shown that the use of high pressures and temperatures and hot pressing, and the addition of SiC to HfB₂ allowed us to achieve a level of mechanical properties of the resulting ceramic materials that, in terms of hardness and crack resistance, surpass the best world analogues. It was also shown that the addition of SiC significantly reduces the melting point and accelerates the oxidation kinetics upon heating. The microhardness, H_V, and fracture toughness, K_1C, (at an indentation load of 9.8 N) of the HfB₂-30 wt.% SiC(5-10 µm) composite material which was hot pressed (under 30 MPa) were H_V =38.6 ±2.5 GPa and K_1C =7.7 ±0.9 MPa m^0.5 when specific density 6.54 g/cm³ (and near zero porosity) was attained. For HfB₂-30 wt.% SiC(30-50 µm) porosity was about 17 % and H_V = 28.1 ±11.3 GPa and K_1C = 6.1 ±2.2 MPa m^0.5. Hot-pressed HfB₂ without additives exhibits H_V = 18.9 ±0.1 GPa and K_1C = 7.65 ±0.6 MPa·m^0.5, porosity 2.4% and specific density 10.79 g/cm³. Ablation tests in air of the samples of ultrahigh-temperature hot-pressed ceramics based on HfB₂ and HfB₂-SiC when heated with a gas burner (into which an O₂/C₂H₂ mixture was fed, and the distance to the sample surface was 13 mm) showed that HfB₂ ceramics with an additive of 30% by weight of SiC with a grain size of 30-50 μm and 5-10 μm turned out to be significantly more stable (up to 2066-2080 °C, respectively, at an internal mass of 0.25 mg/s) than ceramics with HfB₂ without the additive (cracked at 1870 °C). 

The sintering processes of TaB₂ and TaB₂ mixtures with 20 and 30 wt. % SiC, ZrSi₂, Si₃N₄, and MoSi₂ were investigated under hot pressing (HotP) conditions at 30 MPa, 1750-1970 °C, 0.33 - 1.0 h, and high pressure–high temperature (HP-HT) conditions at 4.1 GPa, 1800 °C, 0.33 h,, as well as TaB₂ and its mixtures with 20 and 30 wt.% SiC under spark plasma sintering (SPS) at 45 MPa, 1500-1950 °C, 0.05 h. The highest values of mechanical characteristics of single-phase TaB₂ samples were achieved after sintering by the HotP (1900 °C, 1 h) – Vickers hardness Н_V(9.8 N) = 32.4 ± 0.1 GPa (density r =11.8 g/cm³) and SPS (1950 °C, 0.05 h) - Н_V(49 N) = 20.8 ± 2.0 GPa and K_1C(49 N)= 7.6 ± 1.6 MPa•m^0.5 (r =11.75 g/cm³). A significant improvement in Young's modulus from 532 GPa to 853 GPa was achieved by adding 20 wt.% SiC and HotP at 1900, 1 h. By sintering mixtures with 30 wt.% SiC using the HP-HT and SPS methods at 1800 °C for 0.13 and 0.05 h, respectively, the following materials were obtained: with Н_V(9.8 N)= 39.4 GPa and K_1C(9.8 N)=6.75 MPa•m^0.5 (HotP) and Н_V(49 N)=25.4±2.1 GPa and K_1C(49 N)=10.8±0.8 MPa•m^0.5 (SPS). The variation in the properties of the materials upon addition of additives is explained by the formation of solid solutions due to the diffusion during sintering of the present elements and different porosity. When adding 30 wt.% SiC after HotP (1900 °C, 1 h), the approximate stoichiometric composition of the matrix phase of the sample estimated by SEM EDX was TaB₂Si_0.5O_0.06.

The results of influence of amount of SiC additives to HfB₂ and the physical chemical characteristics of the additives will be under the discussion. Studies of the resistance to ablation of hot-pressed HfB₂ and HfB₂-SiC samples heated by a gas burner showed that HfB₂ ceramics with the addition of 30 wt.% SiC with average grain sizes of 30-50 μm (powder with fragmented grains with sharp edges with approximate average stoichiometry SiC_1.6O_0.1, and  6_H SiC structure) and 5-10 μm (single-crystal grains with a hypercubic shape, close to spherical, practically free of impurities, with approximate stoichiometry SiC_1.5, b-SiC) have significantly higher thermal resistance – up to temperatures of 2766 and 2780 °C, respectively (mass loss of 0.25 mg/s) than HfB₂ ceramics without additives, samples of which cracked already at 1870 °C. The formation of a framework from SiC when 40 wt.% SiC was added resulted in decrease of resistance to ablation, of Young's modulus, and the material cracking at low temperature during heating in air. The composite made from a mixture of HfB₂ - 30 wt.% b-SiC (5-10 μm) by hot pressing under a pressure of 30 MPa, 1950 °C, 30 min. with a specific gravity of 6.54 g/cm³ demonstrated the highest Vickers microhardness H_V(9.8 N)=38.6±2.5 GPa and fracture toughness, K_1c(9.8 N)=7.7 ± 0.9 MPa m^0.5, Young's modulus 510 GPa. The additions of SiC_6H with sharp fragment grains of 1 μm in size with a lamellar or strongly elongated in one direction grains, with an approximate stoichiometry of SiC_4.6O_0.75 or 3-10 μm with an approximate stoichiometry of SiC_2.3O_0.25 added in the same amount (30 wt.%) were cracked during heating in air at a temperature of 1787 and 1455 °C, respectively.

Our previous study has been shown that treatment of coated conductors (CC) under high pressure - high temperature conditions can lead to further increase of critical current density especially in low magnetic fields [1]. Treatment under 100 bar of oxygen for 3 h of GdBCO_CC via Ag layer (which covered superconducting layer of CC) at 600 °C led to an increase in Jc (77 K, 0 T) from 2.57 to 2.67 MA/cm². The charge carrier density n_H(100 K) increased from 6.55´10²¹ to 6.91´10²¹ cm^-3, and  J_c(5 K, 0 T) =28.94 MA/cm² was observed after the treatment. The increase in J_c (77 K, 0 T) from 2.10 to 2.28 MA/cm² for GdBCO_CC which was oxygenated without Ag layer (etched by acid before oxygenation) was observed after treatment at 300 °C under 100 bar of O₂ for 3 h. In the both cases c-parameter of Gd123 decreased from 1.1735(1) to 1.1731(0) nm. The increase of critical current density is connected with overdoping by oxygen and thus by charge carriers of the superconducting GdBCO layer. Oxygenation under 160 bar pressure at 800 ^oC for 3h of tetragonal YBa₂Cu₃O_x film deposited on single crystalline strontium titanate substrate allowed to obtain critical current density J_c (77 K, 0 T) = 4.09 MA/cm² and J_c(5 K, 0 T) = 38.2 MA/cm². The value of Jc (77 K, 0 T) turned out to be twice as high as when saturated with oxygen under a pressure of 1 bar. As a result of oxygenation, c-parameter of YBa₂Cu₃O_x decreased from 1.1711(2) down to 1.1681(3) nm.  This work was supported in part by the funds from MICIU/AEI/FEDER for SUPERENERTECH (PID2021–127297OB-C21), FUNFUTURE “Severo Ochoa” (CEX2019–000917-S); MUGSUP (UCRAN20088) project from CSIC scientific cooperation with Ukraine; Catalan Government 2021 SGR 00440; and NAS of Ukraine Project III-7-24 (0788).

The AlN-based dielectric composite materials with high resistivity values ​​are promising for use in electronics. However, ensuring a sufficient level of their mechanical characteristics is no less important condition for the practical use of products from these composites. For composites of the AlN-C-ZrB2 and AlN-C-TiN systems with resistivity >10⁹ Ohm, which were manufactured under hot pressing conditions at a temperature of 1900 °C and a pressure of 12 MPa, mechanical characteristics, in particular hardness and fracture toughness, were studied. 

Using a FALCON 500 microhardness tester with an optical camera, the Vickers microhardness was determined at a load of 98 N, and the fracture toughness of composite materials was also calculated taking into account the sizes of cracks emanating from the corners of the pyramid imprint. 

The obtained results of the measurements of the mechanical characteristics indicate a slight decrease in the hardness of the AlN-C-ZrB₂ and AlN-C-TiN composites in contrast to the aluminum nitride composite without additional components. The hardness value of AlN-C-ZrB₂ ceramics is 7.99±0.14 GPa, and AlN-C-TiN ceramics is 8.77±0.48 GPa, while for AlN ceramics the HV value is at the level of 11.34 ± 0.7 GPa. It is noted that for the material with a higher hardness value, the level of fracture toughness is also higher and is 5.01±0.34 MPa•m^1/2, while K_1C for the other composite, like the hardness value, is expectedly lower - 4.68±0.3 MPa•m^1/2. The adding components to aluminum nitride to improve electrodynamic characteristics results in a slight decrease in mechanical characteristics, but their level is high enough to withstand loads during material processing or when operating in vibration conditions.