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Solving the problems when developing high-temperature composite materials (HTCM) requires non-standard approaches, as for example, using the long-term high-heat treatment (HHT), which has a significant effect on mechanical properties of the HTCM at high temperatures. To obtain a niobium-based HTCM reinforced with α-Al2O3 single crystal fibers (“Nb – SCF α-Al2O3”) hot pressing technique was used in the research. The HHT effect at 1350 °C of the HTCM on its high-temperature (1300 °C) bending strength, hardness and density at 22 °C after 100 hours HHT with a step in 25 hours was investigated. The samples structure and elements distribution at the interfacial boundaries of HTCMs were studied. It was established that the elements interdiffusion width at the interphase boundary of the continuous composition “Nb – SCF α-Al2O3” doesn’t exceed 2 μm for the whole HHT term; in outside the interphase boundary, only niobium oxides and carbides were detected. It was found that the bending strength after 25 hours HHT slightly exceeded the strength of the initial sample (before HHT); with further high-temperature HHT, the strength increased by 1.7 – 2 times in comparison with the initial sample. The hardness (HV 0.5) after 25 hours HHT remained actually unchanged (70), and subsequently sharply increased and the average hardness in three aging stages (50, 75 and 100 hours) was 330. The density of HTCMs increased with HHT, and after 100 hours HHT increased in comparison with the original sample by 1.3 times.

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By using the magnetic resonance imaging method the nontrivial character of water absorption was demonstrated for the first time in highly porous ceramic materials. The effect of hygroscopic memory was found out which is that the preferable concentration of absorbed water in certain areas within the sample persists regardless the subsequent sample wetting history. Coating the oxide fibres with fluorine-containing hydrocarbons in supercritical CO2 in order to hydrophobize the material has been shown to affect substantially the water transport within the sample that can be referred to as an effective approach to protect the porous materials from humid environment. The results obtained demonstrate the advantages of the magnetic resonance imaging in studying the water absorption processes and visualization of water pathways in highly porous ceramic materials.