Artem Yu. Potanin

A multiphase precursor powder was synthesized by self-propagating high-temperature synthesis (SHS) using Si ₃ N ₄ as a solid nitrogen source in the Ta–Si ₃ N ₄ –C system. Combustion occurred at 1910 °C, and the combustion-front velocity was 1.42 mm/s. The SHS products comprised tantalum carbonitride Ta(C,N), Ta ₅ Si ₃ silicide, and minor amounts of the intermediate phases TaC and Ta ₂ N. Dense Ta(C,N)–Ta ₅ Si ₃ –Si ₃ N ₄ –SiAlON ceramics were fabricated by hot pressing in nitrogen with Si ₃ N ₄ and YAG as sintering aids. X -ray diffraction, scanning electron microscopy, and transmission electron microscopy showed a homogeneous microstructure composed of tantalum carbonitride grains (up to 2 µm), Ta ₅ Si ₃ silicide (up to 3 µm), and β-Si ₃ N ₄ (up to 5 µm); amorphous Y–Si–Al–O–N intergranular films formed along silicon nitride grain boundaries and promoted liquid-phase sintering. The ceramics exhibited a hardness of 17.3 GPa, fracture toughness of 5.9 MPa·m ^0.5 , flexural strength of 410 MPa, and thermal conductivity of 13.26 W/(m·K). Pin-on-disk tests showed that increasing the sliding velocity from 0.3 to 0.8 m/s reduced the coefficient of friction from 0.82 to 0.72 and halved the specific wear rate. This behavior was attributed to a shift in the dominant wear mechanism from fatigue–abrasive wear to tribochemical wear, accompanied by the formation of AlTaO ₄ and SiO _x –TaO _y oxide phases. These results indicate that the developed ceramics are promising for tribological components subjected to high contact loads and sliding velocities.

This work focused on the production of the MoAlB MAB phase through self-propagating, high-temperature synthesis in the thermal explosion mode. The influence of the method of a Mo-Al-B-powder reaction mixture preparation on the combustion temperature, mechanism, and stages of the MAB phase formation in the combustion process was investigated. The combustion temperatures of the mixtures obtained in the rotary ball mill and high-speed planetary ball mill were 1234 and 992 °C, respectively. The formation of intermediate compounds Mo₃Al₈ and α-MoB in the combustion front, along with MoAlB, was established using the time-resolved X-ray diffraction method. In the case of the mixture prepared in a ball mill, the primary interaction in the combustion front occurred through the Al melt, and in the case of using a planetary mill, solid-phase reactions played an important role. The mechanical activation of the mixture in a planetary mill also accelerated the processes of phase formation. The method of a reaction mixture preparation has virtually no effect on the MoAlB MAB phase content in combustion products (92–94%), but it does affect their structure. The synthesis products have a lamellar structure composed of MAB grains with a thickness of ~0.4 μm and a length of ~2–10 μm.

The paper focuses on the study of the combustion kinetics and mechanisms of elemental mixtures in the Zr–Mo–Si–B system, as well as the analysis of phase and structural transformation stages in the combustion wave. A thermodynamic analysis of potential chemical reactions occurring in the combustion wave was carried out. The reaction of ZrB₂ formation is preferred in the range of 298–2500 K. Above 2200 K, the formation of MoB becomes more thermodynamically advantageous as compared to MoSi₂. Phase stability estimates of combustion products showed that ZrB₂, MoSi₂ and MoB phases are in equilibrium. Experimental dependences Т_c(Т₀) and U_c(Т₀) are linear, which implies an unchanged combustion mechanism at T₀ = 298÷800 K. Preheating leads to an increase in U_c. Similarly, an increase in the proportion of Zr and B in the mixture has a similar effect, i.e. an increase in heat emission and T_c. With a minimum content of Zr and B, the interaction between Mo and Si with the formation of MoSi₂ by the reaction diffusion mechanism is decisive. As the proportion of Zr and B increases, the rise of T₀ to 750 K does not affect the T_c. E_eff values (50–196 kJ/mol) confirm the significant influence of liquid-phase processes on the combustion kinetics. The mechanism of structure formation was studied. A Si–Zr–Mo melt is formed in the combustion front. The primary grains of ZrB₂ and MoB crystallize from this melt as it is saturated with boron. At the same time, the melt spreads over the surface of Zr and Mo particles. This leads to the formation of ZrSi_x, MoSi_x films. Core-shell structures are formed behind the combustion front, which disappear as they move towards the post-combustion zone. The phase composition of products is formed in the combustion front in less than 0.25 s.

Работа посвящена исследованию кинетики и механизмов горения реакционных смесей в тройной системе Mo–Al–B, рассчитанных на образование MAB-фазы состава MoAlB. Показано влияние начальной температуры на основные параметры процесса горения. Установлено, что предварительный подогрев реакционной смеси слабо влияет на максимальную температуру горения. Рассчитано значение эффективной энергии активации процесса самораспространяющегося высокотемпературного синтеза (СВС). С применением баз данных AFLOW и Materials Project построены фазовые диаграммы в системе Mo–Al–B. Изучены фазовый состав и структура синтезированной керамики, в которой основной составляющей являются пластинчатые зерна MoAlB толщиной 0,4 мкм и длиной 2–10 мкм. Также идентифицированы линии промежуточных боридов MoB и Mo₂B₅, суммарная доля которых не превышает 3 %. Методами сканирующей электронной микроскопии и энергодисперсионной спектроскопии в межзеренных порах установлено наличие фазы Al₂O₃. Изучена стадийность химических превращений в волне горения и сделано предположение о механизме структурообразования. В процессе СВС первичными фазами могут являться MoO₂ и Al₂O₃, при этом фаза MoAlB образуется из борсодержащего алюминий-молибденового расплава. Выявлено, что субмикронные выделения МoB формируются в зоне дореагирования в результате частичного окисления алюминия по механизму дисперсного упрочнения.