N. A. Kochetov

Effect of mechanical activation on thermal explosion in Ni-Al mixtures

The effect of mechanical activation (MA) on thermal explosion in equimolar Ni-Al mixtures was studied by time-resolved XRD. MA was also found to increase the burning velocity and decrease the ignition temperature. Thermal explosion in non-activated Ni-Al mixtures was found to proceed via formation of liquid (melted) intermediate products, while that in the activated mixtures gave no liquid intermediates.

Mechanically activated SHS of NiAl: Effect of Ni morphology and mechanoactivation conditions

Explored was the effect of the morphology of Ni particles in green mixtures on the mechanoactivated SHS reactions in the Ni-Al system. In case of fluffy Ni particles (produced in a carbonyl process), the mean size of the agglomerates formed in the course of MA was higher than in case of dense Ni particles (produced in an electrolytic process). The absolute values of combustion temperature and burning velocity were higher in case of carbonyl-derived Ni, while an increment in burning velocity caused by MA was higher in case of electrolytically produced Ni.

Characteristic properties of combustion and structure formation in the Ti-Ta-C system

Macrokinetic characteristics of the combustion of mixtures in the (100% − X)(Ti + 0.5C) + X(Ta + C) system with a variable mixing parameter X and initial temperature T0 of charge heating are considered. For compositions with X = 10 and 30%, an abrupt increase in the velocity Uc and temperature Tc of combustion as a result of passing two parallel chemical reactions of titanium and tantalum carbide formation is established. The Uc (T0) and Tc (T0) dependencies are linear for the mixture with X = 50%. By hardening the combustion wave, it is revealed that the primary structure formation in the combustion region starts from the selection of submicron grains of nonstoichiometric titanium carbide from the supersaturated titanium melt. In the investigated range of parameter X, synthetic products are single-phase and represent titanium-tantalum carbide. An increase in X results in a decrease in the size and microhardness of (Ti, Ta) C grains and a reduction of the relative density of compact synthetic products. The kinetics of high-temperature oxidation of alloys on the basis of carbide (Ti, Ta) C is studied. Ceramics produced at X = 10% are most heat-resistant.

Mechanoactivation of SHS systems and processes

Data on the mechanoactivation (MA) of SHS processes and systems obtained over the past 15 years are overviewed. Analysis of these data is used to formulate some recommendations for appropriate application of MA to SHS.

Self-propagating high-temperature synthesis of composite targets based on titanium carbonitride, silicide, and aluminide for ion-plasma deposition of multifunctional coatings

The macrokinetic features of combustion of the mixtures in the Ti-Al-Si3N4-C system calculated for the formation of compact ceramic materials (CCMs), the composition of which is described by the general formula X(TiAl3) + (100 − X)(0.448TiC0.5 + 0.552(Ti5Si3 + 4AlN) with mixture parameter X = 10–50%, are investigated. Compact CCM samples with the main structural components in the form of TiCxNy grains and binding phases TiAl3 and Ti5Si3 are fabricated by the technology of forced self-propagating high-temperature synthesis (SHS) compaction. An increase in X promotes the formation of the Mn+1AXn phase with the composition Ti3SiC2 in the synthesis products. Complex investigations into the physicomechanical properties of the obtained ceramics are performed. Based on their results, the inverse dependence of the density and hardness of compact materials on parameter X is established. Tests of the samples for oxidation resistance showed that the obtained CCMs based on titanium carbonitride, silicide, and aluminide possess excellent resistance to high-temperature oxidation, and their oxidation rate in air at t = 900°C for 30 h does not exceed 7.8 × 10−5 g/(m2 s).

Combustion and structure formation in the mechanoactivated Cr-B system

For Cr + B mixtures, we investigated the influence of mechanoactivation (MA) on the thermal effect of SHS reaction, burning velocity, and combustion temperature, and optimized MA conditions. Using the technique of forced SHS compaction, we manufactured the CrB2-based compacts up to 125 mm in diameter (residual porosity 3–5%) from green mixtures subjected to MA under optimized conditions.

Dependence of burning rate on sample size in the Ni + Al system

Burning rates were measured for samples of the starting mixture of Ni + Al and the same mixture subjected to mechanical treatment in argon and then in water. The dependences of the burning rate on the diameter of the samples of the original and mechanically activated mixtures are similar. The burning rate passes through a maximum as the diameter increases from 8 to 12 mm. It is found that the burning rate of 270–360 µm thick films obtained by rolling the starting mixture of Ni + Al and the mixture mechanically activated and then activated in water (dispersed) is 4–20 times the burning rate of cylindrical samples 8–12 mm in diameter, pressed from the same powders. The data obtained in this study were explained using a convective-conductive model of combustion-wave propagation.

Silicon carbide ceramics SHS-produced from mechanoactivated Si–C–B mixtures

We investigated the influence of mechanical activation (MA) and initial temperature T0 on combustion temperature Tc and burning velocity Uc for 90% (Si + C)–10% (4B + C) mixtures as well as on product morphology. Mechanical activation was found to strongly change the size and morphology of Si and graphite particles (by a factor of 6–8), lead to accumulation of micro strains, and increase the reactivity of green mixture. An increase in T0 was found to proportionally increase the values of Tc and Uc but produce little or no influence on the phase composition of product. The SiC powder consisting of 50–100 nm crystallites was obtained by diminution of combustion product and then used to prepare, by hot pressing, a ceramic PVD target for magnetron sputtering.

Effect of the time of mechanical activation of a Ti + 2B mixture on combustion of cylindrical samples and thin foils

The effect of the activation time of the Ti + 2B mixture on the burning rate of cylindrical samples and thin foils is studied. For cylindrical samples, combustion of samples activated in argon is compared with combustion of samples activated in air. The burning rates are almost identical in these two cases. It is demonstrated that the burning rate of cylindrical samples continuously increases with increasing activation time. The burning rate of thin foils remains almost unchanged as the activation time increases up to 4 min and then drastically increases and reaches a value twice greater than the burning rate of cylindrical samples. For the titanium and boron powders used in this study, the time needed to reach the maximum burning rate is 7 min in the case of activation in air and 5 min in the case of activation in argon; if the activation time is longer, then the product of combustion is formed. The features of combustion observed in this study can be explained from the viewpoint of convective–conductive model of combustion wave propagation.

SHS of TiC-TiNi composites: Effect of initial temperature and nanosized refractory additives

For SHS reactions in Ti-Ni-C powder blends yielding TiC-TiNi composites, investigated was the effect of initial temperature (T0), green composition, and nanosized refractory additive (alloying agent) on combustion parameters and structure formation in combustion products. An increase in T0 elevated combustion temperature T and burning velocity U, while the addition of micro- and nano-sized ZrO2 particles diminished the above parameters due to partial blocking of reactive Ti-Ni contacts by ZrO2 agglomerates. At the same time, the addition of alloying agents enlarged the number of crystallization centers for the growth of primary TiC grains in the melt.