It is known that the presence of oxygen phases in hard materials leads to an undesirable decrease in the mechanical properties. In materials based on AlMgB14, the main oxygen impurity is spinel MgAl2O4; it significantly reduces the hardness of AlMgB14 and its formation during sintering is inevitable. In this work, the ultra-hard spark plasma sintered (SPSed) AlMgB14-TiB2 composite material was fabricated from the AlMgB14-TiB2 precursor obtained by self-propagating high-temperature synthesis (SHS). Due to the high synthesis temperatures, the main oxygen phase in the obtained composite was Al4B2O9 instead of spinel MgAl2O4. It was found that the obtained composite has excellent mechanical properties. The maximum hardness of the sample is 44.1 GPa. The presence of oxygen in the form of the Al4B2O9 phase led to unexpected results: the friction coefficient of the obtained AlMgB14-TiB2 composite under dry conditions against the Al2O3 counter-specimen is approximately four times lower than the friction coefficient of pure ceramic AlMgB14 (0.18 against 0.7, respectively). Based on the observed results, it was found that the Al4B2O9 particles formed during the SHS are responsible for the low friction coefficient. The quantum chemical calculations showed that the elastic moduli of Al4B2O9 are significantly smaller than the elastic moduli of AlMgB14 and TiB2. Thus, during sliding, Al4B2O9 particles are squeezed out onto the composite surface, form the lubricating layer and reduce the friction coefficient.

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The paper presents results of the Rietveld refinement method in X-ray diffraction quantitative phase analysis of calcium mono-aluminate, the mineral. It has been stated that the theoretical calcium mono-aluminate CaO⋅Al2O3. is a high-stable phase, mixing energy of which is equal to 16214,10 eV.Ab-initiocalculations and Rietveld refinement method were used to identify die structural state of CaO⋅Al2O3.

The paper presents the two-dimensional model of Portland cement curing based on the finite element method. The cement curing lasts for 2 days in the temperature gradient conditions ranging from 65 to −20 °C. A cement rod 7×70 cm in size was used for modeling the von Mises stress distribution. It is shown that in the temperature gradient conditions, the stress maximum in curing Portland cement shifts toward the hot edge of the cement rod and exceeds the minimum stress level more than two times. The stress growth at the interfaces between the concrete timber and the cement rod exceeds the stress maximum inside the latter. This stress distribution is connected with the heat flow generation and superposition of direct and reverse heat flows. Superposition of the negative heat flows generates the stresses at the interfaces, which are higher than those generated by the positive heat flows. The concrete timber and the interface fixation are the additional factors that increase the stress level at the interfaces and promote the cement fracture.