Experimental and Theoretical Study of Ultra-Hard AlMgB 14 -TiB 2 Composites: Structure, Hardness and Self-Lubricity.

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 AlMgB 14 , the main oxygen impurity is spinel MgAl 2 O 4 ; it significantly reduces the hardness of AlMgB 14 and its formation during sintering is inevitable. In this work, the ultra-hard spark plasma sintered (SPSed) AlMgB 14 -TiB 2 composite material was fabricated from the AlMgB 14 -TiB 2 precursor obtained by self-propagating high-temperature synthesis (SHS). Due to the high synthesis temperatures, the main oxygen phase in the obtained composite was Al 4 B 2 O 9 instead of spinel MgAl 2 O 4 . 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 Al 4 B 2 O 9 phase led to unexpected results: the friction coefficient of the obtained AlMgB 14 -TiB 2 composite under dry conditions against the Al 2 O 3 counter-specimen is approximately four times lower than the friction coefficient of pure ceramic AlMgB 14 (0.18 against 0.7, respectively). Based on the observed results, it was found that the Al 4 B 2 O 9 particles formed during the SHS are responsible for the low friction coefficient. The quantum chemical calculations showed that the elastic moduli of Al 4 B 2 O 9 are significantly smaller than the elastic moduli of AlMgB 14 and TiB 2 . Thus, during sliding, Al 4 B 2 O 9 particles are squeezed out onto the composite surface, form the lubricating layer and reduce the friction coefficient.