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