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.