Experimental and Theoretical Study of Ultra-Hard AlMgB₁₄-TiB₂ 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 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.