Wear resistance of hot-pressed Si3N4/SiC micro/nanocomposites sintered with rare-earth oxide additives

The wear resistance of hot-pressed silicon nitride/silicon carbide micro/nanocomposites as well as monolithic silicon nitrides sintered with the same rare-earth oxide sintering additives (La2O3, Nd2O3, Y2O3, Yb2O3 and Lu2O3) has been investigated under dry sliding conditions. The friction coefficient decreased with a decreasing ionic radius of rare-earth elements in both the monoliths and the composites. The friction coefficient of composites was always lower in comparison with that of Si3N4 monoliths. Similarly, the specific wear rate significantly decreased with a decreasing ionic radius of rare-earths either in monolithic or composite materials. The composites always exhibited lower specific wear rate compared to the monoliths. Mechanical wear (micro-fracture) and tribochemical reaction were found as the main wear mechanisms in all investigated materials. The higher bonding strength in the case of materials sintered with additives of a smaller ionic radius restricts dropping of the individual silicon nitride grains during wear experiments. This high bonding strength and the high fracture toughness are the reasons why the ceramics doped by Lu exhibited the best wear resistance. The Evans and Marshall lateral-crack chipping model based on the fracture toughness and hardness values well describes the wear rate of the investigated micro/nanocomposites.