Effect of Y ₂ O ₃ and Graphite on the Microstructure, Mechanical Properties, and Wear Resistance of Al ₂ O ₃ –B ₄ C Composites

Alumina (Al 2 O 3 ) cutting tools are the most commonly used ceramic cutting tools for advanced high‐speed machining due to their good mechanical properties and low cost. However, monolithic alumina cutting tools generally suffer due to their low fracture toughness and poor thermal shock resistance, which diminishes the tool life and performance for critical applications. Accordingly, ceramic matrix composites have been developed to replace monolithic alumina. Boron carbide (B 4 C) is a potentially attractive reinforcement due to its extremely high hardness, low density, and high‐temperature resistance. However, the sinterability of Al 2 O 3 –B 4 C composites is limited due to the formation of the deleterious Al 18 B 4 O 33 phase, which restricts densification. Within the scope of this work, the influence of Y 2 O 3 and graphite additions on the microstructure, mechanical properties, and wear resistance of an Al 2 O 3 –30B 4 C composite fabricated using spark plasma sintering has been systematically studied. While Y 2 O 3 addition is beneficial to overall densification, sinterability decreases with increasing graphite addition. The microstructures of the composites display uniform reinforcement distribution, and the presence of B 4 C reinforcements results in significant Al 2 O 3 matrix grain refinement. The developed composites possess much improved mechanical properties in comparison to monolithic Al 2 O 3 , and the following highest mechanical properties have been achieved: Young's modulus of 397 ± 10 GPa, hardness of 22.7 ± 1 GPa, indentation fracture toughness of 6.5 ± 0.5 , and flexural strength of 572 ± 52 MPa. The composite developed with 5 vol.% Y 2 O 3 addition has yielded the best wear resistance, with the specific wear rate one‐fourth that of monolithic alumina and the average coefficient of friction almost two‐thirds that of monolithic alumina. Finally, considering the relative influence of different properties on the overall cutting performance, a simple expression has been established to rank the performance of different compositions developed.