Enhancing the machinability of Si3N4 ceramics through h-BN and ytterbium silicate composite weak interfaces

Machinable Si 3 N 4 ceramics containing 10 vol% h-BN and crystalline ytterbium silicate secondary phases were prepared. • Heat treatment promoted the crystallization of ytterbium-containing secondary phases into ytterbium silicates. • Crystallization was accompanied by the development of compressive residual stress. • 10 vol% h-BN with crystalline ytterbium silicate phases achieved drilling performance comparable to conventional 20 vol% h-BN. Traditionally, the machinability of Si 3 N 4 ceramics has mainly been improved by introducing individual weak-interface phases, while the role of grain-boundary phases was often neglected. In this study, Si 3 N 4 ceramics were fabricated by hot-press sintering with heat treatment using Yb 2 O 3 and SiO 2 as sintering additives. The crystalline ytterbium silicate secondary phases formed in situ during heat treatment, together with the addition of a small amount of h-BN, introduced weak-interface-forming regions into the Si 3 N 4 microstructure and contributed to improved machinability. The effects of sintering additive ratio, h-BN content, and heat-treatment temperature on mechanical and machinable properties were systematically investigated. The results demonstrated that Si 3 N 4 ceramics containing only 10 vol% h-BN could be readily machined using conventional cemented carbide drills while maintaining high strength, with a bending strength of 660 MPa, fracture toughness of 6.9 MPa·m 1/2 , Young’s modulus of 263 GPa, and Vicker’s hardness of 9.3 GPa (under 10 N).