Fabrication and characterization of machinable Si3N4/h-BN functionally graded materials

A new design method of machinable ceramic composites was proposed, which applies the graded-structure concept to the design of machinable Si3N4 ceramics. Silicon nitride/hexagonal boron nitride (h-BN) functionally graded materials (FGMs) were fabricated by hot pressing at 1750°C for 2 h, varying the alignment of the amount of hexagonal BN using powder layering method. The improved machinability of Si3N4/h-BN composite can be attributed to addition of layered structure hexagonal BN. Hexagonal BN possesses excellent cleavage planes perpendicular to the c-axis. Ease of machining depends on degree of crystal interlocking; hence volume content of h-BN crystals and their aspect ratio affect machinability. Such design can improve the machinability of composite, and at the same time can make the mechanical properties of Si3N4 ceramic not to be sacrificed too much. The texture of h-BN and β-Si3N4 was observed during hot pressing sintering.