Novel Nitride Materials Synthesized at High Pressure

Nitride materials including conventional manmade superhard light-element nitrides, such as cubic boron nitride (cBN), cubic silicon nitride (γ-Si3N4), and carbonitrides, have been extensively used for machining (e.g., turning, cutting, grinding, boring, drilling) and coating of ferr ous alloys due to their remarkable performances of high rigidity, high melting-point, and prominent chemical and thermal stabilities. However, to some degree, superhard nitrides merely compensate for the adverse limitations of diamond: reaction (with iron), oxidation, and graphitization at moderate temperatures; they are still unable to dominate the market owing to their relatively low hardness when compared to diamond. Therefore, recent efforts toward the preparation of nitride materials with outstanding mechanical performance and chemical inertness have focused on synthesizing ternary light-element nitride compounds and harvesting the effect of work hardening through microstructure manipulations. These new light-element nitrides are potential candidates to displace diamond in the cutting business. On the other hand, incorporation of transition-metal atoms into the dinitrogen triple-bond can form novel hard transition-metal nitride alloys (TMNAs), such as Mo-N, W-N, Pt-N, Ir-N, Os-N, etc., which are potential candidates for the cutting, coating, and polishing of iron-group metals. However, synthesis of high-crystallinity and stoichiometric TMNAs via traditional routes is challenging, since the embedded nitrogen in the transition-metal lattice is thermodynamically unfavorable at ambient condition. A novel approach involving ion-exchange reactions under moderate pressure and temperature has been developed in recent years for preparation of well-crystallized stoichiometric TMNAs, which have quickly been realized as emergent materials in electronics, catalysts, and superconductors as well.