Self-Lubricating Si₃N₄-based composites toughened by in situ formation of silver

As is well known, it is hard to prepare ceramic-Ag composites, owing to the lower melting temperature of silver. In this paper, Si₃N₄-based composites reinforced with in situ formation of Ag particles were successfully fabricated by sintering Si₃N₄ and AgNO₃, and their microstructure, elevated-temperature mechanical and tribological properties were investigated. It can be found that Ag particles about 1 µm were uniformly dispersed into the Si₃N₄ matrix and there was no outflow of silver during the sintering. Meanwhile, S₃N₄/Ag composite containing 5.1 wt% silver exhibited the best fracture toughness, 7.05 MPa m^1/2 and 6.12 MPa m^1/2 at 25 °C and 600 °C, respectively, and the plastic deformation and crack bridging of silver were responsible for the high toughness. Besides, the friction coefficient and wear rate of composites were continuously reduced as the silver content increases. Especially at 200 °C, the friction coefficients reached the lowest value due to the formation of an amorphous silicon oxide film. And the wear resistance was enhanced by the cooperative strengthening effect of silver toughening and lubricating.