Si(2)N(2)O secondary phase-free, fully dense nano-Si(3)N(4) ceramics (>= 97% of theoretical density) were developed by combining carbothermal reduction treatment and spark plasma sintering (SPS), and their tribological properties were investigated by subjecting to self-mated sliding under unlubricated conditions. Commercially available Si(3)N(4) nanopowder was used as a starting material and phenolic resin was used for carbothermal reduction, which was conducted at 1450 degrees C for 10 h. Fully densified Si(2)N(2)O phase-free Si(3)N(4) ceramics with a wide range of grain size from 90 nm to 1.5 mu m were fabricated by varying SPS temperature from 1550 degrees to 1750 degrees C. The microstructure of the developed Si(3)N(4) ceramics was changed from nano equi-axed at 1550 degrees C to large elongated bimodal grain morphology at 1700 degrees or 1750 degrees C. The frictional behavior was not dependent on the microstructure, but the wear rate was strongly influenced such that it decreased by an order of magnitude (from 9.7 x 10-5 to 0.88 x 10-5 mm3/N center dot m) with decreasing grain size. The dominant wear mechanism was changed from the delamination of tribochemical layer for the ceramics with nano equi-axed grain microstructure to the fracture and grain pull-out for the ceramics with duplex microstructure.