The main objective of this article is to study the tribological behavior of Si 3 N 4 -hBN composites with different hexagonal boron nitride (hBN) volume fraction under distilled water lubrication. Water-lubricated sliding tests were carried out on a pin-on-disc tester, and Si 3 N 4 was used as friction pair. The results showed that the addition of hBN to Si 3 N 4 resulted in a severe decrease of the friction coefficient, from 0.35 for Si 3 N 4 against Si 3 N 4 to 0.01 for Si 3 N 4 -20% hBN against Si 3 N 4 with drip-feed water lubrication; the friction coefficients of Si 3 N 4 -hBN/ Si 3 N 4 pairs sliding with full immersion water lubrication were as low as 0.01. The morphological and chemical characterization of the worn surfaces were conducted using scanning electron microscopy (SEM), laser scanning microscope, X-ray photoelectron spectroscopy (XPS). The analysis indicated that, with drip-feed water lubrication, hBN in Si 3 N 4 -hBN was spalled off during the wearing tests and spalling pits were formed on the wearing surface of Si 3 N 4 -hBN composites, then the wear debris were dropped into the pits and reacted with water, and thus a tribochemical film was formed on the wearing surface. The tribochemical film facilitated the wear surfaces of Si 3 N 4 -hBN and Si 3 N 4 to smooth with drip-feed water lubrication, while the tribochemical remove facilitated the wear surfaces to smooth with full-immersion water lubrication.

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