The tribological behavior of Si

3

N

4

-hBN composites with different hexagonal boron nitride(hBN)volume fractions under distilled water lubrication was investigated. Water-lubricated sliding tests were conducted on a pin-on-disc tester. The results show that the addition of hBN into Si

3

N

4

matrix significantly decreased the friction coefficient from

0.30 for Si

3

N

4

against Si

3

N

4

pair to 0.01 for Si

3

N

4

-20%hBN against Si

3

N

4

pair under titrimetric water lubrication

and the friction coefficient of Si

3

N

4

-hBN/Si

3

N

4

pair decreased to 0.01 in water. The morphological analysis and chemical characterization of the worn surfaces via scanning electron microscopy

laser scanning microscope

and X-ray photoelectron spectroscopy reveal that under titrimetric water lubrication

the hBN in Si

3

N

4

-hBN spalled off during the wearing tests and the spalled pits formed on the wearing surface of Si

3

N

4

-hBN

and then the debris dropped into the pits to react with water

thus a tribochemical film formed on the wearing surface. The formation of the tribochemical film facilitated smoothing the surfaces of Si

3

N

4

-hBN and Si

3

N

4

to create a fine lubrication environment.