Tribological Behavior of Si3N4-hBN Ceramic Materials without Lubrication under Different Test Modes

Unlubricated tribological behaviors of silicon nitride-boron nitride (Si3N4-hBN) composites were investigated with two test modes in air by using a pin-on-disc tribometer. Under upper-disc-on-bottom-pin test mode, the addition of hBN to Si3N4 resulted in a significant decrease of the friction coefficient, from 0.54 for Si3N4 against Si3N4 to 0.19 for Si3N4-20% hBN against Si3N4. The surface analysis indicated that a tribochemical film consisting of SiO2 and H3BO3 was formed on the wear surfaces. The formation of tribochemical film might be attributed to the embedment of wear debris into the spalling pits on the wear surfaces of Si3N4-hBN specimen. The wear debris reacted with moisture in air, and the resultant tribochemical film lubricated the wear surfaces. Under upper-pin-on-bottom-disc test mode, the wear mechanism was dominated by abrasive wear, and no tribochemical products could be detected on the wear surfaces. A slight decrease of the friction coefficient, from 0.85 for Si3N4/Si3N4 to 0.56 for Si3N4/Si3N4-30% hBN, was obtained, which might be attributed to the layered structure of hBN.