Tribooxidational effects on friction and wear behavior of silicon nitride tool steel and silicon nitride gray cast iron contacts

Unlubricated pin-on-disk wear tests of Si3N4 against tool steel and gray cast iron were performed at 5 N of normal load, 0.5 m/s of sliding speed, and environmental temperature in the range 22 degrees-600 degrees C. The friction coefficient of Si3N4 sliding against tool steel and gray cast iron had maximum values of 0.88-0.98 for tests at 100 degrees C. The friction coefficient of Si3N4 sliding against gray cast iron couples had minimum values of 0.48-0.57 at 400 degrees C. Because of the increased third-body protection, the wear coefficient of the Si3N4 pins of the Si3N4/gray cast iron couples decreased by 1 order of magnitude from 1.6 x 10(-5) mm(3)/(N.m) at room temperature to 1.3 x 10(-6) mm(3)/(N.m) at 600 degrees C. Fe2O3 and Fe3O4 resulting from tribooxidation of the metallic disks were the main constituents of the wear debris and adherent tribolayers. Activation energy values (6.3-13.7 kJ/mol) were comparable to those of oxidation wear of steel (7.3-11.8 kJ/mol) but were much lower than the activation energy for oxidation of iron alloys in static conditions. Calculations of the activation energy of the oxidation wear corroborate the morphological observations of a sacrificial action of the metallic surface protecting the ceramic material.