Tribological properties of AlN-CeO2-Si3N4 cutting materials in unlubricated sliding against tool steel and cast iron

Abstract

Ceramic pins of the AIN-CeO2-Si3N4 system were tested in a pin-on-disc tribometer against discs of tool steel and grey cast iron, at room temperature, without lubrication, in different conditions of humidity and sliding speed. Ceramic samples were selected on the basis of their mechanical properties (hardness and fracture toughness), and microstructural characteristics, namely porosity, volume of intergranular phase and nitrogen content of the glass phase. Water vapour increased the weight loss of the nitride by promoting the tribo-oxidation wear mode and by weakening the adhesion of debris to the ceramic surfaces. In dry air, the adhering wear debris provided protection to the sliding surfaces and the wear coefficients of the ceramic converged to similar values for tests with both iron alloys. For the ceramic/tool steel tribopairs, the ceramic surfaces become more protected as the amount of the intergranular glassy layer of the nitride is increased, as the glassy phase gives enhanced debris adhesion. Microcracking of the ceramic surface was the dominant wear mode and the volumic wear rate was found to be dependent on the inverse of hardness and fracture toughness. In humid environments, the effect of roughness of the grey cast iron worn surface surmounted the dependence of the wear rate on microstructural and mechanical properties of the nitride, which was found in the ceramic/steel tribopairs. The sliding speed has a strong effect on the wear behaviour. At low speeds, no protective plates of debris were detected on the worn surfaces. When the speed was increased above 0.5 ms -I, the wear coefficient values fell down almost one order of magnitude. The wear coefficients of porous nitride materials of relative open porosity close to 20%, tested against cast iron, were unexpectedlylower than the values obtained for dense materials of same composition (K ~ 2 x 10- 15 Pa- 1 for porous samples and K~ 1.5 x 10-14 Pa-l, for fully dense samples). SEM observations showed an extensive coating by the metallic rich debris, that performed a solid lubricant action.