TRIBOLOGICAL PROPERTIES OF ALN-CEO2-SI3N4 CUTTING MATERIALS IN UNLUBRICATED SLIDING AGAINST TOOL STEEL AND CAST-IRON

Ceramic pins of the AlN-CeO2-Si3N4 system were tested in a pin-on-disc tribometer against discs of tool steel and grey cast iron, at room te mperature, without lubrication, in different conditions of humidity an d sliding speed. Ceramic samples were selected on the basis of their m echanical properties (hardness and fracture toughness), and microstruc tural characteristics, namely porosity, volume of intergranular phase and nitrogen content of the glass phase. Water vapour increased the we ight loss of the nitride by promoting the tribo-oxidation wear mode an d by weakening the adhesion of debris to the ceramic surfaces. In dry air, the adhering wear debris provided protection to the sliding surfa ces and the wear coefficients of the ceramic converged to similar valu es for tests with both iron alloys. For the ceramic/tool steel tribopa irs, the ceramic surfaces become more protected as the amount of the i ntergranular glassy layer of the nitride is increased, as the glassy p hase gives enhanced debris adhesion. Microcracking of the ceramic surf ace was the dominant wear mode and the volumic wear rate was found to be dependent on the inverse of hardness and fracture toughness. In hum id environments, the effect of roughness of the grey cast iron worn su rface surmounted the dependence of the wear rate on microstructural an d mechanical properties of the nitride, which was found in the ceramic /steel tribopairs. The sliding speed has a strong effect on the wear b ehaviour. At low speeds, no protective plates of debris were detected on the worn surfaces. When the speed was increased above 0.5 ms(-1), t he wear coefficient values fell down almost one order of magnitude. Th e wear coefficients of porous nitride materials of relative open poros ity close to 20%, tested against cast iron, were unexpectedly lower th an the values obtained for dense materials of same composition (K appr oximate to 2 x 10(-15) Pa-1 for porous samples and K approximate to 1. 5 x 10(-14) Pa-1, for fully dense samples). SEM observations showed an extensive coating by the metallic rich debris, that performed a solid lubricant action.