ROUGHNESS EFFECT OF SILICON-NITRIDE SLIDING ON STEEL UNDER BOUNDARY LUBRICATION

The friction and wear behaviour of a silicon nitride ceramic sliding against a 100C6 hard steel (E 52100 steel from SAE-AISI standard) has been investigated under boundary lubrication. The effect of five initial surface roughnesses of silicon nitride is studied. Tests are performed on an alternating ''pin-on-plate'' tribometer. Lubrication is by a commercial oil used as received (new oil) and aged (used oil). The surface finish of the ceramic determines the friction response only during a short transitory stage. The steady stage of friction is characterized by the same value of the coefficient of friction whatever the initial roughness of the ceramic. The worn surfaces of steel exhibit either abrasion grooves when coupled with a rough ceramic (transitory stage), or a continuous metallic layer covering the steel (steady stage). The wear process of the silicon nitride surface includes fracture mechanisms of the upper asperities and polishing by the mechanochemical action of the lubricant and submicronic ceramic wear debris. Photoelectron spectroscopy analyses show the metallic layer strongly oxidized and covered by a reactional film generated through chemical reactions with the oil. The reactional film never occurs on the ceramic and is no longer observed on the steel after long sliding times. Used oil leads to a very smooth sliding surface on the ceramic and allows particularly low friction coefficients and steel wear, but only after a more severe initial degradation process.