The effect of shot peening on the scuffing resistance of Cu-Ni austempered ductile iron

This work presents a study conducted on the scuffing performance of an upper ausferritic Cu-Ni austempered ductile iron (ADI) in the as-austempered and shot peened state. Scuffing tests on the two ADI conditions were carried out under starved lubrication conditions using a pin-on-disk tribometer. Results show that shot peening improved the scuffing wear resistance of the as-austempered specimens by almost an order of magnitude. In fact, shot peened specimens survived 2.1 × 104 cycles while the life of the as-austempered specimens was 0.2 × 104 cycles. The improved scuffing performance might seem anomalous, since the rough surfaces induced by shot peening generally result in low values of the specific film thickness λs, thus inducing scuffing. However, these same indentations acted as oil reservoirs and created a load-bearing hydrodynamic pressure. This resulted in a reduction of surface traction forces, which in turn led to a very low coefficient of friction namely 0.08. This can be considered as an advantage in components sliding under starved lubrication conditions. The higher scuffing resistance of the shot peened specimens was also partly attributed to the high hardness and the high compressive stresses induced in the surface of these specimens. On the other hand, the lower scuffing resistance of the as-austempered specimens was attributed to plastic flow and micro-fracture of the asperities.