Effect of grain size on friction and wear of nanocrystalline aluminum

The friction and wear characteristics of nanocrystalline aluminum were investigated as a function of grain size. Nanocrystalline aluminum samples with an average diameter of 16.4 nm were produced using an r.f. magnetron sputtering technique. The grain size was increased (up to 98.0 nm) by an isothermal annealing treatment at 573 K. Hardness measurements were performed using an ultra-microhardness indentation system and it was observed that within the grain size range of 15-100 nm the hardness-grain size data could be well represented by the Hall-Fetch relationship. Friction and wear measurements were made using a miniature pin-on-disk type tribometer under unlubricated conditions both in air and in vacuum. The coefficient of friction of aluminum tested against a stainless steel pin varied with the sliding distance. At the early stages of sliding the coefficient of friction rose to a peak value, and this was followed by a decrease to a steady-state value. The transition on the friction curve corresponded to a similar transition from a severe wear regime to a mild wear above a characteristic sliding distance on the cumulative volume loss versus sliding distance curve. The value of the peak coefficient of friction decreased from mu(p) = 1.4 for aluminum with a coarse grain size (10(6) nm) to mu(p) = 0.6 for the nanocrystalline aluminum with a grain size of 16.4 nm. The coefficient of friction of nanocrystalline aluminum showed a 30% increase when tested in vacuum. In the nanocryslalline grain range, the wear rates were found to be linearly dependent on the square root of the grain size. An empirical equation based on the Archard's Law is proposed to describe the effect of grain refinement on the wear resistance under unlubricated sliding conditions. A qualitative understanding of wear processes is developed in terms of the variation of the surface morphology and subsurface strength with sliding distance.