High-temperature wear and deformation processes in metal matrix composites

Dry-sliding wear behaviors of a particulate-reinforced aluminum matrix composite 6061 Al-20 pct Al2O3 and an unreinforced 6061 Al alloy were investigated in the temperature range 25 degrees C to 500 degrees C against a SAE 52100 bearing steel counterface. Experiments were carried out at a constant sliding speed of 0.2 m . s(-1) at different test loads. The deformation behavior of the materials was studied by performing uniaxial compression tests in the same temperature range as the wear tests. Both alloys showed a mild-to-severe wear transition above a certain test temperature. In the mild wear regime, the wear rate and the coefficient of friction of the unreinforced 6061 Al decreased slightly with temperature, but the temperature had almost no effect on the wear rate and the coefficient of friction of the 6061 Al-20 pct Al2O3 in the same regime. Particulate reinforcement led to an increase in the transition temperature and a 50 to 70 pct improvement in the wear resistance in the severe wear regime. This was attributed to the formation of tribological layers consisting of comminuted Al2O3 particles at the contact surface. High-temperature compression tests showed that the flow strength of 6061 Al-20 pct Al2O3 and 6061 Al decreased monotonically with temperature and both alloys exhibited a work-softening behavior at temperatures higher than the inflection point on the flow stress vs temperature curves. The logarithmic maximum stress vs reciprocal temperature relationship was not linear, indicating that the deformation processes were too complicated to be characterized by a single activation energy over the whole temperature range. For the range of 250 degrees C to 450 degrees C, the activation energy for deformation was estimated to be 311 kJ . mol(-1) for both the matrix alloy and the composite. Severe wear proceeded by thermally activated deformation processes involving dynamic recrystallization along a subsurface strain gradient. A power-Arrhenius type relationship was found to describe well the observed dependence of severe wear rates on the applied load and temperature. This relationship was used to calculate an apparent activation energy for wear of 87 kJ . mol(-1) for the particulate-reinforced composite and 33 kJ . mol(-1) for the matrix alloy. The wear regimes at elevated temperatures are represented in a deformation mechanism map and the relationship between high-strain deformation processes and severe wear are discussed.