Reversal of favorable microstructure under plastic ploughing vs. interfacial shear induced wear in aged Co_1.5CrFeNi_1.5Ti_0.5 high-entropy alloy

The microscopic tribological behaviour of the aged Co_1.5CrFeNi_1.5Ti_0.5 high entropy alloy (HEA) is examined using nanoindentation and atomic force microscopy (AFM). The effect of aging of the HEA viz. under-aged, peak-aged and over-aged conditions on the tribological properties is investigated under different loading conditions. In nanoindentation based scratch tests, wear is dominated by plastic ploughing and the peak-aged HEA shows the highest wear resistance, owing to its higher hardness. Interestingly, in low load AFM based experiments, in absence of plastic ploughing, the wear behaviour is reversed, i.e., the wear resistance of the peak-aged condition is lowest. Higher interfacial shear strength values, as determined using wearless AFM based sliding experiments, is attributed to the least wear resistance of the peak-aged condition. The observed tribological behaviour can potentially extend the use of HEAs for sliding components by controlling the optimum size and distribution of precipitates under different loading conditions.