Fabrication and characterization of a low-cost Co-free Al0.8CrFeNi2.2 eutectic high entropy alloy based solid self-lubricating composite: Microstructure, mechanical and wear properties

The unique mechanical and physical properties of eutectic high entropy alloys (EHEAs) have attracted increasing attention and research interest in design and development of new advanced materials. However, the relatively high content of Co element in those commonly reported EHEAs based on 1st transition series elements leads to relatively high cost of bulk production and significantly restricts potential applications. Thus, study on development and application of EHEAs without Co addition is essential. In the current work, a novel solid self-lubricating composite has been successfully fabricated by mechanical alloying (MA) and subsequent spark plasma sintering (SPS) processes using the cost-effective Co-free EHEA, Al0.8CrFeNi2.2, as matrix. Its microstructure and phase constitutions were examined by SEM, TEM and XRD techniques, and its mechanical properties were evaluated by Vickers hardness test and compression test. The EHEA-Ag composite exhibited a Vickers hardness of 400 HV, a yield strength of 1136.5 MPa, and an ultimate strength of 1232.9 MPa, respectively. Moreover, its wear properties were investigated in a wide temperature range from room temperature to 900 °C by sliding against silicon nitride Si3N4 balls. Similar friction coefficients of EHEA-Ag were obtained in the studied temperature range. However, the wear rate of EHEA-Ag was found to decrease first when the sliding temperature was lower than 600 °C and then increased significantly when it was increased from 600 to 900 °C. A similar tendency in wear rate was observed during the sliding wear of EHEA matrix, but the comparison revealed a much lower wear rate of EHEA than EHEA-Ag at 900 °C. EHEA-Ag and EHEA exhibit different wear mechanisms at room temperature but the same wear mechanisms when the temperature is higher than 600 °C. The outcome of this study indicates that EHEA-Ag can act as an excellent solid self-lubricating composite candidate in a temperature range from RT to 600 °C.