Effect of hybrid carbonaceous reinforcement on structure, mechanical and wear properties of spark plasma sintered CrCoFeMnNi HEA/GNP+CNT composite

Abstract

For the ultra-fine grained hybrid carbonaceous (graphene nano-plates (GNPs) + carbon nano-tubes (CNTs)) reinforced high entropy alloys (HEAs), there is a lack in understanding of the structure evaluations, mechanical and wear behaviors. In this work, the CrCoFeMnNi HEA and CrCoFeMnNi-X(GNP+CNT) (X = 0.2 and 0.8 wt%) composites were successfully fabricated via mechanical alloying (MA) following by spark plasma sintering (SPS). The effect of various contents of GNPs+CNTs on the phase and microstructure evaluations, the micro-hardness, shear deformation and wear behavior were investigated. The results demonstrated that the major FCC phase was obtained after 50 h milling for CrCoFeMnNi HEA powders, where a two-phase structure was obtained by incorporation of GNPs+CNTs. The Cr7C3 carbide and spinel oxide phases in the SPS processed CrCoFeMnNi-X(GNP+CNT) composites showed higher and lower intensities compared to those of the CrCoFeMnNi HEA, respectively. The considerably effect of GNPs+CNTs to hinder crystal growth was reflected via 31% lower coarsening of crystal size (CS) in the CrCoFeMnNi-0.2 wt% (GNP+CNT) composite than the CrCoFeMnNi HEA. The micro-hardness was improved with increasing the hybrid carbonaceous reinforcement concentration, where the surface roughness showed lessening trend. Ultimate shear stress (USS) of CrCoFeMnNi-0.2 wt% (GNP+CNT) composite (591 MPa) was the highest in comparison with CrCoFeMnNi HEA and CrCoFeMnNi-0.8 wt% (GNP+CNT) composite. The worn surface of the CrCoFeMnNi-0.8 wt% (GNP+CNT) composite presented small debris, some black adhesion and micro cracks where the worn track was the most prominent and oxidation wear resistance was significantly improved. The considerable reduction in the friction coefficient (FC) values about 16% and 43% and improvement in the wear resistance (WR) values by 81% and 91% were achieved with incorporation of 0.2 wt% and 0.8 wt% GNPs+CNTs, respectively. These indicated that hybrid carbonaceous reinforcement obviously developed self-lubricating ability, however, the efficiency of WR was affected by agglomeration of reinforcement phase.