High-entropy FeNiCoCr alloys with improved mechanical and tribological properties by tailoring composition and controlling oxidation

Abstract

Many metal matrix self-lubricating composites possess excellent comprehensive properties of high strength and wear resistance after incorporating various lubricants that usually belong to ceramic phase. However, this improvement is always obtained at the cost of notable decrease in toughness. In order to break through this toughness-tribological properties trade-off, self-lubricating composite alloys based on the matrix of FeNiCoCr0.5 high entropy alloy (HEA) were prepared by milling addition of element Co or Cr and then spark plasma sintering (SPS). Co or Cr is added into the base HEA with the aim of tailoring oxidation behavior, mechanical properties as well as tribological performances. As no ceramic lubricants are used, the alloy composites remain high toughness. Co or Cr element addition leads to the precipitation of a hard phase of σ-Cr and grain refinement, both of which contribute to the increase of mechanical strength. On sliding, oxidation of Fe and Ni is suppressed. Instead, the oxides of cobalt and chromium are formed upon the Co- and Cr-modified HEAs, respectively. These oxidation products, especially the oxides of cobalt that are easy to be sintered on sliding, are in favor of the formation of a lubricating glaze layer at 400 °C on the worn surface, and thus the decrease in friction coefficient and wear rate, which are 0.33 and 4.0 × 10−5 mm3/(N m), respectively.