Dry sliding tribological properties and subsurface structure of nanostructured copper at liquid nitrogen temperature

Abstract

Bulk nanostructured Cu sample with nano-scale twin bundles embedded in nano-sized grains was synthesized by using dynamic plastic deformation (DPD) technique. Dry sliding tribological properties of the DPD Cu and the coarse grained (CG) Cu samples were investigated under liquid nitrogen temperature (LNT) in comparison with room temperature (RT) conditions. Experimental results show that the wear volume under LNT was much larger than that under RT for both sets of Cu samples. The DPD Cu sample exhibits almost same wear volume compared with the CG Cu when sliding at LNT, which is quite different from the enhanced wear resistance for the DPD Cu sample sliding at RT. A similar steady worn subsurface structure was formed for two Cu samples, which was constituted by heavily deformed nanostructured mixing layer (NML) and ultra-fine grained dynamic recrystallization (DRX) layer. Comparing with the worn subsurface structure at RT, lower sliding temperature could effectively suppress grain growth within the DRX layer, resulting in a cracked NML and extremely fine grains in the DRX layer. When sliding at LNT, quick propagation of cracks from NML into DRX layer and flaking rate of NML are determining the wear process which results in a high wear rate.