High load sliding, deformation microstructures, strength, and hardening for cutting and metal forming

Abstract

The plowing action of ridge and wedge shaped micro asperities under large sliding loads is utilized to induce extremely high hardness and strain levels in the subsurface regions of Cu. A unique apparatus was designed and employed to apply high normal stresses (loads) that ranged herein from 0.4 to 3.7 times the yield stress, at either ambient (293 K) or cryogenic (77 K) temperatures. A steep gradient in the size scale of the induced deformation microstructures is coupled to an unprecedented average size (crystallite) scale of 5 nm that includes a high dislocation density near the surface. This structural size is graded in size from very fine at the surface to large with increasing depth and decreasing strain. Due to these size scales, quantitative and statistical measurements of the subsurface deformation microstructures, as a function of depth, load, and temperature were performed with high resolution transmission, scanning electron, and microprobe microscopy. The observed deformation microstructures follow the principal of grain subdivision creating a hierarchical structure, but with a finer size scale, that is similar to deformation structures developing under different deformation modes. The measured structure parameters include the inverse spacing, 1/DavGNB, of deformation induced boundaries (geometrically necessary boundaries GNBs), that have evolved with increasing strain to have medium to high misorientation angles like grain boundaries, and the dislocation density, ρav, in between the GNBs. Dislocation mechanisms were demonstrated to dominate the subsurface deformation at all size scales based on the high density of dislocations, the average refinement that extends to 5 nm, and universal scaling. Subsurface stress estimates use the measured parameters in a linear addition of the classic Hall-Petch formulation for the barrier strength contribution of 1/DavGNB, plus Taylor forest dislocation hardening for the contribution of ρav. A further unification of these parameters is made since ρav is directly proportional to 1/DavGNB. Flow stress estimates indicate that the nearest subsurface layers are extremely hard, 1.7–2.8 GPa with hardness and subsurface refinement increasing with increasing normal load and decreasing temperature. At the highest load and 77 K, the depth at which large shear strains are observed is increased by a factor of about six, i.e. eighty times the average platen asperity height. A quantitative comparison of friction and deformation energies is made together with the stress and strain estimates. Friction and deformation energies increase with increasing the normal loads. At the same time, coefficients of friction decrease due to the creation of a harder surface at higher loads. Large sliding or rubbing loads are present during cutting and metal forming creating subsurface deformation. These loads largely contribute to high friction forces, and strongly influence surface finishes and subsurface hardness.