Nanostructurizing Surface Layer for Improving the Wear and Corrosion Resistance of Roller Knives

The work is devoted to studying roller knives made of Kh12MF tool grade steel with a nickel–copper coating applied by means of diffusion alloying and a subsequent intensive plastic deformation (IPD) under contact stresses ranging from 3.0 to 4.5 GPa. In the course of the studies, such methods as optical and transmission electron microscopy; X-ray diffraction and microrentgenospectral analysis have been used. Tribological and corrosion testing, as well as microhardness and roughness measurements have been carried out. It is shown that after applying Ni–Cu coating and IPD at a stress of 3.5 GPa, not only a twofold decrease in the coefficient of friction, 2.1-fold decrease in wear level, 2.5-fold decrease in the roughness of the roller surface, as to compare to uncoated steel, but also an increase in the seizure load up to 1400 N have been observed. At the same time, a dislocation substructure has been formed in the coating with a minimum size of coherent scattering regions amounting to 23 nm and a maximum level of crystal lattice microdeformation being of 1.25%, which indicats the presence of large elastic distortions in the crystal lattice. The transverse size of dislocation subgrains (small zones inside the crystalline grains separated by small-angle boundaries consisting of dislocation grids formed during the deformation and development of an internal substructure) ranging from 35 to 47 nm exhibit a fivefold decrease times as to compare to the initial state (200–256 nm). In a high-resolution mode, the presence of predominantly small-angle boundaries in the coating structure having a greater penetrating ability for redistributing the alloying elements has been observed. It can be assumed that chromium contained in Kh12MF grade steel, staying in solid solution, is redistributed along the small-angle dislocation boundaries of the elongated polygonized nanostructure. The increase in the concentration of Cr in the coating, as to compare to the initial state, provides an increase in its relative resistance with respect to wear and corrosion.