Effect of preliminary severe plastic deformation on structure and durability of nickel subjected to oxidation

Abstract

Structural transformations in nickel (99.99%) under severe plastic deformation during dry sliding friction and subsequent oxidation in air at temperatures of 400–800°C (1 h of holding) have been studied. The micro-hardness, wear intensity, and coefficient of friction of the strained and oxidized nickel have been measured. It has been shown that plastic deformation leads to the formation of γ crystals with nanocrystalline structures in the surface layer with thicknesses of ∼10 μm; the size of the γ crystal is 10–60 nm and the micro-hardness is about 4 GPa. Oxidation at 500 and 550°C leads to the formation of the two-phase nanocrystalline structure in the strained layer, which consists of the fcc matrix phase and of NiO oxide particles, the volume fraction of which is dozens of percents, and the crystal size is 5–30 nm. The formation of this structure is caused by the accelerated saturation of the strained nickel layer by oxygen atoms, the chemical interaction between nickel and oxygen atoms, and the primary recrystallization in the γ solid solution. The formation of the two-phase nanocrystalline structure, which leads to a considerable increase in hardness, does not have a positive effect on the durability of the nickel surface. Apparently, this is explained by the high brittleness of the analyzed structure. The highest durability is characteristic of a two-phase structure that consists of the γ solid solution supersaturated by oxygen and coarse (up to 200 nm) grains of NiO oxide. This structure forms in the nickel due to the deformation and subsequent oxidation at 800°C. Severe plastic deformation and oxidation have no considerable effect on the coefficient of friction (f = 0.6–0.7).