In-situ Raman Testing And Molecular Dynamic Simulation Of Early Oxidation Behaviors On Metal Surfaces

Oxidation of metal surface is a major cause of equipment failures in petrochemical, power plant, metallurgical, offshore oil and other industries. In order to ensure the safety for equipment during its long-time operation, it is of great importance to illuminate the inducement mechanism and process of oxidation reaction. However, this chemical reaction process turns out to be much more complicated in the presence of service loadings and oxidation-induced intrinsic growth stresses. So far, in this research field, most investigations focus on oxide growth law or the failure of oxide film over a long time span at the macro-scale. There is s a shortage of studies of early oxidation behavior owing to its short time span and small spatial scale (limited to micro/nano level). It is still lacking in scientific understanding of the oxidation law and mechanism in the early stage.Employing in-situ Raman spectrometry and molecular dynamic simulation, investigations on evolution of oxidation growth stresses and stress-dependent oxidation kinetics at micro/nano scale were carried out, and then effects of applied strains or growth strains on oxygen diffusion, orientation growth of oxide islands, oxide surface roughness and origin of defects, were mainly studied. The main contents and conclusions are listed as follows:(1) Investigation of oxygen reaction and the stress evolution in early stage of oxidation by in-situ Raman. A series of in-situ Raman measurements of early stage oxygen reaction on a thin Cu film at 300 ℃ were performed to monitor the oxidation reaction and the stress evolution within oxide layer. Besides* ex-situ atomic force microscope (AFM) was also employed to assess the surface morphology on the thin Cu film before and after the oxygen reaction.The results revealed that the oxidation stress experienced a trend of increase-decrease-increase during the oxidation reaction. Furthermore, the growth compressive stress in the oxide layer was found to reduce the rate of oxidation reaction. A growth model for the oxide layer based on Stranski-Krastonov epitaxy and could explain well the mechanism of stresses relaxation in the oxide layer at the nano level.(2) Study of oxygen diffusion in the early stage of oxidation under applied strains by molecular dynamic simulation. Using molecular dynamic simulation software LAMMPS with embedded reactive force field (reaxff), the early stage oxidation of Fe(1 00) was studied systematically under applied biaxial tensile strains at room temperature. The results indicated that the oxygen diffusion energy barrier varies on the oxide structure. Moreover, the applied strain has a great effect on oxygen diffusion through its induced changes of the oxide structure resulting in the change of the oxygen diffusion barrier and diffusion coefficient. Compared with the effect of the diffusant concentration (composition) and oxidation growth stresses, the evolution of oxide structure played a dominant role on the change of diffusion coefficient in the strain-reaction engineering.(3) Investigation of orientation growth of oxide islands by molecular dynamic simulation. Using molecular dynamic simulation software LAMMPS, the strain-dependent orientation growth of oxide islands was studied during the initial oxidation of Cu(100). The results showed that the growth stress in the oxide island could be relieved more as the island grows along<100>-type directions, compared with the growth along<110>-type directions due to elastic anisotropy in the absence of oxygen diffusion. However, the result was reversed when the oxygen diffusion process in the oxide/substrate interface was involved in the simulation owing to the effect of diffusion-induced stress. The simulation results indicated that the orientation growth of oxide islands is caused by the coupling effect of elastic anisotropy and diffusion induced stress.(4) Study of the evolution of oxide surface roughness in early stage of oxidation under applied strains by Molecular dynamic simulation. Using molecular dynamic simulation software LAMMPS, the oxide surface structure and morphology were studied during the initial oxidation of polycrystalline copper under applied uniaxial tensile strains. The results presented that the applied tensile strains had a great effect on the evolution of surface morphology and could increase the surface roughness of oxide layer. The transformation of oxide wetting layer to islands structure occurred as a result of atoms diffusion triggered by the oxidation-induced growth strain energy. Such transformation process was enhanced by promoting the free energy of copper atoms, especially along grain boundary as the external tensile strain was applied to copper substrate. Moreover, the surface defects were introduced in the process. This new insight provided a fundamental interpretation of the surface roughness of oxide layer and the origin of surface defects under applied tensile loadings.