Protective Nitride Films On Uranium Prepared By Ion Plating

Uranium is widely used in the national defense and nuclear energy engineering fields for its unique properties. However, Uranium has rather active chemical properties, which includes corroding quickly in a natural environment. As a result much attention has been paid to the surface technology of uranium. Minimizing the corrosion of Uranium by using some proper techniques have been studied by scientists all over the world.The corrosion of uranium eventually causes inactivation of components, so it is most important to construct anti-wear and anti-corrosion films on the uranium surface. In the many surface modification techniques, magnetron sputtering and arc ion plating are the most widespread. The former has the following advantages: fast deposition rate, compact films, and low deposition temperature. The latter method has the advantage of: high ionized rate, and good cohesion between the film and substrate. Thus for our experiments we have chosen the two techniques to deposit films.We have utilized an MW-ECR plasma enhanced deposition method to deposit ZrN and TiN films on a simulation material #45 steel. The study mainly involved the change of the bias voltage and the N₂ partial pressure. The results show that the Zr-N structure is influenced by the bias voltage and the partial pressure of N₂, however the influence is not obvious on the Ti-N structure. As the partial pressure of N₂ changes, the hardness values of ZrN films on #45 samples vary between 19.8²6.3GPa, first increasing then decreasing. The hardness values of the TiN films are between 23.0²8.2 GPa, and increase as the partial pressure of N₂ increases. With a bias voltage increase, the hardness of ZrN and TiN films shows the trend of first increasing then decreasing, and the hardness values of the ZrN films are between 20.0 26.0GPa, while the TiN films are between 22.5 28.4 GPa. The anodic polarization experiments in 0.5mol/L NaCl solution indicate that, compared to the substrate, Ecorr of ZrN films can increase to approximately 118mV; the Icorr of the substrate is 9.036uA, and it can decrease to a minimum of 0.142μA after the films are deposited. While the Ecorr TiN films can increase to approximately 111mV, Icorr decreases about 2 orders of magnitude. Within the parameters chosen for the experiments, samples #2 ZrN and 2-#l TiN which have a close chemical ratio have displayed the best combination of wear- and corrosion- resisting properties. We deposited ZrN and TiN films with same the chemical ratio to sample #2 ZrN and 2-#l TiNon hemisphere shells of #45 steel, and exposed them to the atmosphere for about 2 years. At the time of the end of the experiment there was no obvious corrosion.The multi-layered Ti/TiN films, compared to TiN, deposited on Uranium substrates was done by arc ion plating. The results show that the film surfaces are smooth, dense, but have big grains and circular pits. The number of big grains decreases as the bias voltage increases. The multi-layer films also grow layer by layer or by island expansion. As the bias voltage is increased, the directional crystals become fine, and the films become denser. Also, the friction coefficient decreases, while the wear-resistance is enhanced. Each layer of the multi-layer films is a little thicker than the single layer film, the hardness and wear-resistance are slightly less than a single layer film. But the dividing experiments indicate that multi-layer films have better adhesion, and the best adhesion is 72N, which is obtained at a pulse bias of-800V.The anodic polarization experiments in 50ug/g CV solution indicate that Ecorr of the sample at a pulse bias of-800V increases to about 714mV, and Icorr decreases about 2 orders of magnitude. The XPS analyses indicate that the corroded multi-layer films display better anti humidity and thermal corrosion properties in a 50"C, 75%RH humidity and thermal environment. The study on electrochemical corrosion mechanisms indicates that single layer films and gradient films become inactive by deficiency penetration through the substrate and resulting in less corrosion-resistance; while the inactivation of multi-layer films is due to layer malfunction, which makes it difficult for corrosive substances to reach the substrate. In this way the corrosion resistance is enhanced.In our research, we also outline a series of experimental procedures. We have specially designed the clamping fixture and rotational structure and overcome the barrier effect of a hemisphere, which improves the deposition uniformity. According to our results, we have introduced a plasma immersion ion-implantation and deposition system. We have also developed a process of baking and sputtering to clean the samples at the same time, which solves the problem of the oxidation of films/substrate interface. The increasing temperature does not cause acceptable deformation of thin-walled samples. After optimizing the parameters, we have obtained good corrosion resisting protective films. Exposing in moist heat for the duration of the experiment, about 10 months, we observe excellent corrosion resisting properties. It is a promising technique for some engineering components.