High Temperature Behaviors And Aluminum Diffusing Mechanism Of Containing Aluminum Coatings Produced By Thermal Spray

Oxidation and hot corrosion are the main reasons of which the machines or parts used at elevated temperatures degrade. The oxidation resistance of metal materials mainly depends on the properties of the oxide formed on the metal surface. Aluminum is one of the best candidates since alumina film is stable, dense and continuous. Materials, such as steel, copper and nickel, alloyed with aluminum are good oxidation-resistant and hot-corrosion-resistant. It is well known that high-temperature oxidation and hot-corrosion take place on the surface of materials. In order to protect steel and copper parts used at elevated temperatures from oxidation and hot corrosion, arc-sprayed aluminum coatings and aluminum-based coatings were studied in this dissertation.To estimate the behaviors of the coatings at elevated temperature, some hot-corrosion tests and continuous oxidation kinetics tests were performed. By using scanning electron microscopy (SEM), electron probe micro analyzer (EPMA) and X-Ray diffraction analyzer, the surface morphology of coatings and the microstructure of the diffusion layers formed in the coatings and substrates were examined. Also, the relationships between the process parameters, the composition of coating and the thickness of the diffusion layers were studied in this work. In addition, the interdiffusivities of the elements in aluminized layers were calculated by using finite element methods.The results of the experiments revealed that the arc sprayed single coatings of aluminum, aluminum bronze or FeCrAl and the composite coatings of aluminum/aluminum bronze or aluminum/FeCrAl could well protect the steel substrate from high-temperature oxidation, the composite coatings especially being better. The high-temperature sulphuration tests in situ gave a strong proof that aluminum/FeCrAl composite coatings provided a good preservation to gray iron substrate. Under the conditions of high sulphur and low oxygen atmosphere at elevated temperature, the interdiffusion of the elements in the composite coatings resulted in the formation of an intermetallic compound, AlCrFe₂. Therefore, the coatings were oxidation-resistant and sulphuration-resistant since there was a preservative alumina layer formed on their surface.When the specimens coated with aluminum were heated at high temperatures, an intermetallic compound, Fe₂Al₅, was formed on them, because of the interdiffusion of aluminum and iron atoms. It was found that the process parameters and the thickness of as-sprayed aluminum coating could greatly effect on the thickness of the diffused layers. The thicker the arc-sprayed aluminum coating, the thicker the diffusion layers at the temperatures ranging from 700—1100℃.It is much the same as steel that the thickness of diffusion layers was effected on by the thickness of as-sprayed aluminum coating on the copper substrate. There were two layers in the diffused region, the outer layer which originally was an as-sprayed aluminum coating was fomied by the diffusion of copper atoms into the coating upwards and the inner layer formed by the diffusion of aluminum atoms into copper substrate downwards. The results of X-ray diffraction and EDS analyses revealed that both layers consisted of CU₉Al₄ andα-copper solid solution.By using MATLAB software, the interdiffusivities of Fe₂Al₅ phase at temperatures of 800°C, 900°C and 1000°C were calculated to study the diffusing behaviours of elements in aluminid layers. The results of calculation revealed that the interdiffusion coefficients at 800°C increased with increasing of aluminum concentration in the diffusion layer, which ranged from 10^-12 to 10^-11. However, that at 900°C was from 10^-11' to 10^-10, which was tens times more than that at 800°C under the same concentration conditions. By using V. I. Dybkov's experimental formulation, which has relation with interdiffusion coefficients, the thickness of the aluminide was calculated. The calculated results did not have big difference with those from experiments so it proved indirectedly calculated interdiffusion coefficients to be reasonable.