Residual stress development and stability of Cr2AlC MAX phase coatings under thermal loading

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Surface coatings to improve oxidation and wear resistance for components are state of technology since many years. However, a continuous effort in research of new coating material is present, which show an optimal protection and thus lead to an optimization of processes e.g. concerning the efficiency. Cr2AlC, belongs to MAX-phases which have ceramic and metallic properties as well as excellent oxidation resistance. Hence this material has great potential as oxidation and wear resistant coatings.A crucial impact factor on coating characteristics and lifetime are residual stresses. With a selective generation of residual stresses in the component's surface it is possible to extend lifetime in a great amount. On the other hand thermal induced residual stresses caused by large differences in thermal expansion coefficients during thermo-cyclic exposure may result in catastrophic coating failure. Up to now, there is no work published about the stability of residual stresses and therefore the durability of the properties of Cr2AlC coatings at elevated as well as high temperatures. However, the knowledge of this issue is crucial for the application of thin films at thermal exposure. Additional there are no results known about the influence of chromium carbide and alumina phase formation due to the selective oxidation of aluminum as a result of oxidation of Cr2AlC and the residual stress development and film characteristics. Although the great amount in structural changes seem to influence it clearly.In the present proposal the research of the residual stress development and stability in thin Cr2AlC films in dependency of the deposition parameters, the substrate, the thermal and thermomechanical treatment as well as the oxidation including all processes associated with phase formation and transformation is conducted. These investigations are performed to contribute to the understanding in residual stress development in thin films as well as to investigate the ability of Cr2AlC coatings as oxidation and wear resistance coatings. In this project we perform isotherm and thermocyclic in situ high temperature x-ray diffraction (xrd) measurements combined with ex situ xrd analysis of coatings from long term treatments in isotherm and thermocyclic furnaces. Due to the complex processes which occur during oxidation, the experiments will be done in vacuum and oxidation atmosphere, to separate the thermal influence from the impacts through oxidation. Erosion experiments should permit an evaluation of the resistance of Cr2AlC coatings under mechanical and/or thermal exposure, taking into account the influence of residual stresses and phase development through oxidation.The analysis of the relaxation processes of residual stresses in thin films as well as failure mechanism are carried out additionally. Therefore it allows a widespread evaluation of processes during thermal and mechanical load.