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

表面涂层以提高零部件的氧化和耐磨性是多年来的技术状态。然而，在新涂层材料的研究中存在着持续的努力，这表明了最佳的保护，从而导致了例如关于效率的工艺的优化。Cr2AlC属于MAX相，具有陶瓷和金属性能以及优异的抗氧化性。因此，这种材料有很大的潜力作为抗氧化和耐磨涂层。影响涂层特性和寿命的一个关键因素是残余应力。通过在部件表面选择性地产生残余应力，有可能极大地延长寿命。另一方面，在热循环暴露过程中，由于热膨胀系数的巨大差异而引起的热致残余应力可能会导致涂层的灾难性失效。到目前为止，还没有关于残余应力的稳定性以及Cr2AlC涂层在高温和高温下的性能耐久性的研究成果。然而，对这个问题的了解对于薄膜在热曝光中的应用是至关重要的。此外，由于Cr2AlC的氧化导致铝的选择性氧化对碳化铬和氧化铝相形成的影响，以及残余应力的发展和薄膜的特征，目前还没有结果。本文研究了Cr2AlC薄膜中残余应力的发展和稳定性与沉积参数、衬底、热处理和热机械处理以及氧化过程的关系，包括与相形成和转变有关的所有过程。这些研究有助于了解薄膜中残余应力的发展，以及研究Cr2AlC涂层作为氧化和耐磨涂层的能力。在本项目中，我们对在等温炉和热循环炉中长期处理的涂层进行了等温线和热循环原位高温X射线衍射(XRD)测量，并进行了非原位X射线衍射分析。由于氧化过程复杂，实验将在真空和氧化气氛中进行，以分离热影响和氧化影响。腐蚀试验应考虑残余应力和氧化过程中的相发展的影响，评估Cr2AlC涂层在机械和/或热暴露下的耐蚀性。此外，还分析了薄膜中残余应力的松弛过程和失效机理。因此，它允许在热和机械负载期间对工艺进行广泛的评估。

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.