Growth mechanisms and microstructure evolution of MAX phases thin films and of oxide scales on high temperature materials

The knowledge of the exact conditions under which a material forms a microstructure with optimum physical and chemical properties is essential for any technological application. Beyond this, it is also often crucial for reliable and save operation to know how a particular material changes its microstructure under long-term working conditions, e.g. at high-temperature and oxidising atmosphere. Recently, the class of MAX-phases and some Fe-Cr-Al model alloys have gained increasing interest in materials research due to their promising properties for future applications in the high-temperature regime. In order to shed some light on the hitherto unknown growing mechanism of MAX-phases, thin-films of the prominent phases Ti-Al-C and Cr-Al-C were synthesised by magnetron sputtering at various temperatures and their microstructure was subsequently characterised in great detail. Moreover, the effect of oxidation time on the microstructure was studied using thin-film samples with nominal composition Cr2AlC. These investigations were completed by a study on the changes in the microstructure of Fe-Cr-Al model alloys for different thickness and high-temperature oxidation conditions. The microstructural characterisation was performed in all cases using analytical transmission electron microscopy (TEM) on focused ion-beam (FIB) machined cross section specimens. The investigation on as-deposited thin films resulted in several new findings. At high deposition temperatures (850°C), Ti-Al-C thin films form a porous microstructure with many faults and contain some minor phases such as TiC and Ti3AlC2 in addition to Ti2AlC. On the contrary, the microstructure of Cr-Al-C thin-films is free of pores and consists only of Cr2AlC, despite a significant lower deposition temperature (650°C) than used in the Ti-Al-C system. On the other hand, specimens deposited at 450C and 550C are not found any longer as single-phase and decompose into Cr2AlC and Cr23C6 as main constituents. The obtained results confirm that magnetron sputtering of MAX-phases is a complex process which can yield entirely different microstructures depending on the chemistry of the system. However, was possible to prove that good quality Cr-Al-C thin-films can be obtained at temperatures much lower than required to synthesise thin-films in the Ti-Al-C system. This indicates that the nature of the M element likely has the greatest effect on the deposition temperature of a particular MAX-phase and the developed microstructure. Investigations on the oxidation performance of the MAX-phases were carried out using Cr2AlC thin-films as a model system. Notably changes in the microstructure upon oxidation for periods of 4 min, 39 min and 282 min are a rise of the thickness of the upper alumina scale, the increase in the porosity underneath the oxide scale and growing of a Cr-C interlayer at the boundary between substrate and thin-film. In particular the formation of voids and pores below the upper oxide scale appears to be a special feature of the thin-films that is unknown for oxidation of bulk Cr2AlC. Despite the observation that the Cr2AlC thin-film samples undergo decomposition into Cr3C2 and Cr7C3 under oxidation conditions, they still show a fairly good overall oxidation resistance. The series of oxidation studies was completed by a detailed investigation on Fe-Cr-Al alloys doped with Y and Zr. Since incorporation of Zr in the oxide scale became evident during in the early stages of the investigations, specimen with thickness of 0.3 mm and 1.3 mm were later used for studying the effect of the available Zr on the oxidation rate. As a main result it was found that the amount of precipitates in the oxide scale is different only for long oxidation periods. Moreover, EDX analysis revealed that thin specimens always contain more Y-rich precipitates, especially under high-temperature oxidation conditions. The varying thickness of the alumina scale thickness and the finding of Zr-rich precipitates indicates that the extended oxidation life time of the alloy must be directly linked with the presence of a Zr reservoir in the specimen. Based on this a modified model was developed, which describes the mechanism of Zr incorporations into the alumina scale upon oxidation for specimen with finite thickness