Continuous Al-supply in Cr2AlC MAX phase during oxidation at high temperature
AZINA C. 1, HANS M. 1, EKLUND P. 2, GONZALEZ JULIAN J. 3, SCHNEIDER J. 1
1 Materials Chemistry, RWTH Aachen University, Aachen, Germany; 2 Energy Materials Unit, Thin film Physics Division, Dept. of Physics, Chemistry and Biology (IFM), Linköping University, Linköping, Sweden; 3 Chair of Ceramics, Institute of Mineral Engineering, RWTH Aachen University, Aachen, Germany
Phase stability is likely to be one of the most important specifications which determine the lifetime of materials operating in extreme environments. In the case of MAX phases, the weakly bonded A-elements diffuse along the basal planes when a thermal load is applied or when in presence of oxidizing environments. That is the case of Cr2AlC, the loss of Al causes the decomposition of the MAX phase into the binary carbide, Cr7C3. In this work, the possibility of continuously supplying Al to a Cr2AlC coating is investigated, in order to avoid the formation of the carbide layer. To this end, Cr2AlC substrates with different microstructures were produced using Spark Plasma Sintering (SPS) from either elemental powders or Cr2AlC powders obtained by solid-state reaction and molten salt shielded synthesis (MS3). Using different starting powders allowed consolidating bulk samples of different grain sizes and phase purities. The substrates were subsequently coated with Cr2AlC coatings which were sputter-deposited from a powder metallurgical composite target. The oxidation behavior of the MAX/MAX assemblies was investigated both in- and ex-situ in order to assess the feasibility of supplying Al to Cr2AlC during oxidation at high temperatures. Overall, the concept of Al-supply from the substrate to the coating is shown to be highly dependent on the original substrate microstructure.