Investigation of low temperature oxidation behavior of MoNbTaW thin films

MoNbTaW is a promising refractory multi-principal element alloy (RMPEA) that has exhibited good high temperature phase stability in vacuum and exceptional high temperature mechanical properties—even surpassing those seen in Ni superalloys. To date, there have been no detailed studies regarding the environmental resistance of this alloy, which is a primary concern due to the constituent elements being readily oxidizable at elevated temperatures. Therefore, in the current work, in-air anneals of sputtered MoNbTaW films targeting the 200°C–400°C range were conducted to study the low temperature oxidation behavior. An amorphous oxidized layer, which was comprised of three compositionally distinct regions, was observed to form at all in-air annealing temperatures. Furthermore, via nanoindentation, the oxidized layer was determined to be significantly softer and more compliant than the underlying metal. Comparison to pure W samples under the same annealing conditions was also performed. From this, it was determined that MoNbTaW oxidized more quickly and exhibited different oxidation pathways than pure W in the studied temperature range. Through this work, the initial stages of oxidation and their impact on the mechanical properties in this alloy system can begin to be understood.