The long-term viability of machine components operating at high temperatures in an oxidising environment often relies on the integrity of a thin, protective layer of oxide, typically chromia, alumina or silica.
Understanding the mechanisms of the formation of such a protective layer and the processes which affect its mechanical integrity, particularly during temperature changes, are major challenges to this research group as well as to the larger oxidation community.
The compositions of commercial alloys are generally optimised to produce such protective oxide layers but in some applications such as gas turbines, where components may be highly stressed, good oxidation resistance is incompatible with the need for high creep strength. In these cases, oxidation-resistant coatings are applied. Another large area of activity within the Group is the design and testing of new coatings.
The experimental equipment available for work in these areas includes a six-station thermobalance facility capable of undertaking oxidation exposures at temperatures to 1400°C whilst continuously monitoring specimen mass.
This equipment is used to determine the critical temperature drop during cooling to produce loss of surface oxide. The results have provided valuable insights into the oxide spallation process.
A particularly exciting development has been the creation of a coating that can respond in a pseudo-intelligent manner to its local temperature to form an appropriate type of protective oxide. This concept is being developed in collaboration with Cranfield University and various UK industrial companies.
These experimental studies are complemented by extensive theoretical studies, both of the mechanical stability of the oxide layer, and the subsurface depletion of the oxidising element.
These studies are undertaken by use of a unique finite-element model capable of modelling the growth of an oxide/metal interfacial creep crack, and a 2D interdiffusion/oxidation model capable of simulating solute depletion in arbitrary specimen geometries.