Advanced Coatings for Turbine Engines
28 August 2007
Gas turbine engine efficiencies are dictated by their temperatures of operation. Increasing these temperatures leads to reductions in fuel consumption and greenhouse gas output.
Turbine temperatures are now exceeding the capabilities of superalloy components, and it has become necessary to cool them. This is done by pumping air or steam through cooling channels running through the component interiors, and providing thermal insulation (a Thermal Barrier Coating or TBC) on top of an oxidation resistant coating. The TBC is a ceramic made of Y2O3 – stabilized ZrO2 ; the oxidation resistant coating, known as a bondcoat, is an aluminium-rich material (several designs are possible); and the superalloys are complex, nickel-base alloys containing chromium, aluminium and numerous other elements.
Oxide formed on Ni-22Al-30Pt + 1 wt% Hf oxidised in flowing air at 1200°C
An essential function of the bondcoat is to resist oxidation at temperatures in the region of 1000-1200°C. In order to obtain this performance, we have been investigating the oxidation resistance of platinum-modified nickel aluminide coatings. The main discoveries to date have been that oxidation is much worse when the gas contains water vapour, but that the addition of 15-30% Pt to Ni-22Al provides excellent isothermal corrosion resistance. Unfortunately these compositions fail under the thermal cycling conditions which are unavoidable in aero engines.
A collaborative research program involving UNSW with the Japanese National Institute for Materials Science and Hokkaido University, plus the Ames National Laboratory in the US, has centred on these coatings. Work at UNSW has established that the addition of small amounts of Hf improves the cyclic oxidation performance in air-steam mixtures. When Ir is partially substituted for Pt, a highly oxidation resistant coating is produced.
Professor David Young
Fracture cross-section of oxide scale on Ni-22Al-30Pt + 1 wt% Hf oxidised in flowing air at 1200°C
FIB cross section on Ni-20Al-15Pt-10Ir + 1 wt% Hf oxidised in flowing air at 1200°C