The focus in hot section coatings of gas turbines is shifting from temperature to time – so it's all about coating durability, some say. Yet, thickness of thermal barrier coatings (TBC) is still a key issue, as parts of the industry keep pushing for ever-higher firing temperature to boost efficiency rates.
Temperatures in cooled turbines reach around 1250 degrees, enabling efficiency rates of close of just over 60%.
"If we were to reduce cooling air and increase ISO temperature, we could increase relative efficiency by 18% and increase power output," said Matthias Oechsner, professor for materials technology at TU Darmstadt. He cautioned, however, that such a move is not possible at present as some turbine materials cannot withstand higher temperatures.
In his view, it is clearly the thickness, not the thermal conductivity, which is important to minimise the coating temperature. "TBC thickness – that is something you should spend your money on when you are developing gas turbine coating," he told Gas to Power Journal.
Eliminating ‘substances of concern’
PWPS engineers, meanwhile, highlighted the need for R&D to “eliminate substances of concern” in turbine coating. This includes the replacement of lead-containing anti-gallants for blade attachments and disc slots as well the replacement of hexavalent chromium-containing conversion coatings, anodize, seal coats, adhesive primers and plating with alternate coating.
The need for erosion coating is seen to increase as compressor airfoils are becoming thinner which impacts on GT performance. Air quality and ambient temperature is another key issue, notably in Asia. Advanced blade cooling schemes hereby require internal corrosion protection.