Progress in coatings for gas turbine airfoils

The development of ever more efficient gas turbines has always been paced by the results of research and development in the concurrent fields of design and materials technology. Improved structural design and airfoil cooling technology applied to higher strength-at-temperature alloys cast by increasingly complex methods, and coated with steadily improved coating systems, have led to remarkably efficient turbine engines for aircraft propulsion and power generation. For first stage turbine blades, nickel-based superalloys in Various wrought and cast forms, and augmented by coatings since the 1960s, have been singularly successful materials systems for the past 50 years-and still no real world substitutes are on the horizon. This paper traces the history of protective coatings for superalloy airfoils beginning with the simple aluminides, followed by modifications with silicon, chromium and platinum, then MCrAlY overlay coatings, and finally the elegant electron beam vapor deposited ceramic thermal barrier coatings recently introduced to service. The publicly available results of several decades of research supporting these advances are highlighted. These include generic research on oxidation and hot corrosion mechanisms of superalloys and coatings, the intricacies of protective oxide adherence, mechanisms of low temperature (Type II) hot corrosion, and of aluminide coating formation and mechanical properties of alloy-coating systems. With no promising turbine materials beyond coated nickel-base superalloys apparent in the foreseeable future, continued progress will likely be made by further refinement of control of thermally grown oxide adherence, and by more cost effective manufacturing processes for contemporary types of protective coatings. (C) 1998 Elsevier Science S.A. All rights reserved.