Turbine Components Corporation

Introduction

Seventeen years ago the author worked on what was apparently the first application of a thermal barrier coating in a production gas turbine engine. The coating was applied to a louvered sheet metal transition duct in the hot section of the engine to alleviate a severe buckling, cracking and oxidation problem. The coating was bi-layered, consisting of a metallic bond coat and a top layer of zirconium oxide. The presence of the coating doubled the life of the part. For obvious reasons, this re-kindled interest in a broader range of applications in gas turbines. Subsequent development and applications of more durable coatings led to use on gas turbine burner components, vane platforms, vane airfoils and evaluation on diesel engine pistons, valves and heads.

This keynote paper will briefly define thermal barrier coatings and their functions, and will then trace the history of their refinement leading to contemporary and potential future applications. Difficulties and successes will be described within the limits of proprietary or sensitive information. Suggestions for further work to advance the technology base will be made. It is anticipated that following papers will then cover the technical aspects of current applications of, and research on thermal barrier coatings in much more detail.

Definitions

The microstructure of a typical contemporary plasma sprayed thermal barrier coating is shown in Figure 1. It consists of a metallic bondcoat, usually an MCrAlX alloy, where M stands for iron (Fe), cobalt (Co), or nickel (Ni), or

Figure 1. Typical microstructure of plasma sprayed two layer metallic bond coat - zirconium oxide thermal barrier coating (100X).

combinations thereof, and X is an element with a high affinity for oxygen such as yttrium (Y), ytterbium (Yb) and hafnium (Hf). The coating is deposited by injecting particles of the desired composition into a high velocity, partly ionized gas stream wherein the particles are heated and propelled at high velocity to impinge (splat) against the substrate to build up the coating shown.

Alternatively, the zirconia coating can be deposited from the vapor state by electron beam evaporation or sputtering. Such processes produce coatings with columnar structures similar to that shown in Figure 2. When these processes are used it is common practice to deposit both the metallic bond coat and the

0 0

Post a comment