THERMAL BARRIER COATINGS (TBCs) were first successfully tested in the turbine section of a research gas turbine engine in the mid-1970s. By the early 1980s they had entered revenue service on the vane platforms of aircraft gas turbine engines, and today they are flying in revenue service on vane and blade surfaces. Advanced engines, for the foreseeable future, may be expected to rely even more heavily on these coatings. Two processes are used to apply today's TBCs. Plasmasprayed coatings were brought into service first and currently remain in service. By the 1990s, a second type of TBC deposition process--physical vapor deposition (PVD)--was successfully brought into commercial service. The success of both types of coatings in commercial aircraft gas turbine applications, especially the PVD coatings, has played a pivotal role in the acceptance of this technology. However, with this acceptance comes ever-increasing demands on these coatings to perform for longer hot times under increasingly severe conditions.
The earliest ceramic coatings for aerospace applications were frit enamels. The first of these frit coatings were developed by the National Advisory Committee for Aeronautics (NACA) and the National Bureau of Standards (NBS). Frit enamels were used in aircraft engines throughout the 1950s. Later, flame-sprayed ceramic coatings were developed. Of the various early ceramic materials that were evaluated for thermal barrier applications, alumina and zirconia-calcia were the most successful. The bond coat material for these early applications, if one was used at all, was typically nichrome or molybdenum for nonoxidizing environments. Early applications included the protection of sheet metal in jet engines and in rocket engine thrust chambers. The most visible coatings during this period were the flame-sprayed zirconia-calcia coatings on the regeneratively cooled XLR99 thrust chambers for the X-15 experimental rocket planes . A portion of this coating is visible in Fig. 1. With the subsequent development of plasma spray processing--which evolved from research into low-thrust plasma arc engines for spacecraft and from plasma arc test facilities developed for reentry simulation (Ref 11)--the utility of using the very-high-temperature plasmas for spraying ceramics was soon recognized. In 1970, plasmasprayed TBCs began to be used on hot-section transition ducts and other hot-section sheet metal components in commercial gas turbine engines.
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