EB Deposition of Thermal Barrier CoatingsAlbert Feuerstein - Praxair Inc.
Thermal barrier coatings of Yttria stabilized Zirconia and bond coats of Platinum plate diffusion Aluminides have become a standard for industrial gas turbines and aircraft engines operating at the most efficient highest temperatures possible. Praxair has been applying such coatings for several years using technology licensed from General Electric and United Technologies. The coating are used to protect the base metal in engines which actually operate above the melting point of their metallic components. For this to work the coated components must be continuously cooled internally, and contain a vast complex of internal air channels for that purpose. The thermal barrier coating (TBC) keeps the surface of the substrate metal from melting. The bond coating serves two purposes: it allows bonding of the TBC to the substrate, and it protects the substrate from oxidation.
Yttria stabilization of a Zirconia material (about 8-10% Yttria) prevents a phase change in the Zirconia which would overstress and crack up the coating if not stabilized.
There are several methods to apply coatings. Plasma Spray produces a low density coating which would have very limited life. A process called Zircoat helps against this by predisposing the coating to form vertical columns over the substrate, relieving some of the thermal stress across the surface, allowing for a more permanent bond. The Electron Beam Vapor Deposition (EBPVD) coating process creates a very columnar structure well anchored in the bondcoat over the substrate. In terms of performance (as measured by Thermal Conductivity) Plasma spray is by far lowest, with Zircoat reaching about 1 W/m-K and EBPVD reaching about 1.5 - 1.7 W/m-K. Better TC means higher possible operating temperatures and better engine efficiency which justifies the more costly process.
The bondcoat applied for the process is known as an MCrAlY "Em Craly". It can be applied by HVOF, Argon shrouded plasma spray, and low pressure plasma spray processes. It is a predictable failure of the bond coat (due to diffusion of Al into the substrate) that determines the useful life of the TBC. If the remaining Al3O3 exceeds 5 micrometers failure will occur. Yttria in the bondcoat can inhibit this process somewhat. The bond coating process of favor at Praxair is to plate the substrate with platinum and then allow diffusion in a furnace with Aluminum vapor present.
The EBPVD process is a thermal evaporation process. The ceramic is introduced as an ingot and vaporized by the powerful electron beam in a vacuum. Because of the high energies involved the electron beam is preferable to a laser for this purpose which would pose a higher safety risk at a 70 to 100 KW level. Nevertheless radiation safety is a serious concern with the process. If only one ingot and beam were used the deposition of vapor reaching condensation point on the substrate would be quite uneven, but when two ingots are simultaneously vaporized by two beams with the proper spacing, and rectangular pattern of fairly uniform vapor deposition can be achieved. The vaporization of ingot occurs at 4000-5000C. The desire is to deposit vapor to the ceramic at about 1000C onto the substrate surface to achieve the correct columnar buildup. Because it is difficult to get the plumes of vapor to be stable, the deposition equipment is designed so that the plume can be generated continuously, while different loads of parts are introduced into the chamber from one side and then the other in alternation. The parts (which are preheated in side chamber to approx. 2000F prior to introduction to the vapor) are fixtured so they can be rotated for even coating while exposed to the vapor plume in the chamber.
There are many safety issues involved with the process, and no less than 100 lockouts are in place to prevent possible injury to personnel. Operators are people selected for exceptional understanding and attention to detail and quality. Praxair currently operates four of these coaters (which have been made functionally identical to ensure equal product and safe operation of each of them). A fifth coater is planned for the near future.