New Application Technique Improves Efficiency Of Thermal Barrier Coatings

Dr. Jogender Singh, professor of materials science and engineering and head of the Advanced Coatings unit of Penn State's Applied Research Laboratory (ARL), says, "Nearly 75 percent of aircraft engine components have metallic or ceramic coatings to enhance performance and reliability where corrosion, high-temperature oxidation and wear are concerns. Power generator turbines also rely on coatings, as do multilayered ceramic and metallic films used in the fabrication of microelectronic components."

While a variety of industrial thermal barrier coating techniques currently exist, each has disadvantages as well as advantages. Singh says that many of the shortcomings of these techniques can often be overcome through the use of electron beam- physical vapor deposition (EB-PVD). In EB-PVD an electron beam bombards and vaporizes ceramic or metallic coating material and, thereby, controls the way in which the coating is deposited and adheres.

Singh emphasizes that using EB-PVD doesn't change the basic chemistry of the coating material. Rather, the way the coating is applied reduces, for example, its thermal conductivity and enhances its ability to resist corrosion and high-temperature oxidation.

He will describe the process in detail at the International Conference on Metallurgical Coatings and Thin Films in San Diego, Tuesday, May 1, in a paper, "Architecture of Thermal Barrier Coatings Produced by Electron Beam Physical Vapor Deposition." His co-authors are Douglas E. Wolfe, ARL research assistant, and Jason Singh, State College Junior High School student.

Penn State's industrial pilot EB-PVD unit has six electron beam guns housed in a vacuum chamber. Four of the beams are used to evaporate the coating materials and two are used to preheat the object to be coated to facilitate adhesion. The high-energy electron beams, 45 kilowatts each, are focused on the materials to be evaporated and, as the vapor is formed, the object to be coated is maneuvered within the vapor cloud to facilitate uniform coating. Two or more co-evaporated coating materials can be mixed in the vapor cloud.

Coating compositions can be varied via co-evaporation and coatings comprised of alternating layers of different compositions can be made at relatively low temperatures. The process also offers relatively high deposition rates compared with other coating deposition techniques, produces dense coatings, and results in low contamination and high thermal efficiency. In addition, coatings produced by EB-PVD have a good surface finish and a uniform microstructure.

Attaching an ion beam source to the EB-PVD system enables production of textured surfaces. In addition, the ion beam can be used to clean the surface of the object to be coated. The cleaning enhances the coating bonding strength.

"Using EB-PVD and co-evaporation, we can tailor the properties of the coating so that it can be used in a wide variety of applications," says the Penn State researcher.

For example, Singh notes, "It may be possible to coat the surface of cylinders used in heavy truck engines so that they can operate at much higher temperatures and use less fuel. A two percent fuel saving offers a potential of billions of dollars in fuel saving." The EB-PVD technique is being patented by Penn State. The research was supported by grants from the U.S. Navy and NASA as well as internal Applied Research Laboratory funds.