Hard/High Temperature Coatings

With three decades of experience in fabricating and characterizing numerous metal, ceramic, and intermetallic coating systems, the Surface Processing and Mechanics Group has developed broad surface engineering capabilities. Coatings have been developed for applications including automotive wear components, high-temperature super alloy hardware for gas turbine engines, heat exchangers, optically absorbing surfaces, and high-temperature ceramic components. The group has particular expertise in chemical vapor deposition (CVD) of single and multiphase coatings. Advantages of the CVD process include high-purity coatings, non line-of-sight conformal coverage, good microstructural control, excellent adhesion and a large material pallet. Recent research has also involved coatings fabricated by solgel processing and other liquid phase routes.

Thermal barrier coating (TBC) systems are used to reduce the operating temperature of air-cooled superalloy hardware in the hot-section of aircraft and land-based gas turbine engines.  They typically consist of an oxidation-resistant metallic bond coating overlaid with a thermally-insulating ceramic top coating.  Although failure of these complex coating systems is not well understood, the high temperature oxidation behavior of the bond coat typically exerts a major influence on coating durability.  TBC-related work at ORNL has focused on (1) fabrication of high-purity CVD aluminide bond coats for improved oxidation performance, and (2) investigation of the oxidation behavior of various bond coatings and model bond coat materials in order to develop and demonstrate methods of designing improved bond coatings.

• Effects of Platinum Additions on the Adherence of Alumina Scales to CVD Aluminide Bond Coatings
• Effects of Platinum Additions and Sulfur Impurities on the Microstructure and Scale Adhesion Behavior of Single-Phase CVD Aluminide Bond Coatings (PDF 578KBytes)
• Effects of Platinum Additions on the Adherence of Alumina Scales to CVD Aluminide Bond Coatings (PDF 529KBytes)
• Effects of Pt Incorporation on the Isothermal Oxidation Behavior of CVD Aluminide Coatings (PDF 1,535 KBytes)
• A list of TBC Publications

Interface Coatings

Standard Silicon Carbide composite exposed to steam at 1500ºC. High temperature oxidation forms silica scale which spalls off. Fresh layers of silicon carbide are exposed and the process repeats until the part erodes and fails.

Doped SiC layers have been investigated. Recent studies have focused on stochiometric quantities of oxides that will form zircon and mullite when exposed to a steam environment. Powder metallurgy is being investigated as a way to evenly mix these components over a broad surface.

Characterization of Refractory Metal (Al Zr) doped Si₃N₄ and SiC composites

Even ceramic materials such as SiC and Si₃N₄ can be subject to corrosion if exposed to sufficiently aggressive conditions.  For example, silica-forming structural ceramics are very stable in dry oxidation environments, but are rapidly degraded in high temperature oxidizing environments containing water vapor.  Environmental barrier coating (EBC) systems are being developed to protect high temperature structural ceramics from rapid oxidation or volatilization in combustion environments.

Contact: Ted Besmann