Friday, December 31, 2010

United Technologies Develops Rare Earth Nanocoating for Engine Thermal Barriers

United Technologies Corporation (Hartford, CT) earned U.S. Patent 7,858,212 for dispersion strengthened rare earth stabilized zirconia.   The materials relate to a ceramic coating containing dispersion strengthened rare earth stabilized zirconia to be applied to a turbine engine component and a method for forming such a coating. 

According to inventors Kevin W. Schlichting, Paul H. Zajchowski and Susan Manning Meier they have developed a process for forming a coating on a substrate, such as a turbine engine component, is provided. The process comprises the steps of providing a rare earth oxide stabilized zirconia composition, blending the first rare earth oxide stabilized zirconia composition with at least one additional constituent selected from the group consisting of TiO2, Al2O3, a blend of Al2O3--TiO2, La2Zr2O7, and 20 wt % Yttria Stabilized Zirconia; and depositing the blended powder onto the substrate.  

The coatings may be applied to any component of an engine requiring a thermal barrier coating/abradable system or a clearance control system.

An additional benefit of the low thermal conductivity of the coatings is that the coating can be applied at lower thicknesses, thereby saving weight while maintaining equivalent thermal protection as compared to conventional coatings.

The coatings are called dispersion strengthened coatings because they contain a dispersed second phase which improves coating toughness. 

Ceramic thermal barrier coatings have been used for decades to extend the life of combustors and high turbine stationary and rotating components. Zirconia has typically been the base ceramic. Stabilizers have been added to prevent the deleterious phase transformation to the monoclinic phase from the high temperature stable cubic or tetragonal phase. 

Early stabilizers such as 22 wt % magnesia were utilized, but as turbine temperatures increased beyond 1900 degrees Fahrenheit, the durability of the magnesia stabilized zirconia deteriorated since magnesia stabilized zirconia crystallographically destabilizes above 1750 degrees Fahrenheit. 

Compositional improvements led to a 7 wt % yttria stabilized zirconia. With this composition, thermal barrier coatings attained a good balance between durability, i.e. thermal oxidative cycling; stability, i.e. sintering and phase stability; mechanical properties, i.e. fracture toughness, erosion resistance and adherence; and thermal properties, i.e. thermal conductivity and thermal expansion. 

As current engine models continue to increase temperatures and warrant decreased component weight, advanced ceramics are being pursued. A zirconia based coating, such as a gadolinia-zirconia coating as described in commonly owned U.S. Pat. No. 6,177,200 has been developed which provides a reduced thermal conductivity ceramic thermal barrier coating. However, such coating would benefit from improved mechanical properties, such as fracture toughness and erosion resistance. 

The United Technologies material provides a dispersion strengthened rare earth stabilized zirconia coating composition which exhibits desirable mechanical properties for use in jet engines and other high temperature environments. 

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