Quarterly Success Stories

Advancement of a New Application Technique for Improved Thermal Barrier Coatings

An important cause of failure for thermal barrier coatings (TBC) in turbines has been the growth of internal aluminum oxide scales within the TBS at the bond coat. This produces internal stresses and ultimate cracking because of the volumetric growth of the internal oxide layer. The detrimental growth of the alumina scales results from oxygen penetration through the porous outer layer of the TBS to the underlying bond coat.

Under the Advanced Gas Turbine Systems Research (AGTSR) program, Northwestern University (NU) has evaluated a new Small Particle Plasma Spray (SPPS) process to apply a thin YAG (yttrium aluminum garnet) layer on TBC bond coats to block oxygen penetration and thereby alleviate TBC failures. This process also offers the potential for applying graded layers of coating materials to facilitate engineered TBC coatings with other specialized properties. Research in this recently completed project showed that SPPS is a viable process for applying the YAG layer on a bond coat. Experiments on specimens of coated turbine materials indicated that a TBC with the YAG layer on the bond coat experienced internal oxidation rates nearly a factor of two lower than rates for specimens with conventional TBCs. The project has also shown the SPPS process can produce TBC coatings that experience lower fatigue damage (through graded porosity). Lower fatigue damage in addition to lower internal oxidation rates demonstrated in this project indicates that the SPPS process offers the potential for enabling longer TBC lifetime in turbines.

Two patents (#5,744,777 and 5,858,470) were issued concerning the SPPS process during the course of this AGTSR project.

University of Connecticut Determines Effects of Turbine Cycles on TBC Life

There is no accurate measurement technique to predict the expected remaining coating life on turbine parts. Consequently, the great variability of TBC coating lifetimes has resulted in turbine coating failures in the field or parts prematurely taken out of service if removed based on a conservative lower bound of expected coating lifetime.

Under the Advanced Gas Turbine Systems Research (AGTSR) program, the University of Connecticut (UCONN) has been evaluating the use of laser fluorescence (LF) to measure average internal stresses for non-destructive evaluation (NDE) of thermal barrier coatings (TBCs). Experiments in the project subjected TBC coated specimens to thermal cycles up to a temperature of 1121 C (2050 F). The LF technique was able to predict remaining life of coatings to within 5% for specimens that had been exposed to 1 hour thermal cycles and to within 7% for specimens that had been exposed to 24 hour thermal cycles. Useful engineering predictions of remaining TBC lifetimes were consequently shown for 1 hour and 24 hour thermal cycles. However, except for LF measurements taken near the end of coating life, the correlation of LF data with remaining TBC life differed for the two different cycle times. Additional work using LF measurements at times closer to end of life will evaluate whether the prediction method can be used without requiring knowledge of cycle times.

Improved Roughness Information for Turbine Airfoil Design

Roughness characteristics of turbine vane and blade airfoil surfaces change with operation time due to erosion, corrosion, deposition, and spallation of coatings from the parts. These surface changes can degrade the airfoil aerodynamics and cooling from their initial finely tuned, as manufactured, levels.

Under the Advanced Gas Turbine Systems Research (AGTSR) program, the Air Force Institute of Technology (AFIT) and Mississippi State University (MSU) are characterizing the effects of service conditions on turbine vane and blade heat transfer and aerodynamic performance. Over 100 turbine parts have been obtained from Allied-Signal, GE, Siemens-Westinghouse, and Solar Turbines. These components had experienced service in turbines under a wide range of conditions. Scaled models of measured surfaces from turbine parts were produced for wind tunnel experiments in which heat transfer and aerodynamic data were obtained for those surfaces. Comparison of date representing real turbine surfaces and data representing ordered arrays (cones or hemispheres) and equivalent sand grain roughness, which are traditionally used to simulate real surfaces, showed limitations in the traditional methods to characterize roughness of turbine surfaces. A more accurate method was identified to represent turbine surfaces for turbine aerodynamic and heat transfer analyses and design.

The surface roughness measurement database for turbine parts and data from the wind tunnel tests representing the observed roughness are being provided to turbine manufacturers to improve their tools for design and analyses of airfoils.

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