Zaher Mutasim

Effects of alloy composition on the performance of Yttria stabilized zirconia-thermal barrier coatings

Thermal barrier coatings (TBCs) provide an alloy surface temperature reduction when applied to turbine component surfaces. Thermal barrier coatings can be used as a tool for the designer to augment the power and/or enhance the efficiency of gas turbine engines. TBCs have been used successfully in the aerospace industry for many years, with only limited use for industrial gas turbine applications. Industrial gas turbines operate for substantially longer cycles and time between overhauls, and thus endurance becomes a critical factor. There are many factors that affect the life of a TBC including the composition and microstructure of the base alloy and bond coating. Alloys such as Mar-M 247, CMSX-4, and CMSX-10 are materials used for high temperature turbine environments, and usually require protective and/or thermal barrier coatings for increased performance. Elements such as hafnium, rhenium, and yttrium have shown considerable improvements in the strength of these alloys. However, these elements may result in varying effects on the coatability and environmental performance of these alloys. This paper discusses the effects of these elements on the performance of thermal barrier coatings.

Optical Nondestructive Condition Monitoring of Thermal Barrier Coatings

This paper describes recent developments of the thermal barrier sensor concept for nondestructive evaluation (NDE) of thermal barrier coatings (TBCs) and online condition monitoring in gas turbines. Increases in turbine inlet temperature in the pursuit of higher efficiency will make it necessary to improve or upgrade current thermal protection systems in gas turbines. As these become critical to safe operation, it will also be necessary to devise techniques for online condition monitoring and NDE. The authors have proposed thermal barrier sensor coatings (TBSCs) as a possible means of achieving NDE for TBCs. TBSCs are made by doping the ceramic material (currently yttria-stabilized zirconia (YSZ)) with a rare-earth activator to provide the coating with luminescence when excited with UV light. This paper describes the physics of the thermoluminescent response of such coatings and shows how this can be used to measure temperature. Calibration data are presented along with the results of comparative thermal cycle testing of TBSCs, produced using a production standard air plasma spray system. The latter show the durability of TBSCs to be similar to that of standard YSZ TBCs and indicate that the addition of the rare-earth dopant is not detrimental to the coating. Also discussed is the manufacture of functionally structured coatings with discreet doped layers. The temperature at the bond coat interface is important with respect to the life of the coating since it influences the growth rate of the thermally grown oxide layer, which in turn destabilizes the coating system as it becomes thicker. Experimental data are presented, indicating that dual-layered TBSCs can be used to detect luminescence from, and thereby the temperature within, subsurface layers covered by as much as 500 μm of standard TBC material. A theoretical analysis of the data has allowed some preliminary calculations of the transmission properties of the overcoat to be made, and these suggest that it might be possible to observe phosphorescence and measure temperature through an overcoat layer of up to approximately 1.56 mm thickness.

Effects of Alloy Composition on the Performance of Diffusion Aluminide Coatings

Nickel-based superalloys have been used in the gas turbine hot section components for their outstanding mechanical properties at elevated temperatures. Increasing the alloy strength at high temperature is usually achieved at the expense of the alloy’s environmental stability. Oxidation and high heat flux could be limiting factors in the use of these alloys at temperatures above 1800°F. To help overcome these limitations, protective coatings can be applied to the alloy surfaces to provide oxidation and hot corrosion resistance. These coatings are applied to alloys which can be produced in various forms such as equiaxed, directionally solidified or single crystals with varying chemistries. Elemental additions such as hafnium, rhenium, etc. are added to promote the strengthening of these alloys, and could result in varying effects on the coatability and coating performance. This paper discusses the effects of various substrate elements on the processing and stability of diffusion platinum aluminide coatings.© 1998 ASME

Characterization and Performance Evaluation of Pack Cementation and Chemical Vapor Deposition Platinum Aluminide Coatings

Metallurgical evaluation of platinum aluminide coatings applied to industrial gas turbine components, for oxidation and high temperature hot corrosion protection, were conducted. Coatings were processed by electroplating a thin layer of platinum followed by aluminizing using either the pack cementation or the chemical vapor deposition (CVD) processes. Laboratory and field data on the performance of these coatings are presented. Results from these tests showed that both aluminizing processes produced coatings that provided adequate environmental protection. However, the CVD coating experienced less coating growth during engine service and was therefore determined to be thermally more stable than the pack cementation coating in this application.Copyright

Optical Non-Destructive Condition Monitoring of TBC’s

The paper describes recent developments of the thermal barrier sensor concept for non-destructive evaluation (NDE) of thermal barrier coatings and on-line condition monitoring in gas turbines. Increases in turbine inlet temperature in pursuit of higher efficiency will make it necessary improve or upgrade current thermal protection systems in gas turbines. As these become critical to safe operation it will also be necessary to devise techniques for online conditions monitoring and NDE. The authors have proposed thermal barrier sensor coatings (TBSC) as a possible means of achieving NDE for thermal barrier coatings. TBSC’s are made by doping the ceramic material (currently yttria stabilised zirconia) with a rare earth activator to provide the coating with luminescence when excited with UV light. The paper describes the physics of the thermo-luminescent response of such coatings and shows how this can be used to measure temperature. Calibration data is presented along with the results of comparative thermal cycle testing of TBSC’s, produced using a production standard APS system. The latter show the durability of TBSC’s to be similar to that of standard YSZ TBC’s and indicate that the addition of the rare-earth dopant is not detrimental to the coating. Also discussed is the manufacture of functionally structured coatings with discreet doped layers. The temperature at the bond coat interface is important with respect to the life of the coating since it influences the growth rate of the thermally grown oxide layer which in turn destabilises the coating system as it becomes thicker. Experimental data is presented indicating that duallayered TBSC’s can be used to detect luminescence from, and thereby the temperature within, sub surface layers covered by as much as 500μm of standard TBC material. A theoretical analysis of the data has allowed some preliminary calculations of the transmission properties of the overcoat to be made and these suggest that it might be possible to observe phosphorescence and measure temperature through an overcoat layer of up to approximately 1.56mm thickness.Copyright

Intrinsic and Extrinsic Variable Effects on Thermal Barrier Coatings Life

Thermal barrier coating life is dependent on many intrinsic and extrinsic variables within the environment that they operate within. Intrinsic variables include material composition, mechanical and thermal properties, microstructure and ceramic coating thickness. On the other hand, extrinsic variables include cycle time, interface and top surface temperatures, and the gaseous environment, among others. Laboratory testing was conducted to determine the effects of these variables on TBC life. This paper addresses TBC life as a function of microstructure, thickness, and interface temperature.Copyright

Effects of Alloy Composition on the Performance of Yttria Stabilized Zirconia–Thermal Barrier Coatings

Thermal barrier coatings (TBCs) provide an alloy surface temperature reduction when applied to turbine component surfaces. Thermal barrier coatings can be used as a tool for the designer to augment the power and/or enhance the efficiency of gas turbine engines. TBCs have been used successfully in the aerospace industry for many years, with only limited use for industrial gas turbine applications. Industrial gas turbines operate for substantially longer cycles and time between overhauls, and thus endurance becomes a critical factor. There are many factors that affect the life of a TBC including the composition and microstructure of the base alloy and bond coating. Alloys such as Mar-M 247, CMSX-4 and CMSX-10 are materials used for high temperature turbine environments, and usually require protective and/or thermal barrier coatings for increased performance. Elements such as hafnium, rhenium, and yttrium have shown considerable improvements in the strength of these alloys. However these elements may result in varying effects on the coatability and environmental performance of these alloys. This paper discusses the effects of these elements on the performance of thermal barrier coatings.Copyright

Evaluation of Thermal-Sprayed Bronze Abradable Coatings: Effect of Temperature and Time on Coating Microstructure and Performance

Metallurgical evaluation was conducted on thermally-sprayed bronze coatings that are commonly found in gas turbine labyrinth seals. These bronze coatings serve as abradable surfaces for air/air and air/oil seals operating at low temperatures. As-coated and post-oxidation coating microstructures were examined metallographically to establish the coating’s functional life at various operating temperature ranges. Oxidation tests at several temperatures and times were carried out in order to construct a time-temperature map that could be used to predict the useful life of this bronze coating as correlated to gas turbine bronze seals returned from field service. Scanning electron microscopy in conjunction with polarized light microscopy were utilized to identify the presence of red cuprous oxide, a key factor in the determining the extent of internal oxidation.Copyright