Date of Graduation

2012

Document Type

Thesis

Degree Type

MS

Committee Chair

Bruce Kang

Abstract

There is an increase in demand for improved efficiencies in energy conversion in turbine engines for aircraft propulsion and power generation, which has led to a significant increase in inlet temperatures accomplished by nickel-based super alloys with protective thermal barrier coatings (TBCs). TBCs are used to reduce turbine component temperatures and thereby increase turbine component life. TBCs are usually made up of multiple layers, with each layer having a specific role. The topmost layer composed of yttria stabilized zirconia (YSZ), is a ceramic material with low thermal conductivity that provides thermal insulation. The ceramic top layer is deposited on the substrate alloy with intervening oxidation resistant metallic layer called the bond coat. During the ceramic coating deposition, thermally-grown oxides (TGO) forms on the bond coat surface at the ceramic bond-coat interface. This TGO accompanied with residual stresses caused by coefficients of thermal expansion mismatches within the TBCs system are the major causes of its unpredictable spallation failure. In order to achieve improved efficiencies there has to be development in evaluation techniques of the complex nature of the TBC systems. There are currently several non-destructive evaluation (NDE) techniques that have been used to tackle the issue of TBCs failure, but only few have resulted in achieving a true NDE technique capable of predicting failure prior to its occurrence. The purpose of this research is to evaluate the coating surface stiffness responses at both elevated and room temperatures of TBCs exposed to air and carbon dioxide (CO2)/steam conditions. This is achieved by using a load-based micro-indentation method that has been developed for NDE of TBCs exposed to thermal loads. These surface stiffness responses show a buildup both in-plane and out-of-plane residual stresses caused by high interfacial rumpling and non-uniform oxide growth. As a result, an increase in the stiffness response shows an increase in residual stresses, which allows us to determine the period of the TBCs thermal life cycle in which this occurs. Nano indentation is one of the more common indentation testing techniques but is unable to evaluate mechanical properties at elevated temperatures, and harsh environments. Following a classical Hertzian contact mechanics approach, a micro indentation technique that does not require system compliance calibration or the use of high precision depth sensors was used to evaluate the mechanical properties of TBCs material up to 1100oC, in air and CO2/steam conditions.

Download

Share

COinS