Semester

Spring

Date of Graduation

2011

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Bruce Kang.

Abstract

The accumulation of carbon dioxide in the atmosphere continues to be an important ecological issue associated with global warming. The demand for improved efficiencies in energy conversion in recent years has led to the introduction of land-based gas turbines with significantly increased inlet temperatures. To accomplish this, nickel-based superalloys with protective thermal barrier coatings (TBC) are widely used as systems capable of extending the life and increasing firing temperatures of combustor and stationary turbine components. However, coating durability, thermal-fatigue and erratic spallation failure currently limit the continuous operation of turbine engines. Of the present ceramic coating materials used, yttria stabilized zirconia (YSZ) is most prevalent; its low thermal conductivity, high thermal expansion coefficient and outstanding mechanical strength make it ideal for use in TBC systems. However, residual stresses caused by coefficients of thermal expansion mismatches within the TBC system and unstable thermally-grown oxides (TGO) are considered the primary causes for its premature and erratic spallation failure. The development of new materials, coating technologies and evaluation techniques is required if enhanced efficiency is to be achieved. As a result, several non-destructive evaluation (NDE) techniques have been developed to address this problem yet few comprehensive studies have resulted in the development of true NDE techniques capable of predicting failure location prior to its occurrence.;In this research, a load-based micro-indentation method for NDE of TBCs exposed to thermal loads in air has been developed. Coating surface stiffness responses obtained through use of this technique have been found capable of assessing damage accumulation and macroscopic debonding failure sites following thermal exposure of TBC systems to elevated temperatures. Furthermore, microstructural analyses correlating these surface stiffness response to overall YSZ/TGO interface conditions indicate that high interfacial rumpling and non-uniform oxide growth leads to the development of both in-plane and out-of-plane residual stresses. As a result, areas displaying relative increases in surface stiffness response enable early detection of initial TBC spallation locations. Additionally, with the evolution of nanotechnologies, indentation testing techniques have become more common, yet their ability to evaluate material mechanical properties in harsh environments remains a challenge. 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 has been developed. The removal of these constraints has led to the development of both a portable and high-temperature micro-indentation system for TBC materials mechanical property evaluation up to 1000°C.

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