Showing papers by "Robert A. Miller published in 1987"

Current status of thermal barrier coatings — An overview

Abstract: The science and technology of thermal barrier coatings has advanced considerably since reports of the first test on turbine blades in a research engine in 1976. Today thermal barrier coatings are flying in revenue service in a low risk location within the turbine section of certain gas turbine engines. The state-of-the-art coating system for gas turbine applications is currently a plasma-sprayed ZrO 2 -(6%–8%) Y 2 O 3 ceramic layer over an MCrAlY (M ≡ Ni, Co or NiCo) bond coat layer plasma sprayed at low pressure. Although the potential for meeting current and short-term goals is high, longer-range goals may not be attainable with current coating concepts. These longer-range goals will involve high risk designs where coating loss could lead directly to component loss. Several steps must be taken to help meet these goals. Improved understanding of coating failure mechanisms is required. Models are needed to predict lifetimes. Process automation and quality control procedures must be instituted. Finally, new concepts in plasma-sprayed coatings must be developed and alternatives to the plasma- spraying process may be required. The current status of thermal barrier coatings and prospects for future progress in the above areas are summarized.

Behavior of thermal barrier coatings for advanced gas turbine blades

Abstract: The work reported herein deals with a plasma-sprayed zirconia-yttria ceramic coating with a nickel-chromium-aluminum bond coat on a super- alloy substrate. This investigation has as its principal objective the quantitative determination of stress states in a model thermal barrier coating as it cools in air. The effects associated with an idealized rough ceramic-bond interface were investigated. The influence of bond coat oxidation was also determined, together with the impact of initial cracking within the coating. An improved understanding of these coating behaviors is expected to lead to the discovery of coating failure mechanisms which can greatly benefit the designer. In this investigation the powerful finite element method was employed to model the coating which is assumed to be elastic for this initial effort. To obtain the necessary accuracy a very fine finite element grid was developed, utilizing generalized plane-strain elements to model a cylindrical coated specimen. The model which is named TBCOC contains 1316 nodal points and 2140 elements. Using a generic code called MARC, numerical results were obtained from this model. The actual calculations were performed on a CRAY-1S supercomputer. Detailed stress distributions in the coating were obtained to reflect the effects of thermal expansion mismatch and the material properties. Results to date have pinpointed the existence (and location) of large radial tensile stresses in the ceramic layer adjacent to the rough ceramic- bond interface. The effects of oxidation on the stresses are shown together with the influence of initial cracking at specific locations near the ceramic-bond interface. A preliminary failure mechanism for thermal barrier coatings is proposed on the basis of these numerical results and published experimental work.

Finite element thermal stress solutions for thermal barrier coatings

Abstract: The primary objective of this investigation is the quantitative determination of stress states in a model thermal barrier coating (TBC) as it cools in the air to 600 °C from an assumed stress-free state at 700 °C. The TBC under consideration represents a plasma-sprayed zirconia-yttria ceramic layer with a nickel-chromium-aluminum-yttrium bond coat on a cylindrical substrate made of nickel-based superalloy. To account for complex geometry and material properties, the versatile finite element method was used to model the TBC. The elastic model which is an outgrowth of a previously reported one is named TBCG. It contains 1316 nodal points and 2140 elements. An improved version known as TBCGEP is capable of representing the bond coat with an elastic-perfectly plastic material. Numerical results of stresses and strains were obtained from this model by using a generic code called MARC. Actual computations were performed on a CRAY-XMP supercomputer. Based on 15 lengthy computer solutions, quantitative determinations have been made of the influence of the coefficient of thermal expansion of the bond coat, Poisson's ratio of the bond coat, and Young's modulus of elasticity. Also presented in this paper are stress states in both the ceramic layer and the bond coat of an elastoplastic solution.

Progress Toward Life Modeling of Thermal Barrier Coatings for Aircraft Gas Turbine Engines

Abstract: Progress toward developing life models for simulating the behavior of thermal barrier coatings in aircraffft gas turbine engines is discussed. A preliminary laboratory model is described as are current efforts to develop engine-capable models. Current understanding of failure mechanisms is also summarized.

Thermal expansion mismatch and plasticity in thermal barrier coating

Abstract: The basic objective of this investigation is the quantitative determination of stress states in a model thermal barrier coating (TBC) as it cools in the air to 600 C from an assumed stress-free state at 700 C. This model is intended to represent a thin plasma-sprayed zirconia-yttria ceramic layer with a nickel chromium-aluminum-yttrium bond coat on a cylindrical substrate made of nickel-based superalloys typically found in gas turbines.