Showing papers by "Robert A. Miller published in 2000"
TL;DR: In this paper, a laser high heat flux test approach has been established to obtain critical properties of ceramic thermal barrier coatings (TBCs) under near-realistic temperature and thermal gradients that may be encountered in advanced engine systems.
Abstract: Laser high heat flux test approaches have been established to obtain critical properties of ceramic thermal barrier coatings (TBCs) under near-realistic temperature and thermal gradients that may be encountered in advanced engine systems. Thermal conductivity change kinetics of a thin ceramic coating were continuously monitored in real time at various test temperatures. A significant thermal conductivity increase was observed during the laser-simulated engine heat flux tests. For a 0.25 mm thick ZrO2-8% Y2O3 coating system, the overall thermal conductivity increased from the initial value of 1.0 W/m K to 1.15, 1.19, and 1.5 W/m K after 30 h of testing at surface temperatures of 990, 1100, and 1320 °C, respectively, Hardness and elastic modulus gradients across a 1.5 mm thick TBC system were also determined as a function of laser testing time using the laser sintering/creep and microindentation techniques. The coating Knoop hardness values increased from the initial hardness value of 4 GPa to 5 GPa near the ceramic/bond coat interface and to 7.5 GPa at the ceramic coating surface after 120 h of testing. The ceramic surface modulus increased from an initial value of about 70 GPa to a final value of 125 GPa. The increase in thermal conductivity and the evolution of significant hardness and modulus gradients in the TBC systems are attributed to sintering-induced microporosity gradients under the laser-imposed high thermal gradient conditions. The test techniques provide a viable means for obtaining coating data for use in design, development, stress modeling, and life prediction for various TBC applications.
244 citations
TL;DR: In this article, a two-layer TBC system consists of a porous ZrO2-Y2O3 ceramic top coat and an oxidation-resistant metallic bond coat, which can be applied to the metal substrate either by plasma spray or by EB-PVD techniques.
Abstract: Ceramic thermal-barrier coatings (TBCs) have received increasing attention for gasturbine engine applications. The advantages of using TBCs include increased fuel efficiency by allowing higher gas temperatures and improved durability and reliability from lower component temperatures. As illustrated in Figure 1, TBCs can provide effective heat insulation to engine components, thus allowing higher operating temperatures and reduced cooling requirements. Atypical two-layer TBC system consists of a porous ZrO2-Y2O3 ceramic top coat and an oxidation-resistant metallic bond coat. These TBC systems can be applied to the metal substrate either by plasma spray or by electron-beam physical vapor deposition (EB-PVD) techniques.
104 citations
01 Jul 2000
TL;DR: In this paper, the authors address critical issues such as ceramic sintering and creep, thermal fatigue and their relevance to coating life prediction, and establish test approaches to obtain critical thermophysical and thermomechanical properties of the coating systems under near realistic temperature and stress gradients encountered in advanced engine systems.
Abstract: Thermal barrier coatings have been developed for advanced gas turbine and diesel engine applications to improve engine reliability and fuel efficiency. However, the issue of coating durability under high temperature cyclic conditions is still of major concern. The coating failure is closely related to thermal stresses and oxidation in the coating systems. Coating shrinkage cracking resulting from ceramic sintering and creep at high temperatures can further accelerate the coating failure process. The purpose of this paper is to address critical issues such as ceramic sintering and creep, thermal fatigue and their relevance to coating life prediction. Novel test approaches have been established to obtain critical thermophysical and thermomechanical properties of the coating systems under near-realistic temperature and stress gradients encountered in advanced engine systems. Emphasis is placed on the dynamic changes of the coating thermal conductivity and elastic modulus, fatigue and creep interactions, and resulting failure mechanisms during the simulated engine tests. Detailed experimental and modeling results describing processes occurring in the thermal barrier coating systems provide a framework for developing strategies to manage ceramic coating architecture, microstructure and properties.
55 citations
TL;DR: In this paper, the results of cyclic oxidation studies of oxygen barrier/Yttria-stabilized zirconia (YSZ) dual-layer TBCs are presented.
Abstract: Development of advanced thermal barrier coatings (TBCs) is the most promising approach for increasing the efficiency and performance of gas turbine engines by enhancing the temperature capability of hot section metallic components. Spallation of the yttria-stabilized zirconia (YSZ) top coat, induced by the oxidation of the bond coat coupled with the thermal expansion mismatch strain, is considered to be the ultimate failure mode for current state-of-the-art TBCs. Enhanced oxidation resistance of TBCs can be achieved by reducing the oxygen conductance of TBCs below that of thermally grown oxide (TGO) alumina scale. One approach is incorporating an oxygen barrier having an oxygen conductance lower than that of alumina scale. Mullite, rare earth silicates, and glass ceramics have been selected as potential candidates for the oxygen barrier. This paper presents the results of cyclic oxidation studies of oxygen barrier/YSZ dual-layer TBCs.
21 citations