Engineering topics

Miller, Robert A.

Publications and source records attributed to Miller, Robert A..

136 records · Page 8

NASA NTRS1989

TBCs for better engine efficiency

State-of-the-art thermal barrier coatings (TBCs) developed for aircraft engines can achieve both hot-section component operating temperature reductions and superior oxidation resistance. Such TBCs typically consist of two layers: a metallic, often NiCrAlY 'bond' inner layer in contact with the superalloy structural component, and an outer, insulating ceramic layer. A ceramic frequently used in this role due to its high durability is plasma-sprayed ZrO2, partially stabilized with 6-8 wt pct Y2O3. TBCs can also be useful in nonaircraft gas turbines, which frequently use highly contaminated fuels.

Brindley, William J.↗

NASA NTRS1989

Thermal barrier coatings for gas turbine and diesel engines

The present state of development of thin thermal barrier coatings for aircraft gas turbine engines and thick thermal barrier coatings for truck diesel engines is assessed. Although current thermal barrier coatings are flying in certain gas turbine engines, additional advances will be needed for future engines. Thick thermal barrier coatings for truck diesel engines have advanced to the point where they are being seriously considered for the next generation of engine. Since coatings for truck engines is a young field of inquiry, continued research and development efforts will be required to help bring this technology to commercialization.

Miller, Robert A.↗

NASA NTRS1988

Metal/Ceramic Bond Coatings For High Temperatures

Reduced-thermal-expansion bond coatings developed for use at high temperatures in thermal-barrier-coating systems. Bond coatings composed of low-pressure-plasma-sprayed metallic matrices dispersed with low-thermal-expansion high-bulk-modulus ceramic particles. New coatings and application lower thermal-expansion-mismatch strain while maintaining integrity at high temperatures.

Miller, Robert A.↗

NASA NTRS1988

Ceramic Thermal Barriers For Dirty-Fuel Turbines

Report discusses performances of ceramic thermal-barrier coating materials for use in electric-utility gas-turbine engines. Variations of standard coating evaluated in search for coating resistant to dirty fuel. Variations included alterations of level of yttria, replacement of yttria by other stabilizers, controlling surface density (by plasma spray processing, infiltration, laser glazing, or sputtering), and interface treatments.

Miller, Robert A.↗

NASA NTRS1988

Plasma Spraying Of Dense, Rough Bond Coats

Simple modification of plasma torch facilitates spraying of coarse powders. Shape of nozzle changed to obtain decrease in velocity of gas and consequent increase in time particles spend in flame before impact on substrate. Increased residence time allows melting of coarser powders, spraying of which results in rougher bond surfaces.

Miller, Robert A.↗

NASA NTRS1988

High resolution video monitoring of coating thickness during plasma spraying

A new approach to monitoring the thickness of plasma sprayed coatings during application is described. The method employs a high resolution video camera and width analyzer to accurately measure the dimensions of samples having simple geometries. This approach is best suited for cylindrical or flat substrates but it may also work for selected locations on more complex geometries. Measurement accuracy is a function of specimen dimensions and extent of magnification. Tolerances of plus or minus 0.5 mil (0.13 mm) on final coating thickness can be achieved. Additionally, the plot of cumulative coating thickess versus the number of passes has proven to be a useful diagnostic tool. While the ideal plot is linear, strong deviations from linearity - indicating the need for corrective action - may be observed.

Miller, Robert A.↗

NASA NTRS1987

Thermal expansion mismatch and plasticity in thermal barrier coating

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.

Chang, George C.↗

NASA NTRS1986

HOST surface protection R and T overview

Most of the efforts in the HOST Surface Protection Subproject were focused on thermal barrier coating (TBC) life prediction. Also, a small effort, consisting primarily of wrapping up and reporting the work of previous years, remained on the airfoil deposition modeling. The work performed under the airfoil deposition modeling program element was concerned with modeling the deposition of corrodants onto turbine airfoils. Accomplishments included verification of the chemically frozen boundary (CFBL) theory. Encouraging results were also achieved with the recently developed local thermochemical equilibrium (LTCE) theory. The surface protection subprogram was devoted to thermal-barrier-coating life modeling. This modeling is an essential step in the development of TBC's.

Miller, Robert A.↗

NASA NTRS1986

A study on thermal barrier coatings including thermal expansion mismatch and bond coat oxidation

The present investigation deals with a plasma-sprayed thermal barrier coating (TBC) intended for high temperature applications to advanced gas turbine blades. Typically, this type of coating system consists of a zirconia-yttria ceramic layer with a nickel-chromium-aluminum bond coat on a superalloy substrate. The problem on hand is a complex one due to the fact that bond coat oxidation and thermal mismatch occur in the TBC. Cracking in the TBC has also been experimentally illustrated. A clearer understanding of the mechanical behavior of the TBC is investigated. The stress states in a model thermal barrier coating as it cools down in air is studied. The powerful finite element method was utilized to model a coating cylindrical specimen. Four successively refined finite element models were developed. Some results obtained using the first two models have been reported previously. The major accomplishment is the successful development of an elastic TBC finite element model known as TBCG with interface geometry between the ceramic layer and the bond coat. An equally important milestone is the near-completion of the new elastic-plastic TBC finite element model called TBCGEP which yielded initial results. Representative results are presented.

Chang, George C.↗

NASA NTRS1985

The effect of oxidation on the high heat flux behavior of a thermal barrier coating

The effect of oxidation on the high heat flux behavior of a thermal barrier coating was evaluated by cyclically exposing preoxidized specimens to a 3000 C nitrogen plasma. The thermal barrier coatings consisted of a 0.025 cm layer of air-plasma-sprayed ZrO2-7 percent Y2O3 and a 0.012 cm layer of low pressure-plasma-sprayed NiCoCrAlY applied over 0.13 cm diameter B1900+Hf cylindrical substrates. A gradient of 800 C is produced across the ceramic layer in each 0.5 second exposure. This is much more severe than the gradient encountered on a gas turbine engine. Prior to exposure, the specimens were preoxidized at 1200 C for time from 0 to 20 hours. These coatings were found to be tolerant to the high heat flux plasma flame for all but the most severe preoxidations. However, life degraded rapidly for preoxidation times in excess of 15 hours at 1200 C. A log-log plot of cycles-to-failure vs estimated oxidative weight gain yield a straight or nearly straight line, and this line could be rationalized using an oxidation-based model that was developed previously for low heat flux applications.

Miller, Robert A.↗