Showing papers on "Thermal barrier coating published in 1988"

Thermal Barrier Coating Life Prediction Model Development

Abstract: A methodology is established to predict thermal barrier coating life in a environment similar to that experienced by gas turbine airfoils. Experiments were conducted to determine failure modes of the thermal barrier coating. Analytical studies were employed to derive a life prediction model. A review of experimental and flight service components as well as laboratory post evaluations indicates that the predominant mode of TBC failure involves thermomechanical spallation of the ceramic coating layer. This ceramic spallation involves the formation of a dominant crack in the ceramic coating parallel to and closely adjacent to the topologically complex metal ceramic interface. This mechanical failure mode clearly is influenced by thermal exposure effects as shown in experiments conducted to study thermal pre-exposure and thermal cycle-rate effects. The preliminary life prediction model developed focuses on the two major damage modes identified in the critical experiments tasks. The first of these involves a mechanical driving force, resulting from cyclic strains and stresses caused by thermally induced and externally imposed mechanical loads. The second is an environmental driving force based on experimental results, and is believed to be related to bond coat oxidation. It is also believed that the growth of this oxide scale influences the intensity of the mechanical driving force.

Development and Microstructural Characterization of High‐Thermal‐Conductivity Aluminum Nitride Ceramics

Abstract: The effect of several additives, such as CaC2, CaO, Y2O3, and C, on thermal conductivity of hot-pressed AlN ceramics was investigated. The addition of CaC2 reductant was found to be useful for achieving high thermal conductivity of 180 W/(m·K) at room temperature. The characterization of AlN ceramics with CaC2 additive was performed by chemical analysis of Ca, C, and O and microstructural analysis using transmission electron microscopes equipped with an energy-dispersive X-ray analyzer and an electron energy loss spectrometer. The major influence on high thermal conductivity is the disappearance of a thermal barrier caused by oxygen impurities at the grain boundary.

Thin thermal barrier coating for engines

Abstract: Thin thermal barrier coating of a specified thickness of 0.002 to 0.009 inch to insulate the combustion chamber of an internal combustion engine to achieve optimum reduction of transient head flow. The coating is of an optimum thickness to reduce in-cylinder heat loss in the combustion chamber during combustion, thus increasing engine efficiency, specific power output, and reducing emissions. However, the temperature increase is not so great as to adversely affect engine lubricant life or volumetric efficiency. The invention is particularly suitable for gasoline engines as it does not cause preignition or knocking that is generally caused by insulating coatings of greater thickness. In addition, the invention is particulalry suitable for aluminum combustion chamber components. The thinner coating also results in improved reliability and durability by reducing chipping and cracking failure tendencies associated with ceramic coatings.

Current status and future trends in turbine application of thermal barrier coatings

Abstract: This paper provides an overview of the current status and future trends in application of thermal barrier coatings (TBC) to turbine components, and in particular to high turbine airfoils. Included are descriptions of the favorable results achieved to date with bill-of-material applications of plasma deposited TBC, and recent experience with developmental coatings applied by electron beam-physical vapor deposition.

Titania doped ceramic thermal barrier coatings

Abstract: A ceramic thermal barrier coating system for superalloy components subjected to high operating temperatures, such as gas turbine engine airfoils, is disclosed which includes a titania doped interfacial layer between the metallic substrate and the ceramic overcoat in order to better resist failure by spalling.

Advanced thermal barrier coated superalloy components

Abstract: An improved thermal barrier coating system is described. A key feature of the invention is the presence of a thin, adherent aluminum oxide scale on the surface of the substrate. An oxide stabilized ceramic coating is present on the surface of the scale. Preferably, the ceramic coating has a columnar grain structure.

GASCAN—An Interactive Code for Thermal Analysis of Gas Turbine Systems

Abstract: A general, dimensionless formulation of the thermodynamic, heat transfer, and fluid-dynamic processes in a cooled gas turbine is used to construct a compact, flexible, interactive system-analysis program A variety of multishaft systems using surface or evaporative intercoolers, surface recuperators, or rotary regenerators, and incorporating gas turbine reheat combustors, can be analyzed Different types of turbine cooling methods at various levels of technology parameters, including thermal barrier coatings, may be represented The system configuration is flexible, allowing the number of turbine stages, shaft/spool arrangement, number and selection of coolant bleed points, and coolant routing scheme to be varied at will Interactive iterations between system thermodynamic performance and simplified quasi-three-dimensional models of the turbine stages allow exploration of realistic turbine-design opportunities within the system/thermodynamic parameter space The code performs exergy-balance analysis to break down and trace system inefficiencies to their source components and source processes within the components, thereby providing insight into the interactions between the components and the system optimization tradeoffs

Residual stresses in ZrO2-8%Y2O3 plasma-sprayed thermal barrier coatings

Abstract: The successful application of zirconia as thermal barrier coatings in the combustion chamber of diesel engines is dependent on their achieving adequate thermal shock resistance and resistance to attack by combustion gases. Of particular importance is the control of residual stresses in the ceramic coatings if useful lifetimes are to be achieved. An experimental technique for the determination of residual stresses is described, and the effect of varying spraying parameters such as deposition rate and air cooling, and the presence of a bondcoat, are discussed.

Microstructure and Properties of Sprayed Ceramic Coating

Abstract: The potential benefits of a thermal barrier coating on the heated side of diesel or turbine components are well documented, and this as well as other applications of ceramics in heat engines are being actively pursued. The ceramic coatings used in heat engines must be able to withstand severe thermal stress. The microstructure and properties of several sprayed coatings are described, including flame-sprayed alumina coatings, plasma-sprayed alumina coatings, and plasma-sprayed zirconia coatings. The results show that the modulus (E) of the coatings depends mainly on porosity and phase composition, and nonlinear stress-strain behavior is caused by the laminar grain structure of the coatings. The fracture strain of a coating is a major factor in the thermal shock resistance of that coating.

Fracture toughness of plasma-sprayed zirconia coatings

Abstract: Plasma-sprayed ceramic coatings are formed by introducing a powder into a plasma jet to produce a stream of molten particles which then impact, spread and rapidly solidify onto a substrate to form a lamellar microstructure. The cohesive and adhesive fracture toughness of zirconia coatings, fully or partially stabilized with Y2O3 or CeO2, plasma sprayed onto steel, have been determined using the double cantilever beam (DCB) technique. The results show that cohesive fracture toughness is greater than adhesive toughness and that tetragonal coatings have significantly higher toughness than fully stabilized (cubic) zirconia coatings. The highest toughness was achieved with ZrO2-CeO2 coatings which contained a significant proportion of transformable tetragonal phase. The results are interpreted in terms of the influence of grain size and composition on the tetragonal-to-monoclinic phase transformation.

Destabilization of Zirconia Thermal Barriers in the Presence of V2O5

Abstract: Impurities, such as vanadium, degrade the operating performance of yttria-stabilized zirconia thermal-barrier coatings. V2O5 reacts preferentially with Y2O3, forms YVO4, and leads to the destabilization of zirconia thermal-barrier material. A model experiment has been designed to monitor the destabilization of zirconia thermal barriers by directly exposing thin films of yttria-stabilized zirconia to V2O5 vapor. The growth of YVO4 from yttria-stabilized zirconia and the destabilization of cubic yttria-stabilized zirconia into tetragonal and/or monoclinic zirconia polymorphs are monitored by selected-area diffraction and energy-dispersive X-ray spectroscopy in the transmission electron microscope. A special crystallographic orientation relation between YVO4 and cubic zirconia is discussed.

Thermal Barrier Coatings for Jet Engines

Abstract: Thermal barrier coatings (TBC) have been used in jet engine combustors for over 15 years. However, it is only recently that they have been actively used in the harsh turbine environment on nozzle guide vane platforms. It is intended to use TBCs on vane airfoils, and on rotating turbine blades where the maximum payoff will be realized. Much work has been done in the last five years towards this goal. Problem areas that need to be addressed are as follows:1. Prevent coating failure due to:a. thermal cycling of the ceramic layer.b. oxidation of the bond coat.c. erosion due to gas stream solid particles.d. deposition of gas stream molten debris.e. acid leaching of coating phase stabilizers.2. Minimize performance losses due to rough coatings.3. Insure consistent high quality coatings.This report will review some of the work done by Rolls-Royce that would hopefully allow an increased use of TBCs on turbine components in the future.Copyright © 1988 by ASME

Identification of t' Phase in ZrO2-7.5 wt% Y2O3 Thermal-Barrier Coatings

Abstract: The presence of nontransformable tetragonal (t') phase in plasma-sprayed ZrO2-Y2O3 coatings enhances the stability of the coating during high-temperature exposure. In this paper, we describe the importance and procedure for correctly identifying t’phase in X-ray diffraction patterns of the as-sprayed and heat-treated coatings. The results agree with the phase diagram. We also point out that in the heat-treated materials, the durability should be specified in terms of t'-ZrO2.

Gaseous removal of ceramic coatings

Abstract: Removal of ceramic thermal barrier coatings from metallic substrates is achieved by contacting the coated article with fluorine containing gas at an elevated temperature. The gas penetrates the ceramic to attack the interface between the substrate and ceramic causing the ceramic to spall.

Laser Cladding of Thermal Barrier Coatings

Abstract: The feasibility of laser cladding zirconia and alumina thermal barrier coatings on Udimet 700 alloy and AISI 4140 steel substrates was investigated; the objective was to extend the high temperature performance of thermal barrier coatings. Two continuous wave CO2 gas lasers with power levels of 1.2 and 5 kW were employed for cladding. Laser power, powder flowrate, scanning spot size, and travel speed were the major parameters varied during cladding. Zirconia claddings obtained were thin (5–15 μm), dense, hard (800–1700 HV0.2), and crack free, possessing unusually fine microstructures, glassy surfaces, excellent adhesion, and good surface integrity. It was possible to produce thicker claddings, but these tended to crack, delaminate, and chip off from the substrate. The power density was the key process variable that determined the transition from a thicker, cracked coating to a thin, crack free one. The results indicate that a promising approach to generating thicker flawless claddings would be one ...

Thin insulative ceramic coating and process

Abstract: Composition and method providing a chromium oxide densified insulative coating for a substrate comprising an insulative coating comprised of refractory oxide bubbles with a melting point above that of glass bubbles, a refractory oxide and a solution of a binder capable of being converted to an oxide upon being heated, thereby effecting a bond between the refractory oxide and the substrate. The coating is thin and provides increased thermal barrier characteristics.

Thrust chamber thermal barrier coating techniques

Abstract: Methods for applying thermal barrier coatings to the hot-gas side wall of rocket thrust chambers in order to significantly reduce the heat transfer in high heat flux regions was the focus of technology efforts for many years. This paper describes a successful technique developed by the Lewis Research Center that starts with the coating of a mandrel and then builds the thrust chamber around it by electroforming appropriate materials. This results in a smooth coating with exceptional adherence, demonstrated in hot fire rig tests. The low cycle fatigue life of chambers with coatings applied in this manner was increased dramatically compared to uncoated chambers.

Vacuum application of thermal barrier plasma coatings

Abstract: Coatings are presently applied to Space Shuttle Main Engine (SSME) turbine blades for protection against the harsh environment realized in the engine during lift off-to-orbit. High performance nickel, chromium, aluminum, and yttrium (NiCrAlY) alloy coatings, which are applied by atmospheric plasma spraying, crack and spall off because of the severe thermal shock experienced during start-up and shut-down of the engine. Ceramic coatings of yttria stabilized zirconia (ZrO2-Y2O3) were applied initially as a thermal barrier over coating to the NiCrAlY but were removed because of even greater spalling. Utilizing a vacuum plasma spraying process, bond coatings of NiCrAlY were applied in a low pressure atmosphere of argon/helium, producing significantly improved coating-to-blade bonding. The improved coatings showed no spalling after 40 MSFC burner rig thermal shock cycles, cycling between 1700 and -423 F. The current atmospheric plasma NiCrAlY coatings spalled during 25 test cycles. Subsequently, a process was developed for applying a durable thermal barrier coating of ZrO2-Y2O3 to the turbine blades of first stage high-pressure fuel turbopumps utilizing the vacuum plasma process. The improved thermal barrier coating has successfully passed 40 burner rig thermal shock cycles without spalling. Hot firing in an SSME turbine engine is scheduled for the blades. Tooling was installed in preparation for vacuum plasma spray coating other SSME hardware, e.g., the titanium main fuel valve housing (MFVH) and the fuel turbopump nozzle/stator.

Development and Microstructural Characterization of High‐Thermal‐Conductivity Aluminum Nitride Ceramics.

Abstract: The effect of several additives, such as CaC2, CaO, Y2O3, and C, on thermal conductivity of hot-pressed AlN ceramics was investigated. The addition of CaC2 reductant was found to be useful for achieving high thermal conductivity of 180 W/(m·K) at room temperature. The characterization of AlN ceramics with CaC2 additive was performed by chemical analysis of Ca, C, and O and microstructural analysis using transmission electron microscopes equipped with an energy-dispersive X-ray analyzer and an electron energy loss spectrometer. The major influence on high thermal conductivity is the disappearance of a thermal barrier caused by oxygen impurities at the grain boundary.

Thermal spray by design

Abstract: L'enorme intervalle de materiaux deposables par projection a chaud n'apporte pas seulement des reponses aux problemes de conception mais permet egalement d'ameliorer de maniere significative les performances des pieces

Laser drilling of a superalloy coated with ceramic

Abstract: Laser drilling has been developed in advanced aircraft industry in particular to achieve the intricate hole network of the combustion chamber because of several advantages compared to the main competing process, that of electron beam drilling. The combustion chambers for the next generation of engines will be protected on the inner side by a thermal barrier coating ('JJBC) , capable of working at higher temperatures lOO-150°C higher, thus improving the gas turbine efficiency. Consequently, three questions might arise : - Which was the main phenomena involving material under irradiation during laser drilling ? - How could these phenomena be described by models ? - Could laser drilling be applied to multi-layered materials such as a superalloy coated with a TBC ? An attempt, in part, to answer these questions was made by the study of laser drilling of 1.5 mm thick Hastelloy X sheet coated with conventional plasma-sprayed MCrAlY bond coat plus plasma-sprayed zirconia. The study focused on microstructural SEM (Scanning Electron Microscopy) observation coupled with modeling. SEM observations, applied to polished (and, if necessary, etched) axial sections of the holes and to edges of the holes, allowed accurate measurements of relevant microstructural parameters which validated the models. The influences of the principal laser parameters such as pulse length, pulse rate and power density were determined. The nature (metal or ceramic) of the side of the part exposed to the beam, the locus of the beam focus and the beam entrance angle were only considered to lesser degree.

Microlaminate composites as thermal barrier coatings

Abstract: Thick multiple-layered Ni/NiCoCrAlY and Ti/CoCrAlY microlaminate composites are explored as thermal barrier coatings. The method of fabrication of these laminates and the measurement technique used to determine thermal diffusivity and thermal conductivity of these coatings are discussed. Results indicate that the thermal conductivity of the laminate composite perpendicular to the laminate plane decreases with increasing number of interfaces; the drop in thermal conductivity is likely to be associated with interfaces which act as a barrier to the transfer of heat across them; the variation in thermal conductivity with the number of interfaces is not linear.

A Study of the Effects of Thermal Barrier Coating Surface Roughness on the Boundary Layer Characteristics of Gas Turbine Aerofoils

Abstract: Influence de la rugosite de surface d'un revetement isolant thermique sur les caracteristiques de la couche limite d'un profil aerodynamique de turbine a gaz

Durability of thermal barrier coatings in a high heat flux environment

Abstract: Thermal shock is a significant factor in the limited service life of Space Shuttle Main Engine (SSME) high pressure, fuel turbopump (HPFTP) turbine blades. Addition of advanced thermal barrier coatings (TBCs) to the blades could serve to dampen the thermal shock, thereby increasing the life of the blades. However, testing and use of TBCs to date is performed primarily under moderate heat flux conditions which are typical of aircraft turbines. Only limited testing was conducted that addresses high heat flux and severe thermal shock conditions. Therefore, it is not clear if TBCs can survive severe thermal shocks or provide adequate thermal shock protection to the HPFTP turbine blades. The purpose is to experimentally evaluate the potential durability and protective capability of a variety of advanced TBCs in a cyclic thermal shock environment. A secondary goal is to identify significant parameters affecting TBC life during high heat flux testing. Parameters investigated include top coat thickness, bond coat thickness, substrate type, and substrate geometry.

Toward Improved Durability in Advanced Aircraft Engine Hot Sections

Abstract: The conference on durability improvement methods for advanced aircraft gas turbine hot-section components discussed NASA's Hot Section Technology (HOST) project, advanced high-temperature instrumentation for hot-section research, the development and application of combustor aerothermal models, and the evaluation of a data base and numerical model for turbine heat transfer. Also discussed are structural analysis methods for gas turbine hot section components, fatigue life-prediction modeling for turbine hot section materials, and the service life modeling of thermal barrier coatings for aircraft gas turbine engines.

Thermal barrier coatings - Technology for diesel engines.

Abstract: Thermal Barrier Coatings (TBC) are a development of the aerospace industry primarily aimed at hot gas flow paths in turbine engines. TBC consists of zirconia ceramic coatings applied over an (M)CrAlY). These coatings can provide three benefits: a reduction of metal surface operating temperatures, a deterrent to hot gas corrosion, and improved thermal efficiencies. TBC brings these same benefits to reciprocal diesel engines but coating longevity must be demonstrated. Diesels require thicker deposits and have challenging geometries for the arc-plasma spray (APS) deposition process

How to Achieve Strain Tolerant Thermal Barrier Coatings (TBC’s) by Means of Varying Spray Parameters

Abstract: Small gas turbine engines, especially if they are used as automotive propulsion systems, are often compared to diesel engines with regard to their specific fuel consumption. Aside from high gas temperatures and the use of very efficient heat exchangers, there has to be a form of thermal insulation of the turbine casing which can be achieved by means of a thick Thermal Barrier Coating (TBC).Theoretical considerations about the necessary morphology are made on the calculated stress situation within the coating. It will be described, how the TBC morphology can be influenced as desired, by means of a specific selection of spray parameters. In addition, the correlation between spray parameters and the phase distribution, as obtained from neutron diffraction analyses, will be reported.The compatibility between theory and practice will be proven by means of a turbine case dummy with an inner TBC with a thickness of 5 mm which was tested very realistically in a specially designed flame rig. Microsections of this coating are used to confirm the theoretical considerations of the necessary morphology.Copyright © 1988 by ASME

Modeling of thick thermal barrier coatings.

Abstract: Two dimensional finite element analysis of thick thermal barrier coatings has projected that multilayer coatings can survive the predicted thermal stresses imposed at peak operating conditions for diesel engines. Further analysis has predicted that the diesel engine transient heating and cooling cycles are survivable for properly designed thermal barrier coatings.