Hot Sections Of Gas Turbine Engines Research Articles

Oxidation behavior at 1100 °C of NiCrAlY coatings deposited by various PVD techniques

NiCrAlY has been widely used as a protective layer on gas turbine engine hot section components and can be synthesized by different physical vapor deposition techniques. Here, the oxidation behaviors of arc-evaporated, magnetron-sputtered, and hybrid arc/sputtering NiCrAlY coatings were investigated. Preparation techniques can significantly affect the oxidation behavior of NiCrAlY coatings by adjusting the Al content. Arc-evaporated NiCrAlY coating has the lowest resistance against oxidation at 1100 °C, forming a bi-layered scale with outer spinel-rich and inner alumina-rich oxides. In comparison, the other two NiCrAlY coatings with higher Al content were oxidized to form dense scales enriched in α-Al2O3 after 250 h of oxidation. Furthermore, the outward diffusion of Mn from Hastelloy X promotes the production of spinel oxides, which accelerates the oxidation process.

Interaction of ytterbium pyrosilicate environmental-barrier-coating ceramics with molten calcia-magnesia-aluminosilicate glass: Part I, Microstructures

Ytterbium pyrosilicate (or disilicate; β-Yb2Si2O7) is a promising ceramic for environmental barrier coatings (EBCs) for ceramic-matrix composite (CMC) components in the hot-section of gas-turbine engines. In addition to the various requirements an EBC must satisfy, it must be resistant to high-temperature attack by calcia-magnesia-aluminosilicates (CMASs) ingested by the engine in the form of sand, dust, ash, etc. Here, the microstructures of dense, polycrystalline β-Yb2Si2O7 EBC ceramics, before and after high-temperature (1500 °C) interaction with a representative CMAS glass, are characterized, primarily using scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). Since actual EBCs are dense and polycrystalline, sintered pellets of EBC ceramics serve as acceptable proxies for such basic studies. It is confirmed that the CMAS glass wets certain grain boundaries in β-Yb2Si2O7 ceramics, and the mechanisms associated with the penetration of the CMAS glass are elucidated. It is also found that secondary-phase inclusions, that are ubiquitously present in these ceramics, but rarely characterized, are either rounded or faceted. The relatively large, partially-faceted ones are invariably attached to the grain boundaries, and can contain crystalline X2-Yb2SiO5 and Yb2O3, and amorphous SiO2 secondary phases primarily. However, after high-temperature CMAS interaction, these inclusions become occluded within β-Yb2Si2O7 grains, and are filled with Yb-containing CMAS glass. All these new findings contribute towards the broader understanding of β-Yb2Si2O7/CMAS interactions, with implications for the design of EBC microstructures and approaches to mitigate CMAS-induced degradation in EBCs. The accompanying Part II paper presents results from extensive characterization at the atomistic level, and addresses the anisotropic nature of the interfaces in the system, as well as defects along the interfaces, before and after the interaction with the CMAS glass.

High-Temperature Solid Particle Erosion in a Melt-Infiltrated SiC/SiC Ceramic Matrix Composite

Abstract Ceramic matrix composites are an enabling propulsion material system that offer weight benefits over current Ni-based superalloys, and have higher temperature capabilities that can reduce cooling requirements. Incorporating ceramic matrix composites into the hot section of gas-turbine engines therefore leads to an increase in engine efficiency. While significant advancements have been made, challenges still remain for current and next-generation gas turbines; particularly when operating in dust laden or erosive environments. Solid particles entrained in the gas flow can impact engine hardware resulting in localized damage and material removal due to repeated, cumulative impacts. In this study, the erosion behavior of a melt-infiltrated (MI) silicon carbide fiber-reinforced silicon carbide (SiC/SiC) ceramic matrix composite is investigated at high temperature (1200 °C) in a simulated combustion environment using 150 μm alumina particles as erodent. Particle impact velocities ranged from 100 to 200 m/s and the angle of impingement varied from 30 to 90 deg. Erosion testing was also performed on α-SiC to elucidate similarities and differences in the erosion response of the composite compared to that of a monolithic ceramic. Scanning electron microscopy was used to study the posterosion damage morphology and the governing mechanisms of material removal.

Real-time visualization of impact damage in monolithic silicon carbide and fibrous silicon carbide ceramic composite

Impact resistance from foreign debris is a critical requirement for brittle ceramic and ceramic composite materials intended for use in the hot-section of gas turbine engines. A design to mitigate against such impact failures necessitates a detailed understanding of the driving failure mechanisms. The present study introduces a unique time-resolved experimental method which advances the latter effort. Specifically, a pulsed synchrotron X-ray source, in phase contrast imaging (PCI) configuration, is used as a medium, to visually characterize the evolution of damage inside the target materials during impact. As a proof of concept, two types of ceramic materials were tested: monolithic silicon carbide and fibrous silicon carbide composite. Impact was performed using a light-gas gun and 1.5 mm diameter spheres of partially stabilized zirconia (PSZ) and silicon nitride (Si3N4). The retrieved dynamic X-ray image sequences provided clear outlines of the damage features. In the case of the monolithic ceramic, the impact by a PSZ projectile initially produced a cone crack and complete failure resulted by extension of a median crack. By contrast, the fibrous composite deformed readily prior to cone crack formation. Nearly identical damage features were observed in the monolith for the Si3N4 projectile, with the exception of the added vertical tensile crack. For this same projectile, the fibrous ceramic showed very limited surface deformation and enhanced cone cracking and kinking of laminates along the crack path. The latter response is attributed to the change in projectile properties. Some of the target materials were recovered, and post-mortem analysis via scanning electron microscopy (SEM) showed correlation with observed X-ray damage profiles. Moreover, simple Hertzian contact was used to estimate damage for the elastic portion of impact. This approach was found to yield a reasonable match with experimental results for the surface displacement near the contact interface.

Strain Gages for SiC–SiC Ceramic Matrix Composite Engine Components

As increasingly more SiCSiC ceramic matrix composites (CMCs) are being used in the hot sections of gas-turbine engines, there is a greater need for in situ strain measurement in these harsh conditions. Thin-film strain sensors are ideally suited for this task since they have minimal mass and are nonintrusive. However, if the bulk piezoresistive properties of SiCSiC CMCs could be used instead of the thin-film piezoresistive properties, superior resolution and stability could be realized. Toward this end, we have fabricated strain gages in which the active strain element is the SiCSiC CMC itself. As a result of this approach, a Pt:SiC CMC strain gage that is capable of operating at 500C for prolonged periods was demonstrated. The large piezoresistive responses associated with the SiC fibers and SiC matrix resulted in gage factors larger than 100. The performance of the Pt:SiC CMC strain gages under both tension and compression are reported, and the IV characteristics of the strain gages, as well as the piezoresistive response as a function of temperature, are presented. In addition, the potential for embedded strain gages in advanced CMC engine components will be discussed.

Use of the Materials Genome Initiative (MGI) approach in the design of improved-performance fiber-reinforced SiC/SiC ceramic-matrix composites (CMCs)

New materials are traditionally developed using costly and time-consuming trial-and-error experimental efforts. This is followed by an even lengthier material-certification process. Consequently, it takes 10 to 20 years before a newly-discovered material is commercially employed. An alternative approach to the development of new materials is the so-called materials-by-design approach within which a material is treated as a complex hierarchical system, and its design and optimization is carried out by employing computer-aided engineering analyses, predictive tools and available material databases. In the present work, the materials-by-design approach is utilized to design a grade of fiber-reinforced (FR) SiC/SiC ceramic matrix composites (CMCs), the type of materials which are currently being used in stationary components, and are considered for use in rotating components, of the hot sections of gas-turbine engines. Towards that end, a number of mathematical functions and numerical models are developed which relate CMC constituents’ (fibers, fiber coating and matrix) microstructure and their properties to the properties and performance of the CMC as a whole. To validate the newly-developed materials-by-design approach, comparisons are made between experimentally measured and computationally predicted selected CMC mechanical properties. Then an optimization procedure is employed to determine the chemical makeup and processing routes for the CMC constituents so that the selected mechanical properties of the CMCs are increased to a preset target level.

Experimental determination of thermal-fatigue life of turbine blades with various coatings made of an advanced heat resistant nickel alloy

An original CIAM know-how for the experimental determination of thermal-fatigue life of parts of the gas-turbine engine hot section, including parts with heat-resistant and ceramic thermal barrier coatings, in the conditions of superficial uneven heating by high-frequency currents with continuous cooling of the parts at the preset air consumption is outlined in the paper. The possibility of using high-frequency heating of the parts with coatings (including thermal protection coatings) for modeling superficial heating of full-scale parts of gas-turbine engines is shown on the basis of the tests of model samples with coatings controlling the temperature of the surface by means of a thermal imager. The method used makes it possible to simulate the thermal state of the part in the operating conditions. Temperature differences observed both over the surface and the thickness of a part create thermal stresses in the material, similar to those arising during the operation of the engine. If necessary, mechanical loads synchronized with changes in temperature, to simulate for example, the centrifugal force may be applied to the part. The article presents the results of experimental studies of thermal-fatigue life of rotor blades of the 1st stage of the turbine made of an advanced monocrystal alloy VZhM-5 without coating and with various protective coatings, conducted according in the regime Tmin↔Tmax = 350↔1050˚C.

Repair and manufacturing of single crystal Ni-based superalloys components by laser powder deposition—A review

Laser powder deposition is one of the most promising methods for the repairing of the single crystal Ni-based superalloys components used in the hot-section of gas turbine engines in order to extend their lifetime and reduce their overall cost. The microstructure of Ni-based superalloys deposited on single crystal substrates of similar materials depends mainly on the materials involved, on the orientation of the deposited tracks in relation to the substrate and on the deposition parameters. In the present paper these relations are discussed and illustrated for the case of single and multiple layer depositions of NiCrAlY and Rene N4 on (100) single crystal substrates of SRR99 and CMSX-4 Ni-based superalloys. On the other hand, when the aging treatment is applied directly to the solidification microstructure resulting from laser deposition, abnormal γ/γ′ microstructures may result, due to the inhomogeneity created by alloying elements partition during solidification. Performing a homogenization annealing before aging circumvents this difficulty. The homogenizing annealing also eliminates undesirable brittle phases present in the solidification microstructure, such as carbides and topologically close-packed compounds.

The detection and sizing of flaws in components from the hot-end of gas turbines using phase-contrast radiography with neutrons: a feasibility study

In this paper we present a feasibility study of the use of phase contrast radiography in the examination of components from the hot-section of gas turbine engines. These components are usually made from dense materials (nickel or cobalt based superalloys) and, consequently, radiographic examination requires either high energy X-rays (above 60 keV) or neutrons. The relative merits of employing X-rays and neutrons for phase contrast radiography are compared. It is shown that, for similar penetration, neutrons offer better sensitivity and that it should be possible to detect even micron-wide cracks orientated perpendicular to the incident rays. Simulation shows that, for cracks parallel to the incident rays, crack growth in increments of microns can be resolved by monitoring the development of the Fresnel diffraction pattern. Some preliminary experimental results are also presented that demonstrate an improvement over conventional neutron radiography.

Selection of a turbine cooling system applying multi-disciplinary design considerations.

The presented paper describes a multi-disciplinary cooling selection approach applied to major gas turbine engine hot section components, including turbine nozzles, blades, discs, combustors and support structures, which maintain blade tip clearances. The paper demonstrates benefits of close interaction between participating disciplines starting from early phases of the hot section development. The approach targets advancements in engine performance and cost by optimizing the design process, often requiring compromises within individual disciplines.

Crack-resistant Fiber-bonded Ceramic

Low fracture resistance is one of the most serious limitations of ceramic materials. The new SiC fiber-bonded ceramic presented here was developed to address this problem. The thermally conductive material is a candidate for such demanding applications as gas turbine engine hot section components, where its high-temperature capability, high thermal conductivity, and low density make it very attractive for replacement of heavy metal super alloy components. Synthesized by hot-pressing the material with the commercial name Sintered Tyrannohex consists of hexagonal columnar SiC fibers with a thin interfacial carbon layer between them.

High-Temperature Applications of Intermetallic Compounds

One of the greatest challenges currently facing the materials community is the need to develop a new generation of materials to replace Ni-based superalloys in the hot sections of gas-turbine engines for aircraft-propulsion systems. The present alloys, which have a Ni-based solid-solution matrix surrounding Ni3Al-based precipitates, are currently used at temperatures exceeding 1100°C, which is over 80% of the absolute melting temperature. Since Ni3Al melts at 1395°C and Ni at 1453°C, it is clear that significantly higher operating temperatures, with the attendant improvements in efficiency and thrust-to-weight ratio, can only be attained by the development of an entirely new materials system. This problem is a primary reason for the current high level of interest in high-temperature intermetallic compounds.The development of such a material system has important implications for national defense and for spin-offs to civilian technology, as well as for the economy and balance of payments. Obviously it would be a boon to any economy to have these new materials developed domestically, as was the case in the United States for the currently used single-crystal technology applied to Ni-based superalloys. As an example, the aerospace industry is one area where the United States is still the undisputed world leader, with net exports of $29 billion in 1989, twice that of any other U.S. industry.

Structural Analysis Method Development for Turbine Hot Section Components

This paper summarizes the structural analysis technologies and activities of the NASA Lewis Research Center’s gas turbine engine Hot Section Technology (HOST) program. The technologies synergistically developed and validated include: time-varying thermal/mechanical load models; component-specific automated geometric modeling and solution strategy capabilities; advanced inelastic analysis methods; inelastic constitutive models; high-temperature experimental techniques and experiments; and nonlinear structural analysis codes. Features of the program that incorporate the new technologies and their application to hot section component analysis and design are described. Improved and, in some cases, first-time three-dimensional nonlinear structural analyses of hot section components of isotropic and anisotropic nickel-base superalloys are presented.

PROTECTIVE COATINGS FOR AERO ENGINE HOT SECTION COMPONENTS

ABSTRACT Aluminide coatings have been widely used in the aircraft industries for the protection of gas turbine engine hot section components against oxidation and/or hot corrosion. This paper considers modes of coating degradation under conditions of cyclic oxidation, hot corrosion and corrosion-erosion interactions during service, as well as the effects of interdiffusion between coating and substrate alloys either during service or coating application. It also discusses means of improving existing coatings as well as advanced coating systems currently under development. In assessing coating performance, consideration must be given to the influence that coatings may have on substrate properties such as mechanical strength, resistance to creep and resistance to mechanical and thermal fatigue. Finally, it is stressed that proven performance for a given coating/substrate combination is no guarantee that no deleterious reaction will occur when the same coating is used with a different substrate alloy. Therefo...