Ex situ oxidation of IN713 turbine blades in situ deposited with silica layers during operation of a micro-gas turbine

The long-term stability of silica layers deposited in situ on the IN713 turbine blades was investigated by surveying an ex situ thermally cycled oxidation test in air at 1050 °C. Two types of in situ coatings were prepared by burning tetraethylorthosilicate with fuel during a 100-min operation of a 13 kg_f-class gas turbine. The degradation of the blades upon the oxidation test was evaluated by monitoring the weight change and by analyzing the microscopic evolution of surface oxides. One coating, characterized by a uniform and relatively thick porous coating on the pressure side, was chipped off in the early stages of the test. The other coating was ∼4 μm thick at the region near the root, but was too thin (and transparent) at the other parts of the blade to be rendered as protective one. However, this coating was found to be more stable towards chipping than the other coating during the oxidation test. The stability of the coatings was examined in terms of the interaction between the deposited layer and the underlying thermally grown oxide or substrate during the operation of the gas turbine.     

Effect of periodic in situ deposition of silica layer on the long term operation of a micro-gas turbine

Tetraethylorthosilicate (TEOS) diluted in methanol was periodically supplied during operation of a 13 kgf (max. thrust)-class small gas turbine to deposit silica layers on the turbine blades and its effect on the degradation of the turbine blades upon the long term cyclic operation (1 cycle: 30/10 min run/pause at 20,000 rpm) was tested. The exhaust gas temperature (EGT) was set at 800 °C. For comparison, two other gas turbines were also tested without feeding TEOS at the EGT of 800 °C and 700 °C, respectively. The blades were inspected optically and metallographically before and after the test. The supply of TEOS produced a white silica layer indicating a very low level of the apparent density all around the surface of the blades, some of which became dense at the end to a thickness of 5-10 μm. The underlying substrate Inconel 713 was effectively protected, while the blades operated at the EGT of 800 °C, but not in situ coated, were severely degraded in terms of the surface oxidation and microstructural evolution of the substrate. The microstructure of the protected substrate was comparable with that of the unprotected but operated at the EGT of 700 °C. This result was discussed based on the effect of the formation of dense silica layer on the blade by comparing with the previously obtained results.     

Gradient complex protective coatings for single-crystal turbine blades of high-heat gas turbine engines

Complex diffusion-condensation protective coatings characterized by gradient distribution of alloying elements over the thickness due to formation of a diffusion barrier layer on the surface of blades followed by deposition of condensation alloyed layers based on the Ni-Co-Cr-Al-Y system and an external layer based on a NiAl alloyed β-phase and a ZrO₂: Y₂O₃ ceramics are presented. A complex gradient coating possessing unique protective properties at t = 1100–1200°C for single-crystal blades from alloy ZhS36VI for advanced gas turbine engines with gas temperature of 1550°C at the inlet to the turbine is described. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 41–48, May, 2007.     

Effect of Ca addition on the oxidation resistance of AZ91 magnesium alloys at elevated temperatures

AZ91 magnesium alloys containing 0.27–5.22 wt.% Ca, were melted and cast to study the effects of Ca addition on oxidation resistance at elevated temperatures. An ignition temperature test showed that the ignition of AZ91 alloy occurred at about 350–450 °C below the melting point, whereas that of the Ca-containing AZ91 alloys did so at above 650 °C. Weight gain measurements indicated that the oxidation resistance of the AZ91 alloys improved with Ca addition. The oxidation rate was dependent on the oxidation temperature. In the temperature range of 300–400 °C, the oxidation rate increased linearly. By contrast, the weight of 5 wt.% Ca-containing AZ91 alloy increased slowly due to the formation of a protective oxide layer. The oxidized surfaces were analyzed with low-angle XRD, FE-SEM equipped with EDS and AES. Complex structures were found in the oxide layers of the Ca-containing alloys: the outer layer mainly consisted of CaO, which was of uniform thickness, and the inner layer was a mixture of CaO, MgO, and Al₂O₃. In contrast to the loose and porous MgO formed on the surface of AZ91, the compact and dense oxide layers acted as an effective barrier to the further oxidation of the Ca-containing AZ91 alloys.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> content on the oxidation behavior of Fe-Cr-Al-based ODS alloys

A study was conducted to investigate the cyclic oxidation behavior of two oxide dispersion strengthened (ODS) Fe-Cr-Al based alloys containing 0.17 wt.% and 0.7 wt.% Y₂O₃. The alloys were oxidized in air for 100 h at 1200°C based on a 24 h cycle period. X-ray diffraction (XRD) and analytical transmission electron microscopy (TEM) were used to characterize the structure, morphology, and composition of the oxide scales. Both alloys formed highly adherent and continuous layers of α-Al₂O₃ exhibiting a morphology indicative of inward scale growth. The role of Y₂O₃ was to promote adherence by segregating to the grain boundaries within the oxide. Concurrently, Y₂O₃ generated micro-porosity resulting in a scale of comparatively higher thickness in the alloy with 0.7 wt.% Y₂O₃.     

Size and Shape Effects for Gamma Prime in Alloy 738

Cast IN-738 and wrought Inconel 738 are generic applications for most metallurgical designers of gas turbine blades in the Power Generation Industry on a worldwide basis... Particularly, where first stage buckets are concerned. This is the case because both alloy types exhibit outstanding creep and stress rupture properties to provide an extended service period in a harsh environment. Typically, Alloy 738 is fired in the turbine at 1970 °F (1074 °C) which is about 0.9T _m where T _m represents the melting temperature... A very demanding service temperature, indeed. Furthermore, Alloy 738 is expected to endure this high temperature for a duration of 26,000 h at base load before being retired (R) or replaced (R′) or reused (R′′) issues are ever considered. When these three (3) problems (R-R′-R′′) are brought before a given Materials Review Board for appropriate debate, many pro and con arguments are always evident because (1) Gas turbine blades are not inexpensive and (2) The threat of field failures with possible product liability litigation is of maximum interest to all gas turbine repair shop personnel. The intent of this paper is show how gamma prime precipitate particles can be better examined and more efficiently evaluated using a new characterization method. This research is offered as a contribution to the sum of total knowledge.     

Corrosion behaviour of the new gas turbine alloy 2100 GT in hot gases and combustion products

Not only excellent high temperature mechanical properties are needed to establish a new gas turbine alloy, but also a very good oxidation behaviour, together with good resistance to so‐called “hot corrosion”. This paper describes experimental studies on the corrosion behaviour in hot gases and combustion products of a new Ni‐Cr‐Ta alloy 2100 GT in comparison to the commercially established alloys 230, C‐263 and 617. Alloy 2100 GT is a newly developed cobalt, tungsten and molybdenum free Ni‐base superalloy of Krupp VDM. It contains as major alloying elements 25 wt.‐% chromium, 8 wt.‐% tantalum, 2.4–3 wt.‐% aluminium and 0.2–0.3 wt.‐% carbon. High temperature strength is achieved by the addition of tantalum, resulting in significantly increased solid solution strengthening, carbide hardening due to the formation of primary precipitated tantalum carbides, and γ′‐precipitation hardening by aluminium and tantalum. The isothermal oxidation tests showed that the parabolic rate constant of alloy 2100 GT is similar to that of alumina‐forming alloys. This is achieved by the remarkably high aluminium content for a wrought alloy. Additions of yttrium improve the spalling resistance under thermal cycling by the formation of very thin and tightly adherent oxide layers. No deleterious effect caused by the addition of tantalum could be found. In the cyclic oxidation tests performed at temperatures between 700°C and 1200°C alloy 2100 GT showed the lowest mass change of all the alloys investigated. Na₂SO₄ has been found to be a dominant component of alkali salt deposits on gas turbine components at elevated temperatures. Combustion gases contain SO₂ because of the impure nature of the fuel. To investigate the hot corrosion behaviour of alloy 2100 GT, tests were performed with salt deposits containing 0.1 mol Na₂SO₄ and a test gas comprising air and 0.1% SO₂. Test temperatures were 600°C, 700°C, 850°C and 950°C. Alloy 2100 GT exhibited the best performance at all test temperatures. It was the only alloy which did not suffer any fluxing of the oxide layer and only slight internal sulphidation was observed.     

Chromium-oxide Growth on Fe–Cr–Ni Alloy Studied with Grazing-emission X-ray Fluorescence

Using grazing-emission X-ray fluorescence (GEXRF), isothermal oxidation of the alloys 55Fe–25Cr–20Ni and 55Fe–25Cr–20Ni(+0.3Y) (wt.%) were studied as a function of oxidation time at 750 °C in O₂. In addition, the effect of thermal cycling was studied. Using GEXRF, oxide thickness, the Cr-depletion zone in the substrate, and Fe and Ni concentrations in the oxide were monitored as a function of oxidation time. Scanning-electron microscopy was used to independently measure the Cr-depletion zone. Raman spectroscopy was used to measure the concentration of Fe₂O₃ appearing in the oxides in early oxidation (less than 2 h). Both GEXRF and Raman measurements show that the thermally-grown chromium oxide purifies with extended oxidation; initially abundant Fe₂O₃ became undetectable after 2 h of oxidation. However, the total Fe concentration was still ∼3% after 2 h but systematically decreased with further oxidation. Thermal cycling had no effect on these results.      

Influence of service-induced microstructural changes on the aging kinetics of rejuvenated Ni-based superalloy gas turbine blades

Rejuvenation of Ni-based superalloy gas turbine blades is widely and successfully employed in order to restore the material microstructure and properties after service at high temperature and stresses. Application of hot isostatic pressing (HIP) and re-heat treatment can restore even a severely overaged blade microstructure to practically “as-new” condition. However, certain service-induced microstructural changes might affect an alloy’s behavior after the rejuvenated blades are returned to service. It was found that advanced service-induced decomposition of primary MC carbides, and the consequent changes of the γ-matrix chemical composition during the rejuvenation, can cause a considerable acceleration of the aging process in the next service cycle. The paper will discuss the influence of the previous microstructural deterioration on the aging kinetics of rejuvenated gas turbine blades made from IN-738 and conventionally cast GTD-111 alloys.     

Isotope exchange studies of oxidation mechanisms in nickel-base superalloys using FIB-SIMS techniques

The thermo-mechanical stability of the oxide layer that grows between the metallic bond coat and the ceramic top coat on superalloy turbine blades determines the lifetime of the system. Understanding the mechanisms of oxidation of the bond coat that is applied to the superalloy is key to improving the performance of the thermal barrier coatings. FIB-SIMS (Focused Ion Beam-Secondary Ion Mass Spectrometry) techniques in conjunction with tracer diffusion experiments represent a powerful tool in characterizing this oxide layer. This paper presents the results of oxidation studies on single crystal nickel-base superalloys/bond coat systems. In these studies, a two-stage oxidation experiment is used where ¹⁸O₂ serves as a tracer element during the second stage oxidation. The aluminium oxide grown in ¹⁶O₂ during the first stage oxidation represents an initial layer of oxide. Mass spectra collected by FIB-SIMS reveal the counter mass transportation of inward oxygen diffusion and outward diffusion of aluminium. New oxide formation during the second stage oxidation under an ¹⁸O₂ enriched environment is observed at both the gas/oxide interface as well as the oxide/superalloy interface. FIB-SIMS scanning enables high-resolution isotope maps, in particular ¹⁸O⁻, to be captured. These confirm the existence of new oxide forming at the oxide/superalloy interface with clear indications of short circuit diffusion paths through the existing oxide. These data allow the diffusion mechanisms for different superalloy/bond coat systems to be identified and contrasted, allowing the role of alloying additions to be elucidated.     

Experimental analysis and computer‐based simulation of nitridation of Ni‐base alloys—the effect of a pre‐oxidation treatment

Nickel‐base superalloys are commonly used for high‐temperature applications in power‐generation industry, e.g., gas‐turbine blades or heat exchangers. They are designed to resist high creep loading and severe corrosion attack during operation. Nitridation is one of these corrosion processes, in particular when the alloys need to be exposed to a N₂ atmosphere. Based on past assumptions, a dense oxide layer should be an efficient barrier against N₂ ingress. But is this really the case? This work is focused on the nitridation behavior of commercial Ni‐base alloys and the influence of a pre‐oxidation treatment. To model the growth of the internal‐nitridation zone, the diffusion processes were solved using the numerical implicit finite‐difference method in combination with the subroutine ChemApp for thermodynamic calculations.     

Phase and structural transformations in 12% chromium steel ÉP428 due to long-term operation of moving blades

The mechanical properties of LPT moving blades of the 4th and 5th stages of GT-6-750 turbine produced from 12% chromium steel éP428 (20Kh12VNMF) are studied after serving for 100,856 h. The microstructure of the steel is studied by methods of light metallography and analytical electron microscopy with the use of EDX and EELS spectrometers. The phase composition of the metal of moving blades and the chemical composition of the Me₂₃C₆, Me₂(C, N), and Me(C, N) phases after long-term operation at 350–500°C are determined. The phase and structural transformations occurring in the steel under such conditions are investigated. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 23–29, January, 2007.     

Thermal spray manufacturing issues in coating IGT hot section components

The desire to improve the performance of gas turbine engines has led to higher operating temperatures in the turbine sections of the engine. Materials used for hot section turbine blades and vanes are not resistant to hot corrosion, and therefore require protective coatings. This paper reviews the current art and technology of thermally sprayed MCrAlY and TB coatings onto hot section components. The issues in applying such coatings will be discussed, along with references to manufacturing issues on the shop floor. The difficulties inherent in applying a line-of-sight coating to complex geometries will be discussed. The testing, evaluation, and performance characteristics of typical coatings are discussed. Adapted from P. Sahoo et al., “Thermal Spray Manufacturing Issues in Coating IGT Hot Section Components,” Paper No. 97-GT-487, American Society of Mechanical Engineers, 1997.     

Characterization of Si-aluminide coating and oxide scale microstructure formed on γ-TiAl alloy during long-term oxidation at 950 °C

Recently γ-TiAl intermetallics have been successfully applied on low pressure turbine blades of modern jet engines mostly due to their low density, high specific strength and creep resistance, all of which make them excellent alternatives for Ni-based superalloys. However, due to insufficient high temperature oxidation resistance above 800 °C there is the necessity for development of protective coatings that will allow the formation of a thin and slow growing α-alumina oxide scale.The paper presents the results of investigations concerning the behavior and influence of silicon on the substructure of aluminized ɣ-TiAl alloy in the as-deposited state as well as after long-term isothermal high temperature oxidation. The Si-modified aluminide coating was produced by pack cementation method and oxidized isothermally at 950 °C for 3000 h. The detailed microstructural examination was performed using analytical SEM and STEM techniques along with Focused Ion Beam (FIB) technique which was utilized for the preparation of a sample from the metal-scale interface after long-term oxidation test. The addition of silicon to the aluminide coating resulted in the formation of nanometric Ti₅Si₃ precipitates that were found to bind both Ti and Nb from the alloy. It was demonstrated that the investigated ɣ-TiAl with Si-modified aluminide coating is capable of forming a continuous and uniform α-alumina oxide scale at 950 °C that remains adherent for 3000 h. The oxide scale was found to obey the growth mechanism typical for Ni-based superalloys and consisted of outer equiaxed and inner columnar grains.     

Measurements of fluorine depth‐profiles on TiAl turbine blades using ion beam analytical techniques

Intermetallic TiAl alloys are foreseen to substitute Ni‐based alloys in several high‐temperature applications such as turbine blades for aeronautics. Because of their low density the mass of these components could be reduced by half. However, a mixed oxide scale of TiO₂ and Al₂O₃ which provides no oxidation protection is growing at temperatures above 700 °C. By means of the halogen‐effect the high‐temperature oxidation resistance of TiAl alloys can be improved by orders of magnitude. Therefore fluorine was introduced into turbine blades using two different chemical fluorination methods. The application of a fluorine treatment promotes the growth of a pure and dense alumina scale which prevents the alloy from increased oxidation. In previous work it has been shown that an appropriate fluorine content after oxidation and its location beneath the surface are indicators of a successful fluorine effect. In the present work, the fluorine content was measured before and after oxidation of TNB alloy as a function of depth by using proton induced gamma‐ray emission (PIGE) in a specially designed vacuum chamber at the 2.5 MV van‐de‐Graaff accelerator at the IKF. Additionally, composition and thickness of the oxide scale was determined by Rutherford backscattering spectrometry (RBS). The ion beam techniques are non‐destructive and thus offer a method for quality assurance of the halogen treatment.     

Oxidation of Fe-3 wt.% Cr alloy

The oxidation behavior and the oxide microstructure on Fe-3 wt. % Cr alloy were investigated at 800°C in dry air at atmospheric pressure. Two distinct oxidation rate laws were observed: initial parabolic oxidation was followed by nonparabolic growth. The change in the oxidation kinetics was caused by microchemical and microstructural developments in the oxide scale. Several layers developed in the oxide scale, consisting of an innermost layer of (Fe,Cr)₃O₄ spinel, an intermediate layer of (Fe,Cr)₂O₃ sesquioxide, and two outer layers of Fe₂O₃ hematite, each with different morphologies. Wustite (Fe_1–xO) and distorted cubic oxide (-(Fe,Cr)₂O₃) were observed during the iniital parabolic oxidation only.     

Oxidation protection of gamma-titanium aluminide using glass-ceramic coatings

γ-TiAl intermetallic alloys are presently considered an efficient structural material for advanced turbine blades and aero-engine components due to their various advantages compared to the traditionally used superalloys. However, their poor oxidation resistance at temperatures > 750 °C severely limits their wider application. The present study dealt with the improvement of oxidation resistance of this alloy by applying impervious glass-ceramic coatings by vitreous enameling technique. Results showed that MgO-SiO₂-TiO₂ glass-ceramic coating could offer excellent oxidation resistance to γ-TiAl at 800 °C even up to 100 h with negligible weight gain (~ 0.10 mg/cm²) compared to that of the bare alloy (~ 1.3 mg/cm²). The coatings those were belonging from BaO-MgO-SiO₂, ZnO-Al₂O₃-SiO₂ and BaO-SiO₂ systems also extend appreciable improvement in the oxidation resistance of the alloy at 800 °C up to 100 h. At further higher temperature such as at 1000 °C, the ABK-13 and ABK-103 glass-ceramic coatings offered significant protection to the alloy up to 25 h of exposure in air with minimum weight gain (~ 0.34 mg/cm²). However, after that the coated layers started to peel off from the alloy surface.     

Effects of Chromium on the Oxidation Performance of β-FeAlCr Coatings

-FeAl coatings containing various Cr contents of 6.5–45 wt.%were produced with a closed-field, unbalanced magnetron sputter (CFUMS)deposition technique. Cyclic oxidation tests at 1100°C in air for100 1-hr cycles and isothermal exposures at 1000°C in pure O₂ for100 hr were carried out with the coatings and an as-cast FeAlspecimen. All of the coatings showed good scale-spallation resistanceduring cyclic oxidation and the coating with 6.5 wt.% Cr exhibited thelowest oxidation rates in both cyclic and isothermal oxidationexposures. After oxidation, fine-grain ridge-type oxide scales formed onthe coatings, while the oxide scale formed on the cast FeAl showed alarge quantity of -Al₂O₃ blades and large interfacial voids on thebase–alloy surface. The transformation from to -Al₂O₃was accelerated due to the presence of Cr in the coatings. The fasttransformation considerably reduced oxidation rates, suppressed fastoutward Al diffusion for the growth of a -Al₂O₃ scale, and preventedthe formation of interfacial voids that played a major role in causing thescale spallation.     

Diffusion aluminide coatings for TiAl intermetallic turbine blades

Alloys based on intermetallic phases of a Ti–Al system are materials that, thanks to their resistance characteristics, can be widely used in automotive and aerospace applications. The main restriction for the use of Ti–Al materials is their insufficient oxidation resistance above 850 °C. Oxidation parameters might be improved by aluminide coatings based on TiAl₂ and TiAl₃ phases, which could induce the creation of an Al₂O₃ scale in the oxidation process. This type of aluminide could be deposited on the surface of TiAl alloys by various methods such as pack cementation, plasma spraying or magnetron sputtering. This article presents a new method of aluminide coating deposition on TiAl intermetallic alloys: out of pack technology. The investigated coating was deposited on turbine blades made of a Ti45Al5Nb intermetallic alloy. The surface morphology, structure, phase and chemical composition have been investigated using XRD phase analysis, SEM and EDS. The phase analysis showed that TiAl₃ and TiAl₂ were the main components of the deposited coating. An isothermal oxidation test of the TiAl turbine blades was conducted as well. After 1000 h of testing at 950 °C, the scale formed on the surface of the uncoated blades underwent spallation. The scale on the turbine blade with deposited aluminide coatings was very thin and no spallation was observed.     

Synthesis and oxidation behavior of nanocrystalline MCrAlY bond coatings

Thermal barrier coating systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO₂/Y₂O₃). In this work, the oxidation behavior of conventional and nanostructured high-velocity oxyfuel (HVOF) NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Freestanding bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at 1000 °C for different time periods to form the thermally grown oxide layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared with that of the conventional one. The observed behavior is a result of the formation of a continuous Al₂O₃ layer on the surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.