ICMCTF2010 Session A1-3: Coatings to Resist High Temperature Oxidation

Wednesday, April 28, 2010 8:00 AM in Room Sunrise

Wednesday Morning

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Start Invited? Item
8:00 AM A1-3-1 Determination of the DBTT of Aluminide Coatings and its Influence on the Mechanical Behavior of Coated Specimens
Sebastien Dryepondt (Oak Ridge National Laboraotry); Bruce Pint (Oak Ridge National Laboratory)

A high temperature indenter has been developed to determine the Ductile to Brittle Transition Temperature of diffusion aluminide coatings deposited by Chemical Vapor Deposition or pack cementation on martensitic-ferritic steels, austenitic steels and Ni-based alloys. An estimate of the transition temperature was based on hardness measurements as well as characterization of coating deformation and cracking after indentation at increasing temperatures. Creep tests were performed at temperatures below and above the DBTT to assess the impact of that parameter on the lifetime of coated specimens. The influence of the substrate and the coating process will be discussed.

8:20 AM A1-3-2 Oxidation Performance of Low-Temperature Pack Aluminide Coatings on Ferritic-Martensitic Steels
Brian Bates, Ying Zhang (Tennessee Technological University); Bruce Pint (Oak Ridge National Laboratory)
Aluminide coatings were synthesized at temperatures ≤ 700°C via pack cementation on commercial ferritic-martensitic alloys. Binary Cr-Al masteralloys were utilized to reduce the Al activity in the pack aluminizing process to prevent the formation of brittle intermetallic phases such as Fe2Al5. With the Cr-25Al masteralloy, the coating consisted of a thin Fe2Al5 outer layer (~4 μm) and an FeAl inner layer (~12 μm). When the Cr-15Al masteralloy was used, the lower Al activity led to the formation of FeAl coatings of ~12 μm thick. The oxidation performance of these low-temperature pack coatings was evaluated for up to 10,000 h in air + 10 vol.% H2O at 650-700 °C, and compared to the coatings made by chemical vapor deposition at higher temperatures. The thin pack coatings on T91 substrates showed good oxidation resistance in the water vapor environment. Cross-sectional examinations of the coating samples after 10,000 h exposure were conducted to evaluate their microstructural and compositional changes.
8:40 AM A1-3-3 The Dependence of High Temperature Resistance of Aluminized Steel Exposed to Water Vapour Oxidation
Chaur-Jeng Wang, Mohd. Badaruddin (National Taiwan University of Science and Technology, Taiwan)

The oxidation behaviour of hot-dipping aluminized steel was investigated at temperatures ranging from 700 to 800°C in air containing 100% water vapour (steam) at atmospheric pressure. The oxidation tests in 100% water vapour + N(g) were also performed to justify effect of water vapour in the atmosphere without air. After oxidation test, the morphology, composition and structure of the scale were examined by means scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electron probe micro analysis (EPMA). The experimental results show that significant failure of protective coating occurred, resulting in substantial weight increase. The kinetics curve obeys linear law after long exposure times in the steam oxidation. The growth of Al2O3 scale and sporadic iron oxide nodules on the coating surface as well as at the interface are accelerated by the presence of water vapour in the atmosphere. In addition, when the temperature increases up to particularly 800oC, the presence of water vapour greatly gives rise to internal oxidation of Al at the grain boundary in the substrate due to high atomic diffusion rate at this temperature.

9:00 AM A1-3-4 Thin Pure-Metal Films Allowing to Rapid Development of α-Al2O3 Scale on High-Temperature Alloys and Coatings
Shigenari Hayashi, Yuri KItajima, Toshio Narita, Shigeharu Ukai (Hokkaido University, Japan)

Rapid formation of slow growing oxide scale on heat resistant alloys and coatings is one of the most important issues in order to improve reliability and lifetime, particularly of materials which have a low total aluminium content, such as coatings and thin foils. Al2O3 forming alloys and coatings usually form so-called “meta-stable” (γ, θ)-Al2O3 at an initial stage of oxidation, and later transforms to stable (α)-Al2O3. This meta-stable to stable phase transformation is known to require longer time when the alloys and coatings are exposed to temperatures below about 900ºC. Moreover, reactive element additions to alloys and coatings, in order to improve scale spallation performance, are known to be significantly detrimental by delaying this phase transformation. The growth rate of meta-stable Al2O3 is more than two orders of magnitude higher than stable Al2O3, therefore rapid formation of α-Al2O3 and suppression of metastable-Al2O3 scale formation are directly linked to improvement of the performance of heat resistant alloys and coatings.

The aim of this study was to investigate methods to accelerate the meta-stable to α-Al2O3 transformation. In this presentation we will introduce the direct formation of α-Al2O3 scale without the formation of meta-stable Al2O3 on Fe-50Al and Ni-50Al alloys with and without deposition of thin films, ~50nm, of various elements such as Fe, Cr, Ni, Ti, and Al at 900ºC in air. Additionally, the effect of coating elements on initial development of α-Al2O3 as well as the meta-stable to α-Al2O3 phase transformation will be discussed.

Acknowledgements

This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Exploratory Research, 20656115, (2008).

9:40 AM A1-3-6 Development of Thermally Grown Oxide on β-NiAl During Initial Stages of Oxidation at 1100oC
Hyunju Choi (University of Central Florida); Jerzy Jedlinski (AGH University of Science and Technology, Poland); Yongho Sohn (University of Central Florida)
Phase transformations and microstructural evolution of thermally grown oxide (TGO) scale on polycrystalline β-NiAl, with and without Yttrium implantation, were examined by X-ray diffraction, photo-stimulated luminescence (PL), scanning and transmission electron microscopy (SEM & TEM). Implantation of yttrium ion was carried out with a beam energy of 70 keV up to 2 x 1016 ions/cm2. The polycrystalline β-NiAl specimens were oxidized at 1100ºC in air for 15 minutes up to 24 hours. TEM specimens were prepared by using focused ion beam (FIB) in-situ lift-out (INLO) technique. After 15 minutes oxidation, the TGO consisted of islands of flat scale, 300 nm in thickness, in 550 nm-thick scale that exhibited needle-like morphology. This initially-developed scale consisted of θ- and δ-Al2O3 (θ >δ) with traces of equilibrium α-Al2O3. Pores were observed at the TGO/NiAl interface. The islands of flat TGO grew to 500 nm and the needle-like, rough TGO grew to 800 nm after 6 hours. The scale consisted mostly of α-Al2O3 with traces of θ-Al2O3. After 24 hours, two distinct layers in the TGO scale were observed: 1300 nm thick upper layer and 1600 nm thick lower layer in contact with NiAl. Both layers consisted of α-Al2O3. In addition, pores were observed at the interface between these two TGO layers, and Al-depleted γ’-Ni3Al phase, 150 nm in thickness, was observed at the TGO/ Ni3Al interface. With addition of Yttrium, the thickness of the TGO scale was in general thicker, and the transformation to the equilibrium α-Al2O3 was slower. However, the development of the microstructurally distinct two-layer was faster. While the upper TGO layer consisted mostly of the α-Al2O3, the bottom layer in contact with Y-implanted NiAl consisted of α, δ-, and θ-Al2O3 phases after 6 hours of oxidation at 1100ºC (α > δ > θ). Average compressive residual stress within the α-Al2O3, estimated by PL, was determined to be lower for Y-implanted NiAl.
10:00 AM A1-3-7 Influence of Thermal Exposure on the Stability of Non-Equilibrium Microstructures of Sputter Deposited Nanocrystalline 304 and 310 SS Coatings
N. Sastry Cheruvu, Ronghua Wei (Southwest Research Institute); David Gandy (Electric Power Research Institute)

It is well known that the sputter coating deposition technique produces coating with metastable structures. Though the standard 304 and 310 stainless steels (SS) have a face-centered cubic (fcc), the microstructure of the as sputter deposited 304 and 310 SS coatings exhibits metastable bcc (αFe) and fcc (γFe) + bcc structures, respectively [1],[2]. However, the effect of thermal exposure on the stability of the metastable structure as a function of temperature is unknown and hence, a systematic study has been undertaken to identify the metastables phases present in the as sputter deposited 304 and 310 SS coatings and assess the stability of the phases as a function of aging time and temperature. The 304 and 310 SS coatings were deposited on 304 SS samples using a magnetron sputtering technique. The coated samples, along with a 310 SS target sample, were exposed to 500ºC, 650ºC, and 750ºC for up to 5000 hrs. For phase identification, x-ray diffraction (XRD) radiation was conducted on the as-deposited and exposed samples. The XRD results revealed that the microstructures of the as-deposited 304 and 310 SS coatings consisted of σ + αFe and γFe + αFe phases, and as expected, the target material exhibited fcc. Thermal exposure at the three temperatures investigated resulted in the reduction of σ phase and precipitation of γFe phase in the 304 SS coating. On the other hand, thermal exposure of the 310 SS coating led to precipitation of the σ phase and the amount of σ phase in the coating increased with the exposure time. The results will be discussed in terms of variation of composition and alloy segregation between the as-deposited 304 and 310 SS coatings.


[1] Childress, J., Liou, S.H., and Chien, C.L. “Ferromagnetism in Metastable 304 Stainless Steel with bcc Structure.” Journal of Applied Physics, 64(10), 15 Nov 1988, pp.6059-6061.

[2] Malavasi, S., Oueldennaoua, Foos, M., and Frantz, C. “Metastable Amorphous and Crystalline (α,σ) Phase

in Physical Vapor Deposited Fe-(Cr)-Ni-(C( Deposits.” Journal of Vacuum Science Technology A, 5(4), July/August 1987, pp. 1888-1891.

10:20 AM A1-3-8 Microstructure and High Temperature Oxidation Behavior of Hot Dip Aluminized Coating on High Silicon Ductile Iron
Meng-Bin Lin (National Taiwan University of Science and Technology, Taiwan)

In this study, high silicon ductile iron was coated by hot-dipping in pure Al and Al-10Si melt .their high temperature oxidation behaviour was tested at 750°C to 950°C for virous exposure time. After high temperature oxidation tests, the microstructure analysis of all samples were investigated by means of metallographical examination, scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectrometer (EDS) and X-ray diffractometry (XRD).The results showed that hot dipping in Al-10Si could reduce the thinkness of intermetallic layer. Hot dipping Al/Al-Si on high silicon ductile iron improved high temperature oxidation resistence by over 10 times compared with the uncoated high silicon ductile iron.

10:40 AM A1-3-10 A review of Sulphidation and Oxidation Processes in Gasturbines, their Simulation and Modelling via Laboratory Tests
David Rickerby (Rolls-Royce plc, United Kingdom)
Turbine hardware is subjected to a wide range of environmental degradation processes and these will be reviewed and their impact on component life discussed. Progress on the development of laboratory tests aimed at simulating these various environmental factors will be outlined along with their impact on the mechanical performance of coating systems. The extension of these simulation techniques into lifing methodologies for coated turbine hardware will be reviewed and areas for further work discussed.
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