ICMCTF1998 Session A1: Coatings to Resist High-temperature Corrosion

Wednesday, April 29, 1998 8:30 AM in Room Council/Chamber/Cabinet
Wednesday Morning

Time Period WeM Sessions | Abstract Timeline | Topic A Sessions | Time Periods | Topics | ICMCTF1998 Schedule

Start Invited? Item
8:30 AM Invited A1-1 High-Temperature Coatings in the Utility Industry: Problems in the Past and their Resolution; New Challenges for the Future
J. Stringer (Electric Power Research Institute)
Over the last 20-25 years, coatings and claddings have been tested as remedies for a variety of high-temperature corrosion and erosion problems in the electric utility industry. These problems include water-wall corrosion in coal-fired boilers, in-bed tube wastabe in bubbling fluidized bed combustors, wall erosion in circulating fluidized bed combustors, heat exchanger corrosion in coal gasifiers, and erosion and corrosion problems in steam turbines. The coatings have not always been very successful, in part because they have oftern been applied after the fact as problems developed. In this paper, the different approaches that have been tried will also be summarized, together with the problems and issues that have been associated with them. In the future, the introduction of new advanced technologies and increasing demand on the older techniques - higher temperatures and pressures, increased times between maintenance outages - will require a reexamination of surface protection methods, and possible directions for research and development will be suggested.
9:10 AM A1-3 The Characteristics of Alumina Scales formed on HVOF-sprayed MCrAlY Coatings
D. Toma, W. Brandl, J. Krüger (FachhochschuleGelsenkirchen, Germany); H.J. Grabke (Max Planck Institut Düsseldorf, Germany)

The oxidation behaviour of HVOF-sprayed MCrAlY (M=Ni,Co) coatings was examined. The coatings were isothermally oxidized in synthetic air at temperature between 850° and 1050°C for different times.

The HVOF MCrAlY coatings have a microstructure similar to the ODS alloys. During the thermal spraying the aluminium and yttrium were partially oxidized and a fine oxide dispersion was formed in the coating. The oxidation rate of the HVOF MCrAlY coatings was 3-4 times slower than that of the vacuum-plasma sprayed coatings. It seems that the fine oxides hinder the diffusion of the elements from the bulk material and modify the oxidation mechanism. The formation of the oxide dispersion in the coating influenced also the adherence of the oxide scale.

The structure of the oxide scale was examined by X-ray diffraction, scanning electron microscopy and transmission electron microscopy.

9:30 AM A1-4 Parameter Studies on HVOF Spraying of MCrAlY Coatings
E. Lugscheider (Werkstoffwissenschaften, Germany); L. Zhao, C. Herbst (Aachen University of Technology, Germany)
Today's thermally sprayed MCrAlY coatings are commonly manufactured by the Vacuum Plasma Spraying process. This technique provides dense and oxide free coatings. However, mainly due to the vacuum procedures this production is cost intensive and time consuming. Modern HVOF systems nowadays offer processing of materials that are sensitive to oxidation even in atmosphere. In terms of the processing technology this is attributed to further raised kinetic and reduced thermal energy spray particulates are subjected to during passing the high velocity flame. This even allows the manufacturing of MCrAlY coatings with low oxygen contents and dense structures unknown before for applications where high oxidation resistance is required. In addition, investment costs for modern HVOF equipment only is a tenth of VPS. The work presented here focuses the influences of process parameters of a modern gas-driven HVOF system onto microstructure and oxygen-content of MCrAlY coatings. Main parameters were subjected to statistical analyses to estimate single and combination effects. Results show which parameters have to be carefully adjusted in order to get optimized the processing of MCrAlY powders at atmosphere. It turned out that amongst most significant parameter is spray-distance. Also by variation of fuel-gas/oxygen ratio a clear effect onto oxygen- content and microstructure of resulting coating could be observed. Further parameters of significance are substrate temperature, powder feed rate, shroud-gas type, fuel-gas type and powder size fraction. The coatings with lowest oxygen-content were subjected to heat-treatment and oxidation and afterwards metallographically investigated. A comparison with VPS coatings released that HVOF sprayed MCrAlY coatings show equal oxidation behavior.
9:50 AM A1-5 Advanced PVD Coating Technology for Gas Turbines: High Strength MCrAlY, Diffusion Coatings and Coatings for γ-TiAl
R. Wenke, T. Leyendecker, G. Erkens, H.-G. Fuss, St. Esser (CemeCon GmbH, Germany)
High-temperature oxidation and hot corrosion resistant NiCoCrAlY overlay coatings on nickel base turbine blade alloys and on γ-TiAl as well as diffusion barrier coatings based on Pt, Pt-Hf, Pt-Y and Pd were deposited with High Ion Sputtering (H.I.S.TM) PVD technique. The advantage of this process technique is the controlled growth of the coatings. Effects of the used CC800 deposition technique on coating structure, coating composition, strength and microhardness as well as the possibility to add hardphases to the NiCoCrAlY layer by reactive sputtering investigated. For the deposition of Yttrium and Hafnium alloyed Platinum mechanically manufactured targets were designed to achieve a content of 0.5 - 1.0 atomic% of the additional element in the coating. Palladium is seen as a material that could substitute Platinum in future. In comparison to electro-plating pure Palladium was deposited without Nickel bondlayer. The flexibility of the CC800 coating system in view of coating turbine components reproducible and reliable as well as the high quality of the deposited layers will be illustrated.
10:10 AM A1-6 Break

10:30 AM A1-7 X-ray Determination of Stresses in Alumina Scales on High Temperature Alloys
C.N. Sarioglu, E. Schumann, J.R. Blachere, G.H. Meier, F.S. Pettit (The University of Pittsburgh)

The resistance of alumina scales to cracking and spalling under the influence of growth and thermal stresses is a critical aspect of the environmental resistance of high temperature structural alloys, oxidation resistant coatings, and bond coats for thermal barrier coatings. However, the relative magnitudes of the stresses and their distribution are often not known. In this study several x-ray diffraction techniques are being used to measure the strains in alumina scales on a variety of high temperature alloys both during oxidation and after cooling to room temperature. The corresponding stresses are being calculated using appropriate elastic constants. Preliminary results were presented in this conference in 1997. This presentation will highlight the results obtained during the intervening year. These include the observations that:

1. Growth stresses are high in alumina formed on FeCrAl alloys as compared to that formed on nickel-base alloys, such as NiAl or single crystal superalloys.

2. Yttrium additions to alumina-forming alloys do not result in lower growth stresses in the scales.

3. Growth and thermal stresses can be relaxed by plastic deformation of both the alloy and oxide.

The implications of these results with regard to alumina adhesion will be discussed.

10:50 AM A1-8 The Significance of Phase Distribution for the Oxidation Properties of Ni(Co)CrAlY Coatings
W.J. Quadakkers, J. Penkalla (Forschungszentrum Jülich, Germany); W. Stamm, N. Czech (Siemens, Germany); D. Clemens, C. Funke (Forschungszentrum Jülich, Germany)

For achieving increased efficiencies the gas inlet temperatures of advanced land based gas trubines have dramatically been increased in the past decade. Thermal, mechanical and environmentally induced stresses in land based gas turbine applications have become more and more similar to those in aero turbines. Thus, blade life is limited mainly by thermo-mechanical fatigue and oxidation rather than by creep and sulphidation as was often the case in the past.

Protection against oxidation induced damage of the blade materials is obtained by the application of Ni(Co)CrAlY coatings which are applied by thermal spray processes. The oxidation resistance of these coatings relies on protective alumina surface scales which form on the coating surfaces upon exposure to the oxidizing environment at the high service temperatures of around 900°C. In the present study isothermal and cyclic oxidation tests were carried with a number of Ni(Co)CrAlY-coating compositions applied by Vacuum Plasma Spraying. The coating compositions were systematically varied in Co-, Cr- Al and Re-contents. The oxidation tests were carried out in air and in Ar.O2 in the temperature range 900 - 1100°C. Before and after exposure the specimens were examined by optical metallography, SEM, energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and secondary ion mass spectrometry (SIMS). The results showed that in describing the oxication properties of the Ni(Co)CrAlY coatings it is of great importance to take into account that the coating alloys are multiphase. Depending on the exact coating compositon the microstructure can consists of γNi, γ'-Ni3Al, β-NiAl, α-Cr and.or α-CrCo, whereby the mentioned phases are solid solutions with variable compositions tather than stoichiometric compounds. The amount and distribution of these phases affect the coatings oxidation properties in at least two ways: 1) oxides of different composiitons can be formed on the various phases in the early stages of oxidation. The composition and distribution of these transient oxides can strongly affect the long time oxidation behaviour and 2) phase transformations can occur upon temperature changes (e.g. during start up and cool down of the turbine) which can significantly affect the thermal expansion coefficient of the coatings and therfore also the adherence of the alumina based surface scales.

From the obtained results guidelines for coating compositions which are optimized in respect to long term oxidation protection as well as to applications of Ni(Co)CrAlY's as bond coat for thermal barrier coatings can be derived.

11:10 AM A1-9 Use of Experimental Designs to Evaluate Formation of Aluminide and Platinum Aluminide Coatings
T. Kircher, B. McMordie, S. Shankar (Sermatech International); K. Richards (Drexel University)
Aluminide and platinum-modified aluminide coatings are widely used for protecting nickel and cobalt-based superalloy gas turbine engine components from oxidation and corrosion. Conventional methods for producing these coatings include pack cementation, "above-the-pack", and CVD aluminization processes. This paper describes the investigation of an alternative aluminization process utilizing deposition of a metallic slurry on the part surface and formation of the aluminide coating during a high temperature diffusion operation. As part of the initial investigation of this alternative aluminization process, a series of statistically designed experiments was conducted to determine how key process factors influence aluminide coating formation. One experiment utilized a three-level response surface design capable of modeling process factor effects on aluminide coating thickness. A second experiment utilized a two-level screening design to sort process factor effects on platinum aluminide coating structure and thickness. This paper will report on key characteristics of the response surface developed by the first experiment which analyzed process time, process temperature, and applied slurry amount for two candidate slurry formulations. Additionally, the paper will report on the effect of the process factors of platinum diffusion time, process time, and process temperature on two-phase platinum aluminide coating structures formed using the two candidate slurry formulations.
11:30 AM A1-10 Oxidation Resistance of Low-Sulfur NiAl and NiPtAl Coatings on a Desulfurized Ni-based Superalloy
Y. Zhang (University of Tennessee); W.Y. Lee (Stevens Institute of Technology); J.A. Haynes, I.G. Wright, B.A. Pint, K.M. Cooley (Oak Ridge National Laboratory); P.K. Liaw (University of Tennessee)
Previous work demonstrated that reducing the sulfur content of chemical vapor deposition (CVD) NiAl coatings significantly improved Al2O3scale adhesion during cyclic oxidation at 1150°C. The current study investigates the effect of Pt additions on the microstructure and oxidation resistance of low-sulfur CVD aluminide coatings. The NiPtAl coatings were processed by electroplating a thin layer of platinum on a low sulfur (~0.4 ppmw) single-crystal Ni-based superalloy, followed by CVD aluminizing. Laboratory isothermal and cyclic oxidation tests at 1150°C were performed on NiAl- and NiPtAl-coated superalloys. The chemical compositions and microstructures of the NiAl and NiPtAl coatings were compared, before and after oxidation. Depth profiling by glow-discharge mass spectroscopy was used to qualitatively assess sulfur levels. The effects of Pt additions on Al2O3 scale adhesion and diffusion of alloying elements will be discussed.
11:50 AM A1-11 Magnetron Sputtered Ti-Cr-Al Coatings for Oxidation Protection of Titanium Alloys
C. Leyens, J.-W. Van Liere, M. Peters, W.A. Kaysser (DLR-Germany Aerospace Research Establishment, Germany)
Ti-Cr-Al coatings with compositions ranging from 4-22 at.% Cr and 44-67 at% Al were shown to have significant potential for oxidation protection of titanium alloy 1). In this study, Ti-Cr-Al coatings ranging from 12-22 at.% Cr and 44-51 at.% Al were deposited onto a conventional titanium alloy (Ti-6Al-2.75Sn-4Zr-0.4Mo-0.4Si in wt.%) as well as onto an orthorhombic titanium aluminide (Ti-22Al-25Nb in at.%) using magnetron co-sputtering technique with and without substrate bias voltage. As-deposited coatings were annealed in vacuum to establish a two-phase structure preferably consisting of y-TiAl and Laves phase. Oxidation resistance was tested in air at 750 °C showing dependence of environmental resistance on coating composition and bias voltage during deposition. The Ti-Cr-Al coatings were found to be alumina formers, however, in some cases small amounts of TiO2 were detected by XRD. It is suggested that the fine-grained coating structure promotes rutile formation due to the presence of numerous rapid diffusion paths. 1)C. Leyens, M. Schmidt, M. Peters and W.A. Kaysser, Mater. Sci. Eng., in press
Time Period WeM Sessions | Abstract Timeline | Topic A Sessions | Time Periods | Topics | ICMCTF1998 Schedule