ICMCTF2003 Session A1-2: Coatings to Resist High Temperature Corrosion and Wear
Monday, April 28, 2003 1:30 PM in Room Sunset
A1-2-1 Mechanical Behavior and Martensitic Transformation of Platinum Modified Nickel Aluminide TBC Bond Coats
K.J. Hemker (Johns Hopkins University)
Durability of a TBC is directly tied to its ability to resist spallation, and nickel aluminide bond coats play an important role in governing TBC life. The strength of the bond coat is thought to play an important role in governing out-of-plane displacements of the TGO, and efforts to measure elevated temperature bond coat properties with microsample tensile and creep testing will be presented. Moreover, observations of a martensitic transformation in commercial platinum modified nickel aluminide bond coats and related finite element (FE) calculations have highlighted the importance of the transformation strain and temperatures in governing the generation of residual stresses in the TBC as a result of thermal cycling. TEM observations, differential thermal analysis and X-ray diffraction experiments have been used to characterize the martensitic transformation. And, results of FE calculations that incorporate these measurements suggest that suppressing the martensitic transformation, or at least shifting the Ms to lower temperatures, may have an important effect on TBC life. These findings will be outlined and the role of bond coat chemistry discussed.
The support of General Electric Aircraft Engines and the National Science Foundation (Grant No. DMR9986752) are gratefully acknowledged.
A1-2-3 Thermal Expansion in MCrAlY Alloys
T.A. Taylor, P.N. Walsh (Praxair Surface Technologies, Inc.)
A range of common MCrAlY coatings were made by argon-shrouded plasma spray deposition. The samples were hollow tubes, 0.5" diameter, 1" high with 0.025" wall thickness. Sintering effects were removed by heat treatment in vacuum for 4 hours at 1080°C. Then the thermal expansion curves were determined by heating in argon from room temperature to 1100°C at 5°C per minute. The resulting expansion values were compared in terms of the chemical composition of the coatings.
A1-2-4 Differences in Oxidation Behavior of Alumina and Chromia Formers in Air and in Air-H2O Mixtures at Elevated Temperatures
K. Onal, F.S. Pettit, G.H. Meier (University of Pittsburgh)
The results of many oxidation studies in environments containing water vapor have shown that H2O(g) has negative effects on oxidation resistance of many alloys. In this study one Co-based and several Ni-based superalloys were tested in atmospheres with and without water vapor additions in the temperature range 700°C-1000°C at a total pressure of 1 atm. The alloys that rely on chromia formation for oxidation resistance were not affected substantially by the presence of water vapor at 700°C, whereas water vapor caused more transient oxidation in the case of the alloys relying upon alumina formation for oxidation resistance. At higher temperatures, especially at 1000°C, water vapor caused cracking and spalling of oxides for both the chromia and alumina forming alloys. This was much more severe in the case of the chromia formers. The mechanisms by which water vapor promotes transient oxidation in alloys and causes increased spalling of chromia and alumina will be discussed.
A1-2-5 Some Water Vapor Effects During the Oxidation of Ni8Cr6Al Model Alloy
M.C. Maris-Sida, G.H. Meier, F.S. Pettit (University of Pittsburgh)
Oxidation studies were performed at 1100°C and 700°C in dry air and air or inert atmospheres containing fixed partial pressures of water vapor on specimens of a Ni-8wt%-6wt%Al model alloy. Upon investigation of exposed specimens, it has become apparent that water vapor causes accelerated transient oxidation, with thicker oxides formed during oxidation in wet air than in dry air. The more rapid growth of transient oxides may be caused by hydrogen or hydroxyl ions supplied by the water vapor. The accelerated growth of such oxides could hinder the selective oxidation of aluminum. Extensive internal oxidation of Al was observed under certain exposure conditions which could be associated with extensive growth of transient Ni and Cr oxides. An alternative possibility is that, upon water vapor dissociation, some oxygen-containing species develop that could move rapidly into the alloy. The adverse effect of water vapor on the selective oxidation of aluminum is enhanced as the oxidation temperature is lowered. Possible mechanisms for the above observations will be presented.
A1-2-6 Life Time Prediction for MCrAlY Coatings by Means of Inverse Problem Solution (IPS)
V. Kolarik (Fraunhofer-Instut für Chemische Technologie (ICT), Germany); P. Krukovsky, K. Tadlya (Institute of Engineering Thermophysics (ITTF), Ukraine); A. Rybnikov, I. Kryukov, Natalia Mozhayskaya (Polzunov Central Boiler and Turbine Institute (NPO CKTI), Russia); M. Juez-Lorenzo (Fraunhofer-Instut für Chemische Technologie (ICT), Germany)
The method of Inverse Problem Solution (IPS) has been applied to modelling and life time prediction of MCrAlY coatings for stationary gas turbine application. With this technique feasible experimental data like β-NiAl - phase and/or Al concentration profiles acquired after a defined time are used as input values for the model parameter estimation. The life time prediction procedure then calculates the β-NiAl - phase depletion until the complete consumption, which is defined as life time end of the coating. The life time prediction approach has been performed on a NiCoCrAlY coating with 8% Al in air at 900 and 950°C and verified up to 20000 h. Additionally, in situ studies of the oxide scale formation in the first 100 h were carried out. The coating forms at both temperatures α-Al2O3 and initially α-Al2O3 , which influences the initial kinetics and on further oxidation transforms into α-Al2O3 . The β-NiAl - phase and the Al concentration profiles were determined in the initial state and after 100, 300 and 1000 h of oxidation. The profiles measured after 300 and 1000 h were used as input values for the model parameter estimation. With these values the β-NiAl depletion and the oxide scale thickness were calculated as a function of time yielding the life time prediction of the coating. The procedure delivers highly reliable life time prediction and allows easily to evaluate the influence of the coating thickness on the life time.
A1-2-7 Growth Strains in Chromia and Alumina Scales
A.P. Paulikas, M. Beno, J. Linton, G. Jennings, B.W. Veal (Argonne National Laboratory)
We have measured growth strains in chromium and aluminum oxides isothermally grown on 55Fe-25Cr-20Ni alloys (chromia former) and on 25Ni- 25Pt-50Al (alumina former). Strains were measured in-situ in air, as the oxides evolved, using x-ray diffraction, exploiting synchrotron radiation at the Advanced Photon Source. For the chromia former, measurements were obtained at several temperatures between 650 - 1000°C. For the alumina former, measurements were taken at 1100°C (α-Al2O3). For both oxides, compressive growth strains were observed (~0.1%), and some strain relief occurred as oxidation progressed. For the chromia former, growth strains were observed to systematically increase as the oxidation temperature was lowered. Strain relaxation, associated with creep and new growth, was also monitored following an abrupt temperature change. Thermal expansion differences between scale and substrate were exploited to add an incremental compressive stress or to remove a compressive growth stress. Implications to growth mechanisms will be discussed.
A1-2-8 The High-Temperature Corrosion of AISI 1025 Steels with Electroless Nickel/ Aluminizing Coating in the Oxychlorination Environment
C.C. Li, C.J. Wang (National Taiwan University of Science and Technology, Taiwan, ROC)
AISI 1025 steels were chosen to coat with electroless nickel, pack aluminization and hot-dip aluminum, respectively. After surface treatments, specimens were deposited with 2mg/cm2 NaCl and oxidized in a horizontal tube furnace at 850°C in the dry air atmosphere with 600mL/min flow rate for 1 to 25 hr. The results showed that the Ni3P precipitation and the formation of NiO in the electroless nickel layer slightly improved the oxidation resistance, but was not sufficient to inhibit the active oxidation caused by the NaCl deposit. Al2O3 formed on the surfaces of aluminized specimens by both methods after high temperature corrosion tests. Weight gains were reduced tremendously and oxidation resistances were greatly improved. However, microcracks were formed in the brittle aluminized layer during cooling from the post preoxidation. The microcracks provided deposition positions and diffusion shortcuts of molten salt and metals, showing detrimental effects on the oxidation resistance for the aluminized steel. Key words: Electroless Nickel; Aluminizing; NaCl; High-temperature Corrosion.
A1-2-9 Ni and Ni/Cr Diffusion Coatings by Electroplating in Conjunction with Fluidized Bed Reactor Treatment
N. Priyantha, P. Jayaweera, A. Sanjurjo, M. Hornbostel, K.H. Lau (SRI International); K. Krist (Gas Technology Institute)
Nickel diffusion coatings on low-grade alloys such as C1018 and 409SS were prepared by electroplating, followed by heat treatment in a fluidized bed. During this process, electroplated overlay coatings diffused into the bulk substrate to form a Ni/Fe alloyed surface, thus improving the corrosion resistance of the low-grade alloy. In addition, Cr was deposited on electroplated Ni coatings in the fluidized bed reactor at elevated temperature to form diffusion layers consisting of Cr, Ni and Fe. We obtained significant diffusion depths of Cr and Ni in this process. These Cr/Ni/Fe diffusion coatings on low-grade alloys mimicked the behavior of super alloys in aggressive environments resulting in orders of magnitude increase in corrosion resistance.
A1-2-10 Texture, Structure and Phase Transformation in Beta Tantalum Coatings
S.L. Lee, P. Cote, M. Cipollo (US Army Research Development and Engineering Center, Benet Labs)
Properties of beta tantalum are of interest because of the important applications of tantalum in nano-electronics and high temperature wear and erosion protective coatings. In this paper, we report beta tantalum coating, sputter deposited onto inner surface of 45mm and 120mm steel cylinders, and steel and glass plates. Two forms of beta tantalum coatings were observed: One is the conventional more random beta tantalum, with some <002> preferred orientation. The other is a highly (002) fiber-textured beta-tantalum, with predominately <00x> reflections, strong for even x, and weak for odd x. Our pole figure and 2-D XRD analyses showed highly (002) textured tantalum to belong to the same tetragonal structure as more random beta tantalum, and is not a new phase. Calculated tetragonal beta-tantalum cell, using a = 10.211, c = 5.306, space group P-421m, Frank-Kasper structure, showed even and odd <00x> reflections, as observed.
The two forms of beta tantalum showed different phase transformation properties. In a specimen of the first kind, sputtered onto a 45mm diameter cylinder, tetragonal to bcc phase transformation was observed at 750°C-850°C. In another example, a steel cylinder of 45mm diameter was sputter-coated with predominately more random beta tantalum. The cylinder was subjected to a few cycles of high temperature (>1500°C) and pressure (>100 Ksi) test of a few milliseconds duration. XRD showed partial phase transformation from tetragonal to bcc phase, oxidation of tantalum to form Ta2O5, and copper debris from the projectiles.
In a specimen of the second kind, extremely high (002) fiber textured tantalum was sputter deposited onto steel and glass plates. Upon annealing in vacuum, the steel plate specimen showed hot hardness decreased drastically, from 1000KHN, typical for beta tantalum, to 250-300KHN, typical for alpha tantalum. This suggests a phase transformation temperature at 300°C. XRD data further showed that in highly (002) textured beta tantalum, beta to alpha phase transformation began as low as 100°C. Beta to alpha and textured to more random beta tantalum transformation became intense at 300°C.
A1-2-11 High Temperature Oxidation and Microstructural Aspects of Aluminized Coating on Superallloys
F.A. Khalid, F. Nawaz (GIK Insitute of Engineering Sciences and Technology, Pakistan)
The high temperature oxidation behavior of superalloys used for the manufacturing of turbine blade have been examined using optical, scanning electron microscopy (SEM) and fine-probe spot and linescan EDS microanalysis, x-ray mapping and XRD techniques. The role of thermal barrier coating employed has also been examined to evaluate the performance of blade samples under different conditions. IT was found that the formation of adherent coatings is essential for protection against the severe environments. The alloy samples oxidized in air at various temperatures ranging from 800°C to 1100°C were examined. The microchemical and micro structural changes that occurred during oxidation have been analyzed to elucidate the oxidation behavior and the formation of different phases in the samples.