Papers

ICMCTF2001 Session A1-2: Coatings to Resist High Temperature Corrosion

Monday, April 30, 2001 1:30 PM in Room Royal Palm Salon 1-3

Monday Afternoon

Start	Invited?	Item
1:30 PM		A1-2-1 Corrosion Behavior of PVD Coated Tools for the Manufacture of Injection Moulding Polyvinyl Chloride (PVC) Components R.M. Souza (Polytechnic School of the University of Sao Paulo, Brazil); A.P.P. Silveira (Astra S.A., Jundiai Brazil); A. Sinatora, D.K. Tanaka (Polytechnic School of the University of Sao Paulo, Brazil) In this work, the corrosion behavior of titanium nitride (TiN) or chromium nitride (CrN) coated specimens was evaluated under industrial production conditions for the manufacture of polyvinyl chloride (PVC) components. Ten specimens of polymer injection moulding tools were manufactured in AISI H13 steel. Five specimens were coated with TiN by a physical vapor deposition (PVD) process and other five with CrN, also by PVD. TiN or CrN coated specimens were simultaneously mounted in a polymer injection moulding tool and were tested under actual production conditions. The mass loss of the specimens was measured and, after approximately 120,000 injection cycles, the samples were also analyzed in terms of surface roughness and observed under optical and scanning electron microscopes. The analyses indicated that slightly heterogeneous machining processes, prior to the depositions, could be the cause of different corrosion behavior in terms of the amount and morphology of the corrosion pits under injection moulding operation conditions.
1:50 PM		A1-2-2 Protective CVD Mullite Coatings for Corrosive Environments A.K. Pattanaik, V.K. Sarin (Boston University) Silicon based ceramics such as SiC and Si₃N₄, due to their toughness and thermal shock resistances are established leading candidates for the next generation of high temperature materials. However, the applications of these materials are limited by their susceptibility to high temperature oxidation, corrosion, and loss of strength. For example, SiC based heat exchanger have shown considerable promise but are susceptible to aggressive attack by halogen (i.e. Cl₂) containing environments. It is therefore universally agreed that in order to enable the utilization of Si-based ceramics in such aggressive environments, they will have to be protected. A chemical vapor deposition (CVD) process to deposit protective mullite (3Al₂O₃ 2SiO₂) coatings has been developed and patented at Boston University. Preliminary test results in various environments indicate that CVD mullite coated SiC have significantly improved oxidation and corrosion resistance as compared to uncoated SiC. In the present investigation, oxidation behavior of CVD mullite coated SiC in chlorine containing environment will be presented and discussed.
2:10 PM		A1-2-3 Thermal Air Oxidation of Copper in an Applied Electric Field N. Parkansky, B. Alterkop, S. Goldsmith, R.L. Boxman, Z. Barkay (Tel Aviv University, Israel) The structure and composition of copper oxide surface films generated during annealing in an applied electric field were studied. Two copper disks were connected as d.c. capacitor electrodes and were oxidized by annealing in air for one hour at a temperature T in the range of 100-500°C. An electric field of 0-5 kV/cm was applied between the electrodes. After annealing, the plates were allowed to cool to room temperature. The surface structure of the electrodes was examined in a scanning electron microscope using scattered electrons (SE), backscattered electrons (BSE) and energy dispersive spectroscopy (EDS). The electrode masses were measured before and after annealing. At T>300° C the oxide formed on the anode was a dense, continuous, homogenous layer of CuO. The oxide on the cathode and on samples annealed without an applied field consisted of separated grains of CuO on a background of smaller Cu₂O grains. Thus, at T=500 C and the applied field of 5 kV/cm the characteristic grain sizes were 2-4 µm on the anode and 6-10 µm and 1.5 µm on the cathode and 5-7µm and 1 µm on the sample without an applied field. Annealing at T>300° decreased the electrode mass due to spallation of oxide fragments. For applied voltages greater than 1 kV/cm, the cathode mass decreased slightly. The anode mass loss was a factor of 13 more at 5 kV/cm than at 1 kV/cm. A leakage current was observed between the electrodes during annealing for T>=400°c. The current increased up to maximum (<=0.2 mA) and then decreased with time during annealing. The maximum value increased with applied voltage and temperature.
2:30 PM		A1-2-4 Novel Coating Systems Produced by the Combined Cathodic Arc/ Unbalanced Magnetron Sputtering for Environmental Protection of Titanium Alloys and Titanium Aluminides Ch. Leyens, M Peters (DLR-German Aerospace Center, Cologne, Germany); P. Hovsepian, B.D. Lewis (Sheffield Hallam University, Sheffield, UK); W.-D. Münz (Sheffield Hallam University, United Kingdom) The Use of titanium alloys and titanium aluminides in high temperature aerospace and automotive applications is often limited by their insufficient environmental resistance rather than their mechanical properties. Therefore, protective coatings are needed to separate the structural material from the environment, thus minimising oxidation and embrittlement of the alloys. Novel coatings have been developed using a combined cathodic arc/unbalanced magnetron sputtering method, where the substrates are metal ion etched, running for instance Cr target in a cathodic arc mode as an ion source and keeping the substrates at potential of U_b = -1200 V. The metal ion etching process generates an ion implanted subsurface zone as well as a thin layer of the respective metal species depending on the bombardment parameters. In a subsequent step, nitride coatings were deposited by magnetron sputtering. The paper presents initial results on metal ion-etched, near-α titanium alloy IMI 834, overlaid with monolithically grown TiAlCrYN as well as TiAlYN/CrN and CrN/NbN superlattice coatings. Specimens were tested under isothermal and cyclic oxidation in air at 750°C. TiAlCrYN and TiAlYN/CrN coatings demonstrated excellent oxidation resistance up to 1000 hr under these conditions, whereas the CrN/NbN coatings and metal ion implanted only showed no beneficial effect relative to untreated substrate material. Detailed investigation of the coating microstructure in the as-deposited condition and after oxidation testing will be presented to underline the great potential these coating systems appear to have.
2:50 PM		A1-2-5 High Temperature Oxidation Behavior of HVOF-Sprayed Unreinforced and Reinforced Molybdenum Disilicide Powders G. Reisel, B. Wielage, S. Steinhauser (TU Chemnitz, Germany); I. Morgenthal, R. Scholl (Fraunhofer Institute for Applied Materials Research, Germany) Intermetallics like silicides are useful for protective coatings against high-temperature corrosion. Especially molybenum disilicide has great potential as protective coating for example in aircraft engines and gas turbines in the temperature range between 1400 and 1800 Â°C due to its high melting point and its low brittle-ductile transition temperature of approximately 800 - 1100 Â°C. Caused by its moderate density MoSi2 is more economical than commonly used materials. Mixtures of elemental powders, molybdenum and silicon, were prepared by high energy milling (HEM) and subsequent heat treatment to get high dispersed, prereacted powders for further coating treatment. In the case of reinforced powders the reinforcing phase was given to the mixture before heat treatment. For using these powders as feedstock for thermal spraying sifting was executed by means of sieving. The powder were analyzed by X-ray diffraction, scanning electron microscopy (SEM, ESMA) and determination of the oxygen level. The unreinforced molybdenum silicide powder as well as the reinforced mixtures were sprayed for testing onto a carbon steel substrate. High velocity oxyfuel spraying (HVOF) was carried out for producing the coatings instead of the normal way for thermal spraying of molybdenum silicide, that means vacuum plasma spraying. After optimizing the spraying parameters like the spraying distance in order to minimize coating's porosity the high temperature oxidation behavior of the coatings was determined. A comprehensive characterization of the coatings was executed concerning microstructure (SEM, ESMA), phase determination (XRD), oxygen content, microhardness, porosity, surface roughness and wear characteristics before and after the high temperature tests.
3:30 PM		A1-2-7 Corrosion Resistance and Microstructural Evaluation of Chromized Coating Process in Dual Phase Fe-Mn-Al-C Alloy J.W. Lee (Tung Nan Institute of Technology, Taiwan,R.O.C.); J.G. Duh (National Tsing Hua University, Taiwan,R.O.C.) Chromising process is a method for developing a chromized coating for high temperature surface protection, corrosion and wear resistance. A homogenized dual phase Fe-24%Mn-8.3%Al-5%Cr-0.38%Si- 0.34%Mo-0.45%C alloy casting piece was chromized with pack cementation process. A significant increase of the surface hardness was achieved due to the formation of chromium carbide layer. Better corrosion resistance in aqueous solution and high temperature oxidation resistance in air were observed. The phase transformation and microstructure phenomena of chromized coating layer and matrix were studied with X-ray diffractometer and Electron Probe Microanalyzer. Cyclic polarization test in 3.5 wt% NaCl aqueous solution was employed to evaluate the effect of chromising. High temperature oxidation test at 1000°C was further conducted to investigate the oxidation resistance expected to be improved by chromized coating layer. In addition, the hardness of chromized coating layer and matrix were also analyzed and discussed.
3:50 PM		A1-2-8 Steam Resistant Coatings for New Generation High Temperature Steam Turbines A. Agüero, R. Muelas (Instituto Nacional de Técnica Aeroespacial, Spain) The operating temperature of european steam turbines of the XXI century is expected to rise from 550 to 650°C. New ferritic steels with higher creep strength are presently being developed, but their resistance to steam oxidation may not be acceptable at this higher temperature. Therefore, the use of protective coatings is being explored as an alternative within the frame of the european COST action 522. Preliminary studies carried out in our group indicate that a family of Cr and Al containing materials applied by thermal spray or by slurry coating are promising, and protection is achieved for at least 5000 h of pure steam exposure1; however, signs of failure become evident for the slurry aluminide coatings at longer exposures. In order to further increase the life these slurry coatings, Si, Ce and/or Cr have been added and characterization and preliminary testing has been carried out. Longer exposure results of FeCrAl, NiCr and AlCoFeCr cuasicrystalline coatings deposited by HVOF will also be presented, as well as the effects of coating sealants on both HVOF and slurry coated samples. Finally, preliminary results of an investigation aimed at determining the protection mechanism provided by the coatings, will be shown. 1 A. Agüero, R. Muelas y A. Román, Proc. of the 14th Surface Modification Technologies Conference, Paris, Sept 2000, in press
4:10 PM		A1-2-9 PVD Al-Ti and Al-Mn Coatings for High Temperature Corrosion Protection of Sheet Steel W. Lauwerens (WTCM Surface Treatment, Belgium); A. De Boeck (OCAS NV (Research Centre of the Sidmar Group), Belgium); M. Thijs (BOSAL Research, Belgium); S. Claessens (OCAS NV (Research Centre of the Sidmar Group), Belgium); M Van Stappen (WTCM - Surface Treatment, Belgium); P. Steenackers (BOSAL Research, Belgium) Al-Ti and Al-Mn coatings with varying Ti and Mn content (between 0 and 15 at%) were deposited on low carbon steel substrates by ion bombardment assisted magnetron sputtering using substrate bias voltages of 50, 100 and 150 V. The structure, chemical composition and morphology of the coatings have been analyzed and related to functional properties such as adhesion, friction and most important the oxidation and corrosion resistance, especially at high temperatures. For pure Al coatings the oxidation resistance, as measured by the mass increase during isothermal annealing at 600 °C, showed a clear dependence on substrate bias voltage, with a better resistance at high bias. Al-Ti coatings showed the highest oxidation resistance at low substrate bias with mainly a parabolic time dependence of the mass increase. Al-Mn coatings showed no clear dependence on bias voltage and their oxidation behavior showed a linear time dependence. During annealing inter-metallic AlMn phases were formed and the oxidation of the steel substrate under the layer was noticed. Coatings consisting of an Al/Ti bi-layer or multi-layer gave the best results, showing an excellent preservation of their appearance after annealing. For all coatings the corrosion tests, consisting of the exposure to a combination of humidity and salt or to a synthetic concentrated exhaust condensate during several weeks, gave similar results as the oxidation tests. The promising properties of these coating systems are especially illustrated by the fact that many of the PVD coatings with a thickness of 3 to 4 µm showed a higher oxidation and corrosion resistance than a classical 20 µm thick Al-Si coating applied by the hot dipping process.