ICMCTF2002 Session AP-1: Symposium A Poster Session
Time Period MoP Sessions | Topic A Sessions | Time Periods | Topics | ICMCTF2002 Schedule
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AP-1-1 Residual Stress Relaxation Process in Thermal Barrier Coating under Tension at High Temperature
M. Lugovy (Universitat Catholique de Louvain, Belgium); V. Slyunyaev (Institute for Problems of Materials Science, Ukraine); V. Teixeira (Universidade do Minho, Portugal) The residual macrostress in thermal barrier coatings plays key role in TBC failure mechanism. Besides the life time of coating depends on these features. The investigation of residual stress relaxation process in thermal barrier coating is important to control the residual stress level. As a rule such coating is a ceramic material. Therefore the main stress relaxation mechanism in TBC is microcracking. A model of microcracking process is presented which takes into account the statistical distribution of grain size in coating. The authors try to solve a physical problem of the description of failure of a microinhomogeneous solid as stochastic process of the cracking of separate structural elements. The single-phase ceramic-based coating is considered under tension at high temperature. The proposed model allows the relaxation process of residual tensile stresses in thermal barrier coatings at high temperature to be described. |
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AP-1-2 Mullite Coatings on Metal Substrates Produced by Sol-gel Technology
H. Balayan, V. Baghdasaryan, A. Torosyan, S. Hayrapetyan, H. Navasardyan (Yerevan, Institute of General & Inorg. Chemistry, Armenia) Increased interest to mullite-based coatings is stipulated mainly by their high hardness and thermal stability. The obvious advantages of mullite synthesis methods based on sol-gel technology principles are high degree of homogeneity and purity as well as relatively low formation temperature. The work presented discusses the synthesis process and some properties of mullite coatings obtained from water silica sols and alumina sols. The use of water sols obviates difficulties related to both residual carbon influencing the mullite yield and harmful substances that form when Si and Al organic derivatives are used. The kinetics of mullite formation at 800-1000°C is studied by X-ray diffraction using DRON-2 analyzer. The reaction activation energy calculated from the kinetic curves was about 300 KJ mol that is too much lower than when using highly dispersed SiO2 and Al2O3 as initial products. The coatings were deposited on steel substrates by spraying. Electron microscopy of the obtained coatings revealed their homogeneous and uniform structure. The formed coatings have low thermal conductivity, thermal stability up to 1000°C, and high mechanical hardness. |
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AP-1-3 Effects of Post heat Treatment on the Microstructure and Sliding Wear Resistance of HVOF-prayed Nickel Based Coating
M.A. Rodriguez, M.H. Staia (Central University of Venezuela) The effect of the heat treatment carried out using oxyacetylene flame, argon atmosphere and vacuum on the microstructure, sliding wear and hardness of a thermal sprayed coating using HVOF (high velocity oxy-fuel) process has been studied. The powder used, as feed material was a NiCrSiBW alloy commercially named Colmonoy 88. The coating was deposited on AISI 1020 steel using a JP5000 gun using the parameters recommended by powders manufacturer. The samples obtained were heat treated by three different methods. The first one was carried out using an oxyacetylene flame until a "glaze shine" surface was obtained. The other two post heat treatments were carried out in argon atmosphere and vacuum furnaces, respectively and the heating was carried out in two steps. The results have indicated that the post heat treatments performed into the furnaces produced significant changes in the morphology of the hard phases and a better distribution of them. Nevertheless, the microstructure of the coating post heat- treated with oxyacetylene showed no noticeable changes in the morphology of the hard phases. The difference in micro-hardness and sliding wear resistance of the coatings were related to the changes of the hard phases and their better cohesion with the matrix as a consequence of the post-heat treatment. |
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AP-1-4 Failure Analysis of CrN Coated Heat-resistant Steels in Molten Aluminum Alloy
C.-S. Lin, C.-S. Ke, M. Chiang (Da-Yeh University, Taiwan, ROC); H. Peng (Industrial Technology Research Institute, Taiwan, ROC) The corrosion mechanisms of CrN coated heat-resistant steel in molten A356 aluminum alloys were investigated with the emphasis of the effects of the coating microstructure and the surface finish of the steel substrate. A 6-mm CrN coating was deposited by cathodic arc evaporation onto 10-mm-thick heat-resistant steel plates. Dipping tests were conducted in a 700℃ A356 melt for 1 to 21 h at intervals of 3 h. The damage of the coated steel was evaluated by plane-view and cross-sectional metallography. Experimental results indicate that after a certain incubation period, the coated steel was locally attacked; forming hemispherical pits on the coated steel. Once the obvious pits were formed, the pitted areas and average pit size increased with dipping time, of which the CrN coated machined steel sample increased at a larger rate than the CrN coated polished steel sample. In contrast to the increasing pitted area and pit size with dipping time, the immersion time hardly affected the number of the pit. Cross-sectional transmission electron microscopy revealed that the CrN coating retained its columnar grain structure and hardly reacted with molten Al after 21 h of dipping. Meanwhile macroparticles were observed on the surface and interior of the CrN coating. The cracks were observed beneath the macroparticles residing near the CrN/steel interface. Since the surface of CrN coating after dipping is free of cracks and the dipping time has little effect on the number of pits, the macroparticles residing near the CrN/steel interface and those forming during the beginning of deposition and exposing on the coating surface are considered to be the potential sites for the formation of corrosion pits. |
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AP-1-5 Oxidation Performance of CrN Mutilayer Coatings for Aluminum Die Casting Applications
M.-C. Chiu (National Chung Hsing University, Taiwan, ROC); D.-Y. Wang (Mingdao University, Taiwan (ROC)) Mold materials for aluminum die casting experience serious cyclic thermal stress, high temperature oxidation, liquid metal attack, and erosion wear during their service life. For extended mold life, a durable surface layer capable of withstanding high temperature oxidation is needed. In this study, compound CrN coatings consisting of a multilayered structure was deposited by a cathodic arc evaporation process. Intermediate layers between CrN and its substrate were designed to ensure film adhesion under thermal cycling and liquid metal attack. The as-deposited CrN film was post-treated with metal-plasma ion implantation at 45 KeV to incorporate rare earth elements such as Nb and V into the surface layer. The rare earth elements tend to improve the mechanical integrity of the oxide scale. The influence of the implantation dosage on oxidation resistance of CrN films and the mechanism governing the interaction among rare earth elements, CrN films, and oxide films will be investigated. X-ray diffractometry (XRD), electron microscopy, and x-ray photoelectron spectroscopy (XPS) were used for microstructure and phase analyses. |
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AP-1-6 Study on Creamic Coatings for High Temperature Oxidation Behavior
W.-Y. Ho (Ming-Dao University, Taiwan, ROC); M.-C. Chiu (National Chung Hsing University, Taiwan, ROC); D.Y. Wang (National Chung Hsing University, Taiwan, R.O.C.) Applications of thermal barrier coatings have focused on improving the efficiency of the combustion process or potential electrode applications of high temperature operated solid oxide fuel cells. Ceramic coatings suitable for this application must exhibit thermal expansion coefficient comparable to the substrate materials and corrosion resistance. The possible applications processes for thermal barrier coatings are arc-plasma thermal spraying and physical vapor depositions techniques. In this study, we deposited Cr2O3 and CrN duplex coatings, consisting of a thin Cr2O3 oxide film on top of a CrN layer, with PVD cathodic arc deposition technique. Microstructure characterization was conducted with X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). Tribological evaluations were also performed with a scratch tester and ball-on-disc tribometer to demonstrate well-adherent Cr2O3 and CrN duplex coating morphology. Experimental results will also show an oxidation behavior of duplex layer on exposure to high temperature test. |
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AP-1-7 Oxidation Resistance of CrAlN Films
M.K. Kawate, A. Kimura, T. Suzuki (Keio University, Japan) Ti1-XAlXN films have been used for dry machining and high speed cutting operations. However, Ti1-XAlXN films are oxidized over 8000. AlN and CrN films also show good thermal stability and oxidation resistance, but have not been studied very well on ternary nitride of Cr1-XAlXN with changing X. It has been reported that the crystal structure of CrAlN films also changes from the NaCl into wurtzite type between 57 mol% AlN and 75 mol% AlN, but there has been few reports on CrAlN　films comparing with those for TiAlN films. In particular, CrAlN films have not been fully analyzed on the effect of Al content on hardness, lattice parameter, microstructure and oxidation resistance yet. In this paper, Cr1-XAlXN films were synthesized by the arc ion plating method using Cr1-XAlX alloy targets with differing Al contents and investigated on micro-Vickers hardness, lattice parameter and oxidation resistance. X-ray diffraction patterns from films indicated that the NaCl structure for X ≤ 0.6 changed into wurtzite structure for X ≥ 0.7. For films with X ≤ 0.6, the lattice parameter monotonously decreased from 0.416 nm for X = 0 to 0.413 nm for X = 0.6 and correspondingly, the hardness also gradually increased from ~ 1500 HV for X = 0 up to 2700 HV for X = 0.6. On the other hand, the hardness of films with X ≥0.7 abruptly decreased from ~ 2200 HV for X = 0.7 to 1700 HV for X = 1. Oxidation resistance of Cr1-XAlXN and Ti1-XAlXN films was estimated by heating substrates in air at 800 ~ 10000. They were analyzed by the X-ray diffraction method, the X-ray photoelectron spectroscopy and the atomic force microscope. For Ti1-XAlXN films, TiO2 were formed after annealing at 8000 and the surface became quite rough. On the other hand, Cr1-XAlXN films were not oxidized at all, even after annealing 9500. |
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AP-1-8 Effect of the Magnetic Field on the Plasmas Used to Deposited TiN/Ti Multilayers.
M. Flores, S. Muhl (IIM-UNAM, Mexico); E. Andrade (IF-UNAM, Mexico); M. Vite (Sepi, Esime-ipn, Mexico) TiN/Ti multilayers have been deposited on steel substrates by reactive magnetron sputtering with the aim of improving the corrosion and wear resistance. The multilayers were prepared by balanced magnetron sputtering with a coil concentric to the magnetron, in order to produce a variable disbalance of the magnetron and in this way allow us to provide additional plasma bombardment of the substrate. The multilayer period and the use of magnetic fields are powerful tools to control the microstructure and properties of the multiplayer films. Langmuir probe measurements have been used to determined the electron density, electron temperature, plasma and floating potential as a function of the plasma power and coil current with the radial distance. It was observed that the additional magnetic field increased the plasma density as well as altering the other plasma characteristics. The additional field also produced changes in the ion bombardment of the substrate and, this in turn, modified the film microstructure. The corrosion and wear of TiN/Ti multiyers were studied by means of potentiodynamic polarization in 0.5 M NaCl solution and by a ball cratering system respectively. The composition and thickness of the TiNx/Ti layers were determined using ion beam analysis. |
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AP-1-9 Structure and Resistance to Cyclic Oxidation of Al-Si Diffusion Coatings Deposited by Arc-PVD on TiAlCrNb Alloy
L. Swadzba, A. Maciejny, B. Mendala, G. Moskal (Silesian University of Technology, Poland) One of the candidates to replace superalloys in some engine applications isγ-TiAl which is characterized by a density almost half of that for superalloys. Titanium aluminides exhibit a strong TiO2forming tendency rather than formation of the protective Al2O3at high temperatures. The oxidation resistances is further reduced with decreasing Al content. Binary alloys such Ti-48Al at% and Ti-52Al% formed flaking scales with subscale embrittlement and the oxidation rate increased with increasing Al content. The article presents research results of static and cyclic oxidation of TiAlNbCr intermetallic with Al-Si and without coatings. Protective coatings were deposited by Arc-PVD method in two steps. In first the AlSi layer was deposited. In the second step the temperature of samples in vacuum chamber was increased and diffusion TiAlSi coating was formed. After coating deposition the heat treatment of samples in vacuum was made. The temperature of heat treatment was 950°C and the time 2 hours. The cyclic oxidation tests were conducted at temperatures 800°C, 900°C and 950°C. The cycle time was 23 hours (multiplied by). After each cycle mass changes was registered. The number of cycles was 50. At temperature of 950°C cyclic oxidation tests were carried out. The time of reaching the temperature and cooling was 5 minutes. Mass changes of the specimens were recorded every 100 cycles. The total number of cycles amounted to 2400. A critical analysis of the results obtained from two methods of cyclic oxidation was performed. The analysis of Si effect on the structure of the layers and on the oxidation mechanism of TiAlNbCr alloy was made. Phase composition, morphology and the distribution of elements were defined by EDX, XRD and SEM in AlSi layers as well as in the scale. |
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AP-1-10 Apparent Indentation Size Effect in a CVD Aluminide Coated Ni-Base Superalloy
B. Ning, M.L. Weaver (The University of Alabama) Indentation hardness testing has been used to assess the pre-annealing and post-annealing mechanical properties of a low activity CVD NiAl bond coat applied to a single crystal Ni-base superalloy substrate. Tests performed in the nano- and micro-hardness regimes using Berkovich and Knoop indenter geometries revealed the presence of an apparent indentation size effect (ISE) wherein hardness was observed to increase dramatically with decreasing indentation test load. An energy balance analysis was applied to analyze the apparent ISE and to extract the load independent hardness from the experimental data. |
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AP-1-11 Chromising of AISI 304 Steel Steels by the Use of the CVD-FBR Technology
F.J. Pérez, F. Pedraza, M.P. Hierro, M.C. Carpintero, C. Gómez, J.A. Fuente (Universidad Complutense de Madrid, Spain) Austenitic AISI 304 stainless steel may be the material of choice under certain conditions due to its ratio properties/cost. However, the amount of Cr of about 18 wt% may be insufficient to form, grow and regenerate the protective scales formed upon exposure to the aggressive environment because of the presence of a relative high Ni amount. The use of higher alloyed steels on the contrary makes the prices rise due to the need of a higher Ni content so as to maintain the austenitic structure. Therefore, it could be of interest to increase the amount of Cr on the surface of relatively cheap materials by Surface Engineering methods. In this work, we will be presenting the results obtained by applying this technique to a commercial austenitic 18Cr-8Ni stainless steel at temperatures ranging from 825 to 900°C using different H2/HCl ratios so as to obtain the best coating quality (i.e. composition and morphology). Attack from HCl to the alloy will be hindered to some extent by increasing the H2 amount in the coating medium. The deposition rate will be shown to increase with temperature but the quality of the coatings will be somewhat inferior. A solution of compromise is therefore found to fulfil all the requirements. |