Papers by Keyword: Slurry Coating

Abstract: Metal foams have a cellular structure consisting of a solid metal containing a large volume fraction of pores. In particular, open, penetrable pores are necessary for industrial applications such as in high temperature filters and as support for catalysts. In this study, Fe foam with greater than 90% porosity, 2-mm pore size was successfully fabricated using a slurry coating process and the pore properties were characterized. The Fe and Fe₂O₃ powder mixing ratios were controlled to produce Fe foam samples with different pore sizes and porosity. First, the slurry was prepared through the uniform mixing of powders, distilled water, and polyvinyl alcohol (PVA). The amount of slurry coated on the PU foam increased with theFe₂O₃ mixing powder ratio, but the shrinkage and porosity of the Fe foams decreased, respectively, with increasing Fe₂O₃ mixing powder ratio.

Abstract: Fe foam with above 90% porosity and 2 millimeter pore size was successfully fabricated by a slurry coating process. In this study, the binder contents were controlled to produce the Fe foam with different pore size, strut thickness and porosity. Firstly, the slurry was prepared by uniform mixing with Fe powders, distilled water and polyvinyl alcohol (PVA) as initial materials. After slurry coating on the polyurethane (PU) foam the sample was dried at 80°C. The PVA and PU foams were then removed by heating at 700°C for 3 hours. The debinded samples were subsequently sintered at 1250°C with holding time of 3 hours under hydrogen atmosphere. The three dimensional geometries of the obtained Fe foams with open cell structure were investigated using X-ray micro CT(computed tomography) as well as the pore morphology, size and phase.

Abstract: One of the surface modification processes for high-temperature oxidation resistance is slurry aluminizing process, forming protective layer of alumina (Al₂O₃). However, several important parameters such as annealing times and temperatures should be intensively considered. The objective of this study is to improve the process of slurry aluminide coating of ferritic stainless steels type AISI430 (16%Cr) combat to high-temperature oxidation. The specimens were cut, then ground, and finally sprayed with slurry mixture (Al powder + polyvinyl alcohol (PVA)). They were annealed in Ar at 1100°C for 15 minutes in order to eliminate PVA and form aluminide on their suface. The protective layer Al₂O₃ was finally formed in the temperature range of 900-1100 °C for 15-60 minutes. The cyclic oxidation tests were performed at 1000 °C for 24 hours. The surface morphology were then examined by XRD, SEM equipped EDS. The results showed that all oxidation kinetics of coated specimens were parabolic. The oxidation rate of uncoated specimens was apparently higher than that of coated specimens. Comparing with all coated specimens, the oxidation rate decreased with the increasing temperature and annealing time. In this study, the coating process at 1100°C for 60 minutes exhibited the lowest oxidation rate due to the most complete layer of Al₂O₃. The surface morphology showed the formation of continuous layer of Fe₂Al₅ and Al₂O₃, acting as barrier layer to oxide growth. Effect of temperature and time on oxidation resistance were discussed in this study.

Abstract: Carbon-Carbon composite (C/C) substrate materials are prone to severe oxidation and volatilization problems. To address these issues mullite/SiC coatings were deposited on C/C composite. The coatings were applied by a two-step approach; pack cementation and silica sol based slurry-coating processes. The pack mixture composition for SiC coating comprised; 15-50 wt. % Si (75 μm), 0-25 wt. % B (38.2 μm), 0.01-3 wt. % SiO₂ (38 μm) and 40-85 wt. % SiC green (20 μm). Laboratory synthesized mullite via sol-gel (SG) process of different Al₂O₃/SiO₂ ratios (containing approx. 84-72wt. % Al₂O₃) based on stoichiometric calculation were used; these were admixed with silica sol to form a slurry. The pack mixture and the silica sol based slurry coatings were sintered at temperatures of 1800oC and 550°C respectively. The microstructure and isothermal oxidation behavior of the mullite/SiC coatings and C/C substrates during isothermal oxidation cycle at 1500oC were investigated using XRD and SEM mounted with EDS. Results indicate that optimized oxidation protection for mullite-SiC coating was achieved with higher Al₂O₃ content. The weight loss rate of sol-gel mullite with bulk composition 84 wt. % Al₂O₃ (SG84) was 0.09 gcm-²h^-1 as compared to bulk composition 72 wt. % Al₂O₃ (SG72) which had a weight loss rate of 0.20gcm-²h^-1. The oxidation protection for the synthesized sol-gel mullite/SiC coatings offered effective protection as evidenced in the cycle hour exposure time with limited degradation, chemical compatibility between the coating/substrate interface was good and the phases were quite stable after isothermal oxidation cycle.

Abstract: Chromium coatings have been used for the protection of gas turbine blades in power plants during the last years due to their very good resistance against steam corrosion. Microhardness, scratch, adhesion and pin-on-disk sliding tests are commonly used for rapid evaluation of the mechanical properties of these coatings [1,2]. However, very limited information exists on their fatigue resistance. In this paper we present the experimental results of the impact testing examination of the above coatings. This experimental method is capable to assess the fatigue and the impact wear resistance of coatings working under cyclic impact loading conditions. From the experimental results it was concluded that the slurry Cr, Zr coatings deposited on P91 steel showed adequate fatigue strength for the above-mentioned kind of use.

Abstract: Slurry coatings have been used for the protection of gas turbine materials in power plants during the last years. These coatings can be applied by spaying, brushing or dipping. The main constituent elements are silicon, chromium, potassium, borium, carbon and zirconium. They are characterized by high hardness and very good resistance against corrosion, erosion, abrasive and adhesive wear. To guarantee the reliability of coated steam turbines components used in power plants, the lifetime assessment of the coatings and their failure prediction become very important. Microhardness, scratch, adhesion and pin-on-disk sliding tests are commonly used for rapid evaluation of the mechanical properties of these coatings [F. Loeffler: Thin Solid Films, Vol. 339 (1999), p. 181]. However, the above testing methods do not model the dynamic cyclic fatigue. In this paper we evaluate the fatigue resistance of slurry coatings working under cyclic loading conditions by the impact testing method. The coating failure mode and its extent were assessed by SEM observations and EDX analysis. From the experimental results it was concluded that the Si,Cr,O,B,C coatings deposited on P91 steel substrate showed an improved fatigue strength compared to that of Si,Cr,O deposited on the same substrate.