[4]
10 (82. 8%).
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15 (82. 9%).
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5 1600 Graphite-Ar.
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01 (60. 1%).
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46 (66. 4%).
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2 1700 Graphite-Ar.
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07 (80. 1%).
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82 (92. 9%).
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0 Table 1. Physical parameters obtained on Y2O3 and Y2O3 + SiC , at various sintering conditions. Composition (%w/w) 20% ZrSiO4; 80% SiO2 Porosity - Water absorption – Density 29 % - 17% - 1, 71 g/cm3 MOR (3 points)@ RT: 12 MPa (1100 psi) MOR (3 points)@ 1200 °C: 19 MPa (2800 psi) MOR (3 points)@ 1500 °C: 12 MPa (1800 psi) Leacheability Easy in hot caustic solution Table 2. Physical and thermomechanical properties of S820 produced by Avignon Ceramic.
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2 Thermogravimetric measurements. TG-DTA measurements were performed using a Netzsch STA 409A apparatus. These measurements were used in order to assess dewaxing procedures of the unfired bodies (necessary to remove organic additives) and to quantify residual SiC before and after creep measurements. The amount of residual reinforce was studied by weight increase that occurred during oxidation of SiC to SiO2. The quantitative evaluations were complicated by the tendency of yttria to easily form rather stable hydroxides and carbonates: measures on yttria samples were used as blanks, in order to subtract these contributions, but the significant difference on open porosity of reinforced and non-reinforced yttria made these corrections not completely effective.
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3 Compression creep test: equipment and procedure. The compressive tests were performed on a MTS electro-hydraulic testing machine (Fig. 1) equipped with a 5 kN load cell. An Instron furnace made possible to perform the compression testing for 3 hours at 1475°C, under argon flow. The tests were performed using TESTSTAR II basic MTS software, managing the hydraulic piston movement and performing data acquisition, with a sampling rate of 5 samples/second. The load was applied only after reaching the test temperature. Water-cooled metallic parts Furnace Sample Alumina Figure 1. Compressive test assembly: MTS testing machine equipped with an Instron furnace.
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4 Other simpler creep evaluations. Because of the temporary unavailability of MTS apparatus, other creep evaluations were obtained by applying a weight (obtained with W and Mo cylinders) onto the samples, and keeping the assembled apparatus for 3 hours at 1475°C, in argon. The weight is obviously applied also during the warming up (350°C/h) and the cooling down (350°/h). In order to apply the pressure in a reproducible way, a graphite liner was used (Fig. 2). Obviously, with this methodology only the final deformation can be measured. Figure 2. Tungsten and molybdenum weights and graphite liner used to evaluate creep resistance.
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Results and discussion.
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1 Thermogravimetric assessment of the dewaxing step. Since the reinforcing effect would be lost on SiC oxidation, it is extremely important to assess SiC retention during the various steps of the process. The measurement were initially performed on unfired bodies, containing nano and submicronic SiC. Oxidation resistance of not-optimized nano Si/C/N reinforcement appears too low to keep it during the processing (upper curve, in Fig. 3a), while most submicron SiC UF15 gave a promising result (lower curve in fig. 3a). Other measurements were performed after 2 hours dewaxing at 350°C (under flowing air). It was possible to verify that all the submicron-SiC added (corresponding to the measured 1. 6%w/w weight change due to the oxidation of SiC to SiO2) is retained (Fig. 3b). Therefore, only SiC submiconic reinforce was further studied. As stated before, because of the presence of carbonates and hydroxides, curves for SiC reinforced yttria were registered using yttria as a blank. DTA are not reported because of poor sensitivity. SiC-UF15 nano-SiC SiC-UF15 Figure 3a. Evaluation of nano and submicronic SiC oxidation during green body dewaxing. Figure 3b. Evaluation of SiC oxidation on dewaxed SiC reinforced Y2O3.
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2 Thermogravimetric evaluations on sintered cylinders. TG-DTA curves were also registered on sintered yttria and SiC-reinforced yttria samples. Most SiC enclosed in the sintered yttria matrix is protected from oxidation up to 1000°C. This is extremely important for a hypothetical use of reinforced yttria ceramic cores for an industrial production, because of the necessity to burn residual wax from the assembled moulds, before casting. TG curves (Fig. 4) also show that during sintering at 1700°C, SiC-reinforced yttria is partially reduced to form carbides. Consequently oxidation of the samples goes over the expected value (total weight increase: 2. 7% instead of 1. 6%). Unexpectedly, this phenomenon was not observed on non-reinforced yttria, however porosity is completely different in that case. Figure 4. TG on SiC reinforced yttria cylinders sintered in a graphite furnace, at 1600 and 1700°C.
DOI: 10.1016/j.jeurceramsoc.2016.06.025
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3 Creep strain measurements on silica and yttria ceramic cores. The apparent creep strain curves registered with the MTS apparatus on yttria and reinforced yttria are compared to a sample obtained from a commercial core, to demonstrate the better creep resistance of yttria (figure 5). The addition of SiC further improve creep resistance. The data obtained were also used to validate the creep evaluations, obtained with the method described in the paragraph 3. 4, by applying a weight of W and Mo onto the samples (corresponding to a load of 3. 5MPa). After 3h hours at 1475°C, the apparent creep and the total deformation (table 3) were comparable. SiO2 Y2O3 Y2O3+SiC Figure 5. Compressive creep curves (under a stress of 4MPa) for a sample from a commercial silica core, compared to reinforced and non-reinforced yttria samples (both sintered under high vacuum at 1700°C). Sintering temperature 1700°C (W-vacuum) Sintering temperature 1600°C (graphite-Ar) Sintering temperature 1700°C (graphite-Ar) Deformation Open porosity Deformation Open porosity Deformation Open porosity Y2O3.
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0 % Y2O3+5%vol SiC.
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9 % Table 3. Deformation of yttria cylinders, kept for 3 hours at 1475°C, under 3. 5MPa loading. Table 3 shows that submicronic SiC addition induce an improvement of yttria creep resistance of about an order of magnitude, at any sintering temperature or procedure applied. The results obtained on reinforced yttria sintered at 1700°C in a graphite furnace are particularly interesting because high creep resistance (much better than that of the commercial silica core considered) is associated with high open porosity, and hence high leachability. This is extremely important since a complete removal of the ceramic core from turbine blades is compulsory. The same phenomenon was not observed when sintering under vacuum, probably because of earlier decomposition of carbonates.
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Conclusions The present data demonstrate the possibility of obtaining yttria ceramic cores with suitable creep resistance for DS investment casting. The samples were obtained by sintering in a graphite furnace at 1600-1700°C, and this procedure could also be applied for an industrial production. The possibility of reinforcing yttria with submicronic SiC was also demonstrated. The addition of SiC increases both open porosity (and hence leachability) and creep resistance, so it seems an optimal solution for ceramic cores. This type of reinforcement could also be used without significant modifications to the current casting procedures, because of a certain resistance to oxidation in air.
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Acknowledgements The authors wish to thank Dr. Michele Di Foggia (Europea Microfusioni Aerospaziali, Italy) for supplying the commercial silica ceramic cores. This work has been done as a part of the Italian FIRB-MITGEA Project, Project No. RBIP064N2X of the Italian University and Research Ministry).
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References.
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