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SiC composites
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of additives on slip casting rheology, microstructure and mechanical properties of Si3N4/SiC composites Shohreh Shahrestani, Mokhtar Che Ismail, Saeid Kakooei∗, Mohammadali Beheshti Centre for Corrosion Research, Mechanical Engineering Department, Universiti Teknologi Petronas, Seri Iskandar, Malaysia
A R T I C LE I N FO
A B S T R A C T
Keywords: Slurry rheology Additives Slip casting Nitridation Silicon carbide/silicon nitride composite Silicon/silicon carbide powder
The SiC/Si3N4 composites were fabricated with sintering process. To produce SiC/Si3N4 composite components, slurry mixtures containing Si/SiC powders were used by the slip casting method. In order to investigate the effect of dispersants and additives on the rheological properties and the body casted, slurries with concentration of 70% solid weight were prepared. It included a mixture of silicon and silicon carbide with weight ratios of 30 wt% and 70 wt%, respectively, and various weight percentages of Ball clay as lubricant and Tiron (sodium salt of benzene disulfonic acid) as dispersant at pH value of 7. After preparing the green bodies by slip casting method by using plaster mold, the samples were sintered at 1450 °C inside an atmospheric-controlled furnace under a pressure of 0.12 MPa of nitrogen gas for 2 h. By examining the rheological properties of the slurry and the sintering properties, it was concluded that the best slurry was obtained in terms of viscosity, density, porosity and strength using 5 wt% Ball clay and 0.5 wt% Tiron. Phase transformations, microstructure and morphology of the sintered specimens were accomplished by Field Emission Scanning Electron Microscopy (FESEM) examination and X-ray diffraction experimental analysis. XRD and FESEM results demonstrated that the composite fabricated by slurry containing 5 wt% Ball clay and 0.5 wt% Tiron had the least porosity without SiO2 phase.
1. Introduction SiC/Si3N4 composite is a new generation of ceramic materials that was developed a few years ago [1–4]. These ceramic composites are known for their properties such as corrosion resistance, abrasion, high thermal shock resistance and high strength properties. In general, composites can achieve better mechanical and thermal properties than their constituents [5,6]. The major ceramic fields composites include carbon/carbon composites, alumina/silicon carbide composites and composites with silicon nitride reinforced with continuous silicon carbide and carbon fiber [7–9]. An appropriate approach to produce SiC/ Si3N4 composite is the nitridation-sintering of Si/SiC in atmospheric controlled furnaces [8]. Several authors have used Si3N4 with SiC to make SiC–Si3N4 composites [10–12]. In these studies, the use of silicon powders instead of silicon nitride powders to produce SiC/Si3N4 composites could lead to a more economical process. The benefits of using silicon powder are firstly it is cheaper than silicon nitride powder and secondly it requires a low nitride reaction. Slip casting is colloidal and cost-effective method to fabricate the engineering ceramics and their composites in aqueous suspension by pouring in porous mold and draining water from the slurry of a green body ready for sintering process [13,14]. The composite properties ∗
made by slip casting are directly related to the rheological properties of the slurry prepared from them. By controlling the slurry rheology (solid particle behavior in liquid medium, particles stability, etc.), optimal conditions for slurry control and thus the final product are achieved [2,15,16]. Controlling and optimizing factors such as particle size, particle size distribution, particle shape and morphology, volumetric solids content, inter-particle force, pH values and slurry stabilization using desirable type and amount of dispersant are the most important concerns for achieving homogeneity and high density of fabricated green body [1,17]. Colloidal techniques by serving various dispersant can help control the forces between particles in the suspension, thereby reducing the creation and development of agglomerates, facilitating the slip casting process, consequently avoiding the coarsening of the sintered microstructure [18,19]. In the light of the above, various additives such as binders (Kaolin and ball clay) and dispersants (Tiron, Dolapix and TPP) are added to the suspension to provide less viscosity value and consequently higher density of the fabricated bodies. The dispersions improve the stability of the ceramic particles in the suspension electrostatically, sterically or electro-sterically [20,21]. Electrostatic stabilization is based on double layers. Newly absorbed ionic groups are added to the suspension and a charged layer is formed. These double layers are mutually repelled, which provide stability for
Corresponding author. E-mail address: [email protected] (S. Kakooei).
https://doi.org/10.1016/j.ceramint.2019.11.085 Received 23 August 2019; Received in revised form 7 November 2019; Accepted 11 November 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Shohreh Shahrestani, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.085
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various shapes were made. After forming the raw material, the samples were removed from the mold and dried at room temperature (as shown in Fig. 3).
suspension. Steric stabilization is achieved by the addition of macromolecules that produce particles at the surface of the particles and absorb a layer and causing the suspension to stabilize. However, this process can occasionally cause deformation in ceramic materials during sintering. The usual stabilization method is electrostatic stabilization, which combines the two previous stabilized methods [18]. Electrosteric dispersers are usually polyelectrolytes that bind themselves to the surface of the particles and cause enough potential difference, and this causes the particles to disperse. Polyelectrolytes have excellent flexibility for multi-stage processing and more stabilization for slip casting. This system also includes better control of the flocculation and thixotropy conditions and in comparison to inorganic dispersers, they are more stable [21]. Hence, this study focused on optimizing and investigating the effect of type and amount of dispersant on the properties of the slurry, the green body features prepared by slip casting method and consequently on the sintered body structures using an atmospheric-controlled furnace under a nitriding environment. Therefore, the target of this examination was to evaluate the influence of Tiron [22,23] and Ball clay as dispersant and lubricant on the rheometric results, densification, mechanical properties and microstructure analysis of SiC/Si3N4 composite.
2.4. Sedimentation studies
2. Experiments
In order to obtain suitable pH for slip casting, the results obtained from sedimentation studies can be very effective. Under well dispersion conditions there is a minimum amount at sedimentation height. It should be mentioned that the sedimentation height has opposite proportion to the amount of powder dispersion, which means that the greater dispersion of the powder will result in a lower sedimentation height. At first, silicon and silicon carbide powders were dispersed in distilled water with the magnetic stirrer. The pH value was then adjusted using ammonia (NH3) and Hydrochloric acid (HCl) and measured by Metrohm pH meter (827 model, Metrohm International Headquarters, Switzerland). The slurry contains 20 vol% of solids with a 1: 1 ratio of silicon and silicon carbide powders in aqueous solution. Five calibrated containers including slurries prepared with different pH values of 2, 4, 6, 8 and 10 were prepared to study sedimentation studies. After preparation of the slurry, the sedimentation height was measured after 2, 6 and 24 h.
2.1. Raw materials
2.5. Structural analysis
Russian commercial silicon powder and Japanese commercial silicon carbide powder with an average grain size distribution of 9.769 μm (D50 from Anyang Teifa Metallurgy Co. Ltd. China) were used as raw materials in this study as shown in Fig. 1. Tiron with a product code of 1000/00808/1 with a purity more than 99% was used as a dispersant for stabilization the slurry. Tiron was produced by BDH with the chemical formula (C6H4Na2O8S2) and industrial Ball clay (WBB) was used as a lubricant.
X-ray diffraction (XRD) methods were used to determine the phase composition of the primary powder and to discover the phases obtained after heating the samples and studying their structure. X-ray diffraction patterns were performed using X-ray diffraction devices Siemens, Germany, Model 500-D, under a voltage of 30 kV and 25 mA. In all experiments, X-ray CuKα beam with a wavelength of 1/5404 Å was used. The existing phases were identified by comparing the XRD peak diffraction angles and the intensity levels associated with the values presented in the ASTM cards.
2.2. Preparation of slurry 2.6. Morphological study of cross-sectional fracture surface In this study, silicon powder with a relatively fine grain size (micro diameter) and silicon carbide powder with an average grain size of 9.769 μm were used (Fig. 1). Also, silicon powders were used with an average grain size of 7.923 μm (Fig. 2). Powders were dispersed in distilled water for 15 min using a magnetic stirrer. Before mixing the mentioned powders, the different weight ratios of lubricant were dissolved in distilled water, and then different weight ratios of Ball clay and Tiron (a sodium salt of a benzene disulfonic acid) were added to the set with 10 wt% and 5 wt% for Ball clay and 0.5 wt%, 0.2 wt% and 0.1 wt% for Tiron. The slurry was then placed in an ertalonic cup and mixed for 2 h using a planetary mill and alumina balls. In addition, slurries containing 70 wt% solids and various amounts of Tiron and Ball clay were prepared in pH of 7, which solids content is containing 70 wt % of silicon carbide powder and 30 wt% of silicon powder.
To evaluate the cross-sectional fracture of the Field Emission Electron Microscope (FESEM) Mira3-XMU model (TESCAN-Czech Republic) was used including the property of a huge case and high vacuum velocity after the heat treatment and test strength. In order to investigate the cross-section fracture, a gold coating was applied on its surface. By using the images obtained, the changes in its cross-sectional fracture was investigated. 2.7. Mechanical strength measurement Instron 1196 device (Instron, 825 University Ave, Norwood, USA) was used to measure the flexural strength of the sample. The rectangular cube polished samples with dimensions of 38 × 8 × 6 mm were prepared using the plaster molds to measure the flexural strength. The test was carried out by the ASTM D790 standard using a triple-point procedure. The speed of movement of the jaws during the performance of the strength test was constant and equal to 0.5 mm/min. At least 4 samples were used to measure the fracture strength at every test.
2.3. Investigating the rheological behavior of slurries containing the mixture of silicon and silicon carbide powders 20 ml of the prepared slurry was poured into the chamber of the viscometer. In each case, the relation between shear speed and viscosity was plotted using a Brookfield-LV-DV Π Pro viscometer (John Morris group, Australia) and with the help of Rheocalc 32 software, and at cutting speeds from 0.01 s−1 to 56 s−1 and at the temperature of 25 ± 2 °C. The software was programmed in a way that the speed of RPM device was increased exponentially every 3 s, and the data related to the viscosity and cutting speed were recorded every 3 s after automatic fixing of this number, which in total, 60 points would be obtained. For slip casting containing a mixture of silicon and silicon carbide, first gypsum plaster molds with a ratio of plaster to water of 1:3 in
2.8. Density and apparent porosity percentage determination Standard samples were prepared to measure the density of the sintered specimens and their density were determined according to ASTM C373-88 standard. Finally, the density curves were plotted, and the pattern of the samples studied. To calculate the porosity percentage by means of the Archimedean method, dry weight, immersion weight and saturation weight were measured, and obtained by replacing in the formula related to density and porosity percentage. 2
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Fig. 1. a)2.50 kx and (b)10.0 kx of SEM images, (c) XRD and (d) distribution graphs silicon carbide powder with average particle size of d50 ≤ 9.769 μm as raw material for preparing slurries to make SiC/Si3N4 composites.
3. Results and discussion
30–70, and the weight ratio of Si to SiC powder of 30–70 with various percentages of Ball clay and Tiron are shown in Fig. 5. From the obtained curves, it is obvious that the amount of viscosity of all slurries reduced with the increase of shear speed first, and then continued with a less slope and finally reached to a constant amount. The reduction of viscosity by increasing the shear rate means the shear dilution behavior of slurries and keeping the viscosity constant with increasing the speed, or in other words, reaching the gradient of the graph to zero, reflects the Newtonian slurry behavior. By comparing the shear rate and viscosity of each of the four slurries, the slurry containing 5 wt% of Ball clay and 0.5 wt% of Tiron and slurry with 10 wt% Ball clay and 0.1 wt% of Tiron have the lowest and highest amount of viscosity, respectively. Regarding the constant amount of solid materials in the slurry, it will be more desirable for casting, regardless of whether the slurry has a lower viscosity. Considering this matter, from the viewpoint of the viscosity factor, slurry containing 5 wt% of Ball clay and 0.5 wt% of Tiron is a more desirable slurry than other slurries. Viscosity graphs in terms of shear rate with the constant ratios of liquid to solid and of Si to SiC powders for slurry with 10 wt% Ball clay and 0.2 wt% of Tiron and slurry with 10% Ball clay and 0.1 wt% of Tiron are plotted in Fig. 6. Comparing the two slurries, they show that the amount of viscosity decreases when the amount of lubricant (Tiron) is increased. Viscosity curves in terms of shear rate with constant ratios of liquid to solid and Si to SiC powders for slurry with 10 wt% Ball clay
3.1. Sedimentation studies Sedimentation studies can be used to obtain a suitable pH from the slurry. After 2, 6 and 24 h, the sedimentation height was measured, and the corresponding graphs were plotted. The lowest sedimentation heights show a good dispersion of particles, which leads to an ideal compression. With respect to Fig. 4, it is obvious that the lowest t sedimentation heights for slurry with a mixture of silicon and silicon carbide are related to pH greater than value of 5, and at pH 6, 8 and 10 there is no significant difference in sedimentation altitudes. So, by considering that the pH of water is 7 and working in acid pH area will have problems in the production process, slurries can be made in the neutral pH value. 3.2. Investigation the influence of Tiron and Ball clay amounts on the rheological behavior of slurries containing a mixture of Si and SiC powders In order to determine the optimal percentage of dispersants, the slurry rheology behavior was evaluated with different percentages of Tiron and Ball clay. All slurries were comprised of 70 wt% of solid materials containing a weight ratio of 30 and 70 of silicon and silicon carbide powders, respectively. The viscosity graph based on the shear rate for four different slurries with a weight ratio of liquid to solids of 3
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Fig. 2. (a)2.50 kx and (b)10.0 kx of SEM images, (c) XRD and (d) distribution graphs of silicon powder with average particle size of d50 ≤ 2.792 μm as raw material for preparing slurries to make SiC/Si3N4 composites.
Fig. 3. Si–SiC green body fabricated by slip casting.
Fig. 4. Height of the sedimentation vs pH for a mixture of Si + SiC with a weight ratio of 1:1 (including 20 wt% of solids) as a function of time.
and 0.2 wt% of Tiron and slurry containing 5 wt% Ball clay and 0.2 wt% of Tiron are drawn in Fig. 7. By comparing the two slurries, it should be noted that by increasing the percentage amount of Ball clay, the values of viscosity have decreased. With reference to both plotted figures, it is completely deduced that using lower or more amount of additives than
optimum levels will lead to disturbing the double layer, therefore, there is inadequate fluid flow which slows the movement of ions within the electric double layer and suppresses the attraction forces, thereby reducing the fluid viscosity. 4
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Fig. 9. Comparison of the effects of using different percentages of Tiron on rheological behavior including 70 wt% solid and weight ratio of Si/SiC: 30/70 and 10 wt% Ball clay.
Fig. 5. Effect of dispersant materials on slurry rheological behavior including 70 wt% solids and weight ratio of Si/SiC: 30/70.
Fig. 6. Comparison of the effects of using different percentages of Tiron on rheological behavior including weight ratio of Si/SiC: 30/70 and 10 wt% Ball clay.
Fig. 10. Comparison of the effects of using different percentages of Ball clay on rheological behavior including 70 wt% solid weight and weight ratio of Si/SiC: 30/70 and 0.2 wt% Tiron.
The shear stress diagram in terms of the shear rate for four different slurries with various percentages of Ball clay and Tiron were investigated as seen Fig. 8. According to the diagram, the shear stress for all slurries with increasing shear velocity initially increase with a large slope and then decrease in a short period of time and eventually increase again. Slurries with a lower percentage of Ball clay (5% by weight) show Newtonian behavior for shear rates more than 30 s−1, while slurries with a higher percentage of Ball clay (10% by weight) show an approximate behavior of shear dilution. It should also be noted that all slurries at low shear rate reveal the shear dilatation behavior because the amount of shear stress is increasing with a sharp slope. Furthermore, slurry containing 5 wt% Ball clay and 0.5 wt% Tiron nearly discloses a shear-thinning behavior. By comparing all four slurries, it is completely obvious that the amounts of shear stresses for slurry with 10 wt% Ball clay and 0.1 wt% Tiron is higher than other slurries which indicates less fluidity of the slurry compared to other slurries, but this issue is the opposite for slurry containing 5 wt% Ball clay and 0.5 wt% Tiron which means that the amount of shear stresses of this slurry is less than other slurries indicating more fluidity of the slurry compared to the other slurries. The shear stress diagram versus shear rate with constant ratios of liquid to solid and Si to SiC powders for slurry with 10 wt% Ball Clay and 0.2 wt% Tiron and slurry with 10 wt% Ball clay and 0.1 wt% Tiron is plotted in Fig. 9. In this slurry, the amount of Ball clay is fixed, and the percentage of the Tiron has changed. Both slurries initially show shear dilatation behavior and then in speeds more than 30 (s−1) reveals the Newtonian behavior. It is also obvious that with the increase amount of the Tiron, the shear stress value decreases, which indicates the fluidity of these slurries. The shear stress according to the shear rate with fixed liquid to solid ratios and the fixed ratios of Si to SiC for slurry having 5 wt% Ball clay and 0.2 wt% of Tiron and slurry with 10 wt% Ball clay and 0.2 wt% Tiron is plotted in Fig. 10. In these slurries, the amount of Tiron is fixed and the
Fig. 7. Comparison of the effects of using different percentages of Ball clay on rheological behavior including weight ratio of Si/SiC: 30/70 and 0.2 wt% Tiron.
Fig. 8. Influence of dispersant materials on slurry rheological behavior including 70 wt% solids and weight ratio of Si/SiC: 30/70 and different percentage of Tiron and Ball clay.
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Fig. 11. X-ray diffraction pattern of sintered samples including 70 wt% solid-30 wt% liquid, 30 wt% Si-70 wt% SiC; (a) 5 wt% Ball clay-0.5 wt% Tiron; (b) 10 wt% Ball clay-0.2 wt% Tiron, as additives in 1450 °C. Table 1 The data obtained from X-ray diffraction patterns for the sintered sample with 70 wt% of solid materials including 30 wt% Si-70 wt% SiC with different percentage of Ball clay and Tiron as additives. Weight percent of Ball clay and Tiron
I(Si3N4)/I(SiC) (Peak 100)
I(SiO2)/I SiC) (Peak 100)
Temperature (°C)
5 wt% Ball clay-0.5 wt% Tiron 10 wt% Ball clay-0.2 wt% Tiron 10 wt% Ball clay-0.1 wt% Tiron 5 wt% Ball clay-0.2 wt% Tiron
0.06 0.2049 0.155 0.096
~0.00 0.1756 0.1464 0.0875
1450 1450 1450 1450
percentage of the Ball clay has changed. Initially, the shear dilution behavior is more pronounced for slurry with 0.5% Ball clay density. But in the following, for both slurries the slope of curves are unchanged, which reflects the Newtonian behavior. For this slurry, like two slurries as shown in Fig. 10, by increasing the amount of Ball clay, the values of shear stresses in a constant interval of shear rate for each slurry have decreased. Generally, by decreasing the amount of Tiron in the optimal percentage, the viscosity of the slurry reduces and by increasing a greater value of Tiron, the viscosity gradually increases. When the percentage of the lubricant is low, in this case, the dispersing agent cannot show its effect and thus causing the agglomerate in the slurry. By increasing the amount of lubricant, the covering of the absorption surface is increased, and the repulsive forces overcome the van der Waals forces. By continuing to increase the amount of lubricant from the optimum value, surface absorption covering is decreased, and meanwhile, it causes more compression of the double layer, thus resulting in a decrease of the distance between the surfaces and thus reducing the repulsive forces.
Fig. 12. Peak 100 intensity graph of the percentage of silicon nitride to silicon carbide according to the percentage of Tiron at 5 wt% and 10 wt% of Ball clay for the sintered samples with 70 wt% of solid materials including 30 wt% Si −70 wt% SiC in 1450 °C.
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Fig. 13. Ratio intensity graph of ISi3N4/ISiC and ISiO2/ISiC according to the percentage of Ball clay powder for the sintered sample with 70 wt% of solid materials including 30 wt% Si −70 wt% SiC in 1450 °C. Table 2 Characteristics of density and apparent porosity for sintered bodies by 70 wt% of solid and Si/SiC ratio of 30/70 at 1450 °C. Ratio weight of liquid/solid
Ratio weight of Si/SiC
Tiron (wt%)
Ball clay (wt%)
Archimedes Density (g/cm3)
Apparent porosity (%)
Temperature (°C)
70/30 70/30 70/30 70/30
70/30 70/30 70/30 70/30
0.5 0.2 0.2 0.1
5 5 10 10
3.25 3.07 3.4 3.34
12.95 14.3 12.25 12.65
1450
Table 3 Tree-point flexural strength data of sintered bodies. Ratio weight of liquid/ solid
Ratio weight of Si/SiC
Tiron (wt%)
Ball clay (wt%)
Strength (MPa)
Temperature (°C)
70/30 70/30 70/30 70/30
70/30 70/30 70/30 70/30
0.2 0.5 0.2 0.1
5 5 10 10
64.01 66.37 64.76 64.12
1450
Fig. 14. Density according to the percentage of Tiron at 5 wt% and 10 wt% of Ball clay for the sintered samples by 70 wt% of solid and Si/SiC ratio of 30/ 70 at 1450 °C.
Fig. 16. Strength graph in terms of Tiron percentage by 5 wt% and 10 wt% of Ball clay as a function of the additives for slurry containing 70 wt% of solids.
Subsequently, the samples were sintered for 2 h in a nitride atmosphere controlled at1450 °C. The results obtained from XRD analysis and the phases formed in the casting bodies include 70 wt% solids and 30/70 ratios of Si/SiC powders with different ratios of Ball clay and Tiron at 1450 °C is represented in Fig. 11. According to the pattern of X-ray diffraction of the specimen sintered, it is obvious that along with the SiC phase, Si3N4, Si and SiO2 phases are also formed. The presence of the Si3N4 phase reveals the formation of Si3N4–SiC composites, but it is desirable that the identified phases to be only related to SiC and Si3N4, and the presence of Si and SiO2 phases formed along with them reveal
Fig. 15. Apparent porosity according to the percentage of Tiron at 5 wt% and 10 wt% of Ball clay for the sintered samples by 70 wt% of solid and Si/SiC ratio of 30/70 at 1450 °C.
3.3. Investigation the results obtained of sintering samples After evaluating the results of the rheological section, the considered slurries were prepared with optimal conditions and were poured into gypsum molds to carry out the casting process. 7
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Fig. 17. FESEM images of the cross-section area of the sample containing 70 wt% solid materials and 30 wt% Si- 70 wt% SiC with 5 wt% Ball clay and 0.5 wt% Tiron as additives in 1450 °C a)35.0 kx, and b) 500 x magnifications.
Fig. 18. FESEM images of the cross-section area of the sample containing 70 wt% solid materials and 30 wt% Si- 70 wt% SiC with 10 wt% Ball clay and 0.2 wt% Tiron as additives in 1450 °C; a) 35.0 kx, and b) 500 x magnifications.
to the density and porosity percentage of the sintered bodies obtained from slip casting with different rheological properties at 1450 C° are presented in Table 2. The density diagram in terms of weight percentage of Tiron with two constant percentages of Ball Clay 5 wt% and 10 wt% in temperatures of 1450 °C is plotted in Fig. 14. By increasing the percentage of additives, the density has increased. The porosity percentage graph in terms of Ball clay and Tiron as a function of the additives is sketched for slurry containing 30 wt% of solids in Fig. 15. From the diagram, it is recognized that by increasing additives, the porosity percentage has decreased. Generally, the higher density is related to homogeneous, and uniformity of the microstructure of the green body that is realized by using slurry optimization techniques and preventing the formation of agglomeration.
the unfinished reaction of silicon with nitrogen gas and primary silicon oxidation respectively, which by optimizing the slurry rheology features and the sintering temperature, the formation of these phases could be prevented. By details view of Table 1 and Fig. 12 obtained from phase analysis of diffraction patterns, it is noticeable that by increasing the amounts of Ball clay and Tiron, the ratios of ISi3N4/ISiC and ISiO2/ISiC have increased. Therefore, by promoting the percentage of Ball clay, the formation possibility of SiO2 phases get higher. By looking at Fig. 13, the best range of amount of Ball clay and Tiron is 5 wt% and 0.5 wt%, respectively. 3.4. Density and porosity percentage investigation The density and apparent porosity percentage of the specimens after sintering at 1450 °C for 2 h were measured using the Archimedes method and in accordance to ASTM C373-88 standard. The data related 8
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3.5. Investigating the failure strength of the manufactured samples
Declaration of competing interest
Slurries containing a mixing of silicon and silicon carbide powders with a concentration of 70 wt% solids with various amounts of dispersing materials in the manufacturing molds for strength samples were casted and after sintering at 1450 °C, their failure strengths were recorded from an average of 4 samples. The data related to the tree-point flexural strength obtained of the sintered bodies with different additives are presented in Table 3. The strength chart in terms of additives of slurry containing 30 wt% by solids is shown in Fig. 16. It is apparent that with increasing Tiron, the strength has increased, and with increasing Tiron and Ball clay, simultaneously, the strength is changed. The reduction of strength can be attributed to the growth of silicon nitride grains during nitridation. By optimizing the slurry, reducing the free silicon content and higher increase of silicon nitride phase can result in the increase of the uniformity of the microstructure of the sintered body and consequently, its strength.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This research is financially supported by YUTP-FUNDAMENTAL RESEARCH GRANT (YUTP-FRG) (15LC0-165) and Univerisiti Teknologi Petronas. References [1] D.B. Braun, M.R. Rosen, Rheology Modifiers Handbook: Practical Use and Application, William Andrew Publishing, Norwich, New York, USA, 2000, pp. 2–9. [2] B. singh, P. Kumar, B.K. Mishra, Evaluation of primary slurry used in ceramic shell investment casting process, Int. J. Emerg. Technol. Adv. Eng. 2 (2012) 525–529. [3] I.P. Nanda, Z. Ali, M.H. Idris, A. Arafat, A. Pratoto, Shell mould strength of rice husk ash (RHA) and bentonite clays in investment casting, Int. J. Adv. Sci. Eng. Inf. Technol. 8 (1) (2018) 291–297. [4] M.S. Shukri, O.M.F. Marwah, M. Ibrahim, S. Sharif, E.J. Mohamad, M.Y. Hashim, Collapsibility of PMMA based material in direct investment casting, J. Adv. Manuf. Technol. 12 (1–2) (2018) 501–512. [5] ASM Handbook, Composites vol. 21, ASM International, 2001. [6] F. Çalıskana, E. Kocamanb, S. Cömerta, Synthesis of the in-situ Si3N4-SiC composite nano powders by carbothermal reduction, Acta Phys. Pol., A 131 (2017) 601–604. [7] P.H.C. Camargo, K.G. Satyanarayana, F. Wypych, Nanocomposites: sintering, structure, properties and new application opportunities, Mater. Res. 12 (1) (2009) 1–39. [8] J. Chen, N. Li, Y. Wei, B. Han, G. Li, W. Yan, Y. Zhang, Synthesis of Si3N4/SiC reaction-bonded SiC refractories: the effects of Si/C molar ratio on microstructure and properties, Ceram. Int. 43 (2017) 16518–16524. [9] Y. Zhang, Y. Li, Y. Dai, J. Liu, Y. Xu, Hydration evolution of MgO-SiO2 slurries in the presence of sodium metasilicate, Ceram. Int. 44 (6) (2018) 6626–6633. [10] W.T. Kwon, J.S. Park, K.J. Kim, Y.W. Kim, Material Development of Si3N4-SiC Composites for Machining Application, University of Seoul, Korea, 2004, pp. 1–4 Seoul 130-743. [11] A.K. Gain, J.K. Han, H.D. Jang, B.T. Lee, Fabrication of continuously porous SiCSi3N4 composite using SiC powder by extrusion process, J. Eur. Ceram. Soc. 26 (13) (2006) 2467–2473. [12] S. Liu, Y. Li, P. Chen, W. Li, S. Gao, B. Zhang, F. Ye, Residual stresses and mechanical properties of Si3N4/SiC multilayered composites with different SiC layers, Bol. Soc. Espanola Ceram. Vidr. 56 (4) (2017) 147–154. [13] A. Evcin, Investigation of the effects of different deflocculants on the viscosity of slips, Sci. Res. Essays 6 (11) (2011) 2302–2305. [14] H. Sarraf, J. Havrda, Rheological behavior of concentrated alumina suspension: effect of electrosteric stabilization, Ceramics 51 (3) (2007) 147–152. [15] I. Sever, I. Žmak, L. Ćurković, Z. Švagelj, Stabilization of highly concentrated alumina suspension with different dispersant, Trans. FAMENA (2018) 1391–1849. [16] K. Chihara, D. Hiratsuka, Y. Shinoda, T. Akatsu, F. Wakai, J. Tatami, High-temperature compressive deformation of -SiAlON polycrystals containing minimum amount of intergranular glass phase, Mater. Sci. Eng. B 148 (1–3) (2008) 203–206. [17] J. Marchia, G. e Silvab, C. Chaves, B.B. Silvac, J.C. Bressianic, A.H. Bressianic, Influence of additive system (Al2O3-RE2O3, RE= Y, La, Nd, Dy, Yb) on microstructure and mechanical properties of silicon nitride-based ceramics, Mater. Res. 12 (2) (2009) 145–150. [18] H. ÖZDAĞ, V. BOZKURT, H. İPEK, K. BİLİR, A Study of Effects of Different Dispersants on Rheology and Ageing Characteristics of Ceramic Clay Suspensions, Technical University, International Mineral Processing Symposium, İstanbul, Turkey, 2012, pp. 633–639. [19] I. Ganesh, G. Sundararajan, S.M. Olhero, P.M.C. Torres, J.M.F. Ferreira, A novel colloidal processing route to alumina ceramics, Ceram. Int. 36 (4) (2010) 1357–1364. [20] K. Shqau, Electrosteric Dispersants Used in Colloidal Processing of Ceramics, Literature Review, The Ohio State University/Group Inorganic Materials Science, 2005, pp. 1–17. [21] I. Sever, I. Žmak, L. Ćurković, Z. Švagelj, Stabilization of highly concentrated alumina suspension with different dispersants, Trans. FAMENA XLII-3 (2018) 61–70. [22] A. PAPO, L. PIANI, Rheological properties of alumina slurries: effect of deflocculant addition, Part. Sci. Technol. 25 (2007) 375–380. [23] Y. Huang, J. Yang, Chapter 2: gel -tape casting of ceramic substrates, Novel Colloidal Forming of Ceramics, Springer-Verlag Berlin Heidelberg, 2010, pp. 39–46.
3.6. Investigation of samples morphology after sintering According to Fig. 17 and Fig. 18 and comparing them with each other, it can be concluded that with the increase in the amount of Ball Clay, the percentage of porosity increases, and the density of the sample reduces. The observed porosity in the fracture surface, which was promoted with the sintering at 1450 °C, is due to the water drainage resulting from the formation process of the slurry. Clay materials, because of the plasticity property, will cause a more fluidity state of the slurry, but with an increase in their amount from the optimum percentage, it will act in the opposite way, causing the particles to stick together in slurry and increase the agglomeration, which, as a result, reduces the density and strength. In samples with 10 wt% of Ball Clay and 0.2 wt% of Tiron, the porosity is higher than that of 5 wt% of Ball Clay and 0.5 wt% of Tiron. This fact can be attributed to the more desirable properties of the slip casting process including 5 wt% of Ball Clay density and 0.5 wt% of Tiron, due to the lower viscosity and more suitable rheology. Microstructure studies have a homogeneous distribution of the phases formed in composite prepared bodies, which shows a uniform nitridation during the sintering process. 4. Conclusion In order to investigate the effect of using different values of Tiron and Ball clay on rheological properties, viscosity and shear stresses were plotted using rheometric experiments. According to the results obtained from the viscometer and the shear stress tests, it was found that slurry containing 5 wt% Ball clay and 0.5 wt% Tiron nearly reveals shear-thinning behavior and the lowest viscosity and shear stress were related to this slurry, indicating more fluidity of slurry, which will be more suitable for the casting process. Conferring with the three-point flexural strength test of sintered-bodies, the strength of this slurry was higher compared to other bodies. It was also confirmed that by increasing the amount of Ball clay and Tiron, the value of the porosity of sample derived from Archimedes approach increases and the density reduces, where the pictures and results obtained from the FESEM, significantly confirmed this fact. X-ray experiments showed that by increasing the percentage of the Ball clay, the ratios of ISi3N4/ISiC and ISiO2/ISiC increases, as a result the probability of forming the phases of Si3N4 and SiO2 increases.
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Effect of additives on slip casting rheology, microstructure and mechanical properties of Si3N4/SiC composites Shohreh Shahrestani, Mokhtar Che Ismail, Saeid Kakooei∗, Mohammadali Beheshti Centre for Corrosion Research, Mechanical Engineering Department, Universiti Teknologi Petronas, Seri Iskandar, Malaysia
A R T I C LE I N FO
A B S T R A C T
Keywords: Slurry rheology Additives Slip casting Nitridation Silicon carbide/silicon nitride composite Silicon/silicon carbide powder
The SiC/Si3N4 composites were fabricated with sintering process. To produce SiC/Si3N4 composite components, slurry mixtures containing Si/SiC powders were used by the slip casting method. In order to investigate the effect of dispersants and additives on the rheological properties and the body casted, slurries with concentration of 70% solid weight were prepared. It included a mixture of silicon and silicon carbide with weight ratios of 30 wt% and 70 wt%, respectively, and various weight percentages of Ball clay as lubricant and Tiron (sodium salt of benzene disulfonic acid) as dispersant at pH value of 7. After preparing the green bodies by slip casting method by using plaster mold, the samples were sintered at 1450 °C inside an atmospheric-controlled furnace under a pressure of 0.12 MPa of nitrogen gas for 2 h. By examining the rheological properties of the slurry and the sintering properties, it was concluded that the best slurry was obtained in terms of viscosity, density, porosity and strength using 5 wt% Ball clay and 0.5 wt% Tiron. Phase transformations, microstructure and morphology of the sintered specimens were accomplished by Field Emission Scanning Electron Microscopy (FESEM) examination and X-ray diffraction experimental analysis. XRD and FESEM results demonstrated that the composite fabricated by slurry containing 5 wt% Ball clay and 0.5 wt% Tiron had the least porosity without SiO2 phase.
1. Introduction SiC/Si3N4 composite is a new generation of ceramic materials that was developed a few years ago [1–4]. These ceramic composites are known for their properties such as corrosion resistance, abrasion, high thermal shock resistance and high strength properties. In general, composites can achieve better mechanical and thermal properties than their constituents [5,6]. The major ceramic fields composites include carbon/carbon composites, alumina/silicon carbide composites and composites with silicon nitride reinforced with continuous silicon carbide and carbon fiber [7–9]. An appropriate approach to produce SiC/ Si3N4 composite is the nitridation-sintering of Si/SiC in atmospheric controlled furnaces [8]. Several authors have used Si3N4 with SiC to make SiC–Si3N4 composites [10–12]. In these studies, the use of silicon powders instead of silicon nitride powders to produce SiC/Si3N4 composites could lead to a more economical process. The benefits of using silicon powder are firstly it is cheaper than silicon nitride powder and secondly it requires a low nitride reaction. Slip casting is colloidal and cost-effective method to fabricate the engineering ceramics and their composites in aqueous suspension by pouring in porous mold and draining water from the slurry of a green body ready for sintering process [13,14]. The composite properties ∗
made by slip casting are directly related to the rheological properties of the slurry prepared from them. By controlling the slurry rheology (solid particle behavior in liquid medium, particles stability, etc.), optimal conditions for slurry control and thus the final product are achieved [2,15,16]. Controlling and optimizing factors such as particle size, particle size distribution, particle shape and morphology, volumetric solids content, inter-particle force, pH values and slurry stabilization using desirable type and amount of dispersant are the most important concerns for achieving homogeneity and high density of fabricated green body [1,17]. Colloidal techniques by serving various dispersant can help control the forces between particles in the suspension, thereby reducing the creation and development of agglomerates, facilitating the slip casting process, consequently avoiding the coarsening of the sintered microstructure [18,19]. In the light of the above, various additives such as binders (Kaolin and ball clay) and dispersants (Tiron, Dolapix and TPP) are added to the suspension to provide less viscosity value and consequently higher density of the fabricated bodies. The dispersions improve the stability of the ceramic particles in the suspension electrostatically, sterically or electro-sterically [20,21]. Electrostatic stabilization is based on double layers. Newly absorbed ionic groups are added to the suspension and a charged layer is formed. These double layers are mutually repelled, which provide stability for
Corresponding author. E-mail address: [email protected] (S. Kakooei).
https://doi.org/10.1016/j.ceramint.2019.11.085 Received 23 August 2019; Received in revised form 7 November 2019; Accepted 11 November 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Shohreh Shahrestani, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.085
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various shapes were made. After forming the raw material, the samples were removed from the mold and dried at room temperature (as shown in Fig. 3).
suspension. Steric stabilization is achieved by the addition of macromolecules that produce particles at the surface of the particles and absorb a layer and causing the suspension to stabilize. However, this process can occasionally cause deformation in ceramic materials during sintering. The usual stabilization method is electrostatic stabilization, which combines the two previous stabilized methods [18]. Electrosteric dispersers are usually polyelectrolytes that bind themselves to the surface of the particles and cause enough potential difference, and this causes the particles to disperse. Polyelectrolytes have excellent flexibility for multi-stage processing and more stabilization for slip casting. This system also includes better control of the flocculation and thixotropy conditions and in comparison to inorganic dispersers, they are more stable [21]. Hence, this study focused on optimizing and investigating the effect of type and amount of dispersant on the properties of the slurry, the green body features prepared by slip casting method and consequently on the sintered body structures using an atmospheric-controlled furnace under a nitriding environment. Therefore, the target of this examination was to evaluate the influence of Tiron [22,23] and Ball clay as dispersant and lubricant on the rheometric results, densification, mechanical properties and microstructure analysis of SiC/Si3N4 composite.
2.4. Sedimentation studies
2. Experiments
In order to obtain suitable pH for slip casting, the results obtained from sedimentation studies can be very effective. Under well dispersion conditions there is a minimum amount at sedimentation height. It should be mentioned that the sedimentation height has opposite proportion to the amount of powder dispersion, which means that the greater dispersion of the powder will result in a lower sedimentation height. At first, silicon and silicon carbide powders were dispersed in distilled water with the magnetic stirrer. The pH value was then adjusted using ammonia (NH3) and Hydrochloric acid (HCl) and measured by Metrohm pH meter (827 model, Metrohm International Headquarters, Switzerland). The slurry contains 20 vol% of solids with a 1: 1 ratio of silicon and silicon carbide powders in aqueous solution. Five calibrated containers including slurries prepared with different pH values of 2, 4, 6, 8 and 10 were prepared to study sedimentation studies. After preparation of the slurry, the sedimentation height was measured after 2, 6 and 24 h.
2.1. Raw materials
2.5. Structural analysis
Russian commercial silicon powder and Japanese commercial silicon carbide powder with an average grain size distribution of 9.769 μm (D50 from Anyang Teifa Metallurgy Co. Ltd. China) were used as raw materials in this study as shown in Fig. 1. Tiron with a product code of 1000/00808/1 with a purity more than 99% was used as a dispersant for stabilization the slurry. Tiron was produced by BDH with the chemical formula (C6H4Na2O8S2) and industrial Ball clay (WBB) was used as a lubricant.
X-ray diffraction (XRD) methods were used to determine the phase composition of the primary powder and to discover the phases obtained after heating the samples and studying their structure. X-ray diffraction patterns were performed using X-ray diffraction devices Siemens, Germany, Model 500-D, under a voltage of 30 kV and 25 mA. In all experiments, X-ray CuKα beam with a wavelength of 1/5404 Å was used. The existing phases were identified by comparing the XRD peak diffraction angles and the intensity levels associated with the values presented in the ASTM cards.
2.2. Preparation of slurry 2.6. Morphological study of cross-sectional fracture surface In this study, silicon powder with a relatively fine grain size (micro diameter) and silicon carbide powder with an average grain size of 9.769 μm were used (Fig. 1). Also, silicon powders were used with an average grain size of 7.923 μm (Fig. 2). Powders were dispersed in distilled water for 15 min using a magnetic stirrer. Before mixing the mentioned powders, the different weight ratios of lubricant were dissolved in distilled water, and then different weight ratios of Ball clay and Tiron (a sodium salt of a benzene disulfonic acid) were added to the set with 10 wt% and 5 wt% for Ball clay and 0.5 wt%, 0.2 wt% and 0.1 wt% for Tiron. The slurry was then placed in an ertalonic cup and mixed for 2 h using a planetary mill and alumina balls. In addition, slurries containing 70 wt% solids and various amounts of Tiron and Ball clay were prepared in pH of 7, which solids content is containing 70 wt % of silicon carbide powder and 30 wt% of silicon powder.
To evaluate the cross-sectional fracture of the Field Emission Electron Microscope (FESEM) Mira3-XMU model (TESCAN-Czech Republic) was used including the property of a huge case and high vacuum velocity after the heat treatment and test strength. In order to investigate the cross-section fracture, a gold coating was applied on its surface. By using the images obtained, the changes in its cross-sectional fracture was investigated. 2.7. Mechanical strength measurement Instron 1196 device (Instron, 825 University Ave, Norwood, USA) was used to measure the flexural strength of the sample. The rectangular cube polished samples with dimensions of 38 × 8 × 6 mm were prepared using the plaster molds to measure the flexural strength. The test was carried out by the ASTM D790 standard using a triple-point procedure. The speed of movement of the jaws during the performance of the strength test was constant and equal to 0.5 mm/min. At least 4 samples were used to measure the fracture strength at every test.
2.3. Investigating the rheological behavior of slurries containing the mixture of silicon and silicon carbide powders 20 ml of the prepared slurry was poured into the chamber of the viscometer. In each case, the relation between shear speed and viscosity was plotted using a Brookfield-LV-DV Π Pro viscometer (John Morris group, Australia) and with the help of Rheocalc 32 software, and at cutting speeds from 0.01 s−1 to 56 s−1 and at the temperature of 25 ± 2 °C. The software was programmed in a way that the speed of RPM device was increased exponentially every 3 s, and the data related to the viscosity and cutting speed were recorded every 3 s after automatic fixing of this number, which in total, 60 points would be obtained. For slip casting containing a mixture of silicon and silicon carbide, first gypsum plaster molds with a ratio of plaster to water of 1:3 in
2.8. Density and apparent porosity percentage determination Standard samples were prepared to measure the density of the sintered specimens and their density were determined according to ASTM C373-88 standard. Finally, the density curves were plotted, and the pattern of the samples studied. To calculate the porosity percentage by means of the Archimedean method, dry weight, immersion weight and saturation weight were measured, and obtained by replacing in the formula related to density and porosity percentage. 2
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Fig. 1. a)2.50 kx and (b)10.0 kx of SEM images, (c) XRD and (d) distribution graphs silicon carbide powder with average particle size of d50 ≤ 9.769 μm as raw material for preparing slurries to make SiC/Si3N4 composites.
3. Results and discussion
30–70, and the weight ratio of Si to SiC powder of 30–70 with various percentages of Ball clay and Tiron are shown in Fig. 5. From the obtained curves, it is obvious that the amount of viscosity of all slurries reduced with the increase of shear speed first, and then continued with a less slope and finally reached to a constant amount. The reduction of viscosity by increasing the shear rate means the shear dilution behavior of slurries and keeping the viscosity constant with increasing the speed, or in other words, reaching the gradient of the graph to zero, reflects the Newtonian slurry behavior. By comparing the shear rate and viscosity of each of the four slurries, the slurry containing 5 wt% of Ball clay and 0.5 wt% of Tiron and slurry with 10 wt% Ball clay and 0.1 wt% of Tiron have the lowest and highest amount of viscosity, respectively. Regarding the constant amount of solid materials in the slurry, it will be more desirable for casting, regardless of whether the slurry has a lower viscosity. Considering this matter, from the viewpoint of the viscosity factor, slurry containing 5 wt% of Ball clay and 0.5 wt% of Tiron is a more desirable slurry than other slurries. Viscosity graphs in terms of shear rate with the constant ratios of liquid to solid and of Si to SiC powders for slurry with 10 wt% Ball clay and 0.2 wt% of Tiron and slurry with 10% Ball clay and 0.1 wt% of Tiron are plotted in Fig. 6. Comparing the two slurries, they show that the amount of viscosity decreases when the amount of lubricant (Tiron) is increased. Viscosity curves in terms of shear rate with constant ratios of liquid to solid and Si to SiC powders for slurry with 10 wt% Ball clay
3.1. Sedimentation studies Sedimentation studies can be used to obtain a suitable pH from the slurry. After 2, 6 and 24 h, the sedimentation height was measured, and the corresponding graphs were plotted. The lowest sedimentation heights show a good dispersion of particles, which leads to an ideal compression. With respect to Fig. 4, it is obvious that the lowest t sedimentation heights for slurry with a mixture of silicon and silicon carbide are related to pH greater than value of 5, and at pH 6, 8 and 10 there is no significant difference in sedimentation altitudes. So, by considering that the pH of water is 7 and working in acid pH area will have problems in the production process, slurries can be made in the neutral pH value. 3.2. Investigation the influence of Tiron and Ball clay amounts on the rheological behavior of slurries containing a mixture of Si and SiC powders In order to determine the optimal percentage of dispersants, the slurry rheology behavior was evaluated with different percentages of Tiron and Ball clay. All slurries were comprised of 70 wt% of solid materials containing a weight ratio of 30 and 70 of silicon and silicon carbide powders, respectively. The viscosity graph based on the shear rate for four different slurries with a weight ratio of liquid to solids of 3
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Fig. 2. (a)2.50 kx and (b)10.0 kx of SEM images, (c) XRD and (d) distribution graphs of silicon powder with average particle size of d50 ≤ 2.792 μm as raw material for preparing slurries to make SiC/Si3N4 composites.
Fig. 3. Si–SiC green body fabricated by slip casting.
Fig. 4. Height of the sedimentation vs pH for a mixture of Si + SiC with a weight ratio of 1:1 (including 20 wt% of solids) as a function of time.
and 0.2 wt% of Tiron and slurry containing 5 wt% Ball clay and 0.2 wt% of Tiron are drawn in Fig. 7. By comparing the two slurries, it should be noted that by increasing the percentage amount of Ball clay, the values of viscosity have decreased. With reference to both plotted figures, it is completely deduced that using lower or more amount of additives than
optimum levels will lead to disturbing the double layer, therefore, there is inadequate fluid flow which slows the movement of ions within the electric double layer and suppresses the attraction forces, thereby reducing the fluid viscosity. 4
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Fig. 9. Comparison of the effects of using different percentages of Tiron on rheological behavior including 70 wt% solid and weight ratio of Si/SiC: 30/70 and 10 wt% Ball clay.
Fig. 5. Effect of dispersant materials on slurry rheological behavior including 70 wt% solids and weight ratio of Si/SiC: 30/70.
Fig. 6. Comparison of the effects of using different percentages of Tiron on rheological behavior including weight ratio of Si/SiC: 30/70 and 10 wt% Ball clay.
Fig. 10. Comparison of the effects of using different percentages of Ball clay on rheological behavior including 70 wt% solid weight and weight ratio of Si/SiC: 30/70 and 0.2 wt% Tiron.
The shear stress diagram in terms of the shear rate for four different slurries with various percentages of Ball clay and Tiron were investigated as seen Fig. 8. According to the diagram, the shear stress for all slurries with increasing shear velocity initially increase with a large slope and then decrease in a short period of time and eventually increase again. Slurries with a lower percentage of Ball clay (5% by weight) show Newtonian behavior for shear rates more than 30 s−1, while slurries with a higher percentage of Ball clay (10% by weight) show an approximate behavior of shear dilution. It should also be noted that all slurries at low shear rate reveal the shear dilatation behavior because the amount of shear stress is increasing with a sharp slope. Furthermore, slurry containing 5 wt% Ball clay and 0.5 wt% Tiron nearly discloses a shear-thinning behavior. By comparing all four slurries, it is completely obvious that the amounts of shear stresses for slurry with 10 wt% Ball clay and 0.1 wt% Tiron is higher than other slurries which indicates less fluidity of the slurry compared to other slurries, but this issue is the opposite for slurry containing 5 wt% Ball clay and 0.5 wt% Tiron which means that the amount of shear stresses of this slurry is less than other slurries indicating more fluidity of the slurry compared to the other slurries. The shear stress diagram versus shear rate with constant ratios of liquid to solid and Si to SiC powders for slurry with 10 wt% Ball Clay and 0.2 wt% Tiron and slurry with 10 wt% Ball clay and 0.1 wt% Tiron is plotted in Fig. 9. In this slurry, the amount of Ball clay is fixed, and the percentage of the Tiron has changed. Both slurries initially show shear dilatation behavior and then in speeds more than 30 (s−1) reveals the Newtonian behavior. It is also obvious that with the increase amount of the Tiron, the shear stress value decreases, which indicates the fluidity of these slurries. The shear stress according to the shear rate with fixed liquid to solid ratios and the fixed ratios of Si to SiC for slurry having 5 wt% Ball clay and 0.2 wt% of Tiron and slurry with 10 wt% Ball clay and 0.2 wt% Tiron is plotted in Fig. 10. In these slurries, the amount of Tiron is fixed and the
Fig. 7. Comparison of the effects of using different percentages of Ball clay on rheological behavior including weight ratio of Si/SiC: 30/70 and 0.2 wt% Tiron.
Fig. 8. Influence of dispersant materials on slurry rheological behavior including 70 wt% solids and weight ratio of Si/SiC: 30/70 and different percentage of Tiron and Ball clay.
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Fig. 11. X-ray diffraction pattern of sintered samples including 70 wt% solid-30 wt% liquid, 30 wt% Si-70 wt% SiC; (a) 5 wt% Ball clay-0.5 wt% Tiron; (b) 10 wt% Ball clay-0.2 wt% Tiron, as additives in 1450 °C. Table 1 The data obtained from X-ray diffraction patterns for the sintered sample with 70 wt% of solid materials including 30 wt% Si-70 wt% SiC with different percentage of Ball clay and Tiron as additives. Weight percent of Ball clay and Tiron
I(Si3N4)/I(SiC) (Peak 100)
I(SiO2)/I SiC) (Peak 100)
Temperature (°C)
5 wt% Ball clay-0.5 wt% Tiron 10 wt% Ball clay-0.2 wt% Tiron 10 wt% Ball clay-0.1 wt% Tiron 5 wt% Ball clay-0.2 wt% Tiron
0.06 0.2049 0.155 0.096
~0.00 0.1756 0.1464 0.0875
1450 1450 1450 1450
percentage of the Ball clay has changed. Initially, the shear dilution behavior is more pronounced for slurry with 0.5% Ball clay density. But in the following, for both slurries the slope of curves are unchanged, which reflects the Newtonian behavior. For this slurry, like two slurries as shown in Fig. 10, by increasing the amount of Ball clay, the values of shear stresses in a constant interval of shear rate for each slurry have decreased. Generally, by decreasing the amount of Tiron in the optimal percentage, the viscosity of the slurry reduces and by increasing a greater value of Tiron, the viscosity gradually increases. When the percentage of the lubricant is low, in this case, the dispersing agent cannot show its effect and thus causing the agglomerate in the slurry. By increasing the amount of lubricant, the covering of the absorption surface is increased, and the repulsive forces overcome the van der Waals forces. By continuing to increase the amount of lubricant from the optimum value, surface absorption covering is decreased, and meanwhile, it causes more compression of the double layer, thus resulting in a decrease of the distance between the surfaces and thus reducing the repulsive forces.
Fig. 12. Peak 100 intensity graph of the percentage of silicon nitride to silicon carbide according to the percentage of Tiron at 5 wt% and 10 wt% of Ball clay for the sintered samples with 70 wt% of solid materials including 30 wt% Si −70 wt% SiC in 1450 °C.
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Fig. 13. Ratio intensity graph of ISi3N4/ISiC and ISiO2/ISiC according to the percentage of Ball clay powder for the sintered sample with 70 wt% of solid materials including 30 wt% Si −70 wt% SiC in 1450 °C. Table 2 Characteristics of density and apparent porosity for sintered bodies by 70 wt% of solid and Si/SiC ratio of 30/70 at 1450 °C. Ratio weight of liquid/solid
Ratio weight of Si/SiC
Tiron (wt%)
Ball clay (wt%)
Archimedes Density (g/cm3)
Apparent porosity (%)
Temperature (°C)
70/30 70/30 70/30 70/30
70/30 70/30 70/30 70/30
0.5 0.2 0.2 0.1
5 5 10 10
3.25 3.07 3.4 3.34
12.95 14.3 12.25 12.65
1450
Table 3 Tree-point flexural strength data of sintered bodies. Ratio weight of liquid/ solid
Ratio weight of Si/SiC
Tiron (wt%)
Ball clay (wt%)
Strength (MPa)
Temperature (°C)
70/30 70/30 70/30 70/30
70/30 70/30 70/30 70/30
0.2 0.5 0.2 0.1
5 5 10 10
64.01 66.37 64.76 64.12
1450
Fig. 14. Density according to the percentage of Tiron at 5 wt% and 10 wt% of Ball clay for the sintered samples by 70 wt% of solid and Si/SiC ratio of 30/ 70 at 1450 °C.
Fig. 16. Strength graph in terms of Tiron percentage by 5 wt% and 10 wt% of Ball clay as a function of the additives for slurry containing 70 wt% of solids.
Subsequently, the samples were sintered for 2 h in a nitride atmosphere controlled at1450 °C. The results obtained from XRD analysis and the phases formed in the casting bodies include 70 wt% solids and 30/70 ratios of Si/SiC powders with different ratios of Ball clay and Tiron at 1450 °C is represented in Fig. 11. According to the pattern of X-ray diffraction of the specimen sintered, it is obvious that along with the SiC phase, Si3N4, Si and SiO2 phases are also formed. The presence of the Si3N4 phase reveals the formation of Si3N4–SiC composites, but it is desirable that the identified phases to be only related to SiC and Si3N4, and the presence of Si and SiO2 phases formed along with them reveal
Fig. 15. Apparent porosity according to the percentage of Tiron at 5 wt% and 10 wt% of Ball clay for the sintered samples by 70 wt% of solid and Si/SiC ratio of 30/70 at 1450 °C.
3.3. Investigation the results obtained of sintering samples After evaluating the results of the rheological section, the considered slurries were prepared with optimal conditions and were poured into gypsum molds to carry out the casting process. 7
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Fig. 17. FESEM images of the cross-section area of the sample containing 70 wt% solid materials and 30 wt% Si- 70 wt% SiC with 5 wt% Ball clay and 0.5 wt% Tiron as additives in 1450 °C a)35.0 kx, and b) 500 x magnifications.
Fig. 18. FESEM images of the cross-section area of the sample containing 70 wt% solid materials and 30 wt% Si- 70 wt% SiC with 10 wt% Ball clay and 0.2 wt% Tiron as additives in 1450 °C; a) 35.0 kx, and b) 500 x magnifications.
to the density and porosity percentage of the sintered bodies obtained from slip casting with different rheological properties at 1450 C° are presented in Table 2. The density diagram in terms of weight percentage of Tiron with two constant percentages of Ball Clay 5 wt% and 10 wt% in temperatures of 1450 °C is plotted in Fig. 14. By increasing the percentage of additives, the density has increased. The porosity percentage graph in terms of Ball clay and Tiron as a function of the additives is sketched for slurry containing 30 wt% of solids in Fig. 15. From the diagram, it is recognized that by increasing additives, the porosity percentage has decreased. Generally, the higher density is related to homogeneous, and uniformity of the microstructure of the green body that is realized by using slurry optimization techniques and preventing the formation of agglomeration.
the unfinished reaction of silicon with nitrogen gas and primary silicon oxidation respectively, which by optimizing the slurry rheology features and the sintering temperature, the formation of these phases could be prevented. By details view of Table 1 and Fig. 12 obtained from phase analysis of diffraction patterns, it is noticeable that by increasing the amounts of Ball clay and Tiron, the ratios of ISi3N4/ISiC and ISiO2/ISiC have increased. Therefore, by promoting the percentage of Ball clay, the formation possibility of SiO2 phases get higher. By looking at Fig. 13, the best range of amount of Ball clay and Tiron is 5 wt% and 0.5 wt%, respectively. 3.4. Density and porosity percentage investigation The density and apparent porosity percentage of the specimens after sintering at 1450 °C for 2 h were measured using the Archimedes method and in accordance to ASTM C373-88 standard. The data related 8
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3.5. Investigating the failure strength of the manufactured samples
Declaration of competing interest
Slurries containing a mixing of silicon and silicon carbide powders with a concentration of 70 wt% solids with various amounts of dispersing materials in the manufacturing molds for strength samples were casted and after sintering at 1450 °C, their failure strengths were recorded from an average of 4 samples. The data related to the tree-point flexural strength obtained of the sintered bodies with different additives are presented in Table 3. The strength chart in terms of additives of slurry containing 30 wt% by solids is shown in Fig. 16. It is apparent that with increasing Tiron, the strength has increased, and with increasing Tiron and Ball clay, simultaneously, the strength is changed. The reduction of strength can be attributed to the growth of silicon nitride grains during nitridation. By optimizing the slurry, reducing the free silicon content and higher increase of silicon nitride phase can result in the increase of the uniformity of the microstructure of the sintered body and consequently, its strength.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This research is financially supported by YUTP-FUNDAMENTAL RESEARCH GRANT (YUTP-FRG) (15LC0-165) and Univerisiti Teknologi Petronas. References [1] D.B. Braun, M.R. Rosen, Rheology Modifiers Handbook: Practical Use and Application, William Andrew Publishing, Norwich, New York, USA, 2000, pp. 2–9. [2] B. singh, P. Kumar, B.K. Mishra, Evaluation of primary slurry used in ceramic shell investment casting process, Int. J. Emerg. Technol. Adv. Eng. 2 (2012) 525–529. [3] I.P. Nanda, Z. Ali, M.H. Idris, A. Arafat, A. Pratoto, Shell mould strength of rice husk ash (RHA) and bentonite clays in investment casting, Int. J. Adv. Sci. Eng. Inf. Technol. 8 (1) (2018) 291–297. [4] M.S. Shukri, O.M.F. Marwah, M. Ibrahim, S. Sharif, E.J. Mohamad, M.Y. Hashim, Collapsibility of PMMA based material in direct investment casting, J. Adv. Manuf. Technol. 12 (1–2) (2018) 501–512. [5] ASM Handbook, Composites vol. 21, ASM International, 2001. [6] F. Çalıskana, E. Kocamanb, S. Cömerta, Synthesis of the in-situ Si3N4-SiC composite nano powders by carbothermal reduction, Acta Phys. Pol., A 131 (2017) 601–604. [7] P.H.C. Camargo, K.G. Satyanarayana, F. Wypych, Nanocomposites: sintering, structure, properties and new application opportunities, Mater. Res. 12 (1) (2009) 1–39. [8] J. Chen, N. Li, Y. Wei, B. Han, G. Li, W. Yan, Y. Zhang, Synthesis of Si3N4/SiC reaction-bonded SiC refractories: the effects of Si/C molar ratio on microstructure and properties, Ceram. Int. 43 (2017) 16518–16524. [9] Y. Zhang, Y. Li, Y. Dai, J. Liu, Y. Xu, Hydration evolution of MgO-SiO2 slurries in the presence of sodium metasilicate, Ceram. Int. 44 (6) (2018) 6626–6633. [10] W.T. Kwon, J.S. Park, K.J. Kim, Y.W. Kim, Material Development of Si3N4-SiC Composites for Machining Application, University of Seoul, Korea, 2004, pp. 1–4 Seoul 130-743. [11] A.K. Gain, J.K. Han, H.D. Jang, B.T. Lee, Fabrication of continuously porous SiCSi3N4 composite using SiC powder by extrusion process, J. Eur. Ceram. Soc. 26 (13) (2006) 2467–2473. [12] S. Liu, Y. Li, P. Chen, W. Li, S. Gao, B. Zhang, F. Ye, Residual stresses and mechanical properties of Si3N4/SiC multilayered composites with different SiC layers, Bol. Soc. Espanola Ceram. Vidr. 56 (4) (2017) 147–154. [13] A. Evcin, Investigation of the effects of different deflocculants on the viscosity of slips, Sci. Res. Essays 6 (11) (2011) 2302–2305. [14] H. Sarraf, J. Havrda, Rheological behavior of concentrated alumina suspension: effect of electrosteric stabilization, Ceramics 51 (3) (2007) 147–152. [15] I. Sever, I. Žmak, L. Ćurković, Z. Švagelj, Stabilization of highly concentrated alumina suspension with different dispersant, Trans. FAMENA (2018) 1391–1849. [16] K. Chihara, D. Hiratsuka, Y. Shinoda, T. Akatsu, F. Wakai, J. Tatami, High-temperature compressive deformation of -SiAlON polycrystals containing minimum amount of intergranular glass phase, Mater. Sci. Eng. B 148 (1–3) (2008) 203–206. [17] J. Marchia, G. e Silvab, C. Chaves, B.B. Silvac, J.C. Bressianic, A.H. Bressianic, Influence of additive system (Al2O3-RE2O3, RE= Y, La, Nd, Dy, Yb) on microstructure and mechanical properties of silicon nitride-based ceramics, Mater. Res. 12 (2) (2009) 145–150. [18] H. ÖZDAĞ, V. BOZKURT, H. İPEK, K. BİLİR, A Study of Effects of Different Dispersants on Rheology and Ageing Characteristics of Ceramic Clay Suspensions, Technical University, International Mineral Processing Symposium, İstanbul, Turkey, 2012, pp. 633–639. [19] I. Ganesh, G. Sundararajan, S.M. Olhero, P.M.C. Torres, J.M.F. Ferreira, A novel colloidal processing route to alumina ceramics, Ceram. Int. 36 (4) (2010) 1357–1364. [20] K. Shqau, Electrosteric Dispersants Used in Colloidal Processing of Ceramics, Literature Review, The Ohio State University/Group Inorganic Materials Science, 2005, pp. 1–17. [21] I. Sever, I. Žmak, L. Ćurković, Z. Švagelj, Stabilization of highly concentrated alumina suspension with different dispersants, Trans. FAMENA XLII-3 (2018) 61–70. [22] A. PAPO, L. PIANI, Rheological properties of alumina slurries: effect of deflocculant addition, Part. Sci. Technol. 25 (2007) 375–380. [23] Y. Huang, J. Yang, Chapter 2: gel -tape casting of ceramic substrates, Novel Colloidal Forming of Ceramics, Springer-Verlag Berlin Heidelberg, 2010, pp. 39–46.
3.6. Investigation of samples morphology after sintering According to Fig. 17 and Fig. 18 and comparing them with each other, it can be concluded that with the increase in the amount of Ball Clay, the percentage of porosity increases, and the density of the sample reduces. The observed porosity in the fracture surface, which was promoted with the sintering at 1450 °C, is due to the water drainage resulting from the formation process of the slurry. Clay materials, because of the plasticity property, will cause a more fluidity state of the slurry, but with an increase in their amount from the optimum percentage, it will act in the opposite way, causing the particles to stick together in slurry and increase the agglomeration, which, as a result, reduces the density and strength. In samples with 10 wt% of Ball Clay and 0.2 wt% of Tiron, the porosity is higher than that of 5 wt% of Ball Clay and 0.5 wt% of Tiron. This fact can be attributed to the more desirable properties of the slip casting process including 5 wt% of Ball Clay density and 0.5 wt% of Tiron, due to the lower viscosity and more suitable rheology. Microstructure studies have a homogeneous distribution of the phases formed in composite prepared bodies, which shows a uniform nitridation during the sintering process. 4. Conclusion In order to investigate the effect of using different values of Tiron and Ball clay on rheological properties, viscosity and shear stresses were plotted using rheometric experiments. According to the results obtained from the viscometer and the shear stress tests, it was found that slurry containing 5 wt% Ball clay and 0.5 wt% Tiron nearly reveals shear-thinning behavior and the lowest viscosity and shear stress were related to this slurry, indicating more fluidity of slurry, which will be more suitable for the casting process. Conferring with the three-point flexural strength test of sintered-bodies, the strength of this slurry was higher compared to other bodies. It was also confirmed that by increasing the amount of Ball clay and Tiron, the value of the porosity of sample derived from Archimedes approach increases and the density reduces, where the pictures and results obtained from the FESEM, significantly confirmed this fact. X-ray experiments showed that by increasing the percentage of the Ball clay, the ratios of ISi3N4/ISiC and ISiO2/ISiC increases, as a result the probability of forming the phases of Si3N4 and SiO2 increases.
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