Effect of Silicon Nitride (Si3N4) on Mechanical and Dielectric Properties of Fused Silica Ceramic Composites


 Fused silica ceramics composites were widely using for the fabrication of radomes which is the vital component of a missile. The required properties were attained by the addition of small amounts of Silicon Nitride (Si3N4) to the fused silica for enhancing flexural strength and without disturbing the dielectric properties. In the present work, porous fused silica ceramic composites were fabricated using gelcasting process, a near net shaping technique. The experiments were conducted using Response Surface Methodology (RSM) central composite with face centered design with six centre points approach. The process parameters of gelcasting process such as solid loading, monomer ratio, monomer content and additive (Si3N4) were considered as input parameters and flexural strength, porosity and dielectric constant as response parameters. Single parameter and interaction parameter effect on responses were studied. The effectiveness of derived models was compared with Analysis of Variance (ANOVA) at 95% confidence level. Statistical analysis proven that input parameters have critical effect on responses. The derived RSM mathematical models have a higher R2 values (Flexural strength 97.62%, Porosity 95.12% and dielectric constant 95.93%) which shows the critical relation between actual and predicted values. The optimal responses obtained were flexural strength 86.173 MPa, porosity 39.767% and dielectric constant 6.949 corresponding parameters process parameters solid loading 47.95%, additive (Si3N4) 10.43 wt%, monomer content 15 wt% and monomer ratio 3.


Introduction
Ceramics are highly regarded for their mechanical properties such as strength, durability, and hardness. These are widely used in electronic applications such as semiconductors, conductors, insulators, and magnets due to their electrical and magnetic properties. Advanced ceramics have been progressively used in aerospace, automotive engine, defense, construction, biomedical, nuclear industries, chemical, petrochemical, oil/gas, and industrial wear because of their high strength at elevated temperature, high thermal and chemical stability, low density and high wear resistance. Some applications include space shuttle engine components, tank armor, superconductors and piezoelectric devices [1,2].
Fused silica is a non-crystalline form of silicon dioxide (quartz, sand) commonly made by fusion of pure silica sand (Wan et al. 2014a) [3]. Nowadays fused silica is one of the essential materials for many engineering and aerospace applications such as radomes, heat shields, antenna window, and crucibles, because of its interesting and prominent properties such as resistant to thermal shock and corrosion, a low thermal expansion coefficient, high softening temperature, a low and stable dielectric constant, and excellent chemical inertness (Haris and Welsh, 1973) [4]. These properties suggest that the SiO2 ceramics are an ideal candidate material for high-temperature wave-transparent applications and radomes. However, the relatively low mechanical strength of sintered SiO2 ceramics is inadequate to meet the requirements of advanced re-entry vehicles, especially hypersonic spacecraft. In order to overcome these shortcomings, additives including h-BN, Si3N4, fibers, and graphene have been utilized as reinforcements to improve the mechanical strength of ceramics.
Silicon nitride based ceramics are extensively applied in various industrial fields owing to numerous properties, such as high hardness, superior corrosion resistance, excellent mechanical and chemical stability, excellent wear resistance, high decomposition temperature, high strength, and toughness, etc. Si3N4 is produced by reacting SiCl with NH3 (Jong, B.W. et al.,1992) [5]. To attain a high extent of quality, components are spark plasma sintered, hot pressed or reaction sintered as dense parts cannot be produced through direct sintering. At lower temperatures, the bonding of Si3N4 particles is attained by addition of sintering aids which normally encourage liquid-phase sintering. Even at elevated temperatures, Si3N4 ceramics upholds their strength.
Nowadays, various processing techniques have been developed to prepare both the porous and dense Si3N4 ceramics for structural and functional applications (Chen et al., 2010) [6]. These materials can be densified under pressureless sintering conditions by adding a certain amount of sintering aids at high temperatures (1750-2000°C). Si3N4 may be well stabilized electrostatically against agglomeration in aqueous slips either at pH of <5 or >9, having either a positive charge or negative charge, respectively (Greil, 1989) [7].
Gelcasting was first developed at Oak Ridge National Laboratory (ORNL) in the 1960s for hard metals and further developed for ceramic materials by combining traditional slip processing with polymer chemistry (Omatete et al., 1991 andOmatete et al., 1997) [8,9]. The general principle in this process is the ceramic particles suspended are surrounded by a threedimensional network of the cross-linked polymers. For obtaining maximum solid loading typically a dispersant is used which causes the particles in the ceramic slurry to disperse by means of an electric double layer or steric stabilization. Shrinkage can be minimized during sintering process and dense ceramics can be obtained by high solid loading. The free-radical reaction leads to the formation of micro-gels of monomer and cross-linker inside the suspension, which eventually combine to form a macro-gel network. The gel network formed inside the suspension holds the particles together collectively to mold a dense green body that is demoldable and takes the form of the mold cavity. Molds can be made up of metal or plastic materials that are nonporous (Omatete et al., 1991 andOmatete et al., 1997) [8,9]. Over the conventional forming methods gelcasting has distinct advantages such as near-net shape fabrication, low contents of organic monomers, high sintered density, and ease of machinability owing to the intensity of homogeneity and high strength (Nojoomi et al., 2014) [10]. Use of nonporous metal or plastic molds that are reusable makes this process economical (Omatete et al., 1991 andOmatete et al., 1997) [8,9].. Gelcasting has more advantages such as the products produced are consistently defect free, uniformly dense and very strong, and able to form very large parts compared with other forming processes. Gelcasting is attractive for fabricating complex shapes such as radomes, turbine rotors, gears etc. Response surface methodology (RSM) is defined as "a collection of mathematical and statistical techniques useful for the modeling and analysis of problems in which a response (output variable) of interest is influenced by several variables (input variables) and the goal is to optimize the responses that are influenced by the input process parameters" (Montgomery, 2012) [20]. Originally, RSM is developed to model experimental responses and then migrated into the modeling of numerical experiments (Box and Draper, 1987) [21]. They can be applied for modeling and optimization of any engineering problems. Sufficient data is gathered through the experimental design layout and mathematical models for the desired responses as a function of selected variables were developed by applying the multiple regression analysis on the experimental data [22].

Materials and Methods
Commercially available SiO2 (M/s. Ants Ceramics Pvt. Ltd., Thane, India) with an average particle size of 1-5 μm, and Si3N4 (M/s. Denka Company Ltd., Japan) with an average particle size of <10 μm are used in this study. Deionized water is used as a solvent. Commercially Diluted Nitric acid and Ammonium hydroxide (both S.D. fine chemicals, India) were used for pH adjustment.

Slurry preparation
Gelcasting of SiO2 and Si3N4 ceramic composite was carried out at various solid loadings using different monomer content and monomers ratio. A premix solution was prepared by mixing dispersant (1 wt % of monomer content), surfactant (3 wt % of monomer content), monomers MAM and MBAM (10-15 wt% of solid loading) in distilled water by magnetic stirring. SiO2 was added to the premix solution and then Si3N4 was added in regular intervals for fabrication of SiO2-Si3N4 ceramic composite by gelcasting varied from 5 to 15 wt. % of solid loading and stirred for about 6 hrs and pH of the slurry was adjusted to 11 using diluted Nitric acid and Ammonium Hydroxide. Figure 1. A detailed flowchart of gelcasting process of concern ceramic composite The slurry was deaired for 15-20 min and then the initiator APS and catalyst TEMED (1 wt % of monomer content) is added for initiation and polymerization. Finally, the slurry was cast into a glass mold and after the monomers had polymerized, the green bodies were demolded. The samples were then dried in a controlled humidity oven for 24 h. Then binder burnout was carried out in a high-temperature furnace at 600 °C for 1 h with a heating rate of 2 °C/min. Then the samples were sintered at 1250 °C with a heating rate of 4 °C/min for 3h in Nitrogen atmosphere.
The detailed flowchart of gelcasting process for the preparation of ceramic composite concern i.e. SiO2-Si3N4 shown in Figure 1. Sintered samples of SiO2-Si3N4 ceramic composites were shown in Figure 2.

Bulk density and apparent porosity
Bulk density is defined as "the ratio of mass to volume that includes the cavities in a porous material". Archimedes principle states that "when an object is partially or fully immersed in a fluid it experiences an upward force that is equal to the weight of the fluid displaced by it".

Dielectric constant and loss tangent
Dielectric constant is defined as "a quantity measuring the ability of a substance to store electrical energy in an electric field".
The dielectric constant can be calculated by Eqn. 4 by measuring the capacitance: Where K is the dielectric constant; r  is relative permittivity, c is the capacitance; d is the thickness of the specimen; A is the area of the cross-sectional surface;  is permittivity of the medium; 0  is permittivity of free space or vacuum.

Zeta potential and pH value
The zeta potential of SiO2-Si3N4 particles in the slurry is studied by varying the dispersant (Darvan 821A) in the range of 0-0.75 wt% as a function of pH value and is depicted in  The pH value of slurry also play an important role on the rheological properties It is observed that high absolute zeta potential value is obtained by increasing the pH value of the slurry which increases the dispersibility of the slurry. The formation of hydroxide layer on the surface of SiO2 particles is eliminated by the addition of Si3N4 particles.

Solid loading
Higher solid loading of SiO2-Si3N4 slurry is required to improve the mechanical properties of gelcast parts. The solid loading of the slurries are varied from 42 to 50 vol% in which Si3N4 content varies from 5 to 15 wt% and remaining is SiO2. But viscosity increases as the solid loading increases due to agglomeration at higher solid loading which make the slurry difficult to pour into a mold for casting. This is due to flocculation and coagulation caused by the reduction of solvent (water) present in between the ceramic particles. Hence, slurry with 50 vol% solid loading and 15 wt% Si3N4 content is suitable for casting into the mold. The variation of viscosity as a function of the shear rate ranging from 0.1-100 s -1 for slurries of 50 vol% solids loading and with Si3N4 content varying from 5 -15 wt% is shown in Figure 5. Pa.s with an increase in Si3N4 content from 5 to 15 wt% and the gel exhibits shear-thickening behavior. The increase in viscosity is due to the agglomeration at high solid loadings.

X-Ray diffraction analysis and Microstructure
X-ray diffraction patterns of SiO2-Si3N4 ceramic composite sintered at various temperatures are shown in Figure 6. X-ray diffraction analysis is done both on raw ceramics and ceramic composite sintered at 1250 ºC. It can be seen that at 1250 ºC, SiO2 is transformed into cristobalite phase.   Table 1.

Regression model for porosity
A regression model for the porosity of SiO2-Si3N4 ceramic composite is fitted using the experimental results. ANOVA has been applied on the experimental results for porosity and the ANOVA results are given in Table 2.
In Figure 9 (a), it can be seen that the porosity of sintered body decreases with the increase in solid loading. The ceramic particles are compacted, reducing the pores in the composite with an increase in solid loading. This densifies the ceramic composite. From Figure 9 (b), it is observed that Si3N4 content has a significant effect on the porosity. As the Si3N4 content increases there is a rise in the porosity and then declines on further addition of Si3N4 powder.
The rise in the porosity is due to the loose packing of Si3N4 particles in SiO2 ceramics. As the content of Si3N4 powder increases the pores in the composite are reduced that increases the density of the composite causing the lower porosity.
The porosity of sintered body increases with monomer content as shown in Figure 9 (c).
The pores of sintered body largely initiate from the left over micro-space of the organic polymers in the green body during organic binder burnout. The distribution and intensity of pores depend on the monomer content. Thus more the monomer content more is the porosity in the ceramic body. Figure 9 (d) show the decrease in porosity with the increase of monomers ratio. Higher porosity is obtained on further increase in the monomer ratio. The increase in the ratio of monomers to cross-linker sometimes initiates micro-crack propagation that causes higher porosity. The interaction effects of monomer content and ratio of monomers can be seen in Figure 9 (e).

Regression model for dielectric constant
A regression model for dielectric constant of SiO2-Si3N4 ceramic composite is fitted using the experimental results. ANOVA has been applied on the experimental results for dielectric constant and the ANOVA results are given in Table 3.     Table 4. The aim of optimization is to evaluate the best set of inputs for maximization of flexural strength and porosity, and minimization of dielectric constant. This is indicated by the desirability of RSM analysis. The optimum value of responses is to be chosen for maximum desirability index for various sets of inputs. A set of 10 optimal solutions is derived and tabulated in Table 5 for the particular set of input range ( Table 4) The overall desirability of the responses is found to be 0.803.

Conclusions
The following conclusions were drawn from the present work  Fused Silica (SiO2) based ceramic composites SiO2-Si3N4 have been successfully produced using gelcasting method.
 The rheological behavior of the SiO2 suspensions including SiO2-Si3N4 by varying dispersant content, pH value and solid loading has been thoroughly studied and useful ranges of solid loading (SL), monomer content (MC), ratio of monomers (RM) and Si3N4 contents were decided based on initial experiments and they are as follows.