Fabrication, Microstructural, and Mechanical Behavior of SiC Composite with Insitu Formation of BN and Si3N4

Dealing with high-temperature properties and brittleness and is always one of the hot topics in the field of ceramic materials. Extensive research has been carried out for it and Si3N4-SiC composites are one of the most promising composites. In this study, the impact of Si3N4 and BN reinforcement on the microstructure and physio-mechanical properties of SiC-based composites were explored. Two sets of composites, including Si3N4-SiC and BN-Si3N4-SiC aided with Al2O3, were fabricated using nitridation of Si and B2O3 at 1450 °C. The mechanical properties, microstructure, phase transformation, densification, and porosity were depicted and discussed. X-ray diffraction patterns revealed the insitu formation of Si3N4 (both α and β phases) and BN in the final composite. The specimen with 10% Si and 10% B2O3 possessed the superior flexural strength (284.4 MPa) with excellent hardness (23.4 GPa). The incorporation of BN is found to enhance the mechanical properties especially hardness while overcoming the shortcomings of reaction-bonded Si3N4-SiC. The corresponding densification and strengthening mechanism were explained in this paper.


Introduction
Technological developments in aerospace, marine, nuclear, and refractory industries, among others, increase the demand for more robust materials [1][2][3]. Ceramics composites can meet such applications in harsh environments, but further advancements are necessary [4,5].
Silicon carbide (SiC) and Silicon nitride (Si 3 N 4 ) are known for their excellent thermal and mechanical properties. SiC exhibit strong wear, creep, and oxidation resistance at higher temperatures but has a lesser fracture toughness [6]. Si 3 N 4 ceramics, on the other hand, have more substantial flexural strength and fracture toughness but weaker oxidation resistance at extreme temperatures [7,8]. Si 3 N 4 -SiC composite prepared using Si 3 N 4 powders has been widely reported with complex and expensive sintering processes such as SPS [8], Hot-pressed sintering at 1850 °C [9], sintering-post-HIP at around 2000 °C [10]. On the other hand, using Si powders instead of Si 3 N 4 for synthesizing Si 3 N 4 -SiC composites can have economic benefits due to the low cost of the Silicon powder. Thus, the nitridation of silicon in controlled environment furnaces is a suitable method for producing reaction bonded silicon nitride-silicon carbide composites. Direct nitridation of Si powder at 1414 °C (Melting point of Si) could produce a reaction bonded Si 3 N 4 -SiC composite. Additionally, reports suggest SiC-Si 3 N 4 reaction bonded composites fabricated at 1450 °C via pressure-less sintering have better densification with improved overall properties [11,12].
The sintering of Si 3 N 4 -SiC composite is challenging without any additives due to Si's high covalence and low diffusivity [13]. Also, an effective method for further enhancing the high-temperature properties is by promoting the amorphous phase crystallization and increasing the refractoriness of the grain boundary phase [14]. This could be achieved by adding appropriate sintering aids in the composite. MgO, AlN, Y 2 O 3 , Al 2 O 3 , and oxides of some rare earth have been reported as a sintering aid in the hybrid composite system [15][16][17][18][19]. Al 2 O 3 has been known as the most cost-effective in comparison with rest while improving oxidation resistance and reducing linear shrinkage. Al 2 O 3 forms a glassy phase on interacting with SiO 2 formed during the oxidization of Si 3 N 4 and SiC, which shields against further oxidation and corrosion [20][21][22][23].
Reaction-bonded Si 3 N 4 tends to have a porous structure which degrades its mechanical properties and oxidation resistance. To overcome this, B 2 O 3 can be added to the composite, whose nitridation forms boron nitride (BN). BN has excellent hardness, thermal shock resistance, and machinability, which would overcome the shortcomings of Si 3 N 4 -SiC composites [24][25][26].
The development of SiC composites with in-situ formation of BN and Si 3 N 4 is a novel and important topic in the field of advanced ceramics. SiC is a well-known engineering material with excellent mechanical and thermal properties, but its brittle nature limits its application in certain areas. The addition of BN and Si 3 N 4 can enhance the fracture toughness and thermal shock resistance of SiC composites, thereby extending their range of applications. In-situ formation of these phases during the processing of SiC composites eliminates the need for secondary processing steps, making the process more efficient and cost-effective. This topic is being actively researched and several recent studies have reported promising results, indicating the potential for SiC composites with in-situ formation of BN and Si 3 N 4 to be used in high-temperature applications. Overall, this topic holds great potential for developing advanced ceramics with improved properties and wider application ranges. The purpose of the current investigation is to examine the effect of Si and B 2 O 3 nitridation in the SiC composite aided with Al 2 O 3 . Also, the consequences of the formation of Si 3 N 4 and BN in SiC composite on microstructural and physiomechanical properties have been discussed.

Materials
Commercially available raw materials that were utilized and their information attributed to purity, particle size, and supplier are exhibited in Table 1. The Grading method was used in SiC to increase densification and reduce the overall porosity [27]. This grading was done using 30% 80/180 SiC, 30% 220F SiC and 40% 1 µm SiC. These materials were used for different ceramic composites according to the composition indicated in Table 2.

Ball Milling
The above-mentioned raw materials in Table 1 were accurately weighted using a precise digital scale and then were dry mixed in different volume fractions in a ball mill for 4 h. During milling, a small amount of ethanol, 3% by weight, was added to the ball mill to prevent contamination and agglomeration of the powders. The ball milling had a ballto-powder ratio of 1:10, with a cyclic interval of 15 min milling and then 15 min halt to prevent overheating and possible oxidation.

Cold Isostatic Pressing
The milled powders were mixed for 5 min in an agate mortar pestle with a few sprinkled droplets of binder (Dextrine). This was then isostatically pressed through a die in a bar with a dimension of 40 × 10 × 10 mm, with a pressure of 200 MPa. This green compact was dried in a vacuum oven overnight and then sintered.

Sintering
For proper nitridation, the whole sintering of the prepared sample was carried under N 2 pressure of 0.1 MPa. The heating schedule for the samples is at 5 °C/min till it reaches 1000 °C, and post that 3 °C/min till 1450 °C. After holding the samples at 1450 °C for 1 h, they were gradually cooled. The heating rate, cooling rate, and cooling medium can affect densification and result in unwanted phases. The temperature was optimized for the formation of α-Si 3 N 4 since at higher temperatures, α to β phase transformation of Si 3 N 4 occurs more rapidly [28]. The prepared sintered composite samples are depicted in Fig. 1.  A simplified chemical reaction that occurs between the precursors is shown in Eq. (1). Formation of Si 3 N 4 and its α to β phase transformation are denoted in Eqs. (2) and (3). Due to low melting point, B 2 O 3 liquidities first and upon reacting with Silicon nitride forms boron nitride and silicon dioxide. This can be seen in Eqs. (4), (5), and (6).

Phase Evolution
The phases present in the prepared composite were determined by X-Ray Diffraction (Rigaku Miniflex 600 Desktop XRD System) equipped with Ni filtered Cu kα (λ = 1.5417 A o ) using 2 o per minute scan rate, 0.02 o step size within 2θ of 5 o to 90° range. Further, to evaluate the phases present, Rietveld refinement was carried out using the Full-Prof program suite in Match! Software.
Fourier transform infrared (FTIR) spectroscopy was obtained with Thermo electron Scientific instruments LLC (Nicolet iS5) equipment to examine the nature and presence of different functional groups in the wavelength range of 4000-400 cm −1 . (1)

Microstructural analysis
The morphology and microstructure examination of the samples was carried out using a High-Resolution Scanning Electron microscope (HR-SEM, Nova Nano SEM 450, FEI USA). To gain insight into the elemental composition, EDX was conducted using energy-dispersive X-ray spectroscopy (Team Pegasus integrated EDS-EBSD with octane plus and Hikari pro). The hardness indent SEM analysis was done using EVO SEM MA15/18 (Carl Zeiss microscopy ltd.). For all the SEM analysis, the sintered samples were first fine polished and then ultrasonicated in ethanol, followed by thermal etching in an oven at 100 °C overnight. The samples were then gold coated using Au sputtering for 120 s.

Apparent Porosity and Bulk Density
The sample's apparent porosity and bulk density were determined using the Archimedes principle as per the international standard ISO 5017. At first, the dry weight of the prepared samples was measured, following which the sample was suspended in water, and then the soaked weight of the samples was measured. The mathematical calculation for apparent porosity and bulk density were determined using Eqs. (7) and (8), respectively.

Hardness and Flexural Strength
The hardness of the prepared composite samples was measured by the indentation method. The Vickers hardness tester (Bareiss Digi-test, VTP-6046) has been used at a load and dwell time of 10 kgf and 10 s, respectively. The flexural strength of composite samples was determined using a three-point bend test in an Electromechanical universal testing machine (HIECO HL 591) with a capacity of 10 kN.

Effect of Si 3 N 4 on SiC Composite
To examine the influence of Si 3 N 4 on the SiC composite, varying percentages of Si were introduced in the samples denoted as SN1, SN2, SN3, and SN4, indicated with compositions in Table 2. The conversion and grain growth mechanism occurring during the nitridation densifies the overall composite (Depicted in Fig. 2). The brown spherical and rod-like structure denotes α and β phases of Si 3 N 4 , respectively.
In the present sintering condition, both α and β phases of Si 3 N 4 can be formed due to increased α to β phase transformation of Si 3 N 4 at a temperature above 1400 °C. XRD of the sintered samples SN1 to SN4, shown in Fig. 3, was confined to a range of 20° to 26° so that the changes in peak intensity of both α and β-Si 3 N 4 could be visually distinguished. Further, Based on the intensity ratios of the reflection peaks of (101) and (210) in the β phase and (102) and (210) in the α phase, the relative quantities of α and β-phase were measured and estimated using the Gazzara and Messier method [19,29].
The obtained results of the samples are listed in Table 3. As the percentage of Si increased the α phase content of the samples appeared to be in decreasing order, which is due to the rapid phase formation of β-Si 3 N 4 at scattered locations. This β-Si 3 N 4 acts like a seed for more β-Si 3 N 4 formation while suppressing the formation of α-Si 3 N 4 . The peak intensity of α-Si 3 N 4 is also clearly seen in XRD Fig. 3 to be decreasing as Si content increases.

Microstructural Analyses
The addition of Si 3 N 4 can affect the microstructural and mechanical properties of prepared composite samples. The SEM images of sintered SN1, SN2, SN3 and SN4 composite samples are represented in Fig. 4. All samples illustrate good densification with tiny whitish particles denoting the   SiC can also be seen in various shapes and sizes, resulting from different grades of SiC. The samples SN1 to SN4 show an increasing order of rod-like structure, which represents β-Si 3 N 4 structure. Also, it can be seen that the size of this rod structure is also increasing, which is due to the seeding behavior of β phase (Fig. 5). The energy dispersive x-ray (EDX) analysis confirms the constituent elements in the composite system and supports the XRD analyses result. Figure 6 shows SEM image of the nucleation and growth process of elongated rod-shaped β-Si 3 N 4 . Once β-Si 3 N 4 is formed, it encourages the α to β-Si 3 N 4 transformation while behaving like a seed triggering grain growth to form micrometer-sized grain.

Physio-mechanical Characterization
The sample's bulk density and apparent porosity were measured, and Fig. 7 represents the data with varying Si content i.e., SN1 to SN4. The bulk density slightly increased from 2.6149 to 2.6845 gm/cm 3 owing to the increase in Si 3 N 4 formation. The apparent porosity also reduces from 21.85 to 16.92% due to the Si 3 N 4 filling voids mechanism, as illustrated in Fig. 2. Figure 8 depicts the hardness and flexural strength of the sintered SN composite series. The composite SN2 with 10% Si has the highest flexural strength of 214.4 MPa, while SN3 with 15% Si shows the highest hardness of 19.2 GPa. Microstructural properties such as phases, grain size, and porosity should be the main contributors in increasing the hardness and flexural strength. The later decline result from increased β -Si 3 N 4 formation and its elongated grain.

Fourier transform infrared spectroscopy (FTIR) analyses
The presence of functional groups and structural bonds in the prepared composite system was investigated using FTIR spectra. Figure 10 represents the FTIR spectra of the SNB3 composite. The characteristic peaks observed at ~ 401, 437, 460, 503, 570, and 650 cm −1 correspond to alumina [30]. The peaks near 570 and 670 cm −1 represent the stretching vibration of AlO 6 octahedra [31]. The intensity of these peaks increases with the increase in the composition of Al 2 O 3 . The Si-Si stretching mode was observed at 685 cm −1 . The bands observed at 785 and 830 cm −1 correspond to Si-C [32]. The symmetric and asymmetric stretching of Si-N was observed at 480 and 840 cm −1 [33]. The symmetric Si-N stretching has three Si atoms bonded   Figure 11 shows the SEM micrographs of sintered SNB1, SNB2, SNB3, and SNB4 composite samples. SEM images demonstrate good densification, which confirms proper sintering. A spherical shape α-Si 3 N 4 is seen surrounding the SiC. The rod-like structure representing β-Si 3 N 4 is randomly distributed with long elongated grain growth. The energy dispersive x-ray (EDX) analyses confirm the constituent elements in the composite system, which is in range with the Rietveld refinement results. EDX can only detect elements with atomic numbers higher than boron, so EDX results have the remaining constituents apart from Boron (Fig. 12).

Physio-mechanical Characterization of SNB Samples
The physio-mechanical properties, such as bulk density, apparent porosity, hardness, and flexural strength of the SNB samples, were measured post sintering. Figures Fig. 15. It is observed that the depth of indent in samples; SNB1 has maximum and SNB4 demonstrate minimum indent depth.

Conclusion
A method was proposed to prepare the insitu formation of BN and Si 3 N 4 on SiC-based composite with Al 2 O 3 as a sintering additive. The following conclusions were drawn: (1) Insitu formation of Si 3 N 4 and BN with nitridation of Si and B 2 O 3 was successfully achieved and can be used as a cost-effective alternative with improved thermomechanical properties. It is also processed at a much lower temperature, providing a more viable and environment-friendly fabrication.