2.1 Material selection
Agricultural wastes such as sugarcane extracts, peanuts shells, corn cob, and rice husks (Fig. 1) are used for producing silicon carbide (SiC) by pyrolysis process due to their rich silica content. An industrial microwave oven is used to perform the pyrolysis process, for which argon gas is used to create an inert condition that makes the industrial microwave oven chamber free from oxygen. Table 1 provides the chemical content of the rice husk, sugar cane extracts, peanut shells, and corn cob used in this work. Rice husk contains about 75% organic volatile matter (Fig. 1a). Rice husk ash (RHA) contains silica about 85 to 90%. Sugarcane bagasse ash (SCBA) is having a silica content of 96.93% (Fig. 1b). Peanuts are widespread in the tropics and subtropics region and are essential for both small and large commercial producers. The inedible outer covering which is called Peanut shells (Fig. 1c) containing 56.4% of Silica content. Corn cob (Fig. 1d) is composed of cellulose and lignin and also having significant elements such as silicon, calcium, and aluminum. Relatively high silicon content was contained in corn cobs. Corn cob ash (CCA) contains a silica content of 96.6%.
2.2 Sample preparation
Silicon carbide (SiC) powder of particle size 1 µm was produced by pyrolysis process using agricultural wastes such as sugarcane extracts, peanut shells, corn cob, and rice husks which were combined to make powder mixture by grinding machine. This mixture was kept in an industrial microwave oven at a different temp of 160 oC, 170 oC, and 180 oC for a different time duration of 160 min, 170 min, and 180 min respectively. Powdered SiC obtained was kept in a vacuum heat furnace for the sintering process to convert into a solid form. A total of nine samples of each mass 15 g and size of 30 x 5x 3 mm are selected for fracture toughness testing.
2.3 Characterization Techniques
X-ray diffraction (XRD) tests were used to analyze the XRD spectra of powdered SiC by Rigaku 200 B X-ray diffractometer (40 kV and 100 mA). It was recorded from 5 to 90° with a speed of 5°/min. Raman spectroscopy was performed from 200-1300 cm-1 using Raman spectrometer Model- Enspectr R532 to analyze the raman spectra of SiC. Further, the Fourier transform-infra red (FTIR) spectroscopy JASCO 6300 instrument was used to analyze FTIR spectra of the SiC. powdered SiC were mixed with KBr in a ratio of (1:100) by mass for making compressed pellets. It was performed from 450-4000 cm-1 range with 4 cm-1 resolutions and a total scan of 16 for each sample. The morphological structure of powdered SiC was observed by scanning electron microscope (SEM) using MIRA3 LMH microscope operating at 10kV, 10 mm WD, and 30 Pa chamber pressure.
2.4 Indentations fracture (IF) Technique
In this research, the indentations fracture (IF) method is used for calculating the fracture toughness of sintered SiC. Fractured halves of the specimen used for this method are shown in Fig. 2 The diagonal length (2a) and crack length (2c) were recorded for each impression. This fracture toughness test was carried out by INNOVA TEST NEXUS 4303 micro Vickers hardness tester at a load of 290 N for 10s. Crack length (c) and indentation diagonal length (a) on each sample of sintered SiC was measured. The fracture toughness values were calculated by following equation 2 [19].
2.5 Design of experiment (DOE)
In general, traditional experimental design techniques require a large no of experiments which increases the cost. To avoid this, Taguchi experimental design technique is used which has orthogonal arrays to analyze the process parameter with fewer experimental runs [22]. For the design of the experiment, MINITAB software is used. Taguchi technique is used to investigate the impact of the variables and their interactions on process performance [23]. Taguchi's method of the experiment is one of the most popular and useful statistical methods which is used to analyze the influence of more than one variable and its interactions on the process performance. For each parameter, three levels were chosen and an orthogonal array of L9 (33). For each element, the S/N ratios and average responses were plotted against each of its levels in the graphical method. The S/N ratio is the key quantity that has to be determined to achieve an optimal solution like the experiment [24]. Signal to noise ratio is used to determine the mean response for each experiment. The "larger the better" was selected to find the ratio S/N is given in equation 1.
Three process parameters such as heating temperature (0C), time (min), and quantity of agro-waste (g) were taken to perform the pyrolysis process. The sintering process was conducted in a hot press furnace by varying parameters such as heating rate (0C/min), cooling rate (0C/min), and inside chamber pressure (MPa) (Table 2).