Ceramic composites from the polysilazane with the addition of silicon carbide (SiC) was successfully sintered by a hot pressing furnace at 1,600-1,800 ºC for 1 h under the applied pressure of 20 MPa in a nitrogen atmosphere. The addition of SiC did not influence the densification of sintered polysilazane. However, the phases generated during sintering were affected by the addition of SiC, resulting in the multi-phases of sintered polysilazane/SiC composite. The formation of SiC, Si₃N₄, Si₂N₂O, and SiO₂ phases were observed in the sintered polysilazane/SiC composite. The highest hardness of sintered polysilazane was obtained with the addition of SiC sinter at 1,700 ºC, i.e., 24.3 ± 1.1 GPa, and decreased to 21.8 ± 0.7 GPa at 1,800 ºC. This is due to the sintered polysilazane/SiC composite at 1,700 ºC possess fine microstructure compared to 1,800 ºC. Meanwhile, the sintered polysilazane experienced significant grain growth at 1,700 ºC, thus the hardness of this sample was lower than sintered polysilazane/SiC composite.

Keywords: Polysilazane, Silicon carbide, Sintering, Phase, Microstructure

Polysilazane has been used extensively to synthesize Si-based ceramic composites, such as Si₃N₄ and SiC. In general, there are two steps for the sintering of ceramic composites from the polysilazane [1-7]; first the polysilazane was cross-linked and pyrolyzed at temperature 1,000-1,600 ºC to obtain the amorphous powder. Subsequently, the amorphous powder was sintered at high temperatures to obtain a sintered body. The phase that generates during sintering depends on the sintering additives, sintering temperature, and sintering atmosphere that used for the densification of amorphous polysilazane. Moreover, the addition of sintering additives is still required to obtain the dense sintering body from polysilazane [1-3, 5, 6, 8] due to the high covalence bonding and low self-diffusivity of Si-based ceramic, such as Si₃N₄ and SiC, make them difficult to sinter to a high density. For instance, the sintering temperature of 2,500 ºC and applied pressure are needed to densify SiC [9]. The other study reported the use of ultra-high pressure to achieve SiC with density near the theoretical study [10]. Liquid phase sintering is believed as the mechanism for the densification of amorphous polysilazane due to the presence of oxide sintering additives. However, several studies showed that dense ceramics body from polymer precursor could be densified without the addition of sintering additives, which used mechanical milling to activate the amorphous polymer precursor [11-13].
SiC is a ceramic material that widely uses for the application at elevated temperature due to excellent thermal and mechanical properties, such as heat exchanger, gas turbine, and space shuttle [14, 15]. SiC composite is also considered as a structural material for nuclear reactor because of the low neutron irradiation of Si and C. In daily life, SiC is used as abrasive materials due to the high hardness of this material, which suitable for application as wear-resistant materials. SiC is also a promising material for application as electronic devices due to excellent electronic properties [16]. However, the addition of SiC into polysilazane is rarely reported, apart from the work of Noviyanto et al. [8] who reported the addition of SiC into polysilazane (weight ratio = 1) and studied the effect of sintering atmosphere on the microstructure and phase of the sintered ceramic.
In the present study, we investigated the effect of SiC addition on the phase formation and microstructure of Si-based ceramics from polysilazane prepared by a hot-pressing furnace. Three different sintering temperatures were used in this study. The data on phase, microstructure, and relation with the hardness of sintered polysilazane are examined to elucidate the effect of SiC in the polysilazane.

Polysilazane (KiON Ceraset Polysilazane 20, USA) was dissolved in acetone and mixed with β-SiC powder (> 97.5% purity, 4620KE, NanoAmor Inc., USA) with a weight ratio of 5:1. The slurry was dried and subsequently cross-linked on a hot plate at 200 ºC for 90 min. The cross-linked powder was pyrolyzed at 1,300 ºC for 2 h in a nitrogen atmosphere using a tube furnace to form an amorphous powder. According to our previous study, the crystallization of amorphous powder starts at 1,450 ºC [12]. Al₂O₃ (> 99.9% purity, Baikowski, Japan) and Y₂O₃ (> 99.9% purity, Acros Organics, USA) were used as sintering additives with the ratio were 60:40 in weight. Sintering additives were mixed with the pyrolyzed powder using ball milling for 24 h. The number of sintering additives was 5 wt.% in the total mass of mixed powder. The mixed powder was sintered at 1,600, 1,700 and 1,800 ºC for 1 h under an applied pressure of 20 MPa in a nitrogen atmosphere using a hot pressing vacuum furnace. Polysilazane without the addition of β-SiC powder was also prepared for the comparison purpose.
Archimedes principle was used to estimate the density of sintered powder. The microstructure of sintered bodies was observed by scanning electron microscope (SEM: S-4800, Hitachi, Japan). The crystalline phases that formed during sintering were detected by room-temperature X-ray diffraction (XRD: X’Pert-PRO MPD, PANalytical, The Netherlands) using the Cu Kα line. The samples were mirror-polished before measuring the hardness. The Vickers indentation method using a hardness tester (MVK-H1, Mitutoyo, Japan) with a 1 kg load and a loading time of 10 s were used to determine the hardness of sintered samples. 

Fig. 1 displays the XRD patterns of polysilazane/SiC after sintering using the hot pressing furnace at different temperatures. SiC in the form of β-SiC and α-SiC were detected at the 1,600 ºC (Fig. 1(a)). In terms of stability at high temperature, the transformation of β-SiC to α-SiC occurs at temperature > 1,600 ºC [17], however, in our study, the formation of those phases appears simultaneously, as shown in Fig. 1(a). Comparing with polysilazane without the addition of SiC (Fig. 2(a)), it showed that the presence of α-SiC is dominant compared to β-SiC. Therefore, the occurrence of β-SiC in the polysilazane/SiC composite sinter at 1,600 ºC originates from the β-SiC powder since the polysilazane tend to form α-SiC instead of β-SiC. The other phases that exist at 1,600 ºC were Si₂N₂O, SiO₂ and β-Si₃N₄. The origin of those phases came from the polysilazane, since the polysilazane consists of Si, C, N, and O [18], which might form SiC, Si₃N₄, SiO₂. Fig. 3 shows the Gibbs free energy of SiC, Si₃N₄, SiO₂ as a function of temperature. At 1,600 ºC, the Gibbs free energy of SiC, Si₃N₄, and SiO₂ were -50.981, -34.605, and -576.02 kJ mol^-1, respectively. Therefore, the formation of SiO₂ occurred in the beginning. Subsequently, Si₃N₄ started to form after the formation of SiO₂ completed, as observed in Fig. 1(a). However, at a temperature higher than 1,400 ºC, SiC is more stable than Si₃N₄, as shown in Fig. 3. The decomposition of Si₃N₄ to SiC can be suppressed by the use of a nitrogen atmosphere during sintering [19]. Therefore, the existence of Si₃N₄ at sintering temperature > 1,600 ºC, as shown in Fig. 1(b) and (c), due to the sintering atmosphere that used in the present study. Interesting to note is that the formation of Si₂N₂O, which is the result of Si₃N₄ and SiO₂ reaction [8, 20]. It seems the formation of Si₂N₂O complete at 1,700 ºC due to the disappearance of SiO₂ peak at 1,700 ºC, as shown in Fig. 1(b). The phase of polysilazane/SiC composite sinter at 1,700 and 1,800 ºC is similar, as shown in Figs. 1(b) and (c), respectively.
Fig. 2 shows the XRD pattern of the sintered polysilazane at different temperatures. The crystalline phases that detected in sintered polysilazane were β-SiC, α-SiC, α-Si₃N₄, and β-Si₃N₄. At 1,600 ºC (Fig. 2(a)), besides the formation of α-SiC, the other phases that detected were α-Si₃N₄, and β-Si₃N₄. Initially, α-Si₃N₄ formed in the beginning. However, since the α-Si₃N₄ is not stable at high temperatures [21-23], it transformed into β-Si₃N₄. Therefore, the presence of α-Si₃N₄ was not identified at 1,700 ºC, indicates that all α-Si₃N₄ had already transformed into β-Si₃N₄, as shown in Fig. 2(b). Moreover, Fig. 2(b) revealed the existence of β-SiC at 1,700 ºC compared to 1,600 ºC, as indicated by the peak at 35.579º.
Table 1 shows the density and hardness of the sample sinter at different temperatures. It is clear that the density of all samples increased with increasing temperature. It seems the sintering temperature of 1,600 ºC was too low for densification, both for the polysilazane with and without SiC, resulting in a density of 2.41 and 1.73 g/cm³ for the polysilazane/SiC and polysilazane, respectively. However, at 1,700 ºC the density of both samples increased significantly to 3.03-3.05 g/cm³, corresponding to the relative density of 93-95%, if compared to the theoretical density of SiC. Moreover, the relative density higher than 98% was obtained at 1,800 ºC, equal to the density of 3.15-3.17 g/cm³. These results are in agreement with the microstructure analysis, as shown in Fig. 4. The pores were observed in the polysilazane/SiC composite (Fig. 4(a)) and polysilazane (Fig. 4(b)), which indicated a low dense sintered body. However, the particle size of polysilazane sinter at 1,600 ºC is much finer compared to polysilazane/SiC. Since the starting material of polysilazane is an amorphous powder, therefore, the crystallinity of this powder depends on the sintering temperature. As shown in Fig. 2(a), the crystallinity of sintered polysilazane is quite low, indicates that the sintered body in the first stage to form the crystalline phase, resulting in the very fine microstructure. On the other hand, the particle size of polysilazane/SiC was observed very clear. This particle was β-SiC powder, which is intentionally added into polysilazane. Moreover, the peaks of β-SiC were detected even though at 1,600 ºC, as shown in Fig. 1(a). The tremendous change in microstructure was noticeable at 1,700 ºC, as shown in Figs. 4(c) and (d). In particular, for the sintered polysilazane, the particle size increases from the very fine at 1,600 ºC to several hundred nanometers at 1,700 ºC, even though the distribution of the grain size broadens. In contrast, the sintered polysilazane/SiC composite did not show a significant change in the grain size. However, the dense body was obtained at 1,700 ºC, as shown in Fig. 4(c). Furthermore, significant grain growth occurred at 1,800 ºC for both polysilazane/SiC composite (Fig. 4(e)) and polysilazane (Fig. 4(f)).
The hardness of the samples is shown in Table 1. The hardness of polysilazane/SiC composites at 1,700 and 1,800 ºC was 24.3 ± 1.1 and 21.8 ± 0.7 GPa, respectively. Meanwhile, sintered polysilazane at 1,700 and 1,800 ºC was 23.8 ± 0.9 and 23.2 ± 0.2 GPa, respectively. The difference of hardness polysilazane/SiC composite and polysilazane at 1,700 ºC is more likely due to the difference in microstructure, which is the microstructure of polysilazane/SiC composite finer than polysilazane following the Hall-Petch equation. Moreover, the phases have also affected the hardness, while the major phases significantly contributed to the hardness of samples. Indeed, SiC plays an important role in the hardness of sintered samples.

The ceramic composite of polysilazane/SiC was successfully sintered by a hot pressing furnace for 1 h under an applied pressure of 20 MPa in a nitrogen atmosphere. Multi-phases, such as SiC, Si₃N₄, Si₂N₂O, were detected by XRD at any sintering temperature, while SiO₂ only observed at 1,600 ºC. The disappearance of SiO₂ at sintering temperature > 1,600 ºC was likely due to the reaction with Si₃N₄ to form Si₂N₂O. On the other hand, the sintered polysilazane without the addition of SiC showed only SiC and Si₃N₄ phases at any temperature. The density obtains after sintering for the polysilazane/SiC composite was 2.41, 3.03, and 3.15 g/cm³ for the sintering temperature of 1,600, 1,700, and 1,800 ºC, respectively. Similar results are also achieved for the sintered polysilazane without the addition of SiC. However, the microstructure generates after sintering for the polysilazane/SiC composite significantly different compared to polysilazane, in particular for the sintering temperature of 1,600 and 1,700 ºC. The existence of pores was observed for both samples at 1,600 ºC, however, sintered polysilazane possesses a very fine microstructure. Furthermore, the tremendous increase in density and grain size was observed in the sintered polysilazane at 1,700 ºC. In contrast, the sintered polysilazane/SiC composite only increases the density at 1,700 ºC without a significant increase in the grain size. As the results of fine microstructure, polysilazane/SiC composite has the highest hardness, i.e., 24.3 ± 1.1 GPa, among the other samples.

This research was funded by a grant from the INSINAS 2019 (No. 30/INS-1/PPK/E4/2019), the Ministry of Research, Technology, and Higher Education, Republic of Indonesia.