The effects of β-Si3N4 on the formation and oxidation of β-SiAlON

Abstract The influence of the additive β-Si3N4 on the formation and oxidation of Si4Al2O2N6 during the sintering of Al, Si, and Al2O3 powders under flowing nitrogen atmosphere was examined. An increasing molar percentage of β-Si3N4 was shown to alter the morphology of Si4Al2O2N6 from a fiber-like to a rod-like structure and also shortened the time needed to form a dense, continuous oxide layer, which served as a barrier to the diffusion of O2. An optimal molar percentage of β-Si3N4 of 29.9 mol% was discovered, at which the grain growth was enhanced, and the surface area was, in turn, reduced, yielding superior resistance to oxidation. Our results provided a theoretical basis for the formation of β-SiAlON and demonstrated the potential of its use in high-temperature oxidizing environments.

1 Introduction β-Si 3 N 4 has a hexagonal close-packed crystal structure, which is composed of covalently bonded [SiN 4 ] tetrahedral subunits. The Si-N bond length in the crystal is 1.74 Å, which is similar to that of the Al-O bond in crystalline Al 2 [1][2][3][4], in which the Al-O bond is approximately 50% stronger than that in Al 2 O 3 . β-SiAlON is therefore more resistant to decomposition at high temperatures than Si 3 N 4 due to its lower vapor pressure and higher thermodynamic stability [5][6][7]. β-SiAlON exhibits the properties of both Si 3 N 4 and Al 2 O 3, such as excellent thermal shock resistance and mechanical properties and resistance to molten slag corrosion [8][9][10].
The influence of β-Si 3 N 4 on the formation and properties of Si 3 N 4 -based ceramics has been widely studied. Yu et al. prepared graded Si 3 N 4 ceramics with superior wear resistance and a low wear rate by combining two-step sintering and β-Si 3 N 4 seeds [11]. Meanwhile, Lukianova et al. reported that the electrical resistivity of Si 3 N 4 ceramics was linearly dependent on the content of β-Si 3 N 4 in the precursor [12]. Guo et al. studied the effects of β-Si 3 N 4 seeds on the nucleation and growth of Lu 2 O 3 -doped Si 3 N 4 ceramics and showed that seeds with a smaller diameter and a lower aspect ratio induced a finer self-reinforced microstructure, and, in turn, an improved fracture toughness [13]. A similar observation was made by Acikbas et al., but the resulting α/β-SiAlON displayed a poor oxidation resistance due to its inherent thermodynamic instability in an oxidizing environment [14]. It is generally accepted that a pure, dense Si 3 N 4 phase exhibits superior oxidation resistance to SiAlON-based ceramics [15].
The effect of additives on the oxidation resistance of SiAlON has also been widely explored. Li et al. found that β-SiAlON powder prepared using a combustion method displayed a weaker oxidation resistance with an increasing addition of diluents, which was attributed to a decrease in particle size [16]. Li et al. used a mixture of NH 4 F and NH 4 Cl additives to promote the growth of a Ca-α-SiAlON crystal with improved oxidation resistance [17]. The oxidation resistance was related to both the crystal phase and the microstructure and was optimized by adjusting the composition. Finally, Shan et al. reported that the oxidation resistance of Y-α-SiAlON was improved by increasing the nitrogen content (n = 1) and decreasing the Y/Si ratio (0.04) [18].
In this study, to the best of our knowledge, the effects of different β-Si 3 N 4 additions on the oxidation behavior of Si 4 Al 2 O 2 N 6 were explored for the first time. Si 4 Al 2 O 2 N 6 was prepared via high-temperature nitridation in the presence of various molar ratios of the additive β-Si 3 N 4 . The oxidation resistance was assessed using a nonisothermal oxidation test between room temperature and 1,500°C to determine the oxidation onset temperature. Meanwhile, the underlying oxidation mechanism was studied using an isothermal oxidation test between 1,200 and 1,400°C for 2 h. The study showed that the improvement in the oxidation resistance of Si 4 Al 2 O 2 N 6 by the addition of β-Si 3 N 4 will promote its further use in hightemperature industrial applications.
The microstructure of the fractured surfaces was characterized with SEM (Sigma HD, Zeiss, Germany), equipped with an X-ray energy-dispersive spectroscope (EDS; IE250X-Max50, Oxford, UK). Isothermal and non-isothermal oxidations of the Si 4 Al 2 O 2 N 6 powder were conducted on NETZSCH instrument (Setsys Evolution, STA 449 F3; NETZSCH Scientific Instruments Trading Co. Ltd, Germany). For the non-isothermal oxidation experiment, O 2 was injected into the furnace at a flow rate of 80 mL/min, and the Si 4 Al 2 O 2 N 6 powder was heated at a rate of 10°C/min from room temperature to 1,500°C. Based on the results of non-isothermal oxidation, 1,200, 1,300, and 1,400°C were chosen as isothermal oxidation temperatures. Initially, Ar was pumped through the vacuum, then the furnace was heated to the required temperature for 2 h, with a heating rate of 10°C/min. O 2 was injected at a flow rate of 80 mL/min; after completion of the study, the isothermal oxidation was terminated by purging the furnace of O 2 with Ar. Si 4 Al 2 O 2 N 6 grain sizes were assessed by SEM images using "Nano Measurer" software (Fudan University, Shanghai, China). At least 300 grains were counted to obtain average values and size distributions. The specific surface area and pore size distribution were tested using Brunner-Emmet-Teller (BET, ASIQMUTV00U 000-6, Quantachrome, USA) method.

Results and discussion
Initially, the thermodynamics of the reactions in the Si-Al-O-N system were briefly examined to delineate the formation of the Si 4 Al 2 O 2 N 6 phase [19,20]. To achieve this, overlapped phase stability diagram of Si-O-N and Al-O-N systems at 1,623 and 1,823 K, respectively, were     . All peak areas of Si 4 Al 2 O 2 N 6 increased as the initial β-Si 3 N 4 molar percentage increased. Both β-Si 3 N 4 and β-SiAlON have a hexagonal crystal structure. However, due to the inclusion of Al and O in β-SiAlON, the peaks indexed to the (210) plane were shifted to lower diffraction angles with increasing β-Si 3 N 4 molar percentage. This was attributed to an increase in substitution of Si-N bonds with moderately longer Al-O bonds, which increased d spacings of individual lattice planes and ultimately resulted in an increase in lattice parameters. It is known that the formation and mechanical properties of SiAlON ceramics is enhanced with an increasing molar content of β-Si 3 N 4 in the precursor [22]. Our results confirmed that the same effect was observed specifically for the formation of the β-SiAlON form.
However, there existed remarkable differences in the crystal morphology of samples with the increase in the molar percentage of β-Si 3 N 4 . The SEM micrographs of β-SiAlON with different molar percentages of β-Si 3 N 4 are shown in Figure 4. SEM analysis showed that the β-SiAlON crystal morphology changed from fiber-like crystals in sample 1 to a rod-like morphology in sample 2, as the molar percentage of β-Si 3 N 4 in the precursor increased (Figure 4a-d). EDS analysis of samples 1 and 2 showed that the N to O ratio increased from 2.33 to 2.99 when β-Si 3 N 4 was added to the precursor (Figure 4e and f). The N content of sample 2 was 42.59%, which corresponded to a stoichiometry of Si 4 Al 2 O 2 N 6 . Figure 5 shows that the particle size distribution of the β-SiAlON powder increased as the molar percentage of β-Si 3 N 4 in the precursor increased. For example, the radial size of individual particles in sample 1 was within the range 0.15-0.35 µm, while the radial particle sizes in samples 3 and 4 were larger, in the range 0.4-0.8 µm. The structural and size distribution data showed that β-Si 3 N 4 served as a nucleating agent, which increased the rate of non-spontaneous nucleation [23]. Hence, the addition of β-Si 3 N 4 also facilitated the nitridation process, caused increased crystal growth and larger individual grain sizes.
The non-isothermal oxidation behavior of the Si 4 Al 2 O 2 N 6 powder is characterized in Figure 6. The oxidation reaction began at approximately 1,200°C. The weight gain rate increased rapidly between 1,200 and 1,500°C, after which the run was terminated. The degree of oxidation of Si 4 Al 2 O 2 N 6 was calculated from the weight gain according to the following reaction: The mass of the Si 4 Al 2 O 2 N 6 powder was 32.1% higher than the original mass when fully oxidized (i.e., 100% oxidized). The total mass change of samples 1, 2, 3, and 4 were 9.93%, 8.89%, 7.19%, and 6.39%, respectively, which corresponded to oxidation degrees of 30.9%, 27.7%, 22.4%, and 19.9%, respectively. This suggested that a higher β-Si 3 N 4 content in the precursor made the product Si 4 Al 2 O 2 N 6 more oxidation resistant. The specific weight gain due to the oxidation of Si 4 Al 2 O 2 N 6 was then examined as a function of oxidation time between 1,200 and 1,400°C, as shown in Figure 7. The weight gain was higher at 1,400°C than at 1,200°C or 1,300°C, and the specific weight gain for sample 4 was  lower than for all other samples. The specific weight increased linearly with temperature in the early stage of oxidation (within 0.5 h) from 1,200 to 1,400°C. However, after 0.5 h, the specific weight increased according to a parabolic curve relationship, which suggested a change in the oxidation mechanism. During the initial stage, Si 4 Al 2 O 2 N 6 reacted with O 2 at the surface, after which O 2 diffused through the oxide layer into the inside of the material and N 2 produced during the oxidation diffused out. The dense oxide layer was either incomplete or too thin to prevent O 2 diffusion into the matrix, thus the oxidation rate was controlled by the rate of reaction at the Si 4 Al 2 O 2 N 6 surface. At extended oxidation times, the specific weight gain increased, while the degree of weight gain decreased, indicating that the oxide layer formed after 0.5 h was complete or sufficiently thick to prevent O 2 diffusion into the matrix interior. In this regime, the oxidation rate was controlled by the rate of diffusion of O 2 . The parabolic oxidation kinetic curves of Si 4 Al 2 O 2 N 6 powder showed a close fit to the Arrhenius parabolic equation [24]: where W 2 represented the square of the weight gain per unit area; K p was an oxidation rate constant, which was calculated from the slope; t was the oxidation time; and C was constant, which was the intercept and ideally zero. Overall, the W 2 vs. t plots obtained from sample 4 when oxidized at 1,200-1,400°C showed the closest fit to the Arrhenius parabolic model (Figure 8). The calculated oxidation rate constants, K p , for samples 1-4 oxidized between 1,200 and 1,400 are listed in Table 2. As the molar content of β-Si 3 N 4 increased, the obtained K p values decreased, which meant that the dense, continuous oxide layer formed quicker and the oxidation resistance increased. According to the non-isothermal oxidation results, samples 1 and 4 displayed the highest and lowest degrees of oxidation. The XRD analysis of the oxidized products of 1 and 4 indicated that Si 4 Al 2 O 2 N 6 was the major product phase, and Al 2 O 3 was the main oxidation product, shown in Figure 9. The relative intensity of peaks indexed to Al 2 O 3 compared to that indexed to Si 4 Al 2 O 2 N 6 changed with both oxidation temperature and β-Si 3 N 4 molar content. For sample 1, the relative intensity of Al 2 O 3 peaks increased with oxidation temperature, which indicated an increase in the oxidation degree in sample 1. In contrast, the relative intensities of Al 2 O 3 and Si 4 Al 2 O 2 N 6 from sample 4 changed negligibly as the oxidation temperature increased, which indicated that the oxidation resistance of β-SiAlON was improved with the addition of β-Si 3 N 4 .
The pore size distribution and nitrogen absorptiondesorption isotherm of samples 1, 2, 3, and 4 after nitridation at 1,550°C were then analyzed ( Figure 10). A pore size distribution with a maximum 3 nm was observed, which suggested that the material had a mesoporous structure. However, the specific surface area of samples 1, 2, 3, and 4 was 0.888, 0.879, 0.807, and 0.741 m 2 g −1 , respectively, which suggested that an increase in the grain size, as observed by SEM ( Figure 5), leads to a decrease in the specific surface area. The variance in microstructure and oxide layer density across samples leading to different behaviors during oxidation suggested that the efficiency of the oxidation reaction may be dependent on exposed surface area. For instance, samples with larger particle size and larger surface areas, such as sample 4, exhibited the strongest oxidation resistance compared with those with small particle sizes, such as sample 1.

Conclusions
Si 4 Al 2 O 2 N 6 was prepared via high-temperature nitridation of a mixture of α-Al 2 O 3 , metal Al, and Si powders and β-Si 3 N 4 as an additive. As the content of β-Si 3 N 4 increased, the morphology of individual Si 4 Al 2 O 2 N 6 crystallites varied from a fiber-like to a rod-like structure. The addition of β-Si 3 N 4 facilitated the grain growth, which leads to a reduction in surface area and in turn a superior resistance to oxidation. Precursor samples containing 29.9 mol% of β-Si 3 N 4 required the shortest time to form a dense, continuous oxide layer, which prevented the diffusion of O 2 into the inside of material and therefore exhibited the higher oxidation resistance.