J. Cent. South Univ. (2020) 27: 2548-2556

DOI: https://doi.org/10.1007/s11771-020-4480-1

Fabrication and flexural strength of porous Si₃N₄ ceramics with Li₂CO₃ and Y₂O₃ as sintering additives

HU Hai-long(胡海龙)^{1, 2}, LUO Shi-bin(罗世彬)¹

1. School of Aeronautics and Astronautics, Central South University, Changsha 410083, China;

2. Research Center in Intelligent Thermal Structures for Aerospace, Central South University,Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: By employing sintering additives of Li₂CO₃ and Y₂O₃, porous Si₃N₄ ceramics are prepared after experiencing the processes of sintering and post-vacuum heat treatment at 1680 and 1550 °C, respectively. The experimental results demonstrate the completed phase transformation from α to β-Si₃N₄ in Si₃N₄ ceramic samples with a amount of 1.60 wt% Li₂CO₃ (0.65 wt% Li₂O) and 0.33 wt% Y₂O₃ additives. The as-synthesized porous Si₃N₄ ceramics exhibit high flexural strength ((126.7±2.7) MPa) and high open porosity of 50.4% at elevated temperature (1200 °C). These results are attributed to the significant role of added Li₂CO₃ as sintering additive, where the volatilization of intergranular glassy phase occurs during sintering process. Therefore, porous Si₃N₄ ceramics with desired mechanical property prepared by altering the addition of sintering additives demonstrate their great potential as a promising candidate for high temperature applications.

Key words: sintering additive; flexural strength; porosity; glassy phase; Si₃N₄ porous ceramics

Cite this article as: HU Hai-long, LUO Shi-bin. Fabrication and flexural strength of porous Si₃N₄ ceramics with Li₂CO₃ and Y₂O₃ as sintering additives [J]. Journal of Central South University, 2020, 27(9): 2548-2556. DOI: https://doi.org/10.1007/s11771-020-4480-1.

1 Introduction

Porous silicon nitride (Si₃N₄) ceramics have been widely used in many fields, such as high temperature gas filter, catalyst supports, particle filter, gas membrane and optoelectronics owing to their excellent mechanical properties, thermal shock resistance and corrosion resistance, desired spectral emission range with modulated morphology [1-7]. The typical processing routes for the synthesis of high strength porous Si₃N₄ ceramics usually involve the high temperature sintering with a large amount of sintering additives, which are mainly the highly cost rare-earth oxides [8-13]. Such an economic issue is now becoming a hurdle for their practical applications. It is believed that the quantity and chemistry of the glassy phase are determined by the amount of additives, thus affecting mechanical properties including fracture toughness and flexural strength [14-17]. It is generally believed that the formation of eutectic liquid with a large volume induced can result in the substantial degradation of these high-temperature properties, because the modulus of rupture of materials decreases rapidly as a result of the softening glassy phase, resulting in the material deformation by viscous shear in the grain boundary [18-21]. On the other hand, nano-modification approach is also exploited on the surface of Si₃N₄ ceramics by using nano-SiO₂ to reduce the oxidation weight gain [22]. Therefore, it makes the preparation of high-performance porous ceramics with chosen amount of sintering additives at low temperature for high-temperature application to be of high significance.

In order to lower the sintering temperature, the desired sintering additive or an added flux such as rare-earth oxides (Re₂O₃, where Re=Y, Er, Yb) [23-25] and metal multilayers [26] contributes to a low melting eutectic liquid phase formed for the synthesis of ceramics synthesis. However, the quantity of this adding sintering additive played a crucial role in altering the defect chemistry, leading to the change of mechanical property. For instance, if there is sufficient additive, the densification process would occur due to the grain growth and rearrangement caused by the capillary force of formed eutectic liquid phase. On the other hand, the sintering additive disappears or volatilizes during the sintering process, which in turn greatly benefits the fabrication of porous ceramics.

Herein, the low temperature sintering technique with sintering additives of Y₂O₃ and Li₂CO₃ is developed to produce high performance porous Si₃N₄ ceramics for high temperature application. For comparison purpose, samples with sintering additive Y₂O₃ is also prepared under the same conditions. In general, the sintering additives of Li₂CO₃, Li₂O, Li₂CO₃-Y₂O₃ and Li₂O₃-Y₂O₃ are commonly used to fabricate various dense ceramics at low temperatures [27-32]. However, Li₂CO₃ possesses a low melting point of 723 °C, which can diffuse and form the liquid phase before volatilization. Moreover, the rapid volatilization of Li₂O happens above 1300 °C [27, 31]. This character of Li₂O would be conversely beneficial to the preparation of porous Si₃N₄ ceramics. Therefore, when the sintering additive Li₂O is offered to prepare porous Si₃N₄ ceramics, the high temperature properties are expected to be promising, owing to the added amount as well as the consequence of Li₂O volatilization.

In this work, by using Y₂O₃ and Li₂CO₃ as sintering additives, porous Si₃N₄ ceramics are fabricated through sintering and post-vacuum heat treatment approach, where the porosity and microstructure variation of as-prepared Si₃N₄ ceramics in regards to their mechanical property are discussed and analyzed.

2 Experimental procedure

Commercially available α-Si₃N₄ (0.5 mm, purity≥95 wt%, UBE Co., Ltd., Tokyo, Japan) was used as the starting powder. Li₂CO₃ (99% purity, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and Y₂O₃ (5.0 mm; purity ≥99.99wt%; Yuelong Company, Shanghai, China) were used as the sintering additives. Table 1 lists the composition of the slurries performed in this work and the porosity of samples after sintering and post-vacuum heat treatment at 1680 and 1550 °C. Homogeneous slurries with Y₂O₃-Li₂CO₃ (defined as sample LY) and Y₂O₃ (defined as sample Y) as sintering additives were respectively obtained by the high energy planetary ball milling with Si₃N₄ balls in Si₃N₄jars for 2 h. After vacuum drying and sieving through 100 mesh screen, the powder mixture of 2.5 g was uniaxially pressed in a stainless steel die with the pressure of 10 MPa to form a pellet with the dimensions of 5.0 mm×10.0 mm×50.0 mm. The pellets of green compacts were placed in graphite resistance furnace to sinter at 1680 °C for 2 h under a nitrogen pressure of 0.1 MPa with Si₃N₄ powder covered. The sintered samples were then post-heat treated at 1550 °C for 4 h under vacuum.

Table 1 Composition of starting powder using Li₂CO₃ and Y₂O₃ as sintering additives and porosity of samples after sintering and post-vacuum heat treatment at 1680 and 1550 °C

The specimens were machined into rectangles bars with the dimensions of 3.0 mm×4.0 mm×36.0 mm to measure the flexural strength via the three-point bending test (Instron5566, INSTRON, Norwood, MA); the supporting distance of 30.0 mm and a cross-head speed of 0.5 mm/min were used. The open porosity and the bulk density were measured by the Archimedes displacement method. The pore size distribution of sintered samples was measured by intrusion porosimeter (Poresizer 9320, USA). The phase composition was determined by X-ray diffractometer (D/MAX-RBX, Japan) with CuK_α radiation. Scanning electron microscope (FEI Nova SEM 450) was used to observe the morphology of the fracture surface. The percentage of α-Si₃N₄ in specimen was evaluated by analyzing the XRD spectra with the Gazzara and Messier equation [33].

 (1)

where I_β(101) and I_β(210) are the diffraction intensities of the (101) and (210) planes of β-Si₃N₄, respectively; I_α(102) and I_α(210) are the intensities of (102) and (210) planes of α-Si₃N₄, respectively.

3 Results and discussion

Figure 1 shows the open porosity of LY and Y samples after being sintered at 1680 °C with post vacuum heat treatment at 1550 °C. The open porosity has been separately measured for the samples after completing their flexural strength measurement under room temperature (RT) and high temperature (HT). After the vacuum heat treatment at 1550 °C, the open porosity of LY and Y samples both increased compared with the same corresponding samples sintered at 1680 °C. And the Y samples always indicate higher porosity than LY samples, no matter which heat treatment is performed (such as sintering at 1680 °C, or alternatively sintering at 1680 °C combined with post vacuum heat treatment at 1550 °C). It suggests that the densification of LY samples is facilitated by sintering additives LiCO₃-Y₂O₃ at a relatively low sintering temperature. This is related to the formation of low eutectic temperature and low viscosity Li-Si-O-N liquid phase during the sintering processing [27, 32]. It exhibits that the density of LY samples is higher than that of the Y samples. When the vacuum heat treatment is implemented at 1550 °C, the large volatilization of Li is related to intergranular glassy phase accompanied with some β-Si₃N₄ grains growth impingement (in Figure 2) occurring [29], thus leading to the increased porosity for both LY samples. For Y samples, the increased porosity can be ascribed to the change of microstructure morphology after further vacuum heat treatment. In particular, when the mechanical property is measured at high temperature of 1200 °C, the open porosity of both samples significantly decreases, while the total porosity actually shows no apparent difference. This can be explained by the fact that the formation of Li-Si-O-N liquid phase started to flow and refilled up the porosity and aggregated in the grain boundaries [31]. Moreover, when the mechanical property is measured at high temperature of 1200 °C, the open porosity of both samples significantly decreases, resulting in the enhanced strength. This can be explained by the fact that the formation of Li-Si-O-N liquid phase starts to flow and refills up the porosity, and aggregates in the grain boundaries. When measured at 1200 °C, no contribution of the porosity came from the volatilization phenomenon of intergranular glassy phase, as it theoretically would occur above 1200 °C.

Figure 1 Porosity of LY and Y samples after sintering at 1680 °C and post vacuum heat treated at 1550 °C (room temperature abbreviated to RT and high temperature abbreviated to HT)

SEM micrographs of LY and Y samples sintered at 1680 °C with the post vacuum heat treatment at 1550 °C are shown in Figure 2.

Figure 2 Scanning electron microscopy of samples:

Compared with the LY samples, Y samples contain much glassy phase, which is induced by the high amount of 8.26 wt% Y₂O₃ sintering additives, as indicated by the yellow arrow in Figures 2(b), (d) and (f). In addition, a large number of rod-like β-Si₃N₄ grains are observed in both LY and Y samples. Based on Eq. (1), the fractions of β-Si₃N₄ in Figure 3 for LY and Y samples after being sintered at 1680 °C can be determined, showing the values of about 96% and 94%, respectively. However, after post-vacuum heat treatment at 1550 °C, the phase transformation of α→β-Si₃N₄ is almost completed in both samples. This demonstrates that the complete phase of β-Si₃N₄ can be achieved by adding a small amount of Li₂CO₃ and Y₂O₃. After flexural strength was measured at the high temperature of 1200 °C, some large β-Si₃N₄ grains are formed for both LY and Y samples under this high temperature measuring condition, which can be explained by Ostwald ripening mechanism, as shown in Figures 2(e) and (f).

Figure 4 shows the flexural strength of LY and Y samples after being sintered at 1680 °C with post vacuum heat treatment at 1550 °C. After post-vacuum heat treatment at 1550 °C, the flexural strength of sample Y rapidly degrades compared with that of the sample before vacuum heat treatment, which can be affected by the intergranular glassy phase owing to the post heat treatment. However, the flexural strength of sample LY drops slightly. In general, the change of flexural strength is affected by the change of porosity and morphology evolution of grains [13, 14]. This is consistent with the result of this experiment as shown in Figure 1 and Figures 2(c) and (d), showing the slight change of open porosity and the growth of β-Si₃N₄ grains which give rise to the increasing porosity due to the impingement and interlocking of grains. To investigate the effect of residual intergranular glassy phase on the high temperature performance of Si₃N₄ ceramics, the flexural strength of both samples is characterized at 1200 °C.

Figure 3 XRD patterns of samples LY and Y after being sintered at 1680 °C

Figure 4 Flexural strength of samples LY and Y after being sintered at 1680 °C and post-heat treated at 1550 °C

Despite that the open porosity of both LY and Y samples measured at high temperature shows a decrease by 3%-6% compared to that of measured at RT (Figure 1), the flexural strength of the sample LY is significantly enhanced and it reaches up to 126.7 MPa while the sample Y is only 66.5 MPa. It demonstrates that the volatilization of intergranular glassy phase plays a crucial role for the elevated temperature mechanical property because it serves a flowing path for the inward diffusion of oxygen. It is believed that the oxidation of silicon nitride would be quite slow if the sample contained less glassy phase [34]. This is because the passivating layer of silica formed on the surface of silicon nitride for better protection. On the contrary, for the large amount sintering additive added Y samples, the significant degraded flexural strength at high temperature is observed owing to the substantially formed glassy phase, which deteriorates the high temperature mechanical property [10, 17, 18]. Therefore, with less sintering additive addition, after post-vacuum heat treatment, the high flexural strength of 126.7 MPa with open porosity of 50.4% can be achieved for the LY sample. It is observed that the samples with fine pores distributed among the rod-like Si₃N₄ grains. The pore size distributions of the LY and Y samples after being sintered at 1680 °C with post vacuum heat treatment at 1550 °C are plotted in Figure 5. Both samples show narrow pore distributions with pore size of 0.7 mm (LY) and 0.9 mm (Y), respectively. The higher porosity can be attributed to the protruding large fibrous grains, contributing to large inter particle voids.

Consequently, by using low level of sintering additives, the as-synthesized porous Si₃N₄ ceramics sintered at a temperature of 1680 °C exhibit high flexural strength (126.7 MPa), high open porosity of 50.4% at high temperature (1200 °C), and good size distribution. In general, owing to the high eutectic temperature and high viscosity of Y₂O₃-SiO₂ liquid phase, the liquid phase through which the mass transportation occurred is not sufficient enough until high amount of sintering additive Y₂O₃ added up to 8.26 wt%. But the formation of eutectic liquid with a large volume induced by sintering additives would result in the significant degradation of high-temperature properties, because the modulus of rupture of materials rapidly decreases due to the softening of the glassy phase, resulting in the material deformation by viscous shear in the grain boundary [18, 35]. Nevertheless, the Li₂CO₃-Y₂O₃ addition provides low eutectic temperature and low viscosity phase, contributing to the high mass transportation and the homogenization. Furthermore, using sintering additive Li₂CO₃-Y₂O₃ instead of large amount of rare-earth oxides, in conjunction with the volatilized intergranular glassy phase, it demonstrates its promising applications at high temperature with the reduced preparation cost. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) is used to quantitatively determine the concentration of chemical elements of LY samples through identifying the intensity of emission. The residual Li concentrations of LY samples after sintering and heat treatment are 0.061 wt% and 0.014 wt%, showing the volatilization ratio of 77.05%. Therefore, it reveals that significant amount of glass phase is volatilized.

Figure 5 Pore size distribution of LY and Y samples after being sintered at 1680 °C and post vacuum heat treated at 1550 °C

To further analyze the effect of added sintering additive on mechanical property of the fabricated porous ceramics, contribution of this sintering additive can be divided into the following stages as shown in Figure 6. First, initial state; Second, diffusion at the grain boundary, forming the liquid phase, due to the fact that the sintering additive contributes to the formation of intergranular crystalline or glassy phase during the sintering process, which plays a crucial role in promoting the densification of ceramics; Third, solution reprecipitation process, during which the grain growth and rearrangement occur, which is caused by the capillary force of formed eutectic liquid phase; Forth, volatilization, during which as the time and temperature kept holding or increasing, the intergranular glassy phase starts to volatize. In this work, as the amount of 0.98 wt% (0.33 wt% Y₂O₃+0.65 wt% Li₂O) is exploited to prepare porous Si₃N₄ ceramics, thus, very less glassy phase would be remained at the grain boundary after excluding the volatilized part, where the following schematic reaction sequence is about to happen: Si₃N₄+Y₂O₃+Al₂O₃+Li₂CO₃ amorphous Li-Al-Y- Si-O-N. Furthermore, the space or void left by the volatilized glass phase serves a flowing path for the inward diffusion of oxygen, consequently degrading the oxygen content in the ceramics. Therefore, the volatilization of intergranular glassy phase contributes to the elevated temperature mechanical property of porous Si₃N₄ ceramics.

Figure 6 Stages of for transient liquid phase sintering with sintering additive:

To reveal the significance of the results reported in this work, comparison of the flexural strength and porosity is summarized in Figure 7 between our experimental results and those reported in literatures [3, 4, 30, 35-43], showing the high significance of using a small amount of sintering additive to achieve both high porosity and high mechanical strength of porous Si₃N₄ ceramics for high temperature application. For the reported results as indicated in Figure 7, sintering additive with a range of 3 wt%-8 wt% is used to achieve their desirable porosity and mechanical property; however, 0.98 wt% additive including both Y₂O₃ and Li₂O was employed to obtain the relatively high porosity with high flexural strength at both room temperature and high temperature. Therefore, it is revealed that using the developed technique in this work is highly cost-effective and efficient to prepare porous Si₃N₄ ceramics.

Figure 7 Comparison of porosity and flexural strength at room temperature or high temperature for porous Si₃N₄ ceramics

4 Conclusions

Porous Si₃N₄ ceramics are fabricated through sintering process at 1680 °C with a post vacuum heat treatment at 1550 °C by adding the Li₂CO₃ and Y₂O₃ as sintering additives. With the amount of 1.60 wt% Li₂CO₃ (0.65 wt% Li₂O) and 0.33 wt% Y₂O₃ sintering additives being added, the phase transformation of α→β-Si₃N₄ can be almost completed. The results show that porous Si₃N₄ ceramics measured at 1200 °C achieves the highest flexural strength of 126.7 MPa with an open porosity of 50.4%, which is attributed to the specific added sintering additive and its volatilization of intergranular glassy phase. Consequently, the method developed in this work proves to be a cost-effective approach to synthesize porous Si₃N₄ ceramics for high temperature applications.

Contributors

HU Hai-long completed the experimental design, property measurement and original draft-writing of this manuscript. LUO Shi-bin assisted for the whole manuscript reviewing.

Conflict of interest

HU Hai-long and LUO Shi-bin both declare that they have no conflict of interest.

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(Edited by FANG Jing-hua)

中文导读

Li₂CO₃和Y₂O₃烧结助剂对多孔Si₃N₄陶瓷制备及力学性能的影响

摘要：以Li₂CO₃和Y₂O₃为烧结结助剂，通过常规烧结（1680℃）与后烧结真空热处理工艺(1550 °C)协同作用，制备了多孔氮化硅(Si₃N₄)陶瓷。当选取的烧结助剂Li₂CO₃与Y₂O₃含量分别为1.60 wt% Li₂CO₃ (0.65 wt% Li₂O) 与0.33 wt% Y₂O₃时，氮化硅陶瓷由a相完全转变为b相。此外，制备的氮化硅陶瓷在高温1200 °C时，呈现了较高的抗弯曲强度和高的开口孔隙率，分别为(126.7±2.7) MPa和50.4%。进一步探究添加的烧结助剂Li₂CO₃所起的具体作用，发现烧结过程中晶界处形成的非晶玻璃相出现了挥发，降低了陶瓷中非晶相的含量，从而很好地促进了多孔氮化硅陶瓷良好的高温力学性能。因此，通过改变烧结助剂的种类及含量，可以有效地制备高温力学性能优异的多孔氮化硅陶瓷，从而使氮化硅陶瓷材料更好地用于高温结构部件。

关键词：烧结助剂；抗弯强度；孔隙率；玻璃相；多孔氮化硅陶瓷

Foundation item: Project(202045007) supported by the Start-up Funds for Outstanding Talents in Central South University, China

Received date: 2020-03-06; Accepted date: 2020-06-23

Corresponding author: HU Hai-long, PhD, Associate Professor; Tel: +86-731-88877495; E-mail: hailonghu@csu.edu.cn; ORCID: https://orcid.org/0000-0003-4107-2644