Original ArticleCombustion synthesis of high flexural strength, low linear shrinkage and machinable porous β-Si3N4 ceramics
Introduction
Because of the excellent mechanical properties, dielectric properties and chemical stability of porous Si3N4 ceramics, which has been successfully used as high temperature gas filtration materials, radomes, catalyst carriers and diaphragm materials [[1], [2], [3], [4], [5]]. Centering on the preparation of porous Si3N4 ceramics, gel casting method [[6], [7], [8]], freeze-drying method [9,10], adding pore former method [11,12], etc. have been developed successively. All of the above methods are based on expensive α-Si3N4 powder as raw material, accompanied by a long time of high temperature sintering process. As a result of the high cost of raw materials and process, silicon nitride porous ceramics are far from being applied in large scale compared with oxide porous ceramics. Therefore, low cost preparation of porous Si3N4 ceramics is expected.
Reaction bonded silicon nitride (RBSN), developed in the 1960s, is essentially porous Si3N4. It uses cheap Si powder as raw material, utilizes the exothermic characteristics of Si and N2 reaction, realizes Si nitriding and sintering at the same time, and reasonably utilizes the volume expansion when Si is transformed into Si3N4 to reduce the porosity [[13], [14], [15], [16]]. The strength of RBSN is very high, but the porosity is generally less than 20 %. Although RBSN process realizes low-cost manufacturing, it is not comparable with the porosity range of porous oxide ceramics. Based on RBSN, combustion synthesis of Si3N4 technology developed in 1980s has many similarities with RBSN [17]. It is found that the porosity of combustion synthesis of Si3N4 ceramics can be adjusted in a wide range [18,19]. This progress brings hope for low-cost preparation of porous Si3N4 ceramics with high strength and high porosity. However, it should be pointed out that the sintering shrinkage characteristics and dimensional stability mechanism of combustion synthesis of porous Si3N4 ceramics are still unclear.
In this work, Si, α-Si3N4 and Y2O3 powders are used as the raw material. It is hoped that porous Si3N4 ceramics with adjustable strength and porosity can be directly prepared using in-situ densification effect of Y2O3 as sintering aids by combustion synthesis. The sintering shrinkage characteristics and dimensional stability mechanism of as-synthesized porous Si3N4 ceramics were investigated.
Section snippets
Experimental procedure
Si powder (> 99.99 %, D50 ≈ 3 μm, Sinopharm Chemical Reagent Co. Ltd., Beijing, China), α-Si3N4 diluent (α > 93, D50 ≈ 10 μm, self-made by combustion synthesis) and Y2O3 powder (> 99.9 %, D50 ≈ 50 nm, Sinopharm Chemical Reagent Co. Ltd., Beijing, China) in the proportions shown in Table 1 were milled in ethanol for 4 h. After that, the ball milled mixed slurry was completely dried at 100 °C and sieved. The sieved mixed powder was placed in a steel mold with a diameter of 40 mm, and was
Results and discussion
In the range of the addition amount of Si3N4 diluent and Y2O3 additives, all samples have completed combustion synthesis reaction and formed porous Si3N4 ceramics. Because there are only sintering additives in the pellet, and the pellet has a higher density than the powder bed, the densification reaction only occurs in the pellet, so there is almost no reaction between the pellet and the powder bed, and only a layer of loose powder is attached on the surface of the porous ceramic. Fig. 2 shows
Conclusions
Porous β-Si3N4 ceramics with porosity of 49 % and flexural strength of 151 MPa were prepared by combustion synthesis method using Si, α-Si3N4 and Y2O3 powders as raw materials. The linear shrinkage of sintered porous β-Si3N4 ceramics is only about 3%. Due to the low shrinkage, it is possible to fabricate near net shape silicon nitride components with large size and complex shape at low cost. In addition, the porous Si3N4 ceramics have good machinability, which can be drilled and threaded by WC
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The work was supported by the National Key Research and Development Program of China (No. 2017YFB0310303), National Natural Science Foundation of China (Nos. 51702331, 52072381, U1904217) and the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (No. SKL201801SIC).
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