ZHANG Chunpeng ^{1, 2}, ZHU Nannan ^{1, 2}, WEN Guangwu ^{1, 2}

(1. School of Materials Science and Engineering, Shandong University of Technology, Zibo 255000, Shandong, China;

2. Engineering Ceramics Research Institute, Shandong University of Technology, Zibo 255000, Shandong, China)

Extended abstract:

[Background and purposes] Silicon nitride (Si₃N₄) is one of the most promising structural ceramics, due to its outstanding strength, chemical stability, low thermal expansion coefficient and excellent thermal shock resistance. As a result, it is widely used in various fields, such as aerospace industry, automotive industry, electronics industry, medical devices, cutting tools and wear-resistant components. Although metal oxides have been found to be effective sintering aids for Si₃N₄ ceramics, they will increase lattice oxygen, causing defects in ceramics and hence affecting the performance. Therefore, to reduce the influence of impurities and defects, non-oxide sintering additives are selected for sintering of Si₃N₄. Therefore, in this paper, equiaxed grain α-Si₃N₄ powder is used as the main raw material, while non-oxide MgSiN₂ and Y₂O₃ are used as sintering aids. at certain sintering temperatures, the effects of the composite sintering aids on phase composition, relative density and mechanical properties of the Si₃N₄ ceramics, while the toughening mechanism is studied, in order to find a preparation process for high toughness Si₃N₄ ceramics and optimal ratio of sintering aids.

[Methods] Using equiaxed α-Si₃N₄ powder as the raw material, with a fixed Si₃N₄ content of 93 wt.% and a sintering aid content of 7 wt.% of with Y₂O₃-MgSiN₂, Si₃N₄ ceramics were prepared in nitrogen at 1800 ℃ and 30 MPa for 1 h. The samples were respectively labeled as 0Y7M, 1Y6M, 2Y5M, 3Y4M, 4Y3M, 5Y2M, 6Y1M and 7Y0M, corresponding to the ratio of the sintering aids. Phase composition was characterized by using XRD and microstructure of the samples was observed by using SEM. Bending strength of the sintered ceramics was evaluated through the three-point bending test, while density were measured by using the Archimedes displacement method and fracture toughness was measured by using indentation method.

[Results] Relative densities of all the silicon nitride ceramic samples with different ratios of sintering additives are above 96%. Hardness of the ceramics varies with the ratio of sintering additives, showing a trend of increasing first and then decreasing. Among them, hardness of the 3Y4M sample reached 15.84 GPa, which was the highest, while that of the 7Y0M sample was the lowest, only 10.41 GPa. Flexural strength of the 0Y7M, 3Y4M and 7Y0M samples was (639.5±44.02) MPa, (810±14.88) MPa and (669±29.97) MPa, respectively. With varying ratio of sintering aid, flexural strength of the Si₃N₄ ceramics showed a trend of increasing first and then slight decreasing, reaching the maximum value for 3Y4M. The trend of fracture toughness is basically consistent with that of flexural strength. Both increase initially and then slightly decrease with the ratio of sintering additives, reaching the maximum value of 9.63 MPa·m^1/2 for 3Y4M. The Si₃N₄ ceramics only show the characteristic diffraction peaks of β-Si₃N₄, without those of α-Si₃N₄, achieving a full phase transformation. Microstructure of all the samples is characterized by typical long rod-shaped β-Si₃N₄ grains, forming an interlocking structure.

[Conclusions] α-Si₃N₄ powder was used as the raw material and Y₂O₃-MgSiN₂ was employed as the composite sintering aid. With hot pressing sintering process in nitrogen, at 30 MPa and 1800 ℃ for 1 h, high-performance Si₃N₄ ceramics were successfully obtained. The samples with different aid ratios all experienced a complete phase transformation from α-Si₃N₄ to β-Si₃N₄. No obvious impurity phases were detected, indicating that Y₂O₃ and MgSiN₂ can effectively promote the crystal structure transformation of silicon nitride. Microstructure of the 3Y4M and 4Y3M samples presented a typical double-peak grain structure. Relative density of all the Si₃N₄ ceramics reached over 96%, while that of the 3Y4M sample was early 100%. When the sintering aid ratio was 3Y4M, the ceramic had the highest comprehensive mechanical properties, with a hardness of (15.84±0.56) GPa, an anti-bending strength of (810±14.88) MPa and a fracture toughness of (9.63±0.59) MPa·m^1/2.

Key words: Si₃N₄; MgSiN₂; Y₂O₃; hot pressing sintering