Fabrication and strengthening of porous Si 3 N 4 ceramics by replacement of oxide phase with Si 3 N 4 at grain boundary
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State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China (Received 14 January 2017; accepted 6 June 2017)
Porous silicon nitride ceramics are attracting extensive attention due to its high strength and low dielectric loss. However, further strength enhancement at elevated temperatures is hindered by its intergranular phase, forming from sintering additives. This paper describes the fabrication of porous silicon nitride ceramic materials, by using a replacement method of carbothermal nitridation. The initial samples which were obtained from the sintering of mixed powder consisted of 95 wt% Si3N4 and 5 wt% Y2O3. After the removal of the oxide intergranular phase and the infiltration of mixtures of phenolic resins and silica sols, carbothermal nitridation process was carried out at 1550 °C for 2 h under nitrogen. X-ray diffraction and microstructural analysis revealed a complete replacement of oxide intergranular phases by the newly formed Si3N4 intergranular phase. The unmodified ceramic exhibited lower flexural strength at 1400 °C, which was only 50% of the room-temperature strength. Although the modified ceramic attained a slightly lower flexural strength at room temperature after the replacement of intergranular phase, its strength measured at 1400 °C could attain 90% of room-temperature strength.
I. INTRODUCTION
Silicon nitride (Si3N4) ceramics are widely used as electromagnetic waves transparent materials within a wide range of temperature, primarily due to its high strength, good resistance to thermal shock and corrosion, low dielectric and dielectric loss, and other advantages.1–5 Porous Si3N4 ceramics are usually fabricated by liquid phase sintering with the addition of densification additives,6–9 and liquid phase formation from the oxide during sintering at a high temperature promotes the transformation of equiaxed a-Si3N4 to rod-like b-Si3N4. Superior mechanical properties can be obtained due to the elongated b-Si3N4 grains, and oxide grain boundary phases with lower melting temperatures are simultaneously produced. This results in the decrease of hightemperature strength, corrosion resistance, and deterioration of other properties.10–12 Grain boundary phase is a key factor governing the mechanical properties at high temperatures.13 More specifically, the mechanical response of monolithic Si3N4 at high temperatures is mainly dictated by the properties of the intergranular
Contributing Editor: Nahum Travitzky a) Address all correspondence to this author. e-mail: [email protected] b) Present address: School of Materials Science and Engineering, Beifang University of Nationalities, Yinchuan 750021, China. DOI: 10.1557/jmr.2017.252
glassy phase, i.e., its chemistry, amount, viscosity, and crystallization behavior.14 Many researchers who have made exhaustive studies on decreasing the amount of grain boundary phases or modifying them to produce more crystalline phases suggested some methods such as vacuum heat treatment15 and devitrification process.16–18 Vacuu
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