Intergranular Nanostructure Effects on Strength and Toughness of Si₃N₄

Abstract

Strength, toughness, microstructure, and atomic adsorption arrangement in silicon nitrides with MgO and RE 2 O 3 additions (RE = La, Gd, Y, Lu) were examined. Mechanical properties were high for La, Gd, and equal La-Lu additions, but surprisingly were progressively lower for Y-and Lu-doped samples. The lower strength and toughness were associated with fewer visible crack deflections and grain bridges. Detailed microstructural analysis of the Lu-doped material revealed a complex intergranular nanostructure with variable Lu content and Si 3 N 4 nanocrystals. Furthermore, the Lu-rich areas showed an extra Lu-adsorption site on the Si 3 N 4 prismatic planes not previously observed in other studies. This inhomogeneous structure was attributed to grain growth impingement and higher viscosity of the Lu-doped oxynitride glass that slows homogenization. The Y-doped material with nearly identical glass viscosity demonstrates intermediate behavior. Finally, substituting half of the Lu 2 O 3 with La 2 O 3 resulted in a homogenous intergranular structure, attributed to a lower viscosity of the oxynitride glass phase, and high mechanical properties. Overall, care must be taken when adapting Si 3 N 4 processing parameters for the smaller ionic radius rare earth dopants such as Lu and Y.

C0.3N0.7Ti-SiC Toughed Silicon Nitride Hybrids with Non-Oxide Additives Ti3SiC2

“…Remarkable increase of density from 79.20% to 95.48% could be achieved for Si 3 N 4 ceramics with 5 vol.% Ti 3 SiC 2 when sintered at 1600 • C. As expected, higher sintering temperature 1700 • C could further promote addition of rare-earth oxides to promote mass transport and accelerate the rate of α − β transformation, most notably the rare-earth oxides involving Y 2 O 3 [4,31,32]. A combination of various rare-earth oxides and other metallic oxides, such as also are effective sintering aids to densify Si 3 N 4 [25,30,[32][33][34][35].Nevertheless, these oxides additives crystallized to intergranular glassy phase in the cooling stage [31], which deteriorate the high-temperature performance of the ceramics such as creep and high-temperature strength due to the relative low eutectic temperature [32,36]. As a result of the early interest in hot-pressed Si 3 N 4 ceramics as a high-temperature gas turbine material, attention was directed to high-temperature strength and creep resistance.…”

“…Remarkable increase of density from 79.20% to 95.48% could be achieved for Si 3 N 4 ceramics with 5 vol.% Ti 3 SiC 2 when sintered at 1600 • C. As expected, higher sintering temperature 1700 • C could further promote addition of rare-earth oxides to promote mass transport and accelerate the rate of α − β transformation, most notably the rare-earth oxides involving Y 2 O 3 [4,31,32]. A combination of various rare-earth oxides and other metallic oxides, such as also are effective sintering aids to densify Si 3 N 4 [25,30,[32][33][34][35].…”

“…However, due to the high degree of covalent bonding, Si 3 N 4 -based ceramics are very difficult to densify through the solid-state sintering process. Therefore, effective approaches to ensure rapid consolidation and high mechanical performance of Si 3 N 4 -based ceramics are actively being explored, including gas pressure sintering (GPS) [11], hot-pressing sintering (HPS) [20][21][22][23][24][25][26], hot isostatic pressing sintering (HIP) [27], spark plasma sintering (SPS) [26,28,29], and microwave sintering [1,30], etc. However, considering the requirement of high gas pressures for gas pressure sintering and extra current devices for SPS with a significantly higher furnace costs, HPS allows the dense and complex-shaped parts with medium cost [2].…”

C_0.3N_0.7Ti-SiC Toughed Silicon Nitride Hybrids with Non-Oxide Additives Ti₃SiC₂

“…However, due to the high degree of covalent bonding, Si3N4-based ceramics are very difficult to densify through the solid-state sintering process. Therefore, effective approaches to ensure rapid consolidation and high mechanical performance of Si3N4-based ceramics are actively being explored, including gas pressure sintering (GPS) [10] , hot-pressing sintering (HPS) [19][20][21][22][23][24][25] , hot isostatic pressing sintering (HIP) [26] , spark plasma sintering (SPS) [25,27,28] , and microwave sintering [1,29] , etc. However, considering the requirement of high gas pressures for gas pressure sintering and extra current devices for SPS with a significantly higher furnace costs, HP sintering allows the dense and complex-shaped parts with medium cost.…”

Fabrication of dense Si₃N₄ ceramics via coating amorphous Si₃N₄ nano-powders by sodium reduction in liquid ammonia

“…The strength is higher than Si 3 N 4 ceramic prepared with the sintered catalyst of rare earth oxides. 22) Compared to our previous work, the mechanical properties of the sample is closed to the Si 3 N 4 ceramics obtained by sintering of 100% amorphous Si 3 N 4 nano-particals.…”