Microstructural Evolution and Mechanical Properties of Si₃N₄ with Yb₂O₃ as a Sintering Additive

Abstract

Ytterbium oxide (Yb 2 O 3 ) was used as a sintering aid toobtain Si 3 N 4 with excellent high-temperature properties, it is necessary to minimize the amount of grain boundary phases or enhance the mechanical properties of silicon nitride (Si 3 N 4 ) to crystallize them by some treatment. ceramics. The amount of Yb 2 O 3 had significant effects onTo achieve toughening of Si 3 N 4 by a crack bridging or crack microstructural evolution and the composition of secondary deflection process, formation of large elongated grains is necesphases at the grain boundary. When the Yb 2 O 3 added was sary. However, the existence of the elongated grains does not less than 8 wt%, small homogeneous grains were formed. guarantee these processes. These toughening processes occur At the grain boundary, crystalline Yb 2 Si 2 O 7 was formed only when cracks propagate along the grain interfaces. For the along with a glassy phase. As the amounts of Yb 2 O 3 were intergranular fracture mode, debonding at the interface between increased to higher than 8 wt%, large elongated grains the grains and grain boundary phase should occur. The interwere developed in the fine matrix. In those cases, the grain facial strength can be controlled by modifying the chemistry of boundary crystalline phase was changed from Yb 2 Si 2 O 7 to the grain boundary phase. For example, promotion of interfacial Yb 4 Si 2 O 7 N 2 . Mechanical properties were influenced by debonding was observed by increasing the ratio of yttria to these changes in microstructure and grain boundary phase.alumina or by decreasing the nitrogen content in Si-Al-O-N The fracture toughness increased with the Yb 2 O 3 content up glasses at the grain boundaries. Therefore, both the microstructo 8 wt% and decreased slightly thereafter. The increase in ture and the interfacial debonding energy are critical for fracture toughness was apparently due to the formation of increasing the fracture toughness of Si 3 N 4 . 5-7 In addition, there the large elongated grains. When more than 8 wt% of Yb 2 O 3 is one more aspect to be considered, i.e., the high-temperature was added, interfacial debonding energy between the elonproperties. The grain boundary conditions that give excellent gated grains and grain boundary phase became too large,properties at low temperatures are not necessarily suitable for resulting in a decrease in the fracture toughness. The roomhigh-temperature applications. To obtain conditions suitable for temperature flexural strength was not significantly affected high-temperature applications, crystallization of the amorphous by the Yb 2 O 3 content or the microstructure, other than in phase at the grain boundary by heat treatment after sintering the case of 2 wt% addition. The high-temperature strength has been employed. 1,3 in nitrogen, however, increased steadily with Yb 2 O 3 content.Rare-earth oxides have been used as sintering aids to enhance The highest strength, 870 MPa at 1400؇C, was observed the densification and grain growth by in situ phase transformawhen 16 wt% of Yb 2 O 3...

Potential use of only Yb2O3in producing dense Si3N4ceramics with high thermal conductivity by gas pressure sintering

“…At 7 mol% of Yb 2 O 3 , the phase transformation was completed, almost all β-Si 3 N 4 grains became elongated ( figure 3(d)), and the grain size was larger than that in 3 mol% samples. The promotion of β-Si 3 N 4 grain growth upon increasing Yb 2 O 3 content is similar to the findings of Park et al [21]. These results suggest that both the phase transformation and grain growth are dominated by the diffusion-controlled solution-reprecipitation mechanism.…”

“…Although they did not present density data, the microstructure suggested that complete densification was achieved in some of their samples. Yb 2 O 3 is also an effective additive for improving the high-temperature mechanical properties of Si 3 N 4 ceramics because of the easy formation of the crystalline Yb 4 Si 2 O 7 N 2 phase at the grain boundaries [19,21]. Thus, we infer that gas pressure sintering can produce dense β-Si 3 N 4 ceramics with the addition of only Yb 2 O 3 , and such ceramics should have high thermal conductivity and excellent high-temperature mechanical properties.…”

Oxidation and Strength Retention of Monolithic Si3N4 and Nanocomposite Si3N4‐SiC with Yb2O3 as a Sintering Aid

“…15 On the other hand, the oxidation rate of the nanocomposite Si 3 N 4 -SiC containing 20 vol% SiC was similar to that of the monolithic Si 3 N 4 .…”

“…[10][11][12][13][14][15] The improvement was attributed to the extensive formation of crystalline phases at the grain boundary. The high-temperature strength of Si 3 N 4 also was enhanced when fine SiC particles were added to make nanocomposite Si 3 N 4 -SiC.…”

Textured silicon nitride: processing and anisotropic properties

“…The softening of the glassy phase is the major factor detrimental to their HT properties. There are two major strategies for improving the HT properties of Si 3 N 4 ceramics: (i) the crystallization of the secondary amorphous phase by a postheating treatment [18][19][20][21]30] and (ii) the use of more refractory RE oxide additives that can provide crystalline secondary phases with high melting points [24,25,[30][31][32][33][34][35][36][37]. Previous studies have suggested that out of the RE oxides, Yb 2 O 3 and Lu 2 O 3 are the best additives for improving HT resistance up to 1400 and 1500 • C due to the formation of crystalline Yb 4 Si 2 O 7 N 2 [24,25,32,33,36] and Lu 4 Si 2 O 7 N 2 [31,34,37] grain boundary phases, respectively.…”