Sintering of Si₃N₄ with the Addition of Rare‐Earth Oxides

Abstract

The effect of rare‐earth oxide additives on the densification of silicon nitride by pressureless sintering at 1600° to 1700°C and by gas pressure sintering under 10 MPa of N2 at 1800° to 2000°C was studied. When a single‐component oxide, such as CeO2, Nd2O3, La2O3, Sm2O3, or Y2O3, was used as an additive, the sintering temperature required to reach approximate theoretical density became higher as the melting temperature of the oxide increased. When a mixed oxide additive, such as Y2O3–Ln2O3 (Ln=Ce, Nd, La, Sm), was used, higher densification was achieved below 2000°C because of a lower liquid formation temperature. The sinterability of silicon nitride ceramics with the addition of rare‐earth oxides is discussed in relation to the additive compositions.

Pressureless sintering of silicon nitride with lithia and yttria

“…4,5 Y 2 O 3 as an additive leads to excellent mechanical properties at room temperature as well as at elevated temperatures. 6 Furthermore, it enhances the growth of elongated b-Si 3 N 4 grains. 3 However, yttria is not overly beneficial to the kinetics of densification because of the resulting high melting point and viscosity of the glass.…”

Microstructure and mechanical properties of silicon nitride ceramics prepared by pressureless sintering with MgO–Al2O3–SiO2 as sintering additive

“…10 These additives, however, form glassy grain-boundary phases in the dense body which deteriorate high temperature mechanical properties of ceramics such as creep and high temperature strength. [2][3][4] Several methods to improve the mechanical properties at elevated temperature have been proposed: use fewer additives, crystallize the glassy phase by heat treatment, and use an adequate composition through which the additives dissolve in Si 3 N 4 grains after sintering. With these methods, the high temperature properties can be improved by minimizing the volume of the glassy phase.…”

“…[1][2][3][4][5][6][7][8] Pressureless sintering is now commonly used to fabricate dense Si 3 N 4 parts with complex shapes because of its economic advantages over hot pressing and the improvement in thermal mechanical properties as compared to those achieved by reaction-bonded sintering. [6][7][8] However, the covalent nature of Si-N bonding requires the use of sintering additives, such as metal oxides and rare-earth-metal oxides, to promote densification through liquid-phase sintering.…”

Dilatometric studies of (SiO2-RE2O3-Al2O3) silicon carbide ceramics

“…This phase must have adequate viscosity to make material transport more efficient, and consequently improve densification [13][14][15][16] . Sintering additives based on the SiO 2 -Al 2 O 3 -RE 2 O 3 system (RE = rare earth element) have been found to be effective to obtain high density silicon carbide ceramics [17][18][19] , as well as silicon nitride ceramics [20][21][22] , through liquid phase sintering. The liquid phase dissolves the silicon carbide particles at relatively low temperatures.…”