Amorphous Silicoboron Carbonitride Ceramic with Very High Viscosity at Temperatures above 1500°C

Abstract

Recently, the viscosity of a predominantly amorphous silicon carbonitride (Si 1.7 C 1.0±0.1 N 1.5 ) alloy with an apparent glass-transition temperature (T g ) of 1400°-1500°C was studied. In this study, the creep behavior of silicoboron carbonitride (Si 2 B 1.0 C 3.4 N 2.3 ), which seems to have a T g value of >1700°C, was examined. Both materials exhibited a threestage creep behavior. In stage I, the creep rate declined, because of densification. In stage II, the strain rate approaches a steady state. In stage III, it resumes a declining strain rate, which ultimately decreased below the measurement limit of the system. At 1550°C in stage II, the viscosity of silicoboron carbonitride was six orders of magnitude higher than that of fused silica. Among the Si-C-N ceramics, only chemical-vapor-deposited and reaction-bonded silicon carbides seem to have greater creep resistance than the silicoboron carbonitrides at temperatures >1550°C.

Compressive Deformation of Partially Crystallized Amorphous Si-B-C-N Ceramics at Elevated Temperatures

“…The result of X-ray diffraction of the Si-B-C-N specimens after the creep tests by Riedel et al was not significantly altered as compared with the starting material. 14) Such increasing of viscosity without crystallization has been observed in other amorphous materials and usually explained by using free volume models. 25) Baufeld et al discussed that the increase of the flow stress was related to the decrease of free volume in the amorphous phase.…”

“…12,[14][15][16][17] This volume change of the specimen makes it difficult to separate the intrinsic plastic deformation of the amorphous materials from the total deformation. The strain rate of those Si-B-C-N materials decreased with time in the creep tests, while the flow stress increased with strain in the compression tests.…”

“…It indicates that the deformation of this material is non-Newtonian viscous flow. The deformation of amorphous Si-B-C-N monoliths prepared through the warm pressing and pyrolysis route has been studied by creep test (Si 3:0 B 1:0 C 4:3 N 2:0 ), 12,16,17) (Si 2:0 B 1:0 C 3:4 N 2:3 ) 14) and by compression test (Si 1:0 B 0:4 C 3:4 N 2:4 ). 15) In the case of the creep tests by An et al for the Si 1:7 C 1:0 N 1:5 material which did not contain B, the relation between the applied stress and the strain rate in the steady state was Newtonian viscous flow.…”

“…9,[12][13][14][15][16][17] The analysis of intrinsic plastic deformation of such amorphous ceramics is difficult, because both viscous flow of the matrix and shrinkage by the elimination of residual pores, sintering, occurred concurrently. In order to characterize the nature of viscous flow of the covalent amorphous ceramics, it is necessary to study the deformation of dense materials.…”

Atomic Structure and Diffusion in Amorphous Si-B-C-N by Molecular Dynamics Simulation

“…[9][10][11][12] The high temperature stability of amorphous Si 3 N 4 -based materials strongly depends on additional elements such as boron and carbon. Baldus et al reported that amorphous Si-B-N and Si-B-C-N materials retain amorphous up to 1700…”