In situ observations of cracking in constrained sintering

Abstract

In this paper, we examine the long-standing problem of cracking during constrained sintering of a powder aggregate. Using binder jet 3D printing, we prepare ceramic green bodies in the form of center-notched panels, then use in situ imaging to observe how cracks nucleate and grow from the notch as the material sinters under restraint. Quantitative image analysis allows us to identify important characteristics of the sinter-cracking process, indicating a framework for analyzing the problem and developing methods for avoiding it, including representation of sinter-cracking as a creep crack growth process, use of fracture mechanics parameters to design specimen geometries that do not exceed critical stress intensities, and the possibility of exploiting the inherently ductile nature of sinter-cracking to mitigate damage. K E Y W O R D Sfracture, in situ imaging, sinter/sintering, solid freeform fabrication

Crack initiation, propagation, and arrest in sintering powder aggregates

“…All rights reserved specimens. 21 This behavior indicates a regime of notch geometry where catastrophic sintercracking is suppressed: while a crack may initiate, it will grow slowly or not at all.…”

“…20 Recently, the initiation and propagation of sinter-cracks was observed directly via in situ monitoring of a powder aggregate sintering under restraint. 21 The experiment revealed how sinter-cracks initiate following an incubation period of densification, then propagate through a ductile fracture mode, characterized by high material elongation and interrupted by phases of crack arrest and shrinkage. From this macroscopic view of sinter-cracking in progress, new questions arise.…”

“…To determine the relation between crack growth behavior and the stress induced by densification, it is helpful to look at the phenomenon of creep-cracking, to which sinter-cracking has been likened by others and to which the present results suggest a strong similarity. 7,9,21 Clear similarities exist between the two phenomena, including the time-dependent flow of material, cracking under small stresses, and rupture by intergranular failure. 19,66 In creep-crack growth, investigators have observed correlations between the crack growth rate and some power of the stress intensity factor or the net section stress , where n is often equal to the stress exponent in the creep rate deformation law.…”

A Lamination Model for Pressure-Assisted Sintering of Multilayered Porous Structures

“…Equations ( 23) and ( 24) are four simultaneous first-order differential equations to determine the evolutions of thicknesses and porosities h 1 , h 2 , θ 1 , and θ 2 with . ε 0 given in Equation (22). Moreover, the free sintering strains .…”

“…There has also been some work with 3D printed and tape-casted parts utilizing multilayer and multi-material approaches for making bulk structures [19][20][21][22][23], but a better understanding of the sintering and processing with and without an applied mechanical load is needed to achieve the best densities and properties. Fundamentally, 3D printing of materials needing sintering may have the same constraints as multilayers with different densities because of gaps, defects, and layering effects [22,23], but the focus of this research does not discuss this further.…”