Crack Healing in a Silicon Nitride Ceramic

Abstract

A detailed study has been undertaken on crack healing at high temperatures in a silicon nitride containing 10 wt% additives in order to identify the dominant mechanism responsible for the phenomenon. Fracture toughness increased with annealing time and the crack growth rate decreased until arrest with increasing testing time. Differentiation between possible operating mechanisms was obtained using critical experiments involving detailed compliance measurements, crack wake removal, and crack reinitiation tests and a comprehensive TEM study of healed cracks. It was found that crack healing was not uniform in the crack wake. When the original crack path was either transgranular or intergranular, healing was associated with the appearance of a thin layer of silica glass due to the oxidation of Si 3 N 4 grains. But when the crack went through multigrain junctions, the former crack path was completely obliterated and replaced by a new, crystalline phase formed by diffusion of the preexisting glass phase. It is concluded that the increased crack growth resistance and fracture toughness at high temperature is attributable to the partial recovery of the original strength from the crack segments at multigrain junctions due to vitreous phase flow and subsequent crystallization.

Generic principles of crack-healing ceramics

“…The pronounced recovery of strength was attributed to the formation of a silica rich glassy phase, which filled the crack space causing crack tip blunting and crack rebonding. Healing of transgranular cracks in Si 3 N 4 based ceramics exposed to air atmosphere was attributed either to the oxidation of the Si 3 N 4 grains forming a SiO 2 glass at the crack surface or to the oxidation of intergranular phases (2SiO 2 +Y 2 O 3 Y 2 Si 2 O 7 ) [75]. As a result healing could be achieved at temperatures as low as 600 ℃ after 100 h annealing.…”

Characterization of crack self-healing of silicon carbide by hot press sintering

“…하지만 공정 중 지속적으로 발생하는 물리적 및 열적 충격으로 인해 SiC 내 · 외부 에 균열이 발생한다. 재료 내 · 외부의 미세한 균열을 제어하기 위해 세라 믹 재료의 균열자기치유가 연구되고 있다 [6][7][8][9][10][11][12][13]. 균열자 기치유를 이용하면 (1) 공정재료로서 세라믹 재료의 적 합성이 상승하고, (2) 가공 및 연마 비용이 감소하며, (3) 재료의 유지보수 비용 감소 및 수명연장을 기대할 수 있다 [14][15][16].…”

Density changes of iron during morphological healing evolution of internal fatigue microcracks

“…With the data in Tables II and III, the corresponding value of m is calculated by Eq. [5] for the number 2 specimen at annealing times of 2, 3, 5, and 7 hours, respectively. The corresponding results are shown as the filled dot points in Figure 6.…”

“…From Eq. [4], the value of m is evaluated as [5] The SEM observations show that the intragranular cracks have about ϭ 80 pct and the intergranular cracks have 20 pct of the total number of cracks, which are thought to be wholly evolved into arrays of spherical voids at 2 hours of annealing time. [1] In this study, the value of V 0 is acquired by calculating with the size of ⌽5 ϫ 10 mm for the cylinder-shaped specimen.…”

“…[1] Crack healing has also been observed in other nonmetallic materials, such as ceramics, [2][3][4][5] glass, [6] sapphire, [7,8] quartz, [9] or polymers. [10] The morphological evolution of cracks and the evolving mechanisms has been presented in previous studies.…”