Crack Healing of Machining Cracks Introduced by Wheel Grinding and Resultant High‐Temperature Mechanical Properties in a Si₃N₄/SiC Composite

Abstract

as a sintering additive was investigated. The machining cracks were introduced by a wheel grinding process, which is the most common method for finishing ceramic components. A semicircular groove was made at the center of small bending specimens by the machining. The machined specimens were healed at various temperatures and times in air. The optimized crack-healing condition of the machined specimen was found to be a temperature of 13001C and a time of 1 h. The specimens healed by this condition exhibited almost the same strength as the smooth specimens that underwent the healing process. Moreover, the bending strength and fatigue limit of the machined and healed specimens were systematically investigated at temperatures ranging from room temperature to 13001C. The heat-resistance temperature has been determined to be approximately 10001C. Also, the specimens exhibited high static and cyclic fatigue limits at temperatures of 8001 and 10001C. These results demonstrate that crack healing could be an effective method for improving the structural integrity and reducing the manufacturing costs of a Si 3 N 4 /SiC composite ceramic.

Generic principles of crack-healing ceramics

“…Microcracks in crystalline SiO 2 (quartz) with a length of 100 µm and a width of 10 µm were completely healed by exposure to a water pressure of 200 MPa at 600 ℃ for 4 h [50]. Strength recovery by hightemperature oxidation induced crack healing with environmental oxygen at elevated temperatures was reported from a number of non-oxide ceramics as well as oxide ceramics loaded with SiC [26,30,34,[39][40][41]51,[69][70][71][72][73][74]. For example, annealing of SiC reinforced Al 2 O 3 nanocomposites in inert Ar atmosphere resulted in a strength increase of 50% relative to the unannealed specimen whereas annealing in air yielded a three-fold improvement in the indendation strength [69].…”

“…Lower temperatures of 1000-1300 ℃ were reported for oxidation induced crack healing of surface flaws in a variety of Si-containing ceramics where viscous SiO 2 and silicate based reaction products are able to fill the space between crack walls [36][37][38]. Ceramics containing more than 10 vol.% of SiC particles or whiskers were reported to exhibit efficient crack healing ability triggered by SiC oxidation reaction [39] resulting in a pronounced improvement of strength and reliability as well as reduction of the manufacturing costs [40,41]. Oxidation in air as well as viscous flow were successfully demonstrated to trigger healing reaction in ceramic matrix [42] and in glass matrix [43] fiber composites even at relatively low temperatures of 500-600 ℃.…”

Improvement in Contact Strength of Si3N4/SiC Composite by Crack Healing

“…Smooth specimens developed some minute surface defects during the polishing process. Jung et al [8] reported that the optimum conditions for crack healing a Si 3 N 4 /SiC composite in air are 1300 ∘ C and 1 h. Thus, some Smooth specimens were heat-treated at 1300 ∘ C for 1 h in air. Such specimens are referred to as "Smooth + Healed specimens.…”

“…Lee et al [6] reported that the numerous surface cracks resulting from the heavily machining of SiC-reinforced mullite can be completely healed by the crack-healing treatment. Osada et al [7] investigated the high-temperature strength of machined Al 2 O 3 /SiC composite specimens subjected to crack healing and reported that healed specimens exhibit the same level of strength as the smooth specimens do up to 1300 ∘ C. Jung et al [8] stated that surface cracks in a Si 3 N 4 /SiC composite resulting from wheel grinding are completely healed by heat treatment in air at 1200-1400 ∘ C. These results indicate that crack healing is an effective method for increasing the reliability of machined ceramic components.…”

Influence of sintering and sintering additives on the mechanical and microstructural characteristics of Si3N4/SiC wood cutting tools

“…And due to the crack healing behaviour of Si 3 N 4 /SiC ceramics during HIP, which can reduce the defect size introduced by diamond machining. 34 …”