Influence of surface defects on the biaxial strength of a silicon nitride ceramic – Increase of strength by crack healing

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 ℃.…”

Silicon nitride ceramic development in Thales Alenia Space : qualification achievement and further developments for future applications

“…Nevertheless, FCT has experimented another post-treatment on flexure samples, on which some surface defects were created after manufacturing, by grinding (different grit size of the grinding wheel, grinding direction in rectangular direction) or artificial cracks by Vickers indentation [5] [6]. This crack healing treatment have brought very interesting results : the strength decrease caused with the artificial defects was strongly reduced.…”

Synthesis/Processing of Silicon Nitride Ceramics

“…Sometimes a simple heating in a slightly oxidative environment will suffice. Alternatively, one can heat the material surface via laser or infrared heat source, to heal surface cracks (Harrer, Danzer, & Morrell, 2012;Sun, Malshe, Jiang, & McCluskey, 2006).…”