Ceramic microcracking

In polycrystalline ceramics, microcracks can develop at the grain boundaries. The reason for this is the difference in the orientation of two adjacent grains. When the material is heated to a higher temperature, the grains will expand differentially along their different axes. On cooling from this higher temperature, the contraction also will be differential. Because of this differential expansion and contraction, thermal stresses will be built up in the material. 

Fig. 16.7. Microstructural features of a crystalline ceramic grains, grain boundaries, pores, microcracks and second phases.

Vitreous ceramics are made waterproof and strengthened by glazing. A slurry of powdered glass is applied to the surface by spraying or dipping, and the part is refired at a lower temperature (typically 800°C). The glass melts, flows over the surface, and is drawn by capillary action into pores and microcracks, sealing them. 

S. D. Beyea, B. J. Balcom, T. W. Bremn-er, R. L. Armstrong, P. E. Grattan-Bellew 2003, (Detection of microcracking in cementious materials with space resolved 1H nuclear magnetic resonance relaxometry), J. Am. Ceram. Soc. 86 (5), 800-805. 

Figure 5.113 Microcrack parallel to fibers under applied stress, Oa- Reprinted, by permission, from R. W. I. Davidge, and 1. 1. R. Davies, in Mechanical Testing of Engineering Ceramics at High Temperatures, B. F. Dyson, R. D. Lohr, and R. Morrell, eds., p. 253. Copyright 1989 by Elsevier Science Publishers, Ltd.

Fe impurities may cause snow flakes , because of the nucleation and crystallisation of YAG (Y3Al50i2) [532]. The volume change during crystallisation of the grain boundary phase leads to internal stresses which can be cause micropores, or microcracks, or relax by other mechanisms [534, 535]. Such microcracks have only been detected in ceramics with crystallised / -Y2Si207 as grain boundary phase [533]. 

Matrix microcracking may also impose a significant limitation on the engineering use of ceramic composites through its effect on strength at... 

G. F. Raiser, R. J. Clifton, and M. Ortiz, A Soft-Recovery Plate Impact Experiment for Studying Microcracking in Ceramics, Mechanics of Materials, 10, 1-43 (1990). 

---