Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 130

Crack formation in metals subjected to a tensile stress, for example, often occurs by linking up of microvoids already present in the material, resulting in the formation of an elongated crack perpendicular to the loading direction. Ceramics often contain residual pores and crack‐like voids resulting mainly from their fabrication process (Chapter 7). Ceramics can also develop microcracks during cooling from their fabrication temperature due to differences in thermal expansion coefficient between adjacent grains when the grains have a non‐cubic crystal structure. This difference in thermal expansion coefficient generates local stresses during cooling in much the same way that a stress develops upon cooling a sandwich structure composed of two well‐bonded metal sheets that have different thermal expansion coefficients. If sufficiently high, these local stresses can generate microcracks in the ceramic.