Ceramic composites flaws

S. Suresh and J. R. Breckenbrough, A Theory for Creep by Interfacial Flaw Growth in Ceramics and Ceramic Composites, Acta Metall. Mater., 38[1], 55-68 (1990). 

One of the key issues for both ceramics and ceramic composites is the lack of suitable standards and standard reference materials.51 In principle, this issue can be rectified by development of materials containing known types and numbers of flaws. In practice, it is difficult because of our lack of knowledge about the numbers and types of flaws which are important. Techniques suitable for some monolithic ceramics have been developed which incorporate known internal and surface-connected defects. Using these types of specimens, our knowledge of aspects of NDE related to probability of detection of different kinds of flaws, and the procedures which must be followed to optimize detection, will be increased. 

In tension we are concerned with the largest crack, the critical flaw, particularly if it is on the surface. In compression we are concerned with the average flaw size, c. We can estimate the compressive stress to failure by substituting Cav into Eq. 16.5 and using a multiplier between 10 and 15. Teeth are ceramic composites they survive for years even when many cracks are present. 

With device level LTCCs, the glass/ceramic composites themselves are not used separately, but are formed with conductor wiring in each layer and via conductors between layers. Seen from a macro point of view, they can be characterized as composite materials formed of wiring metal and ceramic. For this reason, in order to improve the strength of the LTCC overall, it is effective to reduce micro and macro flaws in the metal/ceramic interface, and to predict and improve strength, with reference to the equation suggested with fiber reinforced resin material below [48],... 

Concrete is a particulate composite of stone and sand, held together by an adhesive. The adhesive is usually a cement paste (used also as an adhesive to join bricks or stones), but asphalt or even polymers can be used to give special concretes. In this chapter we examine three cement pastes the primitive pozzolana the widespread Portland cement and the newer, and somewhat discredited, high-alumina cement. And we consider the properties of the principal cement-based composite, concrete. The chemistry will be unfamiliar, but it is not difficult. The properties are exactly those expected of a ceramic containing a high density of flaws. 

Padture, N.P., Bennison, S.J. Chan, H.M. (1993) Flaw-tolerance and crack-resistance properties of alumina and aluminium titanate composites with tailored microstructures. J. Am. Ceram. Soc. 76, 2312-2320. 

In some applications the lack of toughness of ceramics or CMCs prohibits their use. In cases where enhanced stiffness, wear resistance, or elevated temperature capabilities greater than those provided by metals are necessary, metal matrix composites (MMCs) offer a reasonable compromise between ceramics or CMCs and metals. Typically, MMCs have discrete ceramic particulate or fiber reinforcement contained within a metal matrix. In comparison to CMCs, MMCs tend to be more workable and more easily formed, less brittle, and more flaw tolerant. These gains come primarily at the expense of a loss of high-temperature mechanical properties and chemical stability offered by CMCs. These materials thus offer an intermediate set of properties between metals and ceramics, though somewhat closer to metals than ceramics or CMCs. Nonetheless, like ceramic matrix composites, they involve physical mixtures of different materials that are exposed to elevated temperature processes, and therefore evoke similar thermodyamic considerations for reinforcement stability. 

TADB-derived ceramic fibers, with the idealized composition SiBNsC, do not reach the E-modulus of the most advanced SiC fibers at room temperature. However, they are clearly superior to the latter in a crucial point, namely the drop of the E-modulus and of the creep resistance at high temperatures. SiC fibers already lose a large part of their mechanical strength below 1400 °C, as can be measured by creep resistance. These limitations are fundamental in nature, since they are related to grain boundary sliding and thus to the crystallinity of SiC. In contrast, amorphous SiBNsC fibers do not show any grain boundaries and, moreover, the concentration of microstructural flaws is extremely low. 

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