Ceramics flexural strength

Properties of Dense Silicon Carbide. Properties of the SiC stmctural ceramics are shown in Table 1. These properties are for representative materials. Variations can exist within a given form depending on the manufacturer. Figure 2 shows the flexure strength of the SiC as a function of temperature. Sintered or sinter/HIP SiC is the preferred material for appHcations at temperatures over 1400°C and the Hquid-phase densified materials show best performance at low temperatures. The reaction-bonded form is utilized primarily for its ease of manufacture and not for superior mechanical properties. 

Alumina, or aluminum oxide [1344-28-17, has a thermal conductivity 20 times higher than that of most oxides (5). The flexural strength of commercial high alumina ceramics is two to four times greater than those of most oxide ceramics. The drawbacks of alumina ceramics are their relatively high thermal expansion compared to the chip material (siUcon) and their moderately high dielectric constant. 

Flexural Strength of High Performance Ceramics at Ambient Temperature," ia U.S. Mmry ProposedMilita Standard, MIE-STD-1942A., U.S. Army Materials Technology Laboratory, Watertown, Mass., June 1990 (supersedes MIL-STD-1942(MR) Nov. 1983). 

The flexural strength will depend on sample size, since with increasing size there is an increase in the probability of the existence of a crack-producing flaw, along with a corresponding decrease in the flexural strength. This test, and others like it such as the four-point bend, are common for brittle materials such as ceramics, which we... 

Flexural strength of Si3N4/BN FM ceramic after thermal shock with temperature difference AT (adapted from ref. [29]). 

Baskaran, S., and Halloran, J.W. (1993), Fibrous monolithic ceramics II, Flexural strength and fracture behavior of the silicon carbide/graphite system , J. Am. Ceram. Soc., 76(9) 2217-2224. 

Effect of porosity on flexural strength of composites with code 1723 matrix (reproduced by permission of Am. Ceram. Soc. Bull.) . 

In addition to the initial work in the alumina and mullite matrix systems previously mentioned, SiC whiskers have also been used to reinforce other ceramic matrices such as silicon nitride,9-13 glass,14 15 magnesia-alumina spinel,16 cordierite,17 zirconia,18 alumina/zirconia,18 19 mullite/zirconia,18-21 and boron carbide.22 A summary of the effect of SiC whisker additions on the mechanical properties of various ceramics is given in Table 2.1. As shown, the addition of whiskers increases the fracture toughness of the ceramics in all cases as compared to the same monolithic materials. In many instances, improvements in the flexural strengths were also observed. Also important is the fact that these improvements over the monolithic materials are retained at elevated temperatures in many cases. 

Fig. 2.10 Flexural strength versus temperature for several fiber-reinforced ceramic composites.52,58-60...

K. M. Prewo, Tensile and Flexural Strength of SiC Fiber-Reinforced Glass-Ceramic Composite , J. Mat. Sci., 21, 3590 (1986). 

High temperature deformation is also an extremely important property of ceramic composites for many applications. The techniques required for measuring creep are very similar to testing techniques for determining tensile strength and flexural strength. These techniques, as well as creep properties and mechanisms, are covered in Chapter 4. 

Flexural strength, psi (MPa) Fly ash, wollastonite, and 1-3% glass fiber 1300-1700(9-12) ==700 (4.9) MaCTO-encapsulation of radioactive objects, structural ceramics... 

Silicon nitride ceramics should have high flexural strength and fracture toughness at room temperature and good creep resistance at elevated temperatures. Because these properties are determined by the microstructure of these ceramics, further improvement of these ceramics is possible through alloy design. The following sections will be devoted to micro-... 

Lai and Tien [26] (Fig. 15) have shown that the flexure strength and fracture toughness of silicon nitride ceramics depends on the aspect ratio... 

Table 4.1 further shows essential mechanical properties of several products of CeramTec s BIOLOX family of alumina-based materials for femoral heads of hip endoprostheses as well as those of BIONIT manufactured by Mathys Orthopadie GmbH (Bettlach, Switzerland). It is evident that decreasing the grain size of the ceramic precursor powders increases both the flexural strength and the fracture toughness of the material dramatically. 

It has been known for some time that considerable improvement of the mechanical properties of alumina in terms of flexural strength, fracture toughness, yield strength and elastic modulus can be achieved by adding particles of stabilised zir-conia as a structural reinforcement (see Chapter 4.1.2). Such ceramics are known as ZTA (zirconia-toughened alumina), duplex ceramics or dispersion ceramics. 

Finally, another important ramification of the stochastic nature of brittle fracture is the effect of the stress distribution during testing on the results. When a batch of ceramics is tested in tension, the entire volume and surface are subjected to the stress. Thus a critical flaw anywhere in the sample will propagate with equal probability. In three- or four-point flexure tests, however, only one-half the sample is in tension, and the other one-half is in compression. In other words, the effective volume tested is, in essence, reduced. It can be shown that the ratio of the tensile to flexural strength for an equal probability of survival is... 

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