Ceramics yield strength

Most ceramics have enormous yield stresses. In a tensile test, at room temperature, ceramics almost all fracture long before they yield this is because their fracture toughness, which we will discuss later, is very low. Because of this, you cannot measure the yield strength of a ceramic by using a tensile test. Instead, you have to use a test which somehow suppresses fracture a compression test, for instance. The best and easiest is the hardness test the data shown here are obtained from hardness tests, which we shall discuss in a moment. 

As well as being a good way of measuring the yield strengths of materials like ceramics, as we mentioned above, the hardness test is also a very simple and cheap nondestructive test for (Ty. There is no need to go to the expense of making tensile specimens, and the hardness indenter is so small that it scarcely damages the material. So it can be used for routine batch tests on materials to see if they are up to specification on without damaging them. 

Let us now see whether materials really show this strength. The bar-chart (Fig. 9.2) shows values of Oy/E for materials. The heavy broken line at the top is drawn at the level it/E = 1/15. Glasses, and some ceramics, lie close to this line - they exhibit their ideal strength, and we could not expect them to be stronger than this. Most polymers, too, lie near the line - although they have low yield strengths, these are low because the moduli are low. 

All metals, on the other hand, have yield strengths far below the levels predicted by our calculation - as much as a factor of 10 smaller. Even ceramics, many of them, yield at stresses which are as much as a factor of 10 below their ideal strength. Why is this ... 

The result is work-hardening the steeply rising stress-strain curve after yield, shown in Chapter 8. All metals and ceramics work-harden. It can be a nuisance if you want to roll thin sheet, work-hardening quickly raises the yield strength so much that you have to stop and anneal the metal (heat it up to remove the accumulated dislocations) before you can go on. But it is also useful it is a potent strengthening method, which can be added to the other methods to produce strong materials. 

Because of this, the data listed in Table 15.7 for ceramic materials differ in emphasis from those listed for metals. In particular, the Table shows the modulus of rupture (the maximum surface stress when a beam breaks in bending) and the thermal shoek resist-anee (the ability of the solid to withstand sudden changes in temperature). These, rather than the yield strength, tend to be the critical properties in any design exercise. 

The ultimate goal of assemblies of nanoscale MBBs is to create nanostructures with improved properties and functionality heretofore unavailable to conventional materials and devices. As a result, one should be able to alter and engineer materials with desired properties. For example, ceramics and metals produced through controlled consolidation of their MBBs are shown to possess properties substantially improved and different from materials with coarse microstmctures. Such different and improved properties include greater hardness and higher yield strength in the case of metals and better ductility in the case of ceramic materials [102]. 

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. 

E Young s modulus, YS yield strength, UTS tensile strength, CTE coefficient of thermal expansion, %E1 percent elongation, K thermal conductivity. registered trademark of Morgan Advanced Ceramics, Hayward, CA... 

A model due to Eager et estimates the strain energy in the ceramic in well-bonded ceramic-metal joints. For a small CTE mismatch between the ceramic (C) and the metal substrate (M), but with a large CTE mismatch between the interlayer (I) and the base materials, the elastic strain energy, Uec. >i> the ceramic for a disc-shaped joint is calculated in terms of the yield strength (oyi) of the braze, the radial distance from the center of the joint, and the elastic moduli of the ceramic (Ec) and braze (Ei). Eager et al proposed analytical expressions to calculate Uec as asymptotic approximations (to 1% accuracy) to their finite element calculations these analytical expressions (equations [l]-[3] in ref ) are used here to estimate the strain energy. 

The yield strength (Oy) is expressed in terms of the yield stress, Oo (Oo is related to the intrinsic stress, Oi, resisting dislocation motion) and the grain size, d. When this relationship was deduced, it applied to a situation in which the grains were deformed by plastic deformation and the GBs acted as barriers to dislocation motion. This model is unlikely to be valid in general for ceramics since deformation by dislocation glide is not common. However, the relationship between d and Oy does hold as we saw for polycrystalline MgO in Figure 1.2. 

Figure 17.11 Data from Jiang, B. and Weng, G.J. (2004) A theory of compressive yield strength of nano-grained ceramics , Int. J. Plasticity, 20, 2007.

The relationship between yielding strength and the volume percentage of ceramic phases diameter of the ceramic granules can be described as equation 1... 

Where yielding strength of Ti(C,N)-based cermets, is the volume percentage of ceramic ]diases, [Pg.47]

When tf equals to 3 and 5 pm respectively, one can calculate the volume percentage of ceramic phases at maximum yielding strength using difterential max method. The volume percentage of ceramic phases at... 

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