Compressive ceramic

Calculate the volume of a tablet made of compressed ceramic powder by pressing it in a 54 cm cavity applying a pressure of 316 MPa. The process of pressurization followed the model described by Equation 5.6, and the correlation constants m and b were determined as -4.402 and 4.1157, respectively. 

Stress-strain curves for metals look very similar and provide similar results whether the testing is carried out in tension or compression. Ceramics are generally stronger in compression and can tolerate high compressive loads. Some examples are given in Table 16.6. However, reliable compressive strength data are limited for ceramics. Note that the Young s modulus will be the same because the curves will have the same slope. 

The high elastic modulus, compressive strength, and wear resistance of cemented carbides make them ideal candidates for use in boring bars, long shafts, and plungers, where reduction in deflection, chatter, and vibration are concerns. Metal, ceramic, and carbide powder-compacting dies and punches are generahy made of 6 wt % and 11 wt % Co ahoys, respectively. Another apphcation area for carbides is the synthetic diamond industry where carbides are used for dies and pistons (see Carbon). 

Plastic Forming. A plastic ceramic body deforms iaelastically without mpture under a compressive load that produces a shear stress ia excess of the shear strength of the body. Plastic forming processes (38,40—42,54—57) iavolve elastic—plastic behavior, whereby measurable elastic respoase occurs before and after plastic yielding. At pressures above the shear strength, the body deforms plastically by shear flow. 

Ton-exchange approaches and thermal tempering have been evaluated for strengthening dental ceramics (18,19). Both of these approaches are aimed at placing external surfaces of dental ceramic restoration in compression. Only ion exchange is promoted commercially and is not in extensive use. 

Graphite and ceramic vessels are used fully armored that is, they are enclosed within metal pressure vessels. These materials are also used for boxlike vessels with backiug plates on the sides. The plates are drawn together by tie bolts, thus putting the material in compression so that it can withstand low pressure. 

M.E. Kipp and D.E. Grady, Shock Compression and Release in High-Strength Ceramics, SAND89-1461, Sandia National Laboratories, Albuquerque, NM, 1989. 

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. 

Creep tests require careful temperature control. Typically, a specimen is loaded in tension or compression, usually at constant load, inside a furnace which is maintained at a constant temperature, T. The extension is measured as a function of time. Figure 17.4 shows a typical set of results from such a test. Metals, polymers and ceramics all show creep curves of this general shape. 

Fig. 17.2. Tests which measure the fracture strengths of ceramics, (a) The tensile test measures the tensile strength, CTj. (b) The bend test measures the modulus of rupture, o , typically 1.7 x CTj. (<) The compression test measures the crushing strength, a, typically 15 x...

Why are ceramics usually much stronger in compression than in tension ... 

The compressive strength of composites is less than that in tension. Tills is because the fibres buckle or, more precisely, they kink - a sort of co-operative buckling, shown in Fig. 25.5. So wliile brittle ceramics are best in compression, composites are best in tension. 

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