22 General Improvements In Mechanical Properties

Major progress has been accomplished in the past 20-30 years to increase the capability of ceramics for thermal, wear, corrosion, and structural applications. In particular, the strength and toughness have been dramatically improved to the degree that ceramics are now available that can compete with metals in applications previously thought impossible for ceramics. Figure 2.1 illustrates the level of increases in the key structural characteristics of strength, toughness, and Weibull modulus. [Weibull modulus is the slope of the log-log plot of probability of failure versus fracture stress (strength) for test bars prepared from a block of the material.]

Strength is a measurement of the resistance to formation of a crack or structural damage in the material when a load is applied. Toughness is a measurement of the resistance of the material to propagation of a crack or extension of damage to the point of failure. The Weibull modulus is a measurement of the uniformity in strength. The lower the Weibull modulus, the higher the likelihood that the material will fail at a stress substantially below the average strength. Thus, high Weibull modulus means better material reliability and greater ease in designing with the material.

Most ceramics in the 1960s had strength well below 345 MPa (50,000 psi). Now aluminum oxide, silicon carbide, silicon nitride, and toughened zirconia are available with strength above 690 MPa (100,000 psi). Strength at elevated

0 0

Post a comment