Fracture boron carbides

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. 

The development of ceramic materials for armor since 1970 has been extensive, In addition to alumina and titanium diboride, the most widely used ceramic materials are silicon carbide, boron carbide, and aluminum nitride, as monolithic plates and shapes, which are bonded to a fibrous laminate of fiberglass or Kevlar . A typical impact sequence is shown in Fig. 16.2. On impact, the ceramic plate fractures the projectile core and absorbs a major part of the kinetic energy. The backing material absorbs the residual energy. 

The action of ultrasonic-activated abrasive particle suspension determines the materials removal rate, which is affected by the particles itself, their size and concentration in the suspension. The ideal abrasive particles have high hardness and high compressive strength, a lot of sharp edges and usual fracture behaviour [281,282]. The use of diamond particles allows high ablation rates because of their enormous hardness. However, they are expensive. Boron carbide is an alternative material, which allows an ablation rate of 90% as compared to diamond grains at a significant lower cost. Silicon carbide can also be used however its ablation rate is even lower than those for boron carbide. 

A method was developed for the examination of boron nitride ceramics by automated ultrasonic inspection [57]. A comparison of sliding and abrasive wear mechanisms in ceramics and cemented carbides shows that a-BN ceramics are quite susceptible to indentation fracture [58]. Many more additional applications are reported in Section 4.1.1.10.8, p. 129. 

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