Flexural Strength, Fracture Toughness, and Hardness of Silicon Carbide and Boron Carbide Armor Ceramics

Abstract

Armor‐grade B4C and SiC specimens were analyzed for phase assemblage, microstructure, and mechanical properties. SiC–N showed the highest four‐point fracture strength, and an ∼50% higher notched beam fracture toughness than solid‐state sintered B4C and SiC. This was attributed to preferential crack propagation along a weaker amorphous aluminosilicate grain‐boundary interphase, which also attenuated the effect of surface flaws on bending strength. Verco B4C showed the highest hardness. That material was phase pure, fully dense, and of finer grain size as compared with pressure‐assisted densification (PAD)‐B4C (hot pressed). Verco SiC showed a hardness equal to (Vickers) or higher than (Knoop) PAD‐B4C, and a comparatively narrow distribution in measured hardnesses. This was attributed to a fine‐grained, fully dense, solid‐state sintered microstructure with a fine and well‐distributed graphite second phase. Hardness of all specimens decreased with increasing applied load.

The rate-dependent fracture toughness of silicon carbide- and boron carbide-based ceramics

“…9 and 10) appears contradictory when the major difference between the two categories of materials is the presence of a residual silicon phase, which is a highly brittle material with a lower fracture toughness value than the other constituents (K Ic for polycrystalline Si is ≤1.0 MPa-m 1/2 [44], whereas B 4 C is 2.7-2.9 MPa-m 1/2 [45] and SiC is 2.4-2.6 MPa-m 1/2 measured in this investigation). The results of this investigation also indicate that the fracture toughness increases with an increase in silicon phase content, as illustrated by the quasi-static results in Fig.…”

Mesoscale modeling of nonlinear elasticity and fracture in ceramic polycrystals under dynamic shear and compression

“…Uniform cohesive properties (i.e., fracture strength and fracture energy) are assigned to all grain boundaries in a given microstructure. In real ceramic polycrystals, variability in fracture properties may arise from voids, inclusions, and secondary phases, though in some cases secondary phases may be incorporated deliberately to improve fracture toughness (Faber and Evans, 1983;Shih et al, 1998;Vargas-Gonzalez et al, 2010). Grain boundary misorientation may also influence local fracture properties.…”

“…Greater physical realism would be attained from finite element meshes of microstructures obtained from sectioned material samples, with intial lattice orientation distributions obtained from EBSD measurements, for example (Brahme et al, 2006;Rollett et al, 2007). In particular, SiC-N can often exhibit relatively elongated grains (Lee et al, 2005;Ray et al, 2007;Vargas-Gonzalez et al, 2010), whereas AlON often exhibits relatively equiaxed grains (Corbin, 1989;McCauley et al, 2009;Guo et al, 2011). 2.…”

“…This is a reasonable assumption for certain varieties of SiC (e.g., SiC-N) that contain additives that segregate at grain boundaries, leading to a tendency for grain boundary fracture over cleavage and corresponding to increased toughness (Faber and Evans, 1983;Shih et al, 1998;Lee et al, 2005;Vargas-Gonzalez et al, 2010). On the other hand, this assumption may be less physically reasonable for AlON, in which cleavage fractures have been observed (McCauley et al, 2009).…”

An Analytical Theory for Radial Crack Propagation: Application to Spherical Indentation

“…•^Vargas-Gonzalez and Speyer [22] reported average fracture toughness values of Kc ~ 2.5^.5 MPaV»! for several grades of SiC measured from four-point bend tests.…”