Room temperature fracture toughness improvements in MoSi and NbSi 3 material
Figure 10(a) shows that fracture toughness properties modestly increase with increasing volume fraction of SiC particulates. The toughness seems to saturate to about 4 MPa m I/2 around 50 vol.% SiC, reaching the level of pure SiC. This improvement has been attributed to crack deflection-crack branching processes, or to the residual stresses at the particle-matrix interfaces
. In the XD TM processed composites, SiC is refined to about 2-3 /~m, giving rise to larger increments in toughness at about 20 vol.% SiC. On further increasing the SiC volume fraction, the toughness decreased . In this case, it was observed that the SiC particles fractured and, at volume fraction levels greater than 20%, the fractured SiC particles could have formed a contiguous path, thereby lowering the fracture toughness. On the contrary, TiB2 reinforcements by the same XD TM process seem to give a lower toughening since the TiB 2 particles were pulled out because of weak particle-matrix interfaces . Interestingly, 20 vol.% SiC platelet additions also showed the same amount of toughening as the particles . It is important to point out that these higher toughness measurements were made by short rod tests  or four-point bend tests , while the others were made by indentation fracture [37, 38]. The four-point bend fracture toughness results on MoSi2-20 vol.% VLS SiC whiskers also gave about 8.2 MPa m U2. This may suggest that indentation fracture methods of obtaining fracture toughness may be more conservative than other tests. It should be pointed out that these comparisons are considered to be only qualitative.
In contrast, MoSi2-ZrO 2 composites (Fig. 10(a)) exhibited a monotonic increase in fracture toughness with increasing volume fraction of the ZrO z reinforcement. These composites contain particles of partially stabilized ZrO2, which have a metastable tetragonal crystal structure. In the vicinity of the crack tip stress field, the tetragonal particles transform to the monoclinic crystal structure, with an associated volume increase of about 4%. This crack-induced phase transformation thus produces compressive microstructural stresses which shield the crack tip from the applied external tensile stresses, leading to increased transformation toughening in the composite.