The irradiation effect on nanoindentation hardness of Rohm and Haas CVD SiC in a fluence range of 0.1-18.7 dpa is summarized in Figure 15. It is interest­ing to note that the nanoindentation hardness exhibits relatively small scatter for the individual experiments, and the trend in data as a function of temperature is uniform. This observation is in contrast to both the flexural strength and the indentation fracture tough­ness data, which indicate a broad peak at an intermedi­ate temperature and a relatively large scatter. It is worth noting that nanoindentation hardness of brittle ceramics is, in general, determined primarily by the dynamic crack extension resistance in the near surface bulk material, and therefore should be more relevant to fracture toughness than to plastic deformation resis­tance. However, surface effects of the original sample affect the nanoindentation hardness less, as the samples are generally polished prior to testing.

4.07.4.2 Fracture Toughness of Monolithic SiC

The effect of irradiation on the fracture toughness of Rohm and Haas CVD SiC is summarized in Figure 16. This compilation plots data using the Chevron notched beam technique, although the bulk of the
data sets report Vicker’s or nanoindentation gener­ated data.55-57 The general trend is that the irradiation-induced toughening seems to be signifi­cant at 573-1273 K for the indentation fracture tough­ness data, in spite of the decrease in elastic modulus, which confirms the increase in fracture energy caused by irradiation. The scatter of the indentation fracture toughness data among different experiments is likely caused by both the condition of the sample surface and the lack ofstandardized experimental procedures. Typ­ically, indentation should be applied on the polished surfaces, but conditions of polishing are not always provided in literature. Moreover, the crack length mea­surements are done using optical microscopy, conven­tional scanning electron microscopy (SEM), or field emission SEM, all of which may give very different crack visibility. In addition, a few different models have been used for derivation of the fracture toughness. In conclusion, indentation fracture toughness techni­ques can be used only for qualitative comparison within a consistent set of experiments. It is noted that the experiment employing the Chevron notched beam technique also indicates the irradiation-induced tough­ening, although scatters of toughness values were even greater. These results lead to the conclusion that, in the intermediate irradiation temperature range, the increase of the fracture toughness of SiC exists.