**4.2 Mechanical properties - hardness and fracture toughness**

Hardness is a key performance index of materials and is mainly tested by the indentation method. The Vickers hardness indentations were performed on the polished samples of these materials under an applied load of up to 9.8 N and a dwelling time of 15 s. **Figure 9(a)** and **(b)** show the Vickers hardness (Hv) indenter before testing with a standard indenter made of a single crystal diamond, and the indenter that is damaged after pressing it into a CFCD sample, respectively. The tests were
