## 7 Other Methods for Assessing Diffusion-Bonded Interfaces
### Interfacial Indentation  
Interfacial indentation is similar to the indentation-based technique used to measure the fracture toughness of brittle materials such as ceramics [73; 74]. The test specimen is subjected to loading from a Vickers indenter and cracking is initiated at the corners of the indent if the load exceeds a critical value [75]; the fracture toughness is calculated using the crack length. In interfacial indentation, the specimen is mounted in transverse section and a Vickers indenter is used to propagate cracks along the coating-substrate interface (shown in schematic form in Figure 14) when a critical load is reached. The crack length can then be used to obtain the interfacial fracture toughness [69] which could provide a measure of bond strength.  
[PERSON] et al. [63] used Vickers microhardness indentations to propagate cracks along the interface of NiTi bonded to Ti\({}_{3}\)SiC\({}_{2}\). An example of this can be seen in Figure 15. This indent, which was made at a load of 0.3 kg, generated a crack (approximately 60 \(\upmu\)m in length) that initiated close to the NiTi/Ti\({}_{3}\)SiC\({}_{2}\) interface and propagated between Ti\({}_{3}\)SiC\({}_{2}\) and the interfacial reaction layer and partially through Ti\({}_{3}\)SiC\({}_{2}\). This demonstrated the brittle nature of this interfacial region, although the convoluted paths, which also included crack branching, suggested some ductility in this region despite the higher hardness and modulus than the parent materials.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Bonding Conditions** & **Mean Bond Strength (MPa)** & **Interlayer Structure** \\ \hline
500 \({}^{\circ}\)C/180 min & 262 \(\pm\) 17 &
\begin{tabular}{l} \(\alpha_{\text{c}}\)-Al\({}_{15}\)(Fe,Cr,Mn)\({}_{3}\)Si\({}_{2}\), \\ (0.25 \(\pm\) 0.07 \(\upmu\)m thickness) \\ \end{tabular} \\ \hline
550 \({}^{\circ}\)C/10 min & 239 \(\pm\) 51 &
\begin{tabular}{l} \(\alpha_{\text{c}}\)-Al\({}_{15}\)(Fe,Cr,Mn)\({}_{3}\)Si\({}_{2}\), (-0.8 \(\upmu\)m thickness); AND \\ \(\tau_{1}\)-FeNiAl\({}_{3}\), (\(\leq\)0.5 \(\upmu\)m thickness) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 3: Tensile strength data for Al/Fe bonded under two different conditions and associated details of interlayer structure [19].  
While interfacial indentation may be difficult in cases where the interfacial regions are smaller than the indent size (a point made by [PERSON] et al. [63]), it may be possible for interfaces where there are fewer discrete phases to provide some more quantitative  
Figure 14: Principle of the interface indentation test (**a**); and schematic representation of the results (**b**) [72].  
Figure 15: (**a**) Secondary electron (SE) image of the indent made during a Vickers microhardness test at a load of 0.3 kg on the NiTi–Ti\({}_{3}\)SiC\({}_{2}\) interface bonded at 1000 \({}^{\circ}\)C for 1 h; (**b**) backscattered electron image of the “Area A” marked in (**a**); and (**c**) high-magnification BSE image of the zigzag crack pattern obtained in the interface as shown in (**b**) [63].  
understanding of the bond strength. However, this technique is dependent on the accurate placement of the indents on the interface to propagate the cracks along the bond line.